Oncohistones in Cancer: A Narrative Review of Molecular Mechanisms and Targeted Therapeutic Strategies

Oncohistones in Cancer: A Narrative Review of Molecular Mechanisms and Targeted Therapeutic Strategies Ayoob Radhi Al-Zaalan  Department of Medical Lab Technology, College of Health and Medical Technology, Southern Technical University, Basrah, Iraq Hussam Saadi Aziz  Department of Medical Lab Technology, College of Health and Medical Technology, Southern Technical University, Basrah, Iraq Abstract Oncohistones represent a distinct class of cancer-driving mutations affecting core histone proteins, particularly histone H3 variants. Unlike conventional oncogenes and tumor suppressors, these single amino acid substitutions exert their oncogenic effects through widespread disruption of chromatin architecture and epigenetic regulation. This narrative review examines the molecular mechanisms underlying oncohistone function, with particular focus on H3K27M mutations found in pediatric diffuse midline gliomas and H3K36M mutations characteristic of chondroblastomas. We explore how these mutations interfere with normal chromatin-modifying enzymes, including polycomb repressive complex 2 (PRC2) and SETD2 methyltransferase, leading to aberrant gene expression patterns that promote tumorigenesis. The review discusses the clinical significance of oncohistone mutations in cancer diagnosis and classification, highlighting their integration into current diagnostic frameworks. Additionally, we examine emerging therapeutic approaches targeting oncohistone-driven cancers, including epigenetic modulators, immunotherapeutic strategies, and novel synthetic lethality approaches. Finally, we address current challenges in the field, including therapeutic resistance mechanisms and the complexity of targeting global epigenetic alterations, while identifying key areas for future research to advance our understanding and treatment of oncohistone-driven malignancies. Introduction Cancer research has continually come to the realization that tumorigenesis is not just limited to genetic mutation, but includes massive changes in the regulation of epigenetics. These architectural proteins, known as histones, are dynamic controllers of gene expression as opposed to inactive DNA packaging scaffold. This control is by various post-translational modifications (PTM) such as methylation, acetylation, phosphorylation, and ubiquitination [1]. These alterations interact with each other in a complex network (Collectively called the histone code), which determines the accessibility of chromatin and the active state of transcription by binding particular protein complexes that identify, set, or interfere with these epigenetic modifications [2]. The identification of oncohistones is a new breakthrough in cancer biology. Such mutant forms of the core histone proteins are due to somatic point mutations in histone genes that include H3F3A, HIST1H3B, and HIST1H3C [3]. Contrary to classical oncogenes or tumor suppressors, oncohistones have dominant-negative or neomorphic histone functions, with just one mutant allele capable of disrupting histone function, causing systemic epigenomic ep airplays, which induce tumorigenesis [4]. Such mutation induces changes to the fragile state of epigenetic equilibrium leading to cancer hallmarks such More Information How to cite this article: Al-Zaalan AR, Aziz HS. Oncohistones in Cancer: A Narrative Review of Molecular Mechanisms and Targeted Therapeutic Strategies. Eur J Med Health Res, 2025;3(6):117. DOI: 10.59324/ejmhr.2025.3(6).02 Keywords: Oncohistones, Histone Mutations, H3K27M, H3K36M, Chromatin regulation, Cancer Epigenetics, Targeted Therapy, PRC2 Complex. This work is licensed under a Creative Commons Attribution 4.0 International License. The license permits unrestricted use, distribution, and reproduction in any medium, on the condition that users give exact credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if they made any changes. EUR J MED HEALTH RES Volume 3 | Number 6 | 2025 12 as oncogenes activation or tumor suppressor gene suppression [5]. Oncohistone mutations have been detailed in many malignancies; and have especially high frequencies in childhood and adolescent cancers, and in many cases, oncohistone mutations are associated with specific disease events [6]. Histone mutations are unique because of how they impact hundreds or thousands of genes at once to influence a global impact on transcriptional networks, as compared to conventional mutations. This pan-drug action creates potential appealing treatment opportunities since in a single oncohistone-loaded miscarriage, the potential practicality of restoring a normal level of activity in various other mechanisms would become feasible [7]. It is an evolving region and it follows that there is a need to synthesize recent discovery and clinical implications. It is a literature review, which provides an in depth examination of the tidings already acquired about what has been known on oncohistones, beginning with the working mechanisms to clinical application