A Saccharomyces cerevisiae model and screen to define the functional consequences of oncogenic histone missense mutations

Histone H3 E50K mutation confers oncogenic activity and supports an EMT phenotype

Sequencing of human patient tumors has identified recurrent missense mutations in genes encoding core histones. We report that mutations that convert histone H3 amino acid 50 from a glutamate to a lysine (H3E50K) support an oncogenic phenotype in human cells. Expression of H3E50K is sufficient to transform human cells as evidenced by a dramatic increase in cell migration and invasion, and a statistically significant increase in proliferation and clonogenicity. H3E50K also increases the invasive phenotype in the context of co-occurring BRAF mutations, which are present in patient tumors characterized by H3E50K. H3E50 lies on the globular domain surface in a region that contacts H4 within the nucleosome. We find that H3E50K perturbs proximal H3 post-translational modifications globally and dysregulates gene expression, activating the epithelial to mesenchymal transition. Functional studies using S. cerevisiae reveal that, while yeast cells that express H3E50K as the sole copy of histone H3 show sensitivity to cellular stressors, including caffeine, H3E50K cells display some genetic interactions that are distinct from the characterized H3K36M oncohistone yeast model. Taken together, these data suggest that additional histone H3 mutations have the potential to be oncogenic drivers and function through distinct mechanisms that dysregulate gene expression.

H2B oncohistones cause homologous recombination defect and genomic instability through reducing H2B monoubiquitination in Schizosaccharomyces pombe

Canonical oncohistones are histone H3 mutations in the N-terminal tail associated with tumors and affect gene expression by altering H3 post-translational modifications (PTMs) and the epigenetic landscape. Noncanonical oncohistone mutations occur in both tails and globular domains of all four core histones and alter gene expression by perturbing chromatin remodeling. However, the effects and mechanisms of noncanonical oncohistones remain largely unknown. Here we characterized 16 noncanonical H2B oncohistones in the fission yeast Schizosaccharomyces pombe. We found that seven of them exhibited temperature sensitivities and 11 exhibited genotoxic sensitivities. A detailed study of two of these onco-mutants H2BG52D and H2BP102L revealed that they were defective in homologous recombination (HR) repair with compromised histone eviction and Rad51 recruitment. Interestingly, their genotoxic sensitivities and HR defects were rescued by the inactivation of the H2BK119 deubiquitination function of Ubp8 in the Spt-Ada-Gcn5-Acetyltransferase (SAGA) complex. The levels of H2BK119 monoubiquitination (H2Bub) in the H2BG52D and H2BP102L mutants are reduced in global genome and at local DNA break sites presumably due to enhanced recruitment of Ubp8 onto nucleosomes and are recovered upon loss of H2B deubiquitination function of the SAGA complex. Moreover, H2BG52D and H2BP102L heterozygotes exhibit genotoxic sensitivities and reduced H2Bub in cis. We therefore conclude that H2BG52D and H2BP102L oncohistones affect HR repair and genome stability via the reduction of H2Bub and propose that other noncanonical oncohistones may also affect histone PTMs to cause diseases.

Exploring the Molecular Underpinnings of Cancer-Causing Oncohistone Mutants Using Yeast as a Model

Understanding the molecular basis of cancer initiation and progression is critical in developing effective treatment strategies. Recently, mutations in genes encoding histone proteins that drive oncogenesis have been identified, converting these essential proteins into "oncohistones". Understanding how oncohistone mutants, which are commonly single missense mutations, subvert the normal function of histones to drive oncogenesis requires defining the functional consequences of such changes. Histones genes are present in multiple copies in the human genome with 15 genes encoding histone H3 isoforms, the histone for which the majority of oncohistone variants have been analyzed thus far. With so many wildtype histone proteins being expressed simultaneously within the oncohistone, it can be difficult to decipher the precise mechanistic consequences of the mutant protein. In contrast to humans, budding and fission yeast contain only two or three histone H3 genes, respectively. Furthermore, yeast histones share ~90% sequence identity with human H3 protein. Its genetic simplicity and evolutionary conservation make yeast an excellent model for characterizing oncohistones. The power of genetic approaches can also be exploited in yeast models to define cellular signaling pathways that could serve as actionable therapeutic targets. In this review, we focus on the value of yeast models to serve as a discovery tool that can provide mechanistic insights and inform subsequent translational studies in humans.

H3G34-Mutant Gliomas-A Review of Molecular Pathogenesis and Therapeutic Options

The 2021 World Health Organization Classification of Tumors of the Central Nervous System reflected advances in understanding of the roles of oncohistones in gliomagenesis with the introduction of the H3.3-G34R/V mutant glioma to the already recognized H3-K27M altered glioma, which represent the diagnoses of pediatric-type diffuse hemispheric glioma and diffuse midline glioma, respectively. Despite advances in research regarding these disease entities, the prognosis remains poor. While many studies and clinical trials focus on H3-K27M-altered-glioma patients, those with H3.3-G34R/V mutant gliomas represent a particularly understudied population. Thus, we sought to review the current knowledge regarding the molecular mechanisms underpinning the gliomagenesis of H3.3-G34R/V mutant gliomas and the diagnosis, treatment, long-term outcomes, and possible future therapeutics.