The rate of DNA synthesis in normal human and ataxia telangiectasia cells after exposure to X-irradiation

Evidence for the existence of an actin-derived protein in ataxia-telangiectasia lymphoblastoid cell lines

In a number of lymphoblastoid cell lines from individuals with the genetic disease ataxia-telangiectasia (A-T), a decrease in the levels of actin and a concomitant increase in the levels of a protein species of 37K has been observed to occur following high cell density. In this paper we describe ataxia-telangiectasia lymphoblastoid lines that show this response constitutively, and demonstrate that this 37K protein is most likely an actin-derived peptide. This is because of extensive similarity between the two proteins in the spectrum of peptides produced by proteolytic fingerprinting. It appears likely that this 37K protein is similar to the well documented protease-resistant actin fragments produced by protease digestion of actin in vitro. Examination of a number of possible initiators found no clear signal that promoted this response, although the response is clearly very dependent on cell culture conditions. The possible involvement of a cytoskeletal anomaly in ataxia-telangiectasia is discussed.

X-ray sensitive mutants of Chinese hamster ovary cell line: radio-sensitivity of DNA synthesis

6 mutants of the CHO cell line hypersensitive to ionizing radiation have been previously described (Jeggo et al., 1982; Jeggo and Kemp, 1983). In this report, the degree of radiosensitivity of DNA synthesis in these mutant strains has been investigated. 5 of the 6 mutant strains showed a greater degree of inhibition of DNA synthesis following gamma-irradiation compared to the wild-type strains, one mutant showed a similar response to the parent strain. An analysis of the size of the DNA made after irradiation in mutant and wild-type strains indicated that initiation events were more inhibited than chain elongations. This enhanced radiosensitivity is in contrast to results observed using ataxia telangiectasia fibroblasts, in which DNA synthesis is radioresistant in comparison to normal skin fibroblasts. These results are interpreted on the basis of sensitivity resulting from a defect in DNA repair in these mutant strains.

Effect of caffeine in G2 on X-ray-induced chromosomal aberrations and mitotic inhibition in ataxia telangiectasia fibroblast and lymphoblastoid cells

The effect of post-treatments with caffeine in G2 on the frequency of X-ray-induced chromatid aberrations was studied in normal and ataxia telangiectasia (A-T) fibroblast and lymphoblastoid cells. Caffeine was found to potentiate the X-ray-induced aberration yield in both normal fibroblast and lymphoblastoid cells. An enhancement was also observed in A-T lymphoblastoid cells, whereas the X-ray-induced aberration frequency in A-T fibroblasts was unaffected by the presence of caffeine. The influence of caffeine on the radiation-induced mitotic inhibition was investigated in normal and A-T fibroblasts; in both types of cell less inhibition was obtained in the presence of caffeine.

The effects of ionizing radiation on cell cycle progression in ataxia telangiectasia

Although ataxia telangiectasia (AT) cells are more sensitive than normal cells to killing by ionizing radiation, their DNA synthesis is more resistant to inhibition by radiation. It was thought that this anomaly in DNA synthesis was likely to perturb cell cycle progression. Flow cytometry and the fraction of labelled mitoses (FLM) were used to investigate effects of irradiation in normal and AT cell lines. The FLM indicated that radiation apparently induced a longer G2 delay in normal cells than in AT cells. However, flow cytometry showed that radiation induced much larger and more prolonged increases in the proportion of G2 cells in AT than in normals. AT populations also showed much larger postirradiation decreases in viable cell numbers. These data suggest that a large proportion of the radiosensitive AT cells are not reversibly blocked in G2 but die there, and never proceed through mitosis. The less radiosensitive normal cells are delayed in G2 and then proceed through mitosis. We suggest that the apparently shorter radiation-induced mitotic delay seen in AT cells by FLM is not real but is an artifact arising from perturbation of steady state conditions by selective elimination of a particular cohort of AT cells. Accumulation of AT cells in G2 is compatible with radiosensitivity of these cells and may arise from a defect in DNA repair or an anomaly in DNA replication.

