H3K36 Methylation - a Guardian of Epigenome Integrity

H3K36 methylation is emerging as a key epigenetic modification with strong implications in genetic disease and cancer. However, the mechanisms through which H3K36me impacts the epigenome and asserts its functional consequences are far from understood. Here, we use mouse mesenchymal stem cell lines with successive knockouts of the H3K36 methyltransferases: NSD1, NSD2, SETD2, NSD3, and ASH1L, which result in progressive depletion of H3K36me and its complete absence in quintuple knockout cells, to finely dissect the role of H3K36me2 in shaping the epigenome and transcriptome. We show that H3K36me2, which targets active enhancers, is important for maintaining enhancer activity, and its depletion results in downregulation of enhancer-dependent genes. We demonstrate the roles of H3K36me2/3 in preventing the invasion of gene bodies by the repressive H3K27me modifications. Finally, we observe a previously undescribed relationship between H3K36me and H3K9me3: Following the depletion of H3K36me2, H3K9me3 is redistributed away from large heterochromatic domains and towards euchromatin. This results in a drastic decompartmentalization of the genome, weakening the boundaries between active and inactive compartments, and a catastrophic loss of long-range inter-compartment interactions. By studying cells totally devoid of H3K36 methyltransferase activity, we uncover a broad range of crucial functions of H3K36me in maintaining epigenome integrity.