Abstract C21: Hypoxia-induced down-regulation of BRCA1 nuclear protein in human breast cancer tissues

Abstract

Abstract Background: Poly (ADP-ribose) polymerase (PARP) inhibitors have demonstrated significant anti-tumor activity not only in tumors with BRCA1 or 2 gene mutations due to synthetic lethality, but also in triple-negative breast cancers (TNBC). The triple negative group of tumors may be more sensitive to PARP inhibitors due to loss of BRCA pathway activity, although this has not been proven. Several mechanisms may affect BRCA1 protein level such as epigenetic modification, negative transcriptional regulators, as well as hypoxia. Yale investigators have demonstrated that tumor hypoxia leads to loss of critical DNA repair activities, including BRCA1, RAD-51 and γ-H2AX (Bindra RS, et al. Cancer Res 2005;65(24):11597-604). We propose BRCA1 nuclear protein is the determinant of response to PARP inhibitors in sporadic breast cancer, a phenotype we will term ‘BRCA-like’. This study was designed to explore the relationship of BRCA1 loss and hypoxia in human breast tumors. Methods: Yale archival materials consisting of 660 human breast cancer specimens were used to establish a tissue microarray (TMA). Automated QUantitative Analysis (AQUA) was used to detect the intensity of BRCA1 and CAIXwithin a specific subcellular compartment. Carbonic Anhydrase IX (CA-IX), a downstream target of hypoxia-induced factor 1a (HIF-1a), is an accepted surrogate biomarker for tumor hypoxia. CA-IX antibody M75 was provided by the J Zavada lab (1:10000), BRCA1 monoclonal antibody M110 (Ab-1) was purchased from Calbiochem (1:1000). Results: In the cohort of 660 specimens, 22 were found to have high CA-IX expression score (above cut-off value of 10). Cut-off was based on known positive cell lines and corresponding AQUA scores, as well as visual confirmation of positivity. A negative correlation (Rho=−0.6, p=0.0165) of BRCA1 nuclear protein with CA-IX level was found in hypoxic tumors. Of note, 14 out of 22 hypoxic breast tumors were from triple negative breast cancers (TNBC) (p=0.0034). Total number of TNBC in this cohort is 112 out of 660. BRCA1 nuclear expression alone did not correlate with overall survival (p=0.76). Conclusions: In human breast tumors BRCA1 nuclear expression is negatively correlated with tumor hypoxia, measured by CAIX. Ongoing clinical trial will determine if tumors with high CA-IX or low BRCA1 benefit from PARP inhibitor therapy. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C21.