Advertisement

MYC as a target for cancer treatment

  • Michael J. Duffy
    Correspondence
    Corresponding author at: Clinical Research Centre, St. Vincent’s University Hospital, Dublin, Ireland.
    Affiliations
    UCD School of Medicine, Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Dublin 4, Ireland

    UCD Clinical Research Centre, St. Vincent’s University Hospital, Dublin 4, Ireland
    Search for articles by this author
  • Author Footnotes
    1 Both theses authors contributed equally to the writing of this manuscript.
    Shane O'Grady
    Footnotes
    1 Both theses authors contributed equally to the writing of this manuscript.
    Affiliations
    UCD School of Medicine, Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Dublin 4, Ireland
    Search for articles by this author
  • Author Footnotes
    1 Both theses authors contributed equally to the writing of this manuscript.
    Minhong Tang
    Footnotes
    1 Both theses authors contributed equally to the writing of this manuscript.
    Affiliations
    UCD School of Medicine, Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Dublin 4, Ireland
    Search for articles by this author
  • John Crown
    Affiliations
    Department of Medical Oncology, St Vincent’s University Hospital, Dublin 4, Ireland
    Search for articles by this author
  • Author Footnotes
    1 Both theses authors contributed equally to the writing of this manuscript.
Open AccessPublished:January 18, 2021DOI:https://doi.org/10.1016/j.ctrv.2021.102154

      Highlights

      • The MYC family of proto-oncogenes are frequently altered in human cancers and represent an attractive target for therapy.
      • Therapeutic targeting of MYC has previously been hampered by difficulties in drug development.
      • Newly developed agents can target MYC through multiple mechanisms, including inhibition of MAX binding and blocking MYC-DNA interaction.
      • Some of these agents are now progressing through clinical trials.

      Abstract

      The MYC gene which consists of 3 paralogs, C-MYC, N-MYC and L-MYC, is one of the most frequently deregulated driver genes in human cancer. Because of its high prevalence of deregulation and its causal role in cancer formation, maintenance and progression, targeting MYC is theoretically an attractive strategy for treating cancer. As a potential anticancer target, MYC was traditionally regarded as undruggable due to the absence of a suitable pocket for high-affinity binding by low molecular weight inhibitors. In recent years however, several compounds that directly or indirectly inhibit MYC have been shown to have anticancer activity in preclinical tumor models. Amongst the most detailed investigated strategies for targeting MYC are inhibition of its binding to its obligate interaction partner MAX, prevention of MYC expression and blocking of genes exhibiting synthetic lethality with overexpression of MYC. One of the most extensively investigated MYC inhibitors is a peptide/mini-protein known as OmoMYC. OmoMYC, which acts by blocking the binding of all 3 forms of MYC to their target promoters, has been shown to exhibit anticancer activity in a diverse range of preclinical models, with minimal side effects. Based on its broad efficacy and limited toxicity, OmoMYC is currently being developed for evaluation in clinical trials. Although no compound directly targeting MYC has yet progressed to clinical testing, APTO-253, which partly acts by decreasing expression of MYC, is currently undergoing a phase I clinical trial in patients with relapsed/refractory acute myeloid leukemia or myelodysplastic syndrome.

      Keywords

      Introduction

      MYC is one of the most widely investigated cancer-causing genes, being implicated in the formation, maintenance and progression of several different cancer types [
      • Dang C.V.
      A time for MYC: metabolism and therapy.
      ,
      • Beaulieu M.E.
      • Castillo F.
      • Soucek L.
      Structural and biophysical insights into the function of the intrinsically disordered Myc oncoprotein.
      ,
      • Carroll P.A.
      • Freie B.W.
      • Mathsyaraja H.
      • Eisenman R.N.
      The MYC transcript network: balancing metabolism, proliferation and oncogenesis.
      ]. The MYC gene family consists of 3 members, C-MYC, L-MYC and N-MYC, all of which belong to the superfamily of basic helix-loop-helix leucine zipper (bHLHLZ) DNA binding proteins [
      • Dang C.V.
      A time for MYC: metabolism and therapy.
      ,
      • Beaulieu M.E.
      • Castillo F.
      • Soucek L.
      Structural and biophysical insights into the function of the intrinsically disordered Myc oncoprotein.
      ,
      • Carroll P.A.
      • Freie B.W.
      • Mathsyaraja H.
      • Eisenman R.N.
      The MYC transcript network: balancing metabolism, proliferation and oncogenesis.
      ]. MYC proteins largely function as transcriptional modulators, regulating genes involved in several different cellular processes including cell growth, cell cycle, differentiation, apoptosis, angiogenesis, metabolism, DNA repair, protein translation, immune response and stem cell formation [
      • Dang C.V.
      A time for MYC: metabolism and therapy.
      ,
      • Beaulieu M.E.
      • Castillo F.
      • Soucek L.
      Structural and biophysical insights into the function of the intrinsically disordered Myc oncoprotein.
      ,
      • Carroll P.A.
      • Freie B.W.
      • Mathsyaraja H.
      • Eisenman R.N.
      The MYC transcript network: balancing metabolism, proliferation and oncogenesis.
      ,
      • Kortlever R.M.
      • Sodir N.M.
      • Wilson C.H.
      • Burkhart D.L.
      • Pellegrinet L.
      • Brown Swigart L.
      • et al.
      Myc cooperates with RAS by programming inflammation and immune suppression.
      ]. The specific effects exerted by MYC appear to be at least partly dependent on the cellular context as well as possibly on the cellular levels of MYC protein [
      • Cappellen D.
      • Schlange T.
      • Bauer M.
      • Maurer F.
      • Hynes N.E.
      Novel c-MYC target genes mediate differential effects on cell proliferation and migration.
      ,
      • Murphy D.J.
      • Junttila M.R.
      • Pouyet L.
      • Karnezis A.
      • Shchors K.
      • Bui D.A.
      • et al.
      Distinct thresholds govern Myc's biological output in vivo.
      ].
      Overall, MYC proteins are believed to regulate greater than 15% of the human genome [
      • Li Z.
      • Van Calcar S.
      • Qu C.
      • Cavenee W.K.
      • Zhang M.Q.
      • Ren B.
      A global transcriptional regulatory role for c-Myc in Burkitt's lymphoma cells.
      ,
      • Fernandez P.C.
      • Frank S.R.
      • Wang L.
      • Schroeder M.
      • Liu S.
      • Greene J.
      • et al.
      Genomic targets of the human c-Myc protein.
      ,
      • Zeller K.I.
      • Zhao X.
      • Lee C.W.
      • Chiu K.P.
      • Yao F.
      • Yustein J.T.
      • et al.
      Global mapping of c-Myc binding sites and target gene networks in human B cells.
      ,
      • Dang C.V.
      • O'Donnell K.A.
      • Zeller K.I.
      • Nguyen T.
      • Osthus R.C.
      • Li F.
      The c-Myc target gene network.
      ] and control transcription mediated by all 3 RNA polymerases, i.e., RNA polymerase I, II and III [
      • de Pretis S.
      • Kress T.R.
      • Morelli M.J.
      • Sabò A.
      • Locarno C.
      • Verrecchia A.
      • et al.
      Integrative analysis of RNA polymerase II and transcriptional dynamics upon MYC activation.
      ,

      Grandori C, Gomez-Roman N, Felton-Edkins ZA, Ngouenet C, Galloway DA, Eisenman RN, White RJ. c-Myc binds to human ribosomal DNA and stimulates transcription of rRNA genes by RNA polymerase I. Nat Cell Biol 2005;7:311-8. Erratum in: Nat Cell Biol 2005;7:531.

      ,
      • Gomez-Roman N.
      • Grandori C.
      • Eisenman R.N.
      • White R.J.
      Direct activation of RNA polymerase III transcription by c-Myc.
      ]. Between 2000 and 4000 different genes were found to be regulated by MYC [
      • de Pretis S.
      • Kress T.R.
      • Morelli M.J.
      • Sabò A.
      • Locarno C.
      • Verrecchia A.
      • et al.
      Integrative analysis of RNA polymerase II and transcriptional dynamics upon MYC activation.
      ,
      • Sabò A.
      • Kress T.R.
      • Pelizzola M.
      • de Pretis S.
      • Gorski M.M.
      • Tesi A.
      • et al.
      Selective transcriptional regulation by Myc in cellular growth control and lymphomagenesis.
      ]. Because of its ability to regulate widespread gene expression, MYC is sometimes referred to as a “master gene regulator”. However, whether MYC is a general or specific regulator of gene expression is still unclear.
      Although regulation of transcription is the best-established function of MYC, the oncoprotein has also been reported to play a role in DNA synthesis [
      • Dominguez-Sola D.
      • Ying C.Y.
      • Grandori C.
      • Ruggiero L.
      • Chen B.
      • Li M.
      • et al.
      Non-transcriptional control of DNA replication by c-Myc.
      ] and protein translation [
      • Singh K.
      • Lin J.
      • Zhong Y.
      • Burčul A.
      • Mohan P.
      • Jiang M.
      • et al.
      c-MYC regulates mRNA translation efficiency and start-site selection in lymphoma.
      ]. A specific protein potentially important in MYC-mediated tumorigenesis found to be regulated at the translational stage, at least in a liver cancer model, is the immune checkpoint molecule, programmed-death-ligand 1 (PD-L1) [
      • Xu Y.
      • Poggio M.
      • Jin H.Y.
      • et al.
      Translation control of the immune checkpoint in cancer and its therapeutic targeting.
      ]. PD-L1, following binding to its receptor PD-1, promotes immune evasion, a process that is important in MYC-mediated tumorigenesis (see below).
      Multiple studies extending back over several decades have shown that MYC is causally involved in the growth, progression and maintenance of cancers of diverse origins [
      • Dang C.V.
      A time for MYC: metabolism and therapy.
      ,
      • Beaulieu M.E.
      • Castillo F.
      • Soucek L.
      Structural and biophysical insights into the function of the intrinsically disordered Myc oncoprotein.
      ,
      • Carroll P.A.
      • Freie B.W.
      • Mathsyaraja H.
      • Eisenman R.N.
      The MYC transcript network: balancing metabolism, proliferation and oncogenesis.
      ]. Indeed, dysregulation of MYC gene expression is believed to occur in up to 70% of human cancers [
      • Dang C.V.
      MYC on the path to cancer.
      ,
      • Meyer N.
      • Penn L.Z.
      Reflecting on 25 years with MYC.
      ]. Because of its widespread deregulation and causal role in cancer formation and/or progression, targeting MYC is a potential new strategy for treating malignancy. However, MYC, like RAS and TP53 (p53), has historically been regarded as an “undruggable” or a difficult to drug gene.
      Despite been traditionally referred to as “undruggable”, considerable progress has recently been made in the development of MYC inhibitors. Indeed, several compounds that directly or indirectly target MYC have recently been reported to possess anticancer activity in preclinical tumor models. The aim of this article is to critically review these developments and discuss the current status of MYC inhibitors for cancer treatment. First, however, we briefly review the structure of the MYC protein, its mode of action and its deregulation in malignancy. As C-MYC is the most widely deregulated form of MYC in cancer and the most extensively investigated, most of the article will focus on it. In this article, MYC will refer to C-MYC unless otherwise stated.

      Structure of MYC protein

      All 3 MYC proteins have essentially the same multi-domain-type structure and as mentioned above, all belong to the superfamily of bHLHLZ transcription regulators [
      • Dang C.V.
      A time for MYC: metabolism and therapy.
      ,
      • Beaulieu M.E.
      • Castillo F.
      • Soucek L.
      Structural and biophysical insights into the function of the intrinsically disordered Myc oncoprotein.
      ,
      • Carroll P.A.
      • Freie B.W.
      • Mathsyaraja H.
      • Eisenman R.N.
      The MYC transcript network: balancing metabolism, proliferation and oncogenesis.
      ]. All contain 3 main domains, an N-terminal region containing the transactivation domain, a central region implicated in nuclear localization as well as stability control and a C-terminal region involved in interaction with its obligate partner, MAX as well as binding to DNA [

      Conacci-Sorrell M, McFerrin L, Eisenman RN. An overview of MYC and its interactome. Cold Spring Harb Perspect Med 2014;4:a014357. Review.

      ]. Dimerization with MAX is essential for MYC to regulate gene transcription [
      • Dang C.V.
      A time for MYC: metabolism and therapy.
      ].
      A characteristic of the 3 MYC proteins is the presence of multiple highly conserved sequences, known as MYC or M boxes (MB) [

      Conacci-Sorrell M, McFerrin L, Eisenman RN. An overview of MYC and its interactome. Cold Spring Harb Perspect Med 2014;4:a014357. Review.

      ,
      • Kalkat M.
      • Resetca D.
      • Lourenco C.
      • Chan P.K.
      • Wei Y.
      • Shiah Y.J.
      • et al.
      MYC Protein interactome profiling reveals functionally distinct regions that cooperate to drive tumorigenesis.
      ]. Five MB are present in L-MYC and 6 in both C- and N-MYC [
      • Kalkat M.
      • Resetca D.
      • Lourenco C.
      • Chan P.K.
      • Wei Y.
      • Shiah Y.J.
      • et al.
      MYC Protein interactome profiling reveals functionally distinct regions that cooperate to drive tumorigenesis.
      ]. Extending from the N-terminal region, these 6 MB in C- and N-MYC are dubbed MB0, MBI, MBII, MBIIIa, MBIIIb and MBIV. L-MYC however, lacks the MBIIIa box. MB0, MBI and MBII are located within the transactivation domain of the MYC proteins, while the other M boxes are present in the central region of the proteins (Fig. 1) [
      • Kalkat M.
      • Resetca D.
      • Lourenco C.
      • Chan P.K.
      • Wei Y.
      • Shiah Y.J.
      • et al.
      MYC Protein interactome profiling reveals functionally distinct regions that cooperate to drive tumorigenesis.
      ].
      Figure thumbnail gr1
      Fig. 1Domain structure of C-MYC protein showing the transaction domain, MYC boxes, central region, basic region, helix-loop-helix domain and leucine-zipper domain.
      The different MB domains bind to different proteins and thus have distinct roles in MYC functioning. MB0 interacts with the TFIIF transcription elongation complex to regulate transcription and has been implicated in accelerating tumor growth [
      • Kalkat M.
      • Resetca D.
      • Lourenco C.
      • Chan P.K.
      • Wei Y.
      • Shiah Y.J.
      • et al.
      MYC Protein interactome profiling reveals functionally distinct regions that cooperate to drive tumorigenesis.
      ,
      • Zhang Q.
      • West-Osterfield K.
      • Spears E.
      • Li Z.
      • Panaccione A.
      • Hann S.R.
      MB0 and MBI are independent and distinct transactivation domains in MYC that are essential for transformation.
      ]. MBI acts as a phosphodegron and is involved in the ubiquitination and proteasomal degradation of MYC (e.g. by FBW7) [
      • Yada M.
      • Hatakeyama S.
      • Kamura T.
      • et al.
      Phosphorylation-dependent degradation of c-Myc is mediated by the F-box protein Fbw7.
      ]. MBII is also required for MYC-mediated gene transcription. Activation of transcription occurs following MYC interaction with the TRRAP–HAT complexes [
      • McMahon S.B.
      • Van Buskirk H.A.
      • Dugan K.A.
      • Copeland T.D.
      • Cole M.D.
      The novel ATM-related protein TRRAP is an essential cofactor for the c-Myc and E2F oncoproteins.
      ], while repression can occur following binding to the G9a histone methyltransferase [
      • Tu W.B.
      • Shiah Y.J.
      • Lourenco C.
      • et al.
      MYC Interacts with the G9a histone methyltransferase to drive transcriptional repression and tumorigenesis.
      ]. The MBII domain is also necessary for tumor initiation and in combination with MB0 was shown to be responsible for the cancer-inducing effects of MYC [
      • Kalkat M.
      • Resetca D.
      • Lourenco C.
      • Chan P.K.
      • Wei Y.
      • Shiah Y.J.
      • et al.
      MYC Protein interactome profiling reveals functionally distinct regions that cooperate to drive tumorigenesis.
      ]. The MBIIIa domain has been reported to play a role in gene repression and regulation of apoptosis [
      • Herbst A.
      • Hemann M.T.
      • Tworkowski K.A.
      • Salghetti S.E.
      • Lowe S.W.
      • Tansey W.P.
      A conserved element in Myc that negatively regulates its proapoptotic activity.
      ], while MBIIIb binds to the WD40-repeat protein WDR5 [
      • Thomas L.R.
      • Adams C.M.
      • Wang J.
      • et al.
      Interaction of the oncoprotein transcription factor MYC with its chromatin cofactor WDR5 is essential for tumor maintenance.
      ]. Interaction with WDR5 is necessary for MYC to attach to chromatin [
      • Thomas L.R.
      • Adams C.M.
      • Wang J.
      • et al.
      Interaction of the oncoprotein transcription factor MYC with its chromatin cofactor WDR5 is essential for tumor maintenance.
      ]. Finally, MBIV has been shown to play a role in chromatin binding, induction of apoptosis, G2 cell arrest and interaction with host cell factor-1 (HCF-1) [

      Cowling VH, Chandriani S, Whitfield ML, Cole MD. A conserved Myc protein domain, MBIV, regulates DNA binding, apoptosis, transformation, and G2 arrest. Mol Cell Biol 2006;26:4226-39. Erratum in: Mol Cell Biol 2006;26:5201.

