Ataxia Telangiectasia Lymphoblastoid Cells Research Articles

Modulation of chromatin conformation by the histone deacetylase inhibitor trichostatin A promotes the removal of radiation-induced lesions in ataxia telangiectasia cell lines

Ataxia telangiectasia is a rare autosomal recessive genome instability syndrome caused by mutations in the Ataxia Telangiectasia Mutated gene and characterized by a very high sensitivity to agents inducing double strand breaks such as ionizing radiation. In cells derived from ataxia telangiectasia patients a prominent enhancement of chromosomal aberrations is revealed as a consequence of this radiosensitivity characteristic, arising from defective DNA repair for a small fraction of breaks localized in the less accessible heterochromatin. Moreover, the signaling mediated by ataxia telangiectasia protein kinase also modifies chromatin structure. Even if there is a lot of knowledge concerning biochemical aspects of repair of double strand breaks, no conclusive results on radiosensitivity of structurally- and functionally-different chromatin are available, particularly in ataxia telangiectasia cells. Thus, a wild-type cell line and two ataxia telangiectasia patient derived ones could represent a suitable model to study the possible relationship between chromatin conformation and sensitivity to ionizing radiation. In this context, the effects of both cytosine arabinoside, an inhibitor of DNA repair synthesis, and trichostatin A, a histone deacetylase inhibitor, were tested in normal and ataxia telangiectasia lymphoblastoid cell lines carrying different mutation in the Ataxia Telangiectasia Mutated gene. The response to both inhibitors was investigated analyzing two endpoints, namely, chromosomal aberrations and the removal of DNA lesions by Comet assay, after exposure to X-rays. Results obtained suggest that the modulation of chromatin structure by trichostatin A leading to a more open conformation, decreases radiation-induced chromosomal aberrations in ataxia telangiectasia cells. The reduction in chromosomal instability can be attributed to an enhancement in DNA repair occurring in the presence of the histone deacetylase inhibitor, as its abolishment by the known inhibitor of DNA repair synthesis cytosine arabinoside clearly demonstrates. Data obtained could indicate a pivotal role of chromatin conformation in the radiosensitivity of ataxia telangiectasia cells.

Forward subtractive libraries containing genes transactivated by dexamethasone in ataxia-telangiectasia lymphoblastoid cells

Ataxia telangiectasia (A-T) is a rare autosomal recessive disorder caused by biallelic mutations in the AtaxiaTelangiectasia-mutated gene. A-T shows a complex phenotype ranging from early-onset progressive neurodegeneration to immunodeficiencies, high incidence of infections, and tumors. Unfortunately, no therapy is up to now available for treating this condition. Recently, the short term treatment of ataxia-telangiectasia patients with glucocorticoids was shown to improve their neurological symptoms and possibly reverse cerebellar atrophy. Thus, corticosteroids represent an attractive approach for the treatment of this neurodegenerative disease. However, the molecular mechanism involved in glucocorticoid action in A-T is yet unknown. The aim of our work is to construct cDNA libraries containing those genes which are transactivated by the glucocorticoid analogue, dexamethasone, in A-T human cells. For this purpose, suppression subtractive hybridization has been performed on ATM-null lymphoblastoid cell transcriptome extracted following drug administration. Annotation of whole genes contained in the libraries has been obtained by coupling subtractive hybridization with microarray analysis. Positive transcripts have been validated by quantitative PCR. Through in silico analyses, identified genes have been classified on the basis of the pathway in which they are involved, being able to address signaling required for dexamethasone action. Most of the induced transcripts are involved in metabolic processes and regulation of cellular processes. Our results can help to unravel the mechanism of glucocorticoid action in the reversion of A-T phenotype. Moreover, the induction of a specific region of the ATM transcript has been identified as putative biomarker predictive of dexamethasone efficacy on ataxic patients.

Control of cell respiration by nitric oxide in Ataxia Telangiectasia lymphoblastoid cells

Ataxia Telangiectasia (AT) patients are particularly sensitive to oxidative–nitrosative stress. Nitric oxide (NO) controls mitochondrial respiration via the reversible inhibition of complex IV. The mitochondrial response to NO of AT lymphoblastoid cells was investigated. Cells isolated from three patients and three intrafamilial healthy controls were selected showing within each group a normal diploid karyotype and homogeneous telomere length. Different complex IV NO-inhibition patterns were induced by varying the electron flux through the respiratory chain, using exogenous cell membrane permeable electron donors. Under conditions of high electron flux the mitochondrial NO inhibition of respiration was greater in AT than in control cells ( P ≤ 0.05). This property appears peculiar to AT, and correlates well to the higher concentration of cytochrome c detected in the AT cells. This finding is discussed on the basis of the proposed mechanism of reaction of NO with complex IV. It is suggested that the peculiar response of AT mitochondria to NO stress may be relevant to the mitochondrial metabolism of AT patients.

