Single-strand breaks in DNA of hamster and human cells exposed to methyl methanesulfonate and ultraviolet light

The number of single-strand breaks produced in DNA after exposure to UV light or to methyl methanesulfonate (MMS) was additive when cells were exposed to both agents in close succession. Repair of the damage from either agent was partially inhibited by cytosine arabinoside, resulting in higher break frequencies under all conditions of exposure. Exposure to both agents followed by growth in cytosine arabinoside resulted in break frequencies that were approximately the same as the sum of those from each agent individually. These findings contrast with previous results in which pyrimidine dimer excision and repair replication after exposure to UV light were inhibited by MMS. These observations are not due to cell permeability changes after alkylation, but can be explained if the complex of excision-repair proteins is only partially inactivated by alkylation. Initial incisions to start repair would still occur but only limited amounts of repair replication would ensue without actual removal of the pyrimidine dimers.     

Differential sensitivity of DNA replication and repair in permeable Escherichia coli exposed to various monofunctional methylating or ethylating agents.

Escherichia coli cells made permeable to deoxynucleoside triphosphates by brief treatment with toluene (permeablized) were used to measure the effect of the following chemical alkylating agents on either DNA replication or DNA repair synthesis: methyl methanesulfonate (MMS), ethyl methanesulfonate (EMS), N-methyl-N-nitrosourea (MNU), N-ethyl-N-nitrosourea (ENU), N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) and N-ethyl-N′-nitro-N-nitrosoguanidine (ENNG). Replication of DNA in this pseudo-in vivo system was completely inhibited 10–15 min after exposure to MMS at concentrations of 5 mM or higher or to MNU or MNNG at concentrations of 1 mM or higher. The ethyl derivatives of the alkylating agents were less inhibitory than their corresponding methyl derivatives, and inhibition of DNA replication occurred in the following order: EMS < ENNG < ENU. Maximum inhibition of DNA replication by all of the alkylating agents tested except EMS occurred at a concentration of 20 mM or lower. The extent of replication in cells exposed to EMS continued to decrease with concentrations of EMS up to 100 mM (the highest concentration tested).The experiments in which the inhibition of DNA replication by MMS, MNU, or MNNG was measured were repeated under similar assay conditions except that a density label was included and the DNA was banded in CsCl gradients. The bulk of the newly synthesized DNA from the untreated cells was found to be of the replicative (semi-conservative) type. The amount of replicative DNA decreased with increasing concentration of methylating agent in a manner similar to that observed in the incorporation experiments.Polymerase I (Pol I)-directed DNA repair synthesis induced by X-irradiation of permeablized cells was assayed under conditions that blocked the activity of DNA polymerases II and III. Exposure of cells to MNNG or ENNG at a concentration of 20 mM resulted in reductions in Pol I activity of 40 and 30%, respectively, compared with untreated controls. ENU was slightly inhibitory to Pol I activity, while MMS, EMS, and MNU all caused some enhancement of Pol I activity.These data show that DNA replication in a pseudo-in vivo bacterial system is particularly sensitive to the actions of known chemical mutagens, whereas DNA repair carried out by the Pol I repair enzyme is much less sensitive and in some cases apparently unaffected by such treatment. Possible mechanisms for this differential effect on DNA metabolism and its correlation with current theories of chemically induced mutagenesis and carcinogenesis are discussed.     

Survival and liquid holding recovery in UV-sensitive strains of Saccharomyces cerevisiae after treatment with chemical mutagens and radiation.

