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Take a Step Down and Beware of H3K36me2: The H3K27m Mutation in Glioma Directs H3K27 Methylation

The discovery of H3K27M mutations in pediatric gliomas marked a new chapter in cancer epigenetics. Numerous studies have investigated the effect of this mutation on H3K27 trimethylation, but only recently have we started to realize its additional effects on the epigenome. Here, we use isogenic glioma H3K27M(+/-) cell lines to investigate H3K27 methylation and its interaction with H3K36 and H3K9 modifications. We describe a “Step Down” effect of H3K27M on the distribution of H3K27 methylation: me3 is reduced to me2, me2 is reduced to me1, while H3K36me2/3 delineate the boundaries for the spread of H3K27me marks. We also observe a replacement of H3K27me2/3 silencing by H3K9me3. Using a computational simulation, we explain our observations by reduced effectiveness of PRC2 and constraints imposed on deposition of H3K27me by antagonistic H3K36 modifications. Our work further elucidates the effect of H3K27M in gliomas, as well as general principles of deposition of H3K27 methylation.