      ]. The latter protein has been postulated to participate in cell cycle regulation [
      • Thomas L.R.
      • Foshage A.M.
      • Weissmiller A.M.
      • Popay T.M.
      • Grieb B.C.
      • Qualls S.J.
      • et al.
      Interaction of MYC with host cell factor-1 is mediated by the evolutionarily conserved Myc box IV motif.
      ].

      Mode of action of MYC

      Following MYC-MAX interaction, the dimeric complex preferentially binds to DNA response elements, known as enhancers or E-boxes containing the consensus DNA sequence, CACGTG (or related sequences) [

      Lüscher B, Larsson LG. The basic region/helix-loop-helix/leucine zipper domain of Myc proto-oncoproteins: function and regulation. Oncogene 1999;18:2955-66. Review.

      ,
      • Baudino T.A.
      • Cleveland J.L.
      The Max network gone mad.
      ]. Although these sequences are abundant across the genome, they are particularly enriched at promoter sites of genes involved in regulating cell proliferation. Binding to these sites result in altered gene expression, with some genes being upregulated and others downregulated [
      • Tu W.B.
      • Shiah Y.J.
      • Lourenco C.
      • et al.
      MYC Interacts with the G9a histone methyltransferase to drive transcriptional repression and tumorigenesis.
      ,
      • Walz S.
      • Lorenzin F.
      • Morton J.
      • et al.
      Activation and repression by oncogenic MYC shape tumour-specific gene expression profiles.
      ,
      • Peukert K.
      • Staller P.
      • Schneider A.
      • Carmichael G.
      • Hänel F.
      • Eilers M.
      An alternative pathway for gene regulation by Myc.
      ,
      • Shostak A.
      • Ruppert B.
      • Ha N.
      • et al.
      MYC/MIZ1-dependent gene repression inversely coordinates the circadian clock with cell cycle and proliferation.
      ]. At least some of the genes downregulated result from MYC interacting with the zinc finger protein MIZ1 [
      • Shostak A.
      • Ruppert B.
      • Ha N.
      • et al.
      MYC/MIZ1-dependent gene repression inversely coordinates the circadian clock with cell cycle and proliferation.
      ].
      While binding of the MYC-MAX heterodimer to E-Box sequences in the regulatory regions of target genes appears to be the primary mechanism by which MYC regulates gene expression, MYC has also been reported to attach to non-E-box DNA regions [
      • Blackwell T.K.
      • Huang J.
      • Ma A.
      • Kretzner L.
      • Alt F.W.
      • Eisenman R.N.
      • et al.
      Binding of MYC proteins to canonical and noncanonical DNA sequences.
      ]. Thus, MYC binds to the promoter on ribosome protein genes although such sites lack E boxes [
      • Lorenzin F.
      • Benary U.
      • Baluapuri A.
      • et al.
      Different promoter affinities account for specificity in MYC-dependent gene regulation.
      ]. Furthermore, in some situations, MYC appears to be capable of acting independently of MAX. For example, in N-MYC amplified neuroblastomas, N-MYC was found to bind to p53, resulting in the regulation of novel p53 target genes [

      Agarwal S1, Milazzo G2, Rajapakshe K, Bernardi R, Chen Z, Barberi E, Koster J, Perini G, Coarfa C, Shohet JM. Agarwal S, Milazzo G, Rajapakshe K, Bernardi R, Chen Z, Barberi E, Koster J, Perini G, Coarfa C, Shohet JM. MYCN acts as a direct co-regulator of p53 in MYCN amplified neuroblastoma. Oncotarget 2018;9:20323-20338. Erratum in: Oncotarget 2018;9:30024.

      ].

      Mechanisms activating MYC in cancer

      In healthy adult tissues, expression of MYC is tightly controlled. However, in several different types of human cancers, MYC is overexpressed or structurally altered. Indeed, as mentioned above, such alterations in MYC genes have been reported to occur in approximately 70% of human malignancies [
      • Dang C.V.
      A time for MYC: metabolism and therapy.
      ,
      • Beaulieu M.E.
      • Castillo F.
      • Soucek L.
      Structural and biophysical insights into the function of the intrinsically disordered Myc oncoprotein.
      ,
      • Carroll P.A.
      • Freie B.W.
      • Mathsyaraja H.
      • Eisenman R.N.
      The MYC transcript network: balancing metabolism, proliferation and oncogenesis.
      ]. Mechanisms giving rise to these alterations include gene amplification, chromosomal translocation, retroviral promoter insertion, activation of super enhancers, enhanced cell signalling, altered protein degradation and mutation (Table 1) [
      • Schaub F.X.
      • Dhankani V.
      • Berger A.C.
      • Trivedi M.
      • Richardson A.B.
      • Shaw R.
      • et al.
      Cancer Genome Atlas Network. Pan-cancer alterations of the MYC oncogene and its proximal network across the Cancer Genome Atlas.
      ,
      • Schick M.
      • Habringer S.
      • Nilsson J.A.
      • Keller U.
      Pathogenesis and therapeutic targeting of aberrant MYC expression in haematological cancers.
      ,
      • Adams J.M.
      • Harris A.W.
      • Pinkert C.A.
      • Corcoran L.M.
      • Alexander W.S.
      • Cory S.
      • et al.
      The c-myc oncogene driven by immunoglobulin enhancers induces lymphoid malignancy in transgenic mice.
      ,
      • Xu-Monette Z.Y.
      • Deng Q.
      • Manyam G.C.
      • Tzankov A.
      • Li L.
      • Xia Y.
      • et al.
      MYC mutation profiling and prognostic significance in de novo diffuse large B-cell lymphoma.
      ,
      • Tarrado-Castellarnau M.
      • de Atauri P.
      • Cascante M.
      Oncogenic regulation of tumor metabolic reprogramming.
      ,

      Kalkat M, De Melo J, Hickman KA, Lourenco C, Redel C, Resetca D, Tamachi A, Tu WB, Penn LZ. MYC deregulation in primary human cancers. Genes (Basel) 2017;25;8(6).

      ].
      Table 1Mechanisms of MYC activation in cancer.
      MechanismTumorsRefs.
      AmplificationOvarian, esophagus, uterine, breast
      • Schaub F.X.
      • Dhankani V.
      • Berger A.C.
      • Trivedi M.
      • Richardson A.B.
      • Shaw R.
      • et al.
      Cancer Genome Atlas Network. Pan-cancer alterations of the MYC oncogene and its proximal network across the Cancer Genome Atlas.
      TranslocationB-cell lymphomas, including Burkitt lymphoma
      • Schick M.
      • Habringer S.
      • Nilsson J.A.
      • Keller U.
      Pathogenesis and therapeutic targeting of aberrant MYC expression in haematological cancers.
      Enhancer activationB-cell lymphomas
      • Adams J.M.
      • Harris A.W.
      • Pinkert C.A.
      • Corcoran L.M.
      • Alexander W.S.
      • Cory S.
      • et al.
      The c-myc oncogene driven by immunoglobulin enhancers induces lymphoid malignancy in transgenic mice.
      MutationB-cell lymphomas
      • Xu-Monette Z.Y.
      • Deng Q.
      • Manyam G.C.
      • Tzankov A.
      • Li L.
      • Xia Y.
      • et al.
      MYC mutation profiling and prognostic significance in de novo diffuse large B-cell lymphoma.
      Altered protein stabilityMultiple cancer types

      Kalkat M, De Melo J, Hickman KA, Lourenco C, Redel C, Resetca D, Tamachi A, Tu WB, Penn LZ. MYC deregulation in primary human cancers. Genes (Basel) 2017;25;8(6).

      Increased signallingMultiple cancer types
      • Hayes T.K.
      • Neel N.F.
      • Hu C.
      • Gautam P.
      • Chenard M.
      • Long B.
      • et al.
      Long-term ERK inhibition in KRAS-mutant pancreatic cancer is associated with MYC degradation and senescence-like growth suppression.
      ,
      • Liu P.
      • Cheng H.
      • Santiago S.
      • Raeder M.
      • Zhang F.
      • Isabella A.
      • et al.
      Oncogenic PIK3CA-driven mammary tumors frequently recur via PI3K pathway-dependent and PI3K pathway-independent mechanisms.
      ,
      • Yochum G.S.
      • Sherrick C.M.
      • Macpartlin M.
      • Goodman R.H.
      A beta-catenin/TCF-coordinated chromatin loop at MYC integrates 5' and 3' Wnt responsive enhancers.
      Loss of p53Mammary stem cells
      • Santoro A.
      • Vlachou T.
      • Luzi L.
      • Melloni G.
      • Mazzarella L.
      • D'Elia E.
      • et al.
      p53 loss in breast cancer leads to MYC activation, increased cell plasticity, and expression of a mitotic signature with prognostic value.
      Of these mechanisms, gene amplification appears to be one of the most frequent mechanisms for MYC activation in solid human cancers. Recent studies by The Cancer Genome Atlas (TCGA) network using approximately 9000 samples from 33 tumor types showed that 28% of human cancers have amplification of at least one of the MYC genes [
      • Schaub F.X.
      • Dhankani V.
      • Berger A.C.
      • Trivedi M.
      • Richardson A.B.
      • Shaw R.
      • et al.
      Cancer Genome Atlas Network. Pan-cancer alterations of the MYC oncogene and its proximal network across the Cancer Genome Atlas.
      ]. Amplification of c-MYC occurs most frequently in ovarian cancer (64%), esophageal cancer (45.3%) squamous lung cancer (37.2%) and breast cancer (30%) [
      • Schaub F.X.
      • Dhankani V.
      • Berger A.C.
      • Trivedi M.
      • Richardson A.B.
      • Shaw R.
      • et al.
      Cancer Genome Atlas Network. Pan-cancer alterations of the MYC oncogene and its proximal network across the Cancer Genome Atlas.
      ]. Prevalence of amplification in a specific cancer type, however, may depend on its molecular subtype. For example, in the basal/triple-negative form of breast cancer (TNBC), the most difficult form of breast cancer to treat, C-MYC is amplified in 34–44% of samples in contrast to only 7–13% in luminal A-type (ER/PR-positive) cancers [

      AlSultan D, Kavanagh E, O'Grady S, Eustace AJ, Castell A, Larsson LG, Crown J, Madden SF, Duffy MJ. The novel low molecular weight MYC antagonist MYCMI-6 inhibits proliferation and induces apoptosis in breast cancer cells. Invest New Drugs. 2020 Oct 14. doi: 10.1007/s10637-020-01018-w. Epub ahead of print. PMID: 33052557.

      ] (tumor subtype with a favorable outcome).
      In contrast to c-MYC, neither N-MYC nor L-MYC are frequently amplified in cancer (<7% overall) [
      • Schaub F.X.
      • Dhankani V.
      • Berger A.C.
      • Trivedi M.
      • Richardson A.B.
      • Shaw R.
      • et al.
      Cancer Genome Atlas Network. Pan-cancer alterations of the MYC oncogene and its proximal network across the Cancer Genome Atlas.
      ]. N-MYC however, can be amplified or overexpressed in tumors with neuroendocrine features such as neuroblastoma, retinoblastoma, medullablastoma, small-cell lung cancer and prostate cancer (neuroendocrine type) [
      • Seeger R.C.
      • Brodeur G.M.
      • Sather H.
      • Dalton A.
      • Siegel S.E.
      • Wong K.Y.
      • et al.
      Association of multiple copies of the N-myc oncogene with rapid progression of neuroblastomas.
      ], while L-MYC was reported to be amplified in a small number of small-cell lung cancers [
      • Nau M.M.
      • Brooks B.J.
      • Battey J.
      • Sausville E.
      • Gazdar A.F.
      • Kirsch I.R.
      • et al.
      L-myc, a new myc-related gene amplified and expressed in human small cell lung cancer.
      ].
      While amplification of MYC is mostly found in epithelial-type tumors, activation by translocation is predominantly present in hematological malignancies. Burkitt lymphoma was the first cancer identified with such translocations. In this malignancy, 3 different MYC translocations have been identified, resulting in joining the long arm of chromosome 8 to the immunoglobulin heavy locus [IGH; t(8;14)(q24;q32)], to the kappa light chain locus [IGK; t(2;8)(p11;q24)] or to the lambda light chain locus [IGL; t(8;22)(q24;q11)] (39). The t(8;14)(q24;q32) translocation is found in approximately 80% of patients while the t(2;8)(p11;q24) and t(8;22)(q24;q11) translocations occur in approximately 15% and 5%, respectively. Other lymphomas with MYC translocations include diffuse large B cell lymphoma (5–15%), low grade follicular lymphoma (2–10%) and mantle cell lymphoma (<10%) [
      • Schick M.
      • Habringer S.
      • Nilsson J.A.
      • Keller U.
      Pathogenesis and therapeutic targeting of aberrant MYC expression in haematological cancers.
      ]. Finally, MYC was found to be translocated in 36% of patients with multiple myeloma [
      • Mikulasova A.
      • Ashby C.
      • Tytarenko R.G.
      • Qu P.
      • Rosenthal A.
      • Dent J.A.
      • et al.
      Microhomology-mediated end joining drives complex rearrangements and overexpression of MYC and PVT1 in multiple myeloma.
      ].
      In cancers lacking MYC genetic alterations such as amplification or translocation, MYC mRNA and protein expression and/or stability can be increased as a result of enhanced signalling via the ERK, PI3K or β-catenin signalling pathways [
      • Hayes T.K.
      • Neel N.F.
      • Hu C.
      • Gautam P.
      • Chenard M.
      • Long B.
      • et al.
      Long-term ERK inhibition in KRAS-mutant pancreatic cancer is associated with MYC degradation and senescence-like growth suppression.
      ,
      • Liu P.
      • Cheng H.
      • Santiago S.
      • Raeder M.
      • Zhang F.
      • Isabella A.
      • et al.
      Oncogenic PIK3CA-driven mammary tumors frequently recur via PI3K pathway-dependent and PI3K pathway-independent mechanisms.
      ,
      • Yochum G.S.
      • Sherrick C.M.
      • Macpartlin M.
      • Goodman R.H.
      A beta-catenin/TCF-coordinated chromatin loop at MYC integrates 5' and 3' Wnt responsive enhancers.
      ]. In addition to these signalling mechanisms, loss of p53 function was recently found to activate c-MYC [
      • Santoro A.
      • Vlachou T.
      • Luzi L.
      • Melloni G.
      • Mazzarella L.
      • D'Elia E.
      • et al.
      p53 loss in breast cancer leads to MYC activation, increased cell plasticity, and expression of a mitotic signature with prognostic value.
      ]. Finally, MYC protein stability can be regulated by various post-translational modifications (phosphorylation, acetylation, sumoylation) as well as by E3 ubiquitin ligase recruitment (via FBW7 or SKP2) and proteasomal degradation [

      Kalkat M, De Melo J, Hickman KA, Lourenco C, Redel C, Resetca D, Tamachi A, Tu WB, Penn LZ. MYC deregulation in primary human cancers. Genes (Basel) 2017;25;8(6).

