Abstract A037: POLQ inhibitors induce replication stress and apoptosis in high-grade serous ovarian cancer: Functional and genomic biomarkers of response

Abstract Background: Polymerase theta (POLQ) is essential for theta-mediated end joining (TMEJ), an error-prone DNA repair pathway that is hyperactive in homologous recombination-deficient (HRD) cancers such as high-grade serous ovarian cancer (HGSOC). Inhibiting POLQ is a promising therapeutic strategy, but the molecular determinants of sensitivity and downstream mechanisms remain incompletely defined. Methods: We assessed the growth-inhibitory activity of two structurally distinct POLQ inhibitors, novobiocin and ART558, across 18 well-characterized HGSOC cell lines. Genomic profiling data were integrated with drug response values (GR50s) to identify predictive biomarkers, including mutational status, HRD scores, and copy number signatures (CX1–CX17). Mechanistic assays included POLQ ATPase activity, cell cycle analysis (FUCCI), immunofluorescence for γH2AX and POLQ, apoptosis markers and DNA fiber assays. Results: POLQ inhibitor sensitivity varied independently from olaparib or carboplatin response, indicating a distinct mode of action. Novobiocin sensitivity significantly correlated with CX3 and CX5 copy number signatures, which reflect impaired homologous recombination and replication stress. In contrast, resistant lines were enriched for CX14 and CX6, signatures linked to mitotic errors and telomere shortening. Sensitive cell lines showed increased DNA damage (γH2AX foci), POLQ accumulation on chromatin, and apoptosis, particularly in BRCA1/2-mutant backgrounds. Cell cycle profiling revealed impaired G1/S progression and S-phase depletion. DNA fiber assays demonstrated replication fork degradation upon POLQ inhibition, consistent with POLQ’s role in fork protection. Conclusions: We identify genomic features, specifically CX3 and CX5 copy number signatures as candidate biomarkers of POLQ inhibitor sensitivity in HGSOC. Mechanistically, POLQ inhibition induces replication stress, persistent DNA damage, and apoptosis in HR-deficient settings. These findings support the clinical development of POLQ inhibitors and rational patient stratification based on genome instability signatures. Citation Format: Aleksandra Kruglikov. POLQ inhibitors induce replication stress and apoptosis in high-grade serous ovarian cancer: Functional and genomic biomarkers of response [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Ovarian Cancer Research; 2025 Sep 19-21; Denver, CO. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl):Abstract nr A037.