Identification, Exploitation and Manipulation of BRCA1-Dependent DNA Double-Strand Break and Interstrand Crosslink Repair in Breast and Ovarian Cancer Therapy

Abstract

Expression of functional breast cancer susceptibility gene 1 (BRCA1) in human cancers is associated with resistance to platinum-based chemotherapeutics and poly(ADP-ribose) polymerase (PARP) inhibitors. BRCA1 is a nuclear phosphoprotein with broad tumor suppressor activities that, among other functions, is critical for resolving double-strand DNA breaks (DSBs) and interstrand crosslinks (ICLs) by homologous recombination (HR). In vitro, animal and human clinical data have demonstrated that BRCA1-deficient cancers are highly sensitive to ICL-inducing alkylative chemotherapeutic agents, are amenable to synthetic lethal approaches which exploit defects in DSB/ICL repair (e.g., PARP inhibitors), and are generally associated with more favorable responses to anti-neoplastic therapy and improved survival. Conversely, high expression of wild-type BRCA1 in a number of cancers, as well as frame-restoring intragenic mutations in BRCA1 mutant ovarian cancers, is associated with therapeutic resistance and poor prognosis. Accordingly, there has been much interest in identifying, exploiting and manipulating DSB/ICL repair capacity to restore or enhance sensitivity to cancer therapeutics. In this study, we demonstrate that the heat shock protein 90 (HSP90) inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG (Tanespimycin)), which is currently in Phase II/III clinical evaluation, induces BRCA1 ubiquitination and proteasomal degradation in numerous in vitro models. Mechanistically, we show that loss of HSP90 function completely abolishes both homologous recombination and non-homologous end joining of DSBs, that BRCA1-deficient cells are hypersensitive to 17-AAG due to enhanced replication stress and aberrant entry into mitosis, and that 17-AAG can reverse BRCA1-dependent repair-mediated resistance. Additionally, we assessed the role of BRCA1 promoter methylation in sporadic triple-negative breast cancers (TNBCs) and identify a novel biomarker for poor response to anthracycline regimens in human patients. In summary, we document a novel upstream HSP90-dependent regulatory point in the Fanconi anemia/BRCA DSB/ICL repair pathway, illuminate the role of BRCA1 in regulating damage-associated checkpoint and replication responses to HSP90 inhibitors, specifically identify BRCA1 as a novel, clinically relevant target for enhancing radio- and chemosensitivity in refractory and/or resistant malignancies, and identify a useful biomarker for studies of therapeutic sensitivity in human TNBCs.