      ,

      Yumimoto K, Nakayama KI. Recent insight into the role of FBXW7 as a tumor suppressor. Semin Cancer Biol. 2020:S1044-579X(20)30050-X.

      ].
      Unlike other cancer-causing genes such as RAS and TP53, MYC genes are rarely activated by mutation in its coding sequence. This is consistent with the multiple observations showing that deregulation of MYC expression rather than the acquisition of neomorphic properties is the main mechanism by which MYC drives cancer growth [
      • Carroll P.A.
      • Freie B.W.
      • Mathsyaraja H.
      • Eisenman R.N.
      The MYC transcript network: balancing metabolism, proliferation and oncogenesis.
      ]. MYC mutations however, are found in a subset of Burkitt lymphomas and diffuse large B cell lymphomas [
      • Xu-Monette Z.Y.
      • Deng Q.
      • Manyam G.C.
      • Tzankov A.
      • Li L.
      • Xia Y.
      • et al.
      MYC mutation profiling and prognostic significance in de novo diffuse large B-cell lymphoma.
      ].

      Role of MYC in tumorigenesis

      Although decades of research have shown that deregulation of MYC is causally involved in cancer formation, maintenance and progression, the precise mechanism by which the oncoprotein plays a role in these processes is still unclear. Potential mechanisms by which it may do so include enhancing cell proliferation, inhibition of cell death, modulating metabolism, promoting angiogenesis and regulating stem cell formation [
      • Dang C.V.
      A time for MYC: metabolism and therapy.
      ,
      • Beaulieu M.E.
      • Castillo F.
      • Soucek L.
      Structural and biophysical insights into the function of the intrinsically disordered Myc oncoprotein.
      ,
      • Carroll P.A.
      • Freie B.W.
      • Mathsyaraja H.
      • Eisenman R.N.
      The MYC transcript network: balancing metabolism, proliferation and oncogenesis.
      ]. In most if not all situations where MYC is involved in carcinogenesis, it does not act alone but in collaboration with other cancer causing genes, especially mutant RAS and mutant TP53 [
      • Mahauad-Fernandez W.D.
      • Felsher D.W.
      The Myc and Ras partnership in cancer: indistinguishable alliance or contextual relationship?.
      ,
      • Torchia E.C.
      • Caulin C.
      • Acin S.
      • et al.
      Myc, Aurora Kinase A, and mutant p53(R172H) co-operate in a mouse model of metastatic skin carcinoma.
      ].
      In addition to these cell intrinsic effects, MYC has also been shown to alter the tumor microenvironment and promote immune evasion [
      • Kortlever R.M.
      • Sodir N.M.
      • Wilson C.H.
      • Burkhart D.L.
      • Pellegrinet L.
      • Brown Swigart L.
      • et al.
      Myc cooperates with RAS by programming inflammation and immune suppression.
      ,
      • Xu Y.
      • Poggio M.
      • Jin H.Y.
      • et al.
      Translation control of the immune checkpoint in cancer and its therapeutic targeting.
      ,

      Casey SC, Tong L, Li Y, et al. MYC regulates the antitumor immune response through CD47 and PD-L1 [published correction appears in Science. 2016;352(6282). pii: aaf7984. doi: 10.1126/science.aaf7984]. Science. 2016;352(6282):227-231.

      ,
      • Casey S.C.
      • Baylot V.
      • Felsher D.W.
      The MYC oncogene is a global regulator of the immune response.
      ,

      Marinkovic D, Marinkovic T. The new role for an old guy: MYC as an immunoplayer. J Cell Physiol. 2020 Oct 23. doi: 10.1002/jcp.30123. Epub ahead of print. PMID: 33094851.

      ,
      • Sodir N.M.
      • Kortlever R.M.
      • Barthet V.J.A.
      • et al.
      MYC instructs and maintains pancreatic adenocarcinoma phenotype.
      ]. Thus in a mutant KRAS mouse model of liver cancer, Xu et al [
      • Xu Y.
      • Poggio M.
      • Jin H.Y.
      • et al.
      Translation control of the immune checkpoint in cancer and its therapeutic targeting.
      ] showed that upregulation of MYC resulted in an influx of inflammatory cells such as neutrophils and macrophages into tumors. This increased uptake of inflammatory cells led to enhanced angiogenesis, proliferation, metastasis and therapy resistance. In addition, upregulation of MYC resulted in increased expression of the checkpoint inhibitor, PD-L1 in tumor cells. Binding of PD-L1 to its receptor PD-1 on T cells resulted in immune suppression that in the liver cancer model investigated led to increased metastasis. Similarly, in other cancer models systems, upregulation of MYC was also shown to cause alterations in tumor inflammatory cells and result in immune suppression [
      • Kortlever R.M.
      • Sodir N.M.
      • Wilson C.H.
      • Burkhart D.L.
      • Pellegrinet L.
      • Brown Swigart L.
      • et al.
      Myc cooperates with RAS by programming inflammation and immune suppression.
      ,
      • Casey S.C.
      • Baylot V.
      • Felsher D.W.
      The MYC oncogene is a global regulator of the immune response.
      ,
      • Sodir N.M.
      • Kortlever R.M.
      • Barthet V.J.A.
      • et al.
      MYC instructs and maintains pancreatic adenocarcinoma phenotype.
      ]. The specific effects of MYC on the tumor environment however, seem to vary depending on the specific tumor type. While MYC activation in a lung cancer model caused B cells exclusion [
      • Kortlever R.M.
      • Sodir N.M.
      • Wilson C.H.
      • Burkhart D.L.
      • Pellegrinet L.
      • Brown Swigart L.
      • et al.
      Myc cooperates with RAS by programming inflammation and immune suppression.
      ], it led to B cell influx into the tumor periphery in a pancreatic cancer model [
      • Sodir N.M.
      • Kortlever R.M.
      • Barthet V.J.A.
      • et al.
      MYC instructs and maintains pancreatic adenocarcinoma phenotype.
      ]. As with the intrinsic effects of MYC mentioned above, the extrinsic effects of the oncoprotein on immune cell infiltration, in a least some situations, also depend on co-operating with mutant RAS [
      • Kortlever R.M.
      • Sodir N.M.
      • Wilson C.H.
      • Burkhart D.L.
      • Pellegrinet L.
      • Brown Swigart L.
      • et al.
      Myc cooperates with RAS by programming inflammation and immune suppression.
      ,
      • Sodir N.M.
      • Kortlever R.M.
      • Barthet V.J.A.
      • et al.
      MYC instructs and maintains pancreatic adenocarcinoma phenotype.
      ].
      In summary, it thus seems that MYC can induce tumorigenesis via multiple mechanisms, likely related to its broad ability to regulate expression of a wide number of different genes.
      Consistent with MYC being a driver of tumorigenesis, suppression of its expression or inhibition of its function can reverse tumorigenesis [
      • Jain M.
      • Arvanitis C.
      • Chu K.
      • Dewey W.
      • Leonhardt E.
      • Trinh M.
      • et al.
      Sustained loss of a neoplastic phenotype by brief inactivation of MYC.
      ,
      • Whitfield J.R.
      • Beaulieu M.E.
      • Soucek L.
      Strategies to inhibit myc and their clinical applicability.
      ,
      • Wolf E.
      • Eilers M.
      Targeting MYC proteins for tumor therapy.
      ,
      • Fletcher S.
      • Prochownik E.V.
      Small-molecule inhibitors of the Myc oncoprotein.
      ,
      • Massó-Vallés D.
      • Soucek L.
      Blocking myc to treat cancer: reflecting on two decades of Omomyc.
      ]. This regression of tumorigenesis is mediated by reversal of the carcinogenic processes mentioned above, including, some or all of the following: promotion of cell cycle arrest, triggering of apoptosis, induction of senescence, promotion of differentiation, inhibition of angiogenesis, extrusion of tumor infiltrating inflammatory cells such as macrophages and neutrophils, influx of T and NK cells, reduced levels of PD-L1 and upregulation of an anti-tumor immune response [
      • Kortlever R.M.
      • Sodir N.M.
      • Wilson C.H.
      • Burkhart D.L.
      • Pellegrinet L.
      • Brown Swigart L.
      • et al.
      Myc cooperates with RAS by programming inflammation and immune suppression.
      ,

      Casey SC, Tong L, Li Y, et al. MYC regulates the antitumor immune response through CD47 and PD-L1 [published correction appears in Science. 2016;352(6282). pii: aaf7984. doi: 10.1126/science.aaf7984]. Science. 2016;352(6282):227-231.

      ,
      • Casey S.C.
      • Baylot V.
      • Felsher D.W.
      The MYC oncogene is a global regulator of the immune response.
      ,

      Marinkovic D, Marinkovic T. The new role for an old guy: MYC as an immunoplayer. J Cell Physiol. 2020 Oct 23. doi: 10.1002/jcp.30123. Epub ahead of print. PMID: 33094851.

      ,
      • Sodir N.M.
      • Kortlever R.M.
      • Barthet V.J.A.
      • et al.
      MYC instructs and maintains pancreatic adenocarcinoma phenotype.
      ]. Indeed, in some situations, even brief or partial suppression of MYC has been shown to reverse tumorigenesis [
      • Kortlever R.M.
      • Sodir N.M.
      • Wilson C.H.
      • Burkhart D.L.
      • Pellegrinet L.
      • Brown Swigart L.
      • et al.
      Myc cooperates with RAS by programming inflammation and immune suppression.
      ,
      • Sodir N.M.
      • Kortlever R.M.
      • Barthet V.J.A.
      • et al.
      MYC instructs and maintains pancreatic adenocarcinoma phenotype.
      ,
      • Jain M.
      • Arvanitis C.
      • Chu K.
      • Dewey W.
      • Leonhardt E.
      • Trinh M.
      • et al.
      Sustained loss of a neoplastic phenotype by brief inactivation of MYC.
      ]. For example, in mouse models of pancreatic and lung adenocarcinoma, deactivation of MYC triggered tumor regression after a few hours [
      • Kortlever R.M.
      • Sodir N.M.
      • Wilson C.H.
      • Burkhart D.L.
      • Pellegrinet L.
      • Brown Swigart L.
      • et al.
      Myc cooperates with RAS by programming inflammation and immune suppression.
      ,
      • Sodir N.M.
      • Kortlever R.M.
      • Barthet V.J.A.
      • et al.
      MYC instructs and maintains pancreatic adenocarcinoma phenotype.
      ].

      MYC as a therapeutic target for cancer

      The experiments showing that turning off expression of MYC reverses tumorigenesis provides proof of principle that pharmacological targeting of the oncoprotein should block or decrease tumor cell growth. However, specific pharmacological targeting of MYC with low molecular weight compounds is difficult, as the protein possesses a largely intrinsically disordered structure, lacking a hydrophobic pocket or groove into which such compounds could bind with high affinity. Furthermore, MYC lacks catalytic activity and therefore, unlike several other cancer driver oncoproteins (e.g., EGFR, HER2, BRAF), it cannot be blocked with low molecular weight enzyme inhibitors. Finally, as MYC is located in the nucleus, it is difficult to target with large molecules such as the currently available monoclonal antibodies.
      Despite these challenges, several different pharmaceutical-based strategies have recently been described for targeting MYC with the aim of blocking tumor growth [
      • Whitfield J.R.
      • Beaulieu M.E.
      • Soucek L.
      Strategies to inhibit myc and their clinical applicability.
      ,
      • Wolf E.
      • Eilers M.
      Targeting MYC proteins for tumor therapy.
      ,
      • Fletcher S.
      • Prochownik E.V.
      Small-molecule inhibitors of the Myc oncoprotein.
      ,
      • Massó-Vallés D.
      • Soucek L.
      Blocking myc to treat cancer: reflecting on two decades of Omomyc.
      ,
      • Soucek L.
      • Helmer-Citterich M.
      • Sacco A.
      • Jucker R.
      • Cesareni G.
      • Nasi S.
      Design and properties of a Myc derivative that efficiently homodimerizes.
      ] (Table 2). Of these, the most investigated are inhibition of MYC-MAX interaction, prevention of MYC expression and targeting of genes exhibiting synthetic lethality with overexpression of MYC. The current status of these different approaches is discussed below.
      Table 2Potential strategies with examples in targeting MYC for the treatment of cancer.
      General mode of actionExample(s)Refs.
      Blocking MYC-MAX interactionOmoMYC, 10054-F4, 10074-G5, KJ-Pyr-9, MYCMI-6, MYCi361 and MYCi975

      AlSultan D, Kavanagh E, O'Grady S, Eustace AJ, Castell A, Larsson LG, Crown J, Madden SF, Duffy MJ. The novel low molecular weight MYC antagonist MYCMI-6 inhibits proliferation and induces apoptosis in breast cancer cells. Invest New Drugs. 2020 Oct 14. doi: 10.1007/s10637-020-01018-w. Epub ahead of print. PMID: 33052557.

