DNA repair in lymphoblastoid cell lines established from human genetic disorders

Ataxia-telangiectasia (A-T): An emerging dimension of premature ageing

A-T is a prototype genome instability syndrome and a multifaceted disease. A-T leads to neurodegeneration - primarily cerebellar atrophy, immunodeficiency, oculocutaneous telangiectasia (dilated blood vessels), vestigial thymus and gonads, endocrine abnormalities, cancer predisposition and varying sensitivity to DNA damaging agents, particularly those that induce DNA double-strand breaks. With the recent increase in life expectancy of A-T patients, the premature ageing component of this disease is gaining greater awareness. The complex A-T phenotype reflects the ever growing number of functions assigned to the protein encoded by the responsible gene - the homeostatic protein kinase, ATM. The quest to thoroughly understand the complex A-T phenotype may reveal yet elusive ATM functions.

Genetic and biochemical studies with ataxia telangiectasia. A review

This article summarizes the genetics and clinical features of ataxia telangiectasia (AT) and then reviews recent cytogenetic, cellular, and biochemical studies which support the hypothesis that a defect in DNA repair is responsible for the various manifestations of the disease. The biochemical evidence further indicates that the defect specifically reduces the cellular capacity to remove bases and nucleotides damaged by ionizing radiation, without affecting the cells' ability to scavenge free radicals or to rejoin breaks in the sugar-phosphate backbone of DNA. Suggestions for additional research to more precisely identify the repair defect will also be presented.

Variation in arsenic-induced sister chromatid exchange in human lymphocytes and lymphoblastoid cell lines

This study was undertaken to compare the genotoxic effects of arsenite in cultured human lymphocytes and lymphoblastoid cell lines from a group of normal human volunteers. The goal was to determine whether, as found with other genotoxins, subgroups might exist which showed relative high or low sensitivity to induction of sister chromatid exchanges (SCEs) by this metal. Primary lymphoblast cultures were established by treatment with phytohemagglutinin (PHA-L). Lymphoblastoid cell lines were established by transformation with Epstein-Barr virus. Cultures were exposed for 40 h to sodium arsenite (AsIII) and SCEs assayed by 5-bromo-2'-deoxyuridine incorporation and staining by fluorescence plus Giemsa. SCEs were increased by arsenite in a dose-dependent manner over the concentration range of 10(-7)-10(-5) M. SCEs could not be scored above 10(-5) M because of cytotoxicity. Comparison of SCE frequency in primary lymphocyte cultures among individuals showed substantial variation in sensitivity to arsenite, with some showing no significant effect while others showed a 2-3-fold increase in SCE frequency. In one lymphoblastoid cell line especially sensitive to arsenite, arsenic acid (AsV) or dimethylarsinic acid (DMA) at concentrations up to 10(-5) M did not increase the SCE frequency suggesting that AsIII is the active form of arsenic. When pooled data from the primary lymphocytes was compared to that obtained with the lymphoblastoid cells, the slopes of the dose-response curves for ASIII-induced SCEs were similar. The sensitivity of the majority of the individual primary lymphocyte cultures to SCE induction by arsenite was correlated with the sensitivity of the lymphoblastoid cultures established from the same individual. However, in three individuals no correlation was found. Individual lymphoblastoid cell lines retained their As sensitivity after cryopreservation and subsequent revival. Whether the genotoxic response to As is genetically controlled or the result of phenotypic selection is being explored in these stable lymphoblastoid cell lines.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Flowcytometric analysis of DNA pattern of cells derived from xeroderma pigmentosum A--hypersensitivity to vincristine, etoposide and methotrexate

Xeroderma pigmentosum complementation group A (XPA) is one of the DNA repair deficient syndromes. The cell biological features of XPA were examined by flowcytometry using Epstein Barr (EB) virus-transformed lymphoblastoid cells. Cellular sensitivity to vincristine (VCR), etoposide (VP-16) and methotrexate (MTX) were assayed by DNA pattern changes by flowcytometry. Recently, ataxia-telangiectasia (AT), one of the same kind of disorder, has been reported to have an increased sensitivity to VCR and VP-16. However, AT showed some resistance to MTX according to other reports. Our results showed that XPA had an increased sensitivity to VCR and also to VP-16. Moreover, different from AT, XPA showed some sensitivity to MTX. Thus there is some cell biological similarity between XPA and AT, as well as some difference of the abnormality in the DNA repair pathway.

