In an attempt to move away from toxic and non-specific chemotherapeutic agents, a global effort to develop targeted therapeutic strategies to inhibit oncogenic drivers has dominated the cancer biology field. By interrogating tumor cells at the DNA, RNA, and protein level, we have been able to identify specific cancers or cancer subtypes where a significant percentage of patients express a dominant oncogenic driver. In these cases, researchers have shown that the loss of this dominant driver leads to tumor cell death, and multiple targeted therapeutic agents based on this principle have shown great clinical success. For example, the BCR/ABL1 inhibitor Gleevac
® has increased the 8-year survival of patients with chronic-phase chronic myeloid leukemia (CML) from 6 to ~ 90% and represents one of the most successful targeted kinase inhibitors to date [
1]. Similarly, HER2 (human epidermal growth factor receptor 2) overexpression or amplification has been shown to occur in ~ 20% of breast cancer patients and anti-HER2 therapies such as trastuzumab and lapatinib have significantly increased patient survival in this subset of patients [
2]. These clinical successes have helped fuel translational studies and highlight the potential of utilizing targeted therapies in the clinic. Unfortunately, a large number of tumors are driven by a small number of common oncogenic proteins that lack structural regions amenable to therapeutic inhibition [
3,
4]. Prototypic examples of this are KRAS and MYC, where mutational activation and deregulated oncogenic expression are common driver events in cancer progression in many tissues and, therefore, these oncoproteins are considered highly desirable therapeutic targets [
3,
5‐
7]. However, despite their significant contribution to disease states, these factors are commonly thought to be ‘undruggable’. The generation of therapeutic compounds that could effectively target these drivers would significantly alter the clinical outcome of an extraordinary number of patients. Here we review the biology of MYC deregulation in cancer that supports innovative strategies for therapeutic targeting and the potential for translating these strategies to the clinic.
1.1 MYC Deregulation in Cancer
The MYC transcription factor family consists of c-, L-, and N-MYC. The aberrant expression or activity of any one of these family members has been shown to contribute to tumor development, although the latter two seem to be restricted to specific tissues, most prominently lung and neural, respectively [
8‐
11]. MYC family proteins function as potent transcription factors that regulate multiple cellular processes, including proliferation, differentiation, adhesion, and survival [
9,
10,
12]. Several studies have demonstrated that MYC functions as a master transcriptional regulator, binding to the majority of regulated genes in the genome [
10,
13,
14]. Given the prolific role of MYC in transcriptional regulation, expression of MYC proteins is tightly regulated at the transcriptional, translational, and post-translational levels in normal tissues, with a half-life of ~ 20 min [
15,
16]. The major MYC protein domains include an N-terminal transactivation domain (TAD), MYC box domains (MB0-IV), a PEST domain (Proline, glutamic acid [E], Serine and Threonine rich), a nuclear localization sequence (NLS), and the carboxy-terminus basic-helix-loop-helix-leucine zipper (bHLHZ) [
17‐
21]. Each of these domains facilitates interactions between MYC and a diverse set of binding partners in order to regulate MYC function and gene target specificity. The MB0-II domains are essential to MYC protein stability and activity, and facilitate MYC’s association with co-factors, such as PIN1, FBW7, and P-TEFb [
19,
22‐
26]. MBIII and MBIV regulate the apoptotic function of MYC, as well as protein turnover [
27‐
30]. Finally, the bHLHZ domain facilitates MYC’s interaction with its transcriptional co-factor MYC-associated protein X (MAX), allowing for DNA binding [
8,
17,
18,
31]. Although the complex MYC interactome creates unique challenges for the development of MYC-specific inhibitors, each of these functional domains provides potential points of regulation that can be exploited to reduce the oncogenic function of MYC. Since all three MYC family proteins contain homology in these functional domains and their bHLHZ domains, several of the proposed therapeutic agents are likely to function against multiple MYC proteins.
