The stability of methylated purines and of methylphosphotriesters in the DNA of V79 cells after treatment with N-methyl-N-nitrosourea

Time versus replication dependence of EMS-induced delayed mutation in Chinese hamster cells

We have previously observed in Chinese hamster cells that ethyl methane sulfonate (EMS) induces mutations which are distributed over at least 10-14 cell divisions following treatment. This delayed appearance of mutations could be explained by EMS-induced lesions which remain in DNA and have a probability that is significantly less than 1.0 of producing base mispairing errors during successive replication cycles (replication-dependent). Alternatively, delayed mutation may be a time-dependent process in which a slow acting or damage inducible error-prone repair process removes persistent DNA lesions and replaces them with an incorrect base during the course of 7-10 days of colony growth following EMS exposure. To address this question, the distribution of HGPRT delayed mutation events (fifth division or later) in cells plated immediately for exponential growth after EMS treatment was compared with the distribution in cells which remained under confluent growth conditions for 8 days and then were replated. Both the distribution and rate of accumulation of delayed mutations (mutations/cell division) were similar in the two culture conditions. In contrast, the frequency of early mutations (before the fifth division) in the confluent population was reduced more than 2-fold compared to dividing cells. A comparison of the frequency of EMS-induced DNA lesions in the two populations revealed that the density inhibited population contained one third the DNA lesions of the exponential population. These results argue against a time-dependent process since, if this mechanism applies, one would expect an increase in early mutant events and a decrease in delayed events in the confluent population. The results, however, are consistent with a replication model in which potential early mutant lesions are preferentially removed in the density inhibited culture during the 8 days of incubation while lesions producing late mutants are not removed.

Streptozotocin-induced genotoxic effects in Chinese hamster cells: the resistant phenotype of V79 cells

The genotoxic effects of the methylating agent streptozotocin (STZ) on Chinese hamster cells CHO-9 and V79 were evaluated. The induction of cell killing, chromosomal aberrations, sister-chromatid exchanges (SCEs) and mutations was analyzed. Comparisons were made with the the STZ aglyconic analogue N-methyl-N-nitrosourea (MNU). V79 cells were found to be more resistant than CHO-9 cells to STZ and MNU killing effects, as well as to the induction of chromosomal aberrations and SCEs; however, V79 and CHO-9 cells appeared to be equally sensitive to the induction of 6-thioguanine resistant mutants by STZ. These results suggest that an error-free mechanism that tolerates DNA methylation damage confers a resistant phenotype to V79 cells to the genotoxic effects of methylation damage.

Streptozotocin-induced toxicity in CHO-9 and V79 cells

The cytotoxicity of streptozotocin (STZ) was investigated in Chinese hamster fibroblast lines (CHO-9 and V79) in comparison to two other alkylating agents, methylnitrosourea (MNU) and ethylnitrosourea (ENU), using cell survival as the endpoint. It was found that V79 cells were far more resistant to methylation induced by STZ and MNU than CHO-9 cells (20 and four times, respectively) but equally sensitive to the ethylating agent ENU. The extent of STZ-induced DNA methylation was estimated by analyzing the extent of O6-metG and N7-metG adducts in the DNA of treated cells through high-performance liquid chromatography (HPLC) with electrochemical detection. The number of adducts as well the efficiencies of their removal from the DNA were similar in both cell lines. The response of these cells to the presence of DNA damage was evaluated by analysis of STZ effects on DNA replication and cell cycle progression. Measurement of [3H]-thymidine incorporation showed a similar pattern of response at the level of inhibition of DNA synthesis in both cell lines. However, analysis of metaphase cells 36 hr after STZ exposure showed an accumulation of cells in the second cycle in the CHO-9 line, indicating induction of a cell cycle arrest. Only a slight effect was observed on cell cycle progression in V79 cells, indicating that the methylation resistance of these cells could be related to their ability to progress through the cell cycle despite the presence of DNA lesions.

Effects of cinnamaldehyde on survival and formation of HGPRT- mutants in V79 cells treated with methyl methanesulfonate, N-nitroso-N-methylurea, ethyl methanesulfonate and UV light

The effects of the antimutagenic flavoring cinnamaldehyde on the induction of HGPRT- mutants by methyl methanesulfonate (MMS), N-nitroso-N-methylurea (MNU), ethyl methanesulfonate (EMS) and UV light was investigated in the Chinese hamster V79 cell line. Cinnamaldehyde did not show any mutagenic or toxic effects in this experimental system by itself and did not modify mutation frequency when given to cells simultaneously with chemical mutagens. Under these conditions, the cytotoxicity of MMS, but not that of MNU and EMS, was increased. Cell viability was also reduced in MNU-, EMS-, or UV light-pretreated cells when they were postincubated in the presence of cinnamaldehyde. Moreover, cinnamaldehyde reduced the frequency of mutations induced by MMS but not by the other mutagens. The results obtained suggest that cinnamaldehyde inhibits some cellular function(s) promoting the repair of a variety of different cytotoxic lesions. At the same time, stimulation by cinnamaldehyde of an error-free DNA repair mechanism acting on MMS-induced mutagenic damage was indicated.

Effects of spermine on formation of HGPRT- mutants induced by ethylmethanesulfonate, methylmethanesulfonate, and mitomycin C in V79 Chinese hamster cells

Spermine is a polyamine found in bacteria, animal, and plant tissues. It is involved in a variety of biological processes, and its interaction with DNA stabilizes the secondary structure of the double helix. Spermine is one of the first reported antimutagens, reducing the mutation rate in several prokaryotic test systems, while in eukaryotic organisms conflicting results have been obtained. In light of the significant antimutagenic effect of spermine, it is important to evaluate its activity in mammalian cells in culture. The present study was undertaken to evaluate the ability of spermine to suppress the level of HGPRT- mutants induced by ethylmethanesulfonate, methylmethanesulfonate, and mitomycin C. Spermine reduced the mutation frequency induced by ethylmethanesulfonate and methylmethanesulfonate but did not affect survival; with mitomycin C survival was reduced but mutation rate was not influenced.

