Abstract 1872: MSH6 modulates PARP inhibitor sensitivity in BRCA-proficient high-grade serous ovarian cancer

Abstract Ovarian cancer is the leading cause of gynecologic cancer-related deaths, with high-grade serous ovarian cancer (HGSOC) being the most common subtype. While BRCA1/2 deficiencies are established biomarkers to predict PARP inhibitor (PARPi) sensitivity, some HGSOC patients with BRCA-proficient tumors also respond to PARPi therapy. However, the mechanisms driving PARPi sensitivity in BRCA-proficient HGSOC remain poorly understood. We hypothesized that PARP1 protein complex composition and PARylation-mediated signaling regulate PARPi sensitivity or resistance in BRCA-proficient HGSOC. Through viability screening of BRCA-proficient ovarian cancer cell lines treated with PARPis we identified three sensitive and three resistant lines. Chemical proteomics with rucaparib showed enrichment of PARP1, PARP2, and associated binding proteins, with notably higher MSH6 levels in sensitive cell lines compared to resistant ones. Co-immunoprecipitation further revealed differences in PARP1-MSH6-PARP2 complex compositions between sensitive and resistant cells lines. Genetic knockout or knockdown of MSH6 via CRISPR and siRNA, respectively, conferred resistance to rucaparib, with a more pronounced effect in sensitive cells. To investigate downstream signaling, we performed ADP-ribosylation proteomics using clickable NAD+ analogs, uncovering distinct PARylation profiles between sensitive and resistant lines. CHAF1A, a known MSH6 binder and PARP1 substrate, was identified in PARPi-sensitive cells. More importantly, MSH6 knockdown elevated CHAF1A levels, while rucaparib treatment reduced CHAF1A expression, particularly in sensitive cells. These findings suggest that CHAF1A upregulation through MSH6 inhibition may rescue PARPi-induced cell death. This study integrates chemical proteomics and ADP-ribosylation analysis to delineate the interplay between PARP1 complex composition and signaling dynamics in PARPi response. Our findings highlight MSH6 and CHAF1A as critical modulators of PARPi sensitivity, providing new insights into resistance mechanisms and identifying potential biomarkers to improve PARPi efficacy in BRCA-proficient HGSOC. Citation Format: Ou Deng, Thales Nepomuceno, Bin Fang, Eric A. Welsh, Rachael H. Martin, Erin M. George, Alvaro N. Monteiro, Uwe Rix. MSH6 modulates PARP inhibitor sensitivity in BRCA-proficient high-grade serous ovarian cancer [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 1872.