Sensitivity of a mutator gene in Chinese hamster ovary cell to deoxynucleoside triphosphate pool alterations

YNK1, the yeast homolog of human metastasis suppressor NM23, is required for repair of UV radiation- and etoposide-induced DNA damage

In humans, NM23-H1 is a metastasis suppressor whose expression is reduced in metastatic melanoma and breast carcinoma cells, and which possesses the ability to inhibit metastatic growth without significant impact on the transformed phenotype. NM23-H1 exhibits three enzymatic activities in vitro, each with potential to maintain genomic stability, a 3'-5' exonuclease and two kinases, nucleoside diphosphate kinase (NDPK), and protein histidine kinase. Herein we have investigated the potential contributions of NM23 proteins to DNA repair in the yeast, Saccharomyces cerevisiae, which contains a single NM23 homolog, YNK1. Ablation of YNK1 delayed repair of UV- and etoposide-induced nuclear DNA damage by 3-6h. However, YNK1 had no impact upon the kinetics of MMS-induced DNA repair. Furthermore, YNK1 was not required for repair of mitochondrial DNA damage. To determine whether the nuclear DNA repair deficit manifested as an increase in mutation frequency, the CAN1 forward assay was employed. An YNK1 deletion was associated with increased mutation rates following treatment with either UV (2.6x) or MMS (1.6 x). Mutation spectral analysis further revealed significantly increased rates of base substitution and frameshift mutations following UV treatment in the ynk1Delta strain. This study indicates a novel role for YNK1 in DNA repair in yeast, and suggests an anti-mutator function that may contribute to the metastasis suppressor function of NM23-H1 in humans.

Thymidylate synthase as an oncogene: a novel role for an essential DNA synthesis enzyme

Thymidylate synthase (TS) is an E2F1-regulated enzyme that is essential for DNA synthesis and repair. TS protein and mRNA levels are elevated in many human cancers, and high TS levels have been correlated with poor prognosis in patients with colorectal, breast, cervical, bladder, kidney, and non-small cell lung cancers. In this study, we show that ectopic expression of catalytically active TS is sufficient to induce a transformed phenotype in mammalian cells as manifested by foci formation, anchorage independent growth, and tumor formation in nude mice. In contrast, comparable levels of two TS mutants carrying single point mutations within the catalytic domain had no transforming activity. In addition, we show that overexpression of TS results in apoptotic cell death following serum removal. These data demonstrate that TS exhibits oncogene-like activity and suggest a link between TS-regulated DNA synthesis and the induction of a neoplastic phenotype.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Deoxyribonucleoside triphosphate levels: a critical factor in the maintenance of genetic stability

DNA precursor pool imbalances can elicit a variety of genetic effects and modulate the genotoxicity of certain DNA-damaging agents. These and other observations indicate that the control of DNA precursor concentrations is essential for the maintenance of genetic stability, and suggest that factors which offset this control may contribute to environmental mutagenesis and carcinogenesis. In this article, we review the biochemical and genetic mechanisms responsible for regulating the production and relative amounts of intracellular DNA precursors, describe the many outcomes of perturbations in DNA precursor levels, and discuss implications of such imbalances for sensitivity to DNA-damaging agents, population monitoring, and human diseases.

Mutational specificity of DNA precursor pool imbalances in yeast arising from deoxycytidylate deaminase deficiency or treatment with thymidylate