Mutagenesis and comutagenesis by lead compounds

We have previously reported that lead(II) is weakly mutagenic to Chinese hamster V79 cells. A transgenic cell line G12 containing a single copy of the E. coli gpt gene was developed in this laboratory from Chinese hamster V79 cells. The gpt locus in the G12 cells is more mutable by radiation and oxidative agents compared with the endogenous hprt locus of wild-type V79 cells. We have investigated the mutagenicity of two lead compounds at the gpt locus in G12 cells. Only at a toxic dose is lead acetate significantly mutagenic to G12 cells. Lead nitrate is not significantly mutagenic at any dose. Although both compounds are water-soluble, lead acetate, but not lead nitrate, forms a fine white insoluble precipitate upon addition to growth medium. A nick translation assay on cells treated with lead compounds and then permeabilized indicated that lead nitrate and, to a greater extent, lead acetate causes the appearance of nicks in chromosomal DNA. Lead ions in the presence of hydrogen peroxide, but not alone, introduced nicks into supercoiled plasmid DNA in vitro, suggesting that lead ions can partake in a Fenton reaction and thereby damage DNA. At lower nonmutagenic concentrations, lead acetate enhances the mutagenicity of MNNG and ultraviolet light. DNA damage by ultraviolet light is not enhanced by lead ions in vitro. Our data support the concept that non-toxic concentrations of lead(II) can inhibit DNA repair. Thus, at biologically relevant doses, lead(II) could act as a comutagen and possibly a cocarcinogen, but is not likely to act as an initiating genotoxic carcinogen.

Different modifications by vanillin in cytotoxicity and genetic changes induced by EMS and H2O2 in cultured Chinese hamster cells

The modifying effects of vanillin on the cytotoxicity and 6-thioguanine (6TG)-resistant mutations induced by two different types of chemical mutagens, ethyl methanesulfonate (EMS) and hydrogen peroxide (H2O2), were examined using cultured Chinese hamster V79 cells. The effects of vanillin on H2O2-induced chromosome aberrations were also examined. Vanillin had a dose-dependent enhancing effect on EMS-induced cytotoxicity and 6TG-resistant mutations, when cells were simultaneously treated with vanillin. The post-treatment with vanillin during the mutation expression time of cells after treatment with EMS also showed an enhancement of the frequency of mutations induced by EMS. However, vanillin suppressed the cytotoxicity induced by H2O2 when cells were post-treated with vanillin after H2O2 treatment. Vanillin showed no change in the absence of activity of H2O2 to induce mutations. Post-treatment with vanillin also suppressed the chromosome aberrations induced by H2O2. The differential effects of vanillin were probably due to the quality of mutagen-induced DNA lesions and vanillin might influence at least two different kinds of cellular repair functions. The mechanisms by which vanillin enhances or suppresses chemical-induced cytotoxicity, mutations and chromosome aberrations are discussed.

Genotoxicity of N-acryloyl-N'-phenylpiperazine, a redox activator for acrylic resin polymerization

N-Acryloyl-N'-phenylpiperazine is a promoter of redox reactions synthesized recently, and proposed as an activator for the polymerization of acrylic resins for biomedical use. The chemical was analyzed for different genotoxicity endpoints, to obtain both information on its possible mutagenic/carcinogenic potential and a model analysis of a tertiary arylamine, which belongs to a class of chemicals commonly used as polymerization accelerators in the biomaterial field. The genotoxicity endpoints considered were: gene mutation in the Salmonella test; structural and numerical chromosome alterations in Chinese hamster V79 cells, evaluated by the micronucleus test together with an immunofluorescent staining specific for kinetochore proteins; in vitro and in vivo DNA damage, evaluated in V79 cells and in mouse liver by the alkaline DNA elution technique. On the whole, the results indicate that N-acryloyl-N'-phenylpiperazine is to be regarded not so much as a DNA-damaging agent, but as a genomic mutagen. Indeed, it was not mutagenic in Salmonella (though its toxicity did not allow testing concentrations over 70 micrograms/plate), and it was weakly positive in inducing chromosomal fragmentation in vitro (one positive, not dose-related, result out of five different doses tested) and in vivo DNA damage (increases in DNA elution rate never doubling control values). The chemical was, however, clearly positive (with dose-dependent effects up to about 25 times the control value) in causing numerical chromosome alterations, at the maximal non-toxic doses.

Properties of mer- HeLa cells sensitive or resistant to the cytotoxic effects of MNU; effects on DNA synthesis and of post treatment with caffeine

A line of HeLa cells was shown to be particularly sensitive to N-methyl-N-nitrosurea (MNU) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), but not to a variety of other cytotoxic agents. A resistant line (designated HeLa/A22), was derived by treating HeLa cells repeatedly with MNU. Both the sensitive (HeLa) and resistant (HeLa/A22) cells have a mer- phenotype based both on their reduced rates of loss of O6-methylguanine (O6-MeG) from DNA and their low levels of the enzyme O6-methylguanine methyltransferase (MT). HeLa cells are therefore sensitive to unrepaired O6-MeG in DNA while the HeLa/A22 cells are resistant to unexcised O6-MeG and thus the A22 cells have the mer-rem+ phenotype. MNU produced an immediate dose-dependent inhibition of DNA synthesis in cultures of both sensitive and resistant cells which increased with time until about 4 h after treatment. DNA synthesis then recovered to near control rates in both sensitive and resistant cells before then exhibiting a progressive decrease after about 24 h. DNA synthesis was more depressed at these late times after treatment in cultures of sensitive cells than in those of similarly-treated resistant cells. DNA synthesis remained depressed in sensitive cells but recovered 3 days after treatment in resistant cells. Post treatment incubation of MNU-treated HeLa cells with caffeine did not increase the toxic action of MNU. In contrast, post treatment of the resistant HeLa/A22 cells with caffeine resulted in a dramatic increase in the toxic effects of a higher equitoxic dose of MNU. The depressed rate of DNA synthesis observed in both cell lines after high doses of MNU was partially reversed by post treatment with caffeine in both sensitive and resistant cells. These observations can be interpreted in terms of the effects of caffeine on DNA replication in treated cells.

