Large organized chromatin K9-modifications (LOCKs) distinguish differentiated from embryonic stem cells

Higher eukaryotes must adapt a totipotent genome to specialized cell types with a stable but limited repertoire of functions. One potential mechanism for lineage restriction is changes in chromatin, and differentiation-related chromatin changes have been observed for individual genes1–2. We have taken a genome-wide view of histone H3 lysine-9 dimethylation (H3K9Me2). We find that differentiated tissues exhibit surprisingly large K9-modified regions (up to 4.9 Mb), that are highly conserved between human and mouse, and differentiation-specific, covering only ~4% of the genome in undifferentiated mouse embryonic stem (ES) cells, compared to 31% in differentiated ES cells, ~46% in liver and ~10% in brain. They require histone methyltransferase G9a, and are inversely related to expression of genes within them, and we term them Large Organized Chromatin K9-modifications (LOCKs). LOCKs are substantially lost in cancer cell lines, and they may provide a cell type-heritable mechanism for phenotypic plasticity in development and disease.