Inhibiting MYC has long been considered unfeasible, although its key role in human cancers makes it a desirable target for therapeutic intervention. One reason for its perceived undruggability was the fear of catastrophic side effects in normal tissues. However, we previously designed a dominant-negative form of MYC called Omomyc and used its conditional transgenic expression to inhibit MYC function both in vitro and in vivo. MYC inhibition by Omomyc exerted a potent therapeutic impact in various mouse models of cancer, causing only mild, well-tolerated, and reversible side effects. Nevertheless, Omomyc has been so far considered only a proof of principle. In contrast with that preconceived notion, here, we show that the purified Omomyc mini-protein itself spontaneously penetrates into cancer cells and effectively interferes with MYC transcriptional activity therein. Efficacy of the Omomyc mini-protein in various experimental models of non–small cell lung cancer harboring different oncogenic mutation profiles establishes its therapeutic potential after both direct tissue delivery and systemic administration, providing evidence that the Omomyc mini-protein is an effective MYC inhibitor worthy of clinical development.

It is classically recognized that the physiological and oncogenic functions of Myc proteins depend on specific DNA binding enabled by the dimerization of its C-terminal basic-region-Helix-Loop-Helix-Leucine Zipper (b-HLH-LZ) domain with that of Max. However, a new paradigm is emerging, where the binding of the c-Myc/Max heterodimer to non-specific sequences in enhancers and promoters drives the transcription of genes involved in diverse oncogenic programs. Importantly, Max can form a stable homodimer even in the presence of c-Myc and bind DNA (specific and non-specific) with comparable affinity to the c-Myc/Max heterodimer. Intriguingly, alterations in the Max gene by germline and somatic mutations or changes in the gene product by alternative splicing (e.g. ΔMax) were recently associated with pheochromocytoma and glioblastoma, respectively. This has led to the proposition that Max is, by itself, a tumor suppressor. However, the actual mechanism through which it exerts such an activity remains to be elucidated. Here, we show that contrary to the WT motif, the b-HLH-LZ of ΔMax does not homodimerize in the absence of DNA. In addition, although ΔMax can still bind the E-box sequence as a homodimer, it cannot bind non-specific DNA in that form, while it can heterodimerize with c-Myc and bind E-box and non-specific DNA as a heterodimer with high affinity. Taken together, our results suggest that the WT Max homodimer is important for attenuating the binding of c-Myc to specific and non-specific DNA, whereas ΔMax is unable to do so. Conversely, the splicing of Max into ΔMax could provoke an increase in overall chromatin bound c-Myc. According to the new emerging paradigm, the splicing event and the stark reduction in homodimer stability and DNA binding should promote tumorigenesis impairing the tumor suppressor activity of the WT homodimer of Max.

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