Mechanism of comutagenesis of sodium arsenite with n-methyl-n-nitrosourea

Arsenic compounds are known carcinogens. Although many carcinogens are also mutagens, we have previously shown that sodium arsenite is not mutagenic at either the Na+/K+ ATPase or hprt locus in Chinese hamster V79 cells. It can, however, enhance UV-mutagenesis. We now confirm the nonmutagenicity of sodium arsenite in line G12, a pSV2gpt-transformed V79 (hprt-) cell line, which is able to detect multilocus deletions in addition to point mutations and small deletions. The lack of arsenic mutagenicity has led to studies emphasizing its comutagenicity. Sodium arsenite at relatively nontoxic concentrations (5 microM for 24 h or 10 microM for 3 h) is comutagenic with N-methyl-N-nitrosourea (MMU) at the hprt locus in V79 cells. Using a nick translation assay, which measures DNA strand breaks by incorporating radioactive deoxyribonucleoside monophosphate at their 3'OH ends in permeabilized cells, we found that much more incorporation was seen in cells treated with MNU (4 mM, 15 min) followed by 3-h incubation with 10 microM sodium arsenite compared with cells exposed to the same MNU treatment followed by 3-h incubation without sodium arsenite. This result shows that in the presence of arsenite, strand breaks resulting from MNU or its repair accumulate over a 3-h period. We suggest that the repair of MNU-induced DNA lesions may be inhibited by arsenite either by affecting the incorporation of dNMPs into the MNU-damaged DNA template or by interfering with the ligation step.

Repair of N-methylpurines in specific DNA sequences in Chinese hamster ovary cells: absence of strand specificity in the dihydrofolate reductase gene

We have developed a quantitative method for examining the removal of N-methylpurines from specific genes to investigate their possible differential repair throughout the genome. Chinese hamster ovary cells were exposed to dimethyl sulfate, and the isolated DNA was treated with an appropriate restriction endonuclease. The DNA was heated to convert remaining N-methylpurines to apurinic sites to render them alkaline-labile. Duplicate samples heated in the presence of methoxyamine to protect the apurinic sites from alkaline hydrolysis provided controls to assess total DNA. After alkaline hydrolysis, agarose gel electrophoresis, Southern transfer, and probing for the fragment of interest, the ratios of band intensities of the test DNA sample to its methoxyamine-treated control counterpart were calculated to yield the percentage of fragments containing no alkaline-labile sites. The frequency of N-methylpurines was measured at different times after dimethyl sulfate treatment to study repair. We found no differences between the rates of repair of N-methylpurines in the active dihydrofolate reductase gene and a nontranscribed region located downstream from it in treated cells. Also, similar rates of repair were observed in the transcribed and nontranscribed strands of the gene, in contrast to previous results for the removal of cyclobutane pyrimidine dimers. Thus, there does not appear to be a coupling of N-methylpurine repair to transcription in Chinese hamster ovary cells. However, the repair in the dihydrofolate reductase domain appears to be somewhat more efficient than that in the genome overall. Our method permits the quantifying at the defined gene level of abasic sites or of any DNA adduct that can be converted to them.

Molecular dosimetry studies of forward mutation induced at the yg2 locus in maize by ethyl methanesulfonate

The yg2 assay in Zea mays detects forward mutation in somatic cells within leaf primordia of embryos and it was used in an analysis of the molecular dosimetry of ethyl methanesulfonate (EMS). Parallel genetic and molecular dosimetry experiments were conducted in which the frequency of forward mutation and the level of covalently bound ethyl DNA adducts were determined. Prepared kernels were treated for 8 h at 20 degrees C with 1-10 mM EMS. EMS induced a direct concentration-dependent increase in mutation induction proportional to the exposure concentration (slope = 0.93). The kinetics of mutation induction demonstrated in the intact maize system were consistent with the kinetics observed earlier in in vitro model systems using cultured mammalian cells, and contrasted with the exponential increase in mutation induction characteristic of microbial species. Parallel molecular dosimetry experiments were conducted using [3H]EMS. DNA was extracted and purified from embryonic tissues containing the leaf primordia, the target tissue of the yg2 assay. A linear increase in the molecular dose was observed as a function of EMS concentration. Using concentration as a common parameter between the parallel genetic and dosimetry studies, mutation induction appeared to increase nearly in a direct proportion to the molecular dose. However, studies in other genetic systems indicate that the levels of specific DNA adducts, such as O6-ethylguanine (O6-EtGua) show a better correlation with mutation induction kinetics than molecular dose. Neither molecular dose, nor O6-EtGua levels account for differences in the absolute frequencies of mutation induction observed in different genetic systems. Therefore, reliable assessment of health risks posed to humans by chemical mutagens appears to require consideration of other factors in addition to DNA dose or adduct formation, including differences in repair capabilities and in the size of the genetic targets in humans relative to the model genetic systems under study.

A reduced rate of bulky DNA adduct removal is coincident with differentiation of human neuroblastoma cells induced by nerve growth factor

Human SY5Y neuroblastoma cells which were differentiated in culture by treatment with 7S murine nerve growth factor for 5 weeks and selection with aphidicolin (L. Jensen, Dev. Biol. 120:56-64, 1987) demonstrated a considerably slower rate of removal of DNA adducts of benzo[a]pyrene, benzo[a]pyrenediolepoxide, and N7-methylguanine than did undifferentiated mitotic cells. A dramatic decline in unscheduled DNA synthesis induced by UV radiation was similarly observed. DNA polymerase beta and uracil DNA glycosylase were unchanged after differentiation, DNA polymerase alpha and DNA methylase decreased roughly threefold, and total apurinic-apyrimidinic endonuclease activity increased roughly threefold after treatment.

