Absence of interaction between X-rays and UV light in inducing ouabain- and thioguanine-resistant mutants in Chinese hamster cells

Mutation induction in haploid yeast after split-dose radiation exposure. II. Combination of UV-irradiation and X-rays

Split-dose protocols can be used to investigate the kinetics of recovery from radiation damage and to elucidate the mechanisms of cell inactivation and mutation induction. In this study, a haploid strain of the yeast, Saccharomyces cerevisiae, wild-type with regard to radiation sensitivity, was irradiated with 254-nm ultraviolet (UV) light and then exposed to X-rays after incubation for 0-6 hr. The cells were incubated either on nutrient medium or salt agar between the treatments. Loss of reproductive ability and mutation to canavanine resistance were measured. When the X-ray exposure immediately followed UV-irradiation, the X-ray survival curves had the same slope irrespective of the pretreatment, while the X-ray mutation induction curves were changed from linear to linear quadratic with increasing UV fluence. Incubations up to about 3 hr on nutrient medium between the treatments led to synergism with respect to cell inactivation and antagonism with respect to mutation, but after 4-6 hr the two treatments acted independently. Incubation on salt agar did not cause any change in the survival curves, but there was a strong suppression of X-ray-induced mutation with increasing UV fluence. On the basis of these results, we suggest that mutation after combined UV and X-ray exposure is affected not only by the induction and suppression of DNA repair processes, but also by radiation-induced modifications of cell-cycle progression and changes in the expression of the mutant phenotype.

Mutation induction by different types of radiation at the Hprt locus

Mutation induction at the Hprt locus in Chinese hamster cells was studied after exposure to ultraviolet light, X-rays and alpha particles. While mutant frequency as a function of dose or fluence followed a linear-quadratic relationship with UV and X-rays, it showed a linear dependence for alpha particles. If mutant frequency is plotted vs. the logarithm of surviving fraction, a linear relationship is found in all cases although with different slopes. These are about equal with the two types of ionising radiations but about 10 times larger for UV. They can be used as a measure of mutagenic potential and are termed mutagenicity. It is shown that this parameter is correlated with the maximum of mutant yield, i.e., the number of mutants per cell at risk. It is concluded from this analysis that the maximum mutant yield is always found at doses or fluences which lead to 37% survival irrespective of the kind of radiation. If mutation induction is measured in X-irradiated cells after pre-exposure to UV, mutant frequency is higher than expected on the basis of independent action of the two radiations. Deletion spectra were determined by using multiplex polymerase chain reaction. It was found that the background of spontaneous mutants varied considerably and showed frequently repetitive patterns, presumably because of clonal expansion of pre-formed mutants. UV-induced mutants did not contain any deletions, while those with both X-rays and alpha particles the majority displayed partial and total deletions. Based on a total number of 134 X-ray- and 192 alpha-induced mutants, it is concluded that the total fraction of mutant clones without deletions (partial or total) is about 40% for X-rays and only about 20% for alpha-particles.

Ionizing radiation and genetic risks. III. Nature of spontaneous and radiation-induced mutations in mammalian in vitro systems and mechanisms of induction of mutations by radiation

This paper (1) presents an analysis of published data on the molecular nature of spontaneously arising and radiation-induced mutations in mammalian somatic cell systems and (2) examines whether the molecular nature and mechanisms of origin of radiation-induced mutations, in mammalian in vivo and in vitro systems, as currently understood, are consistent with expectations based on the biophysical and microdosimetric properties of ionizing radiation. Depending on the test system (CHO cells, human T lymphocytes and human lymphoid cell line TK6), 80-97% of spontaneous HPRT mutations show normal Southern patterns; the remainder is due to gross changes, predominantly partial (intragenic) deletions. Total gene deletions at the HPRT locus are rare except in the TK6 cell line. At the APRT locus in CHO cells, 80-97% of spontaneous mutations are due to base-pair changes, the remainder being, mostly, partial deletions. The latter can extend upstream in the 5' direction but not beyond the APRT gene in the 3' direction. At the human HLA-A locus (T lymphocytes), the percentage of mutations with normal Southern patterns is lower than that for HPRT, and in the range of 50-60%. At the HLA-A locus, mitotic recombination contributes substantially to the mutation spectrum (approximately 30% of mutations recovered) and this is likely to be true of the TK locus in the TK6 cell line as well. With a few exceptions, most of the radiation-induced mutations show altered Southern patterns and are consistent with their being deletions and/or other gross changes (HPRT, 70-90% (CHO); 50-85% (TK6); 50-75% (T lymphocytes); TK, 60-80% (TK6); HLA-A, 80% (T lymphocytes); DHFR, 100% (CHO]. The exceptions are APRT mutations in CHO cells (16-20% of mutants with deletions or other changes) and HPRT mutations in T lymphocytes from A-bomb survivors (15-25%); the latter finding is consistent with the occurrence of in vivo selection against HPRT mutant cells. In cases of HPRT intragenic deletions analyzed (CHO cells and V79 Chinese hamster cells), there is evidence for a non-random distribution of breakpoints. The spontaneous mutation frequencies vary widely, from about 0.04/10(6) cells (sickle cell mutations at the human HBB locus) to 30.8/10(6) cells (HLA-A mutations in T lymphocytes) and are dependent on the locus, the system employed and a number of other factors. Those for the other loci fall between these limits.(ABSTRACT TRUNCATED AT 400 WORDS)

