Altered fidelity of a nucleic acid modifying enzyme, T4 polynucleotide kinase, by safrole-induced DNA damage

Genotoxic effects of eugenol, isoeugenol and safrole in the wing spot test of Drosophila melanogaster

In the present study, the phenolic compounds eugenol, isoeugenol and safrole were investigated for genotoxicity in the wing spot test of Drosophila melanogaster. The Drosophila wing somatic mutation and recombination test (SMART) provides a rapid means to evaluate agents able to induce gene mutations and chromosome aberrations, as well as rearrangements related to mitotic recombination. We applied the SMART in its standard version with normal bioactivation and in its variant with increased cytochrome P450-dependent biotransformation capacity. Eugenol and safrole produced a positive recombinagenic response only in the improved assay, which was related to a high CYP450-dependent activation capacity. This suggests, as previously reported, the involvement of this family of enzymes in the activation of eugenol and safrole rather than in its detoxification. On the contrary, isoeugenol was clearly non-genotoxic at the same millimolar concentrations as used for eugenol in both the crosses. The responsiveness of SMART assays to recombinagenic compounds, as well as the reactive metabolites from eugenol and safrole were considered responsible for the genotoxicity observed.

Monitoring of DNA Adducts in Humans and 32P-Postlabelling Methods. A Review

DNA adduct formation in humans is a promising biomarker for elucidating the molecular epidemiology of cancer. For detection of DNA adducts, the most widely used methods include mass spectroscopy, fluorescence spectroscopy, immunoassays and 32P-postlabelling. Among them, the 32P-postlabelling method appears to meet best the criteria of sensitivity and amount of DNA needed, and, therefore, is one of the most appropriate methods for biomonitoring of human DNA adducts. Most classes of carcinogens have been subjected to 32P-postlabelling analysis, ranging from bulky and/or aromatic compounds to small and/or aliphatic compounds; it has also been used, with modifications, to detect apurinic sites in DNA, oxidative damage to DNA, UV-induced photodimers and, to a lesser extent, DNA damage caused by cytotoxic drugs. It has been used in human biomonitoring studies to detect DNA damage from occupational exposure to carcinogens, and also from environmental (i.e. non-occupational) exposures. It has also led to the discovery of the presence of numerous modifications in DNA arising from endogenous processes. The principle of the method is the enzymatic digestion of DNA to nucleotides, 5'-labelling of these nucleotides with an isotopically labelled phosphate group, and the resolution, detection and quantitation of the labelled products. Since the development of the original procedure in the early 1980s, many methods have been developed to increase the sensitivity by enrichment of modified nucleotides prior to labelling. The review presents the individual 32P-postlabelling techniques (standard procedure, enrichment methods) and a critical evaluation of these assays, besides reviewing the applications of the method to different DNA modifications, and its utilization in human biomonitoring studies. A review with 179 references.

Cytotoxicity and genotoxicity of methyleugenol and related congeners-- a mechanism of activation for methyleugenol

Methyleugenol is a substituted alkenylbenzene found in a variety of foods, products, and essential oils. In a 2-year bioassay conducted by the National Toxicology Program, methyleugenol caused neoplastic lesions in the livers of Fischer 344 rats and B6C3F(1) mice. We were interested in the cytotoxicity and genotoxicity caused by methyleugenol and other alkenylbenzene compounds: safrole (a known hepatocarcinogen), eugenol, and isoeugenol. The endpoints were evaluated in cultured primary hepatocytes isolated from male Fischer 344 rats and female B6C3F(1) mice. Cytotoxicity was determined by measuring lactate dehydrogenase (LDH) release, while genotoxicity was determined by using the unscheduled DNA synthesis (UDS) assay. Rat and mouse hepatocytes showed similar patterns of toxicity for each chemical tested. Methyleugenol and safrole were relatively non-cytotoxic, but caused UDS at concentrations between 10 and 500 microM. In contrast, isoeugenol and eugenol produced cytotoxicity in hepatocytes with LC50s of approximately 200-300 microM, but did not cause UDS. Concurrent incubation of 2000 microM cyclohexane oxide (CHO), an epoxide hydrolase competitor, with a non-cytotoxic concentration of methyleugenol (10 microM) resulted in increased cytotoxicity but had no effect on genotoxicity. However, incubation of 15 microM pentacholorophenol, a sulfotransferase inhibitor, with 10 uM methyleugenol resulted in increased cytotoxicity but had a significant reduction of genotoxicity. These results suggest that methyleugenol is similar to safrole in its ability to cause cytotoxicity and genotoxicity in rodents. It appears that the bioactivation of methyleugenol to a DNA reactive electrophile is mediated by a sulfotransferase in rodents, but epoxide formation is not responsible for the observed genotoxicity.

