{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n Histone H4 modifications serve as a central hub for regulating chromatin structure, DNA repair, transcription, and genome stability. For instance, acetylation of histone H4 at lysine 16 (H4‐K16Ac) directly weakens internucleosomal contacts, thereby impeding formation of higher-order chromatin fibers and modulating the engagement of ATP‐dependent remodeling factors – a mechanism that impacts both chromatin compaction and the accessibility of repair factors."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": " In parallel, methylation marks on the H4 tail—such as dimethylation of lysine 20 (H4‐K20me2)—provide a binding platform for repair proteins like 53BP1 whose tandem Tudor domains selectively recognize this mark, ensuring proper targeting of the DNA damage response machinery."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "2"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Enzymes that modulate these H4 marks further highlight their functional importance. For example, the NAD⁺‐dependent deacetylase SIRT2 shows a strong preference for H4‐K16Ac, and its activity during the G2/M transition is essential for promoting chromatin condensation prior to mitosis."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": " In a related context, the acetyltransferase Tip60 influences DNA repair pathway choice by acetylating histone H4 adjacent to repressive methyl marks, thereby modulating the competitive binding between repair factors such as BRCA1 and 53BP1 at double‐strand breaks."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "4"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Beyond these roles in altering chromatin accessibility and repair, the H4 histone also participates in the assembly and maintenance of specialized chromatin domains. In centromeric chromatin, for instance, a unique heterotetramer composed of the centromere‐specific histone CENP‑A and H4 forms a structurally rigid unit that is critical for accurate kinetochore assembly and faithful chromosome segregation"}, {"type": "fg", "children": [{"type": "fg_f", "ref": "5"}]}, {"type": "t", "text": ";."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "6"}]}, {"type": "t", "text": " Furthermore, recognition of specific diacetylated forms of histone H4 by specialized bromodomains—such as that of BRD4 or its testis‐specific homologues—demonstrates how combinatorial modifications on a single H4 tail can serve as a composite epigenetic signal to direct transcriptional programs during cell differentiation."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "7"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Finally, changes in H4 acetylation have been linked to cellular metabolic states and developmental programs. In muscle stem cells, for example, metabolic reprogramming that reduces NAD⁺ levels lowers SIRT1 activity, leading to elevated H4‑K16 acetylation and concomitant activation of myogenic genes, thereby illustrating how H4 modifications integrate environmental cues with epigenetic regulation."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "8"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n "}, {"type": "p", "children": [{"type": "t", "text": "\n Collectively, these studies underscore a multifaceted role for histone H4 modifications—acting as modular signals that govern chromatin architecture, direct DNA repair factor recruitment, and influence transcriptional outcomes—to maintain genomic stability and coordinate cell cycle progression.\n "}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Michael Shogren-Knaak, Haruhiko Ishii, Jian-Min Sun, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Histone H4-K16 acetylation controls chromatin structure and protein interactions."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Science (2006)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1126/science.1124000"}], "href": "https://doi.org/10.1126/science.1124000"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "16469925"}], "href": "https://pubmed.ncbi.nlm.nih.gov/16469925"}]}, {"type": "r", "ref": 2, "children": [{"type": "t", "text": "Maria Victoria Botuyan, Joseph Lee, Irene M Ward, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Structural basis for the methylation state-specific recognition of histone H4-K20 by 53BP1 and Crb2 in DNA repair."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Cell (2006)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/j.cell.2006.10.043"}], "href": "https://doi.org/10.1016/j.cell.2006.10.043"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "17190600"}], "href": "https://pubmed.ncbi.nlm.nih.gov/17190600"}]}, {"type": "r", "ref": 3, "children": [{"type": "t", "text": "Alejandro Vaquero, Michael B Scher, Dong Hoon Lee, et al. "}, {"type": "b", "children": [{"type": "t", "text": "SirT2 is a histone deacetylase with preference for histone H4 Lys 16 during mitosis."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Genes Dev (2006)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1101/gad.1412706"}], "href": "https://doi.org/10.1101/gad.1412706"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "16648462"}], "href": "https://pubmed.ncbi.nlm.nih.gov/16648462"}]}, {"type": "r", "ref": 4, "children": [{"type": "t", "text": "Jiangbo Tang, Nam Woo Cho, Gaofeng Cui, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Acetylation limits 53BP1 association with damaged chromatin to promote homologous recombination."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Nat Struct Mol Biol (2013)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1038/nsmb.2499"}], "href": "https://doi.org/10.1038/nsmb.2499"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "23377543"}], "href": "https://pubmed.ncbi.nlm.nih.gov/23377543"}]}, {"type": "r", "ref": 5, "children": [{"type": "t", "text": "Daniel R Foltz, Lars E T Jansen, Aaron O Bailey, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Centromere-specific assembly of CENP-a nucleosomes is mediated by HJURP."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Cell (2009)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/j.cell.2009.02.039"}], "href": "https://doi.org/10.1016/j.cell.2009.02.039"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "19410544"}], "href": "https://pubmed.ncbi.nlm.nih.gov/19410544"}]}, {"type": "r", "ref": 6, "children": [{"type": "t", "text": "Ben E Black, Daniel R Foltz, Srinivas Chakravarthy, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Structural determinants for generating centromeric chromatin."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Nature (2004)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1038/nature02766"}], "href": "https://doi.org/10.1038/nature02766"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "15282608"}], "href": "https://pubmed.ncbi.nlm.nih.gov/15282608"}]}, {"type": "r", "ref": 7, "children": [{"type": "t", "text": "Jeanne Morinière, Sophie Rousseaux, Ulrich Steuerwald, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Cooperative binding of two acetylation marks on a histone tail by a single bromodomain."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Nature (2009)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1038/nature08397"}], "href": "https://doi.org/10.1038/nature08397"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "19794495"}], "href": "https://pubmed.ncbi.nlm.nih.gov/19794495"}]}, {"type": "r", "ref": 8, "children": [{"type": "t", "text": "James G Ryall, Stefania Dell'Orso, Assia Derfoul, et al. "}, {"type": "b", "children": [{"type": "t", "text": "The NAD(+)-dependent SIRT1 deacetylase translates a metabolic switch into regulatory epigenetics in skeletal muscle stem cells."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Cell Stem Cell (2015)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/j.stem.2014.12.004"}], "href": "https://doi.org/10.1016/j.stem.2014.12.004"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "25600643"}], "href": "https://pubmed.ncbi.nlm.nih.gov/25600643"}]}]}]}