Detection and quantification of 1,N6-ethenoadenine in human urine by stable isotope dilution capillary gas chromatography/negative ion chemical ionization/mass spectrometry

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.

Exocyclic DNA adducts as biomarkers of lipid oxidation and predictors of disease. Challenges in developing sensitive and specific methods for clinical studies

Exocyclic DNA adducts are emerging as potential new tools for the study of oxidative stress-related diseases as well as the determination of cancer etiology and cancer risk. It is important to determine whether levels of exocyclic DNA adducts reflect redox stress in vivo and what role these adducts play in human diseases. To answer these important questions, interindividual differences, tissue distribution, background levels, and repair have to be assessed. This review focuses on recent developments in the use of these adducts as possible biomarkers for disease risk related to oxidative stress and on the challenges in developing sensitive and specific methods for clinical studies.

Simultaneous quantification of three lipid peroxidation-derived etheno adducts in human DNA by stable isotope dilution nanoflow liquid chromatography nanospray ionization tandem mass spectrometry

Etheno DNA adducts are promutagenic DNA lesions derived from exogenous industrial chemicals, as well as endogenous sources including lipid peroxidation. Furthermore, levels of etheno adducts in tissue DNA are elevated in cancer-prone tissues. In this study, we have developed a highly sensitive and specific stable isotope dilution nanoflow LC-nanospray ionization tandem mass spectrometry (nanoLC-NSI/MS/MS) assay for simultaneous detection and accurate quantification of 1,N(6)-etheno-2'-deoxyadenosine (epsilondAdo), 3,N(4)-etheno-2'-deoxycytidine (epsilondCyt), and 1,N(2)-etheno-2'-deoxyguanosine (1,N(2)-epsilondGuo) in tissue DNA. Typically, [(13)C(1),(15)N(2)]epsilondAdo, [(15)N(3])epsilondCyd, and [(13)C(1),(15)N(2)]1,N(2)-epsilondGuo were added to calf thymus, human placenta, or human leukocyte DNA as internal standards, and the mixture was subjected to enzyme hydrolysis to form the nucleosides. The etheno adducts in DNA hydrolysate were enriched by a reversed phase solid-phase extraction column before analysis by nanoLC-NSI/MS/MS under the highly selective reaction monitoring (H-SRM) mode. This nanoLC-NSI/MS/MS assay achieved attomole-level sensitivity with the detection limit of 0.73, 160, and 34 amol for the respective standard epsilondAdo, epsilondCyd, and 1,N(2)-epsilondGuo injected on-column, while the quantification limit for the entire assay was 0.18, 4.0, and 3.4 fmol, respectively. The levels of epsilondAdo, epsilondCyd, and 1,N(2)-epsilondGuo in human placental DNA were 28.2, 44.1, and 8.5 adducts in 10(8) normal nucleosides, respectively. The levels of epsilondAdo, epsilondCyd, and 1,N(2)-epsilondGuo in 11 human leukocyte DNA samples were 16.2 +/- 5.2, 11.1 +/- 5.8, and 8.6 +/- 9.1 (mean +/- S.D.) in 10(8) normal nucleotides, respectively, starting from 30 mug of DNA or 1-1.5 mL of blood, and all the relative standard deviations were within 10%. An aliquot equivalent to 6 mug of DNA hydrolysate was used for analysis by this nanoLC-NSI/MS/MS. Thus, this highly sensitive and specific nanoLC-NSI/MS/MS method is suitable for accurate quantification of the three lipid peroxidation-derived etheno DNA adducts as noninvasive biomarkers in clinical studies for cancer risk assessment and for evaluation of preventive agents.

Challenges in developing DNA and RNA biomarkers of inflammation

Inflammation is now a proven cause of human diseases such as cancer and cardiovascular disease. One potential link between inflammation and disease involves secretion of reactive chemical species by immune cells, with chronic damage to host epithelial cells leading to disease. This suggests pathophysiologically that DNA and RNA damage products are candidate biomarkers of inflammation, both for mechanistic understanding of the process and for risk assessment. Of the current approaches to quantifying DNA damage products, mass spectrometry-based methods provide the most rigorous quantification needed for biomarker development, while antibody-based approaches provide the most practical way to implement biomarkers in a clinical setting. Nonetheless, all approaches are biased by adventitious formation of DNA and RNA damage products during sample processing. Recent studies of tissue-derived DNA biomarkers in mouse models of inflammation reveal significant changes only in DNA adducts derived from lipid peroxidation. These and other observations raise the question of the most appropriate sampling compartment for DNA biomarker studies and highlight the emerging role of lipid damage in inflammation.