Post-irradiation inhibition of scheduled DNA synthesis and stimulation of ADP-ribosylation in sensitive and resistant L5178Y murine lymphoma cells

Post-irradiation changes in DNA synthesis and ADP-ribosyltransferase (ADPRT) activity in L5178YS and L5178YR, radiation sensitive and resistant murine lymphoma cells are described. DNA synthesis was inhibited to a greater extent in L5178YS than in L5178YR cells. The stimulation of ADPRT activity by irradiation was not significantly different between these two cell lines. These observations contribute to other evidence which has failed to confirm a general association of ADP-ribosylation with the DNA synthesis inhibition response. The contrast between the response of L5178Y cells and the corresponding behaviour of ataxia telangiectasia cells and normal human cells indicate that entirely different mechanisms are involved in determining the differences in radiosensitivity in these two systems.

Effect of X-radiation on DNA and histone synthesis in ataxia telangiectasia and normal lymphoblastoid cells

The possibility that the radiosensitivity of lymphoblastoid cell lines from patients with ataxia telangiectasia (A-T) is due to an aberrant content of histones has been examined. The histone pattern of lymphoblastoid cell lines derived from A-T patients was found to be indistinguishable from that obtained from normal individuals. X-ray irradiation led to a greater decrease in cell growth rate in the A-T cells than in the normal cells but was accompanied by a greater decrease of DNA synthesis rate in the normal cells. This difference in radiosensitivity was not reflected in differences in the content or rates of synthesis of histones or of major non-histone proteins in these cells. Reduction in the rate of DNA synthesis was not associated with the appearance of the lysine-rich histone variant H1. We conclude that the hypersensitivity to ionizing radiation in A-T cells is not due to fundamental differences in the composition or synthesis of the major chromosomal proteins.

Inhibition and recovery of DNA synthesis after X-irradiation in radiosensitive mouse-cell mutants

The mouse lymphoma L5178Y cell line and its radiosensitive variants M10 and LX830 were examined for DNA synthesis after X-irradiation. The dose-response curves show that the rates of DNA synthesis immediately after exposure are reduced in a dose-dependent fashion and that the extents of reduction in these 3 cell lines are similar to one another. But a difference was observed in the recovery of DNA synthesis with time of incubation. The recovered levels in M10 and LX830 cells were much higher than those in L5178Y cells at high doses of X-rays. These results are discussed in relation to radioresistant DNA synthesis in ataxia telangiectasia cells.

Ca2+, Mg2+ and (adenosine diphosphate ribose)n in cellular response to irradiation

A model of cellular response to irradiation involving adenosine diphosphate ribosylation (ADP-ribosylation) is proposed. Its main assumptions are (a) control of accessibility of sites for ADP-ribosylation in chromatin by free Ca2+/Mg2+ ratio; and (b) regulation of the Ca2+/Mg2+ ratio by factors affecting intracellular free Ca2+ concentration; the regulation would be mediated by mitochondria. The model seeks to explain the mechanism of action of radiomodifiers such as caffeine, local anaesthetics, polyamines and 2,4-dinitrophenol.

DNA replication and repair in ataxia telangiectasia cells exposed to bleomycin

A marked increase in sensitivity to bleomycin was observed in two ataxia telangiectasia (AT) lymphoblastoid cell lines compared to that in cell lines from two normal individuals. This sensitivity was obtained at two different concentrations of bleomycin. While normal cells showed a rapid recovery of ability to divide, there was no indication of such a recovery in AT cells up to 120 h after bleomycin treatment. A similar level of breakage of DNA occurred in both cell types after incubation with bleomycin. The rate of repair of these breaks was also the same. DNA synthesis was found to be more resistant to bleomycin in AT cells than in control cells. The latter data are in keeping with results previously obtained using ionizing radiation.

Increased clastogenicity and decreased inhibition of DNA synthesis by neocarzinostatin and tallysomycin in ataxia telangiectasia lymphoid cells

Cytogenetic damage in cells cultured from normal individuals and patients with ataxia telangiectasia (A-T) and xeroderma pigmentosum (XP) was induced by the chemotherapeutic antibiotics neocarzinostatin (NCS), tallysomycin (TLM) and bleomycin (BLM). Chromosomal breakage was specifically elevated in A-T cells when compared to the other genotypes tested. Similar results were not observed with the clastogens mitomycin C (MMC) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) as all cells responded similarly. All 5 chemical agents caused a marked suppression of de novo DNA synthesis in normal and XP long-term lymphoid cell lines while the A-T cells seemed resistant to this effect of NCS, TLM and BLM.

