H3K36 Methylation - a Guardian of Epigenome Integrity [ATAC-seq]

H3K36 methylation is a key epigenetic mark with critical roles in development and disease, yet its genome-wide functions remain incompletely understood. Here, we systematically dissect the role of H3K36 methylation using a series of CRISPR-engineered mouse mesenchymal stem cell lines with progressive knockouts of the five H3K36 methyltransferases: NSD1, NSD2, NSD3, SETD2 and ASH1L—culminating in quintuple knockout (QuiKO) cells entirely lacking H3K36me2/3. We demonstrate that H3K36me2 enriched at active cis-regulatory elements predominantly influences enhancer activity and supports the expression of enhancer-dependent genes. Within gene bodies, loss of H3K36me2/3 permits invasion by the repressive H3K27me2/3 modifications, thereby disrupting transcriptional fidelity. Unexpectedly, we uncover a previously undescribed relationship between H3K36me and H3K9me3: upon H3K36me2 depletion, SUV39H1-mediated H3K9me3 redistributes from large constitutive heterochromatic domains into euchromatin, leading to widespread epigenome restructuring and transcriptional changes. This redistribution is accompanied by a collapse of 3D genome organization, including weakened compartment boundaries and a profound loss of long-range interactions. Finally, we validate key findings in two human HNSCC cell lines overexpressing the H3K36M oncohistone, highlighting the conserved role of H3K36 methylation in maintaining epigenome integrity across species and cell types, and providing further mechanistic insights into the downstream consequences of this oncogenic mutation. Together, these data reveal that H3K36me is essential not only for local gene regulation but also for safeguarding the higher-order structure and functional balance of the genome.