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

Kinetics of CHO A L mutant expression after treatment with gamma radiation, EMS, and asbestos

The flow cytometry mutation assay (FCMA) uses hybrid CHO A(L) cells to measure mutations of the cd59 gene located on human chromosome 11 by the absence of fluorochrome-conjugated antibody binding to the CD59 surface antigen. Mutant expression peaks between 6 and 12 days, then decreases to a stable plateau, instead of a constant mutant fraction obtained by clonogenic assays. To evaluate this variable mutant expression time, cells were treated with radiation, EMS or asbestos and cell proliferation and survival were measured at times leading up to peak mutant expression. Potential doubling time (T(pot)) values increased by at least 75% for each agent by 3 h after treatment but returned to control levels after only 3 days. Survival returned to 90% of control within a week, close to the peak expression day for all three agents. The survival of CD59(-) cells sorted on the peak day of expression was roughly half that of CD59(+) cells. Cloned EMS-treated CD59(-) cells had a doubling time of 16.7 vs. 14.1 h for CD59(+) cells. Triple mutants (CD59(-)/CD44(-)/CD90(-)) were preferentially lost from the population over time, while the proportion of CD59(-)/CD90(-) mutants increased. In conclusion, the peak day of mutant expression occurs only when cells recover from the toxic effects of the mutagen. A fraction of cells originally quantified as mutants are lost over time due to lethal deletions and slower growth.

In vitro induction of micronuclei by monofunctional methanesulphonic acid esters

Six monofunctional alkylating methanesulphonates of widely varying structures were investigated in the in vitro micronucleus assay with Syrian hamster embryo fibroblast cells. The results were compared with the alkylating activities measured in the 4-(nitrobenzyl)pyridine test (NBP-test) and the N-methyl mercaptoimidazole (MMI-test) as measures for S(N)2 reactivity as well as in the triflouoroacetic acid (TFA) solvolysis and the hydrolysis reaction as measures for S(N)1 reactivity in order to provide insights into the role of alkylation mechanisms on induction of micronuclei. Moreover we compared the results of micronucleus assay with those of the Ames tests in strain TA 100 and TA1535 and with those of the SOS chromotest with the strains PQ37, PQ243, PM21 and GC 4798. The potency of methanesulphonates to induce micronuclei depended only to a certain degree, on the total alkylating activity (S(N)1 and S(N)2 reactivity). An inverse, significant correlation between the Ames test and the micronucleus assay was observed and an inverse correlation between the micronucleus assay and the SOS chromotest with the different strains. The results indicate that the primary mechanism leading to induction of micronuclei is not O-alkylation in DNA as it is the case in the Ames test with the hisG46 strains TA1535 and TA100 and not N-alkylation as with the SOS chromotest. There is evidence that protein alkylation, e.g. in the spindle apparatus in mitosis is decisive for induction of micronuclei by alkylating compounds. The structurally voluminous methanesulphonates 2-phenyl ethyl methanesulphonate and 1-phenyl-2-propyl methanesulphonate show a clear higher micronuclei inducing potency than the other tested though the bulky methanesulphonates possess a lower total alkylating activity than the others. This effect can be explained by a higher disturbance during mitosis after alkylation of the spindle apparatus with the structurally more bulky methanesulphonates.

Comparison of the mutagenic potential of 17 physical and chemical agents analyzed by the flow cytometry mutation assay

Several methods to assess genotoxicity of physical and chemical agents have been developed, most of which depend on growing colonies in selective medium. We recently published a new method for detecting mutations in the CD59 gene in a Chinese hamster ovary cell line that contains a single copy of human chromosome 11 (CHO A(L)). The assay is based on detecting the surface expression of CD59 with monoclonal antibodies using flow cytometry. The capabilities of this flow cytometry mutation assay (FCMA) to detect mutations from a wide variety of genotoxic agents are described here. There was a 400-fold separation between CD59- and CD59+ populations based on fluorescence intensity. Small numbers of negative cells mixed in with positive cells were detected in a highly linear fashion. Mutation dose response curves over a dose range yielding 80% to 20% survival are shown for ethyl methane sulfonate (EMS), mitomycin C (MMC) and lead acetate. EMS and lead acetate exhibited a threshold in response while MMC had a linear dose response over the full dose range. The mutant fraction was measured over time periods ranging up to 35 days following treatment. The mutant fraction peaked at different times ranging from 6 to 12 days after treatment. An additional 14 chemical and physical agents including point mutagens, heavy metals, ionizing and UV radiation, and DNA intercalators and cross linkers, were analyzed for mutagenic potential after doses giving 80% to 20% survival. The results presented here demonstrate the sensitivity and broad-ranging capability of the FCMA to detect mutations induced by a variety of genotoxic agents.

Screening for genotoxicity using the DRAG assay: investigation of halogenated environmental contaminants