Creating context for the use of DNA adduct data in cancer risk assessment: II. Overview of methods of identification and quantitation of DNA damage

The formation of deoxyribonucleic acid (DNA) adducts can have important and adverse consequences for cellular and whole organism function. Available methods for identification of DNA damage and quantification of adducts are reviewed. Analyses can be performed on various samples including tissues, isolated cells, and intact or hydrolyzed (digested) DNA from a variety of biological samples of interest for monitoring in humans. Sensitivity and specificity are considered key factors for selecting the type of method for assessing DNA perturbation. The amount of DNA needed for analysis is dependent upon the method and ranges widely, from <1 microg to 3 mg. The methods discussed include the Comet assay, the ligation-mediated polymerase reaction, histochemical and immunologic methods, radiolabeled ((14)C- and (3)H-) binding, (32)P-postlabeling, and methods dependent on gas chromatography (GC) or high-performance liquid chromatography (HPLC) with detection by electron capture, electrochemical detection, single or tandem mass spectrometry, or accelerator mass spectrometry. Sensitivity is ranked, and ranges from approximately 1 adduct in 10(4) to 10(12) nucleotides. A brief overview of oxidatively generated DNA damage is also presented. Assay limitations are discussed along with issues that may have impact on the reliability of results, such as sample collection, processing, and storage. Although certain methodologies are mature, improving technology will continue to enhance the specificity and sensitivity of adduct analysis. Because limited guidance and recommendations exist for adduct analysis, this effort supports the HESI Committee goal of developing a framework for use of DNA adduct data in risk assessment.

Human biomonitoring: State of the art

Human biomonitoring (HBM) of dose and biochemical effect nowadays has tremendous utility providing an efficient and cost effective means of measuring human exposure to chemical substances. HBM considers all routes of uptake and all sources which are relevant making it an ideal instrument for risk assessment and risk management. HBM can identify new chemical exposures, trends and changes in exposure, establish distribution of exposure among the general population, identify vulnerable groups and populations with higher exposures and identify environmental risks at specific contaminated sites with relatively low expenditure. The sensitivity of HBM methods moreover enables the elucidation of human metabolism and toxic mechanisms of the pollutants. So, HBM is a tool for scientists as well as for policy makers. Blood and urine are by far the most approved matrices. HBM can be done for most chemical substances which are in the focus of the worldwide discussion of environmental medicine. This especially applies for metals, PAH, phthalates, dioxins, pesticides, as well as for aromatic amines, perfluorinated chemicals, environmental tobacco smoke and volatile organic compounds. Protein adducts, especially Hb-adducts, as surrogates of DNA adducts measuring exposure as well as biochemical effect very specifically and sensitively are a still better means to estimate cancer risk than measuring genotoxic substances and their metabolites in human body fluids. Using very sophisticated but nevertheless routinely applicable analytical procedures Hb-adducts of alkylating agents, aromatic amines and nitro aromatic compounds are determined routinely today. To extend the spectrum of biochemical effect monitoring further methods should be elaborated which put up with cleavage and separation of the adducted protein molecules as a measure of sample preparation. This way all sites of adduction as well as further proteins, like serum albumin could be used for HBM. DNA-adducts indicate the mutagenicity of a chemical substance as well as an elevated cancer risk. DNA-adducts therefore would be ideal parameters for HBM. Though there are very sensitive techniques for DNA adduct monitoring like P32-postlabelling and immunological methods they lack specificity. For elucidating the mechanism of carcinogenesis and for a broad applicability and comparability in epidemiological studies analytical methods must be elaborated which are strictly specific for the chemical structure of the DNA-adduct. Current analytical possibilities however meet their borders. In HBM studies with exposure to genotoxic chemicals especially the measurement of DNA strand breaks in lymphocytes and 8-hydroxy-2'-deoxyguanosine (8-OHdG) in white blood cells has become very popular. However, there is still a lack of well-established dose-response relations between occupational or environmental exposures and the induction of 8-OHdG or formation of strand breaks which limits the applicability of these markers. Most of the biomarkers used in population studies are covered by standard operating procedures (SOPs) as well as by internal and external quality assessment schemes. Therefore, HBM results from the leading laboratories worldwide are analytically reliable and comparable. Newly upcoming substances of environmental relevance like perfluorinated compounds can rapidly be assessed in body fluids because there are very powerful laboratories which are able to elaborate the analytical prerequisites in due time. On the other hand, it is getting more and more difficult for the laboratories to keep up with a progress in instrumental analyses. In spite of this it will pay to reach the ultimate summit of HBM because it is the only way to identify and quantify human exposure and risk, elucidate the mechanism of toxic effects and to ultimately decide if measures have to be taken to reduce exposure. Risk assessment and risk management without HBM lead to wrong risk estimates and cause inadequate measures. In some countries like in USA and in Germany, thousands of inhabitants are regularly investigated with respect to their internal exposure to a broad range of environmentally occurring substances. For the evaluation of HBM results the German HBM Commission elaborates reference- and HBM-values.

