Repair of DNA damaged by alkylating carcinogens is defective in xeroderma pigmentosum-derived fibroblasts

Guanine N7-alkylation in mice in vivo by metrifonate - discussion of possible genotoxic risk in mammals

Following intraperitoneal administration to male mice (strain AB Jena/Halle) of 14CH3-labelled metrifonate, 22 Ci/mol, in dosages of 0.48, 0.40 and 0.065 mmol/kg, DNA from liver and kidneys was analysed for 14C in N-7 methylguanine (7-MeG). The extent of methylation in liver was found to be maximal at 6 hrs after injection in amounts of 6-8 and 0.8 mumol 7-MeG/mol guanine for the high and the low dose, corresponding to a covalent binding index CBI 4-5. The half-life of excretion of 7-MeG was 5 hrs for the high and 15 hrs for the low dose. The extent of methylation at 0-6 of guanine was estimated to be around 0.002-0.01 mumol 0-6 MeG/mol guanine. Data from references concerning methyl methanesulfonate and dimethyl sulfate are compared with those of metrifonate and the genotoxic response of methylating and non-methylating metabolites is discussed.

Defects in antioxidant defense and calcium transport in the epidermis of xeroderma pigmentosum patients

A comparative study of the antioxidant enzymes superoxide dismutase, catalase, glutathione reductase and thioredoxin reductase was undertaken in two families with xeroderma pigmentosum (XP) and in healthy controls of corresponding skin phototypes. Epidermal blister roofs obtained from the XP patients revealed significant decreases in catalase, thioredoxin reductase, and superoxide dismutase, but glutathione reductase was unaffected. In addition, keratinocytes established from XP patients contained a significantly higher than normal intracellular calcium concentration compared with control cells from a corresponding skin type. Keratinocytes established from an XP obligate heterozygote revealed intermediate levels of calcium between XP homozygotes and controls. Previously high intracellular calcium has been shown to compromise the redox status of keratinocytes by allosteric inhibition of the thioredoxin reductase/thioredoxin electron transfer system. In XP homozygous keratinocytes from sun-exposed epidermis, the intracellular concentration of reduced thioredoxin was decreased to 50% compared with these cells from unexposed skin. Taken together, the results from this study indicate that the epidermis in XP patients lacks effective defense against free radicals and peroxides. In addition to the well-established defect in the normal rates of unscheduled DNA repair, these findings provide an even better explanation for the multiple cutaneous neoplasms in these patients.

Genotoxic effects of thiram evaluated by sister-chromatid exchanges in human lymphocytes

The purpose of this investigation was to study the genotoxic potential of thiram (CAS No. 137-26-8) using an in vitro sister-chromatid exchange (SCE) assay with human lymphocytes. The results indicate that thiram and its metabolites increase the SCE frequencies 2-fold over those observed in the negative controls. The standard inducers cyclophosphamide and ethyl methanesulfonate increased SCE frequencies 10- and 4-fold, respectively, over untreated levels.

Carrier detection in xeroderma pigmentosum

We were able to detect clinically normal carriers of xeroderma pigmentosum (XP) genes with coded samples of either peripheral blood lymphocytes or skin fibroblasts, using a cytogenetic assay shown previously to detect individuals with cancer-prone genetic disorders. Metaphase cells of phytohemagglutinin-stimulated T-lymphocytes from eight individuals who are obligate heterozygotes for XP were compared with those from nine normal controls at 1.3, 2.3, and 3.3 h after x-irradiation (58 R) during the G2 phase of the cell cycle. Lymphocytes from the XP heterozygotes had twofold higher frequencies of chromatid breaks or chromatid gaps than normal (P less than 10(-5)) when fixed at 2.3 or 3.3 h after irradiation. Lymphocytes from six XP homozygotes had frequencies of breaks and gaps threefold higher than normal. Skin fibroblasts from an additional obligate XP heterozygote, when fixed approximately 2 h after x-irradiation (68 R), had a twofold higher frequency of chromatid breaks and a fourfold higher frequency of gaps than fibroblasts from a normal control. This frequency of aberrations in cells from the XP heterozygote was approximately half that observed in the XP homozygote. The elevated frequencies of chromatid breaks and gaps after G2 phase x-irradiation may provide the basis of a test for identifying carriers of the XP gene(s) within known XP families.

