Abstract 3186: MDM2 Knockdown Enhances Phosphorylation of MCM2 during Cell Cycle Progression

Abstract MDM2 is an oncogenic E3 ubiquitin ligase that forms heterodimers with the homolog MDMX (also known as MDM4) and is classically known for targeting wild-type p53 for proteasomal degradation. However, in breast cancer harboring mutant p53, MDM2 and MDMX have oncogenic roles independent of p53. The MDM2-MDMX complex interacts with mutant p53 without actively degrading it. In breast cancer models that co-express high levels of mutant p53, MDM2, and MDMX, we identified a tripartite complex that regulates DNA repair protein recruitment of 53BP2 and MDC1. Additionally, mutant p53 colocalizes with replication-associated Mini Chromosome Maintenance2-7 (MCM2-7) helicase and Poly (ADP-ribose) polymerase 1(PARP1), forming what we refer to as the mutant p53-PARP-MCM axis. We are using isogenic estrogen receptor-positive T47D cells expressing missense mutant p53 L194F either with and without MDM2 or MDMX knockdown to dissect if the helicase component MCM2 is regulated by these protein scaffolds. We employed western blotting, immunofluorescence, and proximity ligation assays (PLA) to assess their influences on of MDM family member protein expression on MCM2 phenotype. Preliminary result showed that MDM2 knockdown correlated with increased phosphorylation of MCM2 at serine 108, a marker of replication stress, indicating that the presence of MDM2 mitigates replication stress. Using Proximity Ligation Assay, we observed that MDM2 and MCM2 were in close nuclear proximity. However, co-immunoprecipitation did not detect a direct MDM2-MCM2 protein-protein interaction. Future experiments will address whether the directional interaction between MDM2 and mutant p53 facilitates the role of MDM2 in mitigating replication stress signaling. These evaluations may allow for the mutant p53 and MDM2 overexpression biomarkers to serve as a targetable scaffolding hub for drug targeting. Citation Format: Nikita Meghani, Viola Ellison, Jill Bargonetti. MDM2 Knockdown Enhances Phosphorylation of MCM2 during Cell Cycle Progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3186.