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.

A reduced rate of bulky DNA adduct removal is coincident with differentiation of human neuroblastoma cells induced by nerve growth factor

Human SY5Y neuroblastoma cells which were differentiated in culture by treatment with 7S murine nerve growth factor for 5 weeks and selection with aphidicolin (L. Jensen, Dev. Biol. 120:56-64, 1987) demonstrated a considerably slower rate of removal of DNA adducts of benzo[a]pyrene, benzo[a]pyrenediolepoxide, and N7-methylguanine than did undifferentiated mitotic cells. A dramatic decline in unscheduled DNA synthesis induced by UV radiation was similarly observed. DNA polymerase beta and uracil DNA glycosylase were unchanged after differentiation, DNA polymerase alpha and DNA methylase decreased roughly threefold, and total apurinic-apyrimidinic endonuclease activity increased roughly threefold after treatment.

Comparative analysis of O6-methylguanine methyltransferase activity and cellular sensitivity to alkylating agents in cell strains derived from a variety of animal species

Using 26 cultured cell lines derived from 17 different animal species, we have measured both the activity of O6-methylguanine (O6-MeG) methyltransferase (MT) in cell extracts and the sensitivity of the strains to the lethal effects of the alkylating agents, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU). The MT activity was assayed by measuring the amount of 3H radioactivity transferred from methyl-[3H]-labeled O6-MeG in DNA to acceptor protein molecules in the extracts. In all the 21 mammalian cell strains, lethal sensitivity to ACNU as measured by colony-forming ability correlated well with cellular MT activity, indicating that the major lethal ACNU damage is reparable by the MT. On the other hand, MNNG sensitivity did not necessarily correlate with the MT activity.

Difference in O6-methylguanine methyltransferase activity among transformed NIH3T3 cell clones

We examined the sensitivity to the lethal effects of methylating agents and the O6-methylguanine methyltransferase (MTR) activities of in vitro transformed NIH3T3 cell clones. The sensitivities to the lethal effects of MNNG were not different among all 49 transformed cell clones examined and do not correlate with the MTR activities. All 8 spontaneously transformed cell clones showed the same sensitivities to ACNU as the parental cell line. 2 of 20 transformants induced by UV or MNNG showed higher sensitivities to the ACNU although the MTR activity was normal. One cell clone transformed by UV was sensitive to ACNU and showed about half MTR activity. 5 of 19 cell clones transformed by oncogenes (Ha-ras or SV40 ori-) were sensitive to the lethal effects of ACNU and showed the low MTR activities, but were not as much sensitive as a Ha-MuSV transformed cell clone, Ha821.

Delayed mutation in Chinese hamster cells

The possibility was examined that mutational events can be delayed for more than one or two cell divisions following treatment of Chinese hamster cells with the DNA alkylating agent ethyl methane sulfonate. If mutations in mammalian cells are delayed, the proportion of mutant cells in colonies grown from single mutagen-treated cells will reflect the cell division at which the mutation is genetically fixed, i.e., a first division mutation yields a 1/2 mutant colony, a fifth division mutation produces a 1/32 mutant colony, etc. In the present study, replating of cells from single colonies grown for six to seven days after mutagen treatment resulted in the discrete ratios of glucose-6-phosphate dehydrogenase (G6PD)-deficient mutant to wild-type colonies expected for a delayed mutational process which produces mutations over at least 8-10 cell generations. Further, when cells from 7- to 10-day colonies, grown from ethyl methane sulfonate (EMS)-treated cells were replated into selective medium containing 6-thioguanine (6TG), the number of 6TG-resistant colonies obtained per flask was distributed over a very wide range, consistent with a mutational delay process. These results could not be explained by differences in the number of cells per colony or plating efficiency in selective medium. Assuming that the relative number of 6TG-resistant colonies per flask reflects the time of mutation, EMS treatment produced two groups of mutational events: one which occurred within the first five cell generations and another uniformly distributed over at least the next eight to nine divisions. These results support the conclusion that EMS induces mutants for at least 10-14 cell generations after treatment and raise the possibility that current methods to assess the mutagenic potential of an agent might lead to significant underestimation. The role of delayed mutation in the phenomenon of "mutation expression time" is also discussed.

Reduction of the toxicity and mutagenicity of alkylating agents in mammalian cells harboring the Escherichia coli alkyltransferase gene

The toxic, mutagenic, and carcinogenic effects of alkylating agents have been attributed to their ability to damage DNA. Reaction at the O6 position of guanine results in miscoding during DNA replication, has been shown to be mutagenic in both bacteriophage and bacteria, and may be responsible for malignant transformation. In common with many other prokaryotes and eukaryotes the Escherichia coli B strain contains a protein that repairs O6-alkylation damage in DNA by transferring the alkyl group to one of its own cysteine residues. We have recently cloned the E. coli O6-alkylguanine alkyltransferase gene and shown it to encode a 37-kDa protein containing an additional activity that removes alkyl groups from alkylphosphotriesters in DNA. To examine the biological effects of this gene in mammalian cells, we have now inserted the coding sequence into a retrovirus-based selectable expression vector and transfected it into Chinese hamster V79 cells that lack endogenous alkyltransferase activity. A clone expressing high levels of the bacterial protein was selected and shown to produce a 37-kDa alkyltransferase protein and to rapidly repair O6-methylguanine produced in the host genome following exposure to N-methyl-N-nitrosourea. In comparison with a control population, this clone is considerably more resistant to the toxic and mutagenic effects of alkylating agents that react extensively with oxygen atoms in DNA. The usefulness of these clones in examining the role of DNA alkylation and other biological effects of alkylating agents is discussed.

