Constitutional BRCA1 Methylation is associated with high level of tumoral BRCA1 methylation and homologous recombination deficiency in triple-negative breast cancer

Tumoral BRCA1 promoter methylation occurs frequently in triple-negative breast cancer (TNBC) and contributes to homologous recombination deficiency (HRD). While constitutional BRCA1 methylation has been described, its relationship with tumoral methylation, genomic instability, and prognosis remains unclear. Paired tumor and blood samples from 136 TNBC patients (SCANDARE, NCT03017573) were analyzed for BRCA1 methylation, genomic alterations, HRD and outcomes. Constitutional BRCA1 methylation was detected in 20.6% of patients and tumoral methylation in 31.6%, including 11.5% with somatic-only methylation. In cases with constitutional BRCA1 methylation, tumoral methylation levels increased markedly, with 89% of high-methylation tumors (≥50%) associated with a Loss of Heterozygoty. Tumors with BRCA1 promoter methylation consistently exhibited high HRD (Homologous Recombination Deficiency) scores, comparable to those with pathogenic HRR (Homologous Recombination Repair) gene pathogenic variants (p < 0.001). Conversely, HRD was rare in tumors lacking both BRCA1 methylation and HRR gene alterations. Prognostically, constitutional BRCA1 methylation tended to associate with improved survival, while somatic-only methylation showed a trend toward poorer outcomes (p = 0.06). Constitutional BRCA1 methylation is associated with a high level of tumoral BRCA1 promoter methylation and HRD in TNBC. These findings support integrating constitutional and tumoral BRCA1 methylation into HRD assessment to improve patient stratification and precision treatment in TNBC.