Genomic alterations enable BRCA1 methylation loss and promoter bypass to drive resistance in high-grade serous ovarian cancer

Focal, allele-specific loss of BRCA1 promoter methylation driven by local cis-acting genomic alterations, and restoration of BRCA1 expression through structural variant-mediated promoter bypass or alternative transcription initiation.

BRCA1 promoter methylation predicts sensitivity to PARP inhibitors in high-grade serous ovarian cancer, yet acquired resistance remains mechanistically unresolved. The authors applied long-read direct DNA sequencing to patient-derived xenografts and cell lines, resolving BRCA1 methylation at single-molecule resolution alongside structural and transcriptomic analyses. Two convergent PARP inhibitor and platinum resistance mechanisms were identified and validated in patient tumours: first, focal allele-specific methylation loss arising from local cis-acting genomic alterations rather than global epigenetic reprogramming, with engineered in cis sequence alterations near the methylated promoter sufficient to induce methylation loss, restore homologous recombination and confer resistance; second, restoration of BRCA1 expression despite intact promoter methylation via structural variant-mediated promoter bypass or alternative transcription initiation. The authors conclude that BRCA1 methylation loss is a locus-restricted process rather than a global epigenetic event.

Synthesis written by Geno'X. For the full original abstract, please refer to the source publication.

The mechanism described is essentially somatic and epigenetic: it does not concern germline predisposition and changes neither genetic counselling nor cascade screening. Its relevance to the cancer geneticist is indirect but real, since BRCA1 methylation is the main competitor of germline variants as a homologous recombination deficiency biomarker, and this work shows that measuring it globally risks misclassifying tumours with focal, allele-specific methylation loss. The data come from xenografts and cell lines with only confirmatory work in tumours, in a non-peer-reviewed preprint: a lead for methylation assay design, not a practice-changing result.

Analysis by Dr Thibaut Benquey