{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n Histone H3 and its variants—including H3.3, the testis‐specific H3t, and the primate‐restricted H3.X/H3.Y isoforms—play multifaceted roles in regulating chromatin structure and function. In cancer cells, for example, altered methylation of histone H3 and point mutations in H3.3 disrupt normal epigenetic modifications, thereby perturbing gene expression programs and contributing to tumorigenesis."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "3"}]}, {"type": "t", "text": " In addition, mitochondrial dysfunction can lead to increased H3 methylation at regulatory promoters, providing a potential epigenetic signature in certain tumors."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n Tissue‐ and developmental stage–specific variants further extend the functional diversity of H3. The testis‐specific variant H3t is critical for early spermatogenesis, enabling distinct nucleosome assembly through selective chaperone pathways and influencing chromatin organization via its unique post‐translational modification patterns."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "4", "end_ref": "6"}]}, {"type": "t", "text": " Similarly, structural studies of primate-specific H3.Y reveal that its nucleosomes exhibit increased DNA end flexibility—with consequent effects on accessibility by regulatory factors—while DUX4-induced H3.X/H3.Y incorporation fosters a relaxed chromatin state that enhances reactivation of target genes during development."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "7", "end_ref": "9"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n Furthermore, specific modifications on these variants are functionally important. For instance, trimethylation at lysine 27 on H3t (H3tK27me3) is recognized by chromatin regulators such as PHF1, linking the variant to Polycomb-mediated gene silencing mechanisms."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "10"}]}, {"type": "t", "text": " Studies on plant H3 variants also underscore how subtle amino acid changes can dictate deposition patterns and epigenetic landscapes during development."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "11"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n In summary, these studies collectively demonstrate that histone H3 variants are not merely structural components of nucleosomes but are dynamic regulators of chromatin. Their diverse incorporation, modification, and interaction profiles enable them to orchestrate gene expression programs critical for processes ranging from development and cellular differentiation to responses in metabolic stress and tumorigenesis.\n "}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Ana M Cervera, Jean-Pierre Bayley, Peter Devilee, et al. 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