Background: Euchromatic histone-lysine N-methyltransferase 2 (G9a/Ehmt2) is the main enzyme responsible for the apposition of H3K9 di-methylation on histones. Due to its dual role as an epigenetic regulator and in the regulation of non-histone proteins through direct methylation, G9a has been implicated in a number of biological processes relevant to cell fate control. Recent reports employing in vitro cell lines indicate that Ehmt2 methylates MyoD to repress its transcriptional activity and therefore its ability to induce differentiation of activated myogenic cells.
Methods: To further investigate the importance of G9a in modulating myogenic regeneration in vivo, we crossed Ehmt2 (floxed) mice to animals expressing Cre recombinase from the Myod locus, resulting in efficient knockout in the entire skeletal muscle lineage (Ehmt2 (ΔmyoD) ).
Results: Surprisingly, despite a dramatic drop in the global levels of H3K9me2, knockout animals did not show any developmental phenotype in muscle size and appearance. Consistent with this finding, purified Ehmt2 (ΔmyoD) satellite cells had rates of activation and proliferation similar to wild-type controls. When induced to differentiate in vitro, Ehmt2 knockout cells differentiated with kinetics similar to those of control cells and demonstrated normal capacity to form myotubes. After acute muscle injury, knockout mice regenerated as efficiently as wildtype. To exclude possible compensatory mechanisms elicited by the loss of G9a during development, we restricted the knockout within adult satellite cells by crossing Ehmt2 (floxed) mice to Pax7 (CreERT2) and also found normal muscle regeneration capacity.
Conclusions: Thus, Ehmt2 and H3K9me2 do not play significant roles in skeletal muscle development and regeneration in vivo.
Keywords: Development; Ehmt1; Ehmt2; Euchromatic methyltransferase; G9a; GLP; Myod; Myogenesis; Regeneration; Skeletal muscle.