A cancer persistent DNA repair circuit driven by MDM2, MDM4 (MDMX), and mutant p53 for recruitment of MDC1 and 53BP1 on chromatin.

DNA damage signaling requires functional interactions between 53BP1 and the wild-type p53 tumor suppressor. Cancer cells often express elevated levels of transcriptionally inactive mutant p53 (mtp53) that maintains MDM2 and MDMX (MDM4) binding partners. The ability of mtp53 to functionally interact with additional proteins in the context of a dynamic equilibrium with MDM2-MDMX heterodimers has not been described. Employing a stable isotope labeling in cell culture analysis in T47D breast cancer cells (expressing mtp53 L194F), we uncovered several chromatin-associated DNA replication and repair factors as MDM2-regulated phosphoproteins, including 53BP1. We used proximity ligation analysis in multiple breast cancer cell lines to confirm 53BP1-MDM2 complex formation. We demonstrated that a mtp53-MDM2/MDMX complex promoted 53BP1-MDC1 interactions by showing that mtp53-MDM2/MDMX complex disruptors, Nutlin 3a and ALRN-6924, reduced the 53BP1-MDC1 nuclear interactions (especially in S-phase). Surprisingly, these MDM2-driven MDC1-53BP1 interactions were not ATM dependent, suggesting distinct 53BP1-MDC1 complexes in response to genotoxic stress. We found that MDM2-deficient cells have increased poly-ADP-ribosylation on chromatin which supports the possibility that a mtp53-MDM2/MDMX pathway promotes aberrant DNA repair. Taken together, our data suggest that a mtp53-MDM2/MDMX complex orchestrates DNA repair machinery activity on chromatin, thus priming cancer cells for persistent DNA damage repair (CPR).