Molecular characterization of methyl methanesulphonate (MMS)-induced HPRT mutations in V79 cells

Influence of Kv11.1 (hERG1) K+ channel expression on DNA damage induced by the genotoxic agent methyl methanesulfonate

Besides their crucial role in cell electrogenesis and maintenance of basal membrane potential, the voltage-dependent K⁺ channel Kv11.1/hERG1 shows an essential impact in cell proliferation and other processes linked to the maintenance of tumour phenotype. To check the possible influence of channel expression on DNA damage responses, HEK293 cells, treated with the genotoxic agent methyl methanesulfonate (MMS), were compared with those of a HEK-derived cell line (H36), permanently transfected with the Kv11.1-encoding gene, and with a third cell line (T2) obtained under identical conditions as H36, by permanent transfection of another unrelated plasma membrane protein encoding gene. In addition, to gain some insights about the canonical/conduction-dependent channel mechanisms that might be involved, the specific erg channel inhibitor E4031 was used as a tool. Our results indicate that the expression of Kv11.1 does not influence MMS-induced changes in cell cycle progression, because no differences were found between H36 and T2 cells. However, the canonical ion conduction function of the channel appeared to be associated with decreased cell viability at low/medium MMS concentrations. Moreover, direct DNA damage measurements, using the comet assay, demonstrated for the first time that Kv11.1 conduction activity was able to modify MMS-induced DNA damage, decreasing it particularly at high MMS concentration, in a way related to PARP1 gene expression. Finally, our data suggest that the canonical Kv11.1 effects may be relevant for tumour cell responses to anti-tumour therapies.

Investigation of toxicity and mutagenicity of cold atmospheric argon plasma

Cold atmospheric argon plasma is recognized as a new contact free approach for the decrease of bacterial load on chronic wounds in patients. So far very limited data are available on its toxicity and mutagenicity on eukaryotic cells. Thus, the toxic/mutagenic potential of cold atmospheric argon plasma using the MicroPlaSter β^® , which has been used efficiently in humans treating chronic and acute wounds, was investigated using the XTT assay in keratinocytes and fibroblasts and the HGPRT (hypoxanthine guanine phosphoribosyl transferase) assay with V79 Chinese hamster cells. The tested clinical parameter of a 2 min cold atmospheric argon plasma treatment revealed no relevant toxicity on keratinocytes (viability: 76% ± 0.17%) and on fibroblasts (viability: 81.8 ± 0.10) after 72 hr as compared to the untreated controls. No mutagenicity was detected in the HGPRT assay with V79 cells even after repetitive CAP treatments of 2-10 min every 24 hr for up to 5 days. In contrast, UV-C irradiation of V79 cells, used as a positive control in the HGPRT test, led to DNA damage and mutagenic effects. Our findings indicate that cold atmospheric plasma using the MicroPlaSter β^® shows negligible effects on keratinocytes and fibroblasts but no mutagenic potential in the HGPRT assay, indicating a new contact free safe technology. Environ. Mol. Mutagen. 58:172-177, 2017. © 2017 Wiley Periodicals, Inc.

Data in support of the mutagenic potential of the isoflavone irilone in cultured V79 cells

The isoflavone irilone is found in human plasma after ingestion of red clover-based dietary supplements, but information allowing safety assessment is rare. Here, data in support of the mutagenic potential of irilone in cultured V79 cells [1] are presented. These data include (i) a quantitative assessment of irilone in the culture medium during the cell culture experiments, (ii) changes in the mutation spectrum in cDNA of the hypoxanthine-guanine phosphoribosyltransferase locus of irilone-treated V79 cells, (iii) occurrence of karyorrhexis and apoptosis as well as (iv) number of micronucleated cells containing whole chromosomes or chromosomal fragments. Also exemplary micrographs, used for the fluorescence microscopic assessment of (iii) and (iv) are presented.

