Alkaline sucrose gradient sedimentation studies of DNA from HeLa S3 cells exposed to methyl methanesulphonate or methylazoxymethanol acetate

Contribution of base excision repair, nucleotide excision repair, and DNA recombination to alkylation resistance of the fission yeast Schizosaccharomyces pombe

DNA damage is unavoidable, and organisms across the evolutionary spectrum possess DNA repair pathways that are critical for cell viability and genomic stability. To understand the role of base excision repair (BER) in protecting eukaryotic cells against alkylating agents, we generated Schizosaccharomyces pombe strains mutant for the mag1 3-methyladenine DNA glycosylase gene. We report that S. pombe mag1 mutants have only a slightly increased sensitivity to methylation damage, suggesting that Mag1-initiated BER plays a surprisingly minor role in alkylation resistance in this organism. We go on to show that other DNA repair pathways play a larger role than BER in alkylation resistance. Mutations in genes involved in nucleotide excision repair (rad13) and recombinational repair (rhp51) are much more alkylation sensitive than mag1 mutants. In addition, S. pombe mutant for the flap endonuclease rad2 gene, whose precise function in DNA repair is unclear, were also more alkylation sensitive than mag1 mutants. Further, mag1 and rad13 interact synergistically for alkylation resistance, and mag1 and rhp51 display a surprisingly complex genetic interaction. A model for the role of BER in the generation of alkylation-induced DNA strand breaks in S. pombe is discussed.

Inhibitory effect of theophylline on repair of potentially lethal MMS-induced damages to DNA in V79 cells

In a study of Chinese hamster V79 cells growing in the presence of sublethal concentrations of theophylline, we followed both the nature of DNA replication and the cells' response to toxic and DNA-damaging effects of methyl methanesulfonate (MMS). We found that cells cultured at low concentrations of theophylline (less than or equal to 0.3 mg/ml medium) showed deviations in the rate of DNA replication which, however, did not depress either the growth activity of the cells or their colony-forming ability. Considerable differences as against the controls appear in theophylline-cultured cells after treatment with MMS. Not only are they more sensitive to the toxic effects of this alkylating agent, but also their DNA synthesis is strikingly inhibited. More unrepaired lesions remain in parental DNA, and short fragments of daughter DNA, synthesized following cell treatment with MMS, are not elongated during a 2-h post-MMS treatment. Theophylline obviously belongs among agents inhibiting repair of potentially lethal MMS-induced DNA damages in Chinese hamster V79 cells.

Effects of repair inhibition in the G1 phase of clastogen-treated human lymphocytes on the frequencies of chromosome-type and chromatid-type aberrations and sister-chromatid exchanges

It has been shown that certain types of DNA lesions induced by an S-dependent clastogen are converted to chromosome-type aberrations when their repair is inhibited in the G1 phase of the cell cycle. The purpose of the present study was to investigate which kinds of repair inhibitors have the ability to induce chromosome-type aberrations in cells having DNA lesions and which kinds of DNA lesions will be converted to chromosome-type aberrations when their repair is inhibited. For this purpose, human peripheral blood lymphocytes, which were treated with a clastogen in their G0 phase, were post-treated with one of several kinds of repair inhibitors in the G1 phase, and resulting frequencies of both chromosome-type and chromatid-type aberrations as well as of sister-chromatid exchanges (SCEs) were compared with those of the control cultures: chromatid-type aberrations and SCEs were adopted as cytogenetic indicators of lesions remaining in S and G2 phases. Chemicals used for the induction of DNA lesions were 4-nitroquinoline 1-oxide (4NQO), methyl methanesulfonate (MMS) and mitomycin C (MMC); inhibitors used were excess thymidine (dThd), caffeine, hydroxyurea (HU), 5-fluoro-2'-deoxyuridine (FdUrd), 1-beta-D-arabinofuranosylcytosine (ara C), 9-beta-D-arabinofuranosyladenine (ara A), 1-beta-D-arabinofuranosylthymine (ara T) and aphidicolin (APC). Induction of chromosome-type aberrations was observed in cells pretreated with 4NQO or MMS followed by ara C, ara A, ara T or APC, whereas other combinations of a clastogen and an inhibitor did not induce them. Among the inhibitors, ara C alone induced chromosome-type aberrations in cells without pretreatment. Chromatid-type aberrations were increased only in cells pretreated with MMC and their frequency was enhanced further by post-treatment with ara C. All of the clastogens used in the present experiments induced SCEs. Most inhibitors did not modify the SCE frequencies except for ara C which synergistically increased the frequency in MMC-treated cells. The present study offers further evidence that the lesions responsible for chromosome-type aberrations are those which are repaired quickly, and that they are converted to chromosome-type aberrations when repair by polymerase alpha is inhibited. The effects of ara C on MMC-induced lesions are considered residual effects of ara C treatment in the S or G2 phases rather than repair inhibition in the G1 phase.