      ,
      • Massó-Vallés D.
      • Soucek L.
      Blocking myc to treat cancer: reflecting on two decades of Omomyc.
      ,
      • Yin X.
      • Giap C.
      • Lazo J.S.
      • Prochownik E.V.
      Low molecular weight inhibitors of Myc-Max interaction and function.
      ,
      • Mustata G.
      • Follis A.V.
      • Hammoudeh D.I.
      • et al.
      Discovery of novel Myc-Max heterodimer disruptors with a three-dimensional pharmacophore model.
      ,
      • Follis A.V.
      • Hammoudeh D.I.
      • Wang H.
      • Prochownik E.V.
      • Metallo S.J.
      Structural rationale for the coupled binding and unfolding of the c-Myc oncoprotein by small molecules.
      ,
      • Müller I.
      • Larsson K.
      • Frenzel A.
      • Oliynyk G.
      • Zirath H.
      • Prochownik E.V.
      • et al.
      Targeting of the MYCN protein with small molecule c-MYC inhibitors.
      ,
      • Guo J.
      • Parise R.A.
      • Joseph E.
      • Egorin M.J.
      • Lazo J.S.
      • Prochownik E.V.
      Eiseman Efficacy, pharmacokinetics, tisssue distribution, and metabolism of the Myc-Max disruptor, 10058–F4 [Z, E]-5-[4-ethylbenzylidine]-2-thioxothiazolidin-4-one, in mice.
      ,
      • Zirath H.
      • Frenzel A.
      • Oliynyk G.
      • Segerström L.
      • Westermark U.K.
      • Larsson K.
      • et al.
      MYC inhibition induces metabolic changes leading to accumulation of lipid droplets in tumor cells.
      ,
      • Wang H.
      • Hammoudeh D.I.
      • Follis A.V.
      • Reese B.E.
      • Lazo J.S.
      • Metallo S.J.
      • et al.
      Improved low molecular weight Myc-Max inhibitors.
      ,
      • Hart J.R.
      • Garner A.L.
      • Yu J.
      • Ito Y.
      • Sun M.
      • Ueno L.
      • et al.
      Inhibitor of MYC identified in a Kröhnke pyridine library.
      ,
      • Castell A.
      • Yan Q.
      • Fawkner K.
      • Hydbring P.
      • Zhang F.
      • Verschut V.
      • et al.
      A selective high affinity MYC-binding compound inhibits MYC:MAX interaction and MYC-dependent tumor cell proliferation.
      ,
      • Han H.
      • Jain A.D.
      • Truica M.I.
      • et al.
      Small-molecule MYC inhibitors suppress tumor growth and enhance immunotherapy.
      Blocking MYC-MAX from binding to DNA/preventing expression of MYCOmoMYC, BET inhibitors (JQ1), CDK7 inhibitors (THZ1), ME4, G9a histone transferase inhibitors (UNCO638), MYC-N3A, cardiac glycosides (bufalin, oubain), cytoskeletal disrupters (dolastatins), G-quadraplex inhibitors (APTO-253)
      • Massó-Vallés D.
      • Soucek L.
      Blocking myc to treat cancer: reflecting on two decades of Omomyc.
      ,
      • Soucek L.
      • Helmer-Citterich M.
      • Sacco A.
      • Jucker R.
      • Cesareni G.
      • Nasi S.
      Design and properties of a Myc derivative that efficiently homodimerizes.
      ,
      • Soucek L.
      • Jucker R.
      • Panacchia L.
      • Ricordy R.
      • Tatò F.
      • Nasi S.
      Omomyc, a potential Myc dominant negative, enhances Myc-induced apoptosis.
      ,
      • Soucek L.
      • Whitfield J.
      • Martins C.P.
      • Finch A.J.
      • Murphy D.J.
      • Sodir N.M.
      • et al.
      Modelling Myc inhibition as a cancer therapy.
      ,

      Savino M1, Annibali D, Carucci N, Favuzzi E, Cole MD, Evan GI, Soucek L, Nasi S. The action mechanism of the Myc inhibitor termed Omomyc may give clues on how to target Myc for cancer therapy. PLoS One 2011;6:e22284.

      ,
      • Jung L.A.
      • Gebhardt A.
      • Koelmel W.
      • Ade C.P.
      • Walz S.
      • Kuper J.
      • et al.
      OmoMYC blunts promoter invasion by oncogenic MYC to inhibit gene expression characteristic of MYC-dependent tumors.
      ,
      • Shi J.
      • Vakoc C.R.
      The mechanisms behind the therapeutic activity of BET bromodomain inhibition.
      ,
      • Delmore J.E.
      • Issa G.C.
      • Lemieux M.E.
      • Rahl P.B.
      • Shi J.
      • Jacobs H.M.
      • et al.
      BET bromodomain inhibition as a therapeutic strategy to target c-Myc.
      ,
      • Fu L.L.
      • Tian M.
      • Li X.
      • Li J.J.
      • Huang J.
      • Ouyang L.
      • et al.
      Inhibition of BET bromodomains as a therapeutic strategy for cancer drug discovery.
      ,
      • Zuber J.
      • Shi J.
      • Wang E.
      • Rappaport A.R.
      • Herrmann H.
      • Sison E.A.
      • et al.
      RNAi screen identifies Brd4 as a therapeutic target in acute myeloid leukaemia.
      ,
      • Mertz J.A.
      • Conery A.R.
      • Bryant B.M.
      • Sandy P.
      • Balasubramanian S.
      • Mele D.A.
      • et al.
      Targeting MYC dependence in cancer by inhibiting BET bromodomains.
      ,
      • Dawson M.A.
      • Prinjha R.K.
      • Dittmann A.
      • Giotopoulos G.
      • Bantscheff M.
      • Chan W.I.
      • et al.
      Inhibition of BET recruitment to chromatin as an effective treatment for MLL-fusion leukaemia.
      ,
      • Lockwood W.W.
      • Zejnullahu K.
      • Bradner J.E.
      • Varmus H.
      Sensitivity of human lung adenocarcinoma cell lines to targeted inhibition of BET epigenetic signaling proteins.
      ,
      • Segura M.F.
      • Fontanals-Cirera B.
      • Gaziel-Sovran A.
      • et al.
      BRD4 sustains melanoma proliferation and represents a new target for epigenetic therapy.
      ,
      • Doroshow D.B.
      • Eder J.P.
      • LoRusso P.M.
      BET inhibitors: a novel epigenetic approach.
      ,
      • Wang W.
      • Hu S.
      • Gu Y.
      • et al.
      Human MYC G-quadruplex: From discovery to a cancer therapeutic target.
      ,
      • Asamitsu S.
      • Obata S.
      • Yu Z.
      • Bando T.
      • Sugiyama H.
      Recent progress of targeted G-quadruplex-preferred ligands toward cancer therapy.
      ,
      • Local A.
      • Zhang H.
      • Benbatoul K.D.
      • Folger P.
      • Sheng X.
      • Tsai C.Y.
      • et al.
      APTO-253 Stabilizes G-quadruplex DNA, inhibits MYC expression, and induces DNA damage in acute myeloid leukemia cells.
      ,
      • Huesca M.
      • Lock L.S.
      • Khine A.A.
      • et al.
      A novel small molecule with potent anticancer activity inhibits cell growth by modulating intracellular labile zinc homeostasis.
      ,
      • Cercek A.
      • Wheler J.
      • Murray P.E.
      • Zhou S.
      • Saltz L.
      Phase 1 study of APTO-253 HCl, an inducer of KLF4, in patients with advanced or metastatic solid tumors.
      ,
      • Tsai C.Y.
      • Sun S.
      • Zhang H.
      • Local A.
      • Su Y.
      • Gross L.A.
      • et al.
      APTO-253 is a new addition to the repertoire of drugs that can exploit DNA BRCA1/2 deficiency.
      Exploitation of synthetic lethal partners for MYCInhibitors of CDK1, PIM1 kinase, PARP, aurora B kinase, TRAIL
      • Goga A.
      • Yang D.
      • Tward A.D.
      • Morgan D.O.
      • Bishop J.M.
      Inhibition of CDK1 as a potential therapy for tumors over-expressing MYC.
      ,
      • Horiuchi D.
      • Kusdra L.
      • Huskey N.E.
      • Chandriani S.
      • Lenburg M.E.
      • Gonzalez-Angulo A.M.
      • et al.
      MYC pathway activation in triple-negative breast cancer is synthetic lethal with CDK inhibition.
      ,
      • Blachly J.S.
      • Byrd J.C.
      • Grever M.
      Cyclin-dependent kinase inhibitors for the treatment of chronic lymphocytic leukemia.
      ,
      • Horiuchi D.
      • Camarda R.
      • Zhou A.Y.
      • Yau C.
      • Momcilovic O.
      • Balakrishnan S.
      • et al.
      PIM1 kinase inhibition as a targeted therapy against triple-negative breast tumors with elevated MYC expression.
      ,
      • Carey J.P.W.
      • Karakas C.
      • Bui T.
      • Chen X.
      • Vijayaraghavan S.
      • Zhao Y.
      • et al.
      synthetic lethality of PARP inhibitors in combination with MYC blockade is independent of BRCA status in triple-negative breast cancer.
      ,
      • Yang D.
      • Liu H.
      • Goga A.
      • Kim S.
      • Yuneva M.
      • Bishop J.M.
      Therapeutic potential of a synthetic lethal interaction between the MYC proto-oncogene and inhibition of aurora-B kinase.
      ,
      • Lee H.Y.
      • Cha J.
      • Kim S.K.
      • Park J.H.
      • Song K.H.
      • Kim P.
      • et al.
      c-MYC drives breast cancer metastasis to the brain, but promotes synthetic lethality with TRAIL.
      Preventing MYC translationDactolisib (BEZ235), silvestrol
      • Wiegering A.
      • Uthe F.W.
      • Jamieson T.
      • Ruoss Y.
      • Hüttenrauch M.
      • Küspert M.
      • et al.
      Targeting translation initiation bypasses signaling crosstalk mechanisms that maintain high myc levels in colorectal cancer.
      ,
      • Castell A.
      • Larsson L.G.
      Targeting MYC translation in colorectal cancer.
      Stabilizing MAX homodimersNSC13728, KI-MS2-008
      • Jiang H.
      • Bower K.E.
      • Beuscher 4th, A.E.
      • Zhou B.
      • Bobkov A.A.
      • Olson A.J.
      • et al.
      Stabilizers of the Max homodimer identified in virtual ligand screening inhibit Myc function.
      ,
      • Struntz N.B.
      • Chen A.
      • Deutzmann A.
      • Wilson R.M.
      • Stefan E.
      • Evans H.L.
      • et al.
      Stabilization of the MAX homodimer with a small molecule attenuates myc-driven transcription.

      OmoMYC: The prototype and lead MYC antagonist

      OmoMYC is a 90-amino acid peptide/mini-protein that mimics the bHLHLZ domain of C-MYC [
      • Massó-Vallés D.
      • Soucek L.
      Blocking myc to treat cancer: reflecting on two decades of Omomyc.
      ]. However, it differs from the naturally occurring bHLHLZ domain of MYC in containing 4 point mutations in its leucine zipper domain that were included to alter its dimerization properties [
      • Soucek L.
      • Helmer-Citterich M.
      • Sacco A.
      • Jucker R.
      • Cesareni G.
      • Nasi S.
      Design and properties of a Myc derivative that efficiently homodimerizes.
      ]. OmoMYC appears to antagonize MYC and thus inhibit cancer cell growth via at least 2 mechanisms [
      • Soucek L.
      • Jucker R.
      • Panacchia L.
      • Ricordy R.
      • Tatò F.
      • Nasi S.
      Omomyc, a potential Myc dominant negative, enhances Myc-induced apoptosis.
      ,
      • Soucek L.
      • Whitfield J.
      • Martins C.P.
      • Finch A.J.
      • Murphy D.J.
      • Sodir N.M.
      • et al.
      Modelling Myc inhibition as a cancer therapy.
      ,

      Savino M1, Annibali D, Carucci N, Favuzzi E, Cole MD, Evan GI, Soucek L, Nasi S. The action mechanism of the Myc inhibitor termed Omomyc may give clues on how to target Myc for cancer therapy. PLoS One 2011;6:e22284.

      ,
      • Jung L.A.
      • Gebhardt A.
      • Koelmel W.
      • Ade C.P.
      • Walz S.
      • Kuper J.
      • et al.
      OmoMYC blunts promoter invasion by oncogenic MYC to inhibit gene expression characteristic of MYC-dependent tumors.
      ,
      • Demma M.J.
      • Mapelli C.
      • Sun A.
      • Bodea S.
      • Ruprecht B.
      • Javaid S.
      • et al.
      Omomyc reveals new mechanisms to inhibit the MYC oncogene.
      ]. Firstly, OmoMYC forms homodimers with itself that prevent MYC-MAX heterodimers interaction with DNA and inhibition of MYC-mediated transcriptional [
      • Massó-Vallés D.
      • Soucek L.
      Blocking myc to treat cancer: reflecting on two decades of Omomyc.
      ,
      • Soucek L.
      • Helmer-Citterich M.
      • Sacco A.
      • Jucker R.
      • Cesareni G.
      • Nasi S.
      Design and properties of a Myc derivative that efficiently homodimerizes.
      ,
      • Soucek L.
      • Jucker R.
      • Panacchia L.
      • Ricordy R.
      • Tatò F.
      • Nasi S.
      Omomyc, a potential Myc dominant negative, enhances Myc-induced apoptosis.
      ,

      Savino M1, Annibali D, Carucci N, Favuzzi E, Cole MD, Evan GI, Soucek L, Nasi S. The action mechanism of the Myc inhibitor termed Omomyc may give clues on how to target Myc for cancer therapy. PLoS One 2011;6:e22284.

      ]. Secondly, although OmoMYC interacts with both MYC and MAX it preferentially binds to the latter [
      • Demma M.J.
      • Mapelli C.
      • Sun A.
      • Bodea S.
      • Ruprecht B.
      • Javaid S.
      • et al.
      Omomyc reveals new mechanisms to inhibit the MYC oncogene.
      ]. The binding to MAX represses MYC-induced transcription by replacing MYC-MAX heterodimers with OmoMYC-MAX heterodimers [
      • Demma M.J.
      • Mapelli C.
      • Sun A.
      • Bodea S.
      • Ruprecht B.
      • Javaid S.
      • et al.
      Omomyc reveals new mechanisms to inhibit the MYC oncogene.
      ]. Although OmoMYC inhibits MYC-mediated gene transactivation, it does not affect the MYC-MIZ-dependent promoter binding and gene repression [

      Savino M1, Annibali D, Carucci N, Favuzzi E, Cole MD, Evan GI, Soucek L, Nasi S. The action mechanism of the Myc inhibitor termed Omomyc may give clues on how to target Myc for cancer therapy. PLoS One 2011;6:e22284.

      ].
      Due to its size and peptide structure, OmoMYC was originally believed to be unsuitable for uptake and penetration into cancer cells. Its anticancer activity was therefore initially investigated as a switchable transgene. Using this approach, the OmoMYC was found to exhibit anticancer activity in several different animal tumor models (for review, see ref. 63). Consistent with the ability of MYC to promote immune evasion (see above), OmoMYC appears to induce tumor regression at least in part by altering the tumor microenvironment and inducing an anti-tumor immune response. Thus, Wang et al [
      • Wang E.
      • Sorolla A.
      • Cunningham P.T.
      • Bogdawa H.M.
      • Beck S.
      • Golden E.
      • et al.
      Tumor penetrating peptides inhibiting MYC as a potent targeted therapeutic strategy for triple-negative breast cancers.
      ] found that intra-tumor injection in a TNBC model with OmoMYC linked to a penetrating Phylomer resulted in downregulation of the negative immune checkpoint protein, PD-L1 while Beaulieu et al [

      Beaulieu ME, Jauset T, Massó-Vallés D, Martínez-Martín S, Rahl P, Maltais L, Zacarias-Fluck MF, Casacuberta-Serra S, Serrano Del Pozo E, Fiore C, Foradada L, Cano VC, Sánchez-Hervás M, Guenther M, Romero Sanz E, Oteo M, Tremblay C, Martín G, Letourneau D, Montagne M, Morcillo Alonso MÁ, Whitfield JR, Lavigne P, Soucek L. Intrinsic cell-penetrating activity propels Omomyc from proof of concept to viable anti-MYC therapy. Sci Transl Med 2019;11:(484).

