Decoding BRCA1 promoter hypermethylation: a new frontier in understanding sporadic breast cancer.

The role of epimutations in sporadic oncogenesis has been a hot topic of debate and speculation, as hereditary mutations account for merely 5-10% of cancers. One such epimutation, BRCA1 promoter hypermethylation (BPM), is associated with BRCA1 inactivation at variable frequencies in sporadic breast cancers. Unlike hereditary BRCA1 mutations, the role of BPM in tumorigenesis remains obscure. To investigate this, we employed a modified CRISPR approach to induce site-specific methylations in the wild-type BRCA1 promoter, mimicking BPM under clinical conditions. Our research revealed complex pathways influenced by BPM that accelerate tumor formation and progression. We found that BPM downregulates BRCA1 by modulating ER-α expression, thus affecting the balance between BRCA1 isoforms β and α. BPM also regulates the lncRNA NBR2, which shares the BRCA1 promoter region. Remarkably, silencing NBR2 initiates a feedback loop that exacerbates BRCA1 downregulation and supports tumorigenesis. Shortly after BPM induction, tumor proliferation is enhanced by increased β-hCG and ER-α expression, alongside decreased PR levels. However, prolonged BPM maintenance leads to the remodulation of hormone receptors, resulting in a hormone receptor-negative status. Finally, we demonstrate the higher growth potential and invasiveness of BPM tumors through novel hypermethylation-based xenograft models. Comparative proteomic analysis of BPM cells at two different timepoints post-methylation induction identified diagnostic and prognostic markers. Key molecular biomarkers, including NBR2, β-hCG, ER-α, and associated proteins such as HSP90, STAT1, SPEN, and TFF1, have been identified as potential therapeutic targets for BRCA1-defective breast cancers.