H3.3 K27M depletion increases differentiation and extends latency of diffuse intrinsic pontine glioma growth in vivo

Immune-stimulatory (TK/Flt3L) gene therapy opens the door to a promising new treatment strategy against brainstem gliomas

Diffuse intrinsic pontine glioma (DIPG) is a rare brainstem tumor which carries a dismal prognosis. To date. there are no effective treatments for DIPG. Transcriptomic studies have shown that DIPGs have a distinct profile compared to hemispheric high-grade pediatric gliomas. These specific genomic features coupled with the younger median age group suggest that DIPG is of developmental origin. There is a major unmet need for novel effective therapeutic approaches for DIPG. Clinical and preclinical studies have expanded our understanding of the molecular pathways in this deadly disease. We have developed a genetically engineered brainstem glioma model harboring the recurrent DIPG mutation, activin A receptor type I (ACVR1)-G328V (mACVR1) using the sleeping beauty transposon system. DIPG neurospheres isolated from the genetically engineered mouse model were implanted into the pons of immune-competent mice to assess the therapeutic efficacy and toxicity of immunostimulatory gene therapy using adenoviruses expressing thymidine kinase (TK) and fms-like tyrosine kinase 3 ligand (Flt3L). Immunostimulatory adenoviral-mediated delivery of TK/Flt3L in mice bearing brainstem gliomas resulted in antitumor immunity, recruitment of antitumor-specific T cells, and improved median survival by stimulating the host antitumor immune response. Therapeutic efficacy of the immunostimulatory gene therapy strategy will be tested in the clinical arena in a Phase I clinical trial. We also discuss immunotherapeutic interventions currently being implemented in DIPG patients and discuss the profound therapeutic implications of immunotherapy for this patient populations.

Understanding the epigenetic landscape and cellular architecture of childhood brain tumors

Pediatric brain tumors are the leading cancer-related cause of death in children and adolescents in the United States, affecting on average 1 in 2000 children per year. Recent advances in cancer genomics have led to profound discoveries about the underlying molecular biology and ontogeny of these tumors. In particular, these studies have revealed epigenetic dysregulation to be one of the main hallmarks of pediatric brain tumorigenesis. In this review, we will highlight a number of important recent findings about the nature of this dysregulation in different types of pediatric brain tumors as well as examine their implications for preclinical research and clinical practice. Specifically, we discuss the emergence of methylation signatures as tools for tumor stratification/classification while also highlighting the importance of mutations that directly affect the epigenome and clarifying their impact on risk stratification and pediatric brain tumor biology. We then incorporate recent advances in our understanding of pediatric brain tumor cellular architecture and emphasize the link between epigenetic dysregulation and the "stalled" development seen in many of these malignant neoplasms. Lastly, we explore recentwork investigating the use of these mutated epigenomic regulators as therapeutic targets and extrapolate their utility in overcoming this "stalling" to halt tumor growth.

Epigenetic activation of a RAS/MYC axis in H3.3K27M-driven cancer

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.

Histone H3 at lysine 27 (H3K27M) is often mutated in cancer but its role in tumour initiation is unclear. Here, the authors generated a transgenic model expressing H3.3K27M from the Fabp7 gene promoter, demonstrating that H3.3K27M can initiate diverse tumorigesis on its own, acting through a RAS/MYC transcriptomic programme.

Histone tail analysis reveals H3K36me2 and H4K16ac as epigenetic signatures of diffuse intrinsic pontine glioma

BACKGROUND

Diffuse intrinsic pontine glioma (DIPG) is an aggressive pediatric brainstem tumor. Most DIPGs harbor a histone H3 mutation, which alters histone post-translational modification (PTM) states and transcription. Here, we employed quantitative proteomic analysis to elucidate the impact of the H3.3K27M mutation, as well as radiation and bromodomain inhibition (BRDi) with JQ1, on DIPG PTM profiles.

METHODS

We performed targeted mass spectrometry on H3.3K27M mutant and wild-type tissues (n = 12) and cell lines (n = 7).