      ] reported that administration of purified OmoMYC mini-protein to a mouse model of non-small cell lung cancer led to recruitment of T lymphocytes to the tumor site. Importantly, from a clinical point of view, only mild and fully reversible toxic side effects were observed with OmoMYC in preclinical studies reported to date.
      The ability to systemically administer OmoMYC when combined with its ability to inhibit the growth of diverse experimental tumor types and cause minimal side effects on normal tissues [
      • Soucek L.
      • Whitfield J.
      • Martins C.P.
      • Finch A.J.
      • Murphy D.J.
      • Sodir N.M.
      • et al.
      Modelling Myc inhibition as a cancer therapy.
      ,
      • Soucek L.
      • Whitfield J.R.
      • Sodir N.M.
      • Massó-Vallés D.
      • Serrano E.
      • Karnezis A.N.
      • et al.
      Inhibition of Myc family proteins eradicates KRas-driven lung cancer in mice.
      ,

      Beaulieu ME, Jauset T, Massó-Vallés D, Martínez-Martín S, Rahl P, Maltais L, Zacarias-Fluck MF, Casacuberta-Serra S, Serrano Del Pozo E, Fiore C, Foradada L, Cano VC, Sánchez-Hervás M, Guenther M, Romero Sanz E, Oteo M, Tremblay C, Martín G, Letourneau D, Montagne M, Morcillo Alonso MÁ, Whitfield JR, Lavigne P, Soucek L. Intrinsic cell-penetrating activity propels Omomyc from proof of concept to viable anti-MYC therapy. Sci Transl Med 2019;11:(484).

      ,
      • Wang E.
      • Sorolla A.
      • Cunningham P.T.
      • Bogdawa H.M.
      • Beck S.
      • Golden E.
      • et al.
      Tumor penetrating peptides inhibiting MYC as a potent targeted therapeutic strategy for triple-negative breast cancers.
      ] should now lead to the testing of OmoMYC in clinical trials [
      • Villanueva M.T.
      Long path to MYC inhibition approaches clinical trials.
      ]. However, since OmoMYC is a peptide/small protein, it is potentially susceptible to degradation in vivo [
      • Demma M.J.
      • Mapelli C.
      • Sun A.
      • Bodea S.
      • Ruprecht B.
      • Javaid S.
      • et al.
      Omomyc reveals new mechanisms to inhibit the MYC oncogene.
      ]. It structure may thus have to be modified to prevent such degradation from occurring.

      Blocking MYC-MAX interaction with low molecular weight inhibitors

      Using high-throughput screening methodologies, several low molecular weight compounds have been identified that block the interaction between MYC and MAX [

      AlSultan D, Kavanagh E, O'Grady S, Eustace AJ, Castell A, Larsson LG, Crown J, Madden SF, Duffy MJ. The novel low molecular weight MYC antagonist MYCMI-6 inhibits proliferation and induces apoptosis in breast cancer cells. Invest New Drugs. 2020 Oct 14. doi: 10.1007/s10637-020-01018-w. Epub ahead of print. PMID: 33052557.

      ,
      • Yin X.
      • Giap C.
      • Lazo J.S.
      • Prochownik E.V.
      Low molecular weight inhibitors of Myc-Max interaction and function.
      ,
      • Mustata G.
      • Follis A.V.
      • Hammoudeh D.I.
      • et al.
      Discovery of novel Myc-Max heterodimer disruptors with a three-dimensional pharmacophore model.
      ,
      • Follis A.V.
      • Hammoudeh D.I.
      • Wang H.
      • Prochownik E.V.
      • Metallo S.J.
      Structural rationale for the coupled binding and unfolding of the c-Myc oncoprotein by small molecules.
      ,
      • Müller I.
      • Larsson K.
      • Frenzel A.
      • Oliynyk G.
      • Zirath H.
      • Prochownik E.V.
      • et al.
      Targeting of the MYCN protein with small molecule c-MYC inhibitors.
      ,
      • Guo J.
      • Parise R.A.
      • Joseph E.
      • Egorin M.J.
      • Lazo J.S.
      • Prochownik E.V.
      Eiseman Efficacy, pharmacokinetics, tisssue distribution, and metabolism of the Myc-Max disruptor, 10058–F4 [Z, E]-5-[4-ethylbenzylidine]-2-thioxothiazolidin-4-one, in mice.
      ,
      • Zirath H.
      • Frenzel A.
      • Oliynyk G.
      • Segerström L.
      • Westermark U.K.
      • Larsson K.
      • et al.
      MYC inhibition induces metabolic changes leading to accumulation of lipid droplets in tumor cells.
      ,
      • Wang H.
      • Hammoudeh D.I.
      • Follis A.V.
      • Reese B.E.
      • Lazo J.S.
      • Metallo S.J.
      • et al.
      Improved low molecular weight Myc-Max inhibitors.
      ,
      • Hart J.R.
      • Garner A.L.
      • Yu J.
      • Ito Y.
      • Sun M.
      • Ueno L.
      • et al.
      Inhibitor of MYC identified in a Kröhnke pyridine library.
      ,
      • Castell A.
      • Yan Q.
      • Fawkner K.
      • Hydbring P.
      • Zhang F.
      • Verschut V.
      • et al.
      A selective high affinity MYC-binding compound inhibits MYC:MAX interaction and MYC-dependent tumor cell proliferation.
      ,
      • Han H.
      • Jain A.D.
      • Truica M.I.
      • et al.
      Small-molecule MYC inhibitors suppress tumor growth and enhance immunotherapy.
      ] (Table 2). Consistent with their ability to interfere with MYC-MAX heterodimerization, these compounds have been shown to inhibit cancer cell proliferation in a diverse range of model tumor systems [

      Kalkat M, De Melo J, Hickman KA, Lourenco C, Redel C, Resetca D, Tamachi A, Tu WB, Penn LZ. MYC deregulation in primary human cancers. Genes (Basel) 2017;25;8(6).

      ,
      • Yin X.
      • Giap C.
      • Lazo J.S.
      • Prochownik E.V.
      Low molecular weight inhibitors of Myc-Max interaction and function.
      ,
      • Mustata G.
      • Follis A.V.
      • Hammoudeh D.I.
      • et al.
      Discovery of novel Myc-Max heterodimer disruptors with a three-dimensional pharmacophore model.
      ,
      • Follis A.V.
      • Hammoudeh D.I.
      • Wang H.
      • Prochownik E.V.
      • Metallo S.J.
      Structural rationale for the coupled binding and unfolding of the c-Myc oncoprotein by small molecules.
      ,
      • Müller I.
      • Larsson K.
      • Frenzel A.
      • Oliynyk G.
      • Zirath H.
      • Prochownik E.V.
      • et al.
      Targeting of the MYCN protein with small molecule c-MYC inhibitors.
      ,
      • Guo J.
      • Parise R.A.
      • Joseph E.
      • Egorin M.J.
      • Lazo J.S.
      • Prochownik E.V.
      Eiseman Efficacy, pharmacokinetics, tisssue distribution, and metabolism of the Myc-Max disruptor, 10058–F4 [Z, E]-5-[4-ethylbenzylidine]-2-thioxothiazolidin-4-one, in mice.
      ,
      • Zirath H.
      • Frenzel A.
      • Oliynyk G.
      • Segerström L.
      • Westermark U.K.
      • Larsson K.
      • et al.
      MYC inhibition induces metabolic changes leading to accumulation of lipid droplets in tumor cells.
      ,
      • Wang H.
      • Hammoudeh D.I.
      • Follis A.V.
      • Reese B.E.
      • Lazo J.S.
      • Metallo S.J.
      • et al.
      Improved low molecular weight Myc-Max inhibitors.
      ,
      • Hart J.R.
      • Garner A.L.
      • Yu J.
      • Ito Y.
      • Sun M.
      • Ueno L.
      • et al.
      Inhibitor of MYC identified in a Kröhnke pyridine library.
      ,
      • Castell A.
      • Yan Q.
      • Fawkner K.
      • Hydbring P.
      • Zhang F.
      • Verschut V.
      • et al.
      A selective high affinity MYC-binding compound inhibits MYC:MAX interaction and MYC-dependent tumor cell proliferation.
      ,
      • Han H.
      • Jain A.D.
      • Truica M.I.
      • et al.
      Small-molecule MYC inhibitors suppress tumor growth and enhance immunotherapy.
      ]. The compounds listed in Table 2 however, vary widely in structure, affinity for MYC and ability to inhibit tumor cell proliferation (i.e., have varying IC50 values). Few of the compounds have been investigated for anti-cancer activity in more than one type of animal model.
      Amongst the more promising low molecular weight MYC antagonists are KJ-Pyr-9 [
      • Hart J.R.
      • Garner A.L.
      • Yu J.
      • Ito Y.
      • Sun M.
      • Ueno L.
      • et al.
      Inhibitor of MYC identified in a Kröhnke pyridine library.
      ], MYCMI-6 [

      Kalkat M, De Melo J, Hickman KA, Lourenco C, Redel C, Resetca D, Tamachi A, Tu WB, Penn LZ. MYC deregulation in primary human cancers. Genes (Basel) 2017;25;8(6).

      ,
      • Castell A.
      • Yan Q.
      • Fawkner K.
      • Hydbring P.
      • Zhang F.
      • Verschut V.
      • et al.
      A selective high affinity MYC-binding compound inhibits MYC:MAX interaction and MYC-dependent tumor cell proliferation.
      ] and MYCi975 [
      • Han H.
      • Jain A.D.
      • Truica M.I.
      • et al.
      Small-molecule MYC inhibitors suppress tumor growth and enhance immunotherapy.
      ]. All of these compounds have been shown to inhibit MYC-MAX interaction and decrease cell proliferation in a broad range of cancer cell lines, usually with IC50 values < 10 µM. In addition, all have been shown to exhibit anticancer activity in in vivo MYC-dependent tumor models. Similar to OmoMYC, administration of MYCi975 was found to result in remodelling of the tumor environment and increased tumor uptake of immune cells (e.g., CD3+ T cells, B cells and NK cells) [
      • Han H.
      • Jain A.D.
      • Truica M.I.
      • et al.
      Small-molecule MYC inhibitors suppress tumor growth and enhance immunotherapy.
      ]. Furthermore, combined treatment with MYCi975 and immunotherapy (pembrolizumab) resulted in synergistic tumor growth inhibition in a syngeneic prostate cancer model. Importantly, as with OmoMYC, treatment with MYCi975 did not appear to cause major short-term toxicity [
      • Han H.
      • Jain A.D.
      • Truica M.I.
      • et al.
      Small-molecule MYC inhibitors suppress tumor growth and enhance immunotherapy.
      ]. Outstanding questions with the MYC-MAX antagonists relate to their specificity and potential toxicity in vivo, especially long-term toxicity.

      Preventing expression of MYC

      Rather than directly inhibiting MYC function as described above, another widely investigated approach for targeting MYC involves blocking its expression. While this is an active area of investigation, it is unlikely that any of the compounds discussed below specifically downregulate only MYC. Thus, the potential exists for off-target effects and adverse effects in vivo. Although multiple different strategies are undergoing evaluation for silencing MYC expression, two of the more widely studied involve using inhibitors of bromodomain and extraterminal domain (BET) proteins and use of compounds that stabilize G-quadruplex (G4) motifs in the MYC promoter.

      BET inhibitors

      BET proteins regulate transcription by associating with acetylated chromatin and facilitating the recruitment of transcriptional factors [
      • Shi J.
      • Vakoc C.R.
      The mechanisms behind the therapeutic activity of BET bromodomain inhibition.
      ]. The family consists of 4 members; BRD2, BRD3, BRD4 and bromodomain testis-specific proteins. BRD4 was recently found to bind to the MYC promoter and regulate its transcription [
      • Shi J.
      • Vakoc C.R.
      The mechanisms behind the therapeutic activity of BET bromodomain inhibition.
      ]. Several BET inhibitors, especially the BRD4 inhibitor JQ1, have been shown to downregulate MYC and suppress tumor growth in a diverse range of animal models exhibiting MYC activation [
      • Delmore J.E.
      • Issa G.C.
      • Lemieux M.E.
      • Rahl P.B.
      • Shi J.
      • Jacobs H.M.
      • et al.
      BET bromodomain inhibition as a therapeutic strategy to target c-Myc.
      ,
      • Fu L.L.
      • Tian M.
      • Li X.
      • Li J.J.
      • Huang J.
      • Ouyang L.
      • et al.
      Inhibition of BET bromodomains as a therapeutic strategy for cancer drug discovery.
      ,
      • Zuber J.
      • Shi J.
      • Wang E.
      • Rappaport A.R.
      • Herrmann H.
      • Sison E.A.
      • et al.
      RNAi screen identifies Brd4 as a therapeutic target in acute myeloid leukaemia.
      ,
      • Mertz J.A.
      • Conery A.R.
      • Bryant B.M.
      • Sandy P.
      • Balasubramanian S.
      • Mele D.A.
      • et al.
      Targeting MYC dependence in cancer by inhibiting BET bromodomains.
      ,
      • Dawson M.A.
      • Prinjha R.K.
      • Dittmann A.
      • Giotopoulos G.
      • Bantscheff M.
      • Chan W.I.
      • et al.
      Inhibition of BET recruitment to chromatin as an effective treatment for MLL-fusion leukaemia.
      ]. However, while treatment with BET inhibitors in specific tumor models resulted in MYC downregulation, this association was not consistent across different tumor types [
      • Lockwood W.W.
      • Zejnullahu K.
      • Bradner J.E.
      • Varmus H.
      Sensitivity of human lung adenocarcinoma cell lines to targeted inhibition of BET epigenetic signaling proteins.
      ]. Furthermore, BET inhibitors were found to downregulate expression of genes other than MYC such as the transcriptional factor, FOSL1 [
      • Lockwood W.W.
      • Zejnullahu K.
      • Bradner J.E.
      • Varmus H.
      Sensitivity of human lung adenocarcinoma cell lines to targeted inhibition of BET epigenetic signaling proteins.
      ] and the signalling protein, ERK1 [
      • Segura M.F.
      • Fontanals-Cirera B.
      • Gaziel-Sovran A.
      • et al.
      BRD4 sustains melanoma proliferation and represents a new target for epigenetic therapy.
      ]. Thus, the precise role of MYC downregulation in mediating the anticancer activity of BET inhibitors is unclear and likely to vary, depending on the tumor cell type or cell context. Although several BET inhibitors are currently being investigated in clinical trials, preliminary results suggest that when used alone, the available compounds have limited efficacy as anticancer drugs [
      • Doroshow D.B.
      • Eder J.P.
      • LoRusso P.M.
      BET inhibitors: a novel epigenetic approach.
      ].

