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

DNA Alkylation Damage by Nitrosamines and Relevant DNA Repair Pathways

Nitrosamines occur widespread in food, drinking water, cosmetics, as well as tobacco smoke and can arise endogenously. More recently, nitrosamines have been detected as impurities in various drugs. This is of particular concern as nitrosamines are alkylating agents that are genotoxic and carcinogenic. We first summarize the current knowledge on the different sources and chemical nature of alkylating agents with a focus on relevant nitrosamines. Subsequently, we present the major DNA alkylation adducts induced by nitrosamines upon their metabolic activation by CYP450 monooxygenases. We then describe the DNA repair pathways engaged by the various DNA alkylation adducts, which include base excision repair, direct damage reversal by MGMT and ALKBH, as well as nucleotide excision repair. Their roles in the protection against the genotoxic and carcinogenic effects of nitrosamines are highlighted. Finally, we address DNA translesion synthesis as a DNA damage tolerance mechanism relevant to DNA alkylation adducts.

Mass Spectrometry Analysis of DNA and Protein Adducts as Biomarkers in Human Exposure to Cigarette Smoking: Acrolein as an Example

Acrolein is a major component in cigarette smoke and a product of endogenous lipid peroxidation. It is difficult to distinguish human exposure to acrolein from exogenous sources versus endogenous causes, as components in cigarette smoke can stimulate lipid peroxidation in vivo. Therefore, analysis of acrolein-induced DNA and protein adducts by the highly accurate, sensitive, and specific mass spectrometry-based methods is vital to estimate the degree of damage by this IARC Group 2A carcinogen. This Perspective reviews the analyses of acrolein-induced DNA and protein adducts in humans by mass spectrometry focusing on samples accessible for biomonitoring, including DNA from leukocytes and oral cells and abundant proteins from blood, i.e., hemoglobin and serum albumin.

Quantitation by Liquid Chromatography-Nanoelectrospray Ionization-High-Resolution Tandem Mass Spectrometry of Multiple DNA Adducts Related to Cigarette Smoking in Oral Cells in the Shanghai Cohort Study

We developed a liquid chromatography-nanoelectrospray ionization-high-resolution tandem mass spectrometry (LC-NSI-HRMS/MS) method for simultaneous quantitative analysis of 5 oral cell DNA adducts associated with cigarette smoking: (8R/S)-3-(2'-deoxyribos-1'-yl)-5,6,7,8-tetrahydro-8-hydroxypyrimido[1,2-a]purine-10(3H)-one (γ-OH-Acr-dGuo, 1) from acrolein; (6S,8S and 6R,8R)-3-(2'-deoxyribos-1'-yl)-5,6,7,8-tetrahydro-8-hydroxy-6-methylpyrimido[1,2-a]purine-10(3H)-one [(6S,8S)γ-OH-Cro-dGuo, 2; and (6R,8R)γ-OH-Cro-dGuo, 3] from crotonaldehyde; 1,N⁶-etheno-dAdo (4) from acrylonitrile, vinyl chloride, lipid peroxidation, and inflammation; and 8-oxo-dGuo (5) from oxidative damage. Oral cell DNA was isolated in the presence of glutathione to prevent artifact formation. Clear LC-NSI-HRMS/MS chromatograms were obtained allowing quantitation of each adduct using the appropriately labeled internal standards. The accuracy and precision of the method were validated, and the assay limit of quantitation was 5 fmol/μmol dGuo for adducts 1-4 and 20 fmol/μmol for adduct 5. The assay was applied to 80 buccal cell samples selected from those collected in the Shanghai Cohort Study: 40 from current smokers and 40 from never smokers. Significant differences were found in all adduct levels between smokers and nonsmokers. Levels of 8-oxo-dGuo (5) were at least 3000 times greater than those of the other adducts in both smokers and nonsmokers, and the difference between amounts of this adduct in smokers versus nonsmokers, while significant (P = 0.013), was not as great as the differences of the other DNA adducts between smokers and nonsmokers (P-values all less than 0.001). No significant relationship of adduct levels to risk of lung cancer incidence was found. This study provides a new LC-NSI-HRMS/MS methodology for the quantitation of diverse DNA adducts resulting from exposure to the α,β-unsaturated aldehydes acrolein and crotonaldehyde, inflammation, and oxidative damage which are all associated with carcinogenesis. We anticipate application of this assay in ongoing studies of the molecular epidemiology of cancers of the lung and oral cavity related to cigarette smoking.

