Abstract 3019: Dissecting in vivo functions of R-Loops in Brca1-associated DNA replication stress and mammary tumorigenesis

Abstract Germ-line BRCA1 mutations are associated with a significantly increased incidence of breast cancer in women. Although BRCA1-associated breast tumors are basal-like and estrogen receptor α negative (ERα−), they originate from luminal progenitor cells. At the molecular level, BRCA1 is best known for its roles in resolution of DNA replication stress and promotion of homologous recombination (HR) -based double strand break (DSB) repair. In addition, BRCA1 is implicated in reduction of R-loops, a DNA-RNA hybrid structure and transcriptional byproduct associated with genome instability and transcriptional regulation. Our published findings suggest that BRCA1-dependent R-loop mitigation contributes to luminal cell-specific transcription and differentiation (Zhang et al, 2017, Nature Communications, 8: 15908), which could in turn suppress Brca1-associated tumorigenesis. Using cell type-specific mouse genetics, we investigate a direct impact of R-loop removal on Brca1-associated mammary epithelial tumor formation. R-loop formation in mammary epithelial cells is efficiently reduced by over-expression of RNase H1, an R-loop-specific ribonuclease. R-loop removal does not affect mammary duct development or function. However, it drastically increases ɣH2AX foci in proliferating BRCA1-deficient epithelial cells without external DNA damage. This suggests a role of R-loops in attenuating elevated DNA replication stress caused by the loss of Brca1. While R-loop removal does not appear to affect the overall Brca1-associated mammary tumor incidence, it promotes cell differentiation from ERα− luminal progenitor cells to ERα+ mature luminal cells. Accordingly, unlike their counterparts, a significant percentage of Brca1-associated mammary tumors with RNase H1 overexpression tend to be ERα+. Thus, our findings provide in vivo evidence for a previously unappreciated role of R-loop dynamics on Brca1-associated DNA replication stress and tumor development in mammary epithelium. Citation Format: Huai-Chin Chiang, Leilei Qi, Yanfen Hu, Rong Li. Dissecting in vivo functions of R-Loops in Brca1-associated DNA replication stress and mammary tumorigenesis abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 3019.