Alkyltransferase-mediated toxicity of bis-electrophiles in mammalian cells

The primary function of O(6)-alkylguanine-DNA alkyltransferase (AGT) is to maintain genomic integrity in the face of damage by both endogenous and exogenous alkylating agents. However, paradoxically, bacterial and mammalian AGTs have been shown to increase cytotoxicity and mutagenicity of dihaloalkanes and other bis-electrophiles when expressed in bacterial cells. We have extended these studies to mammalian cells using CHO cells that lack AGT expression and CHO cells stably transfected with a plasmid that expresses human AGT. The cytotoxicity of 1,2-dibromoethane, dibromomethane and epibromohydrin was significantly increased by the presence of AGT but cytotoxicity of butadiene diepoxide was not affected. Mutations caused by these agents were assessed using hypoxanthine-guanine phosphoribosyltransferase (HPRT) as a reporter gene. There was a small (c. 2-3-fold) but statistically significant AGT-mediated increase in mutations caused by 1,2-dibromoethane, dibromomethane and epibromohydrin. Analysis of the mutation spectrum induced by 1,2-dibromoethane showed that the presence of AGT also altered the types of mutations with an increase in total base substitution mutants due to a rise in transversions at both G:C and A:T sites. AGT expression also led to mutations arising from the transcribed strand, which were not seen in cells lacking AGT. Although the frequency of deletion mutations was decreased by AGT expression, the formation of large deletions (> or = 3 exons) was increased. This work demonstrates that interaction of AGT with some bis-electrophiles can cause mutagenicity and diminished cell survival in mammalian cells. It is consistent with the hypothesis that DNA-AGT cross-links, which have been characterized in experiments with purified AGT protein and such bis-electrophiles, can be formed in mammalian cells.

Specific aneusomies in Chinese hamster cells at different stages of neoplastic transformation, initiated by nitrosomethylurea

Aneuploidy is ubiquitous in cancer, and its phenotypes are inevitably dominant and abnormal. In view of these facts we recently proposed that aneuploidy is sufficient for carcinogenesis generating cancer-specific aneusomies via a chain reaction of autocatalytic aneuploidizations. According to this hypothesis a carcinogen initiates carcinogenesis via a random aneuploidy. Aneuploidy then generates transformation stage-specific aneusomies and further random aneusomies autocatalytically, because it renders chromosome segregation and repair mechanisms error-prone. The hypothesis predicts that several specific aneusomies can cause the same cancers, because several chromosomes also cooperate in normal differentiation. Here we describe experiments on the Chinese hamster (CH) that confirm this hypothesis. (i) Random aneuploidy was detected before transformation in up to 90% of CH embryo cells treated with the carcinogen nitrosomethylurea (NMU). (ii) Several specific aneusomies were found in 70-100% of the aneuploid cells from colonies transformed with NMU in vitro and from tumors generated by NMU-transformed cells in syngeneic animals. Among the aneuploid in vitro transformed cells, 79% were trisomic for chromosome 3, and 59% were monosomic for chromosome 10, compared with 8% expected for random distribution of any aneusomy among the 12 CH chromosomes. Moreover, 52% shared both trisomy 3 and monosomy 10 compared with 0.6% expected for random distribution of any two aneusomies. Among the tumor cells, 65% were trisomic for chromosome 3, 51% were trisomic for chromosome 5, and 30% shared both trisomies. Aneuploid cells without these specific aneusomies may contain minor transformation-specific aneusomies or may be untransformed. (iii) Random aneusomies and structurally altered chromosomes increased with the generations of transformed cells to the point where their origins became unidentifiable in tumors. We conclude that specific aneusomies are necessary for carcinogenesis.

Triethylene glycol dimethacrylate induces large deletions in the hprt gene of V79 cells

Acrylate esters are applied in industrial and consumer products often associated with polymers and resins. The difunctional methacrylate, triethylene glycol dimethacrylate (TEGDMA), is also frequently included in dental composite materials. Recently, mutagenicity testing of the compound revealed the induction of gene mutations at the hprt locus in V79 cell [H. Schweikl, G. Schmalz, K. Rackebrandt, The mutagenic activity of unpolymerized resin monomers in Salmonella typhimurium and V79 cells, Mutat. Res. 415 (1998) 119-130]. In the present study, TEGDMA caused a dose dependent increase of the number of micronuclei in V79 cells. Furthermore, the mutation spectra induced in exon sequences of the hprt gene in HPRT-deficient V79 cell clones were analyzed by the polymerase chain reaction (PCR). No DNA sequence deletions were observed in spontaneously occurring HPRT-deficient cell clones at the molecular level after PCR analysis, indicating that all spontaneous mutations were caused by point mutations. However, TEGDMA treated V79 cell cultures exhibited different mutation spectra. Only one cell clone among a total of 25 contained all exon sequences of the hprt gene. Large DNA sequences were deleted in 24 cell clones. Partial gene deletions occurred in four clones from exon 5 through 9, and exon 1 was not amplified in one cell clone. Exon sequences of the hprt gene were totally deleted in 19 HPRT-deficient clones. The induction of mostly large deletions in the genome of mammalian cells, like the mutation spectra induced by TEGDMA in V79 cells here, is probably typical for crosslinking agents, including anticancer drugs. Identical types of mutations including chromosomal aberrations and the formation of micronuclei in vitro were observed for acrylates and methacrylates tested so far in various mutation assays. Therefore, we conclude by analogy that the induction of large DNA sequence deletions as shown here with the reactive dimethacrylate, triethylene glycol dimethacrylate, is probably common for acrylates and methacrylates.