of oncohistones. We combine existing knowledge with recent findings about the effects of oncohistone on the biology of surveillance and heterogeneity of tumors, and the tumor microenvironment. The discussion of emerging therapeutic interventions, such as immunotherapies and epigenetic drugs is discussed, and problems of metacognition therapeutics remain in the modern world to be utilized and challenges and opportunities exist. The review is profoundly holistic regarding the impact of oncohistone mutations on the changes in cancer pathogenesis and therapeutic evolution development [8,9]. Molecular Foundations of Oncohistones Normal Histone Biology and Chromatin Organization It is on this basis that we now come to the molecular basis of functions of histones. Histones are strongly preserved basic proteins, which bundle DNA into nucleosomes, which form repeating packages of chromatid. Each nucleosome has its own octameric core of two copies of histones H2A, H2B, H3 and H4, around which there are approximately wrapped 147 base pairs of DNA [10]. The histone code is made up of post-translational modifications, including methylation, acetylation and phosphorylation, a system of complex regulation that regulates chromatin availability and expression of genes [11]. To illustrate, Polyamination of H3K27 by the Polycomb Repressive Complex 2 (PRC2) and H3K27me3 is a repressive mark related to the silencing and retention of identity in cells under development [7]. On the other hand, the histone H3 lysine 36 tri-methinetation (H3K36me3) on active gene bodies is produced and deposited by biomolecule Methyltransferase SETD2 and has critical roles in transcriptional elongation, DNA repair and mRNA splicing [12]. Definition and Classification of Oncohistones Oncophistones represent the category of histone variants with the mutations of the common skips related to the onset of cancer spreading either by a dominant negative or neomorphic mark of the disruption of the regular chromatin regulation [3]. H3K27M (lysine-to-methionine substitution site 27) H3G34R/V H3K36M are the most widely characterised oncohistones and have patterns of tissue specificity and EU1516 and mechanics [4]. Principal Mutations and Their Molecular Mechanisms H3K27M: Disruption of Polycomb-Mediated Gene Silencing H3K27M mutation is a mutation placing methionine instead of lysine in the 27th position of histone named H3 and forms dominant-negative inhibitor of Polycomb Repressive Complex 2 (PRC2) that normally performs H3K27 trims eathylation [13]. The H3K27M peptide offers a possible competitive inhibitor of PRC2 activity and binds with high affinity to the EZH2 subunit, and does not permit it to methylate wild-type H3K27 residues throughout the genome [14]. The outcome of this process leads to depletion of the repressive (H3K27me3) mark globally which ultimately leads to overt undesirable activity of suppressed developmental genes. H3G34R/V: Perturbation of H3K36 Methylation and DNA Repair The H3F3A gene H3G34R that has undergone a mutation due to the emergence of a particular amino acid, arginine (H3G34R) or valine (H3G34 V) is mutation of these mechanisms [15]. In contrast to the transacting mode of action of H3K27M, these mutations act via cis-actions, changing the local chromatin environment and also the deposition of H3K36 and H3K27 methylation tags. Recent works have shown that mutations of H3G34 are promotors of genomic instability through breaking the processes of DNA repair mechanisms and disrupting the process of normal chromatin response to DNA damage [4]. H3K36M: Inhibition of SETD2 and Transcriptional Dysregulation H3K36M mutation is a special type of oncohistone which acts as a dominant-negative inhibitor of the SETD2 which is the methyltransferase activity of H3K36. The mutation occurs in around 95 percent of chondroblastomas and in a specific number of soft tissue sarcomas, making it one of the most tissuespecific oncohistone mutations as yet known [8]. The H3K36M mutation functions by adenosine diphosphon (AD) activity whereby it captures SETD2 and H3K36 wild-type amid its entire genome transcriptional elongation and DNA repair of DNA repair processes. Common and Divergent Mechanisms Even though all oncohistones change the control of epigenetics, they act in rather different ways. The EUR J MED HEALTH RES Volume 3 | Number 6 | 2025 13 mutation types H3K27M and H3K36M are trans-acting, i.e. they also globally remodell methylation proposals all over the genome, whereas the mutations in H3G34 are cis-acting and the influences are more locally centered in the regalations in the chromatin state and gene expression. These mechanistic disparities are likely to be involved in generating the mechanisms of such unique differentiating tissue specificities, and oncogenic outcomes, which is important to bear in mind in the context of certain cellular system characteristics under which they establish themselves [4]. Role of Oncohistones in Tumorigenesis Impact on Oncogenic Transcription and Gene Expression Networks Taking into consideration such molecular