Characterization and Quantification of Acrolein-Induced Modifications in Hemoglobin by Mass Spectrometry─Effect of Cigarette Smoking

Exposure to acrolein, the smallest α, β-unsaturated aldehyde, in humans originates from cigarette smoking and other environmental sources, food cooking, and endogenous lipid peroxidation and metabolism. The protein modification caused by acrolein is associated with various diseases, including cancer, cardiovascular, and neurodegenerative diseases. In this study, acrolein-modified human hemoglobin was reduced by sodium borohydride. Thus, three types of modifications, that is, Schiff base, Michael addition, and formyl-dehydropiperidion adducts, were converted to the corresponding stable adducts, leading to mass increases of 40.0313, 58.0419, and 96.0575 Da, respectively. These stable acrolein-modified hemoglobin peptides were identified by nanoflow liquid chromatography coupled to a high-resolution nanoelectrospray ionization tandem mass spectrometry. Among the 26 different types and sites of modifications, 15 of them showed a dose-dependent increase with increasing concentrations of acrolein. To investigate the role of acrolein-induced modifications in smoking and oral cancer, the 15 dose-responsive acrolein-modified peptides, together with three ethylated peptides previously identified, were quantified in oral cancer patients, healthy smokers, and healthy nonsmokers. The results reveal that the relative extents of the Michael-type adduct at α-Lys-16, α-His-50, and β-Lys-59 are significantly higher in smokers (oral cancer and healthy) than in nonsmokers. Areas under the receiver operating characteristic curve of these peptides range from 0.7500 to 0.9375, indicating the ability to discriminate smokers from nonsmokers. Additionally, these acrolein-modified peptides correlate with three ethylated peptides at the N-termini of α- and β-globin, as well as β-His-77, and with the number of cigarettes smoked per day. Therefore, measuring the reduced Michael adducts at α-Lys-16, α-His-50, and β-Lys-59 of hemoglobin from one drop of blood by this sensitive and specific method may reflect the increase of acrolein exposure due to cigarette smoking.

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.

Liquid Chromatography-Nanoelectrospray Ionization-High-Resolution Tandem Mass Spectrometry Analysis of Apurinic/Apyrimidinic Sites in Oral Cell DNA of Cigarette Smokers, e-Cigarette Users, and Nonsmokers

Cigarette smoking is an established risk factor for oral cancer. The health effects of e-cigarettes are still under investigation but may disturb oral cavity homeostasis and cause lung and cardiovascular diseases. Carcinogens and toxicants in tobacco products and e-cigarettes may damage DNA, resulting in the formation of apurinic/apyrimidinic (AP) sites and initiation of the carcinogenic process. In this study, we optimized a liquid chromatography-nanoelectrospray ionization-high-resolution tandem mass spectrometry method to analyze AP sites in buccal cell DNA of 35 nonsmokers, 30 smokers, and 30 e-cigarette users. AP sites in e-cigarette users (median 3.3 per 10⁷ nts) were significantly lower than in smokers (median 5.7 per 10⁷ nts) and nonsmokers (median 6.0 per 10⁷ nts). AP sites in smokers were not significantly different from nonsmokers (p > 0.05). The e-cigarette vaporizing solvents propylene glycol and glycerin were tested and did not protect against AP site formation in in vitro control and carcinogen exposed rat liver homogenates. However, propylene glycol may inhibit bacteria in oral cells, resulting in reduced inflammation and related effects, and reduced AP site levels in e-cigarette user DNA. This is the first study to examine AP site formation in e-cigarette users and to evaluate AP sites in human oral cell DNA.

Recent developments in DNA adduct analysis using liquid chromatography coupled with mass spectrometry

The formation of DNA adducts by genotoxic agents is an early event in cancer development, and it may lead to gene mutations, thereby initiating tumor development. The measurement of DNA adducts can provide critical information about the genotoxic potential of a chemical and its mechanism of carcinogenesis. In recent decades, liquid chromatography coupled with mass spectrometry has become the most important technique for analyzing DNA adducts. The improvements in resolution achievable with new chromatographic separation techniques coupled with the high specificity and sensitivity and wide dynamic range of new mass spectrometry systems have been used for both qualitative and quantitative analyses of DNA adducts. This review discusses the challenges in qualitative and quantitative analyses of DNA adducts by liquid chromatography coupled with mass spectrometry and highlights recent developments towards overcoming the limitations of liquid chromatography coupled with mass spectrometry methods. The key steps and new solutions, such as sample preparation, mass spectrometry fragmentation, and method validation, are summarized. In addition, the fundamental principles and latest advances in DNA adductomic approaches are reviewed.

