H3 K27M and EZHIP Impede H3K27-Methylation Spreading by Inhibiting Allosterically Stimulated PRC2

Diffuse midline gliomas and posterior fossa type A ependymomas contain the recurrent histone H3 lysine 27 (H3 K27M) mutation and express the H3 K27M-mimic EZHIP (CXorf67), respectively. H3 K27M and EZHIP are competitive inhibitors of Polycomb Repressive Complex 2 (PRC2) lysine methyltransferase activity. In vivo, these proteins reduce overall H3 lysine 27 trimethylation (H3K27me3) levels; however, residual peaks of H3K27me3 remain at CpG islands (CGIs) through an unknown mechanism. Here, we report that EZHIP and H3 K27M preferentially interact with PRC2 that is allosterically activated by H3K27me3 at CGIs and impede its spreading. Moreover, H3 K27M oncohistones reduce H3K27me3 in trans, independent of their incorporation into the chromatin. Although EZHIP is not found outside placental mammals, expression of human EZHIP reduces H3K27me3 in Drosophila melanogaster through a conserved mechanism. Our results provide mechanistic insights for the retention of residual H3K27me3 in tumors driven by H3 K27M and EZHIP. [Display omitted] •H3 K27M and EZHIP interact with allosterically stimulated PRC2 in vivo•H3 K27M oncohistones reduce H3K27me3 independent of deposition into chromatin•H3 K27M and EZHIP impede PRC2 spreading from CpG islands containing H3K27me3•Human EZHIP reduces H3K27me3 spreading from PREs in Drosophila H3 K27M and EZHIP drive pediatric gliomas by inhibiting Polycomb Repressive Complex 2 activity. Jain et al. show that EZHIP and H3 K27M interact with allosterically stimulated PRC2 at its recruitment sites in vivo and impede its spreading. Chromatin deposition of H3 K27M is not necessary for the reduction of H3K27me3 in vivo.