Two UV-sensitive mutants of Saccharomyces cerevisiae rad 3 and rad 6 were tested for sensitivity to X-rays, MMS, EMS, HNO2 and DEB. Rad 3 mutant is more sensitive than the wild type strain only to HNO2 and DEB, while rad 6 is cross sensitive both to X-rays and all chemicals tested. Liquid holding recovery (LHR) was studied by comparison of cell survival immediately after mutagen treatment and after 5 days of storage in phosphate buffer. LH greatly increases cell survival of rad 3 mutant after DEB and slightly after EMS, MMS and HNO2, while after UV treatment LH significantly decreases survival of this mutant. LH increases survival of rad 6 mutant after exposure to UV, MMS and HNO2, but decreases survival of DEB-treated cells. Exposure of wild type strain to LH results in an increase of survival after UV, and DEB but not after MMS and HNO2. The results suggest that LHR is a strain- and mutagen-specific phenomenon and cannot be explained within the present knowledge of repair processes in yeast.     

The recovery of mammalian cells treated with methyl methanesulfonate, nitrogen mustard or UV light. I. The effect of alkylation products on DNA replication.

CHO cells were synchronized in G1 phase and treated with MMS or HN2. The subsequent rate of DNA replication was found to be reduced in a dose-dependent manner. In addition, 2 X 10(-3 M and 3 X 10(-3) M MMS resulted in a 3--4 h delay prior to the initiation of S phase. If the cells were held for 8 h in hydroxyurea after MMS treatment, no subsequent lag in DNA synthesis was seen after removal of the hydroxyurea. The entry of confluent cells into S phase was found to be delayed 7 h upon trypsinizing and replating. Treatment of these cells with MMS resulted in a reduced rate of DNA replication, but no further delay in its initiation. Repair replication was found to continue at a constant rate for at least 12 h following MMS treatment of cells under all of these conditions. At the concentrations used in these experiments MMS severely inhibited the rate of protein synthesis, but HN2 had little effect. By comparing both the kinetics of repair replication and recovery of protein synthesis with the rate of DNA replication, it was concluded that the initial, severe reduction in rate following MMS treatment was probably due to an inhibition of protein synthesis.     

Prophage induction in Haemophilus influenzae and its relationship to mutation by chemical and physical agents

It is known that UV, X-rays, MMC and MMS are not mutagenic for H. influenzae, whereas HZ, EMS and MNNG are potent mutagens for this bacterium. All of these agents, however, are known to be both mutagenic and able to induce prophage in E. coli. We report here that all the agents except HZ induce prophage in H. influenzae, and EMS even induces in the recombination-defective recl mutant, which is non-inducible by UV, MMC, MNNG and MMS. MMS did not cause single-strand breaks or gaps in DNA synthesized after treatment of H. influenzae, but EMS and MNNG produced them. EMS caused more breaks in DNA synthesized before treatment than in that synthesized after treatment. On the other hand we did observe such breaks or gaps induced in E. coli in DNA synthesized posttreatment by EMS as well as by MMS and MNNG, at comparable survival levels.     

Comparative analysis of deletion and base-change mutabilities of Escherichia coli B strains differing in DNA repair capacity (wild-type, uvrA-, polA-, recA-) by various mutagens.

