R-loop functions in <i>Brca1</i>-associated mammary tumorigenesis.

Abstract

Deleterious accumulation of R-loops, a DNA-RNA hybrid structure, contributes to genome instability. They are associated with <i>BRCA1</i> mutation-related breast cancer, an estrogen receptor α negative (ERα^-) tumor type originating from luminal progenitor cells. However, a presumed causality of R-loops in tumorigenesis has not been established in vivo. Here, we overexpress mouse <i>Rnaseh1</i> (Rh1-OE) in vivo to remove accumulated R-loops in <i>Brca1</i>-deficient mouse mammary epithelium (BKO). R-loop removal exacerbates DNA replication stress in proliferating BKO mammary epithelial cells, with little effect on homology-directed repair of double-strand breaks following ionizing radiation. Compared to their BKO counterparts, BKO-Rh1-OE mammary glands contain fewer luminal progenitor cells but more mature luminal cells. Despite a similar incidence of spontaneous mammary tumors in BKO and BKO-Rh1-OE mice, a significant percentage of BKO-Rh1-OE tumors express ERα and progesterone receptor. Our results suggest that rather than directly elevating the overall tumor incidence, R-loops influence the mammary tumor subtype by shaping the cell of origin for <i>Brca1</i> tumors.