Radiation-induced chromosome breaks in ataxia-telangiectasia cells remain open

Purpose : To determine if broken chromosome-end healing mechanisms through the addition of new telomeric sequences exist in cells having difficulties in rejoining the ends of broken chromosomes. Materials and Methods : A full-colour painting protocol of all human chromosomes by FISH was combined with a telomeric and centromeric labelling using PNA probes to characterize the rejoining pattern and telomere status of radiation-induced chromosome breaks in ataxia-telangiectasia (A-T) and normal lymphoblastoid cell lines. Results : It was first established that the cell lines used for chromosome healing analysis were chromosomally stable. FISH analysis provided evidence that the frequency of deleted chromosomes, apparently unrejoined, was much higher in A-T than in normal cells, as expected by the role of ATM in cell-cycle control, as well as in DNA repair. In spite of their high frequency, broken chromosome ends in A-T cells do not seem to act as substrates for telomerase since additional terminal telomere sequences (more than the 92 expected pairs) indicative of chromosome healing were never observed. Broken chromosome ends in A-T cells remained open. Conclusion : The disability of cells to rejoin broken chromosome ends does not lead to the healing of DSBs by the acquisition of new telomeric sequences.

Variation in radiation-induced apoptosis in ataxia telangiectasia lymphoblastoid cell lines

Purpose : To investigate and compare the ability of Epstein-Barr virus-transformed lymphoblastoid cell lines (LCL) from healthy individuals (normals) and ataxia telangiectasia (A-T) patients to undergo apoptosis after exposure to ionizing radiation. Materials and methods : Four normal and eight A-T LCL were exposed to doses of up to 20 Gy ionizing radiation. Apoptosis induction was studied 24 h after irradiation using three different methods: measurement of caspase-3 activity, PARP-1 cleavage and estimation of the sub-G 1 cell fraction. Results : Of the eight A-T LCL tested, all harbouring truncating ATM mutations, five had a higher level of spontaneous apoptosis than the normal LCL as assessed by the sub-G 1 cell fraction. Four of the eight A-T LCLs showed a similar level of radiation-induced apoptosis after exposure to 5 Gy as the normal LCL. The other four A-T LCL showed a greater radiation-induced apoptotic response, as assessed by at least one of the three techniques. Conclusions : LCL from A-T patients can undergo ionizing radiation-induced apoptosis in spite of a defect in ATM-p53-dependent signalling pathways. However, the apoptotic response is characterized by a large degree of variability between the A-T cell lines, the causes of which remain to be established.

High frequency of deletions at the hypoxanthine-guanine phosphoribosyltransferase locus in an ataxia-telangiectasia lymphoblastoid cell line irradiated with gamma-rays.

The molecular nature of y‐ray‐induced mutations at the hypoxanthine‐guanine phosphoribosyl‐transferase (HPRT) locus in an ataxia‐telangiectasia (A‐T) lymphoblastoid cell line was investigated. Twelve of 15 y‐ray‐induced HPRT‐deficient mutants showed deletions. Eight of them had lost the entire HPRT gene, one showed a 1.9‐kb deletion, and three had deletions of about 40–150 base pairs. Of the eight mutants that lost the entire gene, five had also lost both DXS79 and DXS86, flanking markers of the HPRT locus. The spectrum of mutations induced by y‐irradiation in the A‐T cells showed a high frequency of deletions in comparison with that in a control cell line, WIL2NS. Sequence analysis of breakpoint junctions in four mutants revealed that three of them had junctions between short identical sequences at each breakpoint, leaving one copy at the junction. These results suggest that non‐homologous end‐joining is the major mechanism for deletion formation in A‐T cells.

