Structural alterations of the aprt locus induced by deoxyribonucleoside triphosphate pool imbalances in Chinese hamster ovary cells

Arabidopsis thaliana thymidine kinase 1a is ubiquitously expressed during development and contributes to confer tolerance to genotoxic stress

Thymidine kinase catalyzes the first step in the nucleotide salvage pathway by transferring a phosphate group to a thymidine molecule. In mammals thymidine kinase supplies deoxyribonucleotides for DNA replication and DNA repair, and the expression of the gene is tightly regulated during the cell cycle. Although this gene is phylogenetically conserved in many taxa, its physiological function in plants remains unknown. The genome of the model plant Arabidopsis thaliana has two thymidine kinase genes (AtTK1a and AtTK1b) and microarray data suggest they might have redundant roles. In this study we analyzed the TK1a function by evaluating its expression pattern during development and in response to genotoxic stress. We also studied its role in DNA repair by the characterization of a mutant that contained the T-DNA insertion in the promoter region of the TK1a gene. We found that TK1a is expressed in most tissues during plant development and it was differentially induced by ultraviolet-C radiation because TK1b expression was unaffected. In the mutant, the T-DNA insertion caused a 40 % rise in transcript levels and enzyme activity in Arabidopsis seedlings compared to wild-type plants. This elevation was enough to confer tolerance to ultraviolet-C irradiation in dark conditions, as determined by root growth, and meristem length and structure. TK1a overexpression also provided tolerance to genotoxins that induce double-strand break. Our results suggest that thymidine kinase contributes to several DNA repair pathways by providing deoxythymidine triphosphate that serve as precursors for DNA repair and to balance deoxyribonucleotides pools.

Identification of potentially cytotoxic lesions induced by UVA photoactivation of DNA 4-thiothymidine in human cells

Photochemotherapy-in which a photosensitizing drug is combined with ultraviolet or visible radiation-has proven therapeutic effectiveness. Existing approaches have drawbacks, however, and there is a clinical need to develop alternatives offering improved target cell selectivity. DNA substitution by 4-thiothymidine (S(4)TdR) sensitizes cells to killing by ultraviolet A (UVA) radiation. Here, we demonstrate that UVA photoactivation of DNA S(4)TdR does not generate reactive oxygen or cause direct DNA breakage and is only minimally mutagenic. In an organotypic human skin model, UVA penetration is sufficiently robust to kill S(4)TdR-photosensitized epidermal cells. We have investigated the DNA lesions responsible for toxicity. Although thymidine is the predominant UVA photoproduct of S(4)TdR in dilute solution, more complex lesions are formed when S(4)TdR-containing oligonucleotides are irradiated. One of these, a thietane/S(5)-(6-4)T:T, is structurally related to the (6-4) pyrimidine:pyrimidone [(6-4) Py:Py] photoproducts induced by UVB/C radiation. These lesions are detectable in DNA from S(4)TdR/UVA-treated cells and are excised from DNA more efficiently by keratinocytes than by leukaemia cells. UVA irradiation also induces DNA interstrand crosslinking of S(4)TdR-containing duplex oligonucleotides. Cells defective in repairing (6-4) Py:Py DNA adducts or processing DNA crosslinks are extremely sensitive to S(4)TdR/UVA indicating that these lesions contribute significantly to S(4)TdR/UVA cytotoxicity.

[Notion of threshold in mutagenesis: implications for mutagenic and carcinogenic risk assessment]

During years, it has been widely admitted in the scientific community that there was no threshold in mutagenesis: a compound was or not a mutagen. The meaning of such a proposition was that a risk existed at all exposure level, because, at least theoretically, one molecule is sufficient to cause the formation of a DNA adduct which is able to induce a mutation. However, works carried out in the last few years have shown that in the case of some specific mechanisms of mutagenesis, a threshold could be demonstrated essentially in the case of compounds that do not react directly with DNA. Several types of thresholds exist, and the simple statistical threshold is not sufficient in terms of risk assessment. A biological threshold that is consistent with a mechanism of action of the mutagen should be established. Amongst these mechanisms, we can mention some mechanism with a demonstrated threshold: effects of aneugens, effects of topoisomerases inhibitors, effects of DNA polymerases inhibitors, effects of compounds with a different metabolism at high doses compared to low doses. On the contrary, for some mechanisms, the demonstration of the mechanism is suspected, but not totally demonstrated. It is the case of compounds which induce nucleotides pool imbalance or compounds which are DNA repair inhibitors. In some cases, when a redundancy exists in the repair of damages, like oxidative DNA damage, a threshold is suspected. Some authors even recently proposed the possibility of a threshold in the case of alkylating agents. The majority of mutagenic thresholds were demonstrated in vitro, however some mechanisms were demonstrated in vivo, for example in the case of micronucleus induction by hypo or hyperthermia in rodents bone marrow. The use of threshold in risk assessment requires the use of the most sensitive endpoint for example, non disjunction in the case of aneugens, confusing factors like apoptosis should be eliminated and species sensitivities should be taken into account. A very important point to consider is to demonstrate that the mechanism with threshold was really thee only one involved in the mutagenic effect. The demonstration of such thresholds is of particular interest for human risk assessment in the case of mutagens and of genotoxic carcinogens.