Formation and removal of DNA adducts after treatment of Chinese hamster ovary cells with N-methyl- and N-ethyl-N-nitrosourea

We have studied formation and stability of alkylguanines following treatment of Chinese hamster ovary cells with either N-[3H]methyl-N-nitrosourea (MeNOUr) (applied at 50 microM and 40 microM concentrations) or N-[3H]ethyl-N-nitrosourea (EtNOUr) (applied at 43.1 microM). Analyses of acid hydrolysates of the methylated DNA revealed that 9.3% and 57.0% of the total DNA were O6-methylguanine (m6Gua) and 7-methylguanine (m7Gua), respectively. Analysis of enzymic hydrolysate resulted in 8.2% m6Gua and 50.3% m7Gua. For ethylation, the % of ethylated purines identified as O6-ethylguanine (e6Gua) and 7-ethylguanine (e7Gua) were 20.4% and 31.3%, respectively. Half-lives of the main alkylated purines were determined by analysing DNA of dividing cultures over a time interval of 48 h after treatment with carcinogens. Half-lives measured for methylated DNA bases were: m1Ade, 20.6 h; m3Ade, 25.5 h; m7Ade, 0.9 h; m3Gua, 1.1 h; m6Gua, infinity; m7Gua, 39.1 h. Determinations at the level of deoxyribonucleosides resulted in similar half-lives: m3dA, 15.2 h; m7dA, 2.7 h; m3dG, 2.3 h; m6dG, 224 h; m7dG, 25.6 h. The corresponding values for ethylated purines were: e3Ade, 2.9 h; e7Ade, 7.1 h; e3Gua, 1.4 h; e6Gua, infinity; e7Gua, 42.6 h. The relatively high yields of the premutagenic m6Gua and e6Gua, and their long half-lives (greater than or equal to 224 h) are consistent with the suggestion that these adducts play a dominant role in mutation induction at the hypoxanthine-guanine phosphoribosyltransferase (hgprt) locus in CHO cells.

Effects of antimutagenic flavourings on SCEs induced by chemical mutagens in cultured Chinese hamster cells

Effects of antimutagenic flavourings such as vanillin, ethylvanillin, anisaldehyde, cinnamaldehyde, coumarin and umbelliferone on the induction of SCEs by MMC were investigated in cultured Chinese hamster ovary cells. None of these 6 flavourings showed any SCE-inducing activity by themselves. However, an obvious increase in the frequencies of SCEs was observed when MMC-pretreated cells were cultured in the presence of each flavouring. All these compounds have either an alpha, beta-unsaturated carbonyl group or a carbonyl functionality neighbouring the phenyl group which may react with an enzyme SH-group and cause higher-order structure changes. SCE-enhancing effects of vanillin were further investigated on 6 other kinds of mutagens. Vanillin was also effective on SCEs induced by EMS, ENNG, ENU or MNU. On the other hand, MMS- or MNNG-induced SCEs were not influenced at all by vanillin. SCE-enhancing effects of vanillin seemed to be dependent on the quality of lesions in DNA.

Hypersensitivity of Bloom's syndrome fibroblasts to N-ethyl-N-nitrosourea

Fibroblast cells from two Japanese patients with Bloom's syndrome (BS) and normal donors were studied for the inactivation of colony-forming ability and the induction of sister-chromatid exchanges (SCEs) after N-ethyl-N-nitrosourea (ENU) treatment. The reduction of ENU-induced SCEs as a function of post-treatment incubation time was also compared between BS and normal fibroblasts. BS cells were approximately 4 times more sensitive than normal cells to the lethal effect of ENU and remarkably hypersensitive to the SCE induction by ENU. The post-treatment incubation of ENU-treated normal cells in the fresh medium resulted in a time-dependent decrease of the SCE level until 6 h after which time the SCE level remained the plateau of about 50% of the initial level. In contrast, the ENU-induced SCEs in BS cells decreased much more slowly with post-treatment incubation time and its half life was 24 h. These results collectively support the view that BS cells may be defective in the rapid repair of certain type(s) of DNA damages induced by ENU.

4NQO- or MNNG-resistant variants established from a human cell line, RSb, with high sensitivity to both agents

From a human cell line, RSb, with high sensitivity to the killing effects of 4-nitroquinoline 1-oxide (4NQO), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and 254-nm ultraviolet light, a 4NQO-resistant variant, Qr-10, and an MNNG-resistant one, Gr-10, were established using ethyl methanesulfonate as the mutagen. Cell proliferation studies and colony-formation assays revealed that Qr-10 and Gr-10 cells actively proliferated under conditions where RSb cell proliferation was greatly inhibited by 4NQO and MNNG, respectively. Total cellular DNA synthesis, as estimated by [Me-3H]thymidine uptake into acid-insoluble cell materials, was depressed in 4NQO-treated Qr-10 and MNNG-treated Gr-10 cells as it was in chemical-treated RSb cells, but recovered more markedly from such inhibition in the variants. 4NQO- and MNNG-induced DNA-repair replication synthesis was enhanced to a greater extent in Qr-10 and Gr-10 cells, respectively, than in RSb cells. The Qr-10 and Gr-10 cells showed the same respective susceptibility to the effects of MNNG and 4NQO, on cell growth and DNA synthesis and DNA-repair synthesis as did the parent cells. But, Qr-10 cells had more resistance to UV-killing and higher levels of UV-induced DNA-repair synthesis than did RSb cells, while UV-susceptibility of Gr-10 cells was the same as that of the latter.