Hypersensitivity to ionizing radiation, in vitro, in a new chromosomal breakage disorder, the Nijmegen Breakage Syndrome

The Nijmegen Breakage Syndrome (NBS) is a new chromosomal instability disorder different from ataxia telangiectasia (AT) and other chromosome-breakage syndromes. Cells from an NBS patient appeared hypersensitive to X-irradiation. X-rays induced significantly more chromosomal damage in NBS lymphocytes and fibroblasts than in normal cells. The difference was most pronounced after irradiation in G2. Further, NBS fibroblasts were more readily killed by X-rays than normal fibroblasts. In addition, the DNA synthesis in NBS cells was more resistant to X-rays and bleomycin than that in normal cells. The reaction of NBS cells to X-rays and bleomycin was similar to that of cells from patients with ataxia telangiectasia. Our results indicate that NBS and AT, which also have similar chromosomal characteristics, must be closely related.

Effect of aphidicolin on de novo DNA synthesis, DNA repair and cytotoxicity in gamma-irradiated human fibroblasts. Implications for the enhanced radiosensitivity in ataxia telangiectasia

The antibiotic, aphidicolin, is a potent inhibitor of DNA polymerase alpha and consequently of de novo DNA synthesis in human cells. We report here that in gamma-irradiated normal human cells, aphidicolin (at 5 micrograms/ml and less) had no significant effect on the rate of the rejoining of DNA single strand breaks or rate of removal of DNA lesions assayed as sites sensitive to incising activities present in crude protein extracts of Micrococcus luteus cells. gamma-irradiated human ataxia telangiectasia cells are known to demonstrate enhanced cell killing and exhibit resistance to the inhibiting effects of radiation on DNA synthesis. Under conditions of minimal aphidicolin cytotoxicity but extensive inhibition of de novo DNA synthesis, the radiation responses of neither normal nor ataxia telangiectasia cells were significantly modified by aphidicolin. Firstly, we conclude that human DNA polymerase alpha is not primarily involved in the repair of the two classes of radiogenic DNA lesions examined. Secondly, the radiation hypersensitivity of ataxia telangiectasia cells cannot be explained on the basis of premature replication of damaged cellular DNA resulting from the resistance of de novo DNA synthesis to inhibition by ionizing radiation.

Cellular radiation effects and hyperthermia cell cycle kinetics of radiation sensitive mutants of Saccharomyces cerevisiae after X-irradiation and hyperthermia

Radiosensitive mutants rad2, rad9, and rad51 of Saccharomyces cerevisiae were X-irradiated with 120 Gy or 60 Gy, heated at 50 degrees C for 30 min or treated with a combination of both and incubated in nutrient medium at 30 degrees C. Cell number, percentage of budding cells, and cell cycle progression were determined in 45-min intervals. Cell cycle kinetics were investigated by flow cytofluorometry. Hyperthermia leads mainly to a lengthening of G1, whereas X-rays arrest cells of the rad2 and rad9 mutant in G2 and the rad51--mutant additionaly in a state with DNA contents above G2. Cell division delay is influenced by oxygen in all strains but to a lesser extent in the rad2 mutant. The effect of the combined treatment appears to be merely additive in the rad2 and rad9 mutant while the rad51 mutant is sensitized to X-irradiation by hyperthermia. No selective action of hyperthermia on hypoxic cells was found.

Cell cycle dependence of mitotic delay in X-irradiated normal and ataxia-telangiectasia fibroblasts

The delay in progression of X-irradiated cells through the cell cycle, which is more pronounced in normal (N) than in A-T fibroblasts, is greatest for cells in G2 at the time of irradiation. The greater effect of radiation on the initiation of DNA synthesis in N than in A-T cells is reflected in the shape of the percent labelled mitosis curves after 3H-thymidine treatment. The duration of the S phase in unirradiated A-T cells is greater than in N cells. Any explanation of the underlying defect in A-T must account not only for the reduced radiosensitivity of DNA synthesis but for the lesser delay in G2. Our data support the hypothesis that DNA is the principal target for radiation-induced G2 delay.

