Post-transcriptional regulation of BRCA1 through its coding sequence by the miR-15/107 group of miRNAs

MicroRNAs (miRNAs) are important post-transcriptional regulators of gene expression that act by degrading their RNA targets or by repressing the translation of messenger RNAs (mRNAs). Initially thought to primarily target the 3' untranslated region (3'UTR) of mRNAs, miRNAs have since been shown to also target the 5'UTR and coding sequence (CDS). In this work, we focus on the post-transcriptional regulation of the BRCA1 gene, a major tumor suppressor and regulator of double-stranded break DNA repair and show that its mRNA is targeted by many members of the miR-15/107 group at a site located within the CDS. Ectopic expression of these miRNAs across a panel of nine cell lines demonstrated widespread suppression of BRCA1 mRNA levels. Additionally, by cloning a putative target site from BRCA1's amino acid CDS into a luciferase reporter plasmid we confirmed the direct interaction of these miRNAs with this BRCA1 target. We also examined the relationship between ectopic expression of these targeting miRNAs and BRCA1 protein levels in immortalized pancreatic epithelium (hTERT-HPNE), colorectal adenocarcinoma (HCT-116) and pancreatic adenocarcinoma (MIA PaCa-2) cell lines and found protein abundance to be variably regulated in a cell-type specific manner that was not necessarily concordant with mRNA transcript availability. Our findings reveal a previously unrecognized aspect of BRCA1's miRNA-mediated post-transcriptional regulation, namely the targeting of its amino acid coding region by the miR-15/107 group of miRNAs. The resulting regulation is apparently complex and cell-specific, an observation that may have implications for BRCA1-mediated DNA repair across tissue types.