Regional methylation of the 5 ' end CpG island of BRCA1 is associated withreduced gene expression in human somatic cells

In mammalians, demethylation of specific promoter regions often correlates with gene activation; inversely, dense methylation of CpG islands leads to gene silencing, probably mediated by methyl-CpG binding proteins. In cell L ines and cancers, inhibition of tissue-specific genes and tumor suppressor genes expression seems to be related to such hypermethylation. The 5' end o f the breast cancer predisposition gene BRCA1 is embedded in a large CpG is land of similar to 2.7 kb in length. In human sporadic breast cancers, the down-regulation of BRCA1 does not seem to be related to BRCA1 gene alterati ons. Southern blot analysis and the bisulfite sequencing method indicate th at the BRCA1 CpG island is regionally methylated in all human tissues analy zed and unmethylated in the gametes, suggesting a role for DNA methylation in the control of gene expression. We have therefore investigated the poten tial role of methyl-CpG binding proteins in the regulation of BRCA1 gene ex pression. In vitro, partial methylation of constructs containing this regio n strongly inhibits gene expression in the presence of MeCP2 protein. Moreo ver, iu the five human cell lines analyzed, chemically induced hypomethylat ion is associated with BRCA1 gene activation. These data suggest that methy l-CpG binding proteins might be associated with the control of BRCA1 gene e xpression and that methyl-DNA binding proteins may participate in the regul ation of gene expression in mammalian cells.