      Stabilization of G4 structures

      G4 motifs are secondary DNA structures consisting of ≥ 3 guanine tetrads layers that are located in the promoter and 5′ untranslated regions of highly transcribed genes such as MYC (for review, see refs. 93,94). G4 structures can either positively or negatively regulate gene expression. A specific G4 structure in the nuclease hypersensitive element III region of the MYC promoter was found to downregulate its expression. Consequently, several compounds were identified that bound to and stabilized this structure, resulting in decreased expression of MYC. Consistent with the decreased expression of MYC, several of the compounds (GQC-05, SYUIO-05, DC-34, APTO-253) were shown to inhibit cell proliferation or induce apoptosis in cancer cell lines [
      • Wang W.
      • Hu S.
      • Gu Y.
      • et al.
      Human MYC G-quadruplex: From discovery to a cancer therapeutic target.
      ,
      • Local A.
      • Zhang H.
      • Benbatoul K.D.
      • Folger P.
      • Sheng X.
      • Tsai C.Y.
      • et al.
      APTO-253 Stabilizes G-quadruplex DNA, inhibits MYC expression, and induces DNA damage in acute myeloid leukemia cells.
      ]. Of these, APTO-253 is the most investigated.
      Using acute myeloid leukemia (AML) cell lines, APTO-253 was found to stabilize the G4 structure in the MYC promoter, decrease expression of MYC, induce cell cycle arrest and trigger apoptosis [
      • Local A.
      • Zhang H.
      • Benbatoul K.D.
      • Folger P.
      • Sheng X.
      • Tsai C.Y.
      • et al.
      APTO-253 Stabilizes G-quadruplex DNA, inhibits MYC expression, and induces DNA damage in acute myeloid leukemia cells.
      ]. In addition, the compound has been shown to inhibit tumor growth in a wide range of in vitro and in vivo models as well as freshly isolated leukemic cells from bone marrow samples [
      • Local A.
      • Zhang H.
      • Benbatoul K.D.
      • Folger P.
      • Sheng X.
      • Tsai C.Y.
      • et al.
      APTO-253 Stabilizes G-quadruplex DNA, inhibits MYC expression, and induces DNA damage in acute myeloid leukemia cells.
      ,
      • Huesca M.
      • Lock L.S.
      • Khine A.A.
      • et al.
      A novel small molecule with potent anticancer activity inhibits cell growth by modulating intracellular labile zinc homeostasis.
      ]. These findings led to a phase I clinical trial in which APTO-253 was investigated in 32 patients with diverse metastatic cancers (ClinicalTrials.gov Identifier: NCT123226). Overall, the drug was found to be well tolerated, with fatigue as the only drug-related adverse event reported to occur in greater than 10% of patients [
      • Cercek A.
      • Wheler J.
      • Murray P.E.
      • Zhou S.
      • Saltz L.
      Phase 1 study of APTO-253 HCl, an inducer of KLF4, in patients with advanced or metastatic solid tumors.
      ]. In 21 patients who completed 2 cycles of treatment, stable disease occurred in 5 (23.8%) with durations ranging from 3.6 to 8.4 months. APTO-253 is currently undergoing testing in a further phase I clinical trial involving patients with AML or myelodysplastic syndrome (MDS) (ClinicalTrials.gov Identifier: NCT02267863).
      It is important to state that while APTO-253 decreases expression of MYC, the compound has also been shown to stabilize G4 structures in the KIT promoter, induce DNA damage and increase expression of the transcriptional factor KLF4 (involved in the regulation of proliferation, differentiation, apoptosis and somatic cell reprogramming) [
      • Tsai C.Y.
      • Sun S.
      • Zhang H.
      • Local A.
      • Su Y.
      • Gross L.A.
      • et al.
      APTO-253 is a new addition to the repertoire of drugs that can exploit DNA BRCA1/2 deficiency.
      ,
      • Wang B.
      • Shen A.
      • Ouyang X.
      • Zhao G.
      • Du Z.
      • Huo W.
      • et al.
      KLF4 expression enhances the efficacy of chemotherapy drugs in ovarian cancer cells.
      ]. Whether any of these processes is secondary to the downregulation of MYC is unknown. Furthermore, the relative contribution of these different effects to the antiproliferative/pro-apoptotic actions of APTO-253 is unclear.

      Other compounds downregulating MYC expression

      Other compounds found to downregulate expression of MYC in model systems include the CDK7 inhibitor, THZ1 in MYC-driven tumors such as small cell lung cancer, neuroblastoma and TNBC [
      • Christensen C.L.
      • Kwiatkowski N.
      • Abraham B.J.
      • et al.
      Targeting transcriptional addictions in small cell lung cancer with a covalent CDK7 inhibitor.
      ,
      • Chipumuro E.
      • Marco E.
      • Christensen C.L.
      • et al.
      CDK7 inhibition suppresses super-enhancer-linked oncogenic transcription in MYCN-driven cancer.
      ,
      • Wang Y.
      • Zhang T.
      • Kwiatkowski N.
      • et al.
      CDK7-dependent transcriptional addiction in triple-negative breast cancer.
      ], ME47 in breast cancer [
      • Lustig L.C.
      • Dingar D.
      • Tu W.B.
      • Lourenco C.
      • Kalkat M.
      • Inamoto I.
      • et al.
      Inhibiting MYC binding to the E-box DNA motif by ME47 decreases tumour xenograft growth.
      ], MYC-N-A3 in neuroblastoma [
      • Yoda H.
      • Inoue T.
      • Shinozaki Y.
      • Lin J.
      • Watanabe T.
      • Koshikawa N.
      • et al.
      Direct Targeting of MYCN gene amplification by site-specific DNA alkylation in neuroblastoma.
      ], cardiac glycosides (bufalin, oubain) in myeloma cells [
      • Steinberger J.
      • Robert F.
      • Hallé M.
      • Williams D.E.
      • Cencic R.
      • Sawhney N.
      • et al.
      Tracing MYC expression for small molecule discovery.
      ] and cytoskeletal disruptors (jasplakinolide and dolastatin) in myeloma cells [
      • Steinberger J.
      • Robert F.
      • Hallé M.
      • Williams D.E.
      • Cencic R.
      • Sawhney N.
      • et al.
      Tracing MYC expression for small molecule discovery.
      ].

      Identification of synthetic lethal partners for MYC overexpression

      A further potential approach for targeting MYC involves exploiting the concept of synthetic lethality. Synthetic lethality occurs when a defect (e.g., mutation) in either of 2 genes has little impact on cell viability but defects in both genes leads to cell death. The concept of synthetic lethality has been most successfully exploited in the use of PARP inhibitors for the treatment of patients with BRCA1/2 mutated ovarian and breast cancer [
      • Yi T.
      • Feng Y.
      • Sundaram R.
      • Tie Y.
      • Zheng H.
      • Qian Y.
      • et al.
      Antitumor efficacy of PARP inhibitors in homologous recombination deficient carcinomas.
      ]. In these cancers, the process of synthetic lethality is dependent on the inability of BRCA-mutated cancers to carry out homologous recombination repair of DNA while the PARP-mediated DNA repair process is blocked by specific inhibitors [
      • Yi T.
      • Feng Y.
      • Sundaram R.
      • Tie Y.
      • Zheng H.
      • Qian Y.
      • et al.
      Antitumor efficacy of PARP inhibitors in homologous recombination deficient carcinomas.
      ].
      In the context of aberrant MYC expression, targeting a potentially synthetic lethal protein should theoretically kill only the malignant cells with aberrant MYC expression and spare normal cells. Goga and co-workers were one of the first to explore the synthetic lethality concept for targeting MYC for cancer treatment [
      • Goga A.
      • Yang D.
      • Tward A.D.
      • Morgan D.O.
      • Bishop J.M.
      Inhibition of CDK1 as a potential therapy for tumors over-expressing MYC.
      ,
      • Horiuchi D.
      • Kusdra L.
      • Huskey N.E.
      • Chandriani S.
      • Lenburg M.E.
      • Gonzalez-Angulo A.M.
      • et al.
      MYC pathway activation in triple-negative breast cancer is synthetic lethal with CDK inhibition.
      ]. Using transgenic MYC-driven models of liver cancer, lymphoma or TNBC, these authors reported a synthetic lethal interaction between MYC overexpression and inhibition of CDK1, i.e., treatment of these MYC-dependent models with CDK1 inhibitors (purvalanol and roscovitine) decreased tumor growth and prolonged survival. These preliminary findings however, do not appear to have been further investigated, possibly because the CDK inhibitors used, lacked specificity for CDK1 and were subsequently found to be toxic [
      • Blachly J.S.
      • Byrd J.C.
      • Grever M.
      Cyclin-dependent kinase inhibitors for the treatment of chronic lymphocytic leukemia.
      ].
      More recently, overexpression of MYC was found to be synthetic lethal with PIM1 kinase inhibitors (also in TNBC) [
      • Horiuchi D.
      • Camarda R.
      • Zhou A.Y.
      • Yau C.
      • Momcilovic O.
      • Balakrishnan S.
      • et al.
      PIM1 kinase inhibition as a targeted therapy against triple-negative breast tumors with elevated MYC expression.
      ], with PARP inhibitors (in different cancer types) [
      • Carey J.P.W.
      • Karakas C.
      • Bui T.
      • Chen X.
      • Vijayaraghavan S.
      • Zhao Y.
      • et al.
      synthetic lethality of PARP inhibitors in combination with MYC blockade is independent of BRCA status in triple-negative breast cancer.
      ], with aurora B kinase inhibitors in lymphomas [
      • Yang D.
      • Liu H.
      • Goga A.
      • Kim S.
      • Yuneva M.
      • Bishop J.M.
      Therapeutic potential of a synthetic lethal interaction between the MYC proto-oncogene and inhibition of aurora-B kinase.
      ] and with TRAIL-induced apoptosis (in brain metastasis from breast cancer [
      • Lee H.Y.
      • Cha J.
      • Kim S.K.
      • Park J.H.
      • Song K.H.
      • Kim P.
      • et al.
      c-MYC drives breast cancer metastasis to the brain, but promotes synthetic lethality with TRAIL.
      ]. As with CDK1 inhibition, these results also have not been confirmed in additional model systems.

      Other potential approaches for targeting MYC

      Other potential strategies for targeting MYC for cancer treatment include inhibiting MYC translation [
      • Wiegering A.
      • Uthe F.W.
      • Jamieson T.
      • Ruoss Y.
      • Hüttenrauch M.
      • Küspert M.
      • et al.
      Targeting translation initiation bypasses signaling crosstalk mechanisms that maintain high myc levels in colorectal cancer.
      ,
      • Castell A.
      • Larsson L.G.
      Targeting MYC translation in colorectal cancer.
      ], stabilizing MAX homodimers [
      • Jiang H.
      • Bower K.E.
      • Beuscher 4th, A.E.
      • Zhou B.
      • Bobkov A.A.
      • Olson A.J.
      • et al.
      Stabilizers of the Max homodimer identified in virtual ligand screening inhibit Myc function.
      ,
      • Struntz N.B.
      • Chen A.
      • Deutzmann A.
      • Wilson R.M.
      • Stefan E.
      • Evans H.L.
      • et al.
      Stabilization of the MAX homodimer with a small molecule attenuates myc-driven transcription.
      ] and blocking interaction between MYC and interacting proteins other than MAX. As mentioned above, MYC interacts with several different proteins, in addition to MAX. Thus, it regulates transcription by interacting with co-regulatory proteins such as the TRRAP–HAT complexes [
      • McMahon S.B.
      • Van Buskirk H.A.
      • Dugan K.A.
      • Copeland T.D.
      • Cole M.D.
      The novel ATM-related protein TRRAP is an essential cofactor for the c-Myc and E2F oncoproteins.
      ] and the G9a histone methyltransferase [
      • Tu W.B.
      • Shiah Y.J.
      • Lourenco C.
      • et al.
      MYC Interacts with the G9a histone methyltransferase to drive transcriptional repression and tumorigenesis.
      ]. Preliminary data has shown that low molecular weight inhibitors of G9a such as UNC0642 or A366 decreased proliferation of breast cancer cell lines in vitro [
      • Tu W.B.
      • Shiah Y.J.
      • Lourenco C.
      • et al.
      MYC Interacts with the G9a histone methyltransferase to drive transcriptional repression and tumorigenesis.
      ]. Finally, as the oncogenicity of MYC depends on its interaction with WDR5, blocking this interaction is a further potential approach for targeting MYC [
      • Chacón Simon S.
      • Wang F.
      • Thomas L.R.
      • et al.
      Discovery of WD repeat-containing protein 5 (wdr5)-myc inhibitors using fragment-based methods and structure-based design.
      ].

      Conclusion

      It is clear from above that inhibiting MYC for cancer treatment is a highly active area of preclinical research. Indeed, as indicated, abrogating MYC activity or expression has been shown to result in reduced tumor growth in a diverse range of preclinical tumor models. Furthermore, based on available evidence, it appears that at least some of the MYC inhibitors mentioned above (e.g., OmoMYC) cause minimal toxicity to normal cells. However, it is important to point out that with the exception of OmoMYC, few of the findings with anti-MYC compounds have been reproduced by independent investigators or in multiple model systems. Furthermore, the specificity of the available compounds for MYC is unclear. For example, it is unclear if compounds preventing MYC-MAX interaction also target the bHLHLZ domain present on related transcriptional factors. A problem with available compounds preventing MYC expression is that they are unlikely to be specific in only downregulating MYC. Indeed, as mentioned above, the BET inhibitor, JQ1 has been shown to downregulate expression of multiple genes.
      Future work should continue to investigate the most promising compounds discussed above (e.g., OmoMYC, MYCi975) in additional animal models to further establish efficacy, identify potential predictive biomarkers and assess toxicity. Only when confirmed evidence of efficacy and lack of serious toxicity is seen in several different animal model systems, can these inhibitors move into clinical trials. Finally, since there is increasing evidence that MYC deregulation depresses the immune response to tumors [
      • Kortlever R.M.
      • Sodir N.M.
      • Wilson C.H.
      • Burkhart D.L.
      • Pellegrinet L.
      • Brown Swigart L.
      • et al.
      Myc cooperates with RAS by programming inflammation and immune suppression.
      ] and preliminary evidence that some MYC inhibitor, (i.e., OmoMYC and MYCi361), acts at least in part, by inducing an immune response [
      • Soucek L.
      • Helmer-Citterich M.
      • Sacco A.
      • Jucker R.
      • Cesareni G.
      • Nasi S.
      Design and properties of a Myc derivative that efficiently homodimerizes.
      ], future work should establish the possible generality of MYC inhibitors for enhancing anti-tumor immunity.
      Conflicts of Interest
      MJD, SO’G and MT declare no conflicts of interest. JC has received honoraria from Eisai, Amgen, Puma Biothechnology, Seattle Genetics, Boehringer Ingelheim, Pfizer, Vertex and Genomic Health. He has acted in an advisory/consulting role to Eisai, Puma Biotechnology, Boehringer Ingelheim, Pfizer, Vertex, Roche. He also serves on the Speakers' Bureau for Pfizer, Eisai and Genomic Health and has received Research Funding from Roche, Eisai, Boehringer Ingelheim and Puma Biotechnology. In addition, he has received Travel, Accommodations, Expenses from MSD, Pfizer, Roche, AstraZeneca, Abbvie and Novartis. Finally, he is an employee of OncoMark, has stocks in OncoMark and is named on patent WO2020011770 (A1) - A method of predicting response to treatment in cancer patients.

      Declaration of Competing Interest

      The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

      Acknowledgement

      We thank the Clinical Cancer Research Trust (Ireland) for funding this work.

