R-loop functions in Brca1-associated mammary tumorigenesis

Area of Science:

• Molecular Biology
• Genetics
• Cancer Research

Background:

• R-loops (DNA-RNA hybrids) are implicated in genome instability.
• They are associated with BRCA1-mutation-related estrogen receptor-negative breast cancer originating from luminal progenitor cells.
• The in vivo causality of R-loops in tumorigenesis remains unclear.

Purpose of the Study:

• To investigate the in vivo role of R-loops in BRCA1-deficient mammary tumorigenesis.
• To determine how R-loop removal affects DNA replication stress and repair.
• To analyze the impact of R-loop modulation on mammary tumor subtypes and cell of origin.

Main Methods:

• Overexpression of mouse Rnaseh1 (Rh1-OE) to reduce R-loops in Brca1-deficient (BKO) mouse mammary epithelium.
• Assessment of DNA replication stress and homology-directed repair.
• Analysis of mammary gland cell populations (luminal progenitor vs. mature luminal cells).
• Monitoring of spontaneous mammary tumor incidence and subtype (ERα and progesterone receptor expression).

Main Results:

• R-loop removal in BKO cells exacerbated DNA replication stress.
• Homology-directed repair of double-strand breaks was largely unaffected by R-loop reduction.
• BKO-Rh1-OE mammary glands showed fewer luminal progenitor cells and more mature luminal cells compared to BKO.
• Mammary tumor incidence was similar between BKO and BKO-Rh1-OE, but a significant portion of BKO-Rh1-OE tumors expressed ERα and progesterone receptor.

Conclusions:

• R-loops do not directly increase overall mammary tumor incidence in a BRCA1-deficient context.
• R-loop accumulation influences the cell of origin for BRCA1-related mammary tumors.
• Modulating R-loops can alter the subtype of BRCA1-deficient breast cancer, shifting towards ERα-positive phenotypes.