Comparison of immunoaffinity chromatography enrichment and nuclease P1 procedures for 32P-postlabelling analysis of PAH-DNA adducts

32P-postlabelling analysis for detecting DNA adducts formed by polycyclic aromatic compounds is one of the most widely used techniques for assessing genotoxicity associated with these compounds. In cases where the formation of adducts is extremely low, a crucial step in the analysis is an enrichment procedure for adducts prior to the radiolabelling step. The nuclease P1 enhancement procedure is the most established and frequently used of these methods. An immunoaffinity procedure developed for class specific recognition for polycyclic aromatic hydrocarbon (PAH)-DNA adducts has therefore been compared with the nuclease P1 method for a range of DNA adducts formed by PAHs. The evaluation was carried out with skin DNA from mice treated topically with benzo[a]pyrene, 7,12-dimethylbenz[a]anthracene, 5-methylchrysene or chrysene. The immobilised antibody had the highest affinity for adducts structurally similar to the BPDE-I-deoxyguanosine adduct ([+/-]-N2-(7r,8t,9r-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene-1 0t-yl)-2'-deoxyguanosine) against which the antibody had been raised. Of the PAH-modified DNAs evaluated, the maximum adduct recovery was obtained for DNA containing the BPDE I-deoxyguanosine adduct. With DMBA-modified DNA, the profiles of adducts recovered from the column were similar when the column material was treated either with a digest of DMBA-modified DNA or with 32P-labelled DMBA adducts. I-compounds (endogenous adducts in tissue DNA of unexposed animals), which had similar chromatographic properties to PAH-DNA adducts, were not enriched by the immunoaffinity procedure. Compared to the simple nuclease P1 enhancement procedure, the immunoaffinity methods were lengthier and more labour intensive. Advantages of the immunoaffinity procedure include: specificity, allowing the selective detection of a certain class of adducts: efficient adduct enrichment, providing a viable alternative to other enrichment procedures; adequate sensitivity for model studies and the potential to purify adducts for further characterisation. However, as a general screen for detecting the formation of DNA adducts, the nuclease P1 procedure was viewed as the initial method of choice since it was capable of detecting a wider range of PAH-DNA adducts.

Detection of DNA modifications by the 32P-postlabelling assay

The 32P-postlabelling assay is widely used for detection of carcinogen-DNA adducts and other types of modified nucleotides in DNA. The principle of the method is the enzymatic digestion of DNA to nucleotides, 5'-labelling of these nucleotides with an isotopically labelled phosphate group, and the resolution and detection of the labelled products. Since the development of the original procedure in the early 1980s, many methods have been developed to increase the sensitivity of the method by selection of the modified nucleotides prior to labelling. In favourable circumstances, the method can achieve a level of detection as low as 1 modification in 10(10) nucleotides and requires relatively small quantities of DNA (less than 10 microg). It has been used to detect and characterise DNA adducts formed by numerous genotoxic carcinogens in bacterial and mammalian cells, in animals and, in some cases, in human tissues. Most classes of carcinogen have been subjected to 32P-postlabelling analysis, ranging from bulky and/or aromatic compounds to small and/or aliphatic compounds; it has also been used, with modifications, to detect apurinic sites in DNA, oxidative damage to DNA, UV-induced photodimers and, to a lesser extent, DNA damage caused by cytotoxic drugs. It has provided the first clear evidence for the DNA-damaging properties of several synthetic carcinogenic hormones. It has revealed the DNA-damaging potential of complex mixtures such as coal-tar and tobacco smoke. It has been used in human biomonitoring studies to detect DNA damage from occupational exposure to carcinogens, and also from environmental (i.e. non-occupational) exposures. It has also led to the discovery of the presence of numerous modifications in DNA arising from endogenous processes. The rapid expansion in the use of the assay has resulted in some divergence of procedures and there is a case to be made for the use of more standardised protocols, particularly where human exposure to carcinogens is being measured and where such results may be required for risk assessment. While the procedure is quantifiable, the efficiency of adduct labelling is, in many cases, not quantitative, and the lack of adduct standards has, in many cases, limited the interpretation of data to a demonstration of higher adduct levels in exposed groups compared with unexposed groups. Future developments are expected in automation, standardisation and, in combination with other analytical methods, elucidation of the structures of the many DNA lesions whose existence has been revealed by the 32P-postlabelling technique.

Monophosphate 32P-postlabeling assay of DNA adducts from 1,2:3,4-diepoxybutane, the most genotoxic metabolite of 1,3-butadiene: in vitro methodological studies and in vivo dosimetry

