Methylation of a CTCF-dependent boundary controls imprinted expression of the Igf2 gene

The expression of the insulin-like growth factor 2 (Igf2) and H19 genes is imprinted. Although these neighbouring genes share an enhancer(1), H19 is e xpressed only from the maternal allele, and Igf2 only from the paternally i nherited allele(2,3). A region of paternal-specific methylation upstream of H19 appears to be the site of an epigenetic mark that is required for the imprinting of these genes(4,5). A deletion within this region results in lo ss of imprinting of both H19 and Igf2 (ref. 5). Here we show that this meth ylated region contains an element that blocks enhancer activity. The activi ty of this element is dependent upon the vertebrate enhancer-blocking prote in CTCF. Methylation of CpGs within the CTCF-binding sites eliminates bindi ng of CTCF in vitro, and deletion of these sites results in loss of enhance r-blocking activity in vivo, thereby allowing gene expression. This CTCF-de pendent enhancer-blocking element acts as an insulator. We suggest that it controls imprinting of Igf 2. The activity of this insulator is restricted to the maternal allele by specific DNA methylation of the paternal allele. Our results reveal that DNA methylation can control gene expression by modu lating enhancer access to the gene promoter through regulation of an enhanc er boundary.