Histone methyltransferase G9a crosstalks with H3K36 histone methyltransferases NSD3 and SETD2 to mediate gene activation

The euchromatic histone methyltransferase G9a/KMT1C/EHMT-II functions as a corepressor and a coactivator of transcription, depending upon its association with distinct protein complexes. While the mechanisms of G9a-mediated repression are well understood, the molecular mechanism of G9a-mediated activation remains elusive. In the present study, we report that the coactivator function of G9a involves its association with H3K36 histone methyltransferases, including NSD3/KMT3F/WHSC1L1 and SETD2/KMT3A. Functionally, we demonstrate that the association of G9a with NSD3 and SETD2 is necessary for activating G9a target adult β major globin and β minor globin genes in differentiating adult erythroid cells. Mechanistically, G9a recruits NSD3 to the β major globin gene, and NSD3 activates its expression by stabilizing Mediator complex binding to the promoter and facilitating SETD2-mediated H3K36 trimethylation in the gene body. Knocking down either NSD3 or G9a in differentiating erythroid cells significantly downregulates mediator complex binding at the promoter and the localization of SETD2 and SETD2-mediated H3K36me3 on the coding region of the G9a target β major globin gene, highlighting the necessity of NSD3 in mediating the activation of this gene. Our study reveals a novel crosstalk mechanism in which the histone methyltransferase G9a coordinates with the H3K36 histone methyltransferases NSD3 and SETD2 to mediate gene activation.