Abstract LB-265: Mutations in BRCA1 impair breast epithelial differentiation through the transcriptional repressor Slug

Abstract

Abstract Human breast cancers can be broadly classified based on their molecular and gene expression profiles into luminal and basal-like tumors. These tumor subtypes express markers corresponding to the two major differentiation states of epithelial cells in the breast: luminal cells that line the breast ducts and the outer myoepithelial/basal cells that provide contractile functions. Women with inherited mutations in the BRCA1 gene have increased breast cancer risk and also exhibit a specific predisposition to the development of aggressive basal-like breast cancers. Accumulating evidence suggests that BRCA1 has a role in breast epithelial differentiation and we wanted to further understand how this could contribute to the formation of basal-like tumors. Microarray, flow cytometry and immunohistochemical analysis of breast epithelial cells from disease-free women harboring deleterious mutations in BRCA1 (BRCA1 mut/+) compared to those from BRCA1 +/+ reduction mammoplasties showed an increase in markers of basal differentiation and a decrease in markers of luminal differentiation. We also created breast cancers from single cell suspensions of BRCA1 mut/+ and BRCA1 +/+ epithelial cells that had been transformed with identical oncogenes and injected into humanized mammary fat pads. Tumors derived from BRCA1 mut/+ cells had increased basal differentiation relative to cells obtained from BRCA1 +/+ patients, indicating that the perturbed differentiation evident prior to neoplastic transformation was mirrored in the tumors. Pathway analysis of the microarray data comparing BRCA1 mut/+ and BRCA1 +/+ cells from disease-free tissue indicated that signaling components relating to the transcriptional repressor Slug were overexpressed in BRCA1 mut/+ tissues. We confirmed that Slug protein levels were elevated in BRCA1 mut/+ tissues and in human tumors from BRCA1 mutation carriers. RNAi-mediated downregulation of slug in primary breast epithelial cells and cell lines derived from BRCA1 mut/+ tissue as well as breast cancer cell lines known to harbor BRCA1 mutations, led to a decrease in markers of basal differentiation, indicating that elevated Slug protein levels seen in BRCA1-associated tissues and tumors are contributing to the basal phenotype. Furthermore, RNAi-mediated knockdown of BRCA1 led to an increase in Slug protein expression and breast cancer cell lines harboring BRCA1 mutations showed an increase in Slug protein stability, indicating that loss of BRCA1 protein by mutation contributes to elevated Slug protein levels. These results reveal an important mechanism by which BRCA1 can regulate breast epithelial differentiation and may explain how, in addition to affecting incidence rates, the genetic background of patients could impact tumor phenotype. 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 LB-265.