Regulation of BRCA2 gene expression through CpG methylation of its bi-directional promoter induced by endogenous siRNAs

Human BRCA2 gene expression is tightly regulated. It is not expressed at the G0/G1 phase of human breast cancer cells. Here we provide evidence that BRCA2 gene promoter, through its bidirectional activity, produces two partially overlapping transcripts (BRCA2 and ZAR2) in the G0/G1 breast cells potentially forming a 111 bp RNA duplex. Our data suggest that endogenous siRNAs formed from this RNA duplex suppressed BRCA2 gene expression through CpG methylation of the USF1/2‐binding site at the promoter by transcriptional gene silencing mechanisms. Involvement of DNA methylation in the siRNA‐induced inhibition of BRCA2 gene expression was evident by its dependence on folate and DNMT1. There was hypomethylation of CpG units at the BRCA2/ZAR2 promoter during folate deficiency or knockdown of DNMT1 activity. ChIP analysis revealed a decrease in the recruitment of HDAC1 and increased acetylations of histone H3 and H4 at the BRCA2 promoter DNA under these hypomethylation conditions. These data suggest that siRNA‐induced CpG methylation of BRCA2 promoter DNA regulates the expression of BRCA2 gene in human breast cells at the G0/G1 phase of cell cycle through chromatin remodeling. Supported by the DOD‐CDMRP IDEA Grant# W81XWH‐06‐1‐0466 and the Susan G. Komen Breast Cancer Foundation grant# BCTR0707627 to GC.