7-Alkyldeoxyguanosine adduct detection by two-step HPLC and the 32P-postlabeling assay

Regio and stereospecific DNA adduct formation in mouse lung at N6 and N7 position of adenine and guanine after 1,3 butadiene inhalation exposure

Butadiene monoepoxide (BMO) alkylated guanine N7 and adenine N 6 adducts were prepared and enriched by solid phase extraction and HPLC. The purified adducts were analysed by a modified 32P-postlabelling assay, which utilized one dimensional TLC chromatography and a subsequent HPLC analysis with UV and radioactivity detectors. In vitro with Ct-DNA the formation of N7-dGMP and N 6-dAMP adducts were linear at a concentration range of 44 to 870 nmol of BMO per mg DNA at physiological pH. N7- dGMP and N 6-dAMP adducts were formed in a ratio of 200:1. In dGMP and in dAMP 48 % and 86 % of adducts were covalently bound to the C-2 carbon of BMO. CD-1 mice were inhalation exposed to butadiene for 5 days and 6 h per day. The N7-dGMP adduct level in lung samples of animals exposed to 200, 500 and 1300 ppm was 2.8 +/- 0.9 fmol, 11 +/- 2.0 fmol and 30 +/- 6.7 fmol in 10 mug DNA, respectively. The level of N 6-dAMP adducts in lung samples after 500 ppm and 1300 ppm exposure was 0.09 +/- 0.06 fmol and 0.11 +/- 0.05 fmol in 10 mug DNA. At 200 ppm the adduct level was below the detection limit. A sub-group of animals exposed to 1300 ppm was killed 3 weeks after the last exposure. N7-dGMP adducts were not detected but the level of N 6-dAMP adducts was not affected. N7-dGMP adducts were formed in a clear stereospecific manner in vivo. S -BMO adducts were the main product and represented 77 % (n = 4, SD = 2%) of total BMO adducts. No clear conclusion can be drawn about the enantiospecific DNA binding at the N 6 position of dAMP, because of the poor separation of the enantiomers. However, we could separate regioisomeric adducts which indicated that C-2 adducts represented 69 +/- 3 % of the total N 6 adducts formed in mice lung DNA. This observation is supported by the data derived from in vitro DNA experiments but is different to our previously published data, which indicates the 2:1 (C-1:C-2) ratio in regioisomer formation in nucleotides or nucleosides. We suggest that the data presented in this communication indicate a different mechanism between nucleotides and DNA in BMO-derived adduct formation- Dimroth rearrangement dominates in nucleotides, but in double stranded DNA a direct alkylation is probably the major mechanism of adduct formation.

DNA modification in carcinogen risk assessment in relation to diet: recent advances and some perspectives from a MAFF workshop

Food is one of the ultimate complex mixtures to which man is exposed and which cannot easily be dispensed with. Apart from certain well studied microcomponents for example, food pyrolysates, Sugimura 1990 human exposure to genotoxic agents arising from macrocomponents has been relatively little studied from the standpoint of DNA damage. The results of epidemiological studies into the relationship between diet and cancer have left many researchers with the impression that it is an intrinsically intractable problem which is perhaps best left well alone. However, given the popular conception that the normal human diet is safe and that such risks as there may be are due to contamination by pesticide and other chemical residues, there is clearly a need to evaluate the possible avenues open to investigators and which are likely to yield meaningful results which would enable scientifically based advice to be given to the public as to the best dietary habits. This overview of the current state of methodology for measurement of DNA damage in relation to diet as well as a summary of current MAFF supported work and future prospects in this area arose out of a workshop entitled DNA modification in carcinogen risk assessment held in London on November 18, 1996 . The object of this report is to summarize the results presented at the workshop and also to indicate the significance of the MAFF funded programme within the broader context of recently published studies from the international scientific community. Hence, a comprehensive review of all aspects of diet related DNA damage is beyond the scope of this article. The workshop was organized as part of the MAFF Risk Assessment Research Programme and contributes to an interdepartmental initiative, the Government Research Councils Initiative on Risk Assessment and Toxicology RATSC which aims to bring together regulators and toxicologists to discuss their respective perspectives on current problems in the risk assessment of chemicals. Further aims of RATSC are to identify subjects for future detailed workshops on specific issues and to identify priorities for research into toxic chemical risk assessment. The membership of RATSC is drawn from a wide range of Government Departments and Research Councils and is chaired by Dr David Shannon MAFF Chief Scientist.