RESULTS

We found 29.2 and 26.4% of total H3.3K27 peptides were H3.3K27M in mutant DIPG tumor cell lines and tissue specimens, respectively. Significant differences in modification states were observed in H3.3K27M specimens, including at H3K27, H3K36, and H4K16. In addition, H3.3K27me1 and H4K16ac were the most significantly distinct modifications in H3.3K27M mutant tumors, relative to wild-type. Further, H3.3K36me2 was the most abundant co-occurring modification on the H3.3K27M mutant peptide in DIPG tissue, while H4K16ac was the most acetylated residue. Radiation treatment caused changes in PTM abundance in vitro, including increased H3K9me3. JQ1 treatment resulted in increased mono- and di-methylation of H3.1K27, H3.3K27, H3.3K36 and H4K20 in vitro.

CONCLUSION

Taken together, our findings provide insight into the effects of the H3K27M mutation on histone modification states and response to treatment, and suggest that H3K36me2 and H4K16ac may represent unique tumor epigenetic signatures for targeted DIPG therapy.

Epigenomic Reprogramming as a Driver of Malignant Glioma

Malignant gliomas are central nervous system tumors and remain among the most treatment-resistant cancers. Exome sequencing has revealed significant heterogeneity and important insights into the molecular pathogenesis of gliomas. Mutations in chromatin modifiers-proteins that shape the epigenomic landscape through remodeling and regulation of post-translational modifications on chromatin-are very frequent and often define specific glioma subtypes. This suggests that epigenomic reprogramming may be a fundamental driver of glioma. Here, we describe the key chromatin regulatory pathways disrupted in gliomas, delineating their physiological function and our current understanding of how their dysregulation may contribute to gliomagenesis.

H3K27M in Gliomas Causes a One-Step Decrease in H3K27 Methylation and Reduced Spreading within the Constraints of H3K36 Methylation

The discovery of H3K27M mutations in pediatric gliomas marked a new chapter in cancer epigenomics. Numerous studies have investigated the effect of this mutation on H3K27 trimethylation, but only recently have we started to realize its additional effects on the epigenome. Here, we use isogenic glioma H3K27M^+/− cell lines to investigate H3K27 methylation and its interaction with H3K36 and H3K9 modifications. We describe a “step down” effect of H3K27M on the distribution of H3K27 methylation: me3 is reduced to me2, me2 is reduced to me1, whereas H3K36me2/3 delineates the boundaries for the spread of H3K27me marks. We also observe a replacement of H3K27me2/3 silencing by H3K9me3. Using a computational simulation, we explain our observations by reduced effectiveness of PRC2 and constraints imposed on the deposition of H3K27me by antagonistic H3K36 modifications. Our work further elucidates the effects of H3K27M in gliomas as well as the general principles of deposition in H3K27 methylation.

Harutyunyan et al. use isogenic glioma H3K27M^+/− cell lines to demonstrate the rewiring of the epigenome, specifically H3K27me1/2/3, H3K36me2/3, and H3K9me3. The dynamic deposition of histone marks is simulated by a stochastic model. This work further advances the understanding of the deposition of H3K27 methylation in H3K27M mutant gliomas.

Histone-Mutant Glioma: Molecular Mechanisms, Preclinical Models, and Implications for Therapy

Pediatric high-grade glioma (pHGG) is the leading cause of cancer death in children. Despite histologic similarities, it has recently become apparent that this disease is molecularly distinct from its adult counterpart. Specific hallmark oncogenic histone mutations within pediatric malignant gliomas divide these tumors into subgroups with different neuroanatomic and chronologic predilections. In this review, we will summarize the characteristic molecular alterations of pediatric high-grade gliomas, with a focus on how preclinical models of these alterations have furthered our understanding of their oncogenicity as well as their potential impact on developing targeted therapies for this devastating disease.