Association between cigarette smoking and urinary excretion of 1,N2-ethenoguanine measured by isotope dilution liquid chromatography-electrospray ionization/tandem mass spectrometry

Levels of the promutagenic 1,N2-ethenoguanine (1,N2-epsilonGua), an etheno DNA adduct derived mainly from lipid peroxidation, in experimental animals are associated with risk of cancer formation. Since 1,N2-epsilonGua can be repaired by human glycosylases, it is possible to use it as a biomarker for cancer risk assessment in humans. In the present study, a highly sensitive and specific stable isotope dilution liquid chromatography-electrospray ionization/tandem mass spectrometry (LC-ESI/MS/MS) was developed for accurate quantification of 1,N2-epsilonGua in human urine. The sample pretreatment involved a consecutive strong cation exchange solid-phase extraction (SPE) and reversed phase SPE chromatography. The pretreated sample was analyzed by LC-ESI/MS/MS under multiple reaction monitoring mode (MRM) using a triple quadrupole mass spectrometer. The detection limit of 1,N2-epsilonGua using this LC-ESI/MS/MS assay was 1.0 pg (5.8 fmol) injected on-column. Levels of urine samples collected from healthy volunteers were found to range from 0 to 199 pg/mL, and levels as low as 5.0 pg/mL (29 pM) could be accurately quantified. After adjusting for creatinine levels and body weight, an statistically significant association was observed between urinary levels of 1,N2-epsilonGua and cigarette smoking (p = 0.0006). This highly specific and sensitive assay should be valuable in measuring urinary 1,N2-epsilonGua as a potential noninvasive biomarker for oxidative DNA damage.

Quantification of urinary etheno-DNA adducts by column-switching LC/APCI-MS/MS

Lipid peroxidation induced etheno-DNA adducts are promutagenic and have been suggested to play a causal role in the development of human cancers. Therefore, human biomonitoring of etheno-DNA adducts in urine has been suggested as a potential marker for oxidative stress-related DNA damage. For quantitative determination, a column-switching LC/APCI-MS/MS method was developed for simultaneous analysis of epsilonAde, epsilondC, and epsilondA in human urine. Quantitative validation parameters (precision, within-day repeatability, and between-day reproducibility) yielded satisfactory results below 10%. Limit of quantification for epsilonAde, epsilondC, and epsilondA was 5.3 fmol, 7.5 fmol, and 1.3 fmol on column, respectively. Mean urinary excretion rates of a six healthy volunteers were 45.8 pmol epsilonAde/24 h, 96.8 pmol epsilondC/24 h, and 18.1 pmol epsilondA/24 h. The demonstrated levels of performance suggest a future applicability of this method to studies of cancer and other diseases related to oxidative stress in humans. To our knowledge, this is the first method described that allows simultaneous determination of epsilonAde, epsilondC, and epsilondA in human urine samples.

