Healthy BRCA1/2 mutation carriers exhibit a pre-CAF signature and altered epithelial marker expression in breast tissue.

Germline mutations in BRCA-1 and BRCA-2 significantly increase breast cancer risk. However, the mechanisms driving early tumorigenesis remain incompletely understood. While previous studies have primarily focused on luminal progenitors as tumor-initiating cells in BRCA-1 mut/+and BRCA-2 mut/+carriers, the impact of these mutations on the stromal microenvironment and their role in cancer initiation has notbeen thoroughly explored. We analyzed preneoplastic breast tissues from healthy BRCA-1 mut/+and BRCA-2 mut/+carriers, focusing on expression patterns and alterations of Podoplanin (PDPN), CD10, and programmed cell death ligand 2 (PD-L2) across both epithelial and stromal compartments. Our findings reveal a high prevalence of pre-CAF (PDPNhigh/CD10+) cells in the stroma of BRCA-1mut/+carriers and, for the first time, also in BRCA-2mut/+carriers. These pre-CAFs exhibited overexpression of PDPN, CD10, and PD-L2, supporting their role in early stromal remodeling. Additionally, we identified altered PD-L2 expression across stromal, basal, and luminal progenitor compartments, with distinct localization patterns in BRCA-1mut/+and BRCA-2mut/+ tissues. Our results suggest a potential mechanism through which BRCA1/2 mutations contribute to tumor initiation through early reprogramming of both stromal and epithelial compartments. These findings highlight the pivotal role of the stromal microenvironment in BRCA1/2-associated breast cancer initiation and underscore the importance of simultaneously investigating epithelial and stromal alterations in cancer risk assessment.