Abstract B011: BRCA2 haploinsufficiency induces P53 degradation in primary mammary epithelial cells – a crucial early event in breast tumor promotion

Abstract

Abstract The presence of heterozygous germline BRCA2 mutations is associated with an increased risk of breast and ovarian cancer. In order to design effective preventative strategies, it is critical to understand early tumor promoting events that the mammary epithelial cells undergo on their way to tumorigenesis. To investigate the impact of BRCA2 heterozygosity on tumor formation, we utilized primary BRCA2mut/+ mammary epithelial cells obtained from prophylactic breast tissue of women with BRCA2 mutations. Our findings indicate that BRCA2mut/+ cells display haploinsufficiency for repairing stalled replication forks and resolving RPA32 from these sites, indicative of impaired stalled fork repair. Moreover, they exhibit defects in RAD51 recruitment to double strand break sites indicating a defective homologous recombination dependent double strand break repair efficiency. BRCA2mut/+ cells also show increased sensitivity to both replication stress and double strand break-inducing agents. Interestingly, we find that BRCA2 heterozygous cells downregulate P53 protein levels when compared to BRCA2 wild type cells, and that this downregulation is MDM2-dependent. The decreased P53 levels appear to promote the survival of BRCA2 heterozygous cells under endogenous replication stress conditions. Notably, treatment with nutlin, a P53 protein stabilizer, leads to cell death in BRCA2 heterozygous cells but not in BRCA2 wildtype cells, highlighting the sensitivity of BRCA2 heterozygous cells to endogenous P53 protein levels. Moreover, complete loss of P53 in BRCA2 heterozygous cells results in rescue of sensitivity of these cells to both replication stress and double strand break inducing agents. Furthermore, whole genome sequencing analysis of BRCA2 heterozygous cells deleted for P53 reveal DNA mutational signatures similar to those observed in BRCA2 mutant tumors, implying that loss or downregulation of P53 is an early event driving tumorigenesis in the context of BRCA2 heterozygosity. In summary, our study uncovers the significance of BRCA2 haploinsufficiency in replication stress suppression and highlights the role of P53 loss in promoting BRCA2 heterozygosity-driven cancer. These findings contribute to a better understanding of the molecular mechanisms underlying BRCA2-associated tumorigenesis and may have implications for targeted preventative therapies in the future. Citation Format: Haohui Duan, Margaret Gillis, Deboral Dillon, Judy Garber, Shailja Pathania. BRCA2 haploinsufficiency induces P53 degradation in primary mammary epithelial cells – a crucial early event in breast tumor promotion [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr B011.