Caffeine does not potentiate γ-radiation induced DNA damage in ataxia telangiectasia lymphoblastoid cells

Ataxia telangiectasia (AT) cells display a profound sensitivity to ionizing radiation, exhibiting more frequent chromosomal breaks, increased micronuclei formation and abnormal DNA repair kinetics following exposure. Despite the recent cloning of the ATM gene there remains a need for a simple and rapid means of discriminating AT heterozygotes from normal individuals. Caffeine (1,3,7-trimethyl xanthine), known to inhibit the repair of double-strand DNA breaks following ionizing radiation, increases the frequency of radiation induced chromosomal breaks in normal cells. Here we report that caffeine potentiates the induction of chromosomal breaks in G 2 arrested AT heterozygote and normal lymphoblastoid cells, but not in homozygous AT lymphoblastoid cells. This observation parallels the findings reported by others that caffeine fails to potentiate the effect of ionizing radiation in radiation-sensitive yeast strains and radiation sensitive CHO cells. It also suggests that caffeine may somehow mimic the effect of the ATM gene product in normal cells. We also report that caffeine is unlikely to be useful in helping to discriminate AT heterozygotes from normal individuals.

Sister chromatid exchanges and DNA topoisomerase II inhibitors: effect of low concentrations of etoposide (VP-16) in ataxia telangiectasia lymphoblastoid cell lines

The correlation between etoposide (VP-16) cytotoxicity and the induction of sister chromatid exchanges (SCEs) suggested that the promotion of DNA recombination events may be crucial for the activity of antitopoisomerase drugs. To further evaluate this hypothesis, we investigated the correlation between VP-16 induction of SCEs, chromosomal aberrations and cell cycle alterations in lymphoblastoid cell lines derived from patients affected by ataxia telangiectasia (AT), whose cells are known as hypersensitive to the cytotoxic and clastogenic activity of DNA topoisomerase II inhibitors. Our present study has shown that AT homozygous and heterozygous cell lines exposed to low VP-16 concentrations, although hypersensitive to the induction of chromosomal aberrations, exhibit an induction of SCEs comparable to that found in normal cell lines. Moreover, while the clastogenic effect of the drug was directly correlated to the reduction of the mitotic index, the enhancement of SCE frequencies, obtained over the same range of VP-16 concentrations, was not paralleled by a modification of proliferation index. Thus, these results suggest that etoposide retains in AT cells a strong clastogenic and cytostatic activity which is independent from DNA recombination events and which may be important for the induction of cell death by this kind of drug.

Enhancement of frequencies of restriction endonuclease-induced chromatid breaks by arabinoside adenine in normal human and ataxia telangiectasia cells

. The effect of ara A (9- beta -D-arabinofuranosyladenine) a potent inhibitor of DNA synthesis on the frequencies of chromatid breaks induced by restriction endonucleases (RE) has been investigated in normal human and ataxia telangiectasia (AT) lymphoblastoid cells. PvuII, PstI and BamHI, which cause blunt-ended, 3-overhang and 5-overhang cohesive-ended DNA double-strand breaks (dsb) respectively, were introduced into two AT cell lines (AT-KM and AT-PA) and a normal human (N-SW) cell line by the use of streptolysin-O poration. Controls were exposed to gamma -irradiation and similarly treated with or without ara A. Both AT cell lines were found to exhibit higher frequencies of chromatid breaks when treated with RE alone as compared with the normal cell line. The pattern of chromatid response to the three RE was shown to be similar in all three cell lines i.e. PvuII was most clastogenic while PstI and BamHI were both less effective at inducing chromosomal aberrations. Incubation of cells with ara A resulted in an increase in frequencies of chromatid breaks in PvuII and PstI treated cells but no increase was observed in BamHI treated cells. Normal cells showed most response to ara A following treatment with PvuII and PstI (enhancement ratios 4.63 and 3.75 respectively) while AT cells were affected by ara A to a lesser extent indicating a reduced expression of damage by ara A in these lines. Since ara A is a potent inhibitor of DNA synthesis, it was concluded from the elevated frequency of chromosomal aberrations in the presence of ara A that rejoining of RE-induced dsb in genomic DNA of human cells involves nucleotide insertion at dsb termini prior to ligation.