The elimination of mutagenic lesions induced by alkylating agents and UV in V79 Chinese hamster cells

The mutagenicity of MNU, EMS, BMS and UV light was compared by analyzing the dose-response curve just before and after the replicative process of the HGPRT gene in synchronized V79 Chinese hamster cells. This system makes it possible to compare a 10-h period for repair of different mutagenic lesions with no time for repair. Additional time for repair in synchronized V79 cells resulted in a reduced response for MNU and UV, but not for EMS and BMS. This result suggests that an error-free repair process operates on mutagenic lesions in methylated DNA and on thymine dimers, but not on ethylated and butylated DNA. Based on these results, it is concluded that the repair capacity of V79 cells to remove mutagenic lesions is characterized as low for UV, moderate for MNU and not detectable for the mutagenic lesions induced by EMS and BMS.

The influence of multiple mutagenic treatments on the occurrence of 6-thioguanine-resistant mutants in dividing V79 cells

Repeated treatments of dividing Chinese hamster V79 cells with ultraviolet radiation or a simple monofunctional alkylating agent (methyl methanesulphonate, N-methyl-N'-nitro-N-nitrosoguanidine or N-methyl-N-nitrosourea) were studied with regard to the pattern of cumulative increase of 6-thioguanine-resistant mutants in the surviving population. There were 5-10 divisions of surviving cells between two successive treatments. Repeated treatments with different agents produced different effects. The observed results indicating either a higher or lower than additive effect of single doses may be explained by induced repair processes.

Stereospecific removal of methyl phosphotriesters from DNA by an Escherichia coli ada+ extract

The ada+ gene product, a DNA methyltransferase present in extracts from an Escherichia coli strain constitutive for the adaptive response, removes only half of the methyl phosphotriesters from alkylated DNA. Since DNA phosphotriesters occur in two isomeric configurations (denoted Rp and Sp), we examined whether this reflects a stereospecific mode of repair by the methyltransferase. Analysis by reverse-phase HPLC, phosphorus NMR and circular dichroism established that only triesters in the Sp configuration are acted upon by the E. coli extract.

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.

The effect of thymidine on the induction of micronuclei by alkylating agents in V79 Chinese hamster cells

Incubation in thymidine-containing medium resulted in increased lethality and micronucleus frequency in V79 cells treated with ethyl nitrosourea (ENU), methyl nitrosourea (MNU) and ethyl methane-sulphonate (EMS) but not with methyl methanesulfonate (MMS). Thymidine had no effect in ENU treated HeLa cells. In V79 cells, the presence of thymidine during post-treatment DNA replication was necessary for the effect. It is suggested that the increase in chromosome damage was the result of an increased O6-alkylguanine-thymine mispairing in cells which are defective in the repair of O6-alkylguanine. Treatment of V79 cells with O6-ethylguanine resulted in increased production of both micronuclei and polyploid cells. These effects might be explained by spindle dysfunction caused by the alkylated guanine.

Dependency of the yield of sister-chromatid exchanges induced by alkylating agents on fixation time. Possible involvement of secondary lesions in sister-chromatid exchange induction

Chinese hamster V79 cells were pulse-treated (for 60 min) with various mutagens three, two or one cell cycles before fixation (treatment variants A, B and C, respectively) and the frequencies of induced SCEs were analysed and compared. The degree of increase in frequency of SCEs with dose in the treatment variants depended on the mutagen used. For the methylating agents MNU, MNNG and DMPNU, high yields of SCEs were obtained in the treatment variants A and B, and there was no difference in the efficiency with which these agents induced SCEs in these treatment variants. In the treatment variant C, however, no SCEs were induced with mutagen doses yielding a linear increase in SCE frequency in treatment variants A and B. A slight increase in SCE frequency in treatment variant C was observed only when relatively high doses of MNU or MNNG were applied. Like the above agents, EMS, ENU and MMS induced more SCEs in treatment variants A and B than in C, but for these agents treatment variant B was most effective and SCEs were induced over the entire dose range, also in treatment variant C. As opposed to the methylating and ethylating agents, MMC induced SCEs with high efficiency when treatment occurred one or two generations prior to fixation. There was no difference in SCE frequency between these treatment variants. MMC was completely ineffective for the induction of SCEs when treatment occurred three generations before fixation. The unexpectedly low SCE frequencies induced by the methylating and ethylating agents when treatment occurred one generation before fixation were not due to the exposure of cells to BrdU prior to mutagen treatment. From the results obtained, it is concluded that DNA methylation and ethylation lesions give rise to SCEs only with very low probability during the replication cycle after the lesion's induction, and that subsequent lesions produced during or after replication of the methylated or ethylated template (secondary lesions) are of prime importance for SCE formation after alkylation. For MMC, however, primary lesions seem to be most important for SCE induction.

Quantitation of chemically induced DNA strand breaks in human cells via an alkaline unwinding assay

DNA strand breaks induced in human CCRF-CEM cells by electrophilic chemicals (carcinogens/mutagens) can be readily quantitated via a facile alkaline unwinding assay. This procedure estimates the number of chemically induced DNA strand breaks on the basis of the percentage DNA converted from double-stranded to single-stranded form during an exposure to the alkaline unwinding conditions. The assay is based on the assumption that each strand break serves as a strand unwinding point during the alkaline denaturation. The extent of strand separation can be standardized with respect to the initial level of induced strand breaks by the use of X-rays, which produce known levels of DNA strand breaks per rad in mammalian cells. Subsequent to the alkaline exposure, the single- and double-stranded DNA were separated by use of thermostated hydroxylapatite columns (60 degrees C), and the DNA was quantitated via a fluorescence assay (Hoechst 33258 compound). A correlation was shown between mammalian DNA strand-breaking potential (as measured in this procedure) and the propensity of these chemicals to revert Salmonella typhimurium TA100.

