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

Metabolic Activation and DNA Interactions of Carcinogenic N-Nitrosamines to Which Humans Are Commonly Exposed

Carcinogenic N-nitrosamine contamination in certain drugs has recently caused great concern and the attention of regulatory agencies. These carcinogens-widely detectable in relatively low levels in food, water, cosmetics, and drugs-are well-established and powerful animal carcinogens. The electrophiles resulting from the cytochrome P450-mediated metabolism of N-nitrosamines can readily react with DNA and form covalent addition products (DNA adducts) that play a central role in carcinogenesis if not repaired. In this review, we aim to provide a comprehensive and updated review of progress on the metabolic activation and DNA interactions of 10 carcinogenic N-nitrosamines to which humans are commonly exposed. Certain DNA adducts such as O⁶-methylguanine with established miscoding properties play central roles in the cancer induction process, whereas others have been linked to the high incidence of certain types of cancers. We hope the data summarized here will help researchers gain a better understanding of the bioactivation and DNA interactions of these 10 carcinogenic N-nitrosamines and facilitate further research on their toxicologic and carcinogenic properties.

Mode of action-based risk assessment of genotoxic carcinogens

The risk assessment of chemical carcinogens is one major task in toxicology. Even though exposure has been mitigated effectively during the last decades, low levels of carcinogenic substances in food and at the workplace are still present and often not completely avoidable. The distinction between genotoxic and non-genotoxic carcinogens has traditionally been regarded as particularly relevant for risk assessment, with the assumption of the existence of no-effect concentrations (threshold levels) in case of the latter group. In contrast, genotoxic carcinogens, their metabolic precursors and DNA reactive metabolites are considered to represent risk factors at all concentrations since even one or a few DNA lesions may in principle result in mutations and, thus, increase tumour risk. Within the current document, an updated risk evaluation for genotoxic carcinogens is proposed, based on mechanistic knowledge regarding the substance (group) under investigation, and taking into account recent improvements in analytical techniques used to quantify DNA lesions and mutations as well as "omics" approaches. Furthermore, wherever possible and appropriate, special attention is given to the integration of background levels of the same or comparable DNA lesions. Within part A, fundamental considerations highlight the terms hazard and risk with respect to DNA reactivity of genotoxic agents, as compared to non-genotoxic agents. Also, current methodologies used in genetic toxicology as well as in dosimetry of exposure are described. Special focus is given on the elucidation of modes of action (MOA) and on the relation between DNA damage and cancer risk. Part B addresses specific examples of genotoxic carcinogens, including those humans are exposed to exogenously and endogenously, such as formaldehyde, acetaldehyde and the corresponding alcohols as well as some alkylating agents, ethylene oxide, and acrylamide, but also examples resulting from exogenous sources like aflatoxin B1, allylalkoxybenzenes, 2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline (MeIQx), benzo[a]pyrene and pyrrolizidine alkaloids. Additionally, special attention is given to some carcinogenic metal compounds, which are considered indirect genotoxins, by accelerating mutagenicity via interactions with the cellular response to DNA damage even at low exposure conditions. Part C finally encompasses conclusions and perspectives, suggesting a refined strategy for the assessment of the carcinogenic risk associated with an exposure to genotoxic compounds and addressing research needs.

Mechanism of DNA alkylation-induced transcriptional stalling, lesion bypass, and mutagenesis

Alkylated DNA lesions, induced by both exogenous chemical agents and endogenous metabolites, interfere with the efficiency and accuracy of DNA replication and transcription. However, the molecular mechanisms of DNA alkylation-induced transcriptional stalling and mutagenesis remain unknown. In this study, we systematically investigated how RNA polymerase II (pol II) recognizes and bypasses regioisomeric O²-, N3-, and O⁴-ethylthymidine (O²-, N3-, and O⁴-EtdT) lesions. We observed distinct pol II stalling profiles for the three regioisomeric EtdT lesions. Intriguingly, pol II stalling at O²-EtdT and N3-EtdT sites is exacerbated by TFIIS-stimulated proofreading activity. Assessment for the impact of the EtdT lesions on individual fidelity checkpoints provided further mechanistic insights, where the transcriptional lesion bypass routes for the three EtdT lesions are controlled by distinct fidelity checkpoints. The error-free transcriptional lesion bypass route is strongly favored for the minor-groove O²-EtdT lesion. In contrast, a dominant error-prone route stemming from GMP misincorporation was observed for the major-groove O⁴-EtdT lesion. For the N3-EtdT lesion that disrupts base pairing, multiple transcriptional lesion bypass routes were found. Importantly, the results from the present in vitro transcriptional studies are well correlated with in vivo transcriptional mutagenesis analysis. Finally, we identified a minor-groove-sensing motif from pol II (termed Pro-Gate loop). The Pro-Gate loop faces toward the minor groove of RNA:DNA hybrid and is involved in modulating the translocation of minor-groove alkylated DNA template after nucleotide incorporation opposite the lesion. Taken together, this work provides important mechanistic insights into transcriptional stalling, lesion bypass, and mutagenesis of alkylated DNA lesions.

