Deoxyribonucleotide pools as targets for mutagenesis by N-methyl-N-nitrosourea

A modified nucleoside O6-methyl-2'-deoxyguanosine-5'-triphosphate exhibits anti-glioblastoma activity in a caspase-independent manner

Resistance to temozolomide (TMZ), the frontline chemotherapeutic agent for glioblastoma (GBM), has emerged as a formidable obstacle, underscoring the imperative to identify alternative therapeutic strategies to improve patient outcomes. In this study, we comprehensively evaluated a novel agent, O6-methyl-2'-deoxyguanosine-5'-triphosphate (O6-methyl-dGTP) for its anti-GBM activity both in vitro and in vivo. Notably, O6-methyl-dGTP exhibited pronounced cytotoxicity against GBM cells, including those resistant to TMZ and overexpressing O6-methylguanine-DNA methyltransferase (MGMT). Mechanistic investigations revealed that O6-methyl-dGTP could be incorporated into genomic DNA, disrupting nucleotide pools balance, and inducing replication stress, resulting in S-phase arrest and DNA damage. The compound exerted its anti-tumor properties through the activation of AIF-mediated apoptosis and the parthanatos pathway. In vivo studies using U251 and Ln229 cell xenografts supported the robust tumor-inhibitory capacity of O6-methyl-dGTP. In an orthotopic transplantation model with U87MG cells, O6-methyl-dGTP showcased marginally superior tumor-suppressive activity compared to TMZ. In summary, our research, for the first time, underscores the potential of O6-methyl-dGTP as an effective candidate against GBM, laying a robust scientific groundwork for its potential clinical adoption in GBM treatment regimens.

dNTP concentrations do not increase in mammalian cells in response to DNA damage

A recent publication reported a uniform ∼5- to 6-fold increase in dNTP pools 30 min after exposure to ionizing radiation. Das et al. were not able to reproduce these results. Their data instead agree with earlier publications reporting no increase in dNTP pools in mammalian cells in response to DNA damage.

Intramolecular Charge Transfer in the Azathioprine Prodrug Quenches Intersystem Crossing to the Reactive Triplet State in 6-Mercaptopurine†

The thiopurine prodrugs 6-mercaptopurine and azathioprine are among the world's essential medications for acute lymphoblastic leukemia, immunosuppression and several autoimmune conditions. Thiopurine prodrugs are efficient UVA absorbers and singlet oxygen generators and the long-term treatment with these prodrugs correlates with a high incidence of sunlight-induced skin cancer in patients. In this contribution, we show that the electronic relaxation mechanisms and photochemical properties of azathioprine are remarkably different from those of 6-mercaptopurine upon absorption of UVA radiation. UVA excitation of 6-mercaptopurine results in nearly 100% triplet yield and up to 30% singlet oxygen generation, whereas excitation of azathioprine with UVA leads to triplet yields of 15-3% depending on pH of the aqueous solution and <1% singlet oxygen generation. While photoexcitation of 6-mercaptopurine and other thiopurine prodrugs can facilitate oxidatively generated cell damage, azathioprine's poor photosensitization ability reveals the use of interchromophoric charge-transfer interactions for the molecular design of photostable prodrugs exhibiting a remarkable reduction in photocytotoxic side effects before drug metabolization.