Sensitivity of lymphocytes from patients with systemic lupus erythematosus to the induction of sister chromatid exchanges by alkylating agents and bromodeoxyuridine

Several authors have suggested that the cells of patients with systemic lupus erythematosus (SLE) have defects of DNA repair. Cells from patients with the classical chromosomal instability syndromes, thought to be due to defects in DNA repair, are hypersensitive to the chromosome damaging effects of some DNA damaging agents, measured as sister chromatid exchange (SCE). We have observed that lymphocytes of patients with SLE are not more sensitive than lymphocytes from healthy individuals to the SCE inducing effects of the DNA damaging agents methyl nitrosourea, methyl methanesulphonate, chlorambucil, and bromodeoxyuridine. These observations do not support the suggestion that cells of patients with SLE have an underlying defect of DNA repair.

Comparison of the sensitivity of Fanconi's anemia and normal fibroblasts to the induction of sister-chromatid exchanges by photoaddition of mono- and bi-functional psoralens

The induction of sister-chromatid exchanges (SCE) by photoaddition of a monofunctional furocoumarin (pyrido[3,4-c]psoralen, PyPs) and a bifunctional furocoumarin (8-methoxypsoralen, 8-MOP) in a normal and three Fanconi anemia (FA) fibroblastic cell lines was investigated. When compared to normal cells, the three FA cell lines demonstrated: a higher sensitivity to 8-MOP photoaddition; an equal or reduced sensitivity to PyPs photoaddition in the low dose range. Normal cells demonstrated a higher sensitivity to photoaddition of PyPs than to 8-MOP in the range of doses used; this is likely to be related to the higher amount of lesions induced by PyPs in DNA. Since FA cells were almost equally sensitive to 8-MOP and PyPs photoaddition and demonstrated a higher sensitivity to SCE induction by 8-MOP than normal cells, it can be concluded that this latter difference is mainly due to cross-links.

Inhibition of uracil-DNA glycosylase increases SCEs in BrdU-treated and visible light-irradiated cells

We have approached the study of the ability of different types of lesions produced by DNA-damaging agents to develop sister-chromatid exchanges (SCEs) by analyzing SCE levels observed in Allium cepa L cells with BrdU-substituted DNA and exposed to visible light (VL), an irradiation which produces uracil residues in DNA after debromination of bromouracil and enhances SCE levels but only above a certain dose. We have partially purified an uracil-DNA glycosylase activity from A. cepa L root meristem cells, which removes uracil from DNA, the first step in the excision repair of this lesion. This enzyme was inhibited in vitro by 6-amino-uracil and uracil but not by thymine. When cells exposed to VL, at a dose that did not produce per se an SCE increase, were immediately post-treated with these inhibitors of uracil-DNA glycosylase, a significant increase in SCE levels was obtained. Moreover, SCE levels in irradiated cells dropped to control level when a short holding time (less than 15 min) elapsed between exposure to VL and the beginning of post-treatment with the inhibitor. Thus, our results (1) showed that inhibitors of uracil-DNA glycosylase enhanced SCE levels in cells with unifilarly BrdU-substituted DNA exposed to visible light; (2) pointed to uracils and/or to some products of their repair as lesions responsible for SCE formation under our experimental conditions; and (3) indicated the existence of a very rapid repair of SCE-inducing lesions produced by visible light irradiation of cells with unifilarly BrdU-containing DNA.

In vivo formation and persistence of modified nucleosides resulting from alkylating agents

Alkylating agents are ubiquitous in the human environment and are continuously synthesized in vivo. Although many classes exist, interest has been focused on the N-nitroso compounds, since many are mutagens for bacteria, phage, and cells, and carcinogens for mammals. In contrast to aromatic amines and polyaromatic hydrocarbons which can react at carbons, simple alkylating agents react with nitrogens and oxygens: 13 sites are possible, including the internucleotide phosphodiester. However, only the N-nitroso compounds react extensively with oxygens. In vivo, most possible derivatives have been found after administration of methyl and ethyl nitroso compounds. The ethylating agents are more reactive toward oxygens than are the methylating agents and are more carcinogenic in terms of total alkylation. This is true regardless of whether or not the compounds require metabolic activation. It has been hypothesized that the level and persistence of specific derivatives in a "target" cell correlates with oncogenesis. However, no single derivative can be solely responsible for this complex process, since correlations cannot be made for even a single carcinogen acting on various species or cell types. Some derivatives are chemically unstable, and the glycosyl bond is broken (3- and 7-alkylpurines), leaving apurinic sites which may be mutagenic. These, as well as most adducts, are recognized by different enzymatic activities which remove/repair at various rates and efficiencies depending on the number of alkyl derivatives, as well as enzyme content in the cell and recognition of the enzyme. Evaluation of human exposure requires early and sensitive methods to detect the initial damage and the extent of repair of each of the many promutagenic adducts.