Transcriptional bypass of regioisomeric ethylated thymidine lesions by T7 RNA polymerase and human RNA polymerase II

Alkylative damage to DNA can be induced by environmental chemicals, endogenous metabolites and some commonly prescribed chemotherapeutic agents. The regioisomeric N3-, O(2)- and O(4)-ethylthymidine (N3-, O(2)- and O(4)-EtdT, respectively) represent an important class of ethylated DNA lesions. Using nonreplicative double-stranded vectors containing an N3-EtdT, O(2)-EtdT or O(4)-EtdT at a defined site in the template strand, herein we examined the effects of these lesions on DNA transcription mediated by single-subunit T7 RNA polymerase or multisubunit human RNA polymerase II in vitro and in human cells. We found that O(4)-EtdT is highly mutagenic and exclusively induces the misincorporation of guanine opposite the lesion, whereas N3-EtdT and O(2)-EtdT display promiscuous miscoding properties during transcription. In addition, N3-EtdT and O(2)-EtdT were found to inhibit strongly DNA transcription in vitro and in certain human cells. Moreover, N3-EtdT, but not O(2)-EtdT or O(4)-EtdT, is an efficient substrate for transcription-coupled nucleotide excision repair. These findings provide new important insights into how these alkylated DNA lesions compromise the flow of genetic information, which may help to understand the risk of these lesions in living cells.

Recent developments in DNA adduct analysis by mass spectrometry: a tool for exposure biomonitoring and identification of hazard for environmental pollutants

DNA adducts represent an important category of biomarkers for detection and exposure surveillance of potential carcinogenic and genotoxic chemicals in the environment. Sensitive and specific analytical methods are required to detect and differentiate low levels of adducts from native DNA from in vivo exposure. In addition to biomonitoring of environmental pollutants, analytical methods have been developed for structural identification of adducts which provides fundamental information for determining the toxic pathway of hazardous chemicals. In order to achieve the required sensitivity, mass spectrometry has been increasingly utilized to quantify adducts at low levels as well as to obtain structural information. Furthermore, separation techniques such as chromatography and capillary electrophoresis can be coupled to mass spectrometry to increase the selectivity. This review will provide an overview of advances in detection of adducted and modified DNA by mass spectrometry with a focus on the analysis of nucleosides since 2007. Instrument advances, sample and instrument considerations, and recent applications will be summarized in the context of hazard assessment. Finally, advances in biomonitoring applying mass spectrometry will be highlighted. Most importantly, the usefulness of DNA adducts measurement and detection will be comprehensively discussed as a tool for assessment of in vitro and in vivo exposure to environmental pollutants.

Liver tumor promotion by 2,3,7,8-tetrachlorodibenzo-p-dioxin is dependent on the aryl hydrocarbon receptor and TNF/IL-1 receptors

