Repair of potentially lethal damage in normal cells and ataxia telangiectasia cells; consideration of non-homologous end-joining.

When cell lines are held in a quiescent state after irradiation, survival rates are greater than those from cells that are stimulated to grow immediately after irradiation. These differences in survival rates correspond to rates of potentially lethal damage repair. The effects of confluent holding recovery after gamma-irradiation were investigated using normal human fibroblasts (AG1522) and ataxia telangiectasia fibroblasts (GM02052). Calyculin-A-induced premature chromosome condensation and fluorescent in situ hybridization were applied to study G2/M chromosomal aberrations. Survival results indicated normal capacity for PLDR in AG1522 cells but that PLDR was extremely compromised in GM02052 cells. The chromosomal aberration frequency decreased when AG1522 cells were allowed to repair for 24-h, whereas 24-hour incubation had little effect on the aberration frequency in GM02052 cells. Since the main mechanism for dsbs repair during G0/G1 phases of the cells cycle involve the non-homologous end-joining (NHEJ) process, our study indicates that for AG1522 cells the NHEJ repair process is more likely to induce accurate chromosome repair under quiescent G0 conditions than proliferating G1 phase, while in GM02052 cells the fidelity of NHEJ is similarly defective at either cell cycle phase. Reduced fidelity of NHEJ may be responsible for PLDR defect and its hyper-radiosensitivity in A-T cells.

Caffeine sensitizes nondividing human fibroblasts to x rays by inducing a high frequency of misrepair.

Caffeine sensitizes cells to ionizing radiation, and this effect is believed to be associated with the disruption of DNA damage-responsive cell cycle checkpoints, which is controlled by ATM. Recent studies suggest that misrejoining of DSBs is one of the underlying mechanisms of AT cell hyper-radiosensitivity. In this study, we investigated the effects of caffeine and radiation on nongrowing G0 normal human fibroblast cells by determining cell survival and scoring aberrations in calyculin A-induced G2 chromosomes. Results from the cell survival study indicate that after X-ray exposure G0 cells were sensitized by 24 h treatment with caffeine. Analysis of chromosome aberrations using FISH (fluorescence in situ hybridization) revealed a high frequency of aberrant cells and color junctions in the caffeine-treated cells. Since most DNA repair in nongrowing G0 cells is believed to result from nonhomologous end joining (NHEJ), caffeine may influence the fidelity of the NHEJ pathway in irradiated G0 cells.