G9a Histone Methyltransferase Contributes to Imprinting in the Mouse Placenta

Abstract

Whereas DNA methylation is essential for genomic imprinting, the importance of histone methylation in the allelic expression of imprinted genes is unclear. Imprinting control regions (ICRs), however, are 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 found 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 was unchanged at the domains analyzed, in spite of a global loss of H3-K9 dimethylation (H3K9me2). In contrast, the placenta-specific imprinting of genes on distal chromosome 7 is impaired in the absence of G9a, and this correlates with reduced levels of H3K9me2 and H3K9me3. These findings provide the first evidence for the involvement of an HMT and suggest that histone methylation contributes to imprinted gene repression in the trophoblast.

The expanding role of the Ehmt2/G9a complex in neurodevelopment

“…No apparent requirement for imprinting in embryonic tissue has been found in vivo, however a role has emerged in maintaining repression of imprinted alleles within the placenta. 13,39 Creation of an Ehmt2 mutant lacking the C-terminal half of the protein, including the Ank repeat and SET domains, lead to a partial loss of imprinting at the Kcnq1 locus on chromosome 7 in the trophoblast, but not in the embryo proper. 39 Imprinting at the Kcnq1 locus is therefore more sensitive to a loss of Ehmt2 function in the trophoblast.…”

“…13,39 Creation of an Ehmt2 mutant lacking the C-terminal half of the protein, including the Ank repeat and SET domains, lead to a partial loss of imprinting at the Kcnq1 locus on chromosome 7 in the trophoblast, but not in the embryo proper. 39 Imprinting at the Kcnq1 locus is therefore more sensitive to a loss of Ehmt2 function in the trophoblast. The absence of a detected role for Ehmt2 in embryonic imprinting may be due to residual N-terminal protein function in this deletion strategy, or possibly even redundancy with Ehmt1 or other mechanisms in the embryo proper to guard against a loss of imprinting maintenance.…”

Genomic imprinting in mammals: its life cycle, molecular mechanisms and reprogramming

“…Further studies showed that the silent paternal alleles are marked by repressive histone modifications such as H3K9me2, mediated by G9a, and H3K27me3, mediated by the Polycomb repressive complex 2 (PRC2) [57,58]. Indeed, mice lacking G9a lose the mono-allelic expression patterns of the placenta-specific genes [59]. Also, in embryos lacking Eed, a component of the PRC2 complex, a subset of the paternally repressed genes was aberrantly activated in the trophoblast [60].…”

Macro lncRNAs

“…49,50 However despite the demonstration that the responsible G9A/EHMT2 and PRC2 histone methyltransferases physically interact with Kcnq1ot1, 51 mice lacking G9A/EHMT2 and PRC2 only show stochastic loss of imprinted expression of some genes in the placenta. 52,53 A role for the Kcnq1ot1 macro lncRNA product was recently dismissed based on a siRNA-mediated knockdown experiment that did not change imprinted expression of these genes in undifferentiated embryonic stem cells. 54 However, this may not be relevant since RNAi activity is restricted to the cytoplasm and Kcnq1ot1 is nuclear-localized.…”