Kinetics of chromosome lesion repair in synchronized quiescent and proliferating 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.

Do quiescent human peripheral lymphocytes repair radiation-induced chromosome damage?

Peripheral blood samples from seven healthy donors were exposed to 2 Gy of (60)Co gamma rays. Lymphocyte cultures were prepared from the samples and stimulated to proliferate either immediately after the exposure or after 1, 2 or 4 h. No significant differences were found between the frequencies of chromosome aberrations (breaks or exchange) observed in lymphocytes stimulated after different postirradiation periods. We conclude that unstimulated lymphocytes do not undergo significant levels of recovery from potentially lethal damage at the chromosomal level.

Comparative study of the repair kinetics of chromosomal aberrations and DNA strand breaks in proliferating and quiescent CHO cells

Repair kinetics observable at the level of exchange-type chromosomal aberrations (dicentric chromosomes), using fractionation and delayed-plating techniques, have been compared with repair kinetics of radiation-induced DNA double-strand breaks, measured with PFGE, and with repair kinetics of all strand breaks, measured with the alkali-unwinding technique. Only data from quiescent or proliferating CHO K1 cells obtained in the same laboratory were used. We determined repair kinetics in terms of the time constant tau (equal to half-time/log(e)2). The repair kinetics (tau approximately 11-14 min) observed in the split-dose formation of dicentric chromosomes agrees with fast repair kinetics of double-strand breaks (tau approximately 11-13 min), thus permitting us to identify the latter as the 'primary lesions' whose pairwise interaction leads to the beta D2 yield term of the aberrations. The repair kinetics observed for dicentric chromosomes formed under delayed-plating conditions (tau approximately 75 min), which mainly affects the alpha D yield term, is attributed to an intermediate interchromosomal product temporarily existing in the course of aberration formation; it is suggested that this product is mechanistically correlated with the slow repair kinetics of 'clustered damage' to DNA seen with the applied molecular methods (tau approximately 90 min).

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

CHO K1-cells, arrested in G0/G1 phase were irradiated with graded doses of 150-kV X-rays, and the dicentric chromosome aberrations in the first metaphase after reincubation in full growth medium were evaluated. When 500 microM ara A was administered at least 1 h before irradiation, maximal enhancement of the dicentric yield was obtained under 'immediate plating' conditions as well as under 'delayed plating' conditions, and under both conditions only the linear component alpha D of the aberration yield was affected. Since ara A had no effect upon the number of induced dsbs, as measured with PFGE at doses up to 100 Gy, the action pathway of ara A is suggested to be the repair/misrepair process manifested in the linear component of the dicentric yield. By varying the time at which either ara A at 37 degrees C was administered before irradiation or ara A at 4 degrees C was added after irradiation, it was observed that the repair step sensitive to ara A was completed within about 5 min after irradiation. This is in agreement with other observations of ara A action upon a fast repair step. The conversion of DNA single-strand breaks or base damage sites into double-strand breaks as a consequence of polymerase beta inhibition by ara A would explain the present results.

Laser-light induced chromosome aberrations in Chinese hamster cells

Wild-type Chinese hamster cells CHO K1 and their radiosensitive mutant xrs5 were irradiated at 308 nm, using light pulses of a XeCl excimer laser with total energy fluences of 0.1 kJ/m2 to 4.08 kJ/m2. Chromosome-type and chromatid-type chromosome aberrations have been observed at pulse irradiances of 2.5 x 10(7) W/m2 and 1.7 x 10(8) W/m2, indicating that in mammalian cells DNA double-strand breaks occur already in this irradiance range. The results obtained with laser irradiation are compared with X-ray irradiation.

DNA damage in non-proliferating cells subjected to ionizing irradiation at high or low dose rates

Ionizing radiation damage to the genome of a non-cycling mammalian cell is analyzed using continuous time Markov chains. Immediate damage induced by the radiation is modeled as a batch Poisson arrival process of DNA double strand breaks (DSBs). Different kinds of radiation, for example gamma rays or alpha particles, have different batch probabilities. Enzymatic modulation of the immediate damage is modeled as a Markov process similar to the processes described by the master equation of stochastic chemical kinetics. An illustrative example is the restitution/complete exchange model, which postulates that radiation induced DSBs can subsequently either undergo enzymatically mediated repair (restitution) or can participate pairwise in chromosome exchanges, some of which make irremediable lesions such as dicentric chromosome aberrations. One may have rapid irradiation followed by enzymatic DSB processing or have prolonged irradiation with both DSB arrival and enzymatic DSB processing continuing throughout the irradiation period. A complete solution of the Markov chain is known for the case that the exchange rate constant is negligible so that no irremediable chromosome lesions are produced and DSBs are the only damage to the genome. Using PDEs for generating functions, a perturbation calculation is made assuming the exchange rate constant is small compared to the repair rate constant. Some non-perturbative results applicable to very prolonged irradiation are also obtained using matrix methods: Perron-Frobenius theory, variational methods and numerical approximations of eigenvalues. Applications to experimental results on expected values, variances and statistical distributions of DNA lesions are briefly outlined. Continuous time Markov chain models are the most systematic of those current radiation damage models which treat DSB-DSB interactions within the cell nucleus as homogeneous (e.g. ignore diffusion limitations). They contain most other homogeneous models as special cases, limiting cases or approximations. However, applying the continuous time Markov chain models to studying spatial dependence of DSB interactions, which is generally believed to be very important in some situations, presents difficulties.