Threshold-mediated mechanisms in mutagenesis: implications in the classification and regulation of chemical mutagens

Chemical mutagens are currently regulated and labelled on the basis of their hazardous properties defined in hazard classification schemes. The strength and type of experimental evidence is used as the only criterion for classification in categories which express different levels of concern for the possibility of adverse effects - notably transmissible genetic alterations - in humans. Differently from the classification of carcinogens, no consideration is given to potency, nor to the mechanism of action. The rationale of such hazard based classification is that the hazardous property of a chemical is an intrinsic feature, which is expressed independently of dosing. Changing of dose level results in a mere change in the probability to observe an adverse effect, but not in its potential occurrence. The lack of theoretical threshold underlying this approach can be envisaged, in principle, for stochastic processes such as DNA damage, which can be triggered by single molecular interactions. On the other hand, indirect mechanisms of genotoxicity, involving multiple interactions with non-DNA targets, are expected to show a threshold. At variance to DNA reactive agents, chemicals acting with threshold-mediated mechanism do change also qualitatively their toxic properties depending on the dose level. Possible problems arising in the application of hazard based schemes for the evaluation of chemicals with threshold-mediated mechanism of action are discussed, using the spindle poisons benzimidazole fungicides as an example.

Effect of a dCTP:dTTP pool imbalance on DNA replication fidelity in Friend murine erythroleukemia cells

Nucleotide pool imbalances have been reported to affect the fidelity of DNA replication and repair in prokaryotic and eukaryotic cells. We have reported previously that the mutagen-hypersensitive thymidine kinase (TK)-deficient Friend erythroleukemia (FEL) cells (subclones 707BUF and 707BUE), have a more than sixfold increase in the dCTP:dTTP pool ratio when compared to that of wild-type, TK-positive (TK(+)) clone 707 cells. In this study we present the results of an investigation of the effect of the dCTP:dTTP pool imbalance on the accuracy of DNA replication within 707BUF cells. We examined the spontaneous mutation spectra occurring at the adenine phosphoribosyltransferase (aprt) locus within clone 707 (TK(+)) and 707BUF (TK(-)) FEL cells. Mutations recovered at the aprt locus in FEL cells comprised: base substitutions (43:73), frameshifts (14:13.5), and deletions (43:13.5) [clone 707 (TK(+)):707BUF (TK(-)), respectively, expressed as percentages]. A comparison of the mutation spectra obtained for the two cell lines did not reveal any significant increase in misincorporation of dCTP, the nucleotide in excess, in 707BUF (TK(-)) cells, during DNA replication synthesis. These data suggest that the dCTP:dTTP pool imbalance does not alter the fidelity of DNA replication synthesis in 707BUF (TK(-)) FEL cells. Rather, the predominance of GC --> AT transitions (53%) in the 707BUF (TK(-)) spectrum may reflect a reduced efficiency of repair by uracil DNA glycosylase of uracil residues within these cells.

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.

Nucleotide pools and mutagenic effects of alkylating agents in wild-type and APRT-deficient Friend erythroleukaemia cells

Wild-type Friend mouse erythroleukaemia cells (clone 707) were compared with adenine phosphoribosyltransferase (APRT)-deficient mutant subclones (707DAP8 and 707DAP10) for sensitivity to cell killing and mutagenesis by ethyl methanesulphonate (EMS) and methyl methanesulphonate (MMS). Cells were exposed to 0-300 micrograms/ml EMS and to 0-20 micrograms/ml MMS for a period of 16 h. A slight difference was found between wild-type cells and the two APRT-deficient subclones in terms of sensitivity to cell killing by both mutagens. The APRT-deficient subclones were, however, significantly more sensitive than wild-type cells to mutagenesis to 5-bromo-2-deoxyuridine resistance and 6-thioguanine resistance by EMS and MMS. The APRT-deficient subclones were found to have significantly decreased levels of dATP and dTTP nucleotides and decreased levels of all four ribonucleoside triphosphates (ATP, GTP, CTP and UTP) relative to wild-type cells. Wild-type Friend cells were found to have insignificant levels O6-methylguanine-DNA methyl transferase and it is suggested that the increased mutagen sensitivity of APRT-deficient cells may be due to imbalance of deoxyribonucleoside triphosphate pools during DNA excision-repair processes, or more probably due to deficiency of ATP for ATP-dependent DNA excision-repair enzymes.

