| Abstract: | Metaplastic breast carcinoma (MBC) is a rare histological breast cancer subtype characterized by mesenchymal elements and poor clinical outcome. A large fraction of MBCs harbor defects in breast cancer 1 (BRCA1). As BRCA1 deficiency sensitizes tumors to DNA cross-linking agents and poly(ADP-ribose) polymerase (PARP) inhibitors, we sought to investigate the response of BRCA1-deficient MBCs to the PARP inhibitor olaparib. To this end, we established a genetically engineered mouse model (GEMM) for BRCA1-deficient MBC by introducing the MET proto-oncogene into a BRCA1-associated breast cancer model, using our novel female GEMM ES cell (ESC) pipeline. In contrast to carcinomas, BRCA1-deficient mouse carcinosarcomas resembling MBC show intrinsic resistance to olaparib caused by increased P-glycoprotein (Pgp) drug efflux transporter expression. Indeed, resistance could be circumvented by using another PARP inhibitor, AZD2461, which is a poor Pgp substrate. These preclinical findings suggest that patients with BRCA1-associated MBC may show poor response to olaparib and illustrate the value of GEMM-ESC models of human cancer for evaluation of novel therapeutics.Poly(ADP-ribose) polymerase (PARP) inhibition provides a promising therapeutic strategy for targeting homologous recombination (HR)-deficient tumors, such as breast cancer 1 (BRCA1)-mutated cancers (1). Indeed, clinical phase I and phase II trials have shown potent anticancer activity of small molecule inhibitors of PARP, such as olaparib, in patients with BRCA1-associated breast cancer (2, 3). However, it remains to be established whether different breast cancer subtypes in BRCA1 mutation carriers respond equally to PARP inhibition. Reduced sensitivity of breast cancers to anticancer drugs has frequently been associated with an epithelial-to-mesenchymal transition (EMT) (4–7). Metaplastic breast carcinomas (MBCs) are a subset of triple-negative breast cancers (TNBCs) characterized by a claudin-low and EMT-like phenotype (8) and a poor prognosis compared with other TNBCs (9). More than 60% of MBCs have BRCA1 promoter methylation, raising the question whether these tumors can be effectively targeted by using PARP inhibitors (10). To address this issue in an experimentally controlled setting, we set out to generate a genetically engineered mouse model (GEMM) of BRCA1-deficient MBC by inducing EMT via MET overexpression in a previously established GEMM of BRCA1-mutated breast cancer. We report that EMT is associated with olaparib resistance and can be effectively bypassed by administration of AZD2461, a PARP inhibitor with low affinity for the P-glycoprotein (Pgp) drug efflux transporter. |
