Abstract
Histone H3 lysine 27 (H3K27M) mutations represent the canonical oncohistone, occurring frequently in midline gliomas but also identified in haematopoietic malignancies and carcinomas. H3K27M functions, at least in part, through widespread changes in H3K27 trimethylation but its role in tumour initiation remains obscure. To address this, we created a transgenic mouse expressing H3.3K27M in diverse progenitor cell populations. H3.3K27M expression drives tumorigenesis in multiple tissues, which is further enhanced by Trp53 deletion. We find that H3.3K27M epigenetically activates a transcriptome, enriched for PRC2 and SOX10 targets, that overrides developmental and tissue specificity and is conserved between H3.3K27M-mutant mouse and human tumours. A key feature of the H3K27M transcriptome is activation of a RAS/MYC axis, which we find can be targeted therapeutically in isogenic and primary DIPG cell lines with H3.3K27M mutations, providing an explanation for the common co-occurrence of alterations in these pathways in human H3.3K27M-driven cancer. Taken together, these results show how H3.3K27M-driven transcriptome remodelling promotes tumorigenesis and will be critical for targeting cancers with these mutations.
Highlights
Histone H3 lysine 27 (H3K27M) mutations represent the canonical oncohistone, occurring frequently in midline gliomas and identified in haematopoietic malignancies and carcinomas
CD-1 mice, expressing FLAG/HA-tagged H3.3K27M (H3f3aK27M) under the control of the Fabp[7] promoter, were engineered by microinjection, with colonies established from 3 founders (Supplementary Fig. S1a–c)
The H3.3K27M-FLAG/HA transgene was not overexpressed versus endogenous H3.3 in the mouse samples or when compared to H3K27M levels in human Diffuse Intrinsic Pontine Glioma (DIPG) (Supplementary Fig. S1h, i)
Summary
H3.3K27M increases cancer incidence and leads to early death. CD-1 mice, expressing FLAG/HA-tagged H3.3K27M (H3f3aK27M) under the control of the Fabp[7] promoter, were engineered by microinjection, with colonies established from 3 founders (Supplementary Fig. S1a–c). To further investigate the similarities between the early effects of H3.3K27M on the brainstem and gliomagenesis we compared the transcriptomes of H3.3K27M mutant E14.5 brainstem with H3.3K27M/Trp53+/− mouse HGGs and human DIPGs. The differentially expressed genes and pathways in all 3 datasets were strikingly similar (all two-way overlaps p ≤ 10−7; Fig. 4a, b). RAS signalling was one of the most strongly activated pathways in established H3.3K27M mouse HGG and lymphoma as well as human DIPG (Fig. 4h, i and Supplementary Fig. S5b) To ask if this was an early response to H3.3K27M, we examined E14.5 MB/ HB. In a panel of primary pHGG cells, H3.3K27M-mutant cells had reduced RAS target gene promoter H3K27me[3] compared with H3WT cells (Fig. 6d)[49] Together, this suggests that H3.3K27Mmutant tumours exhibit K27M-mediated epigenetic activation of the RAS/MAPK pathway.
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