Interaction of UV and N-methyl-N'-nitro-N-nitrosoguanidine: cytotoxicity and mutagenicity in V79 cells

Killing and mutation by UV in the MNNG-exposed population of V79 cells, as well as by MNNG in the UV-irradiated population of these cells have been studied. It was observed that pretreatment with MNNG increased the killing and mutation by UV, whereas, pretreatment with UV had no effect upon killing and mutation by MNNG. The increase in sensitivity to UV due to pretreatment with MNNG was lost if UV exposure was delayed for 24 h after MNNG treatment.

Induction of 6-thioguanine-resistant mutants and SCEs by 3 chemical mutagens (EMS, ENU and MMC) in cultured human blood lymphocytes

Human peripheral blood lymphocytes were exposed in vitro to a series of graded concentrations of ethyl methanesulphonate (EMS) or N-ethyl-N-nitrosourea (ENU) or mitomycin C (MMC) to: (a) estimate the frequency of thioguanine-resistant (TGr) cells using the T-cell cloning technique, (b) examine the induction of sister-chromatid exchanges (SCEs) by these chemicals in the lymphocytes of the same blood sample used to study the TGr cells, and (c) assess the nature of correlations between these two biological end-points. The frequencies of TGr cells as well as those of SCEs increased with increasing concentration of the chemicals studied. For EMS and ENU, the increases were consistent with linear dose-effect relationships. There was a linear relationship between SCEs and mutation induction for all 3 chemicals; but the ratio of induced SCEs to induced mutants was different for the different chemicals, being highest for ENU, followed by EMS and MMC, in that order. The basis for these differences is discussed in the light of what is known about the relationships between chemical reactivity patterns and the resultant biological effects of these chemicals.

Regulation of DNA repair in serum-stimulated xeroderma pigmentosum cells

The regulation of DNA repair during serum stimulation of quiescent cells was examined in normal human cells, in fibroblasts from three xeroderma pigmentosum complementation groups (A, C, and D), in xeroderma pigmentosum variant cells, and in ataxia telangiectasia cells. The regulation of nucleotide excision repair was examined by exposing cells to ultraviolet irradiation at discrete intervals after cell stimulation. Similarly, base excision repair was quantitated after exposure to methylmethane sulfonate. WI-38 normal human diploid fibroblasts, xeroderma pigmentosum variant cells, as well as ataxia telangiectasia cells enhanced their capacity for both nucleotide excision repair and for base excision repair prior to their enhancement of DNA synthesis. Further, in each cell strain, the base excision repair enzyme uracil DNA glycosylase was increased prior to the induction of DNA polymerase using the identical cells to quantitate each activity. In contrast, each of the three xeroderma complementation groups that were examined failed to increase their capacity for nucleotide excision repair above basal levels at any interval examined. This result was observed using either unscheduled DNA synthesis in the presence of 10 mM hydroxyurea or using repair replication in the absence of hydroxyurea to quantitate DNA repair. However, each of the three complementation groups normally regulated the enhancement of base excision repair after methylmethane sulfonate exposure and each induced the uracil DNA glycosylase prior to DNA synthesis. These results suggest that there may be a relationship between the sensitivity of xeroderma pigmentosum cells from each complementation group to specific DNA damaging agents and their inability to regulate nucleotide excision repair during cell stimulation.

Relationship between cell killing, chromosomal aberrations, sister-chromatid exchanges and point mutations induced by monofunctional alkylating agents in Chinese hamster cells. A correlation with different ethylation products in DNA

Several monofunctional alkylating agents (AA) were compared for their ability to induce chromosomal aberrations, cell killing, sister-chromatid exchanges (SCE) and point mutations in Chinese hamster cells (CHO and V79 cells). The AAs chosen varied in their reaction kinetics as well as their affinity to nucleophilic sites (different s values). AAs with low s values were more mutagenic in comparison to those with high s values, whereas the reverse was true for induction of cytotoxic effects. Neither SCEs nor chromosomal aberrations correlated with the induction of point mutations, indicating that different primary DNA lesions and repair pathways are involved in these biological processes. Molecular dosimetric studies indicate that O6 alkylation of guanine is the most probable cause of lesions in DNA leading to point mutations following treatment with ethyl methanesulphonate and ethyl nitrosourea.

