Imprinting along the Kcnq1 domain on mouse chromosome 7 involves repressive histone methylation and recruitment of Polycomb group complexes

Imprinted genes are clustered in domains, and their allelic repression is mediated by imprinting control regions(1-3). These imprinting control regions are marked by DNA methylation, which is essential to maintain imprinting in the embryo(4). To explore how imprinting is regulated in placenta, we studied the Kcnq1 domain on mouse distal chromosome 7. This large domain is controlled by an intronic imprinting control region(5,6) and comprises multiple genes that are imprinted in placenta, without the involvement of promoter DNA methylation(7-10). We found that the paternal repression along the domain involves acquisition of trimethylation at Lys27 and dimethylation at Lys9 of histone H3. Eed-Ezh2 Polycomb complexes are recruited to the paternal chromosome and potentially regulate its repressive histone methylation. Studies on embryonic stem cells and early embryos support our proposal that chromatin repression is established early in development and is maintained in the placenta. In the embryo, however, imprinting is stably maintained only at genes that have promoter DNA methylation. These data underscore the importance of histone methylation in placental imprinting and identify mechanistic similarities with X-chromosome inactivation in extraembryonic tissues, suggesting that the two epigenetic mechanisms are evolutionarily linked.