We set out to better understand the signal transduction pathways that mediate liver tumor promotion by 2,3,7,8-tetrachlorodibenzo-p-dioxn ("dioxin"). To this end, we first employed congenic mice homozygous for either the Ahr(b1) or Ahr(d) alleles (encoding an aryl hydrocarbon receptor (AHR) with high or low binding affinity for dioxin, respectively) and demonstrated that hepatocellular tumor promotion in response to dioxin segregated with the Ahr locus. Once we had genetic evidence for the importance of AHR signaling, we then asked if tumor promotion by dioxin was influenced by "interleukin-1 (IL-1)-like" inflammatory cytokines. The importance of this question arose from our earlier observation that aspects of the acute hepatocellular toxicity of dioxin are dependent upon IL1-like cytokine signaling. To address this issue, we employed a triple knock-out (TKO) mouse model with null alleles at the loci encoding the three relevant receptors for tumor necrosis factors α and β and IL-1α and IL-1β (i.e., null alleles at the Tnfrsf1a, Tnfrsf1b, and Il-1r1 loci). The observation that TKO mice were resistant to the tumor promoting effects of dioxin in liver suggests that inflammatory cytokines play an important step in dioxin mediated liver tumor promotion in the mouse. Collectively, these data support the idea that the mechanism of dioxin acute hepatotoxicity and its activity as a promoter in a mouse two stage liver cancer model may be similar, i.e., tumor promotion by dioxin, like acute hepatotoxicity, are mediated by the linked action of two receptor systems, the AHR and the receptors for the "IL-1-like" cytokines.

Cytotoxic and mutagenic properties of regioisomeric O²-, N3- and O⁴-ethylthymidines in bacterial cells

Exposure to environmental agents and endogenous metabolism can both give rise to DNA alkylation. Thymine is known to be alkylated at O(2), N3 and O(4) positions; however, it remains poorly explored how the regioisomeric alkylated thymidine lesions compromise the flow of genetic information by perturbing DNA replication in cells. Herein, we assessed the differential recognition of the regioisomeric O(2)-, N3- and O(4)-ethylthymidine (O(2)-, N3- and O(4)-EtdT) by the DNA replication machinery of Escherichia coli cells. We found that O(4)-EtdT did not inhibit appreciably DNA replication, whereas O(2)- and N3-EtdT were strongly blocking to DNA replication. In addition, O(4)-EtdT induced a very high frequency of T→C mutation, whereas nucleotide incorporation opposite O(2)- and N3-EtdT was promiscuous. Replication experiments with the use of polymerase-deficient cells revealed that Pol V constituted the major polymerase for the mutagenic bypass of all three EtdT lesions, though Pol IV also contributed to the T→G mutation induced by O(2)- and N3-EtdT. The distinct cytotoxic and mutagenic properties of the three regioisomeric lesions could be attributed to their unique chemical properties.

Noninvasive measurement of smoking-associated N(3)-ethyladenine and N(7)-ethylguanine in human salivary DNA by stable isotope dilution nanoflow liquid chromatography-nanospray ionization tandem mass spectrometry

Evidence showed that ethylating agents are contained in cigarette smoke, which damage DNA producing ethylated DNA adducts, including N(3)-ethyladenine (3-EtAde) and N(7)-ethylguanine (7-EtGua). These two ethylpurines can be depurinated spontaneously and be repaired by enzymes and they have been detected in human urine. In this study, a highly specific and sensitive assay based on stable isotope dilution nanoflow liquid chromatography nanospray ionization tandem mass spectrometry (nanoLC-NSI/MS/MS) was used to measure 3-EtAde and 7-EtGua in human salivary DNA. These ethylpurines were released from DNA by neutral thermal hydrolysis and then enriched by a solid-phase extraction column before nanoLC-NSI/MS/MS analysis. The detection limits (S/N≥3) of 3-EtA and 7-EtG were 15 fg (92 amol) and 10 fg (56 amol), respectively, injected on-column. The lower quantification limits of 3-EtAde and 7-EtGua were both 100 fg, i.e. 620 and 560 amol, respectively, corresponding to 9.4 and 8.6 adducts in 10(9) normal nucleotides, respectively, starting with as little as 20 μg of DNA isolated from an average of 3 mL of saliva. The mean (±SD) levels of 3-EtAde in 15 smokers and 15 nonsmokers were 12.6±7.0 and 9.7±5.3 in 10(8) normal nucleotides, respectively, while those of 7-EtGua were 14.1±8.2 and 3.8±2.8 in 10(8) normal nucleotides in smokers and nonsmokers, respectively. Levels of 7-EtGua, but not 3-EtAde, were statistically significantly higher in smokers than in nonsmokers (p<0.0001). Furthermore, salivary 7-EtGua levels are significantly correlated with the number of cigarettes smoked per day as well as with the smoking index. This highly specific and sensitive stable isotope dilution nanoLC-NSI/MS/MS assay might be feasible in measuring 7-EtGua in human salivary DNA as a noninvasive biomarker for DNA damage induced by cigarette smoking.