Combined treatment with CBP and BET inhibitors reverses inadvertent activation of detrimental super enhancer programs in DIPG cells

Diffuse intrinsic pontine gliomas (DIPG) are the most aggressive brain tumors in children with 5-year survival rates of only 2%. About 85% of all DIPG are characterized by a lysine-to-methionine substitution in histone 3, which leads to global H3K27 hypomethylation accompanied by H3K27 hyperacetylation. Hyperacetylation in DIPG favors the action of the Bromodomain and Extra-Terminal (BET) protein BRD4, and leads to the reprogramming of the enhancer landscape contributing to the activation of DIPG super enhancer-driven oncogenes. The activity of the acetyltransferase CREB-binding protein (CBP) is enhanced by BRD4 and associated with acetylation of nucleosomes at super enhancers (SE). In addition, CBP contributes to transcriptional activation through its function as a scaffold and protein bridge. Monotherapy with either a CBP (ICG-001) or BET inhibitor (JQ1) led to the reduction of tumor-related characteristics. Interestingly, combined treatment induced strong cytotoxic effects in H3.3K27M-mutated DIPG cell lines. RNA sequencing and chromatin immunoprecipitation revealed that these effects were caused by the inactivation of DIPG SE-controlled tumor-related genes. However, single treatment with ICG-001 or JQ1, respectively, led to activation of a subgroup of detrimental super enhancers. Combinatorial treatment reversed the inadvertent activation of these super enhancers and rescued the effect of ICG-001 and JQ1 single treatment on enhancer-driven oncogenes in H3K27M-mutated DIPG, but not in H3 wild-type pedHGG cells. In conclusion, combinatorial treatment with CBP and BET inhibitors is highly efficient in H3K27M-mutant DIPG due to reversal of inadvertent activation of detrimental SE programs in comparison with monotherapy.

Clinicopathological characteristics and survival of spinal cord astrocytomas

Background

Due to their rarity, the clinicopathological characteristics and prognostic factors of spinal cord gliomas are still unclear. Here, we aimed to clarify these issues in a cohort of 108 spinal cord astrocytomas.

Methods

We characterized the clinicopathological characteristics, including 2016 World Health Organization (WHO) grade, age, sex, location, segment length, resection, pre‐ and postsurgery, Modified McCormick Scale (MMS), radio‐ and chemotherapy, and Ki‐67 and H3 K27M mutations, in 108 spinal cord astrocytomas through heatmaps. The Cox regression analysis and Kaplan‐Meier curves were used to study the prognostic value of these clinicopathological features.

Results

There are a total 38 H3 K27M‐mutant tumors, including 31 cases with histological grade II/III tumors. The age of low‐grade astrocytoma patients (WHO grade I/II, n = 54) was significantly younger (27.0 vs 35.5 years, P = .001) than those with high‐grade tumors (WHO grade III/IV, n = 54). All patients underwent surgical resection with neurophysiological monitoring, and the surgery did not result in significant changes in MMS. The presurgery MMS was associated with overall survival in the high‐grade subgroup (P = .008) but not in the low‐grade subgroup (P = .312). While, the high content of resection improved the survival of only patients with low‐grade astrocytomas (P = .016) but not those with high‐grade astrocytomas (P = .475). Both the low‐grade and high‐grade astrocytomas had no obvious benefit from neither adjuvant chemotherapy nor radiotherapy (all P > .05).

Conclusions

We characterized the clinicopathological characteristics and their prognostic values in 108 spinal cord astrocytomas, which could help with evidence‐based management of spinal cord astrocytomas.

Improving long-term survival in diffuse intrinsic pontine glioma

INTRODUCTION

Diffuse intrinsic pontine glioma (DIPG) is an almost universally fatal pediatric brain cancer. There has been no improvement in event-free survival (EFS) or overall survival (OS) despite immense effort through a multitude of clinical trials to find a cure. Recently, there has been a surge in the knowledge of DIPG biology, including the discovery of a recurrent H3F3A mutation in over 80% of these tumors.

AREAS COVERED

The authors review the most recent approaches to diagnosis and treatment of DIPG including chemotherapy, biologics, surgical approaches, and immunotherapy.

EXPERT OPINION

The authors propose four main opportunities to improve long-term survival. First, patients should be enrolled in scientifically sound clinical trials that include molecularly profiling either via stereotactic biopsy or liquid biopsy. Second, clinical trials should include more innovative endpoints other than traditional EFS and OS such as MRI/PET imaging findings combined with surrogates of activity (e.g. serial liquid biopsies) to better ascertain biologically active treatments. Third, innovative clinical trial approaches are needed to help allow for the rapid development of combination therapies to be tested. Finally, effort should be concentrated on reversing the effects of the histone mutation, as this malfunctioning development program seems to be key to DIPG relentlessness.

