Abstract 804: Trapped PARP1 cytotoxicity is modulated by the ubiquitin-dependentsegregase p97

Abstract Poly-(ADP-ribose) polymerase inhibitors (PARPi) elicit anti-tumor activity in homologous recombination defective cancers by promoting cytotoxic, chromatin-bound, “trapped” PARP1. Here we have elucidated a pathway that process trapped PARP1. By exploiting wild-type or trapping-resistant PARP1 transgenes combined with either a rapid immunoprecipitation mass-spectrometry of endogenous proteins (RIME)-based approach or PARP1 Apex2-proximity labelling linked to mass-spectrometry, we generated proteomic profiles of trapped and non-trapped PARP1 complexes. These experiments demonstrated an increase in the SUMO1/2 modifications upon trapping, as well as an interaction with the ubiquitin-regulated p97 ATPase (aka VCP). Subsequent experiments demonstrated that upon trapping, PARP1 is SUMOylated by the SUMO-ligase PIAS4 and subsequently ubiquitinated by the SUMO-targeted E3-ubiquitin ligase, RNF4, events that promote p97 recruitment and p97 ATPase-mediated removal of trapped-PARP1 from chromatin. Consistently, small molecule p97 complex inhibitors, including a metabolite of the clinically-used drug disulfiram, CuET that acts as a p97 sequestration agent, prolong PARP1 trapping and thus enhance PARPi-induced cytotoxicity in homologous recombination defective tumor cells and patient-derived tumor organoids. Taken together, these results suggest that p97 ATPase plays a key role in the processing of trapped PARP1 from chromatin and the response of homologous recombination defective tumor cells to PARPi. Citation Format: Dragomir B. Krastev, Chris Lord. Trapped PARP1 cytotoxicity is modulated by the ubiquitin-dependentsegregase p97 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 804.