Disruption of the dCMP deaminase (DCD1) gene, or provision of excess dTMP to a nucleotide-permeable strain, produced dramatic increases in the dCTP or dTTP pools, respectively, in growing cells of the yeast Saccharomyces cerevisiae. The mutation rate of the SUP4-o gene was enhanced 2-fold by the dCTP imbalance and 104-fold by the dTTP imbalance. 407 SUP4-o mutations that arose under these conditions, and 334 spontaneous mutations recovered in an isogenic strain having balanced DNA precursor levels, were characterized by DNA sequencing and the resulting mutational spectra were compared. Significantly more (greater than 98%) of the changes resulting from nucleotide pool imbalance were single base-pair events, the majority of which could have been due to misinsertion of the nucleotides present in excess. Unexpectedly, expanding the dCTP pool did not increase the fraction of A.T----G.C transitions relative to the spontaneous value nor did enlarging the dTTP pool enhance the proportion of G.C----A.T transitions. Instead, the elevated levels of dCTP or dTTP were associated primarily with increases in the fractions of G.C----C.G or A.T----T.A. transversions, respectively. Furthermore, T----C, and possibly A----C, events occurred preferentially in the dcd1 strain at sites where dCTP was to be inserted next. C----T and A----T events were induced most often by dTMP treatment at sites where the next correct nucleotide was dTTP or dGTP (dGTP levels were also elevated by dTMP treatment). Finally, misinsertion of dCTP or dTTP did not exhibit a strand bias. Collectively, our data suggest that increased levels of dCTP and dTTP induced mutations in yeast via nucleotide misinsertion and inhibition of proofreading but indicate that other factors must also be involved. We consider several possibilities, including potential roles for the regulation and specificity of proofreading and for mismatch correction.

The molecular basis of mutations induced by deoxyribonucleoside triphosphate pool imbalances in mammalian cells

Alterations of the balanced supply of the precursors of DNA synthesis, the deoxyribonucleoside triphosphates, have dramatic genetic consequences for mammalian cells including the induction of mutations, the sensitization to DNA damaging agents, and the production of gross chromosomal abnormalities. The use of recombinant DNA techniques has allowed the analysis of some of these effects and has revealed further mechanisms by which mammalian cells control the accuracy of DNA replication.

The effects of thymidine on deoxyribonucleotide pool levels, cytotoxicity and mutation induction in Friend mouse erythroleukaemia cells

The ability of excess thymidine (10(-6)-10(-3) M) to enhance the frequency of 6-thioguanine (6-TG) resistant cell mutants and 2,6-diaminopurine (DAP) resistant cell mutants in Friend mouse erythroleukaemia cells, clone 707, was investigated. A significant increase in mutant frequency for both markers was observed at the higher (10(-4) and 10(-3) M) thymidine treatments. Measurements of deoxyribonucleoside triphosphate pool sizes in the cells revealed a dramatic elevation of the deoxythymidine triphosphate and deoxyguanosine triphosphate pools, an increase in the deoxyadenosine triphosphate pool and an almost complete disappearance of the deoxycytidine triphosphate pool at the higher thymidine treatments. This complemented the mutagenesis data. These results support the view that increases in mutant frequency may take place following perturbations in DNA precursor pools through a resultant decrease in the fidelity of DNA synthesis. Measurements of deoxyribonucleoside triphosphate pools were also carried out on clone 707 Friend cells and a thymidine kinase-deficient subclone, 707 BUF. The thymidine kinase-deficient subclone had significantly reduced deoxythymidine triphosphate and deoxyguanosine triphosphate pools relative to those observed in-clone 707 cells. The previously observed mutagen hypersensitivity in thymidine kinase-deficient Friend cells may result through pool imbalance rendering DNA excision repair error prone.

Deoxynucleoside triphosphate pool perturbation is not a general feature in mutagen-treated mammalian cells

Deoxynucleoside triphosphate (dNTP) and ribonucleoside triphosphate (rNTP) pools were analyzed in 4 mammalian cell lines following treatment with UV-C (254 nm), UV-A (365 nm) or the carcinogen, 1-methyl-3-nitro-1-nitrosoguanidine (MNNG). No substantial alterations in dNTP pool levels were observed in any treatment group. However, the cellular conversions of exogenously added deoxycytidine and deoxyguanosine to the corresponding triphosphates were inhibited 30-97% by UV-C and MNNG treatment. In addition, the conversion of dGuo to GTP and deoxyadenosine to ATP were inhibited 25-50% in CHO cells by mutagen treatment. The data do not support the notion that modulation of specific dNTP pools is a general feature of mutagen treatment in mammalian cells, but so suggest a mutagen-sensitivity of deoxynucleoside metabolism.

