Abstract A072: BRCA1 depletion enhances progesterone receptor-driven cytoskeletal remodeling in ovarian cancer

Abstract Pathogenic mutations in the DNA repair genes, BRCA1/2, are responsible for approximately 11-15% of ovarian cancers (OC). Notably, BRCA1/2 mutations significantly increase the risk of developing an aggressive OC subtype known as high-grade serous ovarian cancer (HGSOC). The role of progesterone (P4) in the initiation and progression of OC subtypes remains poorly understood. Research suggests a potential interplay between Progesterone Receptor (PR) signaling and BRCA1/2 actions. BRCA1 regulates PR expression and transcriptional activity, while P4 can drive estrogen receptor/PR complexes to BRCA1 DNA binding motifs. Normal fallopian tube tissues from healthy BRCA1/2 carriers (the cells of origin for HGSOC) exhibit gene signatures similar to HGSOC only during the luteal phase of the menstrual cycle, when circulating P4 levels are elevated. This suggests that PR/BRCA complexes may modulate the initiation and progression of OC. To explore this PR/BRCA crosstalk, CRISPR-mediated BRCA1 knockdown (KD) was performed in ES-2 OC cell models that express the PR-B isoform. We hypothesized that depletion of BRCA1 would modulate PR-driven signaling and reveal novel regulatory mechanisms. Parental and KD cell pools were treated with vehicle or 10 nM R5020 (a synthetic progestin) for 6 hr, followed by RNA collection for analysis of R5020-induced gene expression using RNA sequencing. A high-confidence list of BRCA1-dependent, R5020-induced genes was generated for pathway analysis, which revealed that migration and cytoskeletal organization were the predominant PR-regulated processes. Significant genes of interest included ROCK2 and PIP5K1C, both of which are kinases and regulators of the cytoskeletal organization. Transwell migration assays showed that R5020 treatment promotes migration, which is further enhanced in the context of BRCA1 depletion. Co-treatment with a pan-ROCK1/2 inhibitor Y-27632 resulted in an increase in R5020-induced migration, which was further enhanced with BRCA1 KD. In contrast, inhibition of PIP5K1C using UNC3230 blocked R5020-induced migration in one of the BRCA1-depleted cell pools. Treatment with pan-Rho inhibitor CT04 led to a slight reduction in R5020-induced migration in the context of BRCA1 KD. Finally, the Rac1 inhibitor EHT-1864 suppressed R5020-induced migration. Analysis of known BRCA1 and PR DNA-binding sites revealed several binding motifs within genes related to cytoskeleton function, like ARHGEF37 and COL16A1. Additionally, some BRCA1 and/or PR binding sites also contain motifs for p130, a component of the DREAM complex, which regulates cell cycle genes. This suggests that a novel complex of BRCA1, PR, and DREAM proteins may exist which drives cytoskeleton rearrangement towards a more pro-metastatic state. Ongoing co-immunoprecipitation (IP) and chromatin-IP assays should identify these novel complexes and their target DNA binding sites. Future goals include validation in additional models of BRCA1/2 loss and biological assays, such as collagen invasion. Citation Format: Jocelyn Baquier, Noelle E. Gillis, Caroline H. Diep, Laura J. Mauro, Carol A. Lange. BRCA1 depletion enhances progesterone receptor-driven cytoskeletal remodeling in ovarian cancer [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 A072.