Altered temporal expression of DNA repair in hypermutable Bloom's syndrome cells

The temporal regulation of DNA repair during synchronous cell proliferation was examined in normal human skin fibroblasts and in Bloom's syndrome skin fibroblasts. Normal human cells regulated DNA repair in a defined temporal sequence prior to the induction of DNA replication. Nucleotide-excision repair was stimulated prior to the induction of base-excision repair, which itself was increased prior to the induction of DNA replication. This temporal sequence was observed (i) by quantitation of the induction of the base-excision repair enzyme uracil DNA glycosylase during cell proliferation in the absence of cellular insult and (ii) by quantitation of nucleotide-excision repair after UV irradiation or base-excision repair after exposure to methylmethane sulfonate. In contrast, Bloom's syndrome cells were characterized by specific alterations in this temporal sequence of gene regulation, such that DNA repair was not enhanced prior to the induction of DNA replication. Nucleotide-excision repair, base-excision repair, and the uracil DNA glycosylase were induced in a temporal sequence identical to that observed for DNA polymerase and for DNA replication. The inability of Bloom's syndrome cells to enhance DNA repair prior to DNA replication suggests that miscoding lesions remain in DNA and are replicated during cell proliferation.

The induction of SCE with relation to specific base methylation of DNA in Chinese hamster cells by N-methyl-N-nitrosourea and dimethylsulfate

The relationship between the formation of alkylated purines in DNA and sister chromatid exchange (SCE) induction has been studied. Both exponentially growing and density-inhibited Chinese hamster (V79) cultures were treated with various doses of N-methyl-N-nitrosourea (MNU) or dimethylsulfate (DMS). The colony-forming ability and induced frequencies of SCEs were assayed. Following the exposure of density-inhibited cells to radiolabeled methylating agents these phenomena were related to the levels of 7-methylguanine (7-meGua), 0(6)-methylguanine (0(6)-meGua) and 3-methyladenine (3-meAde) in the DNA. At equitoxic doses MNU and DMS induced similar frequencies of SCEs. Since, at equitoxic doses MNU produces about 20 times more 0(6)-meGua in V79-cell DNA than does DMS, this indicates that the formation of this adduct in DNA is not critical for the induction of SCEs by these alkylating agents. Dimethylsulfate-induced SCEs may be mediated via the production of both 3-meAde and 7-meGua in the DNA; these 2 methylated purines may also be responsible for MNU-induced SCE. No one specific methylated purine was identified, therefore, as being solely accountable for the formation of SCEs. The repair of lesions in the DNA of nonreplicating V79 cells lead to a reduction in the SCE frequency on their subsequent release from the density-inhibited state. This suggests that excision repair is not responsible for the formation of SCEs.

Transient structural change in hepatic DNA of rats chronically exposed to dimethylnitrosamine

Structural analysis by chromatography on benzoylated-DEAE-cellulose (BD-cellulose) has been made of hepatic DNA from rats treated for up to 21 weeks with dimethylnitrosamine (DMN). The carcinogen (1 mg/kg body wt) was administered on a daily basis by intraperitoneal injection. By comparison with preparations from saline-treated controls, the proportion of DNA retained by benzoylated-DEAE-cellulose in the presence of 1.0 M NaCl was increased by administration of the carcinogen. Variation in the result, dependent upon the time after the final dose, suggested a complex relationship between structural damage to DNA and duration of treatment. Structural damage was confirmed by S1 nuclease digestion. The observations imply that in the course of chronic administration, increasing time is required to complete DNA repair processes operative in rat liver.

Reactivity, SCE induction and mutagenicity of benzyl chloride derivatives

Benzyl chloride, benzyl bromide, p-methylbenzyl chloride, and p-nitrobenzyl chloride were used to study chemical reactivity with 4-(p-nitrobenzyl)-pyridine (NBP), and with guanosine in vitro, in relation to mutagenic potency in S. typhimurium and sister chromatid exchange (SCE) induction in CHO cells. Benzyl bromide was found to be the most reactive compound, followed by p-methylbenzyl chloride, benzyl chloride and p-nitrobenzyl chloride. The order of mutagenicity was p-nitrobenzyl chloride much greater than benzyl bromide greater than benzyl chloride approximately equal to p-methylbenzyl chloride. The compounds tested caused base-pair mutations only. The order of SCE-inducing ability decreased as follows: benzyl bromide greater than benzyl chloride approximately equal to p-nitrobenzyl chloride approximately equal to p-methylbenzyl chloride. The particularly high mutagenicity of p-nitrobenzyl chloride in bacteria may be due to reactions other than direct aralkylation, or it may react particularly actively with DNA. Among the other compounds, benzyl bromide was the most active aralkylating compound, mutagen and SCE inducer. The results suggested that reaction of N2 of guanine, as compared with N-7 of guanine, failed to show any remarkable mutagenicity or SCE induction, since p-methylbenzyl chloride, reacting preferentially at N2 of guanosine, failed to show unexceptional potency.