Mutagenesis and deoxyribonucleotide pool imbalance

Numerous studies have demonstrated that DNA-precursor pool imbalances are mutagenic and can modulate the lethality and mutagenicity of DNA-damaging agents. In addition, physical and chemical mutagens can induce alterations in DNA-precursor levels. Such findings suggest that regulation of intracellular concentrations of DNA precursors may be an important factor in environmental mutagenesis. In this article, results linking mutation and disturbances in DNA-precursor pools are reviewed.

Germ and somatic cell abnormalities following in vivo administration of thymidine and adenine

It is known that an excess of or a depletion in bases and nucleosides produce genetic effects in vitro, and a similar effect has been found with the nucleoside thymidine in this laboratory in vivo. To confirm this effect and to see if this occurs with the base adenine, thymidine and adenine were administered to male mice by i.p. injection and the sperm examined for head-shape abnormalities 4 and 5 weeks later. Treated males also were mated to untreated females for the provision of an F1 generation. The F1 males were subjected to the sperm morphology assay when they reached 14 weeks of age. Amongst those F0 males given adenine, there was a dose-related increase in the frequency of abnormal sperm and the group given thymidine also showed increases, confirming the results of previous studies in this laboratory. In the F1 generation, the fraction of mice from treated males showing increases in numbers of abnormal sperm was greater than that of the controls. In a micronucleus test with mice treated with thymidine, mitosis was delayed and there was a marginal increase in micronuclei, suggesting that an imbalance in nucleoside pools may be responsible for chromosomal damage in somatic cells in vivo. Therefore it is considered that similar effects to those produced in vitro can be demonstrated in vivo. Furthermore, the results of the sperm morphology assay show that the damage is transmissible.

Direct selection of Chinese hamster ovary strains deficient in CTP synthetase activity

Mutant CHO cell strains dependent upon cytidine for growth and survival were isolated by a direct selection protocol. The mutants obtained are deficient in CTP synthetase activity (less than 2% residual activity) and have low reversion frequencies (less than 10(-7)). Cytidine deprivation of these stains leads to rapid depletion of intracellular CTP pools, but not dCTP pools, and a surprisingly rapid loss of cell viability. These properties should make the cytidine auxotrophs useful for a number of biochemical and genetic studies.

Synergism between U.V. and thymidine treatments in the induction of cytogenetic damage in wild-type Friend erythroleukaemia cells

The cytogenetic aberration-inducing effect of excess thymidine and U.V. light in Friend erythroleukaemia (clone 707) cells was investigated. Three doses of U.V. were utilised, namely 2.4, 4.8 and 7.2 J/m2. Thymidine, at 1 X 10(-5) M, was present for 48 h prior to U.V. treatment and for 15 h following it. Although no significant increase in metaphase aberrations was observed following thymidine treatment alone, relative to the spontaneous frequency, clone 707 exhibited increased sensitivity to U.V.-induced cytogenetic damage, when grown in the presence of 1 X 10(-5) M thymidine. The observed synergism between U.V. and thymidine treatments may be due to thymidine-induced nucleotide pool imbalance with consequent inaccuracies in DNA repair.

Disturbances in DNA precursor metabolism associated with exposure to an inhibitor of poly(ADP-ribose) synthetase

3-Aminobenzamide (3AB) is widely used as an inhibitor of poly(ADP-ribose) synthetase to study the effect of protein ribosylation on various cellular processes, but the specificity of its inhibition has not been demonstrated. We found that 3AB has a wide range of effects on DNA precursor metabolism, as determined by high-performance liquid chromatographic separation of deoxynucleosides derived from enzymatic digestion of cellular DNA. 3AB (10-20 mM) significantly reduced cell growth in human lymphoblastoid cells. Furthermore, the incorporation of [3H]deoxycytidine into DNA was significantly enhanced relative to incorporation of [3H]deoxythymidine, [3H]deoxyguanosine, and [3H]deoxyadenosine. Incorporation of fragments of [3H]glucose into the pyrimidine fraction of DNA was significantly inhibited relative to incorporation into the purine fraction. At only 1 mM, 3AB had a major inhibitory effect on the incorporation of the methyl group from [3H]methionine into deoxyguanosine, deoxyadenosine, and deoxycytidine, with 50% inhibition into deoxyguanosine and deoxyadenosine and 90% inhibition into deoxycytidine. The specificity of 3AB inhibition to poly(ADP-ribose) synthetase is therefore doubtful in view of this variety of metabolic effects, involving pyrimidine synthesis and de novo synthesis via the one-carbon pool.

