Resistance to therapy caused by intragenic deletion in BRCA2

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. Cells with loss of BRCA2 function are defective in homologous recombination (HR) and are highly sensitive to inhibitors of poly(ADP-ribose) polymerase (PARP)1,2, which provides the basis for a new therapeutic approach. Here we show that resistance to PARP inhibition can be acquired by deletion of a mutation in BRCA2. We derived PARP-inhibitor-resistant (PIR) clones from the human CAPAN1 pancreatic cancer cell line, which carries the protein-truncating c.6174delT frameshift mutation. PIR clones could form DNA-damage-induced RAD51 nuclear foci and were able to limit genotoxin-induced genomic instability, both hallmarks of a competent HR pathway. New BRCA2 isoforms were expressed in the resistant lines as a result of intragenic deletion of the c.6174delT mutation and restoration of the open reading frame (ORF). Reconstitution of BRCA2-deficient cells with these revertant BRCA2 alleles rescued PARP inhibitor sensitivity and HR deficiency. Most of the deletions in BRCA2 were associated with small tracts of homology, and possibly arose from error-prone repair caused by BRCA2 deficiency3,4. Similar ORF-restoring mutations were present in carboplatin-resistant ovarian tumours from c.6174delT mutation carriers. These observations have implications for understanding drug resistance in BRCA mutation carriers as well as in defining functionally important domains within BRCA2.