Imprinting on distal chromosome 7 in the placenta involves repressive histone methylation independent of DNA methylation

Imprinted genes are expressed from only one of the parental chromosomes and are marked epigenetically by DNA methylation and histone modifications. The imprinting center 2 (IC2) on mouse distal chromosome 7 is flanked by several paternally repressed genes, with the more distant ones imprinted exclusively in the placenta. We found that most of these genes lack parent-specific DNA methylation, and genetic ablation of methylation does not lead to loss of their imprinting in the trophoblast (placenta). The silent paternal alleles of the genes are marked in the trophoblast by repressive histone modifications (dimethylation at Lys9 of histone H3 and trimethylation at Lys27 of histone H3), which are disrupted when IC2 is deleted, leading to reactivation of the paternal alleles. Thus, repressive histone methylation is recruited by IC2 (potentially through a noncoding antisense RNA) to the paternal chromosome in a region of at least 700 kb and maintains imprinting in this cluster in the placenta, independently of DNA methylation. We propose that an evolutionarily older imprinting mechanism limited to extraembryonic tissues was based on histone modifications, and that this mechanism was subsequently made more stable for use in embryonic lineages by the recruitment of DNA methylation.