Characterization Of A Functional Brca1 Variant: A Marker Of Aggressive Breast Cancer

The single nucleotide polymorphism (SNP), termed rs8176318 G\u3eT, within the BRCA1 3\u27-Untranslated Region (3\u27-UTR) has previously been associated with increased breast cancer risk. We investigate the biological function of this variant on the molecular level and further test its association with cancer risk and tumor biology in a genetically homogenous West-Irish patient cohort. To establish whether the SNP affects BRCA1 protein expression, we created BRCA1 3\u27-UTR dual luciferase reporters harboring either the wild type allele (G) or the variant allele (T). We found that the luciferase reporters containing the variant allele displayed reduced protein expression in triple negative breast cancer cell lines and in cells exposed to anti-estrogen. Furthermore, tissue core biopsies from triple negative breast cancers also exhibited decreased BRCA1 expression in those tumors harboring the variant allele. It is hypothesized that this 3\u27-UTR SNP lies within a microRNA binding site, and that the variant allele enhances microRNA binding, thereby suppressing BRCA1 expression. Although we identified three candidate microRNAs whose binding would be strengthened by the presence of the variant allele, we were unable to demonstrate decreased protein expression in the presence of these microRNAs. Finally, we genotyped DNA from 728 breast cancer cases and 387 controls for the rs8176318 SNP in a West-Ireland cohort. We found a modest increased risk for developing breast cancer among patents carrying the variant allele. More significantly, we found a four-fold increased risk of presenting with Stage IV or metastatic disease at the time of diagnosis (OR 3.73, 95% CI 1.3-11.0, p=0.018). Our results show that the rs8176318 SNP is a functional variant that affects BRCA1 expression and associates clinically with aggressive breast cancer. This polymorphism represents a novel class of mutations within the untranslated region of the genome that are increasingly being found to have clinical significance