MYC-Targeting PROTACs Lead to Bimodal Degradation and N-Terminal Truncation.

MYC is a master regulatory transcription factor whose sustained dysregulation promotes the initiation and maintenance of numerous cancers. While MYC is a regarded as a potenial therapeutic target in cancer, its intrinsically disordered structure has proven to be a formidable barrier toward the development of highly effective small molecule inhibitors. We rationalized that proteolysis targeting chimeras (PROTACs), which might accomplish the targeted degradation of MYC, would achieve more potent cell killing in MYC-driven cancer cells than reversible inhibitors. PROTACs are bifunctional small molecules designed to produce a ternary complex between a target protein and an E3 ligase leading the target's ubiquitination and degradation by the 26S proteasome. We generated PROTAC MTP3 based on modifications of the previously reported MYC-targeting compound KJ-Pyr-9. We found that MTP3 depletes endogenous full-length MYC proteins and uniquely induces increasing levels of a functional, N-terminally truncated MYC species, tMYC. Furthermore, MTP3 perturbs cellular MYC levels in favor of a tMYC-dominated state whose gene regulatory landscape is not significantly altered compared to that of wild type MYC. Moreover, although it lacks ∼10 kDa of MYC's N-terminal transactivation domain, tMYC is sufficient to maintain an oncogenic proliferative state. Our results highlight the complexities of proximity-inducing compounds against highly regulated and conformationally dynamic protein targets such as MYC and indicate that PROTACs can induce alternative outcomes beyond target protein degradation.