Abstract 5413: Temporal regulation of c-Myc in endocycling cancer cells facilitates mitotic bypass in response to chemotherapy

Abstract Chemotherapy resistance remains a major challenge in clinical oncology. The mechanisms that regulate dynamic movement through adaptive cell states to survive through chemotherapeutic stress need to be understood to advance treatment options for patients and move towards curative therapies. In response to various stressors, cancer cells can reprogram their cell cycle, uncoupling DNA replication from mitotic division to acquire a large polyploid phenotype. This alternative cell cycle of repeated G and S phases is known as an endocycle, observed across the tree of life during both development and stress response. Cells exhibiting this phenotype exist in the transient endocycling cancer cell state. Endocycling cancer cells (ECCs) may serve as an actuator of chemotherapy resistance and disease recurrence. Understanding the mechanisms regulating the mitotic-to-endocycle switch and back are crucial to enhance treatments for patients with chemotherapy resistant cancer. A major driver of endocycling described in the literature is the transcription factor, c-Myc, due to its role in governing the G1/S transition. C-Myc is dysregulated in >70% of cancers, functioning as a critical cell cycle driver through transcriptional regulation of cyclins and CDKs, as well as essential metabolic programs necessary for cycle progression. Despite the published roles of Myc in both cancer and endocycling, we have shown through single cell RNA sequencing and western blotting that ECCs exhibit a low-Myc phenotype across multiple contexts. Myc protein levels can be rescued using the proteasome inhibitor, MG132, demonstrating that the low Myc status is driven by proteasomal degradation. To test if Myc degradation serves a mechanistic function, the small molecule inhibitor, MYCi975, was used to inhibit Myc activity at various points in the mitotic cell cycle. Our data shows that loss of Myc activity in cells beyond the restriction point induces mitotic bypass, generating a 4N G1 population. Notably, cells actively replicating their DNA in S phase show high sensitivity to Myc inhibition after skipping to G1, displaying cytotoxic effects not seen in other phases of the cell cycle. Similarly, when ECCs are challenged with the Myc inhibitor, the treatment promotes mitotic bypass and enriches for G0 and G1 populations, primed to enter the next S phase upon release from Myc inhibition. Together, these data suggest a temporal aspect of Myc regulation that has not been described in the literature. Myc activity beyond the restriction point appears to commit cells to mitotic entry, playing a functional role in coupling DNA replication to mitosis. This data supports our hypothesis that the low-Myc status of endocycling cells plays a mechanistic role in permitting mitotic bypass through premature loss of activity in G2. Preliminary data suggests that rescuing Myc activity will force mitotic entry producing mitotic progeny cells. Citation Format: Michael Loycano, Kenneth J. Pienta, Sarah R. Amend. Temporal regulation of c-Myc in endocycling cancer cells facilitates mitotic bypass in response to chemotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5413.