The DRAG test is a rapid high-throughput screening assay for detection of repairable adducts by growth inhibition of Chinese hamster ovary cells (CHO) characterized by different defects in DNA repair. A more pronounced growth inhibition caused by a certain DNA-reactive substance in a repair-deficient cell line (EM9, UV4 and UV5) as compared to wild-type cells (AA8) is interpreted as a consequence of their inability to repair induced DNA lesions. Thus, the use of such cell lines in the DRAG test may provide information of the type of DNA lesions induced by a certain genotoxic substance. To select optimal assay conditions, as well as to provide a mechanistic basis for interpreting the results, the model compounds benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide (BPDE), ethyl methanesulfonate (EMS), mitomycin C (MMC) and hydrogen peroxide (H2O2) were used. These agents can induce bulky adducts, alkyl adducts, cross-links and oxidative damage, respectively. The specificity of the DRAG test constitutes an important prerequisite for its practical use in a broader context. To assess this aspect, we have investigated the genotoxic and cytotoxic properties of a selection of metabolites of and isomers from polychlorinated biphenyls (PCB) and polybrominated diphenyl ethers (PBDE), along with a few other halogenated compounds. All these compounds have been detected as pollutants in the external environment, and for most of them there is no convincing evidence of mutagenicity from conventional assays. As could be predicted from their mode of action, BPDE, MMC, and EMS were all found to be more toxic in the repair-deficient cell lines compared with wild-type cells. The results with H2O2 were inconclusive, and the PCB metabolite 4,4'-diOH-CB80 only exhibited borderline activity, while all other halogenated compounds, or their metabolites, were found to be inactive. In conclusion, the DRAG assay could provide a robust and useful tool when screening large numbers of potentially genotoxic agents, while in addition providing mechanistic information. However, the usefulness of the selected cell lines to detect oxidative damage may be limited.

On the role of alkylating mechanisms, O-alkylation and DNA-repair in genotoxicity and mutagenicity of alkylating methanesulfonates of widely varying structures in bacterial systems

The Ames test and the SOS-chromotest are widely used bacterial mutagenicity/genotoxicity assays to test potential carcinogens. Though the molecular mechanisms leading to backmutations and to the induction of SOS-repair are in principle known the role of alkylation mechanisms, of different DNA-lesions and of DNA-repair is in parts still unknown. In this study we investigated 14 monofunctional methanesulfonates of widely varying structures for mutagenicity in Salmonella typhimurium strain TA 1535 sensitive for O(6)-guanine alkylation for comparison with strain TA 100 in order to obtain additional information on the role of alkylation mechanisms, formation of the procarcinogenic DNA-lesion O(6)-alkylguanine and the role of DNA-repair in induction of backmutation. The substances were also tested in the SOS-chromotest with Escherichia coli strain PQ 37 and strain PQ 243 lacking alkyl base glycosylases important for base excision repair in order to examine the role of alkylation mechanisms, of base excision repair and the role of O-alkyl and N-alkyl DNA-lesions on the induction of SOS-repair. The secondary methanesulfonates with very high S(N)1-reactivity isopropyl methanesulfonate and 2-butyl methanesulfonate showed highest mutagenicities in both strains. The higher substituted methanesulfonates with very high S(N)1-reactivity had lower mutagenic activities because of reduced half lives due to their high hydrolysis rates. A clear increase in mutagenicities in strain TA 100 was observed for the primary compounds methyl methanesulfonate and allyl methanesulfonate with very high S(N)2-reactivity. The primary compound phenylethyl methanesulfonate has a relatively high mutagenicity in both Salmonella strains which can be explained by an increased S(N)1-reactivity and by low repair of the O(6)-phenylethylguanine. Highest SOSIPs (SOS inducing potency) in strains PQ 37 and PQ 243 were found for methyl methanesulfonate and for the secondary compounds with high S(N)1-reactivity. The ratios in the SOSIPs between strain PQ 243 and PQ 37, indirectly indicative for the role of O- and N-alkylation in the induction of SOS-repair, was high for the primary methanesulfonates and lower for the secondary, indicating that the SOS-repair is, to a certain extent, also induced by other lesions than O(6)-alkylation. The results indicate that O(6)-alkylation is also a predominant lesion for backmutation in strain TA 100 and that in the case of monofunctional alkylating agents high S(N)2-reactivities are required to induce error prone repair mediated backmutations. The O(6)-alkylguanine lesion is also important for induction of SOS-repair in the SOS-chromotest, however, other sites of alkylation which are repaired by the base pair excision repair system can also efficiently contribute to the induction of SOS-repair.

Genetic damage by bifunctional agents in repair-active pre-meiotic stages of Drosophila males

Most of our understanding of germline mutagenesis in Drosophila is based on the DNA repair-inactive, haploid post-meiotic stages. The diploid, repair-active pre-meiotic stages are more relevant to the situation encountered in somatic cells. DNA mono-adducts induced by agents like methyl methanesulphonate (MMS) and ethylene oxide (EO) are well repaired in the pre-meiotic cell stages, and these agents show therefore, no or considerable lower mutagenic activity in these stages. In contrast, in this study the two bifunctional nitrogen mustards chlorambucil (CAB) and mechlorethamine (MEC) show significantly elevated mutant frequencies of both post- and pre-meiotic germ cells. Results were similar for the X-chromosomal and the autosomal (2nd) recessive lethal (RL) test. CAB and MEC were also active in stem cells, but in comparison with post-stem cell stages they seem to be better protected. The germ cell specific response in post- and pre-meiotic cell stages was for both nitrogen mustards comparable to mutagenic activity patterns observed in the specific locus test in the mouse. It was reported that for diepoxybutane (DEB), another cross-linking agent, the ratio of the RL frequency for the 2nd- and the X-chromosome was increased from 2.1 for post-meiotic stages to 9.5 for pre-meiotic stages. In own experiments aiming to confirm this observation, a high ratio was indeed found. The induction of large deletions by DEB could be the reason for this difference, since such lesions might include both a sex-linked lethal and a vital gene required for the development of spermatocytes into mature sperm. Similar differences were expected for CAB and MEC since they are also inducers of large deletions. But unexpectedly, no differences in 2nd/X RL ratio between post- and pre-meiotic cell stages were found for the nitrogen mustards. Possible causes such as distinct proportions of multi-locus deletions (MLDs), mitotic recombination and the formation of persistent lesions, are discussed.