Checkpoint Abrogation in G2 Compromises Repair of Chromosomal Breaks in Ataxia Telangiectasia Cells

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Abstract
Checkpoint abrogation in G2 compromises repair of DNA double-strand breaks (DSB) and confers enhanced G2 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 G2 checkpoint in irradiated primary AT and normal human lymphocytes and their lymphoblastoid derivatives. Direct analysis of radiation damage in G2 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 G2 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 G2 checkpoint without significantly affecting DSB rejoining increases metaphase chromosome damage of normal cells to the AT level but leaves unchanged interphase chromosome damage in G2. Caffeine has no effect on any of these end points in AT cells. These observations represent the first direct evidence that the G2 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 G2-to-M phase transition. (Cancer Res 2005; 65(24): 11292-6)