Analysis of bromodeoxyuridine-induced single and twin sister chromatid exchanges in tetraploid Chinese hamster ovary cells

Culture of cells in high exogenous levels (greater than 10(-4) M) of bromodeoxyuridine (BrdUrd) or thymidine will increase the baseline sister chromatid exchange (SCE) frequency. The effect is thought to be related to the balance of the DNA precursors thymidine and deoxycytidine. Exogenous addition of deoxycytidine will reverse this effect. Single and twin SCEs were analysed in Colcemid-induced tetraploid Chinese hamster ovary cells exposed to different concentrations of BrdUrd to determine at what stage SCEs are induced by high levels of BrdUrd. In cells exposed to low concentrations of BrdUrd (10(-5) M), equal numbers of SCEs were induced in each of the two cell cycles. With increasing concentrations of BrdUrd (10(-4) to 2 X 10(-4) M), SCE frequency increased in both cell cycles, but far more SCEs were induced in the second cell cycle. Deoxycytidine (2 X 10(-4) M) reduced the frequency of SCEs primarily by reducing the frequency of SCEs induced in the second cell cycle. Treatment with 3-aminobenzamide (3AB), a potent inhibitor of poly(ADP-ribose) polymerase, produced effects similar to exposure to high levels of BrdUrd including inducing SCEs in the second replication cycle. This suggests a similar mechanism of action. Deoxycytidine had no effect on 3AB-induced SCEs, however, and there was no interaction between 3AB and high exogenous levels of BrdUrd in SCE induction. Thus these two agents probably act through different mechanisms.

Quantitative and molecular analyses of ethyl methanesulfonate- and ICR 191-induced mutation in AS52 cells

A pSV2gpt-transformed Chinese hamster ovary (CHO) cell line has been used to study mutation at the molecular level. This cell line, designated AS52, was constructed from a hypoxanthine-guanine phosphoribosyl transferase (HPRT)-deficient CHO cell line, and has been previously shown to contain a single, functional copy of the E. coli xanthine-guanine phosphoribosyl transferase (XPRT) gene (gpt) stably integrated into the Chinese hamster genome. In this study, conditions for its use in the study of mammalian cell mutagenesis have been stringently defined. The spontaneous mutation rate (2 X 10(-6)/cell division) and phenotypic expression time (7 days) of the gpt locus compare favorably with those of the hprt locus in wild-type CHO-K1-BH4 cells. While both cell lines exhibit similar cytotoxic responses to ethyl methanesulfonate (EMSO and ICR 191, significant differences in mutation induction were observed. Ratios of XPRT to HPRT mutants induced per unit dose of EMS and ICR 191 are 0.70 and 1.6, respectively. Southern blot hybridization analyses revealed that most XPRT mutant cell lines which arose following treatment with EMS (20/22) or ICR 191 (20/24) exhibited no alterations of the gpt locus detectable by this technique. Similar observations were made for the hprt locus in EMS-(21/21) and ICR 191-induced (22/22) HPRT mutants. In contrast, most spontaneous gpt mutants (14/23) contained deletions, while most spontaneous hprt mutants (18/23) exhibited no detectable alterations. Results of this study indicate that the AS52 cell line promises to be useful for future study of mutation in mammalian cells at the DNA sequence level.

