H3K9 Methyltransferase EHMT2/G9a Controls ERVK-Driven Noncanonical Imprinted Genes

Aim: Paternal allele-specific expression of noncanonical imprinted genes in the extraembryonic lineages depends on an H3K27me3-based imprint in the oocyte, which is not a lasting mark. We hypothesized that EHMT2, the main euchromatic H3K9 dimethyltransferase, also has a role in controlling noncanonical imprinting. Methods: We carried out allele-specific total RNA-seq analysis in the ectoplacental cone of somite-matched 8.5 days post coitum embryos using reciprocal mouse crosses. Results: We found that the maternal allele of noncanonical imprinted genes was derepressed from its ERVK promoter in the Ehmt2-/- ectoplacental cone. In Ehmt2-/- embryos, loss of DNA methylation accompanied biallelic derepression of the ERVK promoters. Canonical imprinting and imprinted X chromosome inactivation were generally undisturbed. Conclusion: EHMT2 is essential for repressing the maternal allele in noncanonical imprinting.