Split-dose exposure to N-methyl-N'-nitro-N-nitrosoguanidine in BALB/3T3 C1 a31-1-1 cells: evidence of DNA repair by alkaline elution without changes in cell survival, mutation and transformation rates

Dose fractionation of a direct-acting chemical carcinogen, the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), was studied for its concurrent effects on survival, DNA damage and repair, ouabain resistance (Ouar) mutations and neoplastic transformation, in the mouse embryo cell line BALB/3T3 C1A31-1-1. MNNG doses of 0.5, 1 and 2 micrograms/ml were added to the cells either as a single exposure or in two equal fractions separated by 1, 3 or 5 h intervals. No significant difference in cytotoxicity was found when single and split-dose treatments were compared. No recovery from sublethal damage was therefore found in this cell line by split-dose administration of MNNG, although such an effect was found when the same cell line was treated with single and split doses of X-rays. Repair of DNA damage as measured by alkaline elution was studied up to 24 h after a single MNNG exposure (0.5 micrograms/ml). DNA repair was rapid during the first 5 h after treatment and slow thereafter. DNA damage detected after split doses of MNNG at 1 and 5 h intervals was significantly lower than after a corresponding single dose. With both single and split doses, rejoining of single-strand breaks (ssb) was nearly complete after 24 h of repair time. Ouar mutation and neoplastic transformation frequencies were determined for single and split doses of MNNG with the second treatment being given during (1 h) or after (5 h) the period of rapid DNA repair. No significant differences in either effect were detected for dose splitting at any tested dose.

Timing of mutation-fixation events in ethyl methane sulfonate-treated Chinese hamster cells

Exposure of single Chinese hamster ovary (CHO) cells to the mutagen, ethyl methane sulfonate, produces two types of mutant colonies lacking glucose-6-phosphate dehydrogenase activity: colonies uniformly deficient in enzyme activity, and mosaic colonies containing both mutant and nonmutant cell phenotypes in various relative proportions and sectored patterns (1/8, 1/4, 1/2). We find that the relative size of the mutant sector in these mosaic colonies primarily reflects the cell division at which the mutation was genetically fixed. Thus, the mutation-fixation event occurs before the first cell division in 1/2 sector and pure mutant colonies, between the first and second divisions for 1/4 sectors, and between the second and third divisions for 1/8 sectors. Delay in the formation of mutations could also explain the phenomenon of "mutation expression time" which is observed when drug resistance is used to select for mutants. Colony sectoring offers for the first time in mammalian cells the opportunity to observe an agent's effect on the timing of the mutational process.

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.

Lack of effect of hydroxyurea on base excision repair in mammalian cells

The effect of hydroxyurea on the initial steps of base excision repair has been examined in mammalian cells in 3 different proliferative states: i.e., quiescent cells, asynchronously growing cells undergoing multiple divisions prior to confluence; and synchronous cell populations undergoing the first cell cycle(s) after release from quiescence. Two parameters of the base excision repair pathway were examined: (1) The direct excision of 7-methylguanine from cellular DNA in the presence of increasing hydroxyurea concentrations was quantitated by high performance liquid chromatography; (2) the effects of hydroxyurea on the uracil DNA glycosylase were examined by quantitating the levels of this base excision repair enzyme in quiescent and proliferating cells. In quiescent cells, hydroxyurea at concentrations routinely used to quantitate DNA repair had no effect on the excision rates of 7-methylguanine examined over a span of 3 days; nor was there any effect on the specific activity of uracil DNA glycosylase in confluent cells. In asynchronously proliferating mammalian cells, identical hydroxyurea concentrations had no effect on the induction of the glycosylase. In synchronous growing cells HU had no effect on the temporal sequence of induction of uracil DNA glycosylase prior to DNA replication, nor on the extent of this induction. These results suggest that hydroxyurea at concentrations generally used to measure DNA repair has no effect on base excision repair.

Unscheduled DNA synthesis tests. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The utility of unscheduled DNA synthesis (UDS) testing for screening potentially hazardous chemicals was evaluated using the published papers and technical reports available to the UDS Work Group. A total of 244 documents were reviewed. Based on criteria defined in advance for evaluation of the results, 169 were rejected. From the 75 documents accepted, results were reviewed for 136 chemicals tested using autoradiographic approaches and for 147 chemicals tested using liquid scintillation counting (LSC) procedures; 38 chemicals were tested by both approaches to measure UDS. Since there were no documents available that provided detailed recommendations of UDS screening protocols or criteria for evaluating the results, the UDS Work Group presents suggested protocols and evaluation criteria suitable for measuring and evaluating UDS by autoradiography in primary rat hepatocytes and diploid human fibroblasts and by the LSC approach in diploid human fibroblasts. UDS detection is an appropriate system for inclusion in carcinogenicity and mutagenicity testing programs, because it measures the repair of DNA damage induced by many classes of chemicals over the entire mammalian genome. However, for this system to be utilized effectively, appropriate metabolic activation systems for autoradiographic measurements of UDS in human diploid fibroblasts must be developed, the nature of hepatocyte-to-hepatocyte variability in UDS responses must be determined, and the three suggested protocols must be thoroughly evaluated by using them to test a large number of coded chemicals of known in vivo mutagenicity and carcinogenicity.