Dose-response curves were compared for deletions [ColBR (resistant to colicin B) mutations being more than 80% deletions] and base changes (reversion of argFam to prototrophy argplus) induced in the same set of E. coli strains (wild-type for DNA repair, uvrA-, polA- and recA-) by N-methyl-N'-nitro-N-nitrosoguanidine (NTG), ethyl methanesulfonate (EMS), hydroxylamine (HA), 4-nitroquinoline I-oxide (4NQO), mitomycin C (MTC, UV and X-rays. All these agents induced deletions as well as base changes in the wild-type strain. Thus chemical mutagenesis differed in E. coli and bacteriophages in vitro, for HA, NTG, EMS and perhaps UV produced only point mutations in phage Tr. The patterns of deletion and base-change mutability in E. coli were surprisingly similar. (I) The recombination less recA- strain was mutable by only three (NTG, EMS, HA) of the seven mutagens for either deletions or base changes. (2) The uvrA- strain, unable to excise pyrimidine dimers, was very highly mutable by 4NQO and UV but immutable by MTC for both deletions and base changes. (3) The polA- strain, defective in DNA polymerase I due to a non-suppressible mutation, was very highly mutable by HA and highly mutable by MTC and 4NQO for both deletions and base changes but was highly mutable only for deletions by UV and X-rays, remaining normally mutable by the other agents for both deletions and base changes despite its high sensitivity to their inactivating action. We conclude that errors in the recA-dependent repair of induced DNA damage (after 4NQO, MTC, UV and X-rays) or errors in replication enhanced by damage to the replication system or to the template strands (after NTG, EMS, and HA) give rise to deletions as well as to base changes. From a comparative analysis of 14 dose-response curves for deletions and base changes, we conclude that the order of mutagenic efficiency relative to killing is (EMS, NTG) greater than (UV, 4NQO) greater than HA greater than (X-rays, MTC), and that X-rays, 4NQO, HA and MTC induce more ColBR deletions than Argplus base changes, whereas UV and EMS induce ColBR deletions and Argplus base changes at nearly equal rates and the specificity of NTG is intermediate between these two types.     

Unscheduled DNA synthesis induced in mouse spermatids after combined treatment with methyl methanesulfonate and X-rays.

Unscheduled DNA synthesis (UDS), which is considered to be DNA repair, has been studied in early- to mid-spermatid stages of the mouse after combined treatments with X-rays and methyl methanesulfonate (MMS). UDS in spermatids was detected by giving testicular injections of [methyl-3H]thymidine ([3H]dThd) and making use of the fact that no scheduled DNA synthesis occurs in the germ cells after the last S period in primary spermatocytes. X-rays and MMS are each able to induce UDS in mouse spermatids. However, there was a statistically significant reduction in the amount of UDS observed when X-ray exposures of from 200 to 600 R were given 4 h before an i.p. injection of 75 mg/kg of MMS and concurrent testicular injections of [3H]dThd. This reduction in UDS is more than can be explained by the completion of repair of X-ray-induced DNA lesions. We suggest that the reduction in UDS is the result of an X-ray-produced impairment of a least a part of the repair mechanism involved in correcting MMS-induced DNA lesions. When the time interval between a 600-R X-ray exposure and MMS treatment was between 3 and 20 h (latest time interval s;udied) there was a statistically significant reduction of UDS in the spermatids. No significant decrease in UDS response occurred when the time interval between radiation exposure and MMS treatment was less than approximately 3 h.     

Choloroquine inhibition of repair of DNA damage induced in mammalian cells by methyl methanesulfonate

Chloroquine (ClQ) inhibited the repair of DNA damage produced in cultured rat liver cells by methyl methanesulfonate (MMS). MMS caused fragmentation of single-strand DNA in alkaline sucrose gradients. Repair of the damage was followed by observing the restoration of the normal sedimentation pattern at intervals after treatment. Repair was significant by 7 h and nearly complete at 24 h. Addition of ClQ during the repair peiod markedly reduced the rate of repair. Also, ClQ increased the lethality of MMS, which could be due to the inhibition of repair. ClQ was found to inhibit protein synthesis, but the effect on repair is probably not due entirely to this action since comparable inhibition of protein synthesis by cycloheximide produced a lesser degree of delay in repair.     

Retardation of cell cycle progression in yeast cells recovering from DNA damage: A study at the single cell level