Detection and simultaneous quantification of three smoking-related ethylthymidine adducts in human salivary DNA by liquid chromatography tandem mass spectrometry

Smoking cigarette increases levels of certain ethylated DNA adducts in certain tissues and urine. Cigarette smoking is a major risk factor of various cancers and DNA ethylation is involved in smoking-related carcinogenesis. Among the ethylated DNA adducts, O(2)-ethylthymidine (O(2)-edT) and the promutagenic O(4)-ethylthymidine (O(4)-edT) are poorly repaired and they can accumulate in vivo. Using an accurate, highly sensitive, and quantitative assay based on stable isotope dilution nanoflow liquid chromatography-nanospray ionization tandem mass spectrometry (nanoLC-NSI/MS/MS), O(2)-edT, N(3)-edT (N(3)-ethylthymidine), and O(4)-edT adducts in human salivary DNA were simultaneous detected and quantified. Saliva is easily accessible and available and it can be a potential target in searching for noninvasive biomarkers. Under the highly selected reaction monitoring (H-SRM) mode, salivary samples from 20 smokers and 13 nonsmokers were analyzed. Starting with 50 μg of DNA isolated from about 3.5 mL of saliva, levels of O(2)-edT, N(3)-edT, and O(4)-edT in 20 smokers' salivary DNA samples were 5.3±6.2, 4.5±5.7, 4.2±8.0 in 10(8) normal nucleotides, respectively, while those in 13 nonsmokers were non-detectable. In addition, statistically significant correlations (p<0.0001) were observed between levels of O(2)-edT and N(3)-edT (γ=0.7388), between levels of O(2)-edT and O(4)-edT (γ=0.8839), and between levels of N(3)-edT, and O(4)-edT (γ=0.7835). To the best of our knowledge, this is the first report of detection and quantification of these three ethylthymidine adducts in human salivary DNA, which might be potential biomarkers for exposure to ethylating agents and possibly for cancer risk assessment.

Simultaneous quantitative analysis of N3-ethyladenine and N7-ethylguanine in human leukocyte deoxyribonucleic acid by stable isotope dilution capillary liquid chromatography-nanospray ionization tandem mass spectrometry

Cigarette smoke contains ethylating agents which damage DNA producing ethylated DNA adducts, such as N(3)-ethyladenine (3-EtAde), N(7)-ethylguanine (7-EtGua), and regioisomers of ethylthymine. Among them, 3-EtAde and 7-EtGua are present in human urine and their levels are higher in smokers than in nonsmokers. The amount of ethylated DNA adducts in tissue DNA represents the steady-state levels of DNA adducts resulting from the ethylating agent after repair in vivo. In this study, we have developed a highly sensitive, accurate, and quantitative assay for simultaneous detection and quantification of 3-EtAde and 7-EtGua by stable isotope dilution capillary liquid chromatography-nanospray ionization tandem mass spectrometry (capLC-NSI/MS/MS). Under the highly selective reaction monitoring (H-SRM) mode, the detection limit of 3-EtAde and 7-EtGua injected on-column was 5.0 fg (31 amol) and 10 fg (56 amol), respectively. The quantification limit for the entire assay was 50 and 100 fg of 3-EtAde and 7-EtGua, corresponding to 4.7 and 8.6 adducts in 10(9) normal nucleotides, respectively, starting with 20 μg of DNA isolated from <1 mL of blood and injecting an equivalent of 4 μg of DNA on-column. The mean (±SD) levels of 3-EtAde and 7-EtGua in leukocyte DNA from 20 smokers were 16.0±7.8 and 9.7±8.3 in 10(8) normal nucleotides, respectively, which were statistically significantly higher than those of 5.4±2.6 3-EtAde and 0.3±0.8 7-EtGua in 10(8) normal nucleotides from 20 nonsmokers (p<0.0001). The levels of 3-EtAde and 7-EtGua in these 40 leukocyte DNA samples are positively correlated (γ=0.6970, p<0.0001). Furthermore, there are statistically significant associations between the number of cigarettes smoked per day, as well as the smoking index, and the levels of 3-EtAde and 7-EtGua. Levels of 3-EtAde and 7-EtGua are compared to those of ethylthymidine adducts. To our knowledge, this is the first assay for simultaneous quantification of 3-EtAde and 7-EtGua in the same DNA sample and is the first report of 3-EtAde in human DNA. This highly sensitive and specific stable isotope dilution capLC-NSI/MS/MS assay should be useful in measuring 3-EtAde and 7-EtGua in human leukocyte DNA as potential biomarkers for smoking-related cancers.

