title: HP1-mediated formation of alternative lengthening of telomeres-associated PML bodies requires HIRA but not ASF1a.
abstract: Approximately 10% of cancers use recombination-mediated Alternative Lengthening of Telomeres (ALT) instead of telomerase to prevent telomere shortening. A characteristic of cells that utilize ALT is the presence of ALT-associated PML nuclear bodies (APBs) containing (TTAGGG)n DNA, telomere binding proteins, DNA recombination proteins, and heterochromatin protein 1 (HP1). The function of APBs is unknown and it is possible that they are functionally heterogeneous. Most ALT cells lack functional p53, and restoration of the p53/p21 pathway in these cells results in growth arrest/senescence and a substantial increase in the number of large APBs that is dependent on two HP1 isoforms, HP1α and HP1γ. Here we investigated the mechanism of HP1-mediated APB formation, and found that histone chaperones, HIRA and ASF1a, are present in APBs following activation of the p53/p21 pathway in ALT cells. HIRA and ASF1a were also found to colocalize inside PML bodies in normal fibroblasts approaching senescence, providing evidence for the existence of a senescence-associated ASF1a/HIRA complex inside PML bodies, consistent with a role for these proteins in induction of senescence in both normal and ALT cells. Moreover, knockdown of HIRA but not ASF1a significantly reduced p53-mediated induction of large APBs, with a concomitant reduction of large HP1 foci. We conclude that HIRA, in addition to its physical and functional association with ASF1a, plays a unique, ASF1a-independent role, which is required for the localization of HP1 to PML bodies and thus for APB formation.
authors.affiliation: Cancer Research Unit, Children's Medical Research Institute, Westmead, New South Wales, Australia.
authors.forename: Wei-Qin
authors.initials: WQ
authors.lastname: Jiang
authors.forename: Akira
authors.initials: A
authors.lastname: Nguyen
authors.forename: Ying
authors.initials: Y
authors.lastname: Cao
authors.forename: Andy C-M
authors.initials: AC
authors.lastname: Chang
authors.forename: Roger R
authors.initials: RR
authors.lastname: Reddel
mesh_terms: D021581:Active Transport, Cell Nucleus; D000818:Animals; D018797:Cell Cycle Proteins / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D002466:Cell Nucleolus / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism*; D016922:Cellular Senescence; D000090266:Chromobox Protein Homolog 5; D002868:Chromosomal Proteins, Non-Histone / Q000378:metabolism*; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000378:metabolism; D055785:Gene Knockdown Techniques; D006367:HeLa Cells; D056488:Histone Chaperones / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D018832:Molecular Chaperones; D059505:Telomere Homeostasis; D014157:Transcription Factors / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism
chemical_list: C491641:ASF1A protein, human; C000715594:CBX5 protein, human; D018797:Cell Cycle Proteins; D002868:Chromosomal Proteins, Non-Histone; D050759:Cyclin-Dependent Kinase Inhibitor p21; C086057:HIRA protein, human; D056488:Histone Chaperones; D018832:Molecular Chaperones; D014157:Transcription Factors; D016159:Tumor Suppressor Protein p53; D000090266:Chromobox Protein Homolog 5