Recent Studies on DNA Adducts Resulting from Human Exposure to Tobacco Smoke

DNA adducts are believed to play a central role in the induction of cancer in cigarette smokers and are proposed as being potential biomarkers of cancer risk. We have summarized research conducted since 2012 on DNA adduct formation in smokers. A variety of DNA adducts derived from various classes of carcinogens, including aromatic amines, polycyclic aromatic hydrocarbons, tobacco-specific nitrosamines, alkylating agents, aldehydes, volatile carcinogens, as well as oxidative damage have been reported. The results are discussed with particular attention to the analytical methods used in those studies. Mass spectrometry-based methods that have higher selectivity and specificity compared to 32P-postlabeling or immunochemical approaches are preferred. Multiple DNA adducts specific to tobacco constituents have also been characterized for the first time in vitro or detected in vivo since 2012, and descriptions of those adducts are included. We also discuss common issues related to measuring DNA adducts in humans, including the development and validation of analytical methods and prevention of artifact formation.

Age-Associated Methylation in Human Hemoglobin and Its Stability on Dried Blood Spots As Analyzed by Nanoflow Liquid Chromatography Tandem Mass Spectrometry

Methylation of biomolecules is involved in many important biological processes. The contributing methylating agents arise from endogenous and exogenous sources (such as cigarette smoking). Human hemoglobin is easily accessible from blood and has been used as a molecular dosimeter for monitoring chemical exposure. We recently developed a method for characterization and quantification of the extents of methylation and ethylation in hemoglobin by nanoflow liquid chromatography tandem mass spectrometry under the selected reaction monitoring mode. Using this method, the relative extents of methylated and ethylated peptides in hemoglobin were quantified in nonsmoking subjects at various ages in this study. Among the nine methylation sites, we found that the extents of methylation were significantly higher in elderly subjects at the N-terminal and His-20 of α-globin, and at the N-terminal and Glu-26 of β-globin. Moreover, the extents of methylation at these sites were significantly correlated with the age of the subjects. On the other hand, no statistically significant difference was found in the ethylated peptides. We also examined the stability of methylated and ethylated hemoglobin when stored on dried blood spot cards. The extents of these modifications on hemoglobin are stable for at least 4 weeks stored at room temperature. Our results suggest that age should be considered as a factor when measuring hemoglobin methylation and that dried blood spot is a valuable biomonitoring technique for hemoglobin modifications in epidemiological studies.

Direct-acting DNA ethylating agents associated with tobacco use primarily originate from the tobacco itself, not combustion

Unburnt tobacco and tobacco smoke contain a variety of carcinogens, exposure to which are causally associated with the incidence of several human cancers. Herein, we used isotope-dilution LC-MS/MS for the quantification of alkylated purines in DNA, following in vitro exposure to aqueous extracts of tobacco itself, and tobacco smoke. Our results demonstrated the presence of direct-acting ethylating agent(s) in unburnt tobacco, which 4.0-6.3 times exceeded that in the particulate phase of sidestream cigarette smoke and 6.8-8.9 times exceeded that in mainstream smoke. Interestingly, particulate phase of sidestream cigarette smoke exhibited higher ethylating potency than that in mainstream smoke. This finding refutes the previous assumptions that the ethylating agent(s) associated with smoking, are derived from cigarette smoke. Indeed, our data show that combustion of tobacco actually decreases the ethylating potency of tobacco. Although the identity of this agent(s) remains unknown, our data suggest that it is highly hydrophilic, and hence likely to be easily extracted by saliva. This would allow intimate contact with the tissues of the oropharyngeal cavity. Taken together, these results have profound implications for tobacco use, in particular for tobacco chewers and passive smokers, whose exposure to ethylating agent(s) is greater than previously thought.

The Saliva Exposome for Monitoring of Individuals' Health Trajectories

BACKGROUND

There is increasing evidence that environmental, rather than genetic, factors are the major causes of most chronic diseases. By measuring entire classes of chemicals in archived biospecimens, exposome-wide association studies (EWAS) are being conducted to investigate associations between a myriad of exposures received during life and chronic diseases.

OBJECTIVES

Because the intraindividual variability in biomarker levels, arising from changes in environmental exposures from conception onwards, leads to attenuation of exposure-disease associations, we posit that saliva can be collected repeatedly in longitudinal studies to reduce exposure-measurement errors in EWAS.

METHODS

From the literature and an open-source saliva-metabolome database, we obtained concentrations of 1,233 chemicals that had been detected in saliva. We connected salivary metabolites with human metabolic pathways and PubMed Medical Subject Heading (MeSH) terms, and performed pathway enrichment and pathway topology analyses.

RESULTS

One hundred ninety-six salivary metabolites were mapped into 49 metabolic pathways and connected with human metabolic diseases, central nervous system diseases, and neoplasms. We found that the saliva exposome represents at least 14 metabolic pathways, including amino acid metabolism, TCA cycle, gluconeogenesis, glutathione metabolism, pantothenate and CoA biosynthesis, and butanoate metabolism.

CONCLUSIONS

Saliva contains molecular information worthy of interrogation via EWAS. The simplicity of specimen collection suggests that saliva offers a practical alternative to blood for measurements that can be used to characterize individual exposomes. https://doi.org/10.1289/EHP1011.