Summary Pedigree analyses of individual yeast cells recovering from DNA damage were performed and time intervals between morphological landmark events during the cell cycle (bud emergence and cell separation), were recorded for three generations. The associated nuclear behavior was monitored with the aid of DAPI staining. The following observations were made: (1) All agents tested (X-rays, MMS, EMS, MNNG, nitrous acid) delayed the first bud emergence after treatment, which indicates inhibition of the initiation of DNA replication. (2) Cells that survived X-irradiation progressed further through the cell cycle in a similar way to control cells. (3) Progress of chemically treated cells became extremely asynchronous because surviving cells stayed undivided for periods of varying length. (4) Prolongation of the time between bud emergence and cell separation was most pronounced for cells treated with the alkylating agents MMS and EMS. This is interpreted as retardation of ongoing DNA synthesis by persisting DNA adducts. (5) Cell cycle prolongation in the second and third generation after treatment was observed only with MMS treated cells. (6) In all experiments, individual cells of uniformly treated populations exhibited highly variable responses.Abbreviations DAPI 4,6-diamidino-2-phenyl-indole - EMS ethyl methanesulfonate - MMS methyl methanesulfonate - MNNG N-methyl-N-nitro-N-nitrosoguanidine     

Cell cycle regulation of DNA repair in normal and repair deficient human cells

The regulation of nucleotide excision repair and base excision repair by normal and repair deficient human cells was determined. Synchronous cultures of WI-38 normal diploid fibroblasts and Xeroderma pigmentosum fibroblasts (complementation group D) (XP-D) were used to investigate whether DNA repair pathways were modulated during the cell cycle. Two criteria were used: (1) unscheduled DNA synthesis (UDS) in the presence of hydroxyurea (HU) after exposure to UV light or after exposure to N-acetoxy-acetylaminofluorene (N-AcO-AAF) to quantitate nucleotide excision repair or UDS after exposure to methylmethane sulfonate (MMS) to measure base excision repair; (2) repair replication into parental DNA in the absence of HU after exposure to UV light. Nucleotide excision repair after UV irradiation was induced in WI-38 fibroblasts during the cell cycle reaching a maximum in cultures exposed 14–15 h after cell stimulation. Similar results were observed after exposure to N-AcO-AAF. DNA repair was increased 2–4-fold after UV exposure and was increased 3-fold after N-AcO-AAF exposure. In either instance nucleotide excision repair was sequentially stimulated prior to the enhancement of base excision repair which was stimulated prior to the induction of DNA replication. In contrast XP-D failed to induce nucleotide excision repair after UV irradiation at any interval in the cell cycle. However, base excision repair and DNA replication were stimulated comparable to that enhancement observed in WI-38 cells. The distinctive induction of nucleotide excision repair and base excision repair prior to the onset of DNA replication suggests that separate DNA repair complexes may be formed during the eucaryotic cell cycle.     

Coordinate inhibition of DNA synthesis and thymidylate synthase activity following DNA damage and repair

Two agents, 3-aminobenzamide (3-AB) and beta lapachone, that inhibit repair of mammalian cell DNA damaged by methyl methane sulfonate (MMS), also coordinately blocked both DNA replication (incorporation of 3H-thymidine) and thymidylate synthase (TS) activity. Aphidicolin also inhibited both 3H-TDR incorporation and TS in damaged cells, the former more strongly than the latter, in a manner not coordinated with lethality. It is proposed that the DNA lesions created by MMS and modified by repair inhibit semiconservative DNA synthesis by allosterically interacting with the DNA replication replitase complex, so as to block its overall function and also the activity of TS, one of its enzymes.     

Somatic cell mutagenicity in Drosophila melanogaster in comparison with genetic damage in early germ-cell stages