The current dogma regards
MYC amplification as the primary method by which MYC is deregulated in disease states. However, the post-translational regulation of MYC has emerged as an important mechanism, irrespective of amplification, by which MYC is stabilized and activated [
32‐
34]. Research has identified two interdependent phosphorylation sites that are critical for the regulation of MYC stability and function. While these sites are conserved across MYC family members, we focus here on c-MYC (‘MYC’, unless otherwise specified). Downstream of growth-stimulatory signals, activation of the RAS/MEK/ERK cascade or cyclin-dependent kinases (CDKs) leads to the phosphorylation of MYC at Serine 62 (pS62-MYC) [
32,
33,
35,
36]. This modification supports isomerization of Proline 63 in MYC from the
trans to
cis conformation by the phospho-serine/threonine-directed peptidyl-prolyl isomerase, PIN1, and these events increase MYC DNA binding and target gene regulation. Phosphorylation of Serine 62 (S62) also primes MYC for glycogen synthase kinase 3 (GSK3)-mediated phosphorylation at Threonine 58 (pT58-MYC), which initiates MYC turnover. Dual phosphorylated MYC (pS62/pT58-MYC) then undergoes a second isomerization by PIN1, returning Proline 63 MYC to the
trans conformation. This second isomerization event results in the association of MYC with the
trans-specific phosphatase Protein Phosphatase 2A (PP2A), which dephosphorylates the stabilizing S62 residue and targets MYC for ubiquitin-mediated proteosomal degradation through the E3 ubiquitin ligase SCF
FBW7 [
33,
37‐
40]. Considering that MYC has a very short half-life, the balance of these phosphorylation and isomerization states provides controlled activity and rapid turnover of the MYC protein, allowing an expedited response to cellular signals while preventing the persistent expression of gene targets in normal cells.
It is now well-appreciated that a high percentage of cancers develop mechanisms to increase MYC activity in order to globally increase cell survival, proliferation, and invasiveness [
9]. In disease states, studies have shown that aberrant MYC expression results in promoter invasion, with MYC binding to both high- and low-affinity consensus sequences, altering the expression of a large number of target genes [
41,
42]. Consistent with these results, amplified or high
Myc expression can drive tumorigenesis in multiple mouse models and
MYC amplification is observed to various degrees in almost every human cancer type [
11,
43]. Although amplification or overexpression of MYC commonly occurs in cancers, this is not the only mechanism by which MYC is deregulated. In fact, the majority of solid tumors do not display significant
MYC amplification [
44]. We and others find elevated levels of pS62-MYC and lower levels of pT58-MYC, consistent with a more active and stable form of MYC, in a large percentage of tested human tumors [
33,
45‐
50]. Moreover, mutation of the Threonine 58 (T58) residue (Myc
T58A) results in constitutive S62 phosphorylation and increased tumorigenic potential compared to wild-type MYC [
48,
51]. These studies suggest that the post-translational regulation of MYC in cancer may be wildly underestimated and play a significant role in tumor phenotypes. Importantly, in mouse models, low-level constitutive expression of
Myc alone does not induce transformation, but rather exacerbates tumorigenic phenotypes when combined with oncogenes such as HER2 and mutant KRAS that can enhance S62 phosphorylation [
51,
52]. Conversely, the genetic loss of
Myc can prolong survival in aggressive KRAS-driven tumors, highlighting the contribution of endogenous MYC activity to oncogenic signaling pathways and supporting the rationale for therapeutic inhibition of MYC in a large number of cancers [
52‐
55].
Given that MYC has been implicated in global gene regulation, one would predict that MYC suppression would result in large toxicities, with decreased proliferation and survival in normal cells. Surprisingly, the genetic inhibition of MYC in mice, through switchable transgenes or expression of a dominant negative form called OmoMYC, has resulted in dramatic losses of tumor phenotypes in lung adenocarcinomas, glioblastomas, skin papillomatosis, and pancreatic tumors with little to no toxicities [
12,
54,
56‐
59]. OmoMYC is a mutated bHLHZ dimerization domain that is able to form OmoMYC homodimers that bind to DNA and compete with endogenous MYC:MAX complexes, reducing MYC promoter occupancy and effectively suppressing transcription. A recent study from Jung et al. [
59] demonstrated that under physiologic levels of MYC, expression of recombinant OmoMYC protein minimally suppresses MYC at high-affinity binding sites. In contrast, the oncogenic, low-affinity MYC binding sites are acutely responsive to OmoMYC expression [
59]. This study suggests that therapeutic targeting of oncogenic-specific MYC functions may be possible and highlight the importance of understanding the contribution of MYC signaling to oncogenic phenotypes.