Abstract PS3-01-04: Single-cell transcriptomic and phenotypic profiling reveals T cell dysfunction in cancer-free BRCA1 germline mutation carriers

Abstract Women harboring germline mutations in BRCA1 have an approximately 80% lifetime risk of developing breast cancer. Extensive research on the breast epithelium, the cell of origin for BRCA1-related tumors, has shed light on the molecular functions of BRCA1 in tumor suppression. However, all somatic cells in a germline BRCA1 mutation carrier contain the same mutant allele, yet it remains unclear whether BRCA1 heterozygosity in stromal compartments could also contributes to the increased risk of BRCA1-associated cancer. To determine the impact of BRCA1 heterozygosity on T cells at the transcriptional and functional levels, we first investigated a cohort of T cells isolated from healthy individuals who are BRCA1 mutation carriers (BRCA1 mut/+ ) and non-carriers, using single cell RNA sequencing (scRNA-seq) and high-parametric spectral flow cytometry. Single-cell transcriptomic analysis revealed a significant enrichment of a gamma-delta (γδ) T cell subpopulation in BRCA1 mut/+ samples. A similar trend, albeit not statistically significant, was observed for a SOX4high T cell subpopulation. Functionally, BRCA1 mut/+ T cells exhibited a delayed response to CD3/CD28 costimulation and showed an enrichment of naïve cells in the CD4 compartment. In addition, the production of cytokine IL-17 is significantly reduced in BRCA1 mut/+ T cells after 24 hours of activation. To determine a causal effect of T-cell BRCA1 heterozygosity on tumorigenesis, we next deleted one allele Brca1 in mouse T lymphocytes. Implanted mammary tumors grew more robustly in T cell-specific Brca1 -/+ mice than their Brca +/+ counterparts, supporting the notion that Brca1 heterozygosity in mouse T lymphocytes compromises the antitumor immunity. Furthermore, mechanistic studies indicate that BRCA1 regulates T cell-related gene expression in conjunction with other transcriptional coregulators. In summary, our data from clinical samples and preclinical models suggest that the heterozygous BRCA1 mutation status leads to T cell-intrinsic functional alterations and compromised antitumor immunity. We propose that germline BRCA1 mutations exert a “double whammy” effect—contributing to elevated cancer risk through their presence in both the cell of origin and the surrounding stromal compartments. Citation Format: H. Chiang, R. Araya, L. Qi, H. Arestake, S. Martinez, A. Sanchez, I. Kalia, Y. Hu, A. Horvath, R. Li. Single-cell transcriptomic and phenotypic profiling reveals T cell dysfunction in cancer-free BRCA1 germline mutation carriers [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-01-04.