Cross-resistance studies with V79 Chinese hamster cells adapted to the mutagenic or clastogenic effect of N-methyl-N'-nitro-N-nitrosoguanidine

When V79 cells are exposed to a single low dose of MNNG or MNU they acquire resistance to the mutagenic or to the clastogenic effect of the agents. Here the effect of MNNG pretreatment on mutagenesis (6-thioguanine resistance) and aberration formation in cells challenged with various mutagens/clastogens is reported. MNNG-adapted cells were resistant to the mutagenic effects of MNU and, to a lower extent, of EMS. No mutagenic adaptation was observed when MNNG-pretreated cells were challenged with MMS, ENU, MMC or UV. Cells pretreated with a dose of MNNG which makes them resistant to the clastogenic effect of this compound were also resistant to the clastogenic activity of other methylating agents (MNU, MMS), but not so with respect to ethylating agents (EMS, ENU). Cycloheximide abolished the aberration-reducing effect of pretreatment. However, when given before the challenge dose of MNNG, MNU or MMS, it drastically enhanced the aberration frequency in both pretreated and non-pretreated cells. No significant enhancement of aberration frequency by cycloheximide was found for ethylating agents. The results indicate that clastogenic adaptation is due to inducible cellular functions. It is concluded that mutagenic and clastogenic adaptation are probably caused by different adaptive repair pathways.

Biological effects of incorporation of O6-methyldeoxyguanosine into Chinese hamster V79 cells

Analysis of the biological effects of specific DNA alkylations by simple alkylating agents is complicated by the variety of sites involved. It is, therefore, of value to be able to incorporate into cellular DNA nucleosides alkylated in a single position, e.g., O6-methyldeoxyguanosine. Such cellular incorporation is particularly difficult to achieve because this nucleoside is rapidly demethylated by adenosine deaminase. We have attempted to achieve such incorporation into the DNA of V79 cells by using coformycin, an inhibitor of adenosine deaminase, and by forcing the cells to depend on exogenous purines by the use of medium containing aminopterin. The DNA of V79 cells exposed to O6-methyl-[8-3H]deoxyguanosine (2.4 microM, sp. act. 14500 Ci/mole) showed an incorporation level of 4 X 10(-8) nucleotides. When 1000-fold higher concentrations were employed (3-15 mM, sp. act. 1.6 Ci/mole), significant cytotoxicity and inhibition of DNA synthesis was observed. However, because it was not economically feasible to administer high specific activity O6-methyldeoxyguanosine to the cells at these concentrations, we could not determine the amount of labeled nucleoside incorporated into DNA. Examination of the frequency of 6-thioguanine-resistant cells in these treated populations showed no significant increase above the background level. Comparison of the cytotoxic effect of O6-methyldeoxyguanosine with deoxyadenosine showed that the toxicity induced by O6-methyldeoxyguanosine could have resulted from mimicry of deoxyadenosine, rather than by incorporation of the alkylated nucleoside itself.

Mutagenicity of cyanate, a decomposition product of MNU

Knox reported that the short-term effects of the carcinogen methylnitrosourea (MNU) were due to the formation of its decomposition product, the cyanate ion. He showed that cell survival and DNA synthesis decreased as the concentration of MNU and the cyanate ion (NCO-) increased in the medium. Further, the product of MNU decomposition comigrated with NCO- when added to his chromatographic test system. However, Knox did not study the mutagenicity of MNU or its breakdown products. We compared the mutagenicity of MNU and potassium cyanate (KNCO) in mammalian cells. Our results demonstrate that, although it is toxic to cells, KNCO does not induce ouabain-resistant mutants in cultured Chinese hamster cells (V79).

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.

Induction of mutations and sister-chromatid exchanges in Chinese hamster ovary cells by ethylating agents

Chinese hamster ovary (CHO) cells were exposed to [3H]ethyl nitrosourea (ENU) or [3H]ethyl methanesulfonate (EMS) and the following DNA ethylation products were quantitated: 3- and 7-ethyladenine, O2-ethylcytosine, 3-, 7- and O6-ethylguanine, O2- and O4-ethyldeoxythymidine and the representative ethylated phosphodiester, deoxythymidylyl (3'-5')ethyl-deoxythymidine. When mutations at the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus induced by these same treatments were compared with the observed ethylation products, mutations were found to correlate best with 3- and O6-ethylguanine. EMS induced approximately twice as many sister-chromatid exchanges (SCEs) as ENU at doses yielding equal mutation frequencies. When SCEs were indirectly compared with DNA ethylation products, 3-ethyladenine and ethylated phosphodiesters related best to SCE formation. Because mutation and SCE induction appear, at least in part, to be related to different DNA adducts, SCE induction by simple ethylating agents may not be a quantitative indicator of potentially mutagenic DNA damage.

Adaptive resynthesis of O6-methylguanine-accepting protein can explain the differences between mammalian cells proficient and deficient in methyl excision repair

Mammalian cells have been classified as proficient (Mer(+)) or deficient (Mer(-)) in methyl excision repair in terms of their cytotoxic reactions to agents that form O(6)-alkylguanine and their abilities to reactivate alkylated adenoviruses. O(6)-Methylguanine (O(6)MeGua) is considered to be a lethal, mutagenic, and carcinogenic lesion. We measured the abilities of cell extracts to transfer the methyl group from an exogenous DNA containing O(6)MeGua to acceptor protein. The constitutive level of acceptor activity was independent of the Mer phenotype and was approximately 100,000 acceptor sites per cell. Treatment of cells with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) results in a dose-dependent decrease in the acceptor activity in extracts because the rapid reaction between endogenous O(6)MeGua and acceptor protein makes the latter unavailable for further reaction. Treatment of cells with 1 muM MNNG for 15 min or 2 muM for approximately 2 min uses up >95% of the constitutive activity. However, Mer(+) cells, which are resistant to MNNG, rapidly resynthesize new acceptor protein, and the activity returns to the basal level in approximately 90 min. In Mer(-) tumor cells and Chinese hamster cells, which are sensitive to MNNG, resynthesis is not detectable in 90 min. Mer(-) simian virus 40-transformed fibroblasts, known to have an intermediate sensitivity to MNNG, have an intermediate resynthesis rate. Treatment of cells with multiple low doses of MNNG results in the enhanced production of O(6)MeGua-accepting protein in levels 2.5-fold above the constitutive values for Mer(+) tumor cells and to approximately 1.5-fold for Mer(+) fibroblasts or Mer(-) simian virus 40-transformed cells. Such treatments reduce the activities in Mer(-) tumor cells and Chinese hamster cells. We conclude: (i) estimates of O(6)MeGua in cellular DNA shortly after treatment may be seriously in error because of the rapid repair of this lesion, and (ii) the adaptive resynthesis of acceptor protein, not its constitutive level, is the important correlate of cell resistance to methylating agents.