Salvage of oxidized guanine derivatives in the (2'-deoxy)ribonucleotide pool as source of mutations in DNA

Recent evidence suggests that salvage of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) and 8-oxo-7,8-dihydro-guanine (8-oxoGua) can contribute substantially to levels of 8-oxoGua in DNA and RNA. However, it remains to be determined if this mechanism contributes to mutagenesis and disease. This review covers the predominant methods for detecting 8-oxoGua and its derivatives, summarizes some of the relevant recent DNA repair studies and discusses the mechanisms for metabolism of oxidized guanine derivatives in the (2'-deoxy)ribonucleoside and (2'-deoxy)ribonucleotide pools.

The formation and biological significance of N7-guanine adducts

DNA alkylation or adduct formation occurs at nucleophilic sites in DNA, mainly the N7-position of guanine. Ever since identification of the first N7-guanine adduct, several hundred studies on DNA adducts have been reported. Major issues addressed include the relationships between N7-guanine adducts and exposure, mutagenesis, and other biological endpoints. It became quickly apparent that N7-guanine adducts are frequently formed, but may have minimal biological relevance, since they are chemically unstable and do not participate in Watson Crick base pairing. However, N7-guanine adducts have been shown to be excellent biomarkers for internal exposure to direct acting and metabolically activated carcinogens. Questions arise, however, regarding the biological significance of N7-guanine adducts that are readily formed, do not persist, and are not likely to be mutagenic. Thus, we set out to review the current literature to evaluate their formation and the mechanistic evidence for the involvement of N7-guanine adducts in mutagenesis or other biological processes. It was concluded that there is insufficient evidence that N7-guanine adducts can be used beyond confirmation of exposure to the target tissue and demonstration of the molecular dose. There is little to no evidence that N7-guanine adducts or their depurination product, apurinic sites, are the cause of mutations in cells and tissues, since increases in AP sites have not been shown unless toxicity is extant. However, more research is needed to define the extent of chemical depurination versus removal by DNA repair proteins. Interestingly, N7-guanine adducts are clearly present as endogenous background adducts and the endogenous background amounts appear to increase with age. Furthermore, the N7-guanine adducts have been shown to convert to ring opened lesions (FAPy), which are much more persistent and have higher mutagenic potency. Studies in humans are limited in sample size and differences between controls and study groups are small. Future investigations should involve human studies with larger numbers of individuals and analysis should include the corresponding ring opened FAPy derivatives.

Validation of biomarkers for the study of environmental carcinogens: a review

There is a need for validation of biomarkers. Our aim is to review published work on the validation of selected biomarkers: bulky DNA adducts, N-nitroso compounds, 1-hydroxypyrene, and oxidative damage to DNA. A systematic literature search in PubMed was performed. Information on the variability and reliability of the laboratory tests used for biomarkers measurements was collected. For the evaluation of the evidence on validation we referred to the ACCE criteria. Little is known about intraindividual variation of DNA adduct measurements, but measurements have a good repeatability irrespective of the technique used for their identification; reproducibility improved after the correction for a laboratory factor. A high-sensitivity method is available for the measurement of 1-hydroxypyrene in urine. There is consensus on validation of biomarkers of oxidative damage DNA based on the comet assay and chromatographic measurement in blood while urinary measurements by chromatographic assays are well validated, and ELISA-based assays appear to lack specificity. Immunoassays for the quantification of adducts of N-nitroso compounds are useful for large epidemiological studies, given their sensitivity, the small amount of DNA required and their potential for rapid and high-throughput analysis.

