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. (Cancer Res 2005; 65(24): 11292-6)

Amifostine reduces the mean yield of chromatid breaks in normal cells and in cells from cancer prone patients when present during G(2) irradiation. Although the precise mechanisms of radioprotection caused by amifostine remain unclear, the results obtained using premature chromosome condensation reveal that amifostine does not act on cells only as a free radical scavenger and as a repair enhancer of DNA damage.