Analysis of urinary 8-nitroguanine, a marker of nitrative nucleic acid damage, by high-performance liquid chromatography-electrochemical detection coupled with immunoaffinity purification: association with cigarette smoking

We have developed an analytical method to quantitate urinary 8-nitroguanine, a product of nitrative nucleic acid damage caused by reactive nitrogen species such as peroxynitrite and nitrogen dioxide. 8-Nitroguanine was purified from human urine using immunoaffinity columns with an anti-8-nitroguanine antibody, followed by quantitation by high-performance liquid chromatography-electrochemical detection. Four sequential electrodes were used to (a) oxidize interfering compounds (+250 mV), (b) reduce nitrated bases (two online electrodes at -1000 mV), and (c) quantitate reduced derivatives (+150 mV). Using this system 8-nitroxanthine can also be detected, with the detection limits being 25 and 50 fmol/injection for 8-nitroguanine and 8-nitroxanthine, respectively. The method was used to analyze both adducts in the urine of smokers (n=12) and nonsmokers (n=17). We found that smokers excrete more 8-nitroguanine [median, 6.1 fmol/mg creatinine; interquartile range (IQR), 23.8] than nonsmokers (0; IQR, 0.90) (p=0.018), and although 8-nitroxanthine was detected in human urine, its level was not related to smoking status. This is the first report to show that 8-nitroguanine is present in human urine and the methodology developed can be used to study the pathogenic roles of this adduct in the etiology of cancers associated with cigarette smoking and inflammation.

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.

7-Alkylguanine adduct levels in urine, lungs and liver of mice exposed to styrene by inhalation

This study describes urinary excretion of two nucleobase adducts derived from styrene 7,8-oxide (SO), i.e., 7-(2-hydroxy-1-phenylethyl)guanine (N7alphaG) and 7-(2-hydroxy-2-phenylethyl)guanine (N7betaG), as well as a formation of N7-SO-guanine adducts in lungs and liver of two month old male NMRI mice exposed to styrene by inhalation in a 3-week subacute study. Strikingly higher excretion of both isomeric nucleobase adducts in the first day of exposure was recorded, while the daily excretion of nucleobase adducts in following time intervals reached the steady-state level at 4.32+1.14 and 6.91+1.17 pmol/animal for lower and higher styrene exposure, respectively. beta-SO-guanine DNA adducts in lungs increased with exposure in a linear way (F=13.7 for linearity and 0.17 for non-linearity, respectively), reaching at the 21st day the level of 23.0 adducts/10(8) normal nucleotides, i.e., 0.74 fmol/microg DNA of 7-alkylguanine DNA adducts for the concentration of 1500 mg/m3, while no 7-SO-guanine DNA adducts were detected in the liver after 21 days of inhalation exposure to both of styrene concentrations. A comparison of 7-alkylguanines excreted in urine with 7-SO-guanines in lungs (after correction for depurination and for missing alpha-isomers) revealed that persisting 7-SO-guanine DNA adducts in lungs account for about 0.5% of the total alkylation at N7 of guanine. The total styrene-specific 7-guanine alkylation accounts for about 1.0x10(-5)% of the total styrene uptake, while N1-adenine alkylation contributes to this percentage only negligibly.

Measurement of urinary excretion of 5-hydroxymethyluracil in human by GC/NICI/MS:

5-Hydroxymethyluracil (5-HMU) is derived from radiation in addition to endogenous oxidative DNA damage and it is one of the most abundant DNA adducts. Human 5-HMU-DNA-glycosylase has been shown to repair this lesion. Whether urinary levels of 5-HMU is a valid biomarker for oxidative DNA damage in vivo has been investigated. However, controversial results on its relation to cigarette smoking were reported. To facilitate analysis of urinary 5-HMU in epidemiological studies, a highly sensitive and specific assay based on stable isotope dilution gas chromatography/negative ion chemical ionization/mass spectrometry was developed. The limit of detection for N1,N3-bis(pentafluorobenzyl)-HMU is 10 fg (20 amol) (S/N=4) injected on column and the limit of quantification in urine was 0.7 nM of 5-HMU. Using as little as 10 microL of human urine samples, levels of urinary 5-HMU in 21 healthy volunteers were accurately quantified. No correlation was observed between urinary 5-HMU levels and cigarette smoking. However, there was a statistically significant association between urinary levels of 5-HMU and thiobarbituric acid-reactive substances (r=0.71, p=0.0003). In addition, urinary 5-HMU levels also correlated with urinary levels of 1,N6-ethenoadenine (r=0.54, p=0.01). These findings suggest that this assay should be valuable in assessing the role of urinary 5-HMU as a biomarker of oxidative DNA damage and repair.