The effects of exogenous thymidine on endogenous deoxynucleotides and mutagenesis in mammalian cells

The intracellular deoxyribonucleoside triphosphate pools in mammalian cells affect diverse biological functions including the spontaneous or induced mutability. We have isolated from murine T-lymphosarcoma S49 cells, a mutant that is unable to convert dCMP to dUMP, contains deranged intracellular dNTP pools, and exhibits a mutator phenotype. The enzymatic defect in araC-6-1 cells is a deficiency of deoxycytidylate deaminase, which accounts for the high dCTP and low TTP intracellular pools. The addition of increasing concentrations of exogenous thymidine to araC-6-1 cells alters these dNTP pools in a predictable manner: increasing the TTP and diminishing the dCTP. Concomitant with this reversal of the dCTP:TTP ratio is a marked decrease in the mutation rate followed by an increase in the mutation rates at higher exogenous thymidine concentrations. This response of the mutation rate is in contrast to that seen in the control cell line containing normal deoxycytidylate deaminase. In the latter case, increasing thymidine concentration induces an enhanced mutation rate that parallels the later phase of the thymidine-induced mutation rate in araC-6-1 cells. The deficiency of deoxycytidylate deaminase, the endogeneous dNTP pool alterations, and the mutator phenotype of araC-6-1 cells are all recessive traits in cell-cell hybrids. These observations allow one to predict whether exogenous thymidine will be mutagenic, antimutagenic, or both for a given cell line and provide a basis for understanding conflicting reports in the literature concerning the effects of the thymidine on genomic stability.

The effects of pyrimidine nucleotides on alkylating agent induced cytotoxicity and spontaneous and induced mutation to purine analog resistance in V79 cells

Exposure of three V79 cell lines to dT after treatment with monofunctional alkylating agents resulted in potentiation of alkylation induced cytotoxicity. The degree of potentiation achieved was dependent on the concentration and duration of exposure to dT and was reversed by equimolar concentrations of dCyd. Exposure to dT after UV or X-irradiation or treatment with HN2 or MMC did not affect the cytotoxic response. dT exposure at non-cytotoxic concentrations did not affect DNA synthesis as measured by [3H]-dT incorporation when allowance was made for reductions in specific activity of labelled thymidine. However, dT post treatment reversed the alkylation induced inhibition of DNA synthesis. Toxic concentrations of dT caused an increase in frequency of TGR colonies but this increase was shown to be due to effects of dT on cell growth rate, and differential sensitivity ot HGPRT- and HGPRT+ cells. The frequency of spontaneous and alkylation induced AZR and to a lesser extent TGR colonies was also increased by non-toxic dT concentrations. Evidence was obtained which suggests that this increase is more likely to be due to alterations in the selective efficiency of the purine analogs than alterations in coding fidelity due to altered dNTP pools.

Antipain-mediated suppression of sister chromatid exchanges induced by an inhibitor of poly (ADP-ribose) polymerase

Exposure of mammalian cells to inhibitors of poly(ADP-ribose) polymerase, such as 3-aminobenzamide (3AB) results in the induction of sister chromatid exchanges (SCEs). The mechanism for the induction of SCEs by 3AB is unknown but is thought to be related to the incorporated halogenated pyrimidine used in SCE analysis. In this characteristic, 3AB-mediated SCE induction is similar to the elevated SCE frequency found in Bloom's syndrome (BS) cells. Recently, it has been reported that certain protease inhibitors, such as antipain, will inhibit SCE induction in BS cells. We now report that antipain will also suppress 3AB-induced SCE frequency. As has been reported for BS cells, the effects of antipain on SCE induction are partial, reducing SCE frequency by 0.15 to 0.40 SCE/chromosome (5-25% of the total induced frequency), and 30 microM concentrations of antipain are saturating. Antipain has no effect on baseline SCE frequency. These effects appear to involve free-radical production because dimethylsulfoxide (DMSO), a free-radical scavenger, will mimic the effects of antipain on 3AB-induced SCEs. Both antipain and DMSO will also reduce the elevated SCE frequency found in cells exposed to high (100 microM or more) levels of bromodeoxyuridine (BrdUrd). High exogenous levels of BrdUrd produce some of the same biological effects as 3AB exposure. Thus, a minor fraction of the elevated SCE frequency seen in cells exposed to 3AB or to high levels of BrdUrd appears to be similar to that found in cultured BS cells and is probably due to some free-radical-producing process.