Quantitation of DNA adducts by stable isotope dilution mass spectrometry

Exposure to endogenous and exogenous chemicals can lead to the formation of structurally modified DNA bases (DNA adducts). If not repaired, these nucleobase lesions can cause polymerase errors during DNA replication, leading to heritable mutations and potentially contributing to the development of cancer. Because of their critical role in cancer initiation, DNA adducts represent mechanism-based biomarkers of carcinogen exposure, and their quantitation is particularly useful for cancer risk assessment. DNA adducts are also valuable in mechanistic studies linking tumorigenic effects of environmental and industrial carcinogens to specific electrophilic species generated from their metabolism. While multiple experimental methodologies have been developed for DNA adduct analysis in biological samples, including immunoassay, HPLC, and ³²P-postlabeling, isotope dilution high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (HPLC-ESI-MS/MS) generally has superior selectivity, sensitivity, accuracy, and reproducibility. As typical DNA adduct concentrations in biological samples are between 0.01-10 adducts per 10⁸ normal nucleotides, ultrasensitive HPLC-ESI-MS/MS methodologies are required for their analysis. Recent developments in analytical separations and biological mass spectrometry, especially nanoflow HPLC, nanospray ionization MS, chip-MS, and high resolution MS, have pushed the limits of analytical HPLC-ESI-MS/MS methodologies for DNA adducts, allowing researchers to accurately measure their concentrations in biological samples from patients treated with DNA alkylating drugs and in populations exposed to carcinogens from urban air, drinking water, cooked food, alcohol, and cigarette smoke.

Mutagenicity of 5-hydroxymethylfurfural in V79 cells expressing human SULT1A1: identification and mass spectrometric quantification of DNA adducts formed

5-Hydroxymethylfurfural (HMF), a heterocyclic product of the Maillard reaction, is a ubiquitous food contaminant. It has demonstrated hepatocarcinogenic activity in female mice. This effect may originate from sulfo conjugation of the benzylic alcohol yielding 5-sulfooxymethylfurfural (SMF), which is prone to react with DNA via nucleophilic substitution. Indeed, we showed that HMF induces gene mutations in Chinese hamster V79 cells engineered for the expression of human (h) sulfotransferase (SULT)1A1 but not in parental V79 cells. In order to identify potential DNA adducts, we incubated DNA samples with SMF or HMF in aqueous solution. Modified DNA was digested and surveyed by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) for adducts that may be formed by nucleosides either via nucleophilic substitution at the electrophilic carbon atom of SMF or via imine formation with the aldehyde group present in HMF and SMF. The most abundant adducts formed from SMF, N(6)-((2-formylfuran-5-yl)methyl)-2'-deoxyadenosine (N(6)-FFM-dAdo) and N(2)-((2-formylfuran-5-yl)methyl)-2'-deoxyguanosine (N(2)-FFM-dGuo), were synthesized, purified, and characterized by (1)H NMR. Imine adducts were only detected when DNA was incubated with very high levels of HMF following reduction of the imines to corresponding secondary amines by NaBH(3)CN. Sensitive techniques based on LC-MS/MS multiple reaction monitoring for the quantification of the adducts in DNA samples were devised using isotope-labeled [(15)N(5)]N(6)-FFM-dAdo and [(13)C(10),(15)N(5)]N(2)-FFM-dGuo as internal standards. Both 5-methylfurfuryl adducts were detected in DNA from V79-hSULT1A1 treated with HMF but not in DNA from V79 control cells. Considering the lack of other known mutagenic metabolites, we hypothesize that the hepatocarcinogenic potential of HMF originates from the formation of mutagenic SMF.