Antimutagenic action of cobaltous chloride on radiation-induced mutations in cultured Chinese hamster cells

The effects of cobaltous chloride on 8-azaguanine (8AG)-resistant mutations induced by gamma-rays or ultraviolet (UV) light in cultured Chinese hamster V79 cells were examined. Cobaltous chloride alone had no significant effects on survival and mutations of V79 cells at concentrations less than 1 x 10(-5) M. Cobaltous chloride at a concentration of 3 x 10(-6) M had a marked effect in reducing 8AG-resistant mutations induced by gamma-rays of 2-6 Gy, when cells were incubated for 6-7 days in the presence of cobaltous chloride after gamma-ray irradiation (posttreatment). The pretreatment of cells with cobaltous chloride for 6 days before gamma-ray irradiation reduced 8AG-resistant mutations induced by gamma-rays. Pre- or post-treatment with cobaltous chloride had no such effect on UV-induced mutations, however. The difference in responsiveness to cobaltous chloride between bacterial and mammalian cell systems is discussed.

HGPRT- mutants of V79 cells that revert specifically by base pair substitution and frameshift mutations

In order to determine the mutagenic specificity of mutagenic and carcinogenic agents in mammalian cells, a reversion system capable of distinguishing between frameshift mutations and various kinds of base pair substitutions would be useful. We report here a method for the isolation and characterization of HGPRT- Chinese hamster V79 cell mutants that might form the basis for such a system. Two mutants of different specificity have been partially characterized. DEW-1, isolated following N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) treatment, is revertible by the base pair substitution mutagens MNNG and ethyl methanesulfonate (EMS), but not by frameshift mutagens. DSW-3, isolated following ICR-191 treatment, is specifically reverted by frameshift mutagens, but not by EMS or MNNG. With the further characterization of these and other mutants, it should be feasible to determine not only whether an agent is mutagenic in V79 cells, but also to determine the type(s) of mutation(s) it produces.

Inhibition of DNA synthesis is not sufficient to cause mutagenesis in Chinese hamster cells

Experiments were designed to test whether the inhibition of DNA synthesis in Chinese hamster V79 cells would result in increased mutagenesis by a mechanism similar to "SOS repair" in E. coli. Treatment of cells for 16 hours with excess of the deoxynucleosides TdR, UdR, AdR and GdR was mutagenic, whereas treatment with hydroxyurea demonstrated no mutagenic effect. The mutagenicity of TdR could be reversed by the addition of CdR. In E. coli, inhibition of DNA synthesis by a short exposure to hydroxyurea resulted in the induction of lambda prophage and increased mutagenesis. These results show that whereas the presence of a stalled replication fork in E. coli can result in mutagenesis via induction of the "SOS system", the same phenomenon does not seem to occur in Chinese hamster V79 cells. The mutagenic mechanism of high concentrations of deoxynucleosides in Chinese hamster V79 cells is likely to be due to replication errors which result from alterations in deoxynucleotide pools.

Effect of split doses of radiation on mutation frequency in rodent cell lines

Mutation frequencies in mouse cells and Chinese hamster cells were measured following single or split doses of UV light or X-rays. Split doses separated by a few to over 24 h induced no more ouabain- or 6-thioguanine-resistant mutants than did comparable single doses. These data lend no support to the possible existence of an inducible error-prone repair system in rodent cells.

Enhanced survival and reduced mutation and aberration frequencies induced in V79 chinese hamster cells pre-exposed to low levels of methylating agents

Exposure of V79 Chinese hamster cells to a single very low (sub-toxic and sub-clastogenic) dose of MNU or MNNG made these cells resistant to the toxic, mutagenic and clastogenic activities of the same agents given 6 h later. Cell survival was increased nearly 2-fold under optimal conditions when compared with the non-pretreated controls. Aberration frequencies were reduced to nearly half the control values (cells not pretreated). This was observed for a wide range of pretreatment concentrations and at different recovery times. The effect of mutagen pretreatment was most pronounced with respect to the induction of TG resistance, which became drastically reduced. The data indicate the existence of an adaptive repair pathway in V79 cells which may be induced by very low levels of methylating agents and which is error-free in handling lesions responsible, at least partially, for reproductive cell death, mutations and chromosomal aberrations.

