Cells expressing high levels of breast cancer susceptibility gene 1 (BRCA1) are resistant to ionizing radiation and alkylative chemotherapeutic agents. Ablation of BRCA1 expression in vitro and in vivo can enhance and/or restore sensitivity to these agents.

BRCA1 is a nuclear phosphoprotein that is critical for resolving double-strand DNA (dsDNA) breaks. Thus, its overexpression likely confers an advantage to cancerous cells targeted by anti-neoplastic agents that induce this form of DNA damage. In vitro evidence suggests that ablating BRCA1 expression may present a novel mechanism to enhance or restore radio- and chemosensitivity in refractory cancers. Like BRCA1, high expression of heat shock protein 90 (Hsp90) has been associated with resistance to radiotherapy and chemotherapy, though the mechanism(s) behind this phenotype are poorly characterized. Hsp90 is a cytoplasmic chaperone that is upregulated or exists in a hyperfunctional state in many human malignancies. Previous studies have demonstrated that Hsp90 regulates the assembly of several molecules involved in the homology-directed repair of dsDNA breaks, including BRCA2/FANCD1, FANCA, MRE11/RAD50/NBS1 (MRN), and RAD51. Using in vitro culture of breast, ovarian, and colorectal cancer cell lines, we demonstrate that inhibition of Hsp90 by the pharmacologic agent 17-allylamino-17-demethoxygeldanamycin (17-AAG), which is currently in clinical evaluation, targets BRCA1 for ubiquitylation and proteasome-mediated degradation. Furthermore, inhibition of Hsp90 and the subsequent loss of BRCA1 expression are associated with decreased repair of ionizing radiation-induced DNA damage and increased tumor cell death. Using lentiviral shRNA expression, we also demonstrate that loss of BRCA1 is epistatic to the ability of 17-AAG to sensitize cells to the effects of ionizing radiation, consistent with the central role of BRCA1 in assembling the dsDNA break repair machinery.

These results suggest that inhibition of Hsp90 using an existing therapeutic agent (17-AAG) may be an effective mechanism to transiently inhibit BRCA1 expression and improve or restore radio- and/or chemosensitivity in breast and ovarian cancers.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1391.