X-ray-induced G2 arrest in ataxia telangiectasia lymphoblastoid cells

Sensitivity to X-ray-induced G2 arrest was compared between ataxia telangiectasia (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 AT 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 G2 arrest as a function of dose in AT cells was indistinguishable from that in normal cells. Moreover, no significant difference in cell viability was noted 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 more stringent irreversible blockage of cell-cycle progression at the G2 phase and eventual cell death there. The possible reasons for this greater accumulation are discussed.

Host cell reactivation of CAT-expression vectors as a method to assay for cloned DNA-repair genes

We demonstrate the feasibility of using passive host-cell reactivation of a shuttle-vector pRSVcat to detect cloned DNA-repair genes. As models, a transient expression vector, pRSVdenV, and a positive-selection vector, pRSVdenV/SVgpt, were constructed containing the T4 coliphage denV gene, coding for an ultraviolet-specific endonuclease, under promotion of the Rous sarcoma virus (RSV) long-terminal repeat. Cotransfection of one or three copies of pRSVdenV per UV-irradiated pRSVcat molecule into xeroderma pigmentosum (XP) cells (XP12Ro[M1]) resulted in a dramatic increase in transient expression of chloramphenicol acetyl transferase (CAT) activity. XP clones stable transformed by pRSVdenV/SVgpt but not the parent cell line rescued CAT activity from this UV-irradiated reporter gene. The ability to express CAT activity from a UV-irradiated pRSVcat correlated with the presence of the structural denV gene as detected by Southern blot analysis. Post-UV irradiation colony-forming ability and DNA nucleotide excision-repair synthesis were partially restored in XP clones which rescued CAT activity. These results demonstrate the feasibility of using the cloned denV gene with its well characterized pyrimidine cyclobutane dimer-specific endonuclease activity to reconstitute UV-induced DNA repair in human cells deficient in DNA repair. Measuring CAT expression from pRSVcat affords a rapid, sensitive procedure to screen for functional cloned DNA-repair genes and to test mutant cells for defects in DNA repair.

Structural alterations of DNA ligase I in Bloom syndrome

Cell lines derived from seven patients with Bloom syndrome all contain a DNA ligase I with unusual properties. Six lines were shown to have a reduced level of this enzyme activity and the residual enzyme was anomalously heat-labile. The seventh line contained a dimeric rather than monomeric form of ligase I. Several cell lines representative of other inherited human syndromes have apparently normal DNA ligases. The data indicate that Bloom syndrome is due to a defect in the structure of DNA ligase I caused by a "leaky" point mutation occurring at one of at least two alternative sites.

DNA repair, DNA synthesis and cell cycle delay in human lymphoblastoid cells differentially sensitive to the cytotoxic effects of nitrogen mustard

Two cloned human lymphoblastoid cell lines, Raji and TK6, differ in their sensitivity to the cytotoxic effects of nitrogen mustard (HN2). Raji cells exhibit a biphasic response with an initial D value of 0.06 microgram/ml and a final slope of 0.25 microgram/ml. TK6 cells were considerably more sensitive, D0 value 0.02 microgram/ml. Dose-response relationships for delay in cell cycle progression were measured using flow cytometry. Delay in S-phase traverse was concentration-dependent in both cell lines, and at a given concentration was 2-fold greater in TK6 than in Raji. Numbers of crosslinks (determined by alkaline elution) increased linearly with increasing HN2 concentration and were approximately 2-fold higher in TK6 than in Raji. At equal levels of DNA crosslinks, rates of removal were similar in both cell lines. Inhibition of [3H]TdR uptake was concentration-dependent and the extent of inhibition was similar in both cell lines. Recovery from HN2-induced inhibition of cell cycle progression markedly preceded recovery from inhibition of [3H]TdR incorporation suggesting that nucleotide pools are markedly perturbed in HN2-treated cells. The difference in sensitivity of these two cell lines cannot be adequately explained by differences in amounts of initial DNA damage, rates of repair, differential S-phase delay or rate of loss of DNA crosslinks.

