Dual interactions of K27M mutant nucleosomes with PRC2 and MLL1 shape the cancer epigenetic landscape (MARS-seq)

Cancer associated mutations in genes encoding histones dramatically reshape chromatin patterns and support tumorigenesis, despite their low prevalence. Lysine to Methionine substitution of residue 27 on histone H3 (K27M) is a driver mutation in high-grade pediatric gliomas, known to abrogate Polycomb activity. We applied single-molecule systems to image individual nucleosomes and delineate the combinatorial chromatin patterns associated with K27M expression. We find that K27M-mutant nucleosomes not only sequester PRC2, but also bind preferentially to MLL1, thus leading to genome-wide redistribution of H3K4me3. K27M-mediated deregulation of both repressive and activating chromatin marks maintains ‘bivalent’ chromatin, supporting a poorly differentiated cellular state. This study provides evidence for widespread effects of the K27M oncohistone on multiple core epigenetic pathways, and highlights the use of single-molecule tools to reveal mechanisms of chromatin deregulation in cacner. Bulk MARS-seq of DIPG cell culture