With the intention of assessing the general performance, sensitivity and the underlying mechanisms of somatic cell mutagenicity assays in Drosophila, a study was undertaken to compare the effectiveness of 5 procarcinogens and 4 direct-acting agents in the white/white-coral eye mosaic assay (SMART) with their activity in early (premeiotic) male and female germ-cell stages, after exposure of Drosophila larvae. The outcome indicated a lack of agreement in the results from recessive lethal assays (SLRL) in comparison with the somatic mutation and recombination test (SMART). The procarcinogens 2-naphthylamine (NA), 3-methylcholanthrene (MC), 9,10-dimethylanthracene (DA) and 7,12-dimethylbenz[a]anthracene (DMBA), and the direct-acting mutagens bleomycin (BM), methyl methanesulfonate (MMS) and ethyl methanesulfonate (EMS), were quite efficient in producing somatic recombination and mutations in white/white-coral larvae, as opposed to only weak effects in early germ-cell stages. 2-Acetylaminofluorene (2AAF) showed marginal effects in both germ cells and somatic tissue after exposure of female larvae, but was inactive in testis. The discrepancy in mutational response between somatic cells and premeiotic germ cells is most impressive for MMS and BM. There is sufficient evidence for attributing a good sized proportion of the encountered variation to efficient error-free DNA repair of premutational damage and to segregational elimination during meiosis of deleterious mutations: (1) The efficient point mutagen ENU was the but one agent producing high levels of viable genetic alterations in early germ cells and in somatic cells. A similar behaviour was previously described for diethylnitrosamine, which ethylates DNA in the same fashion as ENU. (2) In early germ-cell stages of mei-9L1 male larvae, MMS induced multiple mutations (putative clusters) at a low dose differing by a factor 20-40 from those needed to produce an equivalent response in repair-competent strains. This is consistent with the concept of an active excision repair in premeiotic cells. (3) In the case of EMS, next to DNA repair, germinal selection seems to restrict the realization of EMS-induced genetic damage in premeiotic cells. (4) Bleomycin-induced chromosome aberrations caused high mortality rates in males (hemizygous for an X-chromosome) but not in females. MMS and BM, agents known to show preference for chromosome aberration induction, produced 3-6-fold higher rates of somatic mutational events (SME) in female genotypes as compared with the other sex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inducible DNA-repair systems in yeast: competition for lesions

DNA lesions may be recognized and repaired by more than one DNA-repair process. If two repair systems with different error frequencies have overlapping lesion specificity and one or both is inducible, the resulting variable competition for the lesions can change the biological consequences of these lesions. This concept was demonstrated by observing mutation in yeast cells (Saccharomyces cerevisiae) exposed to combinations of mutagens under conditions which influenced the induction of error-free recombinational repair or error-prone repair. Total mutation frequency was reduced in a manner proportional to the dose of 60Co-gamma- or 254 nm UV radiation delivered prior to or subsequent to an MNNG exposure. Suppression was greater per unit radiation dose in cells gamma-irradiated in O2 as compared to N2. A rad3 (excision-repair) mutant gave results similar to wild-type but mutation in a rad52 (rec-) mutant exposed to MNNG was not suppressed by radiation. Protein-synthesis inhibition with heat shock or cycloheximide indicated that it was the mutation due to MNNG and not that due to radiation which had changed. These results indicate that MNNG lesions are recognized by both the recombinational repair system and the inducible error-prone system, but that gamma-radiation induction of error-free recombinational repair resulted in increased competition for the lesions, thereby reducing mutation. Similarly, gamma-radiation exposure resulted in a radiation dose-dependent reduction in mutation due to MNU, EMS, ENU and 8-MOP + UVA, but no reduction in mutation due to MMS. These results suggest that the number of mutational MMS lesions recognizable by the recombinational repair system must be very small relative to those produced by the other agents. MNNG induction of the inducible error-prone systems however, did not alter mutation frequencies due to ENU or MMS exposure but, in contrast to radiation, increased the mutagenic effectiveness of EMS. These experiments demonstrate that in this lower eukaryote, mutagen exposure does not necessarily result in a fixed risk of mutation, but that the risk can be markedly influenced by a variety of external stimuli including heat shock or exposure to other mutagens.     

Effects of DNA replication inhibitors on UV excision repair in synchronised human cells.

