Since the discovery of recurrent mutations in histone H3 variants in pediatric brain tumours, so-called 'oncohistones' have been identified in various cancers. While their mechanism of action remains under active investigation, several studies have shed light on how they promote genome-wide epigenetic perturbations. These findings converge on altered post-translational modifications on two key lysine (K) residues of the H3 tail, K27 and K36, which regulate several cellular processes, including those linked to cell differentiation during development. We will review how these oncohistones affect the methylation of cognate residues, but also disrupt the distribution of opposing chromatin marks, creating genome-wide epigenetic changes which participate in the oncogenic process. Ultimately, tumorigenesis is promoted through the maintenance of a progenitor state at the expense of differentiation in defined cellular and developmental contexts. As these epigenetic disruptions are reversible, improved understanding of oncohistone pathogenicity can result in needed alternative therapies.
Accepted ArticleThis article is protected by copyright. All rights reserved A number of cancers carry recurrent, somatic, gain-of-function, heterozygous mutations in different histone 3 (H3)-encoding genes, which lead to amino acid substitutions on key residues of the H3 tail. These hotspot H3 mutations, oncohistones as we label them, were first identified in a deadly brain cancer, pediatric high-grade gliomas (pHGGs), where they account for a large proportion of these tumours. Notably, they show remarkable spatio-temporal specificity, indicating that their pathogenesis may be closely linked to aberrant development [1,2]. Indeed, HGGs of the central nervous system midline (which includes the pons, thalamus and spine) target younger children, and show a high frequency (~80%) of H3 lysine to methionine, or rarely to isoleucine, substitutions (K27M/I). These K27M/I mutations occur in either canonical H3.1/H3.2 variants mainly in the pons, or in the non-canonical H3.3 variant across all brain midline structures (Figure 1) [2][3][4][5][6]. By contrast, in HGGs of the brain hemispheres, glycine 34 to arginine or valine (G34R/V) substitutions are specific to H3F3A, which encodes the H3.3 variant, and target primarily the temporo-parietal cortex in adolescents and young adults, where they account for ~30% of these tumours [2][3][4][5][6]. Other hemispheric H3 wild-type gliomas of adolescents and young adults (mean age of 33 years) occur primarily in fronto-parietal lobes and carry non-overlapping truncating mutations in SETD2 [7], the only H3K36 tri-methyltransferase in humans, and/or hotspot somatic mutations in isocitrate dehydrogenases 1/2 (IDH1/2) [3,6,[8][9][10]. IDH mutations generate a neomorphic enzyme and excess production of the oncometabolite 2-hydroxyglutarate, which competitively inhibits histone and DNA demethylases to affect the methylation of K residues on the H3 tail and promote a CpG island methylator phenotype (CIMP). Together, ...