SETDB1/NSD dependent H3K9me3/H3K36me3 dual heterochromatin maintains gene expression profiles by bookmarking poised enhancers

Gene silencing by heterochromatin plays crucial roles in cell identity and development. However, the function of heterochromatin components is not fully understood. Here, we characterize the localization, the biogenesis and the function of an atypical heterochromatin, which is simultaneously enriched in the typical heterochromatin mark H3K9me3 as well as in H3K36me3, histone mark usually associated with gene expression. This dual heterochromatin forms on 3,015 regions in the genome of mouse embryonic stem cells and relies on the histone methyltransferases SET Domain Bifurcated 1 (SETDB1) and Nuclear Set Domain containing proteins (NSD) to generate H3K9me3 and H3K36me3, respectively. Upon SETDB1 removal, dual domains loose both trimethyl marks, gain epigenomic signatures of active enhancers, and come into long range contact with upregulated genes, suggesting that it is a major pathway by which gene expression is controlled by heterochromatin. In differentiated tissues, a large subset of these dual domains is destabilized and become enriched in active enhancer marks, providing a mechanistic insight into the involvement of heterochromatin in development and the maintenance of cell identity. Characterization of H3K9me3/H3K36me3 dual heterochromatin in mouse embryonic stem cells using ChIP-seq, CIDOP-seq, RNA-seq, ATAC-seq and HiC methods