Liquid chromatography-electrospray ionization tandem mass spectrometry analysis of 7-ethylguanine in human liver DNA

Considerable evidence in the literature demonstrates the exposure of humans to an unknown ethylating agent. Previous studies have demonstrated the presence of 7-ethyl-Gua and 3-ethyl-Ade in urine, 7-ethyl-dGuo and O4-ethyl-dThd in human lung, and ethylvaline in hemoglobin. Some studies also report higher levels of ethyl adducts in smokers than in nonsmokers, and there is convincing evidence for an uncharacterized ethylating agent in cigarette smoke. To further investigate this question, we have developed a liquid chromatography-electrospray ionization tandem mass spectrometry-selected reaction monitoring method for analysis of 7-ethyl-Gua in human liver DNA. To our knowledge, there are no previous reports of MS analyses of 7-ethyl-Gua in human tissues. [15N 5]7-Ethyl-Gua was synthesized and used as the internal standard. Human liver DNA was heated to release 7-ethyl-Gua. After partial purification by solid-phase extraction, analysis was carried out using the transition m/z 180 [M+H]+-->m/z 152 [Gua+H]+ for 7-ethyl-Gua and m/z 185-->m/z 157 for the internal standard. The method was accurate and precise. The detection limit was approximately 8-9 fmol/micromol Gua, starting with 1-2 mg of DNA. Clear coeluting peaks for 7-ethyl-Gua and the internal standard were observed in the human liver DNA samples. Twenty-six human liver DNA samples (0.77+/-0.40 mg) were analyzed, and 25 were positive for 7-ethyl-Gua. The mean level of 7-ethyl-Gua was 42.2+/-43.0 fmol/micromol Gua (8.4+/-8.6 adducts per 10(9) nucleotides). These results demonstrate that 7-ethyl-Gua is a common DNA adduct in human liver with likely endogenous sources that require further investigation.

Lung cancer risk and variation in MGMT activity and sequence

O(6)-Alkylguanine-DNA alkyltransferase (MGMT) repairs DNA adducts that result from alkylation at the O(6) position of guanine. These lesions are mutagenic and toxic and can be produced by a variety of agents including the tobacco-specific nitrosamines, carcinogens present in cigarette smoke. Here, we review some of our work in the context of inter-individual differences in MGMT expression and their potential influence on lung cancer risk. In humans there are marked inter-individual differences in not only levels of DNA damage in the lung (N7-methylguanine) that can arise from exposure to methylating agents but also in MGMT activity in lung tissues. In the presence of such exposure, this variability in MGMT activity may alter cancer susceptibility, particularly as animal models have demonstrated that the complete absence of MGMT activity predisposes to alkylating-agent induced cancer while overexpression is protective. Recent studies have uncovered a series of polymorphisms that affect protein activity or are associated with differences in expression levels. The associations between these (and other) polymorphisms and cancer risk are inconsistent, possibly because of small sample sizes and inter-study differences in lung cancer histology. We have recently analysed a consecutive series of case-control studies and found evidence that lung cancer risk was lower in subjects with the R178 allele.

Simultaneous determination of N7-alkylguanines in DNA by isotope-dilution LC-tandem MS coupled with automated solid-phase extraction and its application to a small fish model

In the present study, we report the development of a sensitive and selective assay based on LC (liquid chromatography)-MS/MS (tandem MS) to simultaneously measure N7-MeG (N7-methylguanine) and N7-EtG (N7-ethylguanine) in DNA hydrolysates. With the use of isotope internal standards (15N5-N7-MeG and 15N5-N7-EtG) and on-line SPE (solid-phase extraction), the detection limit of this method was estimated as 0.42 fmol and 0.17 fmol for N7-MeG and N7-EtG respectively. The high sensitivity achieved here makes this method applicable to small experimental animals. This method was applied to measure N7-alkylguanines in liver DNA from mosquito fish (Gambusia affinis) that were exposed to NDMA (N-nitrosodimethylamine) and NDEA (N-nitrosodiethylamine) alone or their combination over a wide range of concentrations (1-100 mg/l). Results showed that the background level of N7-MeG in liver of control fish was 7.89+/-1.38 mmol/mol of guanine, while N7-EtG was detectable in most of the control fish with a range of 0.05-0.19 mmol/mol of guanine. N7-MeG and N7-EtG were significantly induced by NDMA and NDEA respectively, at a concentration as low as 1 mg/l and increased in a dose-dependent manner. Taken together, this LC-MS/MS assay provides the sensitivity and high throughput required to evaluate the extent of alkylated DNA lesions in small animal models of cancer induced by alkylating agents.