Elevated frequency of p53-independent apoptosis after irradiation increases levels of DNA breaks in ataxia telangiectasia lymphoblasts

. Ataxia telangiectasia is a recessive genetic disease featuring cerebellar degeneration, developmental abnormalities, high cancer risk, immunodeficiency, and radiosensitivity. Increased levels of unrepaired DNA breaks have been observed in irradiated ataxia telangiectasia cells compared to normal cells but no specific DNA break rejoining rate deficiency has been defined. Alterations in radiation-induced p53-dependent apoptosis have been reported for ataxia telangiectasia cells. This study investigated the radiation response of ataxia telangiectasia lymphoblastoid cells using the comet assay and uncovered a new feature of this technique, namely its capacity to preferentially detect living cells. It is shown here that early after exposure to gamma -rays, ataxia telangiectasia lymphoblasts exhibit an elevated frequency of cells committed to die via apoptosis. The observed apoptosis, which is likely to be independent of p53, leads to a higher number of DNA breaks during the first 3 h post irradiation in ataxia telangiectasia cells, relative to controls. Apart from cells undergoing apoptosis, ataxia telangiectasia lymphoblasts have an identical capacity to rejoin radiation-induced DNA breaks as controls. Results suggest that p53-independent apoptosis may contribute to the radiosensitivity and the immune defects of ataxia telangiectasia patients.

Chromosome end associations, telomeres and telomerase activity in ataxia telangiectasia cells.

Cells derived from individuals with ataxia telangiectasia (AT) show enhanced spontaneous levels of chromosomal abnormalities and are sensitive to ionizing radiations and radiomimetic drugs, as evidenced by decreased survival and increased chromosome aberration frequencies at mitosis when compared with normal cell lines. The higher base line frequencies of chromosome aberrations in part involve chromosome end-to-end associations as seen at metaphase. Since telomeres of tumor cells and aging tissues are often reduced in length, chromosome end associations may be due to loss of telomeric repeats. We studied the chromosome behavior and telomeres of two ataxia telangiectasia lymphoblastoid cell lines compared to two normal control cell lines. The ataxia telangiectasia cell lines showed higher frequencies of chromosome end associations both at metaphase and in interphase, determined in prematurely condensed chromosomes of G1 and G2 cells. They also showed higher frequencies of chromosomal breaks at metaphase and fewer telomeric signals determined using fluorescent in situ hybridization with a (TTAGGG)n probe. The frequency of telomeric repeats was variable in the ataxia telangiectasia cell lines (4.3 and 8.2 kb) compared to the normal cell lines (9.6 and 12 kb) and an inverse correlation between telomere length and chromosome end associations was observed. Both ataxia telangiectasia cell lines showed more robust telomerase activity than the normal cell lines, precluding defective enzymatic capacity as the basis for the chromosome end associations. It is possible that chromatin structure in the form of telomere-nuclear matrix interactions are variant in ataxia telangiectasia cells negatively influencing telomerase function and contributing to telomere associations.

Ionizing Radiation and Cell Cycle Progression in Ataxia Telangiectasia

Exposure of mammalian cells to ionizing radiation causes delay in normal progress through the cell cycle at a number of different checkpoints. Abnormalities in these checkpoints have been described for ataxia telangiectasia cells after irradiation. In this report we show that these abnormalities occur at different phases in the cell cycle in several ataxia telangiectasia lymphoblastoid cells. Ataxia telangiectasia cells, synchronized in late G1 phase with either mimosine or aphidicolin and exposed to radiation, showed a reduced delay in entering S phase compared to irradiated control cells. Failure to exhibit G1-phase delay in ataxia telangiectasia cells is accompanied by a reduced ability of radiation to activate the product of the tumor suppressor gene p53, a protein involved in G1/S-phase delay. When the progress of irradiated G1-phase cells was followed into the subsequent G2 and G1 phases ataxia telangiectasia cells showed a more pronounced accumulation in G2 phase than control cells. When cells were irradiated in S phase the extent of delay was more evident in G2 phase and ataxia telangiectasia cells were delayed to a greater extent. These results suggest that the lack of initial delay in both G1 and S phases contributes to the radiosensitivity observed in this syndrome.

Enhanced chromosomal response of ataxia-telangiectasia cells to specific types of DNA double-strand breaks