When HeLa cells are irradiated with UV and treated with the DNA synthesis inhibitors hydroxyurea (HU) and 1-beta-D-arabinofuranosylcytosine (ara C), DNA strand breaks accumulate at sites where excision repair of DNA damage has been inhibited after the incision step. This break accumulation occurs in mitotic, G1 and S phase cells. But UV-induced repair synthesis of DNA, as measured by [3H]thymidine incorporation into unreplicated DNA, is not inhibited by HU and ara C in G1 or S phase cells, even though replicative synthesis is virtually abolished. Repair and replication must therefore utilise different DNA precursor pools, or different DNA synthetic systems; and the action of Hu and ara C in causing strand break accumulation may occur at the ligation step of excision repair.     

Biological effect of low doses of alkylating agents in drosophila studies

The low dose (0.05-0.1 mM) influence of alkylating agents on germ cell survival and male fertility, the level of embryonic and postembryonic lethality as well as the sex-linked recessive lethal (SLRL) frequency induced by high alkylating agent doses was studied in Drosophila melanogaster. The pretreatment of adult males with low doses of methyl and ethyl methanesulfonate (MMS and EMS) did not change or even enhanced EMS cytotoxicity and mutagenicity in both mature sperm and premeiotic cells. On the contrary, the low EMS dose pretreatment of larvae protected them against higher mutagen doses increasing male fertility, decreasing embryonic and postembryonic lethality in F1, and leading to three-fold reduction in the SLRL frequency in F2. The adaptive response was dependent on the Drosophila developmental stage exposed to challenge mutagen doses, since the protection was maximal in larvae and practically absent when the high dose was administered to adult males. The adaptive response observed does not seem to be associated with DNA repair, but it is rather due to other protective mechanisms.     

Repair replication and unscheduled DNA synthesis in mammalian cell lines showing differential sensitivity to alkylating agents

Repair replication has been measured by CsCl density gradient centrifugation in cell lines showing differential sensitivity to mono- and bifunctional alkylating agents. A correlation between cellular sensitivity as measured by the D₀ value and amount of repair replication was demonstrated after exposure of Yoshida cells to nitrogen mustard (HN2) and methylene dimethanesulphonate (MDMS). No differences in the amount of repair replication after methyl methanesulphonate (MMS) were observed in two L5178Y cell lines which differed in sensitivity by virtue of the shoulder size only. The Yoshida cell lines showed no difference in sensitivity to MMS and no difference in amount of repair replication. Incorporation of tritiated thymidine 9[³H]TdR) after drug treament was also measured by autoradiography. The qualitative differences observed between the two cell lines were similar to those obtained in density gradient experiments. The temporal pattern of [³H]TdR uptake indicated that the reduced repair replication observed in the sensitive line after HN2 and MDMS is not due to slower synthesis. The kinetics of [^3H]TdR incorporation differed for all three mutagens suggesting that different enzymes may be involved in each case.     

Excision repair of UV-damaged plasmid DNA in Xenopus oocytes is mediated by DNA polymerase alpha (and/or delta).

We studied DNA repair by injecting plasmids containing random pyrimidine dimers into Xenopus oocytes. We demonstrated excision repair by recovering plasmids and analyzing them with T4 UV endonuclease treatment and alkaline agarose gel electrophoresis. The mechanism for excision repair of these plasmids appears to be processive, rather than distributive, since repair occurs in 'all or none' fashion. At less than 4-5 dimers/plasmid, nearly all repair occurs within 4-6 hours (approximately 10(10) dimers repaired per oocyte); the oocyte, therefore, has abundant repair activity. Specific antibodies and inhibitors were used to determine enzymes involved in repair. We conclude that DNA polymerase alpha (and/or delta) is required because repair is inhibited by antibodies to human DNA polymerase alpha, as well as by aphidicolin, an inhibitor of polymerases alpha (and/or delta). Repair was not inhibited by hydroxyurea, cytosine beta-D-arabinofuranoside, or inhibitors of topoisomerase II (novobiocin). Oocyte repair does not activate semi-conservative DNA replication, nor is protein synthesis required. Photoreactivation cannot account for repair because dimer removal is independent of exogenous light.