Comparative analysis of human chromosome 7q21 and mouse proximal chromosome 6 reveals a placental-specific imprinted gene, TFPI2/Tfpi2, which requires EHMT2 and EED for allelic-silencing

Genomic imprinting is a developmentally important mechanism that involves both differential DNA methylation and allelic histone modifications. Through detailed comparative characterization, a large imprinted domain mapping to chromosome 7q21 in humans and proximal chromosome 6 in mice was redefined. This domain is organized around a maternally methylated CpG island comprising the promoters of the adjacent PEG10 and SGCE imprinted genes. Examination of Dnmt3l(-/+) conceptuses shows that imprinted expression for all genes of the cluster depends upon the germline methylation at this putative « imprinting control region » (ICR). Similarly as for other ICRs, we find its DNA-methylated allele to be associated with trimethylation of lysine 9 on histone H3 (H3K9me3) and trimethylation of lysine 20 on histone H4 (H4K20me3), whereas the transcriptionally active paternal allele is enriched in H3K4me2 and H3K9 acetylation. Our study reveals a novel placenta-specific transcript, TFPI2, which is expressed from the maternal allele in both humans and mice. Deficiency for the histone methyltransferase EHMT2 (also known as G9A) or for the Polycomb group protein EED, involved in repressive H3K9me2 and H3K27me3 respectively, leads to biallelic expression of Tfpi2 in the extra-embryonic lineages, whereas the other genes in the cluster maintain correct imprinting. Apart from the putative ICR, however, no other promoter regions within the domain exhibited allele-specific repressive histone modifications. This unexpected general lack of repressive histone modifications suggests that this domain may utilize a different silencing mechanism as compared to other imprinted domains.