Liquid chromatography-electrospray ionization-mass spectrometry: the future of DNA adduct detection

Over the past 40 years considerable emphasis has been placed on the development of accurate and sensitive methods for the detection and quantitation of DNA adducts. The formation of DNA adducts resulting from the covalent interaction of genotoxic carcinogens with DNA, derived from exogenous and endogenous sources, either directly or following metabolic activation, can if not repaired lead to mutations in critical genes such as those involved in the regulation of cellular growth and subsequent development of cancer. The major analytical challenge has been to detect levels of DNA adducts at the level of 0.1-1 adducts per 10(8) unmodified DNA bases using only low microgram amounts of DNA, and with high specificity and accuracy, in humans exposed to genotoxic carcinogens derived from occupational, environmental, dietary and life-style sources. In this review we will highlight the merits as well as discuss the progress made by liquid chromatography coupled to electrospray ionization mass spectrometry as a method for DNA adduct detection.

Quantitative determination of urinary N7-ethylguanine in smokers and non-smokers using an isotope dilution liquid chromatography/tandem mass spectrometry with on-line analyte enrichment

Previous studies demonstrated the presence of unknown direct-acting ethylating agents arising from cigarette smoke. We hypothesized that such agents would also lead to ethylation of guanine in DNA followed by depurination/repair and excretion of N7-ethylguanine (N7-EtG) in urine. In this study, a highly specific and sensitive liquid chromatographic/tandem mass spectrometric (LC/MS/MS) method was firstly developed for measuring urinary N7-EtG. With the use of an isotope internal standard (15N5-N7-EtG) and on-line enrichment techniques, the detection limit of this method was estimated as 0.59 pg/ml (0.33 pmol) on-column. This method was then applied to measure urinary samples obtained from 35 non-smokers and 32 smokers with dietary control. The results showed that the mean urinary levels of N7-EtG were 85.5+/-105 and 28.1+/-19.4 pg/mg creatinine for smokers and non-smokers, respectively. Smokers had about three times higher level of N7-EtG than non-smokers (P<0.005). It was further noted that the urinary level of N7-EtG was significantly associated with cotinine for smokers (r=0.49, P<0.005). Taken together, this is the first study that demonstrated the presence of N7-EtG in urine, and that cigarette smoke was highly responsible for the increased urinary excretion of N7-EtG. This non-invasive measurement of urinary N7-EtG would be useful for the surveillance of ethylating agent exposure and its associated cancer risk in the future.

Associations between smoking, GST genotypes and N7-methylguanine levels in DNA extracted from bronchial lavage cells

N7-Methylguanine (N7-MeG) DNA adducts are markers of human exposure to methylating agents including tobacco-specific nitrosamines (TSNAs). Repair of this adduct is poor, so levels in lung tissue should reflect variation in both intensity of exposure and in metabolism. N7-MeG adducts in lung DNA from bronchial lavage samples were measured to determine whether levels were higher in smokers than non-smokers, and if levels were modified by genetic variation in carcinogen-metabolising enzymes. Adducts were detected in 38 out of 44 DNA samples by 32P post-labelling of the N7-methyldeoxyguanosine-3'-monophosphate (N7-MedGp) isolated from DNA digests by two-stage HPLC. N7-MeG adduct levels were higher in smokers than in never smokers ((9.99 +/-20.3)x10(-7) versus (0.58+/-0.50)x10(-7) N7-MedGp/deoxyguanosine-3'-monophosphate (dGp); P=0.02) and intermediate in ex-smokers ((5.59+/-15.6)x10(-7) N7-MedGp/dGp). Adduct levels tended to be higher in individuals with GSTM1 null, GSTT1 null or GSTP1 ile/ile genotypes. When genotypes were combined, N7-MedGp levels among GSTM1 null/GSTT1 null individuals (n=6) were higher than among those having at least one wild-type allele of these two genes ((26.1+/-38.0)x10(-7) versus (2.73+/-4.07)x10(-7) N7-MedGp/dGp), although the results were not statistically significant (P=0.13). Adduct levels were highest in individuals with three unfavourable genotypes (GSTM1 null/GSTT1 null and GSTP1 ile/ile) compared with others ((74.5+/-13.1)x10(-7) versus (2.64+/-3.89)x10(-7) N7-MedGp/dGp, P=0.02). N7-MeG adduct levels in DNA isolated from lung tissue thus reflect exposure to cigarette smoke, and genetic variation in carcinogen-metabolising enzymes may modify these levels.

