Abstract PO3-15-10: Enhanced Cancer Cell Proliferation and Aggressive Phenotype Counterbalance in Breast Cancer with High BRCA1 Gene Expression

Abstract Introduction: It is estimated that 5–10% of all breast cancer cases have a hereditary component with BRCA1 mutation being one of the most frequently observed gene mutations in breast cancer patients. BRCA1 plays a crucial role in DNA repair, and its mutation has been extensively studied. However, the clinical significance of BRCA1 gene expression remains largely unexplored. Given that heightened DNA repair mechanisms can potentially increase cancer cell proliferation, we hypothesized that breast cancer with high BRCA1 gene expression might be associated with aggressive tumor biology and worse survival. Methods: The clinical, pathological and gene expression levels of a total of 6,245 breast cancer patients were analyzed from three large independent cohorts: METABRIC (n = 1,903), GSE96058 (n = 3,273), and The Cancer Genome Atlas (TCGA, n = 1,069). The data was retrieved through cBioPortal. The xCell algorithm was used to correlate the BRCA1 gene expression with the stromal and immune cell fractions infiltrating the tumor microenvironment (TME). Results: BRCA1 gene expression was higher in breast cancer patients without BRCA1 mutation, and it correlated with the DNA repair activity (p = 0.026 and p 0.01 respectively), however, it did not correlate with BRCA2 gene expression (p = 0.38). Breast cancer with high BRCA1 gene expression was associated with cancer cell proliferation as evidenced by higher Nottingham histological grade (p 0.001), Ki67 expression (p 0.001), significant enrichment of cell proliferation-related gene sets such as E2F Targets, G2M Checkpoint, MYC Targets V1 and V2, Mitotic Spindle (all FDR 0.25), and significantly less infiltration of stromal cells such as adipocytes, fibroblasts, microvascular and lymphatic endothelial cells, and pericytes (all p 0.05). High BRCA1 gene expression was associated with higher homologous recombination deficiency, intratumor genomic heterogeneity, and fraction altered (all p 0.001); however, there was no correlation observed with mutation rates or neoantigens. It was observed that breast cancer with high BRCA1 gene expression had less infiltration of CD8 T cells, dendritic cells, regulatory T cells, and B cells and had higher infiltration of Th1 cells (all p 0.05). To our surprise, there was no significant difference in overall survival regardless of the subtypes (except for ER+/HER2- subtype in METABRIC cohort, p = 0.03). Breast cancer with low BRCA1 gene expression enriched cancer aggravating pathway gene sets such as Cancer Stem Cell-related signaling pathways (NOTCH and HEDGEHOG), Angiogenesis, Epithelial Mesenchymal Transition, Inflammatory Response, and TGF-beta signaling (all FDR 0.25). Conclusion: Contrary to our initial hypothesis, we did not find any significant association between BRCA1 gene expression and survival outcomes in breast cancer patients. Based on the results of our study, we speculate that the interplay between enhanced cancer cell proliferation and aggressive tumor characteristics may counterbalance the effect of high BRCA1 expression in breast cancer. Citation Format: Kohei Chida, Masanori Oshi, Arya Mariam Roy, Itaru Endo, Kazuaki Takabe. Enhanced Cancer Cell Proliferation and Aggressive Phenotype Counterbalance in Breast Cancer with High BRCA1 Gene Expression abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-15-10.