Review: putative mutagens and carcinogens in foods. V. Cycad azoxyglycosides

Cycasin is a member of a family of azoxyglycosides produced by cycads. It is mutagenic and carcinogenic only when deglucosylated to release its principal metabolite, methylazoxymethanol (MAM). Methylazoxymethanol is also the aglycone of other cycad azoxyglycosides and is responsible for their toxicologic properties. The way in which people can be exposed to cycad azoxyglycosides is through the consumption of foods prepared from cycads. MAM induces genetic alterations in various test systems in bacteria, yeast, plants, Drosophila, and mammalian cells. An important aspect of the biological activities of cycasin and MAM is the intimate connection between their metabolism and their toxicologic effects. In adult mammals, the deglucosylation of cycasin is catalyzed only by enzymes of the microflora of the gut. Cycasin is therefore active when administered orally but not when administered parenterally. In contrast, MAM is active regardless of the route of exposure. Major uncertainties remain regarding the intermediates generated from MAM spontaneously and metabolically. More knowledge of these intermediates is required for a better understanding of the molecular mechanisms underlying the toxicity of cycasin, MAM, and related compounds.

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.

The effect of caffeine upon cell survival and post-replication repair of DNA after treatment of BHK 21 cells with either UV irradiation or N-methyl-n-nitrosoguanidine

It has been found that in BHK 21 cells caffeine potentiates cell killing by both UV irradiation and N-methyl-N-nitrosoguanidine (MNNG). The potentiating effect is greater with UV than with MNNG. While non-toxic concentrations of caffeine inhibit the joining of newly-replicated DNA fragments into large molecular weight DNA (post-replication repair) after UV irradiation, they have no such effect after MNNG treatment. Furthermore, the joining of DNA fragments continues in the cells treated with 3 microgram/ml of MNNG, a dose which leads to less than 5% cell survival. While inhibition of the synthesis of large molecular weight DNA can explain the synergistic effect of caffeine upon cell survival after UV irradiation, it cannot explain the similar effect after MNNG treatment.

Repair of MMS-induced DNA double-strand breaks in haploid cells of Saccharomyces cerevisiae, which requires the presence of a duplicate genome

The formation and repair of double-strand breaks induced in DNA by MMS was studied in haploid wild type and MMS-sensitive rad6 mutant strains of Saccharomyces cerevisiae with the use of the neutral and alkaline sucrose sedimentation technique. A similar decrease in average molecular weight of double-stranded DNA from 5--6 X 10(8) to 1--0.7 X 10(8) daltons was observed following treatment with 0.5% MMS in wild type and mutant strains. Incubation of cells after MMS treatment in a fresh drug-free growing medium resulted in repair of double-strand breaks in the wild type stain, but only in the exponential phase of growth. No repair of double-strand breaks was found when cells of the wild type strain were synchronized in G-1 phase by treatment with alpha factor, although DNA single-strand breaks were still efficiently repaired. Mutant rad6 which has a very low ability to repair MMS-induced single-strand breaks, did not repair double-strand breaks regardless of the phase of growth. These results suggest that (1) repair of double-strand breaks requires the ability for single-strand breaks repair, (2) rejoining of double-strand breaks requires the availability of two homologous DNA molecules, this strongly supports the recombinational model of DNA repair.