Evaluation of fecal mutagenicity and colorectal cancer risk

Colorectal cancer is one of the most common internal malignancies in Western society. The cause of this disease appears to be multifactorial and involves genetic as well as environmental aspects. The human colon is continuously exposed to a complex mixture of compounds, which is either of direct dietary origin or the result of digestive, microbial and excretory processes. In order to establish the mutagenic burden of the colorectal mucosa, analysis of specific compounds in feces is usually preferred. Alternatively, the mutagenic potency of fecal extracts has been determined, but the interpretation of these more integrative measurements is hampered by methodological shortcomings. In this review, we focus on exposure of the large bowel to five different classes of fecal mutagens that have previously been related to colorectal cancer risk. These include heterocyclic aromatic amines (HCA) and polycyclic aromatic hydrocarbons (PAH), two exogenous factors that are predominantly ingested as pyrolysis products present in food and (partially) excreted in the feces. Additionally, we discuss N-nitroso-compounds, fecapentaenes and bile acids, all fecal constituents (mainly) of endogenous origin. The mutagenic and carcinogenic potency of the above mentioned compounds as well as their presence in feces, proposed mode of action and potential role in the initiation and promotion of human colorectal cancer are discussed. The combined results from in vitro and in vivo research unequivocally demonstrate that these classes of compounds comprise potent mutagens that induce many different forms of genetic damage and that particularly bile acids and fecapentaenes may also affect the carcinogenic process by epigenetic mechanisms. Large inter-individual differences in levels of exposures have been reported, including those in a range where considerable genetic damage can be expected based on evidence from animal studies. Particularly, however, exposure profiles of PAH and N-nitroso compounds (NOC) have to be more accurately established to come to a risk evaluation. Moreover, lack of human studies and inconsistency between epidemiological data make it impossible to describe colorectal cancer risk as a result of specific exposures in quantitative terms, or even to indicate the relative importance of the mutagens discussed. Particularly, the polymorphisms of genes involved in the metabolism of heterocyclic amines are important determinants of carcinogenic risk. However, the present knowledge of gene-environment interactions with regard to colorectal cancer risk is rather limited. We expect that the introduction of DNA chip technology in colorectal cancer epidemiology will offer new opportunities to identify combinations of exposures and genetic polymorphisms that relate to increased cancer risk. This knowledge will enable us to improve epidemiological study design and statistical power in future research.

Tobacco smoke carcinogens and lung cancer

The complexity of tobacco smoke leads to some confusion about the mechanisms by which it causes lung cancer. Among the multiple components of tobacco smoke, 20 carcinogens convincingly cause lung tumors in laboratory animals or humans and are, therefore, likely to be involved in lung cancer induction. Of these, polycyclic aromatic hydrocarbons and the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone are likely to play major roles. This review focuses on carcinogens in tobacco smoke as a means of simplifying and clarifying the relevant information that provides a mechanistic framework linking nicotine addiction with lung cancer through exposure to such compounds. Included is a discussion of the mechanisms by which tobacco smoke carcinogens interact with DNA and cause genetic changes--mechanisms that are reasonably well understood--and the less well defined relationship between exposure to specific tobacco smoke carcinogens and mutations in oncogenes and tumor suppressor genes. Molecular epidemiologic studies of gene-carcinogen interactions and lung cancer--an approach that has not yet reached its full potential--are also discussed, as are inhalation studies of tobacco smoke in laboratory animals and the potential role of free radicals and oxidative damage in tobacco-associated carcinogenesis. By focusing in this review on several important carcinogens in tobacco smoke, the complexities in understanding tobacco-induced cancer can be reduced, and new approaches for lung cancer prevention can be envisioned.