Hypersensitivity of ataxia telangiectasia skin fibroblasts to DNA alkylating agents

3 ataxia telangiectasia (AT) fibroblast cell strains, AT4BI, AT5BI and AT2BE (CRL1343) were studied for their colony-forming ability after treatment with various concentrations of 4 different DNA alkylating agents. The results were compared to the response of fibroblast strains from 3 normal individuals. None of the AT strains were abnormally sensitive to N-methyl-N'-nitro-N-nitrosoguanidine. 1 strain (AT5BI) was significantly more sensitive to treatment with methyl methanesulfonate (MMS) based on a survival curve D0 value of 0.29 mM vs. the normal average D0 of 0.38 mM (P less than 0.02) and a D10 value of 0.85 mM vs. the normal average D10 of 1.2 mM (P less than 0.025). Strain AT4BI was also significantly more sensitive to MMS treatment when D10 values were compared (0.73 mM, P less than 0.01). All 3 AT cell strains were significantly more sensitive to treatment with ethyl methanesulfonate when D10 values were the criterion of sensitivity, AT4BI 16 mM, AT5BI 13 mM and AT2BE 15 mM vs. the normal human fibroblast average D10 value of 28 mM (P less than 0.01 for all 3 AT strains). 2 of the 3 AT cell strains (AT4BI and AT2BE) were abnormally sensitive to treatment with 4-nitroquinoline-1-oxide; the D0 values were 0.045 microM and 0.05 microM, respectively, vs. the normal average D0 value of 0.11 microM (P less than 0.01 for both AT strains). The corresponding D10 values were 0.08 microM and 0.11 microM, respectively, vs. the normal average D10 value of 0.27 microM (P less than 0.01 for AT4BI and P less than 0.025 for AT2BE). These results indicate that there is a heterogeneity in the response of AT fibroblast cell strains to treatment with DNA alkylating agents, except possibly in the case of ethylating compounds.

Ultraviolet radiation sensitivity of proliferating and differentiated human neuroblastoma cells

The effects of ultraviolet (U.V.) radiation were studied on a cloned line of human neuroblastoma cells in proliferative and differentiated growth modes, the latter being induced by serum deprivation. The neuroblastoma cells were found to be unusually sensitive in comparison with HeLa cells when survival was measured by colony formation in soft agar, the differentiated mode being the most sensitive. Ultraviolet radiation sensitivity was associated with very low DNA repair capacity as measured by DNA repair synthesis and by removal of M. luteus endonuclease-sensitive sites from irradiated DNA. The greater sensitivity of the differentiated cells appeared to be related to a greater degree of DNA damage at a given U.V. dose, resulting from altered cell geometry in the growth mode. The neuroblastoma cells showed little or no post-irradiation inhibition of DNA replication at low U.V. doses, suggesting that it is the repair process rather than the DNA damage which is responsible for inhibiting replication.

DNA-repair deficiencies do not affect intercalator-induced cytotoxicity or DNA scission in human cells

The ability of the DNA intercalator m-AMSA to produce protein-associated DNA-strand breaks in normal, xeroderma pigmentosum and ataxia-telangiectasia fibroblasts was compared with m-AMSA uptake and cytotoxicity. No differences were detected between the cytotoxicity and DNA breakage produced by this antineoplastic acridine derivative among these three human cell lines. Uptake studies confirmed that no actual increased sensitivity was being masked by decreased intracellular drug. m-AMSA appears to be unique in its ability to produce breaks in cellular DNA that are not associated with an enhanced sensitivity in repair-deficient cells. Intercalator-induced, protein-associated DNA breaks are probably the result of a novel cellular response which differs from that which is abnormal in xeroderma pigmentosum or ataxia-telangiectasia cells.

Genetic heterogeneity in ataxia-telangiectasia studied by cell fusion

The effect of x-rays on the rate of semiconservative DNA replication was investigated by autoradiography in single cells obtained from normal individuals and from patients having ataxia-telangiectasia (AT). In the five AT cell strains studied, the rate of DNA synthesis was inhibited to a lesser extent that in two normal cell strains. By using this abnormal regulation of DNA replication in AT cells as a marker, an experimental procedure was developed that allowed genetic complementation analysis of AT. After Sendai virus-induced fusion of AT cells, the grains were counted over binucleate cells with both nuclei in S phase. In some cases, the inhibition of DNA synthesis caused by x-rays in the heterodikaryons was more pronounced than that in the parental homodikaryons and was comparable to that in normal binucleate cells, indicating complementation. By using this approach, the five AT cell strains that were investigated could be assigned to three complementation groups. The data suggest that extensive genetic heterogeneity exists in AT.