Analysis of ethylated thymidine adducts in human leukocyte DNA by stable isotope dilution nanoflow liquid chromatography-nanospray ionization tandem mass spectrometry

Studies showed that levels of ethylated DNA adducts in certain tissues and urine are higher in smokers than in nonsmokers. Because cigarette smoking is a major risk factor of various cancers, DNA ethylation might play an important role in cigarette smoke-induced cancer formation. Among the ethylated DNA adducts, O(2)-ethylthymidine (O(2)-edT) and O(4)-ethylthymidine (O(4)-edT) are poorly repaired and are accumulated in the body. In addition, O(4)-edT possesses promutagenic properties. In this study, we have developed a highly sensitive, accurate, and quantitative assay for simultaneous detection and quantification of O(2)-edT, N(3)-edT (N(3)-ethylthymidine), and O(4)-edT adducts by isotope dilution nanoflow liquid chromatography-nanospray ionization tandem mass spectrometry (nanoLC-NSI/MS/MS). Under the highly selected reaction monitoring (H-SRM) mode, the detection limit of O(2)-edT, N(3)-edT, and O(4)-edT injected on-column was 5.0, 10, and 10 fg, respectively. The quantification limit for the entire assay was 50, 100, and 100 fg of O(2)-edT, N(3)-edT, and O(4)-edT, respectively, corresponding to 1.1, 2.3, and 2.3 adducts in 10(9) normal nucleotides, respectively, starting with 50 μg of DNA (from 1.5-2.0 mL of blood). Levels of O(2)-edT, N(3)-edT, and O(4)-edT in 20 smokers' leukocyte DNA were 44.8 ± 52.0, 41.1 ± 43.8, 48.3 ± 53.9 in 10(8) normal nucleotides, while those in 20 nonsmokers were 0.19 ± 0.87, 4.1 ± 13.3, and 1.0 ± 2.9, respectively. Levels of O(2)-edT, N(3)-edT, and O(4)-edT in human leukocyte DNA are all significantly higher in smokers than in nonsmokers, with pvalues of 0.0004, 0.0009, and 0.0004, respectively. Furthermore, levels of O(2)-edT show a statistically significant association (γ = 0.4789, p = 0.0327) with the smoking index in smokers. In the 40 leukocyte DNA samples, the extremely significant statistical correlations (p < 0.0001) are observed between levels of O(2)-edT and O(4)-edT (γ = 0.9896), between levels of O(2)-edT and N(3)-edT (γ = 0.9840), and between levels of N(3)-edT and O(4)-edT (γ = 0.9901). To our knowledge, this is the first mass spectrometry-based assay for ethylated thymidine adducts. Using this assay, the three ethylated thymidine adducts were detected and quantified for the first time. Therefore, this highly sensitive, specific, and accurate assay should be clinically feasible for simultaneous quantification of the three ethylated thymidine adducts as potential biomarkers for exposure to ethylating agents and for cancer risk assessment.

N7-glycidamide-guanine DNA adduct formation by orally ingested acrylamide in rats: a dose-response study encompassing human diet-related exposure levels

Acrylamide (AA) is formed during the heating of food and is classified as a genotoxic carcinogen. The margin of exposure (MOE), representing the distance between the bench mark dose associated with 10% tumor incidence in rats and the estimated average human exposure, is considered to be of concern. After ingestion, AA is converted by P450 into the genotoxic epoxide glycidamide (GA). GA forms DNA adducts, primarily at N7 of guanine (N7-GA-Gua). We performed a dose-response study with AA in female Sprague-Dawley (SD) rats. AA was given orally in a single dosage of 0.1-10 000 μg/kg bw. The formation of urinary mercapturic acids and of N7-GA-Gua DNA adducts in liver, kidney, and lung was measured 16 h after application. A mean of 37.0 ± 11.5% of a given AA dose was found as mercapturic acids (MAs) in urine. MA excretion in urine of untreated controls indicated some background exposure from endogenous AA. N7-GA-Gua adduct formation was not detectable in any organ tested at 0.1 μg AA/kg bw. At a dose of 1 μg/kg bw, adducts were found in kidney (around 1 adduct/10(8) nucleotides) and lung (below 1 adduct/10(8) nucleotides) but not in liver. At 10, respectively, 100 μg/kg bw, adducts were found in all three organs, at levels close to those found at 1 μg AA/kg, covering a range of about 1-2 adducts/10(8) nucleotides. As compared to DNA adduct levels from electrophilic genotoxic agents of various origin found in human tissues, N7-GA-Gua adduct levels within the dose range of 0.1-100 μg AA/kg bw were at the low end of this human background. We propose to take the background level of DNA lesions in humans more into consideration when doing risk assessment of food-borne genotoxic carcinogens.