Simultaneous analysis of six aldehyde-DNA adducts in salivary DNA of nonsmokers and smokers using stable isotope dilution liquid chromatography electrospray ionization-tandem mass spectrometry

A stable method, using isotope dilution liquid chromatography-tandem mass spectrometry (LC-MS/MS), to simultaneously determine six aldehyde-DNA adducts was developed and applied to the analysis of human salivary DNA samples. The detection limit of these six DNA adducts was in the range of 0.006-0.014ng/mL and that of the quantification limit was 0.017-0.026ng/mL. The intra-day and inter-day precision of all aldehyde-DNA adducts was <10%. The analysis was completed within 25min. Additionally, a noninvasive technique was used to collect the DNA samples from human saliva. The new method was successfully applied for the analysis of salivary DNA of nonsmokers and smokers. Five aldehyde-DNA adducts were detected in both smoker and nonsmoker salivary DNA, while α-Acr-dG was not detected in all the samples. Among these detected DNA adducts, no significant differences were found between smoker and nonsmoker (p>0.05). This may due to the individual detoxifying differences or environmental and endogenous exposure. Our study provides a rapid and selective method to simultaneously detect six aldehyde-DNA adducts and to assess potential DNA damage induced by aldehydes.

Oral Cell DNA Adducts as Potential Biomarkers for Lung Cancer Susceptibility in Cigarette Smokers

This perspective considers the use of oral cell DNA adducts, together with exposure and genetic information, to potentially identify those cigarette smokers at highest risk for lung cancer, so that appropriate preventive measures could be initiated at a relatively young age before too much damage has been done. There are now well established and validated analytical methods for the quantitation of urinary and serum metabolites of tobacco smoke toxicants and carcinogens. These metabolites provide a profile of exposure and in some cases lung cancer risk, but they do not yield information on the critical DNA damage parameter that leads to mutations in cancer growth control genes such as KRAS and TP53. Studies demonstrate a correlation between changes in the oral cavity and lung in cigarette smokers, due to the field effect of tobacco smoke. Oral cell DNA is readily obtained in contrast to DNA samples from the lung. Studies in which oral cell DNA and salivary DNA have been analyzed for specific DNA adducts are reviewed; some of the adducts identified have also been previously reported in lung DNA from smokers. The multiple challenges of developing a panel of oral cell DNA adducts that could be routinely quantified by mass spectrometry are discussed.

Mass spectrometry for the assessment of the occurrence and biological consequences of DNA adducts

Exogenous and endogenous sources of chemical species can react, directly or after metabolic activation, with DNA to yield DNA adducts. If not repaired, DNA adducts may compromise cellular functions by blocking DNA replication and/or inducing mutations. Unambiguous identification of the structures and accurate measurements of the levels of DNA adducts in cellular and tissue DNA constitute the first and important step towards understanding the biological consequences of these adducts. The advances in mass spectrometry (MS) instrumentation in the past 2-3 decades have rendered MS an important tool for structure elucidation, quantification, and revelation of the biological consequences of DNA adducts. In this review, we summarized the development of MS techniques on these fronts for DNA adduct analysis. We placed our emphasis of discussion on sample preparation, the combination of MS with gas chromatography- or liquid chromatography (LC)-based separation techniques for the quantitative measurement of DNA adducts, and the use of LC-MS along with molecular biology tools for understanding the human health consequences of DNA adducts. The applications of mass spectrometry-based DNA adduct analysis for predicting the therapeutic outcome of anti-cancer agents, for monitoring the human exposure to endogenous and environmental genotoxic agents, and for DNA repair studies were also discussed.

Biomarkers of tobacco smoke exposure

Diseases and death caused by exposure to tobacco smoke have become the single most serious preventable public health concern. Thus, biomarkers that can monitor tobacco exposure and health effects can play a critical role in tobacco product regulation and public health policy. Biomarkers of exposure to tobacco toxicants are well established and have been used in population studies to establish public policy regarding exposure to second-hand smoke, an example being the nicotine metabolite cotinine, which can be measured in urine. Biomarkers of biological response to tobacco smoking range from those indicative of inflammation to mRNA and microRNA patterns related to tobacco use and/or disease state. Biomarkers identifying individuals with an increased risk for a pathological response to tobacco have also been described. The challenge for any novel technology or biomarker is its translation to clinical and/or regulatory application, a process that requires first technical validation of the assay and then careful consideration of the context the biomarker assay may be used in the regulatory setting. Nonetheless, the current efforts to investigate new biomarker of tobacco smoke exposure promise to offer powerful new tools in addressing the health hazards of tobacco product use. This review will examine such biomarkers, albeit with a focus on those related to cigarette smoking.