processes, now we have time to speak about their specific role in cancer. Transcriptional mutations based on H3K27M mutations are particularly dramatic because of all the effects they have on Polycomb-mediated silencing. H3K27M mutants lead to global reductions in the concentration of H3K27me3 and unsuitable release of repressed genes and, consequently, the overall transcriptional landscape, which eventually favors tumour formation [13]. Stunningly PRC2 complexes are trapped on the untapped promoters and not on the activated promoters, which intensifies wonky marks which assists oncogenic gene expression projects. Cancer Stem Cells and Cellular Differentiation Defects H3K27M mutations have also been shown to promote the activation of transcriptional networks, typically linked to neural stem cells and embryonic development including those regulated by key pluripotent transcription factors such as SOX2, NANOG and OCT4 [16,17] In particular, it was revealed that specifically the quantification of OCT4 transcripts in glioblastoma versus controls showed an increase by a factor of 14.77, and SOX2 expression signatures are a constant in such tumors and this fact contributes to the embryonic nature of glithero astropodes [17]. It is triggered by epigenetic reprogramming of cells into cells projected to self-renew and resistance to apoptosis, which are crucial features of cancer stem cells [18,19]. Recent studies had shown that H3K27M oncohistones promote maintenance of stem-like states via particular molecular axes. In their study, discovered that a loop of H3.3K27M/CREB5/ID1 maintains the diffuse intrinsic pontine glioma (DIPG) cells in the stem-like state, which promotes malignancy [20]. The researchers demonstrated that H3.3K27M has a direct positive effect of CREB5 through altering the H3K27me3 landscape, especially at super-enhancers regions, to control levels of ID1 to preserve stemness qualities. It is a mechanism by which H3K27M mutations help keep cells in their underdeveloped condition of the existing condition before they have been further differentiated [16]. Maintaining stem like characteristics in variousiated cell types is a cardinal strategy through which oncohistones play a role in tumorigenesis and therapy resistance. It has also been shown that only tumor xenografts can be produced by oligodendrocyte precursor cell (OPC) like tumor cells with stem cell features, thus indicating an inverse relationship between differentiation and tumorigenicity. Moreover, ability to increase proliferation and multipotency properties of the cancer cell (typical of cancer stem cell) caused by the dysbalance of ID proteins, that would stop differentiation and favor stemness by occupying differentiation-related transcription factors, gives the cancer cell increased proliferation and multipotency attributes [20]. Interaction with the Tumor Microenvironment More recent findings have shown that oncohistones are very important in regulating the microenvironment of the tumor, affecting how immune cells infiltrate into the tumor and immunotherapy outcomes. Andrade et al., showed that both H3.3K27M and G34R/V-mutant glioma are highly infiltrative with different myeloid populations and high levels of immune checkpoint markers and a dearth of lymphoid cells [21]. Tumors with oncohistone mutations undergo the altered chromatin environment to influence the expression levels of the immune-related genes, providing not only a potential area of application of an immunotherapeutic solution but a part of the immune evasion arsenal [22]. These myeloid populations tremendously interact with H3-mutant tumor cells, eliciting immunosuppression and tumor maintenance [21]. Notably, jointly suppressing myeloid-cells, as well as, immune checkpoint signaling, had important therapeutic value in preclinical models. That has already resulted in the first-in-human H3K27Mtargeted vaccine (H3K27M-vac) of eight adult patients with progressive diffuse midline glioma showing immunotherapy against mutations has a promising future because of its potential [23]. Clinical Applications and Therapeutic Strategies Oncohistones as Biomarkers for Diagnosis and Prognosis H3K27-altered status is now included among the diagnostic criteria of the world health organization in 2021 (WHO) to classify tumors of the central nervous system, and it made a new diagnostic pattern of diffuse midline gliomas which replaced former histology-based classifications [24]. This type of molecular classification has facilitated better diagnostic accuracy and has significant prognostic consequences because H3K27Mmutant tumors usually pursue aggressive phenotypes and have worse outcomes than their wild-type counterparts [25]. In H3K27M-mutant diffuse midline gliomas, the PDGFRA amplifications, chromosome 17p loss and intricate chromosomal patterns have been EUR J MED HEALTH RES Volume 3 | Number 6 | 2025 14 identified as specific prognostic markers that additionally stratify patient outcomes and inform the treatment decisions [26]. However, overall, the median survival of H3K27M-mutant diffuse