Hypoxia Inducible Factors' Signaling in Pediatric High-Grade Gliomas: Role, Modelization and Innovative Targeted Approaches

The brain tumor microenvironment has recently become a major challenge in all pediatric cancers, but especially in brain tumors like high-grade gliomas. Hypoxia is one of the extrinsic tumor features that interacts with tumor cells, but also with the blood-brain barrier and all normal brain cells. It is the result of a dramatic proliferation and expansion of tumor cells that deprive the tissues of oxygen inflow. However, cancer cells, especially tumor stem cells, can endure extreme hypoxic conditions by rescheduling various genes' expression involved in cell proliferation, metabolism and angiogenesis and thus, promote tumor expansion, therapeutic resistance and metabolic adaptation. This cellular adaptation implies Hypoxia-Inducible Factors (HIF), namely HIF-1α and HIF-2α. In pediatric high-grade gliomas (pHGGs), several questions remained open on hypoxia-specific role in normal brain during gliomagenesis and pHGG progression, as well how to model it in preclinical studies and how it might be counteracted with targeted therapies. Therefore, this review aims to gather various data about this key extrinsic tumor factor in pHGGs.

Mutant ACVR1 Arrests Glial Cell Differentiation to Drive Tumorigenesis in Pediatric Gliomas

Diffuse intrinsic pontine gliomas (DIPGs) are aggressive pediatric brain tumors for which there is currently no effective treatment. Some of these tumors combine gain-of-function mutations in ACVR1, PIK3CA, and histone H3-encoding genes. The oncogenic mechanisms of action of ACVR1 mutations are currently unknown. Using mouse models, we demonstrate that Acvr1G328V arrests the differentiation of oligodendroglial lineage cells, and cooperates with Hist1h3bK27M and Pik3caH1047R to generate high-grade diffuse gliomas. Mechanistically, Acvr1G328V upregulates transcription factors which control differentiation and DIPG cell fitness. Furthermore, we characterize E6201 as a dual inhibitor of ACVR1 and MEK1/2, and demonstrate its efficacy toward tumor cells in vivo. Collectively, our results describe an oncogenic mechanism of action for ACVR1 mutations, and suggest therapeutic strategies for DIPGs.

Combined Therapy of AXL and HDAC Inhibition Reverses Mesenchymal Transition in Diffuse Intrinsic Pontine Glioma

PURPOSE

Diffuse intrinsic pontine glioma (DIPG) is an incurable type of pediatric brain cancer, which in the majority of cases is driven by mutations in genes encoding histone 3 (H3K27M). We here determined the preclinical therapeutic potential of combined AXL and HDAC inhibition in these tumors to reverse their mesenchymal, therapy-resistant, phenotype.

EXPERIMENTAL DESIGN

We used public databases and patient-derived DIPG cells to identify putative drivers of the mesenchymal transition in these tumors. Patient-derived neurospheres, xenografts, and allografts were used to determine the therapeutic potential of combined AXL/HDAC inhibition for the treatment of DIPG.

RESULTS

We identified AXL as a therapeutic target and regulator of the mesenchymal transition in DIPG. Combined AXL and HDAC inhibition had a synergistic and selective antitumor effect on H3K27M DIPG cells. Treatment of DIPG cells with the AXL inhibitor BGB324 and the HDAC inhibitor panobinostat resulted in a decreased expression of mesenchymal and stem cell genes. Moreover, this combination treatment decreased expression of DNA damage repair genes in DIPG cells, strongly sensitizing them to radiation. Pharmacokinetic studies showed that BGB324, like panobinostat, crosses the blood-brain barrier. Consequently, treatment of patient-derived DIPG xenograft and murine DIPG allograft-bearing mice with BGB324 and panobinostat resulted in a synergistic antitumor effect and prolonged survival.

