Influence of ara A on the formation of dicentrics in irradiated quiescent CHO cells

For X-irradiated normal human fibroblasts, only half of cell inactivation results from chromosomal damage

PURPOSE

To study the relationship between residual double-strand breaks (dsbs), chromosomal damage, and cell inactivation for X-irradiated normal human fibroblasts.

METHODS AND MATERIALS

The experiments were performed with 12 normal human fibroblast strains and, for comparison, a fibroblast line from a LiFraumeni patient (LFS2800), a squamous cell carcinoma line (FaDu), and CHO cells. Cells were irradiated in plateau phase, which was followed by immediate or delayed (14 h) plating. Chromosomal damage was measured by metaphase technique and loss of proliferative capacity by colony-forming assay. The data obtained were compared with residual double-strand breaks measured previously (Dikomey et al. IJROBP 2000;46:481-490).

RESULTS

For each fibroblast strain, the number of lethal chromosome aberrations (CAs) increased with dose, but with a substantial variation among the strains (coefficient of variation = 20%-26%). The number of lethal aberrations was significantly correlated with the number of residual dsbs measured for the same strain (r(2) = 0.71, p = 0.0006). The residual dsbs were assumed to represent both non- and also mis-rejoined dsbs. There was a significant correlation between lethal aberrations and cell survival, but only for delayed and not immediate plating (r(2) = 0.69, p < 0.0008 vs. r(2) = 0.19, p = 0.16). For delayed plating, the ratio between lethal events (LEs) and CAs amounted to LE:CA = 2.0 +/- 0.05:1, indicating that on average, only half of cell inactivation resulted from chromosomal damage. The other 50% was attributed to the p53-dependent permanent G1 arrest, because cells lacking in functional p53 (LFS2800, FaDu, CHO) showed a ratio of LE:CA = 1.01 +/- 0.02:1.

CONCLUSION

On average, up to 50% of the inactivation of X-irradiated normal human fibroblasts is a result of lethal chromosome aberrations, whereas the rest is due to a p53-dependent process, probably permanent G1 arrest.

Large-mutation spectra induced at hemizygous loci by low-LET radiation: evidence for intrachromosomal proximity effects

A mathematical model is used to analyze mutant spectra for large mutations induced by low-LET radiation. The model equations are based mainly on two-break misrejoining that leads to deletions or translocations. It is assumed, as a working hypothesis, that the initial damage induced by low-LET radiation is located randomly in the genome. Specifically, we analyzed data for two hemizygous loci: CD59- mutants, mainly very large-scale deletions (>3 Mbp), in human-hamster hybrid cells, and data from the literature on those HPRT- mutants which involve at least deletion of the whole gene, and often of additional flanking markers (approximately 50-kbp to approximately 4.4-Mbp deletions). For five data sets, we estimated f, the probability that two given breaks on the same chromosome will misrejoin to make a deletion, as a function of the separation between the breaks. We found that f is larger for nearby breaks than for breaks that are more widely separated; i.e., there is a "proximity effect". For acute irradiation, the values of f determined from the data are consistent with the corresponding break misrejoining parameters found previously in quantitative modeling of chromosome aberrations. The value of f was somewhat smaller for protracted irradiation than for acute irradiation at a given total dose; i.e., the mutation data show a decrease that was smaller than expected for dose protraction by fractionation or low dose rate.