midline glioma is estimated as 9-12 months after the diagnosis, significantly shorter than that of H3-wild type tumours [27]. Targeted Therapeutic Strategies EZH2 Inhibitors and Polycomb-Targeted Therapies The clinically most advanced inhibitor of EZH2 is tazemetostat (EPZ-6438) that was approved by the FDA as Restoration medicine against epithelioid sarcoma and follicular lymphoma. Tazemetostat has previously participated in preclinical 133 trials of a variety of types of cancer, such as H3K27M-mutated tumors, but mutation-permissive and mutation-forbidden interplay may differ [28]. It is paradox to observe how EZH2 inhibitors are used in cancers with already impaired PRC2 activity due to H3K27M in an attempt to impair oncohistones in their target. Immunotherapeutic Approaches The most promising line of immunotherapeutic treatment of H3K27M-mutant diffuse midline gliomas is CAR-T cell therapy using GD2. The initial clinical experience of GD2-CAR T cells in four patients with H3K27M-mutated DIPG or spinal cord diffuse midline glioma reported by Majzner et al., showed feasibility and initial evidence of efficacy [29]. Recent findings of this continuing Stanford trial indicate impressive results; 9 out of 11 patients were improved in neurology with 7 having tumor reduction and some of them having a dramatic response [30]. The mode of treatment is repeated infusions; the first intravenous injection is done and the subsequent can occur in the presence of an Ommaya reservoir which helps bypass the bloodbrain barrier restriction and fix systemic inflammation [29]. The median patient survival time is 20.6 months after the diagnosis, one patient recorded 100% loss of the tumor, and despite being over 30 months since enrollment, he has no recurrence of the cancer [30]. These have been the first reported results on a successful treatment of a type of tumor that is nearly one hundred percent fatal and has an average survival of less than one year once diagnosed. Novel Therapeutic Targets: EZHIP and Synthetic Lethality The new smooth has been introduced by the discovery of EZHIP (Enhancer of Zeste Homologs Inhibitory Protein) that could recreate the effects of H3K27M mutations, and it marked a new therapeutic opportunity. Jain et al., showed that EZHIP has a conserved sequence that directly interacts with the active site of EZH2 in a process similar to H3K27M oncahistone [31], resulting in comparable chromatin profiles of loss of broad domains of H3K27me3 and the retention of H3K27me3 at CpG Island sites. EZHIPpositive tumors and, most notably, posterior fossa type A (PFA) ependymomas share comparable epigenetic changes with tumors harboring H3K27M mutations and therefore, pharmacological EZHIP or its downstream inhibition may offer alternative lines of therapeutic intervention to those without H3K27M mutations. Oncohistone mutations are also being pursued, which now is being actively studied using synthetic lethality strategies that take advantage of vulnerabilities induced by oncohistone mutations. Ngoi et al., pointed out that synthetic lethal interactions present therapeutic potential in cancers with mutations in pathways that would no longer be regarded as being undruggable [32]. These strategies specifically target DNA repair programs and metabolic outputs, in which the perturbed chromatin setup of oncohistone-mutant tumours presents an exact set of robotable liabilities to be capitalized on clinically by means of targeted suppression of the restorative pathways. Challenges and Future Directions Current Research Gaps and Limitations Many important gaps are still left in the study of the biology of oncohistone and the preparation of effective treatment. Although the various oncohistone mutations greatly prefer one or another tissue type, the cellular processes involved in this tissue specificity are still not well understood [33]. The cellular address where these mutations occur such as the stage of development and the lineage of the impacted cells has an essential impact on the possibility of the mutation having oncogenic capabilities, as has been demonstrated in the study by Nagarajan et al., which demonstrated that an identical H3K27M mutation exhibited specific patterns of oncogenic reprogramming in response to histone variant and cell context [34]. The gaps in studies are an incomplete knowledge of the mechanism of lysine-to-methionine inhibition of cognate methyltransferases, structural and biochemical studies present conflicting evidence regarding the exact mechanism of enzyme inhibition [33]. Besides, although the reversible aspect of oncohistone effects has been revealed in experimental knockout studies in which the normal histone methylation patterns can be restored, it is difficult to translate these observations into practicable therapies. Signs of regional and developmental specificity in these mutations act to indicate that some type of cellular environment is more tolerant of epigenetic insults resulting in cancer but the factors dictating such tolerance are yet to be understood. Technical and Methodological