title: Checkpoint abrogation in G2 compromises repair of chromosomal breaks in ataxia telangiectasia cells.
abstract: Checkpoint abrogation in G(2) compromises repair of DNA double-strand breaks (DSB) and confers enhanced G(2) chromosomal radiosensitivity in ataxia telangiectasia (AT) cells. To directly test this hypothesis, we combined calyculin A-induced premature chromosome condensation with conventional cytogenetics to evaluate chromosome damage before and after the G(2) checkpoint in irradiated primary AT and normal human lymphocytes and their lymphoblastoid derivatives. Direct analysis of radiation damage in G(2) by premature chromosome condensation reveals practically indistinguishable levels of chromosomal breaks in AT and normal cells. Yet a 4-fold increase in metaphase chromosome damage is observed in AT cells as compared with normal cells which, in contrast to AT cells, exhibit a strong G(2) arrest manifest as an abrupt reduction in the mitotic index. Thus, an active checkpoint facilitates repair of chromosomal breaks in normal cells. Treatment with caffeine that abrogates the G(2) checkpoint without significantly affecting DSB rejoining increases metaphase chromosome damage of normal cells to the AT level but leaves unchanged interphase chromosome damage in G(2). Caffeine has no effect on any of these end points in AT cells. These observations represent the first direct evidence that the G(2) checkpoint facilitates repair of chromosome damage, presumably by supporting repair of DNA DSBs. Failure to arrest will lead to chromatin condensation and conversion of unrepaired DNA DSBs to chromosomal breaks during G(2)-to-M phase transition.
authors.affiliation: Institute of Nuclear Technology and Radiation Protection, National Centre for Scientific Research Demokritos, Aghia Paraskevi Attikis, Athens, Greece.
authors.forename: Georgia I
authors.initials: GI
authors.lastname: Terzoudi
authors.forename: Kaliopi N
authors.initials: KN
authors.lastname: Manola
authors.forename: Gabriel E
authors.initials: GE
authors.lastname: Pantelias
authors.forename: George
authors.initials: G
authors.lastname: Iliakis
mesh_terms: D001260:Ataxia Telangiectasia / Q000235:genetics*; D002110:Caffeine / Q000494:pharmacology; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D042002:Chromatin Assembly and Disassembly; D019457:Chromosome Breakage; D002877:Chromosomes, Human / Q000528:radiation effects*; D004247:DNA / Q000528:radiation effects*; D004260:DNA Repair; D016195:G2 Phase / Q000528:radiation effects*; D006801:Humans; D008214:Lymphocytes / Q000166:cytology / Q000378:metabolism / Q000528:radiation effects; D008387:Marine Toxins; D008940:Mitotic Index; D010080:Oxazoles / Q000494:pharmacology; D011836:Radiation Tolerance; D014407:Tumor Cells, Cultured
chemical_list: D000697:Central Nervous System Stimulants; D008387:Marine Toxins; D010080:Oxazoles; D002110:Caffeine; C059041:calyculin A; D004247:DNA