Endogenous and background DNA adducts by methylating and 2-hydroxyethylating agents

Detection of 7-alkylguanine DNA adducts is useful to assess human exposure to and the resulting DNA damage caused by simple alkylating agents. The background 7-methylguanine (7-MG) and 7-hydroxyethylguanine (7-HEG) adduct levels were determined in human and rat tissues, using thin-layer chromatography (TLC) combined with high pressure liquid chromatography (HPLC). In addition, these two adduct levels were also compared in various tissues between smokers and non-smokers. The results demonstrated that the background level of 7-alkylguanine adducts in WBC and lung tissues of non-smokers was 2.9 and 4.0 adducts/107 nucleotides, respectively. In smokers with lung cancers 7-MG adduct level in lung samples (6.3+/-1.9 adducts/107 nucleotides) and in bronchus samples (6.1+/-1.5 adducts/107 nucleotides) was significantly higher than that in WBC samples (3.3+/-0.9 adducts/107 nucleotides). 7-HEG adduct levels obtained from the same individuals were 0.8+/-0.3 in lung, 1.0+/-0.8 in bronchus and 0.6+/-0.2 adducts/107 nucleotides in WBC, respectively. Animal studies showed that background levels of 7-MG (2.1-2.5 adducts/107 nucleotides) in control rats were approximately 2-4-fold higher than 7-HEG levels (0.6-0.9 adducts/107 nucleotides). After a 3-day exposure to 300 ppm ethene, 7-HEG adducts accumulated to a similar extent in different tissues of rats, with the mean adduct level of 5.6-7.0 in liver, 7.4 in lymphocytes and 5.5 adducts/107 nucleotides in kidney.

DNA adduct formation from tobacco-specific N-nitrosamines

Tobacco-specific N-nitrosamines are a group of carcinogens derived from the tobacco alkaloids. They are likely causative factors for cancers of the lung, esophagus, pancreas, and oral cavity in people who use tobacco products. The most carcinogenic tobacco-specific nitrosamines in laboratory animals are 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), and N'-nitrosonornicotine (NNN). DNA adduct formation from NNK and NNN has been studied extensively and is reviewed here. NNK is metabolically activated by cytochromes P450 to intermediates which methylate and pyridyloxobutylate DNA. The resulting adducts have been detected in cells and tissues susceptible to NNK carcinogenesis in rodents. The methylation and pyridyloxobutylation pathways are both important in carcinogenesis by NNK. NNK also induces single strand breaks and increases levels of 8-oxodeoxyguanosine in DNA of treated animals. NNAL, which like NNK is a potent pulmonary carcinogen, is also metabolically activated to methylating and pyridyloxobutylating intermediates. NNN pyridyloxobutylates DNA in its rat target tissues, esophagus and nasal mucosa. Methyl and pyridyloxobutyl DNA adducts are detected in human tissues. The methyl adducts most likely result in part from exposure of smokers to NNK, but these adducts are also detected in non-smokers. Some of the methyl adducts detected in non-smokers may be due to environmental tobacco smoke exposure. There are also potential dietary and endogenous sources of these adducts. Pyridyloxobutyl DNA adducts in human tissues result mainly from exposure to tobacco-specific N-nitrosamines. In laboratory animals, DNA adduct formation and carcinogenicity of tobacco-specific N-nitrosamines are closely correlated in many instances, and it is likely that similar relationships will hold in humans.

Tissue distribution of DNA adducts in male Fischer rats exposed to 500 ppm of propylene oxide: quantitative analysis of 7-(2-hydroxypropyl)guanine by 32P-postlabelling

