Germline <i>BRCA1</i> Mutation Deregulates Irradiation–Induced Cell Cycle Checkpoints in Human Mammary Luminal Progenitors

<jats:title>Abstract</jats:title> <jats:p> Mammary ductal tree comprises two epithelial lineages, basal and luminal, which harbour stem and progenitor cell subsets essential to breast physiology and are cancer-initiating precursors. Dysregulated cellular stress response is an early event in tumorigenesis, yet lineage-rooted study of primary breast epithelium pertinent to familial mutation carriers is limited. Here, we establish a lineage-resolved monolayer culture system that enables propagation of primary human mammary epithelial populations while preserving lineage identity across passages. We identify intrinsically distinct cell cycle response of luminal progenitors and basal cells to irradiation stress where the former engages both G1/S and G2/M checkpoints to achieve cell cycle arrest and the latter relies on the G2/M checkpoint. These lineage distinctions are diminished in germline <jats:italic>BRCA1</jats:italic> mutation carriers with an attenuated G1/S checkpoint in luminal progenitors. Paired transcriptomic-proteomic profiling of acute irradiation response uncovers <jats:italic>BRCA1+/−</jats:italic> luminal progenitor population with sustained AKT-mTOR signaling and compromised cell cycle arrest marking an early deviation in the stress response of these purported cells-of-origin of aggressive breast cancers known to arise in <jats:italic>BRCA1</jats:italic> germline mutation carriers. Our study provides a functional framework for determining critical events in the expansion of genomically altered mammary epithelial cells in the high-risk breast to enable future preventive interventions. </jats:p>