Challenges Effective therapeutic approaches to oncohistoneinduced cancers have a number of technical challenges. It is impossible to place specific therapeutic targets (i.e., particular cells, skate erythropoietin) without impairing proper cellular functions, which is why it is hard to EUR J MED HEALTH RES Volume 3 | Number 6 | 2025 15 discover the actual globality of epigenetic alterations caused by these mutations [35]. As identified by Gold and Shilatifard, there are complex, and clinical barriers to epigenetic therapies involving the histone lysine methylation, portions of which include drug resistance related to somatic mutations, possible side effects, including bone marrow disorders; as well as limitations in accessing and delivering the drug or drug action [36]. Moreover, the complexity of oncohistone-mutant tumors, including those found in a single patient and in various types of cancers, makes it difficult to generate universal methods of therapy. As pointed out by Mitchener and Muir, histone mutants, despite a low dosage, can corrupt chromatin states, which are hard to predict and treat [35]. It is further complicated by the observation that oncohistones constitute just 5-10 percent of the total histone pool but have prominent global effects on chromatin organization so it has remained challenging to design interventions sufficiently powerful to override their impact without interfering with normal epigenetic processes. Future Research Directions To advance oncohistone-targeted therapies, future studies also ought to emphasize some of the key areas. To begin with, it is of importance to delineate the mechanisms of tissue specificity to gain information about why some oncohistone mutations are selectively produced in particular cell types and situations during development. Second, combination therapy aimed at multiple pathways at once can be potentially useful, which Morel et al., also emphasized, cautioning that epigenetic drugs have potentially significant roles in combinations with other anticancer therapies such as chemotherapy, radiation therapy, and immunotherapy [37]. Third, strategies to overcome therapeutic resistance in this model should be designed, in line with inferences by Yamagishi et al., who reported that with many years of consistent anti-PRC2 therapy, the appearance of therapeutically resistant clones begins to appear, either as results of mutations of the TET2 and DNMT3A proteins [38]. A combination of superior genomic tools, such as single-cell sequencing and spatial transcriptomics applications, will be essential to determine the cellular heterogeneity of -ontic histonemutant tumors and distinguish subpopulations, which have different metabolic and gene-translation profiles, and which determine predictability to any therapeutic intervention. Implications for Personalized Medicine One of the goals of the field is the development of personalized medicine methods conditional on certain oncohistone profiles. Epigenetic biomarkers, as stressed by Kronfol et al., represent a completely different type of characteristics of cells and processes, and the developmental background when the genotype information cannot encompass them due to their values being unique [39]. Patterns of epigenetics can be programmed to reveal what occurred to you and just to you, perhaps permitting indeed a calibration of medication program and forecasting of therapeutic achievement. This will suggest the creation of systems of universal molecular profiling that can determine the delicate systems of epigenetic history resulted by various mutations in oncohistones, the design of predictive biomarkers of treatment outcomes, and adaptable clinical research strategies. Hau and Fruehwald, found that goal holder trials are in a promising model with rare cancers, as the multitude of potentially targeted can be simultaneous using an adaptive design in which the possible heterogeneity of oncohistone-dependent cancers can be addressed [40]. With the help of such platforms, it is possible to reach the maximum amount of knowledge in comparison with the comparatively few patients and to reduce costs, as well as accelerate the process of finding practical treatment options. Conclusion and Recommendations Oncohistone research has identified the way in which tumorigenesis is propagated in tumours by a single amino acid replacement in central histone proteins to bring about universal epigenomic restructuring and reconfiguration in core histones to drive tumorigenesis. H3K27M mutations play a dominant-negative role against the PRC2 activity that results in the global loss of H3K27me3 and developmental transcription factor hyperactivity in pediatric diffuse midline gliomas. Chondroblastoma-specific H3K36M mutations are competitive binding alternatives which sequester SETD2 on a genome-wide scale. The effect has been immensely clinical, oncohistone status is now characterized as a part of the 2021 WHO tumor classification, and molecular diagnostic tests have become standard of care. 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