A transcriptional response to replication stress selectively expands a subset ofBRCA2-mutant mammary epithelial cells

BRCA2 mutation carriers preferentially develop luminal-like breast cancers, but it remains unclear how BRCA2 mutations affect mammary epithelial subpopulations. Here, we report that Brca2 mut/WT mammary organoids subjected to replication stress activated a transcriptional response that selectively expands Brca2 mut/WT luminal cells lacking hormone receptor expression (HR-). While CyTOF analyses revealed comparable epithelial compositions among wildtype and Brca2 mut/WT mammary glands, Brca2 mut/WT HR- luminal cells exhibited greater organoid formation and preferentially survived and expanded under replication stress. ScRNA-seq analysis corroborated the expansion of HR- luminal cells which express elevated levels of Tetraspanin-8 ( Tspan8 ) and Thrsp mRNA, and pathways implicated in replication stress survival including Type I interferon responses. Notably, CRISPR/Cas9-mediated deletion of Tspan8 or Thrsp prevented Brca2 mut/WT HR- luminal cell expansion. Our findings indicate that Brca2 mut/WT cells have an activate a transcriptional response after replication stress that preferentially favours outgrowth of HR- luminal cells through the expression of interferon-responsive and mammary alveolar genes.