Alkaline sucrose sedimentation studies of MMS-induced DNA single-strand breakage and rejoining in the wild type and in UV-sensitive mutants of Saccharomyces cerevisiae

MMS-induced DNA single-strand breakage and rejoining was studied in the RAD strain and in rad6 and rad21 mutants, both very sensitive to this treatment as compared with the wild type. Alkaline sucrose gradient centrifugation showed that MMS treatment reduced the molecular weight of DNA in the RAD strain and in rad6 and rad21 mutants to the same extent. Four hours of post-incubation in synthetic growth medium after treatment with a dose of 0.4% MMS which reduces cell survival of RAD, rad21 and rad6 to 50, 20 and less than 0.01%, respectively, resulted in a significant increase in the molecular weight of DNA in the wild type, but in only slight increase in mutant strains. When the strains were exposed to a lower dose of MMS (0.04%) which led to 100% survival of RAD and 50 and 20% survival of rad21 and rad6, respectively, wild-type DNA sedimented to the position of control DNA, while in both mutants the increase in molecular weight of DNA was less pronounced.

Evidence for caffeine-sensitive damage in methylazoxymethanol acetate-treated L5178Y cells

The effects of methylazoxymethanol (MAM) acetate on colony survival, cell proliferation and DNA synthesis of murine lymphoma L5178Y cells are studied. Decreased sensitivity and immediate depression of cell proliferation and DNA synthesis were found in L5178Y cells in contrast to the reports on HeLa cells. Pre-labelling with 5-bromodeoxyuridine (BUdR) did not enhance significantly the carcinogen-induced cell lethality. Post-treatment with caffeine greatly enhanced cell lethality and depression of cell proliferation. These effects of caffeine were diminished when the cells had passed through two generations following the MAM acetate treatment. Experiments with synchronized cells showed that the action of caffeine was located primarily in S phase following the MAM acetate-treatment. These results strongly suggest that in L5178Y cells, MAM acetate induces damage, which is repaired by a mechanism analogous to post-replication repair of UV light-induced damage.

Investigations into the mechanism of action of anti-tumour platinum compounds: time- and dose-dependent changes in the alkaline sucrose gradient sedimentation profiles of DNA from hamster cells treated with cis-platinum (II) diamminedichloride

Under the conditions of low speed centrifugation used in this study, the proportion of radioactively labelled DNA from Chinese hamster V79-379A cells sedimenting to the 700S region of an alkaline sucrose gradient was increased in a dose-dependent manner by prior treatment of the cells for 2 h with cis-platinum (II) diamminedichloride [cis-Pt(II)]. This increase was at the expense of material sedimenting in the 400-650S region. This profile was not modified by a 2-h post-treatment incubation prior to centrifugation. 6 h after treatment, the DNA from treated cells sedimented in a narrow band at a position corresponding to 350S. 21 h after exposure to the drug, a dose-dependent restitution of the DNA species sedimenting in the 450-650S range was observed. These results combined with other data relating to platinum binding allow the following conclusions to be reached: (1) cis Pt(II) treatment does not lead to the rapid formation of single-stranded breaks or alkali-labile sites in cellular DNA. (2) The time-dependent changes in sedimentation rate of DNA from treated cells may reflect the transient appearance of gaps following endonuclease attack at platinum-bound sites in DNA. (3) The likely ratio of inter to intra-strand DNA-platinum interactions suggests that such endonuclease attack is primarily at platinum induced inter-strand DNA cross-links.