Adaptive increase of O6-methylguanine-acceptor protein in HeLa cells following N-methyl-N'-nitro-N-nitrosoguanidine treatment

We have assayed in extracts of HeLa cells the amount of acceptor protein that removes O6-methylguanine adducts from alkylated DNA. Cells were treated with single or multiple nontoxic doses of N-methyl-N'-nitrosoguanidine (MNNG) and the extracts were analyzed up to 32 h after the last exposure. The acceptor activity assayed immediately (1 h) after single exposures decreases linearly with dose indicating that the acceptor protein is used up by endogenous O6-methylguanine adducts in a stoichiometric reaction. Multiple exposures, assayed 8-24 h after the last exposure, increase the amount of acceptor protein in a dose dependent fashion followed by a decrease above a cumulative dose of 100 ng/ml. Under conditions of maximum induction, there are about 300,000 acceptor protein sites per cell, approximately 3 fold above the constitutive level. Both in adapted and unadapted cells the methyl group from O6-methylguanine adducts in the alkylated DNA is transferred to cysteine residues of the acceptor protein(s).

A CHO-cell strain having hypersensitivity to mutagens, a defect in DNA strand-break repair, and an extraordinary baseline frequency of sister-chromatid exchange

A mutant of CHO cells (strain EM9) previously isolated on the basis of hypersensitivity to killing by ethyl methanesulfonate (EMS) is approx. 10-fold more sensitive than the parental line, AA8, to killing by both EMS and MMS. It is also hypersensitive to killing by other alkylating agents (ethyl nitrosourea and N-methyl-N'-nitro-N-nitrosoguanidine), X-rays, and ultraviolet radiation. The production and repair of DNA single-strand breaks (SSB) were studied using the technique of alkaline elution of DNA from filters. After exposure to 4 Gy of X-rays at 0 degrees C and subsequent incubation at 25 degrees C, SSB were repaired within 12 min in AA8, but little repair occurred in EM9. Similarly, with doses of EMS or MMS that produced comparable numbers of SSB in AA8 and EM9 at the end of a 10-min exposure, repair of SSB occurred more rapidly in AA8 than in EM9, suggesting that individual SSB are longer lived in EM9. EM9 was found to be hypersensitive also to the induction of mutations and sister-chromatid exchanges (SCE) by EMS; per unit dose the mutant had twice as many mutations to thioguanine resistance, 3 times as many mutations to azaadenine resistance, and a 7-fold enhancement in SCE, compared to AA8. Moreover, the baseline frequency of SCE in the mutant was extraordinarily high, i.e., 8.6 +/- 0.6 vs. 107 +/- 5 SCE/cell for AA8 and EM9, respectively, with 10 microM BrdUrd in the medium. The high SCE frequency in EM9 did not vary significantly with BrdUrd concentration over the range examined from 2.5 to 20 microM, and the percentage of 5-bromouracil substitution in the DNA was the same in EM9 and AA8 under these conditions. These data, however, do not rule out the possibility that the high SCE frequency in EM9 is a consequence of an altered sensitivity to incorporated BrdUrd. Thus, EM9 may carry a pleiotropic mutation affecting some function in DNA replication and/or DNA repair and causing the variety of phenotypic properties described in this study.

Methods for the detection of single-strand breaks in DNA under neutral conditions and their application in a study on the mechanism of repair of N-methylated purines in mouse cells

Considering enzymatic activities found in bacteria and in animal cells, there are two possible mechanisms for repair of N-methylated purines produced by methylating agents such as the mutagen and carcinogen N-methyl-N'-nitro-N-nitrosoguanidine. Both mechanisms involve first an enzymatic removal of the methylated bases by glycosylases. The resulting apurinic sites could then be repaired by (a) direct insertion of the correct bases purine insertases or (b) opening of the polynucleotide chain by apurinic endonuclease followed by repair synthesis. As the methods commonly used to detect lesions induced by methylating agents involve alkali, it was thus far not possible to decide between the above possibilities because apurinic sites are by themselves alkali labile. In this paper I describe two methods which avoid alkali and therefore allow the clarification of some aspects of the repair reaction. One of these methods makes use of 95% formamide at 40 degrees C in place of alkali to denature DNA with pre-existing single-strand breaks, the other measures the capacity of DNA scissions with free 3'-OH groups to act as primer for Escherichia coli DNA polymerase I. Results obtained with both methods make it unlikely that purine insertases play a major role in the repair of apurinic sites. Kinetics of production and repair of single-strand breaks, produced in 3T6 mouse fibroblasts by incubation with N-methyl-N'-nitro-N-nitrosoguanidine, were also examined using the methods of alkaline elution and alkaline sucrose gradient centrifugation.

The alkaline hydrolysis of phosphotriesters in alkylated mammalian DNA

Isolated DNA was alkylated with N-[14C]methyl-N-nitrosourea or N-[14C]ethyl-N-nitrosourea. Sedimentation analysis of the alkylated DNA before and after alkaline hydrolysis was used to determine the number of single-strand breaks introduced by hydrolysis of the triesters. Vacuum distillation from alkylated DNA solutions before and after alkaline hydrolysis was used to determine the numbers of triesters hydrolysing to the alcohol.

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.