Secondary mutations of BRCA1/2 and drug resistance.

Inherited mutations in the tumor suppressor genes BRCA1 and BRCA2 cause increased risk of developing various cancers, especially breast and ovarian cancers. Tumors that develop in patients with inherited BRCA1/2 mutations are generally believed to be BRCA1/2-deficient. Cancer cells with BRCA1/2 deficiency are defective in DNA repair by homologous recombination and sensitive to interstrand DNA crosslinking agents, such as cisplatin and carboplatin, and poly(ADP-ribose) polymerase inhibitors. Therefore, these agents are logical choices for the treatment for BRCA1/2-deficient tumors and have shown to be clinically effective. However, BRCA1/2-mutated tumors often develop resistance to these drugs. Restoration of BRCA1/2 functions due to secondary BRCA1/2 mutations has been recognized as a mechanism of acquired resistance to cisplatin and poly(ADP-ribose) polymerase inhibitors in BRCA1/2-mutated cancer cells. This indicates that even disease-causing inherited mutations of tumor suppressor genes can be genetically reverted in cancer cells, if the genetic reversion is advantageous for the cells' survival. In this review, we will discuss this drug resistance mechanism.