BRCA Gene Mutations and Poly(ADP-Ribose) Polymerase Inhibitors in Triple-Negative Breast Cancer

Breast cancer is the most common cancer in women worldwide. Treatment is chosen according to its hormone receptor status and human epidermal growth factor receptor 2 (HER2) status. Among the four main clinically set subtypes, hormone receptor-negative/HER2-negative subtype, also called triple-negative subtype (TNBC), is the most aggressive type with limited choices of therapy. However, recent research has provided important new insights into effective treatments for this subtype. One molecular target that has gained attention is the BRCA gene. BRCA proteins are involved in the maintenance of genomic integrity, therefore playing an important role as a "caretaker" DNA repair protein. Approximately 5% of all breast cancer patients are BRCA mutation carriers, and among the patients with BRCA mutations, 57.1% have the clinical TNBC subtype, showing a high association between BRCA mutations and TNBCs. When cells lack either BRCA1 or BRCA2, all types of homology-directed repairs are compromised, and poly(ADP-ribose) (PAR) polymerase (PARP) acts as a backup system to maintain the genome, consequently making the cells highly sensitive to PARP1 inhibitors. PARP inhibitors have shown promising activity in preclinical and early clinical trials, and today, phase III trials are ongoing. In this chapter, we discuss the mechanism and the role of PARP inhibitors in BRCA-mutated breast cancers and further elaborate the clinical potential of PARP inhibitors as well as their barriers.