Pl02.1.a delineating and targeting a novel metabolism-based post-translational mechanism regulating the abundance of the ‘undruggable’ oncoprotein c-myc in medulloblastoma

Abstract BACKGROUND Brain tumors are the leading cause of cancer death in children, and medulloblastoma (MB) is one of the most common pediatric central nervous system malignancies. Amplification of the c-MYC oncogene is frequently observed in the most aggressive and lethal subgroup of this disease, group 3 (G3), but not in other subgroups. Patients that have G3 MB tumors with high c-MYC abundance are more likely to present as metastatic and are prone to develop fatal recurrent tumors. Unfortunately, the functional ubiquity and disordered structure of c-MYC makes it difficult to target for cancer treatment. Therefore, it is critical to identify novel, out-of-the-box strategies to suppress oncogenic c-MYC in highly aggressive G3 MB brain tumors. Recently, metabolism has emerged as a major regulator of overall cellular signaling processes through post-translational and epigenetic mechanisms. While c-MYC is known to regulate cellular metabolism, whether metabolism plays a role in reciprocally supporting enhanced c-MYC abundance in cancer is unknown. We hypothesize that an intrinsic feedback mechanism may exist where metabolic activity modulates c-MYC abundance that could be exploited as a therapeutic strategy to improve outcomes for G3 MB patients. MATERIAL AND METHODS Using various well-characterized G3 MB cells, patient-derived G3 MB cells, orthotopic intracerebellar xenograft models, patient tumor bioinformatics, detailed biochemical characterization, and point-mutation analyses, we have identified a novel metabolism-dependent post-translational modification that regulates c-MYC stability in G3 MB. RESULTS In-depth molecular analyses unveiled that c-MYC is susceptible to oxidation and proteasomal degradation under conditions of metabolic stress. Targeting mitochondrial respiration via inhibition of complex-I promotes the accumulation of reactive oxygen species (ROS) and leads to rapid cysteine oxidation and proteasomal degradation of c-MYC in G3 MB cells, but not normal human brain astrocytes or neural stem cells (NSCs). Point mutation analysis combined with biochemical oxidation assays identified the specific cysteine residues of c-MYC that are susceptible to oxidation and ultimately responsible for enhanced c-MYC degradation following complex-I inhibition. Importantly, oral administration of a blood-brain barrier permeable complex-I inhibitor impaired the growth of intracerebellar MB xenograft tumors in mice, significantly prolonging animal survival. CONCLUSION Altogether, these findings unveil a novel mechanism through which metabolism regulates the post-translational stability of c-MYC and provides insights for designing rationale therapeutic strategies for the treatment of MB patients. SUPPORT This work is supported by a project grant from the Canadian Institutes of Health Research (CIHR).