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.

Elimination of MNU-induced mutational lesions in V79 Chinese hamster cells

Studies were performed on the non-linear dose response for gene mutations induced by low doses of monofunctional methylating agents in V79 Chinese hamster cells. When treatment with methylnitrosourea was applied at the beginning of the S phase in synchronized cells, a linear dose-response curve was obtained, whereas application of the dose after gene replication resulted in a strong reduction of the number of induced mutations. Additional time for repair resulted in reduced dose response of MNU, indicating that an error-free repair process operates on methylated DNA in V79 Chinese hamster cells.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

Mutation, DNA labeling, and transformation of BHK-21/CL 13 cells by MNNG, and nitrosocimetidine

N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG) has been reported to induce BHK-21/Cl 13 cell growth in agar suspension. To determine if MNNG was also mutagenic to BHK cells, an ouabain-resistance mutation assay was established using these cells. In this system MNNG was compared to nitrosocimetidine (NC). MNNG and NC did induce ouabain-resistant mutations in BHK cells. The ability of the test compounds to methylate DNA in BHK cells was also determined, and both MNNG and NC yielded detectable levels of 7-methylguanine in treated cells. MNNG and NC were tested for the ability to transform BHK cells, and did. NC was found to be as effective a mutagen and transforming agent in BHK cells as MNNG when administered at equitoxic concentrations; approx. 4-fold less effective at equimolar concentrations.

DNA repair assays as tests for environmental mutagens. A report of the U.S. EPA Gene-Tox Program

A literature review was undertaken to determine the usefulness of DNA repair assays, other than unscheduled DNA synthesis, as screening techniques for mutagenic carcinogens. 92 reports were found to contain useful data for 49 chemicals using 6 techniques, namely, (1) cesium chloride equilibrium density gradients to study repair replication, (2) benzoylated naphthoylated diethylaminoethyl cellulose columns to study repair replication, (3) 313-nm irradiation of DNA containing bromodeoxyuridine to study repair replication, (4) alkaline elution to study repair of single-strand breaks and crosslinks, (5) alkaline sucrose gradients to study repair of single-strand breaks, and (6) direct assays for removal of adducts from DNA. Almost all of the 49 chemicals studied were known mutagens or carcinogens and/or known inducers of DNA repair, 9 compounds failed to elicit DNA repair by at least 1 assay technique, and at least 3 of these were not tested by the most appropriate and sensitive method. Nevertheless, although valid for studying repair phenomena in eukaryotic cells, these assays are not considered useful for screening. They are time-consuming, expensive, and/or require highly specialized skills. Despite the high frequency of positive reports, it is obvious from the literature that repair assays will fail to detect, or will detect with low efficiency, those agents whose main action is either intercalation or induction of strand breaks. For these and other reasons, DNA repair as a basis for screening for mutagenic carcinogens is not considered to be a useful concept.

A model for the effect of excision repair on the survival of human cells exposed to chemical carcinogens

A model is presented which correlates the survival of normal human fibroblasts (NF) after exposure to N-acetoxy-2-acetylaminofluorene (N-AcO-AAF) with the rate of excision of carcinogen residues bound to DNA. Measurements of the rate of excision of carcinogen residues suggest that this is a first-order process with 37% of the adducts remaining after about 70 h. From this information and the dose-response relationship for survival of NF and repair deficient cells we can determine the mean number of adducts required to produce a potentially lethal lesion and the effective time available for repair. When these adjustable parameters have been determined, we can use the model to predict the rate of excision in partially repair-deficient cells and the effect of extending the repair period by arresting cell growth after treatment. Survival studies in which cells were held at confluence for varying amounts of time between treatment and replating at low density show good agreement with the predictions of the model.

