The Msx1 Homeoprotein Recruits G9a Methyltransferase to Repressed Target Genes in Myoblast Cells

Abstract

Although the significance of lysine modifications of core histones for regulating gene expression is widely appreciated, the mechanisms by which these modifications are incorporated at specific regulatory elements during cellular differentiation remains largely unknown. In our previous studies, we have shown that in developing myoblasts the Msx1 homeoprotein represses gene expression by influencing the modification status of chromatin at its target genes. We now show that genomic binding by Msx1 promotes enrichment of the H3K9me2 mark on repressed target genes via recruitment of G9a histone methyltransferase, the enzyme responsible for catalyzing this histone mark. Interaction of Msx1 with G9a is mediated via the homeodomain and is required for transcriptional repression and regulation of cellular differentiation, as well as enrichment of the H3K9me2 mark in proximity to Msx1 binding sites on repressed target genes in myoblast cells as well as the developing limb. We propose that regulation of chromatin status by Msx1 recruitment of G9a and other histone modifying enzymes to regulatory regions of target genes represents an important means of regulating the gene expression during development.

Sumoylation of the Basic Helix-Loop-Helix Transcription Factor Sharp-1 Regulates Recruitment of the Histone Methyltransferase G9a and Function in Myogenesis

“…While a few transcription factors such as the homeoprotein Msx1 and myocyte enhancer factor (MEF2) are known to be sumoylated, the role of SUMO modification of these transcription factors in regulation of muscle gene expression remains to be clarified. For instance, Msx1 is SUMO-modified and was recently shown to repress myogenesis by recruiting G9a (28,29). However, the functional relevance of SUMO modification of Msx1 in recruitment of G9a or in the control of myogenesis has not been reported.…”

Sound of silence: the properties and functions of repressive Lys methyltransferases

“…Furthermore, G9A directly interacts with basic helix-loop-helix family member E41 (bHLHE41; also known as SHARP1), which is a transcription factor and a potent repressor of skeletal muscle differentiation, and enhances its ability to transcriptionally repress MYOD target genes 184 . Intriguingly, MSX1 was found to mediate G9A-dependent deposition of H3K9 dimethylation marks (H3K9me2) at muscle-specific regulatory regions 185 , suggesting a possible functional interplay with PRC2 in ensuring stable transcriptional repression (FIG. 3a).…”

G9a, a multipotent regulator of gene expression

“…25,45 Recruitment of G9a to muscle promoters is mediated by the bHLH transcription factor Sharp-1, as well as by the homeoprotein Msx1. 33,46 Similar to its impact on skeletal myogenesis, G9a represses differentiation of adipocytes, which is dependent upon H3K9me2-mediated inhibition of PPARγ expression. In addition, G9a enhances Wnt10b expression, an inhibitor of adipogenesis, in a SET-domain independent manner.…”