7-(2-Hydroxypropyl)guanine (7-HPG) constitutes the major adduct from alkylation of DNA by the genotoxic carcinogen, propylene oxide. The levels of 7-HPG in DNA of various organs provides a relevant measure of tissue dose. 7-Alkylguanines can induce mutation through abasic sites formed from spontaneous depurination of the adduct. In the current study the formation of 7-HPG was investigated in male Fisher 344 rats exposed to 500 ppm of propylene oxide by inhalation for 6 h/day, 5 days/week, for up to 20 days. 7-HPG was analyzed using the 32P-postlabelling assay with anion-exchange cartridges for adduct enrichment. In animals sacrificed directly following 20 days of exposure, the adduct level was highest in the respiratory nasal epithelium (98.1 adducts per 10(6) nucleotides), followed by olfactory nasal epithelium (58.5), lung (16.3), lymphocytes (9.92), spleen (9.26), liver (4.64), and testis (2.95). The nasal cavity is the major target for tumor induction in the rat following inhalation. This finding is consistent with the major difference in adduct levels observed in nasal epithelium compared to other tissues. In rats sacrificed 3 days after cessation of exposure, the levels of 7-HPG in the aforementioned tissues had, on the average, decreased by about one-quarter of their initial concentrations. This degree of loss closely corresponds to the spontaneous rate of depurination for this adduct (t 1/2 = 120 h), and suggests a low efficiency of repair for 7-HPG in the rat. The postlabelling assay used had a detection limit of one to two adducts per 10(8) nucleotides, i.e. it is likely that this adduct could be analyzed in nasal tissues of rats exposed to less than 1 ppm of propylene oxide.

Methods for predicting carcinogenic hazards: new opportunities coming from recent developments in molecular oncology and SAR studies

Without epidemiological evidence, and prior to either short-term tests of genotoxicity or long-term tests of carcinogenicity in rodents, an initial level of information about the carcinogenic hazard of a chemical that perhaps has been designed on paper, but never synthesized, can be provided by structure-activity relationship (SAR) studies. Herein, we have reviewed the interesting strategies developed by human experts and/or computerized approaches for the identification of structural alerts that can denote the possible presence of a carcinogenic hazard in a novel molecule. At a higher level of information, immediately below epidemiological evidence, we have discussed carcinogenicity experiments performed in new types of genetically engineered small rodents. If a dominant oncogene is already mutated, or if an allele of a recessive oncogene is inactivated, we have a model animal with (n-1) stages in the process of carcinogenesis. Both genotoxic and receptor-mediated carcinogens can induce cancers in 20-40% of the time required for classical murine strains. We have described the first interesting results obtained using these new artificial animal models for carcinogenicity studies. We have also briefly discussed other types of engineered mice (lac operon transgenic mice) that are especially suitable for detecting mutagenic effects in a broad spectrum of organs and tissues and that can help to establish mechanistic correlations between mutations and cancer frequencies in specific target organs. Finally, we have reviewed two complementary methods that, while obviously also feasible in rodents, are especially suitable for biomonitoring studies. We have illustrated some of the advantages and drawbacks related to the detection of DNA adducts in target and surrogate tissues using the 32P-DNA postlabeling technique, and we have discussed the possibility of biomonitoring mutations in different human target organs using a molecular technique that combines the activity of restriction enzymes with polymerase chain reaction (RFLP/PCR). Prediction of carcinogenic hazard and biomonitoring are very wide-ranging areas of investigation. We have therefore selected five different subfields for which we felt that interesting innovations have been introduced in the last few years. We have made no attempt to systematically cover the entire area: such an endeavor would have produced a book instead of a review article.

Measurement of DNA adducts in humans after complex mixture exposure

In contrast to acute or chronic dosing experiments with a single chemical in animals, man is exposed to thousands of chemicals during a lifetime. Each of these may act alone, additively, synergistically or antagonistically in terms of biological effects, but most current risk assessment procedures fail to recognize such interactions. In carcinogenesis, a mutational process that is thought to occur through DNA damage by endogenous and/or exogenous agents, a wide variety of host factors is involved in disease outcome. These include absorption of chemicals, their distribution, metabolism and excretion. In addition, once metabolic activation has occurred, there is an array of protective mechanisms that cells have evolved to maintain DNA integrity, such as DNA repair, genetic redundancy and programmed cell death. One approach to risk assessment is to regard all DNA-damaging events as potentially leading to cancer and to measure DNA damage as the biologically relevant endpoint. The main method, if not the only method, presently available to assay a wide range of DNA adducts is 32P-postlabelling. This method has high sensitivity (limit of detection > 1 adduct per 10(10) nucleotides) and is capable of visualizing many different DNA adducts in a single analysis. Postlabelling is best suited for detecting hydrophobic adducts--low molecular weight adducts usually need a preliminary separation procedure prior to being postlabelled. This chromatographic procedure has been used to study DNA samples from human tissues of cigarette smokers, occupationally exposed groups and individuals living in polluted environments. Correlations have been found between the severity of exposure and the level of DNA adducts detected for human samples. However, most studies are single-time point studies, whereas for risk assessment purposes it may be better to use more quantitative and representative measures of long-term exposure, for example the number of adducts formed per annum. This article reviews methods of DNA adduct measurement, with particular reference to the 32P-postlabelling technique, which has been used to determine DNA adduct levels in populations exposed to complex mixtures.