Genotoxicity profile of azidothymidine in vitro

Azidothymidine (Zidovudine, AZT) is part of the standard care of treatment for acquired immunodeficiency syndrome since many years. A great number of studies on the genotoxic potential of AZT have been published, but no comprehensive hypothesis yet explains all observations. We investigated a multitude of genotoxic endpoints, both in vitro and in vivo, with the goal to complete the picture. The mutagenic potential of AZT in bacteria was found to be restricted to strains with an "ochre" target sequence and could be abrogated both by thymidine supplementation and rat liver S9 mix. Single-strand breaks in mammalian cells were detected in the comet assay after short-term treatment (3h) with AZT, which did not induce micronuclei. The latter were mainly seen after prolonged exposure (24 and 48h) and are probably not directly related to AZT incorporation into DNA. Our data demonstrate that short-term exposure to low AZT concentrations does not induce biologically relevant micronucleation. Only treatment with high concentrations of AZT for prolonged time periods manifests in substantial micronucleus induction. Furthermore, we found that high concentrations of thymidine have no effect in the comet assay but increase micronucleus frequency in a manner very similar to AZT. These results lead us to the following hypothesis: AZT is triphosphorylated and then incorporated into DNA strands, leading to mutations and cytotoxicity. Cellular attempts to repair these DNA lesions as well as stalled replication forks due to chain termination are detectable with the comet assay. Increased micronucleus frequency is likely related to nucleotide pool imbalance.

Effects of biological DNA precursor pool asymmetry upon accuracy of DNA replication in vitro

Deoxyguanosine triphosphate is underrepresented among the four common deoxyribonucleoside triphosphates (dNTPs), typically accounting for just 5-10% of the total dNTP pool. We have asked whether this pool asymmetry affects the fidelity of DNA replication, by use of an in vitro assay in which an M13 phagemid containing the Escherichia coli lacZalpha gene and an SV40 replication origin is replicated by extracts of human cells. By monitoring reversion of either a TGA or TAA codon within the lacZalpha gene, we found that replication in "biologically biased" dNTPs, representing our estimate of the concentrations in HeLa cell nuclei, is not significantly more accurate than when measured in reaction mixtures containing the four dNTPs at equimolar concentrations. However, sequence analysis of revertants revealed significantly different patterns of mispairing events leading to mutation. During replication at biased dNTP levels, mutations at the site 5' to C in the template strand for the TGA triplet were less frequent than seen in equimolar reaction mixtures, suggesting that extension from mismatches at this site is relatively slow, and proofreading efficiency high, when dGTP is the next nucleotide to be incorporated. Mismatches opposite template C, which might have been favored by the low physiological concentrations of dGTP, were not favored in our in vitro system, although one particular substitution at this site, TGA-->TTA, was strongly favored at low [dGTP]. An excess of one dNTP was found in our system to be more mutagenic than a corresponding deficiency. We also estimated dNTP concentrations in non-transformed human fibroblasts and found that in vitro replication at these levels caused significantly fewer mutations than we observed under equimolar conditions (100 microM each dNTP). This increased replication fidelity may result from increased proofreading efficiency at the lower dNTP levels; however, replication rates were decreased only slightly at these non-transformed fibroblast concentrations.

Ribonucleotide reductase, a possible agent in deoxyribonucleotide pool asymmetries induced by hypoxia

While investigating the basis for marked natural asymmetries in deoxyribonucleoside triphosphate (dNTP) pools in mammalian cells, we observed that culturing V79 hamster lung cells in a 2% oxygen atmosphere causes 2-3-fold expansions of the dATP, dGTP, and dTTP pools, whereas dCTP declines by a comparable amount. Others have made similar observations and have proposed that, because O(2) is required for formation of the catalytically essential oxygen-bridged iron center in ribonucleotide reductase, dCTP depletion at low oxygen tension results from direct or indirect effects upon ribonucleotide reductase. We have tested the hypothesis that oxygen limitation affects ribonucleotide specificity using recombinant mouse ribonucleotide reductase and an assay that permits simultaneous monitoring of the reduction of all four nucleotide substrates. Preincubation and assay of the enzyme in an anaerobic chamber caused only partial activity loss. Accordingly, we treated the enzyme with hydroxyurea, followed by removal of the hydroxyurea and exposure to atmospheres of varying oxygen content. The activity was totally depleted by hydroxyurea treatment and nearly fully regained by exposure to air. By the criterion of activities regained at different oxygen tensions, we found CDP reduction not to be specifically sensitive to oxygen depletion; however, GDP reduction was specifically sensitive. The basis for the differential response to reactivation by O(2) is not known, but it evidently does not involve varying rates of reactivation of different allosteric forms of the enzyme or altered response to allosteric effectors at reduced oxygen tension.