Ribonucleotide reductase activity in intact mammalian cells: stimulation of enzyme activity by MgCl2, dithiothreitol, and several nucleotides

An intact cell assay system based on Tween-80 permeabilization was used to investigate ribonucleotide reductase activity in Chinese hamster ovary cells. Dithiothreitol, a reducing agent, is required for optimum activity. Analysis of dithiothreitol stimulation of CDP and ADP reductions indicated that in both cases the reducing agent served only to increase the reaction rate without altering the affinity of the enzyme for substrates. Magnesium chloride significantly stimulated the reduction of CDP but not ADP; this elevation in CDP reduction was due to an increase in both the affinity of the enzyme for substrate and the Vmax. In addition to ATP and dGTP, well-known activators of CDP and ADP reductase activities, it was found that dCTP and GTP were also able to activate CDP and ADP reductase activities, respectively. For the dCTP-activated reaction the Vmax was 0.158 nmol dCDP formed 5 X 10(6) cells-1 h-1 and the Km was 0.033 mM CDP, while for the GTP-activated reduction a Vmax of 0.667 nmol dADP formed 5 X 10(6) cells(-1) h-1 and Km of 0.20 mM ADP were observed. Kinetic analysis revealed that dCTP, dGTP, and GTP stimulate ribonucleotide reduction solely by increasing the affinity of the enzyme for substrate without affecting the Vmax of the respective reactions. ATP behaves in a different manner as it stimulates CDP reduction by altering both the affinity of the enzyme for substrate and the Vmax. Cellular concentrations of ribo- and deoxyribonucleoside di- and triphosphate pools were measured to help evaluate the relative physiological importance of the nucleotide activators. These determinations, along with the reaction kinetic studies, strongly imply that ATP is a much more important regulator of CDP reduction that dCTP, whereas GTP may serve as well or better than dGTP as the in vivo activator of ADP reduction.

Deoxycytidine kinase-deficient mutants of Chinese hamster ovary cells are hypersensitive to DNA alkylating agents

Chinese hamster ovary cell strains deficient in deoxycytidine kinase activity were selected by isolating mutants resistant to high concentrations of the analogue arabinosyl cytosine. Mutants isolated were deficient in the pool of dCTP, supporting earlier a suggestion that the deoxycytidine kinase may play a role in the turnover and maintenance of the dCTP pool. Consistent with earlier observations that increased intracellular levels of dTTP relative to dCTP lead to increased sensitivity to monofunctional DNA alkylating agents, deoxycytidine kinase-deficient mutants showed a 2-5-fold increase in sensitivity to the cytotoxic and mutagenic effects of one agent, ethyl methanesulfonate (EMS). The survival of the two kinase-deficient strains after mutagen treatment was clearly related to dCTP level as the strain with lowest dCTP was most sensitive to EMS. Thus hypersensitivity to this class of DNA damaging agents can result from cellular mutations decreasing the intracellular level of dCTP.

The role of DNA polymerase in base substitution mutagenesis on non-instructional templates

In vitro DNA synthesis on phi X174 or M13 templates with non-instructional lesions such as UV dimers or AP (apurinic/apyrimidinic) sites terminates one base before the site of the lesion when synthesis is catalyzed by T4 DNA polymerase or E. coli polymerase I. E. Coli polymerase I also produces termination bands at the site of AP lesions. Substitution of Mn2+ for Mg2+ and increasing the concentration of dNTP's results in elongation of the newly synthesized strand opposite the site of the lesion and beyond. Purine deoxynucleoside triphosphates are utilized for insertion opposite lesions to a greater extent than are pyrimidine deoxynucleoside triphosphates. Deoxy ATP is used almost exclusively for elongation opposite AP sites with pol I-Klenow fragment in the presence of Mg2+. We suppose that these results illustrate the previously observed greater affinity of polymerases under template-free conditions for purine nucleotides. We also suppose that the results can be used to account for mutagenic base selection on noninstructional DNA templates. If purines are preferentially selected by polymerases, then treatments which inactivate pyrimidines will lead to an excess of transitions whereas inactivation of purines will produce more transversions. Data in the literature support this hypothesis.