{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n Histone H3 variants—including the replication‐dependent form (H3.1, encoded by genes within the HIST1 cluster, sometimes referred to using nomenclature such as “H3C11”) and the replication‐independent form H3.3—are essential components of nucleosomes that not only package DNA but also impart regulatory information to the chromatin. Several of the abstracts demonstrate that these variants play distinct roles in cellular function. For instance, H3.3 is deposited independent of DNA replication by specialized chaperones such as DAXX and ATRX, and it marks transcriptionally active chromatin; mutations in H3.3 (for example, K27M or G34V) lead to widespread disruption of epigenetic marks, notably inhibiting polycomb repressive complex 2 (PRC2)–mediated trimethylation of lysine 27, thereby reprogramming gene expression in pediatric high‐grade gliomas and diffuse intrinsic pontine gliomas (DIPG)."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "3"}]}, {"type": "t", "text": " \n In contrast, histone H3.1—typically incorporated during S phase—is associated with different chaperone complexes and, when mutated (as seen with K27M substitutions in certain tumors), defines a separate molecular subgroup with a mesenchymal/astrocytic phenotype and distinct clinical outcomes."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "4"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Beyond their deposition and incorporation into chromatin, these histone H3 variants serve as platforms for post‐translational modifications that regulate gene expression, cell identity, and genome stability. In the context of oncogenesis, mutant forms of H3 disrupt normal epigenetic programming by altering the balance between activating and repressive marks. For example, the K27M mutation directly impairs the activity of EZH2—the catalytic subunit of PRC2—resulting in a global reduction of the repressive H3K27me3 mark while allowing focal retention at specific loci that may silence tumor suppressor genes."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "6"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Moreover, tumor type specificity has been observed with different H3 variant mutations: chondroblastomas predominantly harbor H3.3 mutations encoded by H3F3B, whereas giant cell tumors of bone display alterations in H3F3A."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "7"}]}, {"type": "t", "text": " Together with the findings that proper deposition of H3.3 by the DAXX/ATRX complex is integral to maintaining active chromatin states"}, {"type": "fg", "children": [{"type": "fg_f", "ref": "8"}]}, {"type": "t", "text": ", these studies underline that histone H3 variants are not merely structural components but dynamic regulators of chromatin state whose mutation drives distinct, clinically relevant oncogenic programs.\n "}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Dong-Anh Khuong-Quang, Pawel Buczkowicz, Patricia Rakopoulos, et al. 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