Secondary mutations as a mechanism of cisplatin resistance in BRCA2-mutated cancers

Abstract

The platinum chemotherapeutics such as cisplatin and carboplatin are in clinical use in patients with BRCA2-mutated ovarian cancer. The initial response is generally good but most ovarian carcinomas ultimately become resistant to therapy. Two papers in this issue have identified a possible cause of this resistance as further mutation of the BRCA2 gene. Mutations in BRCA2 are associated with familial breast and ovarian cancer. Loss of BRCA2 function impairs DNA repair by homologous recombination and renders cells particular sensitive to cisplatin and also to PARP (poly (ADP-ribose) polymerase) inhibitors. The secondary 'resistance' mutations act by restoring the wild-type BRCA2 reading frame. Ovarian carcinomas with mutations in the tumour suppressor BRCA2 are particularly sensitive to platinum compounds1. However, such carcinomas ultimately develop cisplatin resistance. The mechanism of that resistance is largely unknown2. Here we show that acquired resistance to cisplatin can be mediated by secondary intragenic mutations in BRCA2 that restore the wild-type BRCA2 reading frame. First, in a cisplatin-resistant BRCA2-mutated breast-cancer cell line, HCC1428, a secondary genetic change in BRCA2 rescued BRCA2 function. Second, cisplatin selection of a BRCA2-mutated pancreatic cancer cell line, Capan-1 (refs 3, 4), led to five different secondary mutations that restored the wild-type BRCA2 reading frame. All clones with secondary mutations were resistant both to cisplatin and to a poly(ADP-ribose) polymerase (PARP) inhibitor (AG14361). Finally, we evaluated recurrent cancers from patients whose primary BRCA2-mutated ovarian carcinomas were treated with cisplatin. The recurrent tumour that acquired cisplatin resistance had undergone reversion of its BRCA2 mutation. Our results suggest that secondary mutations that restore the wild-type BRCA2 reading frame may be a major clinical mediator of acquired resistance to platinum-based chemotherapy.