The Transcriptomic Profile Underlying Somatic Monoallelic <i>BRCA1</i> Inactivation: A Biomarker for Breast Cancer Prognosis.

<b>Background and Objectives</b>: Most of the research on the role of the <i>BRCA1</i> gene in breast cancer is focused on monoallelic germline alterations and loss of heterozygosity in tumors. The aim of this study was to identify the characteristic transcriptomic pattern of monoallelic somatic <i>BRCA1</i> inactivation and estimate its correlation with event-free breast cancer survival. <b>Materials and Methods</b>: We conducted global transcriptome sequencing of breast cancer tissue samples to identify differentially expressed genes and signaling pathways associated with monoallelic somatic <i>BRCA1</i> inactivation. The study group involved 36 patient samples categorized based on <i>BRCA1</i> inactivation status. Subsequently, the differential gene expression and Kaplan-Meier analyses in the groups with and without monoallelic somatic <i>BRCA1</i> inactivation were performed. <b>Results</b>: Kaplan-Meier analysis showed a tendency for longer event-free survival in patients with monoallelic somatic <i>BRCA1</i> inactivation, suggesting somatic <i>BRCA1</i> inactivation to be a favorable prognostic. Differential gene expression analysis followed by the STRING tool enrichment analysis showed significant enrichment of proteins in the extracellular region and extracellular space. <b>Conclusions</b>: In this study, we identified transcriptomic profiles of differentially expressed genes <i>TPSD1</i>, <i>FABP4</i>, <i>CARTPT</i>, and <i>MMP9</i> as indicative of homologous recombination-impaired tumors with a tendency for better therapy results.