Molecular genetic analysis of recessive mutations at a heterozygous autosomal locus in human cells

We have investigated the genotypic changes that lead to expression of a recessive allele at a heterozygous autosomal locus in a human cell line. Mutant clones lacking thymidine kinase activity were derived from a B-cell lymphoblastoid line initially heterozygous at the tk locus, and restriction mapping was performed to detect intragenic structural alterations in the tk gene. In addition, informative molecular markers located elsewhere on chromosome 17 were analysed in order to detect large-scale (multilocus) events. We report that among 325 spontaneous and induced mutants, allele loss was more common than intragenic rearrangements or point mutations; in many cases, loss of heterozygosity appears to have extended well beyond the locus under selection. Cytogenetic analysis of a subset of these mutants showed that expression of the recessive TK-deficient phenotype and the associated loss of heterozygosity for chromosome 17 markers was not typically associated with detectable chromosomal changes.

The genetic consequences of DNA precursor pool imbalance: sequence analysis of mutations induced by excess thymidine at the hamster aprt locus

To determine the effect of deoxyribonucleoside triphosphate pool imbalances on the accuracy of DNA replication within the cell, we examined the base pair alterations induced by excess intracellular dTTP at the adenine phosphoribosyl transferase (aprt) locus of CHO cells. The mutations were predominantly simple (C----T) transitions (38/44) and transversions (G----T, 5/44) explicable by the misincorporation of the DNA precursor supplied in excess (dTTP). Only one small deletion was observed. The context of the mutations is notable as the nucleotide incorporated after the error was usually the nucleotide in excess for the great majority of the transitions but not the transversions. As next nucleotide effects are characteristic of replication complexes having proofreading exonuclease activity, our data indicate that this mechanism functions within the cell to control the occurrence of some types of replicational errors.

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.

Denaturing gradient gel analysis of single-base substitutions at a mouse adenine phosphoribosyltransferase splice acceptor site

Denaturing gradient gel electrophoresis (DGGE) can detect single-base changes in DNA. We used site-directed mutagenesis to produce all six possible single-base substitutions at a splice acceptor consensus AG dinucleotide within the mouse adenine phosphoribosyltransferase (aprt) gene. Studies of mouse and Chinese hamster cell aprt indicate a high level of both spontaneous and induced mutations in this region. We systematically evaluated each of the six mutations by DGGE. Five of the six mutant sequences could be distinguished from wildtype by DGGE analysis of a 560-bp fragment containing the mutation. However, one mutant could not be distinguished from wild-type, and some of the mutant constructs could not be distinguished from each other. Analysis of DNA heteroduplexes consisting of wild-type and mutant strands or two different mutant strands enabled all mutant constructs to be distinguished from wild-type and from each other. The pairwise mixtures resulted in 24 different heteroduplexes, all of which were less stable than the parental homoduplexes. End labeling of DNA prior to heteroduplex formation and subsequent DGGE analysis enabled us to determine the relative destabilization caused by different types of single-base mismatches.

A genetic analysis of the adenine phosphoribosyl transferase locus in Chinese hamster V79-AP4 cells: relevance to mutagenesis studies

The chromosomal location of the autosomal locus aprt has been investigated in the permanent Chinese hamster cell line V79-AP4 by standard somatic cell genetics methodologies. Aprt is functionally dizygous in V79-AP4 and the 2 alleles map on 2 chromosome 3 homologs, in agreement with the chromosome assignment of the gene in Chinese hamster primary cells. Chromosome G-banding and a Southern blot analysis of V79-AP4 DNA, using as a probe the cloned Chinese hamster aprt gene, have not revealed any structural alteration at either of the 2 aprt alleles. One of the chromosomes 3 has, however, a terminal deletion in its long arm and is therefore morphologically marked. These findings could make V79-AP4 an interesting cell system for the study of mutational mechanisms at the aprt locus in Chinese hamster.

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.

Somatic mutations at a heterozygous autosomal locus in human cells occur more frequently by allele loss than by intragenic structural alterations

A human B-cell lymphoblastoid cell line heterozygous at the thymidine kinase (TK) locus (i.e., carrying one functional and one nonfunctional thymidine kinase allele) was used to study the molecular nature of mutations leading to loss of TK activity. A total of 113 mutant clones, both spontaneous and induced, were examined by restriction enzyme mapping and by the use of a restriction fragment length polymorphism (RFLP) at the TK locus. A majority (71%) of all mutant clones examined had lost the entire functional TK allele, becoming either homozygous or hemizygous for the nonfunctional allele. The remaining mutants had either no detectable changes (26%) or had obvious structural alterations (less than 5%) in the active TK gene. These results emphasize the importance of allele loss, presumably by mitotic chromosomal mechanisms, in mutagenesis at autosomal loci, and suggest that in vitro models for recessive somatic mutation which are based at hemizygous loci may ignore a large category of genetically significant events.

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.