Spindle disturbances induced by benzo[a]pyrene and 7, 12-dimethylbenz[a]anthracene in a Chinese hamster cell line (V79-MZ) and the stability of the numerical chromosome aberrations that follow

We previously reported that benzo[a]pyrene (BP) and 7, 12-dimethylbenz[a]anthracene (DMBA) induce aneuploidy and polyploidy, respectively, in the Chinese hamster cell line V79-MZ in the absence of S9 mix. In the present study we investigated the effect of BP and DMBA on the mitotic spindle. BP caused incomplete spindle formation and DMBA inhibited spindle formation completely. The combined results indicate that incomplete spindles caused by BP resulted in aneuploidy, and the absence of spindle formation caused by DMBA resulted in polyploidy. The induced polyploidy was stable for several serial passages in fresh medium. BP and DMBA induced different chromosome number distributions. After BP treatment, the normal distribution of chromosome number was restored in 4 days. After DMBA treatment, on the other hand, a tetraploid peak was maintained for 2 months following an initial transient broad distribution of chromosome number after 1 day. The results suggest that different mechanisms were involved in the induction of numerical aberrations by BP and DMBA. Furthermore the induction of numerical aberrations by BP and DMBA was reproducible over 5 months of passages. In four clones tested, the frequency of cells with the modal chromosome number in control cultures gradually decreased from 82% on the average just after cloning to 62% 5 months later. BP and DMBA induced characteristic ploidy changes following succeeding cell passages for up to 5 months, indicating that the ability to respond to BP and DMBA was stable for that period of time. Because these findings were specific to V79-MZ cells, this cell line might be a good tool for studying chemicals that induce numerical chromosome aberrations.

Methyl methanesulfonate-induced hprt mutation spectra in the Chinese hamster cell line CHO9 and its xrcc1-deficient derivative EM-C11

The Chinese hamster cell mutant EM-C11, which is hypersensitive to the cell killing effects of alkylating agents compared to its parental line CHO9, has been used to study the impact of base excision repair on the mutagenic effects of DNA methylation damage. This cell line has a defect in the xrcc1 gene. XRCC1 can interact with DNA polymerase-beta, thereby suppressing strand displacement, and DNA ligase III, both of which have been implicated in base excision repair. XRCC1 may, therefore, allow efficient ligation of single-strand breaks generated during base excision repair. Both EM-C11 and CHO9 cells were treated with methyl methanesulfonate (MMS), a DNA-methylating agent reacting predominantly with nitrogen atoms generating adducts which are substrates for the base excision repair pathway. EM-C11 cells are much more sensitive to the cytotoxic effects of MMS than CHO9: for EM-C11, the dose of MMS inducing 10% survival is 6-fold lower than that for CHO9. In contrast, mutation induction at the hprt locus following MMS is similar in EM-C11 and CHO9. Molecular analysis of hprt gene mutations showed that although the largest class of hprt mutations, both in EM-C11 and CHO9 cells, consisted of GC > AT transitions, most likely caused by O6-methylguanine, the size of this class was smaller in EM-C11. The fraction of deletion mutants in EM-C11, however, was twice as large as that found in CHO9 cells. These results suggest that reduced ligation efficiency of single-strand breaks generated during base excision repair, as result of a defect in XRCC1, may lead to the formation of deletions.

Mammalian germ cell mutagenicity of ENU, IPMS and MMS, chemicals selected for a transgenic mouse collaborative study

A collaborative study to systematically assess transgenic mouse mutation assays as screens for germ cell mutagens has been conducted. Three male mouse germ cell mutagens (ENU, iPMS and MMS) were selected for testing. This paper provides a brief review of the effects reported for those 3 chemicals in the most commonly used non-transgenic germ cell mutagenicity assays, namely the dominant lethal, heritable translocation, and specific locus tests. Additionally, information on the DNA reactivity and the molecular nature of mutations induced by these chemicals is summarized.