Effect of metals on mutagenesis and DNA repair

Unlike the situation with organic compounds, metals do not show a high correlation between carcinogenicity and mutagenicity. An agent may be mutagenic by causing misreplication of DNA due to alterations of the DNA template, decreased fidelity of DNA polymerase, or inhibition of the proofreading of DNA replication. In addition, bacteria have an inducible, error-prone DNA repair system (SOS repair) whose activity results in mutagenesis. In the best studied example of metal mutagenesis, chromate, there is little evidence for the involvement of the SOS system. Metals may act as comutagens by inhibiting the repair of damage to DNA caused by another agent. This has been demonstrated for arsenite. Comutagens would not be detected by standard screening methods.

Protease inhibitors neither damage DNA nor interfere with DNA repair or replication in human cells

Human fibroblasts were exposed to antipain or leupeptin at concentrations up to 2.5 mM and the presence of DNA damage and repair was assayed by several different methods. These did not reveal DNA damage or repair after exposure to either antipain or leupeptin, even in the presence of rat-liver microsomal S9 mix. Antipain also had no significant effects on the repair or replication of DNA after ultraviolet or X-irradiation. The demonstrated potentiation of radiation-induced transformation of human cells and the selective killing of repair-deficient cells by antipain must therefore occur by mechanisms that do not involve direct interaction of antipain with DNA.

Antipain and 12-O-tetradecanoyl-phorbol-13-acetate: phenomenology of effects on UV mutagenesis in V79 Chinese hamster cells

Antipain (AP) and 12-O-tetradecanoyl-phorbol-13-acetate (TPA) were tested in V79 Chinese hamster cells for cytotoxicity and effects on survival and 6-thioguanine-resistant (6-TGr) mutation after UV-irradiation. AP and/or TPA were relatively non-cytotoxic and had no significant effects on UV survival. Despite their non-mutagenicity, the recovery of UV-induced 6TGr colonies was significantly enhanced by the pretreatment with either AP (0.5-2 mM) or TPA (0.1-1 microgram/ml) only during the expression period before the 6TG selection at a low density of cells in the absence of AP or TPA. Such enhancing effects were maximal when AP or TPA was present during the late expression period after the mutation fixation and extensive dilution of DNA lesions. Reconstruction experiments revealed the antagonistic actions that TPA and AP tended to eliminate and increase, respectively, the metabolic co-operation. In the TPA-plus-AP treatment, AP abolished the TPA-enhanced recovery of induced mutants. Thus, it seems that TPA increases the mutant recovery largely through decreased metabolic co-operation and AP could modulate the mutation expression. Further, an error-prone inducible repair may not exist, or, if it exists, AP may not inhibit it in V79 Chinese hamster cells.

Effects of a tumor promoter and an anti-promoter on spontaneous and UV-induced 6-thioguanine-resistant mutations and sister-chromatid exchanges in V79 Chinese hamster cells

The effects of a tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and/or an anti-promoter antipain (protease inhibitor) on spontaneous and ultraviolet-induced sister-chromatid exchanges (SCEs) and 6-thioguanine-resistant (6TGr) recessive mutations were examined in V79 Chinese hamster cells in culture. TPA and/or antipain neither significantly altered base-line and UV-induced immediate SCE frequencies, nor decreased the level of delayed SCEs which persisted 6-7 days after irradiation. TPA and/or antipain appeared to enhance the recovery of UV-induced 6TGr colonies at the plateau expression phase despite non-mutagenicity by themselves and unaltered metabolic cooperation. Thus, the results conceivably imply that the 6TGr-recessive mutation expression, but not fixation, can be modulated at the cell level by TPA and/or antipain. Our results, together with the recent results of Loveday and Latt, may argue against the notion that TPA enhances the antipain-suppressible SCEs as an index of mitotic recombination in relevance with a tumor-promotion mechanism.

Ultraviolet light induction of diphtheria toxin-resistant mutations in normal and DNA repair-deficient human and Chinese hamster fibroblasts

The role of unrepaired DNA lesions in the production of mutations is suspected of contributing to the initiation phase of carcinogenesis. Since the molecular basis of mutagenesis is not understood in eukaryotic cells, development of new genetic markers for quantitative in vitro measurement of mutations for mammalian cells is needed. Furthermore, mammalian cells, genetically deficient for various DNA repair enzymes, will be needed to study the role of unrepaired DNA lesions in mutagenesis. The results in this report relate to preliminary attempts (1) to characterize the diphtheria toxin resistance marker as a useful quantitative genetic marker in human cells and (2) to isolate and characterize various DNA repair-deficient Chinese hamster cells.