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.

DNA precursor asymmetries, replication fidelity, and variable genome evolution

Balanced pools of deoxyribonucleoside triphosphates (dNTPs) are essential for DNA replication to occur with maximum fidelity. Conditions that create biased dNTP pools stimulate mutagenesis, as well as other phenomena, such as recombination or cell death. In this essay we consider the effective dNTP concentrations at replication sites under normal conditions, and we ask how maintenance of these levels contributes toward the natural fidelity of DNA replication. We focus upon two questions. (1) In prokaryotic systems, evidence suggests that replication is driven by small, localized, rapidly replenished dNTP pools that do not equilibrate with the bulk dNTP pools in the cell. Since these pools cannot be analyzed directly, what indirect approaches can illuminate the nature of these replication-active pools? (2) In eukaryotic cells, the normal dNTP pools are highly asymmetric, with dGTP being the least abundant nucleotide. Moreover, the composition of the dNTP pools changes as cells progress through the cell cycle. To what extent might these natural asymmetries contribute toward a recently described phenomenon, the differential rate of evolution of different genes in the same genome?

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.

Single cell analysis in toxicity testing: the mitogenic activity of thioacetamide in cultured rat hepatocytes analyzed by DNA/protein flow cytometry

Rat hepatocytes were cultured at 4% O2 and 13% O2 and exposed to the nongenotoxic rodent carcinogen thioacetamide (TA) from 24 to 72 h after isolation at exposure levels between 0.01 and 0.33 mM. Hepatocytes and isolated nuclei were analyzed by DNA-protein flow cytometry. An aggregate correction procedure was applied and the proportion of S-phase, diploid, tetraploid or octoploid hepatocytes as well as binucleated cells, were measured or calculated. The proportion of S-phase cells within the diploid hepatocytes increased with increasing concentration of TA up to 3.9-fold, whereas the corresponding increase in S-phase mononucleated tetraploid cells was only 1.8-fold. S-phase binucleate tetraploid cells showed no increase. In the tetraploid hepatocytes, the mitogenic stimuli was detectable only in cultures maintained at 4% O2. The relative contribution of binuclear cells was increased 1.5-fold in the octoploid cells. It is concluded that the mitogenic activity of TA initiates DNA synthesis in diploid hepatocytes in the G1 and in the following G2 cell-cycle phase, omitting karyogenesis. The cellular protein content is not affected which indicates that the mitogenic activity of the chemical is not necessarily associated with an increase in cellular protein content. The results obtained correspond well with data of in vivo studies. The method applied therefore allows the mitogenic activity of nongenotoxic carcinogens to be detected in vitro within 48 h and their mode of action to be elucidated.

Cell cycle-dependent variations in deoxyribonucleotide metabolism among Chinese hamster cell lines bearing the Thy- mutator phenotype

Deoxyribonucleotide pool imbalances are frequently mutagenic. We have studied two Chinese hamster ovary cell lines, Thy- 49 and Thy- 303, that were originally characterized by M. Meuth (Mol. Cell. Biol. 1:652-660, 1981). In comparison with wild-type CHO cells, both lines have elevated dCTP/dTTP ratios, resulting from loss of feedback control of CTP synthetase. While asynchronous cultures of both cell lines contain nearly identical deoxyribonucleoside triphosphate (dNTP) pools and both display elevated spontaneous mutation frequencies, the mutation frequencies between the two cell lines differ by as much as 10-fold. We asked whether differences in dNTP pools could be seen in extracts of rapidly isolated nuclei. Small differences, probably not large enough to account for the differences in mutation frequencies, were seen. However, when synchronized S-phase-enriched cell populations were examined, substantial differences were seen, both in whole-cell extracts and in nuclear extracts. Thy- 303 cells, which have higher mutation frequencies than do Thy- 49 cells, also showed the more aberrant dNTP pools. These data indicate that the Thy- 303 line contains a second mutation in addition to the mutation affecting CTP synthetase control. Evidence suggests that this putative second mutation affects an allosteric regulatory site of ribonucleotide reductase. The data on intranuclear dNTP pools in synchronized S-phase cells indicate that higher proportions of cellular dATP and dGTP are found in the nucleus than are corresponding amounts of dCTP and dGTP. Thus, despite the porous nature of the nuclear membrane, there are conditions under which the distributions of deoxyribonucleotides across this membrane are not random.

