Abstract 1188: BRCA1 regulates EGFR expression through microRNAs
Authors: Kumaraswamy Easwari , Stecklein Shane R. , Augustine Laura A. , Jensen Roy A. 
Published: June 2012

Germline mutations in the tumor suppressor BRCA1 (breast cancer 1, early onset) are associated with a substantially higher risk of developing basal-like breast cancer (BLBC). Many sporadic breast cancers express decreased levels of BRCA1 and show a strong resemblance to BRCA1-hereditary tumors. BRCA1-associated tumors are triple-negative (TN), basal-like high grade ductal carcinomas that display an undifferentiated phenotype and frequently overexpress epidermal growth factor receptor (EGFR/ERBB1/HER1) and basal cytokeratins (CK5, CK6, CK14 and CK17) and confer a poor clinical prognosis. Although upregulation of EGFR is commonly observed in BRCA1-associated breast cancer, the mechanism of EGFR regulation by BRCA1 is not understood. EGFR upregulation could occur at multiple transcriptional and/or post-transcriptional levels. MicroRNAs (miRNAs) are an evolutionarily conserved class of small non-coding RNAs that regulate gene expression post-transcriptionally and/or translationally. Several miRNAs are found to be differentially expressed in breast cancer and studies show their involvement in stem cell differentiation, epithelial-to-mesenchymal transition (EMT), invasion and metastasis. Identifying the functional relationship between loss of BRCA1 and miRNAs will help us understand the post-transcriptional mechanism by which BRCA1 deficiency leads to aberrant EGFR expression.

In the current study, we show for the first time, a direct inverse correlation between BRCA1 and EGFR protein expression levels in breast cancer cell lines representing the different intrinsic subtypes and in primary mammary epithelial cells. By overexpression and knockdown of BRCA1, we demonstrate that BRCA1 loss results in increased stability of EGFR protein. Using our in vitro system of immortalized primary mammary epithelial cells, we evaluated the effect of functional loss of BRCA1 on the expression of miRNAs in secondary mammospheres which are enriched for stem-like cells and in cells differentiated in vitro. We observed that BRCA1 deficiency leads to dysregulation of miRNAs linked to stem cell renewal, EGFR signaling and EMT. miRNA gain-of-function experiments using Pre-miR™ miRNA precursors and loss-of-function experiments using miRCURY LNA™ miRNA inhibitors were performed with the selected miRNAs to further confirm their effect on EGFR expression. Results suggest that miRNA mediated EGFR deregulation could be a critical step in tumor initiation and progression in BRCA1 deficiency.

Since EGFR upregulation is an early event in breast cancer, additional functional studies will provide a novel molecular mechanism by which BRCA1 deficiency leads to the development of BLBC with aberrant EGFR expression mediated by miRNAs. The study will offer new therapeutic approaches for BLBC, a highly aggressive disease with limited therapeutic options.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1188. doi:10.1158/1538-7445.AM2011-1188