      References

        • Dang C.V.
        A time for MYC: metabolism and therapy.
        Cold Spring Harb Symp Quant Biol. 2016; 81: 79-83
        • Beaulieu M.E.
        • Castillo F.
        • Soucek L.
        Structural and biophysical insights into the function of the intrinsically disordered Myc oncoprotein.
        Cells. 2020; 9: 1038
        • Carroll P.A.
        • Freie B.W.
        • Mathsyaraja H.
        • Eisenman R.N.
        The MYC transcript network: balancing metabolism, proliferation and oncogenesis.
        Front Med. 2018; 12: 412-425
        • Kortlever R.M.
        • Sodir N.M.
        • Wilson C.H.
        • Burkhart D.L.
        • Pellegrinet L.
        • Brown Swigart L.
        • et al.
        Myc cooperates with RAS by programming inflammation and immune suppression.
        Cell. 2017; 171e14
        • Cappellen D.
        • Schlange T.
        • Bauer M.
        • Maurer F.
        • Hynes N.E.
        Novel c-MYC target genes mediate differential effects on cell proliferation and migration.
        EMBO Rep. 2007; 8: 70-76
        • Murphy D.J.
        • Junttila M.R.
        • Pouyet L.
        • Karnezis A.
        • Shchors K.
        • Bui D.A.
        • et al.
        Distinct thresholds govern Myc's biological output in vivo.
        Cancer Cell. 2008; 14: 447-457
        • Li Z.
        • Van Calcar S.
        • Qu C.
        • Cavenee W.K.
        • Zhang M.Q.
        • Ren B.
        A global transcriptional regulatory role for c-Myc in Burkitt's lymphoma cells.
        Proc Natl Acad Sci U S A. 2003; 100: 8164-8169
        • Fernandez P.C.
        • Frank S.R.
        • Wang L.
        • Schroeder M.
        • Liu S.
        • Greene J.
        • et al.
        Genomic targets of the human c-Myc protein.
        Genes Dev. 2003; 17: 1115-1129
        • Zeller K.I.
        • Zhao X.
        • Lee C.W.
        • Chiu K.P.
        • Yao F.
        • Yustein J.T.
        • et al.
        Global mapping of c-Myc binding sites and target gene networks in human B cells.
        Proc Natl Acad Sci U S A. 2006; 103: 17834-17839
        • Dang C.V.
        • O'Donnell K.A.
        • Zeller K.I.
        • Nguyen T.
        • Osthus R.C.
        • Li F.
        The c-Myc target gene network.
        Semin Cancer Biol. 2006; 16: 253-264
        • de Pretis S.
        • Kress T.R.
        • Morelli M.J.
        • Sabò A.
        • Locarno C.
        • Verrecchia A.
        • et al.
        Integrative analysis of RNA polymerase II and transcriptional dynamics upon MYC activation.
        Genome Res. 2017; 27: 1658-1664
      1. Grandori C, Gomez-Roman N, Felton-Edkins ZA, Ngouenet C, Galloway DA, Eisenman RN, White RJ. c-Myc binds to human ribosomal DNA and stimulates transcription of rRNA genes by RNA polymerase I. Nat Cell Biol 2005;7:311-8. Erratum in: Nat Cell Biol 2005;7:531.

        • Gomez-Roman N.
        • Grandori C.
        • Eisenman R.N.
        • White R.J.
        Direct activation of RNA polymerase III transcription by c-Myc.
        Nature. 2003; 421: 290-294
        • Sabò A.
        • Kress T.R.
        • Pelizzola M.
        • de Pretis S.
        • Gorski M.M.
        • Tesi A.
        • et al.
        Selective transcriptional regulation by Myc in cellular growth control and lymphomagenesis.
        Nature. 2014; 511: 488-492
        • Dominguez-Sola D.
        • Ying C.Y.
        • Grandori C.
        • Ruggiero L.
        • Chen B.
        • Li M.
        • et al.
        Non-transcriptional control of DNA replication by c-Myc.
        Nature. 2007; 448: 445-451
        • Singh K.
        • Lin J.
        • Zhong Y.
        • Burčul A.
        • Mohan P.
        • Jiang M.
        • et al.
        c-MYC regulates mRNA translation efficiency and start-site selection in lymphoma.
        J Exp Med. 2019; 216: 1509-1524
        • Xu Y.
        • Poggio M.
        • Jin H.Y.
        • et al.
        Translation control of the immune checkpoint in cancer and its therapeutic targeting.
        Nat Med. 2019; 25: 301-311
        • Dang C.V.
        MYC on the path to cancer.
        Cell. 2012; 149: 22-35
        • Meyer N.
        • Penn L.Z.
        Reflecting on 25 years with MYC.
        Nat Rev Cancer. 2008; 8: 976-990
      2. Conacci-Sorrell M, McFerrin L, Eisenman RN. An overview of MYC and its interactome. Cold Spring Harb Perspect Med 2014;4:a014357. Review.

        • Kalkat M.
        • Resetca D.
        • Lourenco C.
        • Chan P.K.
        • Wei Y.
        • Shiah Y.J.
        • et al.
        MYC Protein interactome profiling reveals functionally distinct regions that cooperate to drive tumorigenesis.
        Mol Cell. 2018; 72e7
        • Zhang Q.
        • West-Osterfield K.
        • Spears E.
        • Li Z.
        • Panaccione A.
        • Hann S.R.
        MB0 and MBI are independent and distinct transactivation domains in MYC that are essential for transformation.
        Genes (Basel). 2017; 8: 134
        • Yada M.
        • Hatakeyama S.
        • Kamura T.
        • et al.
        Phosphorylation-dependent degradation of c-Myc is mediated by the F-box protein Fbw7.
        EMBO J. 2004; 23: 2116-2125
        • McMahon S.B.
        • Van Buskirk H.A.
        • Dugan K.A.
        • Copeland T.D.
        • Cole M.D.
        The novel ATM-related protein TRRAP is an essential cofactor for the c-Myc and E2F oncoproteins.
        Cell. 1998; 94: 363-374
        • Tu W.B.
        • Shiah Y.J.
        • Lourenco C.
        • et al.
        MYC Interacts with the G9a histone methyltransferase to drive transcriptional repression and tumorigenesis.
        Cancer Cell. 2018; 34: 579-595.e8
        • Herbst A.
        • Hemann M.T.
        • Tworkowski K.A.
        • Salghetti S.E.
        • Lowe S.W.
        • Tansey W.P.
        A conserved element in Myc that negatively regulates its proapoptotic activity.
        EMBO Rep. 2005; 6: 177-183
        • Thomas L.R.
        • Adams C.M.
        • Wang J.
        • et al.
        Interaction of the oncoprotein transcription factor MYC with its chromatin cofactor WDR5 is essential for tumor maintenance.
        Proc Natl Acad Sci U S A. 2019; 116: 25260-25268
      3. Cowling VH, Chandriani S, Whitfield ML, Cole MD. A conserved Myc protein domain, MBIV, regulates DNA binding, apoptosis, transformation, and G2 arrest. Mol Cell Biol 2006;26:4226-39. Erratum in: Mol Cell Biol 2006;26:5201.

        • Thomas L.R.
        • Foshage A.M.
        • Weissmiller A.M.
        • Popay T.M.
        • Grieb B.C.
        • Qualls S.J.
        • et al.
        Interaction of MYC with host cell factor-1 is mediated by the evolutionarily conserved Myc box IV motif.
        Oncogene. 2016; 35: 3613-3618
      4. Lüscher B, Larsson LG. The basic region/helix-loop-helix/leucine zipper domain of Myc proto-oncoproteins: function and regulation. Oncogene 1999;18:2955-66. Review.

        • Baudino T.A.
        • Cleveland J.L.
        The Max network gone mad.
        Mol Cell Biol. 2001; 21: 691-702
        • Walz S.
        • Lorenzin F.
        • Morton J.
        • et al.
        Activation and repression by oncogenic MYC shape tumour-specific gene expression profiles.
        Nature. 2014; 511: 483-487
        • Peukert K.
        • Staller P.
        • Schneider A.
        • Carmichael G.
        • Hänel F.
        • Eilers M.
        An alternative pathway for gene regulation by Myc.
        EMBO J. 1997; 16: 5672-5686
        • Shostak A.
        • Ruppert B.
        • Ha N.
        • et al.
        MYC/MIZ1-dependent gene repression inversely coordinates the circadian clock with cell cycle and proliferation.
        Nat Commun. 2016; 7: 11807
        • Blackwell T.K.
        • Huang J.
        • Ma A.
        • Kretzner L.
        • Alt F.W.
        • Eisenman R.N.
        • et al.
        Binding of MYC proteins to canonical and noncanonical DNA sequences.
        Mol Cell Biol. 1993; 13: 5216-5224
        • Lorenzin F.
        • Benary U.
        • Baluapuri A.
        • et al.
        Different promoter affinities account for specificity in MYC-dependent gene regulation.
        Elife. 2016; 5e15161
      5. Agarwal S1, Milazzo G2, Rajapakshe K, Bernardi R, Chen Z, Barberi E, Koster J, Perini G, Coarfa C, Shohet JM. Agarwal S, Milazzo G, Rajapakshe K, Bernardi R, Chen Z, Barberi E, Koster J, Perini G, Coarfa C, Shohet JM. MYCN acts as a direct co-regulator of p53 in MYCN amplified neuroblastoma. Oncotarget 2018;9:20323-20338. Erratum in: Oncotarget 2018;9:30024.

        • Schaub F.X.
        • Dhankani V.
        • Berger A.C.
        • Trivedi M.
        • Richardson A.B.
        • Shaw R.
        • et al.
        Cancer Genome Atlas Network. Pan-cancer alterations of the MYC oncogene and its proximal network across the Cancer Genome Atlas.
        Cell Syst. 2018; 6e2
        • Schick M.
        • Habringer S.
        • Nilsson J.A.
        • Keller U.
        Pathogenesis and therapeutic targeting of aberrant MYC expression in haematological cancers.
        Br J Haematol. 2017; 179: 724-738
        • Adams J.M.
        • Harris A.W.
        • Pinkert C.A.
        • Corcoran L.M.
        • Alexander W.S.
        • Cory S.
        • et al.
        The c-myc oncogene driven by immunoglobulin enhancers induces lymphoid malignancy in transgenic mice.
        Nature. 1985; 318: 533-538
        • Xu-Monette Z.Y.
        • Deng Q.
        • Manyam G.C.
        • Tzankov A.
        • Li L.
        • Xia Y.
        • et al.
        MYC mutation profiling and prognostic significance in de novo diffuse large B-cell lymphoma.
        Clin Cancer Res. 2016; 22: 3593-3605
        • Tarrado-Castellarnau M.
        • de Atauri P.
        • Cascante M.
        Oncogenic regulation of tumor metabolic reprogramming.
        Oncotarget. 2016; 7: 62726-62753
      6. Kalkat M, De Melo J, Hickman KA, Lourenco C, Redel C, Resetca D, Tamachi A, Tu WB, Penn LZ. MYC deregulation in primary human cancers. Genes (Basel) 2017;25;8(6).

      7. AlSultan D, Kavanagh E, O'Grady S, Eustace AJ, Castell A, Larsson LG, Crown J, Madden SF, Duffy MJ. The novel low molecular weight MYC antagonist MYCMI-6 inhibits proliferation and induces apoptosis in breast cancer cells. Invest New Drugs. 2020 Oct 14. doi: 10.1007/s10637-020-01018-w. Epub ahead of print. PMID: 33052557.

        • Seeger R.C.
        • Brodeur G.M.
        • Sather H.
        • Dalton A.
        • Siegel S.E.
        • Wong K.Y.
        • et al.
        Association of multiple copies of the N-myc oncogene with rapid progression of neuroblastomas.
        N Engl J Med. 1985; 313: 1111-1116
        • Nau M.M.
        • Brooks B.J.
        • Battey J.
        • Sausville E.
        • Gazdar A.F.
        • Kirsch I.R.
        • et al.
        L-myc, a new myc-related gene amplified and expressed in human small cell lung cancer.
        Nature. 1985; 318: 69-73
        • Mikulasova A.
        • Ashby C.
        • Tytarenko R.G.
        • Qu P.
        • Rosenthal A.
        • Dent J.A.
        • et al.
        Microhomology-mediated end joining drives complex rearrangements and overexpression of MYC and PVT1 in multiple myeloma.
        Haematologica. 2020; 105: 1055-1066
        • Hayes T.K.
        • Neel N.F.
        • Hu C.
        • Gautam P.
        • Chenard M.
        • Long B.
        • et al.
        Long-term ERK inhibition in KRAS-mutant pancreatic cancer is associated with MYC degradation and senescence-like growth suppression.
        Cancer Cell. 2016; 29: 75-89
        • Liu P.
        • Cheng H.
        • Santiago S.
        • Raeder M.
        • Zhang F.
        • Isabella A.
        • et al.
        Oncogenic PIK3CA-driven mammary tumors frequently recur via PI3K pathway-dependent and PI3K pathway-independent mechanisms.
        Nat Med. 2011; 17: 1116-1120
        • Yochum G.S.
        • Sherrick C.M.
        • Macpartlin M.
        • Goodman R.H.
        A beta-catenin/TCF-coordinated chromatin loop at MYC integrates 5' and 3' Wnt responsive enhancers.
        Proc Natl Acad Sci U S A. 2010; 107: 145-150
        • Santoro A.
        • Vlachou T.
        • Luzi L.
        • Melloni G.
        • Mazzarella L.
        • D'Elia E.
        • et al.
        p53 loss in breast cancer leads to MYC activation, increased cell plasticity, and expression of a mitotic signature with prognostic value.
        Cell Rep. 2019; 26e8
      8. Yumimoto K, Nakayama KI. Recent insight into the role of FBXW7 as a tumor suppressor. Semin Cancer Biol. 2020:S1044-579X(20)30050-X.

        • Mahauad-Fernandez W.D.
        • Felsher D.W.
        The Myc and Ras partnership in cancer: indistinguishable alliance or contextual relationship?.
        Cancer Res. 2020; 80: 3799-3802
        • Torchia E.C.
        • Caulin C.
        • Acin S.
        • et al.
        Myc, Aurora Kinase A, and mutant p53(R172H) co-operate in a mouse model of metastatic skin carcinoma.
        Oncogene. 2012; 31: 2680-2690
      9. Casey SC, Tong L, Li Y, et al. MYC regulates the antitumor immune response through CD47 and PD-L1 [published correction appears in Science. 2016;352(6282). pii: aaf7984. doi: 10.1126/science.aaf7984]. Science. 2016;352(6282):227-231.

        • Casey S.C.
        • Baylot V.
        • Felsher D.W.
        The MYC oncogene is a global regulator of the immune response.
        Blood. 2018; 131: 2007-2015
      10. Marinkovic D, Marinkovic T. The new role for an old guy: MYC as an immunoplayer. J Cell Physiol. 2020 Oct 23. doi: 10.1002/jcp.30123. Epub ahead of print. PMID: 33094851.

        • Sodir N.M.
        • Kortlever R.M.
        • Barthet V.J.A.
        • et al.
        MYC instructs and maintains pancreatic adenocarcinoma phenotype.
        Cancer Discov. 2020; 10: 588-607
        • Jain M.
        • Arvanitis C.
        • Chu K.
        • Dewey W.
        • Leonhardt E.
        • Trinh M.
        • et al.
        Sustained loss of a neoplastic phenotype by brief inactivation of MYC.
        Science. 2002; 297: 102-104
        • Whitfield J.R.
        • Beaulieu M.E.
        • Soucek L.
        Strategies to inhibit myc and their clinical applicability.
        Front Cell Dev Biol. 2017; 5: 10
        • Wolf E.
        • Eilers M.
        Targeting MYC proteins for tumor therapy.
        Annu Rev Cancer Biol. 2020; 4: 61-75
        • Fletcher S.
        • Prochownik E.V.
        Small-molecule inhibitors of the Myc oncoprotein.
        Biochim Biophys Acta. 2015; 1849: 525-543
        • Massó-Vallés D.
        • Soucek L.
        Blocking myc to treat cancer: reflecting on two decades of Omomyc.
        Cells. 2020; 9: 883
        • Soucek L.
        • Helmer-Citterich M.
        • Sacco A.
        • Jucker R.
        • Cesareni G.
        • Nasi S.
        Design and properties of a Myc derivative that efficiently homodimerizes.
        Oncogene. 1998; 17: 2463-2472
        • Soucek L.
        • Jucker R.
        • Panacchia L.
        • Ricordy R.
        • Tatò F.
        • Nasi S.
        Omomyc, a potential Myc dominant negative, enhances Myc-induced apoptosis.
        Cancer Res. 2002; 62: 3507-3510
        • Soucek L.
        • Whitfield J.
        • Martins C.P.
        • Finch A.J.
        • Murphy D.J.
        • Sodir N.M.
        • et al.
        Modelling Myc inhibition as a cancer therapy.
        Nature. 2008; 455: 679-683
      11. Savino M1, Annibali D, Carucci N, Favuzzi E, Cole MD, Evan GI, Soucek L, Nasi S. The action mechanism of the Myc inhibitor termed Omomyc may give clues on how to target Myc for cancer therapy. PLoS One 2011;6:e22284.