Sister chromatid exchange induced by short-lived monoadducts produced by the bifunctional agents mitomycin C and 8-methoxypsoralen

To see if DNA crosslinks are involved in the induction of sister chromatid exchange (SCE), Chinese hamster ovary cells were exposed to two bifunctional alkylating agents, mitomycin C and 8-methoxypsoralen, and their monofunctional derivatives, decarbamoyl mitomycin C and angelicin. The data indicate that monoadducts, rather than crosslinks, are responsible for SCE formation. Furthermore, all agents but angelicin produced short-lived lesions that led to SCEs in the first period of DNA replication after treatment (twin SCEs), but not in the second (single SCEs). In contrast, angelicin, like methyl methanesulfonate and N-acetoxyacetylaminofluorene, produced lesions that lasted more than one cycle, indicating that several different types of DNA lesions are capable of SCE induction.

Repair of O6-methyl-guanine residues in DNA takes place by a similar mechanism in extracts from HeLa cells, human liver, and rat liver

Extracts from HeLa S3 cells, human liver, and rat liver were found to contain an activity that transfers the methyl group from O6-methyl-guanine residues in DNA to a cysteine residue of an acceptor protein. The molecular weights of the acceptor proteins in HeLA cells and human liver are 24,000 +/- 1,000 and 23,000 +/- 1,000, respectively. Assuming that each acceptor molecule is used only once, the average number of acceptor molecules in HeLa cells was calculated to be about 50,000. The extracts also contained 3-methyl-adenine-DNA glycosylase activity and 7-methyl-guanine-DNA glycosylase activity, although the latter activity was not detected in extracts from human liver in our assay system. Thus, the three major alkylation products resulting from the effect of methylating agents, such as N-methyl-N-nitroso urea, can all be repaired in animal cells. Pretreatment of HeLa cells with N-methyl-N'-nitro-N-nitrosoguanidine (0.1 micrograms/ml) strongly reduced the capacity of HeLa cell extracts to repair O6-methyl-guanine residues, while the activity of three DNA-N-glycosylases was essentially unaltered. This inactivation was not caused by a direct methylation of the enzyme by the carcinogen. The results demonstrate that the mechanism of repair of O6-methyl-guanine residues in DNA is strikingly similar in E coli and animal cells, including humans.

Effect of partial hepatectomy on removal of O6-methylguanine from alkylated DNA by rat liver extracts

1. The activity of an enzyme catalysing the loss of O6-methylguanine from methylated DNA was increasing during liver regeneration after partial hepatectomy. Activity was increased 3-fold by 24h and was maximal (6-fold increase) over the period 48-72h after operation. 2. This activity could also be induced by chronic treatment with dimethylnitrosamine, but the maximal response amounted to a 2-3-fold change (with the greater effect in male rats) after 4-6 weeks of exposure to daily doses of 2 mg of dimethylnitrosamine/kg. 3. Neither partial hepatectomy nor treatment with dimethylnitrosamine increased the activities of two other enzymes repairing alkylated DNA, DNA (7-methylguanine-)glycosylase and DNA (3-methyladenine-)glycosylase. 4. These results therefore indicate that there is a selective induction of the O6-methylguanine removal system during hepatocyte proliferation. Since this product is known to lead to mutations and its persistence in DNA throughout cell replication has been implicated in tumour initiation, this induction may play a role in resistance to carcinogenesis by alkylating agents.

Human lymphoblasts contain DNA glycosylase activity excising N-3 and N-7 methyl and ethyl purines but not O6-alkylguanines or 1-alkyladenines

Cultured human lymphoblasts (CCRF-CEM line) have DNA glycosylase activities, the specificities of which were investigated by using high-performance liquid chromatography. In addition to 3-methyladenine, 3-ethyladenine, 7-methylguanine, 7-ethylguanine, 3-methylguanine, and 3-ethylguanine also were excised from alkylated double-stranded DNA and deoxypolynucleotides, but 1-methyladenine, 1-ethyladenine, O6-methylguanine, and O6-ethylguanine were not. The glycosylase activity was generally greater for the methylated than for the corresponding ethylated purines and was also greater toward 3-alkyladenine than toward 3-alkyladenine than toward 3-alkylguanine. 7-Methylguanine and 7-ethylguanine were excised to similar but low extents. However, in molar terms, the release of 7-methylguanine was similar to that of 3-methyladenine.

Removal of O6-methylguanine from DNA of normal and xeroderma pigmentosum-derived lymphoblastoid lines

The ability to excise (repair) UV-induced pyrimidine dimers in Escherichia coli is not related to its ability to remove N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced O6-methylguanine (O6-MeG) from DNA. It was therefore surprising that certain xeroderma pigmentosum cell lines, deficient in dimer excision, were also unable to remove O6-MeG. We find that removal of O6-MeG occurs rapidly with a half life of less than 1 h. Two cell types can be distinguished: mex+, which remove O6-MeG residues produced by incubation with 0.5 microgram ml-1 MNNG, and mex- cells, which are unable to remove the adduct. Xeroderma pigmentosum-derived lymphoblastoid lines of complementation groups A, C or D may be either mex+ or mex-. The biochemical mechanism for the removal of O6-MeG in human cells is distinct from the excision of adducts produced by compounds such as N-acetoxy-N-2-acetylaminofluorene (AAAF) or by UV irradiation but it is not clear whether the distinction between mex+ and mex- lines is genetic or epigenetic.

