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H3.3K27M mutant proteins reprogram epigenome by sequestering the PRC2 complex to poised enhancers

ABSTRACT: Expression of histone H3.3K27M mutant proteins in diffuse intrinsic pontine glioma (DIPG) results in a global reduction of tri-methylation of H3K27 (H3K27me3), and paradoxically, H3K27me3 peaks remain at hundreds of genomic loci, a dichotomous change that lacks mechanistic insights. Here we show that the PRC2 complex is sequestered at poised enhancers, but not at active promoters with high levels of H3.3K27M proteins, thereby contributing to the global reduction of H3K27me3. Moreover, the levels of H3.3K27M proteins are low at the retained H3K27me3 peaks and consequently having minimal effects on the PRC2 activity at these loci. H3K27me3-mediated silencing at specific tumor suppressor genes, including Wilms Tumor 1, promotes proliferation of DIPG cells. These results support a model in which the PRC2 complex is redistributed to poised enhancers in H3.3K27M mutant cells and supports the proliferation of DIPG cells in part through silencing of tumor suppressor gene WT1.

ORGANISM(S): Mus musculus Homo sapiens

PROVIDER: GSE94834 | GEO | 2018/07/02

REPOSITORIES: GEO

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Project description:Diffuse Intrinsic Pontine Glioma (DIPG) is a universally fatal childhood brain tumour which frequently carries a mutation in genes encoding histone H3. These mutations lead to a lysine-to-methionine (K-to-M) substitution at position 27 (H3K27M) within the histone H3 tail and initiate global shifts in the abundance of Polycomb Repressive Complex 2 (PRC2) mediated H3K27 methylation (me) and P300/CBP mediated acetylation (ac). Acquisition of the H3K27M mutation is the founding genetic event in DIPG tumours, making it likely that these global shifts in H3K27me/ac are instrumental in promoting the early steps toward gliomagenesis. Here, we show that the H3.3K27M oncogene directly disrupts the function of tissue specific gene enhancers and increases Polycomb target gene repression limiting developmental potential of neural stem cells (NSCs). To study the initial functional consequences of H3K27M in glioma, we developed an isogenic human model system. To do this, we modelled the earliest stages of DIPG development by introducing either K27M or wildtype (WT) H3.3 into hindbrain derived NSC cultures. Importantly, this model faithfully recapitulated the global H3K27ac/me dynamics observed in patient samples. Moreover, epitope tagging of the K27M and WT histones allowed us to globally assess the localisation of K27M and WT H3.3. This for the first time facilitated a global analysis of H3.3K27M distribution, as it relates to the WT histone in a biologically relevant context. We found that the presence of the K27M mutation does not alter the localisation of the mutant histone, which is most abundant at enhancer elements with lower levels at gene promoters. Remarkably, the H3.3K27M oncogene limits the developmental potential of NSCs by directly impeding tissue specific enhancer function. Moreover, we found that PRC2 function is primarily altered outside of stably bound Polycomb target sites, with the majority of H3K27me3 lost outside of these regions. Finally, we provide in vivo evidence for sequestration of the PRC2 complex at a subset of Polycomb target promoters. Taken together, these findings provide important new mechanistic insights on the initial steps of DIPG development.

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H3.3K27M mutant proteins reprogram epigenome by sequestering the PRC2 complex to poised enhancers

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