The chromosomal response of two ataxia-telangiectasia (A-T) lymphoblastoid cell lines (A-T-PA and A-T-KM) to restriction endonucleases (RE) is compared with that of a normal (N-SW) lymphoblastoid cell line. The RE used were PvuII (generating DNA double-strand breaks with blunt termini), BamHI (cohesive termini with 4 base, 5' overhangs) and PstI (cohesive termini with 4 base 3' overhangs). Chromatid aberrations were analysed in cells 5 h after treatment. Cells were porated using streptolysin O to allow entry of RE. Both A-T lines showed an enhanced frequency of chromatid breaks in G2 phase compared with normal cells in response to RE. The enhanced response of A-T cells was most marked in the case of PvuII treatment when the enhancement ratios were 2.5 and 4.2 for A-T-PA and A-T-KM respectively. However, the frequency of DNA double-strand breaks (dsb), measured by neutral filter elution, were considerably lower in A-T-PA cells than N-SW, due to a lower efficiency of poration. When A-T-PA cells were treated with streptolysin O at a higher concentration (0.3 Units/ml), a condition that apparently led to a similar level of poration in A-T-PA as in N-SW cells treated with 0.06 Units/ml as judged by the similar number of dsb induced in the two lines for a given PvuII concentration, the enhancement ratio for A-T-PA cells treated with PvuII increased from 2.5 to 5.8. BamHI and PstI were found to be less clastogenic in all three cell lines as found previously for Chinese hamster cells, although part of this effect may be due to a lower activity, particularly in the case of PstI. However, even at a 4-6-fold higher concentration, BamHI was still less clastogenic than PvuII. It is concluded that dsb with blunt termini are more clastogenic than those with cohesive termini. The results suggest that the chromosomal sensitivity of A-T cells may result from a defect causing a higher rate of conversion of dsb into chromatid aberrations.

Enhanced Chromosomal Response of Ataxia-telangiectasia Cells to Specific Types of DNA Double-strand Breaks

The chromosomal response of two ataxia-telangiectasia (A-T) lymphoblastoid cell lines (A-T-PA and A-T-KM) to restriction endonucleases (RE) is compared with that of a normal (N-SW) lymphoblastoid cell line. The RE used were PvuII (generating DNA double-strand breaks with blunt termini), BamHI (cohesive termini with 4 base, 5' overhangs) and PstI (cohesive termini with 4 base 3' overhangs). Chromatid aberrations were analysed in cells 5 h after treatment. Cells were porated using streptolysin O to allow entry of RE. Both A-T lines showed an enhanced frequency of chromatid breaks in G2 phase compared with normal cells in response to RE. The enhanced response of A-T cells was most marked in the case of PvuII treatment when the enhancement ratios were 2.5 and 4.2 for A-T-PA and A-T-KM respectively. However, the frequency of DNA double-strand breaks (dsb), measured by neutral filter elution, were considerably lower in A-T-PA cells than N-SW, due to a lower efficiency of poration. When A-T-PA cells were treated with streptolysin O at a higher concentration (0.3 Units/ml), a condition that apparently led to a similar level of poration in A-T-PA as in N-SW cells treated with 0.06 Units/ml as judged by the similar number of dsb induced in the two lines for a given PvuII concentration, the enhancement ratio for A-T-PA cells treated with PvuII increased from 2.5 to 5.8. BamHI and PstI were found to be less clastogenic in all three cell lines as found previously for Chinese hamster cells, although part of this effect may be due to a lower activity, particularly in the case of PstI. However, even at a 4-6-fold higher concentration, BamHI was still less clastogenic than PvuII. It is concluded that dsb with blunt termini are more clastogenic than those with cohesive termini. The results suggest that the chromosomal sensitivity of A-T cells may result from a defect causing a higher rate of conversion of dsb into chromatid aberrations.

High recurrence of rearrangements involving chromosome 14 in an ataxia telangiectasia lymphoblastoid cell line and in its mutagen-treated derivatives

The cytogenetic characterization of CH cell line obtained by Epstein-Barr-virus transformation of the lymphocytes of a patient affected by ataxia telangiectasia is reported. Control CH cells and 2 subcultures treated with the mutagens R7000 or NQO were developed in parallel and studied. A common chromosome anomaly, a der(14) t(11;14) (q13.2;q32), was found in all the studied karyotypes, indicating that it occurred either in vivo or early in vitro. In non-treated cultures, additional anomalies were present in 6 derived subclones. All R-7000 treated cells had the same karyotype corresponding to one of the subclones observed without prior treatment. All NQO-treated cells acquired 2 common anomalies, and could be differentiated into 2 subclones because of the addition of a t(7;14) or a t(11;14). Chromosome 14 was involved in various rearrangements after breakage in band q11.2 or q12 in 6/8 subclones. This was not correlated with tumorigenicity, which was clearly increased in mutagen-treated cells as tested by in vitro growth in semi-solid medium and in vivo by grafts into nude mice or growth on the chorio-allantoic membrane of chick embryos. The CH cell line and its derivatives appear to be a promising in vitro system, showing various stages progressing towards malignancy, and reproducing a number of chromosome anomalies spontaneously occurring in AT patients.