Cell cycle-dependent variations in deoxyribonucleotide metabolism among Chinese hamster cell lines bearing the Thy- mutator phenotype

Deoxyribonucleotide pool imbalances are frequently mutagenic. We have studied two Chinese hamster ovary cell lines, Thy- 49 and Thy- 303, that were originally characterized by M. Meuth (Mol. Cell. Biol. 1:652-660, 1981). In comparison with wild-type CHO cells, both lines have elevated dCTP/dTTP ratios, resulting from loss of feedback control of CTP synthetase. While asynchronous cultures of both cell lines contain nearly identical deoxyribonucleoside triphosphate (dNTP) pools and both display elevated spontaneous mutation frequencies, the mutation frequencies between the two cell lines differ by as much as 10-fold. We asked whether differences in dNTP pools could be seen in extracts of rapidly isolated nuclei. Small differences, probably not large enough to account for the differences in mutation frequencies, were seen. However, when synchronized S-phase-enriched cell populations were examined, substantial differences were seen, both in whole-cell extracts and in nuclear extracts. Thy- 303 cells, which have higher mutation frequencies than do Thy- 49 cells, also showed the more aberrant dNTP pools. These data indicate that the Thy- 303 line contains a second mutation in addition to the mutation affecting CTP synthetase control. Evidence suggests that this putative second mutation affects an allosteric regulatory site of ribonucleotide reductase. The data on intranuclear dNTP pools in synchronized S-phase cells indicate that higher proportions of cellular dATP and dGTP are found in the nucleus than are corresponding amounts of dCTP and dGTP. Thus, despite the porous nature of the nuclear membrane, there are conditions under which the distributions of deoxyribonucleotides across this membrane are not random.

Linkage of neutrophil response to a mutagen gene (Nrm-1) to the agouti locus in the rat

The ACI or BDIV rats responded with decreased or increased neutrophil levels in blood after the N-methyl-N-nitrosourea (MNU) administration. The F1 hybrids had decreased neutrophil counts, and the BDIV x (BDIV x ACI)F1 backcross offspring showed two phenotypes. The sex of the rats and the neutrophil response to MNU assorted independently. The results indicated that the neutrophil response to MNU was regulated by autosomal gene Nrm-1 with two alleles. The Nrm-1d regulates the decrease and the Nrm-1i regulates the increase of neutrophils in blood after the MNU administration. The results were confirmed by the SKUMIX computer program. We found that the Nrm-1 gene was linked to the agouti locus (chi-square = 10.3, P less than 0.001). The map distance between two genes was 33 +/- 5 cM. The Nrm-1 gene thus resides on the linkage group IV of the rat.

Deoxyribonucleoside triphosphate pools in mutagen sensitive mutants of Neurospora crassa

Deoxyribonucleoside triphosphate (dNTP) levels were measured in wild type Neurospora and nine mutagen-sensitive mutants, at nine different genes. Eight of these mutants are sensitive to hydroxyurea and histidine and show chromosomal instability, a phenotype which could result from altered levels of dNTPs. Two patterns were seen. Five of the mutants had altered ratios of dNTPs, with relatively high levels of dATP and dGTP and low levels of dCTP, but changes in the dTTP/dCTP ratio did not correlate with changes in spontaneous mutation levels. During exponential growth all but two of the mutants had small but consistent increases in dNTP pools compared to wild type. DNA content per microgram dry hyphae was altered in several mutants but these changes showed no correlation with the dNTP pool alterations.