Use of lymphoblastoid cell lines to evaluate the hypersensitivity to ultraviolet radiation in Cockayne syndrome

Cockayne syndrome (CS) is a rare autosomal recessive disease characterized by acute sun sensitivity, cachectic dwarfism, and neurologic and skeletal abnormalities. Cultured skin fibroblasts from patients with this disease are known to be hypersensitive to the lethal effects of 254-nm UV radiation. We have studied the sensitivity of 254-nm UV radiation of lymphoblastoid lines derived from 3 typical CS patients, 1 atypical CS patient who had a very late age of onset of clinical manifestations, 2 patients who had both xeroderma pigmentosum (XP) and typical CS, and 3 heterozygous parents of these patients. Post-UV survival was determined by the trypan-blue dye-exclusion method. The lymphoblastoid lines from the 3 typical CS patients, the atypical CS patient, and the 2 patients with both CS and XP had decreased post-UV viability in comparison with lines from normal donors. Lines from the heterozygous parents had normal post-UV viability. The post-UV viability of the typical CS lines was similar to that of a XP complementation group C line. The relative post-UV viability of lymphoblastoid lines from the typical CS patients was similar to the relative post-UV survival of their fibroblast lines. The lymphoblastoid line from the atypical CS patient had a post-UV viability similar to that of the typical CS patients. Thus, the relative hypersensitivity of CS patients' cells in vitro does not reflect the severity or age of onset of the patients' clinical manifestations. The lymphoblastoid lines from the 2 patients who had both CS and XP were significantly more sensitive to the UV radiation than those from patients with only CS. Our studies demonstrate that lymphoblastoid lines from patients with CS are appropriate and useful cell lines for the study of the inherited hypersensitivity to UV radiation.

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.

Similar repair of O6-methylguanine in normal and ataxia-telangiectasia fibroblast strains. Deficient repair capacity of lymphoblastoid cell lines does not reflect a genetic polymorphism

The ability of human fibroblast strains to repair the mutagenic DNA adduct O6-methylguanine (O6-MeG) induced by brief exposure to N-methyl-N'-nitroso-N-nitrosoguanidine (MNNG) was investigated. The repair reaction proceeded rapidly during the first hour after alkylation, followed by a slow, continuous phase of repair, and both processes were saturated by low doses of carcinogen. This was similar to what had previously been found in human lymphoblastoid lines. Three fibroblast strains from healthy donors and six strains from patients with ataxia telangiectasia were all proficient in their capacity to repair O6-MeG and had the same sensitivity to the cytotoxicity of MNNG and methyl methanesulphonate as normal cells. Three of these cell strains were derived from individuals whose lymphoblastoid lines were deficient in their ability to repair O6-MeG. These lymphoblastoid lines were also extremely hypersensitive to killing by methylating carcinogens. Because non-transformed cells from the same donors behaved normally with regard to both parameters, we concluded that the repair deficiency accompanied by carcinogen hypersensitivity of the lymphoblastoid lines does not indicate a genetic deficiency in the donor. These findings imply that lymphoblastoid lines may not always be the appropriate cell type for investigating genetic susceptibility to chemical mutagens.

Cellular DNA damage by nitrosocimetidine: a comparison with N-methyl-N'-nitroso-nitrosoguanidine and x-irradiation

Permanently proliferating lymphoblastoid cell lines (LCLs) and normal unstimulated peripheral blood leukocytes have been used to study the effects of nitrosocimetidine (NC) on cultured human lymphoid cells. The approaches that were used to assess the cells' ability to cope with NC were: (i) determination of cell survival as measured by colony formation in microtiter plates; (ii) quantitation of DNA synthesis and DNA-repair replication by isopyknic sedimentation of DNA density labeled with 5-bromo-2-deoxyuridine (BrdU); (iii) measurement of the induction of alkali labile lesions and strand breaks by NC in 3H-labeled DNA using velocity sedimentation in alkaline sucrose. In summary, treatment with NC was found to inhibit both replicative DNA synthesis and colony formation in LCLs. At the molecular level, NC treatment induced alkali labile lesions in LCL DNA and elicited DNA-repair replication in proliferating LCLs as well as unstimulated lymphocytes. Considered in total, these data indicate that NC is reactive with human DNA in the cellular environment in a manner similar to methylating nitroso compounds which have been shown to be carcinogenic. The significance of these findings will be discussed.