Abstract 4196: Over-expression of miR-146a in basal-like breast cancer cells confers enhanced tumorigenic potential in association with altered p53 status

Over-expression of miR-146a in basal-like breast cancer cells confers enhanced tumorigenic potential in association with altered p53 status p53 is the most frequently mutated gene in human cancers, mutated in 26-30% of breast cancers. However, mutation rates differ according to breast cancer subtype, being more prevalent in aggressive ER negative tumors, basal-like and HER2 amplified subtypes. This heterogeneity suggests that p53 signaling may differ across molecular subtypes of breast cancer. Understanding the inherent differences in p53 pathway in luminal versus basal-like breast cancers is important and could provide a target for development of new treatment strategies. We used RNAi-mediated p53 knockdown in a panel of four cell lines (2 basal-like, two luminal), together with antagomir-mediated knockdown of a p53-regulated microRNA, to study how gene expression and cellular phenotypes of p53 loss differ in luminal vs. basal-like breast cancer. We observed that p53 knockdown significantly altered gene expression and miRNA expression. We identified a common response shared by all cell lines indicating that certain established p53 targets (e.g. p21 and miR-34 family) are down-regulated and proliferation is upregulated. Interestingly, some p53-dependent changes were subtype-specific. Basal-like cells lines demonstrated increased correlation with a Claudin-low breast cancer signature in presence of p53 KD, while luminal cell lines did not. Basal-like cell lines with p53 KD also exhibited decreased levels of apoptosis, while luminal cell-lines did not. Several miRs were differentially expressed in luminal vs. basal-like knock down cell lines including miR-134, miR-146a, and miR-181b. miR-146a was chosen for further analysis based on the most significant fold change. Antagomir knockdown of miR-146a ablated several p53-dependent phenotypic changes observed in basal-like cells, including increased population doubling times and decreased steady state apoptosis levels. The mechanism underlying these cellular phenotypes involves NF-kB signaling alterations, with downregulation of the extrinsic apoptotic pathway including TNFR, FADD, and TRADD. Antagomir-mediated knockdown of miR-146a restored expression of these components in basal-like breast cancer cell lines with p53 KD. This study documents that an alternate p53-dependent survival pathway exists for basal-like breast cancer cells with p53 loss, mediated by upregulation of miR-146a. Targeting miR-146a expressio