The pattern of mutations induced by neocarzinostatin and methyl methanesulfonate in the ataxia telangiectasia-like Chinese hamster cell line V-E5

The Chinese hamster cell line V-E5 is a mutant cell line isolated from V79 cells. The phenotypic characteristics of V-E5 strongly resemble those of cells from patients suffering from the genomic instability syndrome ataxia telangiectasia. In order to further characterize the mutant cell line and to get insight into the underlying genetic defect we compared the clastogenic and mutagenic effects of neocarzinostatin (NCS) and methyl methanesulfonate (MMS) in V-E5 and V79 wild-type cells (V79-LE). V-E5 cells were 2-3 times more sensitive to the cytotoxic effect of NCS or MMS. The clastogenic action of NCS was characterized by the predominant induction of chromosome breaks and dicentrics in both cell lines, whereas MMS mainly induced chromatid-type aberrations. The frequency of mutations induced by NCS as well as MMS was slightly enhanced in V-E5 cells compared to V79 cells treated with the same dose. However, the mutant cell line was found to be hypomutable when considering the same survival level as in the parental cell line. Molecular analysis of mutants induced by NCS revealed a high frequency of total deletions of the hprt gene in both cell lines. In contrast, among MMS-induced mutations only 11% deletion mutations were found in V79-LE, whereas in V-E5 MMS-induced deletions were seen in 52% of the hprt-deficient mutants. These results are discussed with respect to a possible relation between genomic instability, cell cycle control and mutational spectra.

Differences in the response to mutagens between two V79 sublines

Two V79 Chinese hamster sublines (V79-UL and V79-MZ) which differed markedly with respect to their spontaneous pattern of mutations at the HPRT locus were comparatively investigated in genotoxicity tests with ethyl methanesulphonate (EMS). EMS-induced frequencies of HPRT mutations and sister chromatid exchanges (SCEs) were much higher in V79-MZ than in V79-UL. V79-MZ were not hypersensitive against EMS and had normal frequencies of spontaneous gene mutations and chromosome aberrations. Baseline SCE frequencies at various BrdUrd concentrations were slightly increased compared to V79-UL. EMS induced a similar amount of chromosome aberrations in both cell lines but exchange figures occurred with lower frequency in V79-MZ. The results indicate that specific and significant differences in the response to mutagens may exist between 'normal' Chinese hamster cell lines which might be relevant for genotoxicity testing.

Induction of DNA strand breaks by ethylene oxide in human diploid fibroblasts

In vitro exposure of normal human diploid fibroblasts (strain VH-10) to ethylene oxide (EtO) induced DNA strand breaks in the dose range of 2.5-30 mMh of EtO. Alkaline DNA unwinding (ADU), neutral filter elution (NFE), pulsed field gel electrophoresis (PFGE), and the comet assay were used to measure DNA single (SSBs) and double strand breaks (DSBs). Different induction rates of SSBs and DSBs, depending on applied method and also on treatment conditions (cells in monolayer or in suspension were used), were found. A dose-dependent increase of DNA strand breaks was found by the ADU method in the dose range of 2.5-20 mMh of EtO when treatment was performed in monolayer and in suspension. DSBs were detected by NFE only when the cells were treated with EtO in suspension (doses 10-30 mMh). The highest induction rate of DSBs (about 4 DSBs per 100 Mbp per 1 mMh of EtO) was detected in suspension with PFGE applied. We have shown that heat-labile sites are formed by EtO. Presumably, the different DSB levels detected by PFGE and NFE result from the conversion of these sites to DSBs during cell lysis at elevated temperature in the PFGE method. The results of the comet assay confirmed that apoptotic processes are not involved in the formation of DSBs in our experimental conditions (less than 1% of apoptotic cells were observed at all doses studied). Possible mechanisms for the induction of DNA strand breaks by EtO-treatment are discussed. The capacity to repair DSBs in EtO-exposed (5-7.5 mMh) cells was studied, and it was found that a considerable part of the damage (about 50%) could be repaired during 18 hr of incubation.

Mutagenicity of methyl methanesulfonate (MMS) in vivo at the Dlb-1 native locus and a lacI transgene

Methyl methanesulfonate (MMS) is an extraordinarily poor mutagen compared to ethylnitrosourea (ENU) or even X-rays. In lung fibroblasts in vivo, MMS has been shown to induce many micronuclei but few, if any, mutations at the hpt locus. We wondered if the lack of mutations might be due to the lack of division and DNA synthesis in fibroblasts in vivo, which would permit substantial time for differential repair of DNA lesions. This idea was tested in the small intestine, a tissue in which the cells are actively dividing. Two loci were examined: a native locus (Dlb-1) which determines the presence or absence of a lectin binding site on the surface of the epithelial cells, and a lacl transgene which controls beta-galactosidase synthesis. Locl mutations were detected after in vitro packaging of DNA isolated from the intestinal epithelium into lambda phage and expression in suitable bacteria. Although the epithelial cells are proliferating, acute treatments produced no significant increase in mutations at either locus. Subacute treatments produced low but significant increases in mutation frequency at both loci. The results confirm that MMS is a far more potent clastogen than it is a mutagen and should be regarded as a super-clastogen in the same manner as ENU is a super-mutagen. The carcinogenicity of MMS is probably the result of its potent clastogenicity rather than its weak activity as a point mutagen.