        • Jung L.A.
        • Gebhardt A.
        • Koelmel W.
        • Ade C.P.
        • Walz S.
        • Kuper J.
        • et al.
        OmoMYC blunts promoter invasion by oncogenic MYC to inhibit gene expression characteristic of MYC-dependent tumors.
        Oncogene. 2017; 36: 1911-1924
        • Demma M.J.
        • Mapelli C.
        • Sun A.
        • Bodea S.
        • Ruprecht B.
        • Javaid S.
        • et al.
        Omomyc reveals new mechanisms to inhibit the MYC oncogene.
        Mol Cell Biol. 2019; 39: e00248-e319
        • Soucek L.
        • Whitfield J.R.
        • Sodir N.M.
        • Massó-Vallés D.
        • Serrano E.
        • Karnezis A.N.
        • et al.
        Inhibition of Myc family proteins eradicates KRas-driven lung cancer in mice.
        Genes Dev. 2013; 27: 504-513
      12. Beaulieu ME, Jauset T, Massó-Vallés D, Martínez-Martín S, Rahl P, Maltais L, Zacarias-Fluck MF, Casacuberta-Serra S, Serrano Del Pozo E, Fiore C, Foradada L, Cano VC, Sánchez-Hervás M, Guenther M, Romero Sanz E, Oteo M, Tremblay C, Martín G, Letourneau D, Montagne M, Morcillo Alonso MÁ, Whitfield JR, Lavigne P, Soucek L. Intrinsic cell-penetrating activity propels Omomyc from proof of concept to viable anti-MYC therapy. Sci Transl Med 2019;11:(484).

        • Wang E.
        • Sorolla A.
        • Cunningham P.T.
        • Bogdawa H.M.
        • Beck S.
        • Golden E.
        • et al.
        Tumor penetrating peptides inhibiting MYC as a potent targeted therapeutic strategy for triple-negative breast cancers.
        Oncogene. 2019; 38: 140-150
        • Villanueva M.T.
        Long path to MYC inhibition approaches clinical trials.
        Nat Rev Cancer. 2019; 19: 252
        • Yin X.
        • Giap C.
        • Lazo J.S.
        • Prochownik E.V.
        Low molecular weight inhibitors of Myc-Max interaction and function.
        Oncogene. 2003; 22: 6151-6159
        • Mustata G.
        • Follis A.V.
        • Hammoudeh D.I.
        • et al.
        Discovery of novel Myc-Max heterodimer disruptors with a three-dimensional pharmacophore model.
        J Med Chem. 2009; 52: 1247-1250
        • Follis A.V.
        • Hammoudeh D.I.
        • Wang H.
        • Prochownik E.V.
        • Metallo S.J.
        Structural rationale for the coupled binding and unfolding of the c-Myc oncoprotein by small molecules.
        Chem Biol. 2008; 15: 1149-1155
        • Müller I.
        • Larsson K.
        • Frenzel A.
        • Oliynyk G.
        • Zirath H.
        • Prochownik E.V.
        • et al.
        Targeting of the MYCN protein with small molecule c-MYC inhibitors.
        PLoS ONE. 2014; 9e97285
        • Guo J.
        • Parise R.A.
        • Joseph E.
        • Egorin M.J.
        • Lazo J.S.
        • Prochownik E.V.
        Eiseman Efficacy, pharmacokinetics, tisssue distribution, and metabolism of the Myc-Max disruptor, 10058–F4 [Z, E]-5-[4-ethylbenzylidine]-2-thioxothiazolidin-4-one, in mice.
        Cancer Chemother Pharmacol. 2009; 63: 615-625
        • Zirath H.
        • Frenzel A.
        • Oliynyk G.
        • Segerström L.
        • Westermark U.K.
        • Larsson K.
        • et al.
        MYC inhibition induces metabolic changes leading to accumulation of lipid droplets in tumor cells.
        Proc Natl Acad Sci U S A. 2013; 110: 10258-10263
        • Wang H.
        • Hammoudeh D.I.
        • Follis A.V.
        • Reese B.E.
        • Lazo J.S.
        • Metallo S.J.
        • et al.
        Improved low molecular weight Myc-Max inhibitors.
        Mol Cancer Ther. 2007; 6: 2399-2408
        • Hart J.R.
        • Garner A.L.
        • Yu J.
        • Ito Y.
        • Sun M.
        • Ueno L.
        • et al.
        Inhibitor of MYC identified in a Kröhnke pyridine library.
        Proc Natl Acad Sci U S A. 2014; 111: 12556-12561
        • Castell A.
        • Yan Q.
        • Fawkner K.
        • Hydbring P.
        • Zhang F.
        • Verschut V.
        • et al.
        A selective high affinity MYC-binding compound inhibits MYC:MAX interaction and MYC-dependent tumor cell proliferation.
        Sci Rep. 2018; 8: 10064
        • Han H.
        • Jain A.D.
        • Truica M.I.
        • et al.
        Small-molecule MYC inhibitors suppress tumor growth and enhance immunotherapy.
        Cancer Cell. 2019; 36e15
        • Shi J.
        • Vakoc C.R.
        The mechanisms behind the therapeutic activity of BET bromodomain inhibition.
        Mol Cell. 2014; 54: 728-736
        • Delmore J.E.
        • Issa G.C.
        • Lemieux M.E.
        • Rahl P.B.
        • Shi J.
        • Jacobs H.M.
        • et al.
        BET bromodomain inhibition as a therapeutic strategy to target c-Myc.
        Cell. 2011; 146: 904-917
        • Fu L.L.
        • Tian M.
        • Li X.
        • Li J.J.
        • Huang J.
        • Ouyang L.
        • et al.
        Inhibition of BET bromodomains as a therapeutic strategy for cancer drug discovery.
        Oncotarget. 2015; 6: 5501-5516
        • Zuber J.
        • Shi J.
        • Wang E.
        • Rappaport A.R.
        • Herrmann H.
        • Sison E.A.
        • et al.
        RNAi screen identifies Brd4 as a therapeutic target in acute myeloid leukaemia.
        Nature. 2011; 478: 524-528
        • Mertz J.A.
        • Conery A.R.
        • Bryant B.M.
        • Sandy P.
        • Balasubramanian S.
        • Mele D.A.
        • et al.
        Targeting MYC dependence in cancer by inhibiting BET bromodomains.
        Proc National Acad Sci USA. 2011; 108: 16669-16674
        • Dawson M.A.
        • Prinjha R.K.
        • Dittmann A.
        • Giotopoulos G.
        • Bantscheff M.
        • Chan W.I.
        • et al.
        Inhibition of BET recruitment to chromatin as an effective treatment for MLL-fusion leukaemia.
        Nature. 2011; 478: 529-533
        • Lockwood W.W.
        • Zejnullahu K.
        • Bradner J.E.
        • Varmus H.
        Sensitivity of human lung adenocarcinoma cell lines to targeted inhibition of BET epigenetic signaling proteins.
        Proc Natl Acad Sci U S A. 2012; 109: 19408-19413
        • Segura M.F.
        • Fontanals-Cirera B.
        • Gaziel-Sovran A.
        • et al.
        BRD4 sustains melanoma proliferation and represents a new target for epigenetic therapy.
        Cancer Res. 2013; 73: 6264-6276
        • Doroshow D.B.
        • Eder J.P.
        • LoRusso P.M.
        BET inhibitors: a novel epigenetic approach.
        Ann Oncol. 2017; 28: 1776-1787
        • Wang W.
        • Hu S.
        • Gu Y.
        • et al.
        Human MYC G-quadruplex: From discovery to a cancer therapeutic target.
        Biochim Biophys Acta Rev Cancer. 2020; 1874188410
        • Asamitsu S.
        • Obata S.
        • Yu Z.
        • Bando T.
        • Sugiyama H.
        Recent progress of targeted G-quadruplex-preferred ligands toward cancer therapy.
        Molecules. 2019; 24: 429
        • Local A.
        • Zhang H.
        • Benbatoul K.D.
        • Folger P.
        • Sheng X.
        • Tsai C.Y.
        • et al.
        APTO-253 Stabilizes G-quadruplex DNA, inhibits MYC expression, and induces DNA damage in acute myeloid leukemia cells.
        Mol Cancer Ther. 2018; 17: 1177-1186
        • Huesca M.
        • Lock L.S.
        • Khine A.A.
        • et al.
        A novel small molecule with potent anticancer activity inhibits cell growth by modulating intracellular labile zinc homeostasis.
        Mol Cancer Ther. 2009; 8: 2586-2596
        • Cercek A.
        • Wheler J.
        • Murray P.E.
        • Zhou S.
        • Saltz L.
        Phase 1 study of APTO-253 HCl, an inducer of KLF4, in patients with advanced or metastatic solid tumors.
        Invest New Drugs. 2015; 33: 1086-1092
        • Tsai C.Y.
        • Sun S.
        • Zhang H.
        • Local A.
        • Su Y.
        • Gross L.A.
        • et al.
        APTO-253 is a new addition to the repertoire of drugs that can exploit DNA BRCA1/2 deficiency.
        Mol Cancer Ther. 2018; 17: 1167-1176
        • Wang B.
        • Shen A.
        • Ouyang X.
        • Zhao G.
        • Du Z.
        • Huo W.
        • et al.
        KLF4 expression enhances the efficacy of chemotherapy drugs in ovarian cancer cells.
        Biochem Biophys Res Commun. 2017; 484: 486-492
        • Christensen C.L.
        • Kwiatkowski N.
        • Abraham B.J.
        • et al.
        Targeting transcriptional addictions in small cell lung cancer with a covalent CDK7 inhibitor.
        Cancer Cell. 2014; 26: 909-922
        • Chipumuro E.
        • Marco E.
        • Christensen C.L.
        • et al.
        CDK7 inhibition suppresses super-enhancer-linked oncogenic transcription in MYCN-driven cancer.
        Cell. 2014; 159: 1126-1139
        • Wang Y.
        • Zhang T.
        • Kwiatkowski N.
        • et al.
        CDK7-dependent transcriptional addiction in triple-negative breast cancer.
        Cell. 2015; 163: 174-186
        • Lustig L.C.
        • Dingar D.
        • Tu W.B.
        • Lourenco C.
        • Kalkat M.
        • Inamoto I.
        • et al.
        Inhibiting MYC binding to the E-box DNA motif by ME47 decreases tumour xenograft growth.
        Oncogene. 2017; 36: 6830-6837
        • Yoda H.
        • Inoue T.
        • Shinozaki Y.
        • Lin J.
        • Watanabe T.
        • Koshikawa N.
        • et al.
        Direct Targeting of MYCN gene amplification by site-specific DNA alkylation in neuroblastoma.
        Cancer Res. 2019; 79: 830-840
        • Steinberger J.
        • Robert F.
        • Hallé M.
        • Williams D.E.
        • Cencic R.
        • Sawhney N.
        • et al.
        Tracing MYC expression for small molecule discovery.
        Cell Chem Biol. 2019; 26e6
        • Yi T.
        • Feng Y.
        • Sundaram R.
        • Tie Y.
        • Zheng H.
        • Qian Y.
        • et al.
        Antitumor efficacy of PARP inhibitors in homologous recombination deficient carcinomas.
        Int J Cancer. 2019; 145: 1209-1220
        • Goga A.
        • Yang D.
        • Tward A.D.
        • Morgan D.O.
        • Bishop J.M.
        Inhibition of CDK1 as a potential therapy for tumors over-expressing MYC.
        Nat Med. 2007; 13: 820-827
        • Horiuchi D.
        • Kusdra L.
        • Huskey N.E.
        • Chandriani S.
        • Lenburg M.E.
        • Gonzalez-Angulo A.M.
        • et al.
        MYC pathway activation in triple-negative breast cancer is synthetic lethal with CDK inhibition.
        J Exp Med. 2012; 209: 679-696
        • Blachly J.S.
        • Byrd J.C.
        • Grever M.
        Cyclin-dependent kinase inhibitors for the treatment of chronic lymphocytic leukemia.
        Semin Oncol. 2016; 43: 265-273
        • Horiuchi D.
        • Camarda R.
        • Zhou A.Y.
        • Yau C.
        • Momcilovic O.
        • Balakrishnan S.
        • et al.
        PIM1 kinase inhibition as a targeted therapy against triple-negative breast tumors with elevated MYC expression.
        Nat Med. 2016; 22: 1321-1329
        • Carey J.P.W.
        • Karakas C.
        • Bui T.
        • Chen X.
        • Vijayaraghavan S.
        • Zhao Y.
        • et al.
        synthetic lethality of PARP inhibitors in combination with MYC blockade is independent of BRCA status in triple-negative breast cancer.
        Cancer Res. 2018; 78: 742-757
        • Yang D.
        • Liu H.
        • Goga A.
        • Kim S.
        • Yuneva M.
        • Bishop J.M.
        Therapeutic potential of a synthetic lethal interaction between the MYC proto-oncogene and inhibition of aurora-B kinase.
        Proc Natl Acad Sci U S A. 2010; 107: 13836-13841
        • Lee H.Y.
        • Cha J.
        • Kim S.K.
        • Park J.H.
        • Song K.H.
        • Kim P.
        • et al.
        c-MYC drives breast cancer metastasis to the brain, but promotes synthetic lethality with TRAIL.
        Mol Cancer Res. 2019; 17: 544-554
        • Wiegering A.
        • Uthe F.W.
        • Jamieson T.
        • Ruoss Y.
        • Hüttenrauch M.
        • Küspert M.
        • et al.
        Targeting translation initiation bypasses signaling crosstalk mechanisms that maintain high myc levels in colorectal cancer.
        Cancer Discov. 2015; 5: 768-781
        • Castell A.
        • Larsson L.G.
        Targeting MYC translation in colorectal cancer.
        Cancer Discov. 2015; 5: 701-703
        • Jiang H.
        • Bower K.E.
        • Beuscher 4th, A.E.
        • Zhou B.
        • Bobkov A.A.
        • Olson A.J.
        • et al.
        Stabilizers of the Max homodimer identified in virtual ligand screening inhibit Myc function.
        Mol Pharmacol. 2009; 76: 491-502
        • Struntz N.B.
        • Chen A.
        • Deutzmann A.
        • Wilson R.M.
        • Stefan E.
        • Evans H.L.
        • et al.
        Stabilization of the MAX homodimer with a small molecule attenuates myc-driven transcription.
        Cell Chem Biol. 2019; 26e14
        • Chacón Simon S.
        • Wang F.
        • Thomas L.R.
        • et al.
        Discovery of WD repeat-containing protein 5 (wdr5)-myc inhibitors using fragment-based methods and structure-based design.
        J Med Chem. 2020; 63: 4315-4333