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.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/ .

ACKNOWLEDGMENTS

We thank Sheila Webb for blastocyst injections, Patricia Cavelier for histology, and Philippe Arnaud for discussion and comments.

R.F. and W.B. thank the 6th Framework NoE EPIGENOME (LSHG-CT-2004-503433) for support and encouragement. R.F. acknowledges grant support from ESF EUROCORES program EuroSTELLS, the ARC program ARECA, the ANR (Programme Blanc), the Cancéropôle Grand Sud-Ouest, and the ACI program of the French Ministry of Education and Science. K.W. received a Peter Baker traveling fellowship, and A.W. acknowledges an ARC Ph.D. Fellowship. R.S. was the recipient of an EMBO long-term fellowship.