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Hox genes play important roles in haematopoietic development in mammals. ASH1 is a member of the trithorax group (trxG) that is required for proper expression of Hox genes and is preferentially expressed in haematopoietic stem cells. We have recently reported that ASH1 methylates histone H3 at lysine 36 (K36) but its biological function has remained elusive. Here we show that ASH1 regulates Hox gene expression positively and negatively in a leukemic cell line K562 and is required for myelomonocytic differentiation of murine haematopoietic stem cells. ASH1 binds to endogenous Hox loci in K562 cells and its knockdown causes reduced expression of Hox genes. In addition, ASH1 and MLL1 induce more than 100-fold activation of Hox promoters in HeLa cells if expressed simultaneously but not individually. Notably, ASH1 harbouring a point mutation that kills methyltransferase activity is more efficient than wild type ASH1 in Hox gene activation, indicating that K36 methylation is not a prerequisite for Hox gene expression. Moreover, tethering wild type or catalytically inactive methyltransferase domain of ASH1 to a heterologous promoter causes downregulation or upregulation, respectively, of transcription, supporting a hypothesis that K36 methylation imparts repression. Knockdown of ASH1 in K562 cells  in vitro  causes increased expression of ε-globin gene and reduced expression of myelomonocytic markers GPIIb and GPIIIa, whereas knockdown of ASH1 in murine haematopoietic stem cells  in vivo  results in decreased number of macrophages and granulocytes, a phenotype similar to that induced by loss of  mll1  function. Taken together, our data suggest that ASH1 and MLL1 synergize in activation of Hox genes and thereby regulate development of myelomonocytic lineages from haematopoietic stem cells.

Dual Function of Histone H3 Lysine 36 Methyltransferase ASH1 in Regulation of Hox Gene Expression