Lysine methyltransferase G9a is required for de novo DNA methylation and the establishment, but not the maintenance, of proviral silencing

Department of Medical Genetics, University of British Columbia, Vancouver, BC, Canada.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 03/2011; 108(14):5718-23. DOI: 10.1073/pnas.1014660108
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Methylation on lysine 9 of histone H3 (H3K9me) and DNA methylation play important roles in the transcriptional silencing of specific genes and repetitive elements. Both marks are detected on class I and II endogenous retroviruses (ERVs) in murine embryonic stem cells (mESCs). Recently, we reported that the H3K9-specific lysine methyltransferase (KMTase) Eset/Setdb1/KMT1E is required for H3K9me3 and the maintenance of silencing of ERVs in mESCs. In contrast, G9a/Ehmt2/KMT1C is dispensable, despite the fact that this KMTase is required for H3K9 dimethylation (H3K9me2) and efficient DNA methylation of these retroelements. Transcription of the exogenous retrovirus (XRV) Moloney murine leukemia virus is rapidly extinguished after integration in mESCs, concomitant with de novo DNA methylation. However, the role that H3K9 KMTases play in this process has not been addressed. Here, we demonstrate that G9a, but not Suv39h1 or Suv39h2, is required for silencing of newly integrated Moloney murine leukemia virus-based vectors in mESCs. The silencing defect in G9a(-/-) cells is accompanied by a reduction of H3K9me2 at the proviral LTR, indicating that XRVs are direct targets of G9a. Furthermore, de novo DNA methylation of newly integrated proviruses is impaired in the G9a(-/-) line, phenocopying proviral DNA methylation and silencing defects observed in Dnmt3a-deficient mESCs. Once established, however, maintenance of silencing of XRVs, like ERVs, is dependent exclusively on the KMTase Eset. Taken together, these observations reveal that in mESCs, the H3K9 KMTase G9a is required for the establishment, but not for the maintenance, of silencing of newly integrated proviruses.

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Available from: Matthew Lorincz, Oct 05, 2015
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    • "However, deacetylated histone H3 and H1 can still be associated with silent retrovirus in Dnmt3a and 3b null ES cells,14 and H3K9me3 marks established by SetDB1 in ES cells are also independent of DNA methylation.15 The enzymes G9a/GLP write H3K9me2 marks but also promote DNA methylation of LTR elements and other genomic regions independently from their histone methyltransferase activity.16,17 Inhibiting G9a/GLP activity with a chemical probe (UNC0638) can reactivate the silent internal promoter of a retroviral vector and trigger DNA demethylation.18 "
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    • "Maternal DPPA3/Stella, TRIM28 and H3K9me2 may be required for maintaining imprints in the zygote (Nakamura et al. 2007, Messerschmidt et al. 2012, Szabo & Pfeifer 2012). Interestingly, G9a, a euchromatin-localised histone methyltransferase, catalyses H3K9 dimethylation and is required for the establishment of methylation at specific loci (Dong et al. 2008, Tachibana et al. 2008, Ikegami et al. 2011, Leung et al. 2011). Combined, these studies led to speculation that histone modifications marking allele-specific methylation may contribute to the maintenance of imprinting during zygotic reprogramming (Delaval et al. 2007, Abramowitz & Bartolomei 2012). "
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    • "Many of the post-translational modifications of histones discussed above are specifically induced at particular residues on specific histones, while others are not affected (e.g., Chwang et al. 2006; Gupta et al. 2010; Gupta-Agarwal et al. 2012; Mahan et al. 2012), indicating that upstream signaling cascades may differentially regulate the activity of multiple HATs, HDACs, histone methylases, and histone demethylases that regulate specific modifications at distinct residues. For example, specific HATs, including HPA2 and Gcn, specifically acetylate H3K14 (Angus-Hill et al. 1999; P Cheung et al. 2000a,b; WL Cheung et al. 2000), and the histone methyltransferase Mll specifically methylates H3K4 (Milne et al. 2002), whereas the G9a/G9a-like protein (GLP) lysine dimethyltransferase complex catalyzes methylation of K3K9 (Kubicek et al. 2007; Leung et al. 2011; Shinkai and Tachibana 2011). Different enzymes that catalyze the addition and removal of post-translational modifications appear to have at least partially independent effects on memory formation. "
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