Poster Sessions

CANCER-31

Suhwan Chang

S. Chang, K. Biswas, S. Sharan

Control of miR-155 by BRCA1 and Its Effect on the ES Cell Differentiation

Breast and ovarian cancers are one of the most frequent malignancies in woman. 5-10% of the breast and ovarian cancers are known to be associated with mutations in either BRCA1 (Breast Cancer Associated 1) or BRCA2 gene. The lifetime cancer risk for the mutation carrier of BRCA1 or BRCA2 is 60-85% for the breast cancer and 26-54% for ovarian cancer. Even though there are variety of many BRCA1 mutations found from cancer patients, there are very little mutations being determined to be deleterious or neutral, mainly due to the large numbers of unclassified variants scattered throughout the BRCA1 gene and the lack of a suitable functional assay. In order to study the function of unclassified variants in BRCA1, an ES cell-based assay system was developed. Among the mutant BRCA1 ES cell lines which has been generated in this assay system so far, a cell line with R1699Q mutation in BRCT1 domain of BRCA1 showed marked differentiation defects, as revealed by histological analysis of embryoid body generated from mutant ES cell as well as teratoma from nude mice injected with the mutant ES cells. This phenotype is very similar to that of ES cells deficient with Dicer, a key enzyme in the processing of siRNA and miRNA. Inspired by this fact, miRNA microarray analysis was performed. The result revealed that miRNA-155, one of the oncogenic miRNA implicated in B-cell lymphoma, is up-regulated in R1699Q mutant embryoid body cells and it is confirmed by real-time PCR analysis. As R1699Q mutation is reported to inactivate the transcriptional regulatory function of BRCA1, we hypothesized that the expression of miRNA-155 is de-repressed in R1699Q mutant cells. Indeed, our data from the miRNA-155 reporter assay in HCC1937 cells supported this idea, showing dose-dependent down regulation of miRNA-155 by wild type BRCA1. To understand the effect of miR-155 disregulation on the ES cell differentiation, target of miR-155 was screened. By matching down-regulated genes in R1699Q embryoid body cells with predicted target of miR-155, we identified Cyr61 as a target of miR-155 in embryoid body. Ths datais confirmedsome predicted targets of miR-155 to understand how BRCA1 mediated miR-155 regulation affects cellular differentiation. Our study strongly suggests that a novel regulation of an oncogenic miRNA by a tumor suppressor, providing new insights to understand the mechanism of BRCA1 associated tumorigenesis.