G9a histone methyltransferase maintains genomic imprinting in the mouse placenta.

Whereas DNA methylation is essential for genomic imprinting, the importance of histone methylation in the allelic repression of imprinted genes is unclear. Imprinting control regions (ICRs), however, are consistently marked by histone H3 K9 methylation on their DNA-methylated allele. In the placenta, the paternal silencing along the Kcnq1 domain on distal chromosome 7 also correlates with the presence of H3-K9 methylation, but imprinted repression at these genes is maintained independently of DNA methylation. To explore which histone methyltransferase (HMT) could mediate the allelic H3-K9 methylation on distal chromosome 7, and at ICRs, we generated mouse conceptuses deficient for the SET-domain protein G9a. We find that in the embryo and placenta, the differential DNA methylation at ICRs and imprinted genes is maintained in the absence of G9a. Accordingly, in embryos, imprinted gene expression is unchanged at the domains analysed, in spite of a global loss of H3-K9 di-methylation (H3K9me2). In contrast, the placenta-specific imprinting of genes on distal chromosome 7 is lost in the absence of G9, and this correlates with a loss of H3K9me2 and H3K9me3. These findings provide the first in vivo evidence for the involvement of a SET domain protein in imprinting and highlight the importance of histone lysine methylation rather than DNA methylation in the maintenance of imprinting in the trophoblast lineage.

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Wagschal A, Sutherland HG, Woodfine K, Henckel A, Chebli K, Schulz R, Oakey RJ, Bickmore WA, Feil R