Diverse BRCA1 and BRCA2 Reversion Mutations in Circulating Tumor DNA of Therapy-Resistant Breast or Ovarian Cancer

Purpose: Resistance to platinum-based chemotherapy or PARP inhibition in germline BRCA1 or BRCA2 mutation carriers may occur through somatic BRCA1 or BRCA2 reversion mutations or intragenic deletions that restore BRCA1 or BRCA2 function. We assessed whether BRCA1/2 reversion mutations can be identified in circulating tumor DNA (ctDNA) of patients with ovarian or breast cancer previously treated with platinum or PARP inhibitors.Experimental Design: ctDNA from 24 BRCA1- or BRCA2 germline mutant patients, including 19 prospectively accrued platinum-resistant/refractory ovarian cancer and five prospectively accrued platinum and PARP inhibitor pre-treated metastatic breast cancer patients, was subjected to massively parallel sequencing targeting all exons and introns of BRCA1 and BRCA2, and 141 additional genes. Functional studies were performed to assess the impact of the putative BRCA1/2 reversion mutations on BRCA1/2 function.Results: Diverse and often polyclonal BRCA1 or BRCA2 reversion mutations were identified in ctDNA from four patients with ovarian cancer (21%) and from two patients with breast cancer (40%) previously treated with platinum or PARP inhibitors. BRCA2 reversion mutations were detected prior to PARP inhibitor treatment in a breast cancer patient who did not respond to treatment, with reversion mutations enriched in serial plasma samples after PARP inhibitor treatment. Foci formation and immunoprecipitation assays revealed that a subset of the reversion mutations restored BRCA1/2 function.Conclusions: BRCA1/2 reversion mutations can be detected by ctDNA sequencing analysis. Detection of reversion mutations prior to PARP inhibitor therapy suggests that ctDNA screening may aid the selection of patients likely to benefit from PARP inhibition.