Quantification of urinary excretion of 1,N6-ethenoadenine, a potential biomarker of lipid peroxidation, in humans by stable isotope dilution liquid chromatography-electrospray ionization-tandem mass spectrometry: comparison with gas chromatography-mass spectrometry

Etheno DNA adducts are promutagenic DNA lesions derived from exogenous as well as endogenous sources. The levels of etheno adducts in tissue DNA are elevated in cancer prone tissues, and the urinary excretion of etheno adducts is associated with oxidative stress. In this report, a new assay based on isotope dilution liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) is developed for the quantification of 1,N(6)-ethenoadenine (epsilonAde) in human urine samples without the need for derivatization. Sample purification before analysis by MS only requires a reversed phase solid phase extraction column. Two multiple reaction monitoring transitions with two product ion fragments generated from a common parent ion were used to quantify urinary epsilonAde. The detection limit of epsilonAde using LC-ESI-MS/MS is 2 pg injected standard epsilonAde on-column, and the assay allows accurate quantification of urinary epsilonAde at concentrations higher than 10 pg/mL. The presence of epsilonAde in human urine is confirmed by the collision-induced daughter ion spectrum. Using this assay, the levels of epsilonAde in the 24 h urine samples from 18 healthy individuals are determined, and the results are in very good agreement with those obtained using isotope dilution gas chromatography-negative ion chemical ionization-mass spectrometry. The high specificity and simple sample pretreatment of this LC-ESI-MS/MS method render it a valuable tool in measuring epsilonAde in the complex mixture of human urine as a promising noninvasive biomarker for DNA damage associated with oxidative stress and for cancer chemoprevention studies.

Urinary excretion of 3,N4-etheno-2'-deoxycytidine in humans as a biomarker of oxidative stress: association with cigarette smoking

Smokers are known to have elevated levels of lipid peroxidation, a form of oxidative stress. Etheno DNA adduct formation can originate from endogenous lipid peroxidation or from exogenous exposure of carcinogens. Using a modified stable isotope dilution GC/negative ion chemical ionization/MS assay originally developed for urinary 3,N(4)-ethenocytosine (epsilonCyt), the nucleoside 3,N(4)-etheno-2'-deoxycytidine (epsilondCyd) was detected for the first time in human urine. The presence of epsilondCyd in human urine was confirmed by LC/electrospray ionization/tandem MS. Concentrations of epsilondCyd in the 24 h urine samples from healthy individuals not occupationally exposed to industrial chemicals were in the range between 0 and 0.80 nM. A statistically significant correlation was established between cigarette smoking and urinary excretion of epsilondCyd after being adjusted for creatinine (p = 0.004). Furthermore, the urinary total antioxidant capacity was found to correlate inversely with the epsilondCyd levels (r = -0.50, p = 0.02). The results indicate that urinary epsilondCyd may provide a valuable noninvasive biomarker for oxidative DNA damage.

Quantification of 1,N6-etheno-2'-deoxyadenosine in human urine by column-switching LC/APCI-MS/MS

1,N6-etheno-2'-deoxyadenosine (epsilondA) is one of several promutagenic DNA modifications arising from cellular oxidative metabolism. It is believed that these background DNA lesions may contribute to various diseases, such as cancer. Therefore, human biomonitoring of epsilondA in urine could be a potential marker for oxidative stress-related DNA damage. Existing methods for quantifying urinary epsilondA use 32P postlabeling. We have developed a nonradioactive, fast, and easier method based on column-switching liquid chromatography with atmospheric pressure chemical ionization tandem mass spectrometry (LC/APCI-MS/MS) in the positive mode. Differences in column temperatures were used to influence analyte retention and sample focusing. With multiple reaction monitoring (MRM) mode the afforded limit of detection was about 0.7 pM when starting with 3 ml of urine. The urinary excretion rates of epsilondA from 28 nonsmoking and 5 smoking men were 10.0-99.6 pmol/24 h, and did not correlate with body weight, age, or plasma vitamin C concentration. The 5 smokers excreted 30.5 +/-8.5 and the 28 nonsmokers excreted 38.6 +/- 2.4 pmol epsilondA per 24 h, p=.37 (mean +/- SEM). The demonstrated level of performance suggests the future applicability of this method to studies of cancer and other diseases related to oxidative stress in humans.