Abnormal levels of UV-induced unscheduled DNA synthesis in ataxia telangiectasia cells after exposure to ionizing radiation

In cultured cells from normal individuals and from patients having ataxia telangiectasia (AT) the rate of unscheduled DNA synthesis (UDS) induced by UV light was investigated by autoradiography. The number of grains in 6 different AT cell strains was similar to that observed in normal cells. Exposure of normal cells to doses of X-rays up to 20 krad had no influence on the rate of UV-induced UDS. In contrast, the UV-induced UDS was significantly modified in AT cells by treatment with X-rays. In AT cell strains that were reported to have reduced levels of gamma-ray-induced repair DNA synthesis ('excision-deficient' AT cells) the effect of X-rays on UV-induced UDS was inhibitory, whereas UV-induced UDS was stimulated by X-ray exposure in 'excision-proficient' AT cell strains. Different UV and X-ray dose-response relationships were seen in the two categories of AT cell strains. These results strongly suggest that different molecular defects are present in excision-deficient and excision-proficient At cells. They also indicate that the altered levels of repair DNA synthesis after exposure to UV in AT cells may be a secondary consequences of the way such cells handle DNa damage caused by ionizing radiation.

The effect of bleomycin on DNA synthesis in ataxia telangiectasia lymphoid cells

Bleomycin, a radiomimetic glycopeptide, inhibits de novo DNA synthesis in ataxia telangiectasia lymphoblastoid B cells to a markedly lesser extent than in normal and xeroderma pigmentosum lymphoid cells. This observation is similar to that following ionizing radiation; however, the effect is slower following the chemical treatment. Recovery of the normal cells occurs 15-18 hours after treatment, whereas the ataxia telangiectasia lines do not attain normal levels of DNA synthesis during the entire 24-hour observation period. Similar differences were not observed following treatment with mitomycin C, a bifunctional alkylating agent, indicating a specific effect of bleomycin on DNA synthesis in ataxia telangiectasia cells. Following bleomycin treatment and preincubation with hydroxyurea, residual DNA synthesis in ataxia telangiectasia cells was similar to that in both normal and xeroderma pigmentosum lymphoid cells, suggesting that the capacity to repair the induced DNA lesion is present.

DNA-repair synthesis in ataxia telangiectasis lymphoblastoid cells

The ability of a number of Epstein-Barr virus-transformed lymphoblastoid cells from ataxia telangiectasis (AT) patients to repair gamma-radiation damage to DNA was determined. All of these AT cells were previously shown to be hypersensitive to gamma-radiation. Two methods were used to determine DNA-repair synthesis: isopycnic gradient analysis and a method employing hydroxyurea to inhibit semiconservative DNA synthesis. Control, AT heterozygote and AT homozygote cells were demonstrated to have similar capacities for repair of radiation damage to DNA. In addition at high radiation doses (10-40 krad) the extent of inhibition of DNA synthesis was similar in the different cell types.

Radioresistant DNA synthesis: an intrinsic feature of ataxia telangiectasia

DNA synthesis in 6 ataxia telangiectasia (AT) cell strains was much more resistant to X-irradiation than was DNA synthesis in normal human diploid cells. 3 of the cell strains tested have been classified as proficient in repair replication. These data, along with those reported elsewhere, strongly suggest that radioresistant DNA synthesis is an intrinsic feature of this disease. The radioresistance of DNA synthesis in AT cells is primarily due to a reduced inhibition of replicon initiation compared to that occurring in normal cells, but DNA chain elongation is also more radioresistant in AT cells. The small inhibition of DNA synthesis that does occur in AT cells at doses up to 2000 rad is almost exclusively due to inhibition of replicon initiation, and not to inhibition of chain elongation, as would be expected from results with normal human cells or from previous studies with established cell lines.