CONCLUSIONS

Combined inhibition of AXL and HDACs in DIPG cells results in a synergistic antitumor effect by reversing their mesenchymal, stem cell-like, therapy-resistant phenotype. As such, this treatment combination may serve as part of a future multimodal therapeutic strategy for DIPG.

Epigenetic reprogramming and chromatin accessibility in pediatric diffuse intrinsic pontine gliomas: a neural developmental disease

Diffuse intrinsic pontine glioma (DIPG) is a rare but deadly pediatric brainstem tumor. To date, there is no effective therapy for DIPG. Transcriptomic analyses have revealed DIPGs have a distinct profile from other pediatric high-grade gliomas occurring in the cerebral hemispheres. These unique genomic characteristics coupled with the younger median age group suggest that DIPG has a developmental origin. The most frequent mutation in DIPG is a lysine to methionine (K27M) mutation that occurs on H3F3A and HIST1H3B/C, genes encoding histone variants. The K27M mutation disrupts methylation by polycomb repressive complex 2 on histone H3 at lysine 27, leading to global hypomethylation. Histone 3 lysine 27 trimethylation is an important developmental regulator controlling gene expression. This review discusses the developmental and epigenetic mechanisms driving disease progression in DIPG, as well as the profound therapeutic implications of epigenetic programming.

The growing role of epigenetics in childhood cancers

PURPOSE OF REVIEW

Altered epigenetics is central to oncogenesis in many pediatric cancers. Aberrant epigenetic states are induced by mutations in histones or epigenetic regulatory genes, aberrant expression of genes regulating chromatin complexes, altered DNA methylation patterns, or dysregulated expression of noncoding RNAs. Developmental contexts of dysregulated epigenetic states are equally important for initiation and progression of many childhood cancers. As an improved understanding of disease-specific roles and molecular consequences of epigenetic alterations in oncogenesis is emerging, targeting these mechanisms of disease in childhood cancers is increasingly becoming important.

RECENT FINDINGS

In addition to disease-causing epigenetic events, DNA methylation patterns and specific oncohistone mutations are being utilized for the diagnosis of pediatric central nervous system (CNS) and solid tumors. These discoveries have improved the classification of poorly differentiated tumors and laid the foundation for future improved clinical management. On the therapeutic side, the first therapies targeting epigenetic alterations have recently entered clinical trials. Current clinical trials include pharmacological inhibition of histone and DNA modifiers in aggressive types of pediatric cancer.

SUMMARY

Targeting novel epigenetic vulnerabilities, either by themselves, or coupled with targeting altered transcriptional states, developmental cell states or immunomodulation will result in innovative approaches for treating deadly pediatric cancers.

Invited Review: Emerging functions of histone H3 mutations in paediatric diffuse high-grade gliomas

Paediatric diffuse high-grade gliomas (pHGG) are rare, but deadly tumours. The discovery of recurrent mutations in the tail of histone H3, changing lysine 27 to methionine, or glycine 34 to arginine or valine, has illuminated a critical role for epigenetic dysregulation in the aetiology of childhood gliomas and opened new avenues of exploration that have resulted in numerous advances for the field. In this review, we describe the current models of H3K27M mutant cancer that are available to the research community and the insights they have provided on tumour biology and the epigenetic and transcriptional effects of histone mutations. We also review the current understanding of the H3G34R/V mutation and the therapeutic outlook for the treatment of pHGG.

Forty-five patient-derived xenografts capture the clinical and biological heterogeneity of Wilms tumor

The lack of model systems has limited the preclinical discovery and testing of therapies for Wilms tumor (WT) patients who have poor outcomes. Herein, we establish 45 heterotopic WT patient-derived xenografts (WTPDX) in CB17 scid^-/- mice that capture the biological heterogeneity of Wilms tumor (WT). Among these 45 total WTPDX, 6 from patients with diffuse anaplastic tumors, 9 from patients who experienced disease relapse, and 13 from patients with bilateral disease are included. Early passage WTPDX show evidence of clonal selection, clonal evolution and enrichment of blastemal gene expression. Favorable histology WTPDX are sensitive, whereas unfavorable histology WTPDX are resistant to conventional chemotherapy with vincristine, actinomycin-D, and doxorubicin given singly or in combination. This WTPDX library is a unique scientific resource that retains the spectrum of biological heterogeneity present in WT and provides an essential tool to test targeted therapies for WT patient groups with poor outcomes.

