The role of MLL1 in post natal myogenesis

The trithorax H3K4 histone methyltransferase (HMT) MLL1 has important roles for early embryonic development, hematopoiesis and neurogenesis through regulation of Hox and homeodomain factor expression. MLL1 has been previously implicated in activation of Myf5 expression through an interaction with Pax7. Here, we find that in vivo, MLL1 is necessary for efficient muscle regeneration, and for maintenance of muscle stem and progenitor cells. Loss of MLL1 in cultured myoblasts reveals an essential role for proliferation and expression of both Myf5 and Pax7. Loss of Myf5 is conditional on loss of Pax7 and exogenous Pax7 rescues Myf5 in the absence of MLL1 suggesting a role for MLL1 in Pax7 expression but not Pax7 activity. Importantly, Mll1 knockout results in a minor decrease to H3K4me3 at Pax7, a 40% decrease in Pax7 mRNA and an 85% decrease in Pax7 protein, suggesting a previously proposed non-HMT role for MLL1 may involve linking transcription to translation. For in-vitro analysis of loss of MLL1, myoblasts were isolated from Mll1fl/fl mice (Jude et al., 2007) kindly provided by Patricia Ernst, crossed with Rosa26-CreERT2 mice (Seibler et al., 2003). Myoblast line was established in culture for 10 days followed by treatment to excise exon 2 of MLL1 for knockout. Control and MLL1 KO samples are from the same cell line treated or not treated with 4-OHT.