The Response of Ataxia-Telangiectasia Lymphoblastoid Cells to Neutron Irradiation

The response of control and ataxia-telangiectasia (A-T) cells to increasing doses of high-linear-energy-transfer (LET) ionizing radiation (neutrons) was compared. Ataxia-telangiectasia cells were markedly more sensitive to neutron irradiation than were control cells. The D0 value for the two A-T cell lines was 0.4 Gy while the value for controls was approximately 1.4 Gy. Fast neutrons were considerably more effective than gamma rays in inducing cell death in both cell types, but the sensitivity factor remained approximately the same as with gamma rays. A minimal depression of DNA synthesis was observed in ataxia-telangiectasia cells after neutron irradiation, similar to that reported previously after gamma irradiation. The extent of inhibition was not significantly greater in control cells, contrary to that seen with gamma rays. In time-course experiments a significant difference in degree of inhibition of DNA synthesis was observed between the cell types. Low doses of fast neutrons induced a G2-phase delay in both cell types, but the degree and extent of this delay was greater in ataxia-telangiectasia cells as observed previously with low-LET radiation.

X-ray-induced G 2 arrest in ataxia telangiectasia lymphoblastoid cells

Sensitivity to X-ray-induced G 2 arrest was compared between ataxia telangiecstasia (AT) lymphoblastoid cells and normal human cells. Flow cytometrical analysis of cells following X-ray irradiation revealed that the fraction of cells with 4n DNA content was greater in T cells than in normal cells as previously reported by other investigators. However, the other parameters for cell-cycle progression kinetics including mitotic indices, cumulative mitotic indices and cumulative labelled mitotic indices indicated that X-ray-induced G 2 arrest as a function of dose in AT cells was indistunguishable from that in normal cells. Moreover, no significant difference in cell viability was notd between AT and normal cells until 48 h following X-irradiation up to 2.6 Gy, although X-irradiated AT cells, compared to normal cells, showed a significantly decreased survival in terms of cell multiplication in growth medium and colony formation in soft agar. These data collectively suggest that the greater accumulation of AT cells with 4n DNA content in flow cytometry cannot be attributed to mor stringent irreversible blockage of cell-cycle progression at the G 2 phase and eventual cell death there. The possible reasons fro this greater accumulation are discussed.

A defect in DNA topoisomerase II activity in ataxia-telangiectasia cells

DNA topoisomerase type I and II activities were determined by serial dilution in nuclear extracts from control and ataxia-telangiectasia lymphoblastoid cells. Topoisomerase I activity, assayed by relaxation of supercoiled plasmid DNA, was found to be approximately the same in both cell types. In order to remove interference from topoisomerase I, the activity of topoisomerase II was measured by the unknotting of knotted P4 phage DNA in the presence of ATP. The activity of topoisomerase II was markedly reduced in two ataxia-telangiectasia cell lines, AT2ABR and AT8ABR, compared to controls. This reduction in activity was detected with increasing concentration of protein and in time course experiments at a single protein concentration. A third cell line, AT3ABR, did not have a detectably lower activity of topoisomerase II when assayed under these conditions. The difference in topoisomerase II activity in the ataxia-telangiectasia cell lines examined may reflect to some extent the heterogeneity observed in this syndrome.

Comparison of radiation-induced cell cycle effects in ataxia telangiectasia fibroblasts and lymphoblastoid cells

Comparison of radiation-induced cell cycle effects in ataxia telangiectasia fibroblasts and lymphoblastoid cells

Ionizing radiation and DNA-chain elongation in ataxia telangiectasia lymphoblastoid cells

DNA-chain elongation rates, determined by sedimentation analysis, were found to be similar in control and ataxia-telangiectasia lymphoblastoid cells. A γ-radiation dose of 6 Gray, which had previously been shown to have a marked inhibitory effect on initiation of DNA replication, had no appreciable effect on elongation rates in either cell type. Elongation rates were also determined at 20 Gray of γ-rays by pulsing cells with [ 3H]thymidine prior to irradiation to avoid anomalous sedimentation behaviour. At this radiation dose elongation was almost completely inhibited in control cells while little or no inhibition was observed in ataxia-telangiectasia cells. Deoxyribonucleoside triphosphate pool equilibration times were not altered at either dose.