Quantitation of 7-ethylguanine in leukocyte DNA from smokers and nonsmokers by liquid chromatography-nanoelectrospray-high resolution tandem mass spectrometry

There is considerable evidence for the exposure of humans to an unknown ethylating agent, and some studies indicate that cigarette smoking may be one source of this exposure. Therefore, we have developed a liquid chromatography-nanoelectrospray-high resolution tandem mass spectrometry-selected reaction monitoring (LC-NSI-HRMS/MS-SRM) method for the analysis of 7-ethyl-Gua in human leukocyte DNA, a readily available source of DNA. [(15)N(5)]7-Ethyl-Gua was used as the internal standard. Leukocyte DNA was isolated and treated by thermal neutral hydrolysis. The hydrolysate was partially purified by solid-phase extraction. The fraction containing 7-ethyl-Gua was analyzed by LC-NSI-HRMS/MS-SRM using the transition m/z 180 [M + H](+)→ m/z 152.05669 [Gua + H](+) for 7-ethyl-Gua and m/z 185 → m/z 157.04187 for the internal standard. The detection limit was approximately 10 amol on column, while the limit of quantitation was about 8 fmol/μmol Gua starting with 180 μg DNA (corresponding to 36 μg DNA on-column). Leukocyte DNA samples from 30 smokers and 30 nonsmokers were analyzed. Clear peaks for 7-ethyl-Gua and the internal standard were observed in most of the samples. The mean (±SD) level of 7-ethyl-Gua measured in leukocyte DNA from smokers was 49.6 ± 43.3 (range 14.6-181) fmol/μmol Gua, while that from nonsmokers was 41.3 ± 34.9 (range 9.64-157) fmol/μmol Gua. Although a significant difference between smokers and nonsmokers was not observed, the method described here is unique in the use of high resolution mass spectrometry and establishes for the first time the presence of 7-ethyl-Gua in human leukocyte DNA.

Detection of 7-(2'-carboxyethyl)guanine but not 7-carboxymethylguanine in human liver DNA

7-Carboxymethylguanine (7-CMGua) and 7-(2'-carboxyethyl)guanine (7-CEGua) are DNA adducts that potentially could be formed upon the metabolism of the carcinogenic nitrosamines N-nitrososarcosine (NSAR) and 3-(methylnitrosamino)propionic acid (MNPA), respectively, or from other sources such as nitrosation of glycine (7-CMGua) or reaction of DNA with acrylic acid (7-CEGua). Since both NSAR and MNPA have been detected in human urine and there are plausible sources of exposure to other precursors to these adducts, we analyzed human liver DNA for 7-CMGua and 7-CEGua, using liquid chromatography-electrospray ionization-tandem mass spectrometry-selected reaction monitoring (LC-ESI-MS/MS-SRM). Human hepatic DNA was mixed with [15N5]7-CMGua and [15N5]7-CEGua as internal standards and enzymatically hydrolyzed. The hydrolysate was partially purified by solid-phase extraction, and the resulting fraction was treated with acetyl chloride in methanol to convert 7-CMGua and 7-CEGua to their methyl esters. After a second solid-phase extraction, LC-ESI-MS/MS-SRM analysis was carried out using the transitions m/z 224 [M + H](+) --> m/z 164 [(M + H)-HCOOCH3]+ and m/z 238 [M + H]+ --> m/z 152 [BH]+ for the methyl esters of 7-CMGua and 7-CEGua, respectively. The method was sensitive, accurate, precise, and apparently free from artifact formation. 7-CEGua, as its methyl ester, was detected in all 24 human liver samples analyzed, mean +/- SD, 373 +/- 320 fmol/mumol Gua (74.6 adducts per 10(9) nucleotides), range 17-1189 fmol/mumol Gua, but the methyl ester of 7-CMGua was not detected in any sample. These results demonstrate the ubiquitous presence of 7-CEGua in human liver DNA. Acrylic acid may be a likely endogenous precursor to 7-CEGua.

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.