DNA and protein adducts

Application of methods for the measurement of DNA and protein adducts in environmental studies was surveyed. The methods included the 32P-postlabelling assay, immunoassay and synchronous fluorescence spectroscopy for DNA adducts. Additionally, methods for detecting excreted urinary RNA and DNA adducts were discussed. The protein adduct techniques included both immunological and chemical assays. The techniques have been applied in occupational and environmental studies, but usually one assay at a time. As specific DNA adducts can now be assayed for, it would be important to use these methods and specific protein adduct assays in the same studies. It is important to develop further specific adduct tests. This can be done with the help of standard compounds, which also allow quantitation in the assays. An international bank of standard compounds would be a major advancement to human biomonitoring.

A 32P-postlabeling method for detecting unstable N-7-substituted deoxyguanosine adducts in DNA

Many antitumor agents, including the mustards, form N-7 deoxyguanosine adducts in DNA that are difficult to quantitate by the 32P-postlabeling procedure because of their instability. We have developed a method that is successful for the analysis of such adducts using, as a prototype mustard, 14C-labeled bis(2-chloroethyl)sulfide. This agent forms the unstable product 7-hydroxyethylthioethyldeoxyguanosine in DNA. By performing enzymatic digestions to 3'-deoxynucleotides at 10 degrees C, including a second N-7-substituted guanine deoxynucleotide as an internal standard, removing most of the unmodified nucleotides and [32P]ATP on disposable anion columns, and measuring the labeled products after separation on a C18 column, we are able to detect 1 unstable N-7 deoxyguanosine adduct in 10(7) normal nucleotides with good precision.

7-Methyldeoxyguanosine as a marker of exposure to environmental methylating agents

7-Methyldeoxyguanosine (7-medG) may be a useful marker of human exposure to methylating agents at the individual level. Methods of detection, including 32P-postlabeling, HPLC with electrochemical or fluorescence detection, and immunoassays, are critically summarized. These methods have been applied to the analysis of 7-medG in human samples, resulting from environmental exposure or endogenous processes. Detectable levels of adduct have been demonstrated in several tissues (liver, lung, bronchial, pancreas) and peripheral white blood cells. Studies are in progress to verify the possible use of white blood cells as a surrogate for internal organs.

Adduct detection by acylation with [35S]methionine: analysis of DNA adducts of 4-aminobiphenyl

Reaction of synthetic N-(2'-deoxyguanosin-8-yl)-4-aminobiphenyl (dGuo-8-ABP) with t-butoxycarbonyl-L-[35S]methionine, N-hydroxysuccinimidyl ester (35S-labeled TBM-NHS), under optimized conditions produced mono-, bis-, and tris-TBM-acylated nucleosides that were separable by HPLC. Reaction of different amounts of N-(2'-deoxy-1',2'-[3H]guanosin-8-yl)-4-aminobiphenyl ([3H]dGuo-8-ABP) with 35S-labeled TBM-NHS established that total 35S content of acylated products was linearly related to adduct concentration (r = 0.992) over the range of 10 fmol to 30.6 pmol. Additionally, the N-(deoxyguanosin-8-yl)-4-[3H]aminobiphenyl (dGuo-8-[3H]ABP) adduct was isolated from calf thymus DNA adducted in vitro and from rat liver DNA adducted in vivo and similarly reacted with 35S-labeled TBM-NHS. Acylation products of dGuo-8-ABP from all three sources showed HPLC retention times identical to those of authentic TBM-dGuo-8-ABP, and 35S incorporation into acylated products was linearly related to amount of adduct reacted. These results indicate that the procedure, to which we have referred as adduct detection by acylation with methionine (ADAM), has potential applicability as an analytical procedure for detection and quantification of DNA adducts in human tissues in the molecular epidemiology of cancer.