Polymerase iota coordinates adaptation to replication stress in BRCA2-deficient mammary epithelial cells.

BRCA2 deficiency induces chronic replication stress, yet how human mammary epithelial cells adapt before malignant transformation remains poorly understood. Here, we identify polymerase iota (Pol ι) as a context-dependent regulator of replication stress responses in BRCA2-deficient mammary epithelium. Single-cell transcriptomic analysis of preneoplastic breast epithelium from BRCA2 mutation carriers revealed enrichment of POLI-positive hormone-sensing cells, while BRCA2 depletion increased nuclear Pol ι in mammary epithelial models. Functionally, Pol ι promoted post-S-phase DNA synthesis specifically in BRCA2-deficient cells. Unexpectedly, POLI depletion reduced chromosomal instability and replication stress markers, while enhancing replication fork progression under replication stress. POLI depletion was also associated with expansion of a luminal-like epithelial population, linking altered replication stress processing to epithelial cell state remodeling. These findings identify Pol ι as a regulator of replication stress adaptation and provide a framework for understanding early cellular changes preceding BRCA2-associated breast tumorigenesis.