The progress in pre-clinical drug discovery for Wilms tumor (WT) is limited by a lack of disease models. Here, the authors develop 45 heterotopic WT patient-derived xenografts including several anaplastic models that recapitulate the biological heterogeneity of WT, and propose this as a resource for evaluating future therapeutics for WT.

Oncohistone Mutations in Diffuse Intrinsic Pontine Glioma

Diffuse intrinsic pontine glioma (DIPG) is a lethal pediatric tumor with no currently available treatment options. More than 60-70% DIPG tumors harbor heterozygous mutations at genes encoding histone H3 proteins that replace lysine 27 with methionine (K27M). In this review, we discuss how K27M mutation reprograms the cancer epigenome to lead to tumorigenesis, and highlight potential drug targets and therapeutic agents for DIPG.

Liquid biopsy for diffuse intrinsic pontine glioma: an update

Diffuse intrinsic pontine glioma (DIPG), otherwise known as diffuse midline glioma with H3K27M mutation, is a devastating brainstem glioma without a cure. Efforts are currently underway to better optimize molecular diagnoses through biological sampling, which today remains largely limited to surgical biopsy sampling. Surgical intervention is not without its risks, and therefore a preference remains for a less invasive modality that can provide biological information about the tumor. There is emerging evidence to suggest that a liquid biopsy, targeting biofluids such as CSF and blood plasma, presents an attractive alternative for brain tumors in general. In this update, the authors provide a summary of the progress made to date regarding the use of liquid biopsy to diagnose and monitor DIPG, and they also propose future development and applications of this technique moving forward, given its unique histone biology.

CATACOMB: An endogenous inducible gene that antagonizes H3K27 methylation activity of Polycomb repressive complex 2 via an H3K27M-like mechanism

Using biochemical characterization of fusion proteins associated with endometrial stromal sarcoma, we identified JAZF1 as a new subunit of the NuA4 acetyltransferase complex and CXORF67 as a subunit of the Polycomb Repressive Complex 2 (PRC2). Since CXORF67's interaction with PRC2 leads to decreased PRC2-dependent H3K27me2/3 deposition, we propose a new name for this gene: CATACOMB (catalytic antagonist of Polycomb; official gene name: EZHIP ). We map CATACOMB's inhibitory function to a short highly conserved region and identify a single methionine residue essential for diminution of H3K27me2/3 levels. Remarkably, the amino acid sequence surrounding this critical methionine resembles the oncogenic histone H3 Lys27-to-methionine (H3K27M) mutation found in high-grade pediatric gliomas. As CATACOMB expression is regulated through DNA methylation/demethylation, we propose CATACOMB as the potential interlocutor between DNA methylation and PRC2 activity. We raise the possibility that similar regulatory mechanisms could exist for other methyltransferase complexes such as Trithorax/COMPASS.

Histone H3 Mutations: An Updated View of Their Role in Chromatin Deregulation and Cancer

In this review, we describe the attributes of histone H3 mutants identified in cancer. H3 mutants were first identified in genes encoding H3.3, in pediatric high-grade glioma, and subsequently in chondrosarcomas and giant cell tumors of bone (GCTB) in adolescents. The most heavily studied are the lysine to methionine mutants K27M and K36M, which perturb the target site for specific lysine methyltransferases and dominantly perturb methylation of corresponding lysines in other histone H3 proteins. We discuss recent progress in defining the consequences of these mutations on chromatin, including a newly emerging view of the central importance of the disruption of H3K36 modification in many distinct K to M histone mutant cancers. We also review new work exploring the role of H3.3 G34 mutants identified in pediatric glioma and GCTB. G34 is not itself post-translationally modified, but G34 mutation impinges on the modification of H3K36. Here, we ask if G34R mutation generates a new site for methylation on the histone tail. Finally, we consider evidence indicating that histone mutations might be more widespread in cancer than previously thought, and if the perceived bias towards mutation of H3.3 is real or reflects the biology of tumors in which the histone mutants were first identified.