Among the main DNA-reactive metabolites of 1,3-butadiene (BD), both 1,2:3,4-butadiene diepoxide (BDE) and 1,2-epoxy-3-butene (BME) have been reported in mice and rats exposed to BD, but blood and tissue levels of these metabolites are much higher in mice than in rats under similar exposure conditions. BDE, being more reactive and genotoxic than BME, is thought to be responsible for the greater susceptibility of mice to BD carcinogenicity. While BDE is a DNA-alkylating agent and some BDE adducts have been characterized, no sufficiently sensitive method has been reported for studying BDE-DNA binding in vivo. In the present investigation, a modified dinucleotide/monophosphate version of the 32P-postlabeling assay was applied to detect BDE-DNA adducts, which were prepared by reacting BDE with calf thymus DNA or deoxyribooligonucleotides [(AC)10, (AG)10, (CCT)7 and (GGT)7] in vitro or with skin DNA of mice in vivo upon topical treatment. Optimal resolution by 2-D PEI-cellulose TLC of the highly polar 5'-monophosphate adducts was achieved at +4 degrees C using 0.3 M LiCI (DI) and 0.4 M NaCl, 0.04 M H3BO3, pH 7.6 (D2). The profiles of the 32P-postlabeled adducts were similar for calf thymus and skin DNA, with 3 major spots being detected. Adducts obtained in in vitro and in vivo experiments were compared by re- and cochromatography in 4 or 5 different solvents, and these experiments provided evidence that corresponding BDE adducts, for the most part, were identical and represented adenine derivatives. Guanine adducts were not detected by this method although literature data indicate their formation. Quantitatively, the assay responded linearly to adduct concentration, as shown in an experiment where BDE-modified skin DNA was serially diluted up to 81-fold with control DNA. The limit of detection was approximately 1 adduct in 10(8) normal nucleotides. Further, in an in vivo dosimetry study, skin DNA from groups of 8 individual mice treated with different doses of BDE (1.9, 5.7, 17, 51 and 153 mumol/mouse) for 3 days exhibited a linear relationship (r > or = 0.992) between adduct levels and dose. The results suggest that the 32P-postlabeling assay described herein will have utility in mechanistic studies and biomonitoring of DNA adduct formation from BDE and possibly other polar epoxides.

Effect of eugenol on the mutagenicity of benzo[a]pyrene and the formation of benzo[a]pyrene-DNA adducts in the lambda-lacZ-transgenic mouse

To study the possible reduction by eugenol of the mutagenicity and genotoxicity of benzo[a]pyrene (B[a]P) in vivo, the lambda-lacZ-transgenic mouse strain 40.6 (Muta Mouse) was used. Male mice were fed a diet containing 0.4% (w/w) eugenol or a control diet for 58 days. On day 10, half of the mice received an i.p. dose of 100 mg/kg b.w. B[a]P. The lacZ mutants were recovered by packaging of DNA isolated from liver into lambda phage, and expressed in E. coli C lacZ-recA-galE- bacteria. In both control mice and mice fed the eugenol diet, B[a]P treatment resulted in a similar, significant increase in lacZ mutant frequency. Eugenol was not mutagenic by itself. By 32P-postlabelling analysis of the liver DNA using an analysis method with chromatographic conditions for B[a]P-DNA adducts, no effect of eugenol on the formation of B[a]P-DNA adducts in the lambda-lacZ-transgenic mouse was found. By 32P-postlabelling analysis using an alkenylbenzene solvent system the amount of B[a]P-DNA adducts was lower in mice fed the eugenol diet than in mice fed the control diet but the decrease was not statistically significant. However, one spot indicative of an eugenol-associated DNA adduct was detected. The present data provide no evidence for antimutagenic or antigenotoxic potential of eugenol in vivo. Furthermore, they suggest genotoxicity in vivo of eugenol per se.

In vitro and in vivo (32)P-postlabeling analysis of 4-vinyl-1-cyclohexene (butadiene dimer) diepoxide-DNA adducts

4-Vinyl-1-cyclohexene diepoxide (VCD), and industrial chemical, and its parent compound, 4-vinyl-1-cyclohexene (VCH), are potential health hazards, a they destroy oocytes in follicles in rodents. VCD is also a skin carcinogen at the site of application in both female and male mice and rats and after gavage, induces ovarian tumors in mice and forestomach tumors in rats. A (32)P-postlabeling assay was developed for the detection and measurement of VCD-DNA adducts. VCD, a direct-acting carcinogen, was reacted with DNA in vitro, as well as through mouse skin painting for 3 days with different doses of VCD. (32)P-Labeled adducts were separated by polyethyleneimine (PEI)-cellulose TLC and detected by screen-enhanced autoradiography. Comparable adduct profiles were obtained in vitro and in vivo. At higher doses (36-225 micro mol/mouse), adduct levels in vivo showed a linear dose response, while there was no difference between 14 and 36 micro mol/mouse. The limit of detection was estimated to be 1-3 adducts in 10(8) DNA nucleotides. The results show that VCD exposure gives rise to (presumably pre-mutagenic) DNA adducts in vivo and that (32)P-postlabeling can be applied to biomonitoring of VCD exposure.

Postlabeling: a sensitive method for studying DNA adducts and their role in carcinogenesis

The covalent binding of xenobiotics to DNA is an important trigger of the multistage process that leads to carcinogenesis. 32P-postlabeling represents a highly sensitive method for biomonitoring exposure to genotoxic agents and for cancer risk assessment; it is capable of detecting less than one DNA adduct per human genome. Recent improvements to the technique have shown that the resistance of adducted DNA to enzyme digestion may lead to an overestimation of the number of different adducts present in a sample.

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.