Repair of ultraviolet damage in human cells also exposed to agents that cause strand breaks, crosslinks, monoadducts and alkylations

Excision repair of UV damage in human cells was measured by the incorporation of new bases into DNA after exposure to UV light and variety of other carcinogens including X-rays, furocoumarins plus 360 nm light (8-methoxypsoralen (8-MOP), 4'-aminomethyl 4,5',8-trimethylpsoralen hydrochloride (AMT) and angelicin), methyl methanesulfonate (MMS), dimethyl sulfate (DMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Repair of UV damage was unaffected by concomitant exposure to X-rays. Furocoumarin adducts and alkylating agents, however, interacted with UV repair and reduced the amount of repair replication observed. The interaction between repair of furocoumarin and UV damage is consistent with the involvement of a common DNA repair pathway: its saturation with respect to repair of UV damage also results in saturation with respect to other lesions involving that pathway. The observed effect of alkylating agents on UV repair, however, cannot be due to saturation of a common pathway because damage from these agents are repaired by different mechanisms than UV damage. Instead, it appears likely that the effect is due to alkylation damage to repair enzymes. From a consideration of the degree of protein alkylation at millimolar concentrations of alkylating agents, we estimate that the UV repair system could consist of an extremely large complex of protein subunits in the order of 1 million molecular weight. Whereas some previous studies have used concomitant exposures to pair of different agents to determine whether the damage they produce is repaired by common or independent pathways, our results indicate that such a method is an unreliable indicator of the number of repair pathways.

Defective repair of alkylated DNA by human tumour and SV40-transformed human cell strains

We have identified a group of 8 (among 39) human tumour cell strains deficient in the ability to support the growth of adenovirus 5 preparations treated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), but able to support the growth of non-treated adenovirus normally. This deficient behaviour defines the Mer- phenotype. Strains having the Mer- phenotype were found to arise from tumours originating in four different organs. Relative to Mer+ strains, Mer- tumour strains showed greater sensitivity to MNNG-produced killing, greater MNNG-stimulated "DNA repair synthesis and a more rapid MNNG-produced decrease in semi-conservative DNA synthesis. Here we report that (1) Mer- strains are deficient in removing O6-methylguanine (O6-MeG) from their DNA after [Me-14C]MMNG treatment (Table 1); (2) Mer- tumour strains originate from tumours arising in patients having Mer+ normal fibroblasts (Fig. 1a, b); (3) SV40 transformation of (Mer+) human fibroblasts often converts them to Mer- strains (Fig. 1c, d); (4) MNNG produces more sister chromatid exchanges (SCEs) in Mer- than in Mer+ cell strains (Fig. 2).

Relationship between chemical damage of DNA and mutations in mammalian cells. I. Dose-response curves for the induction of 6-thioguanine-resistant mutants by low doses of monofunctional alkylating agents, X-rays and UV radiation in V79 Chinese hamster cells

The shape of the dose-response curve for mutations induced at low doses of mutagenic agents in mammalian cells was studied. With the exception of X-rays and MMS, which are very toxic in relation to their mutagenic potency, dose-response studies with EMS, MNU, ENU and UV radiation were performed at doses giving about 100% survival. The results from several experiments were pooled for each agent to get higher resolution power at low doses, and the result was compared with what could be expected when linear interpolation is performed from higher doses. The dose response for induction of mutations by UV- and X-irradiation did not deviate from linearity at low doses. In contrast with irradiation and the ethylating agents EMS and ENU, the methylating agents MMS and MNU showed a significantly lower effect at low doses as compared with estimation from a dose 5 times as high. The extent of alkylation of DNA by various doses of MMS was linear in the same dose intervals. It is suggested that the decreased response at low doses of the methylating agents found here may be connected with changes in the mechanism of repair of the lesions induced at different dose levels.

Dna repair: pathways and defects

Damaged DNA can be repaired by three different mechanisms: photoreactivation, excision repair and postreplication repair. Each mechanism is regulated by a highly specific set of enzymes. Defects within these systems result in diseases which have one common feature: affected individuals are cancer prone. Recently, newly developed methods not only make it possible to diagnose affected patients but also to detect individuals at risk. Furthermore, the results obtained elucidate some mechanisms of carcinogenesis. Clinical applications are discussed.