Analysis of DNA adducts of acetaldehyde by liquid chromatography-mass spectrometry

LC-MS/MS Analysis of the Formation and Loss of DNA Adducts in Rats Exposed to Vinyl Acetate Monomer through Inhalation

Formation of DNA adducts is a key event during carcinogenesis. DNA adducts, if not repaired properly, can lead to mutations and cancer. DNA adducts have been frequently used as biomarkers to evaluate chemical exposure. Vinyl acetate monomer (VAM) is widely used in the manufacture of various industrial polymers. Previous studies have documented that VAM induced nasal tumors in rodents exposed to high exposure levels of VAM. VAM is metabolized by carboxylesterase to acetaldehyde (AA), which subsequently results in DNA adducts. However, AA is also an endogenous metabolite in living cells, which impedes accurate assessment of the contribution of VAM exposure under the substantial endogenous background. To address this challenge, we exposed rats to stable isotope labeled [¹³C₂]-VAM at 50, 200, and 400 ppm through inhalation for 6 h, followed by DNA adduct analysis in nasal respiratory and olfactory epithelia with highly sensitive mass spectrometry. Our results show that exogenous N²-ethyl-dG adducts were present in all rats exposed to [¹³C₂]-VAM, with over 2-fold higher DNA adducts in nasal respiratory epithelium than olfactory epithelium. Our data also show that N²-ethyl-dG is a more sensitive biomarker to assess VAM exposure than 1,N²-propano-dG adducts. Moreover, a very low amount of exogenous N²-ethyl-dG adducts were detected in peripheral blood mononuclear cell samples of exposed rats, suggesting that only an extremely small percentage of [¹³C₂]-VAM or its metabolite may enter into systemic circulation to potentially damage tissues beyond nasal epithelium. Furthermore, exogenous N²-ethyl-dG DNA adducts undergo rapid repair or spontaneous loss in nasal epithelium of exposed rats. Taken together, the results presented herein provide novel quantitative data and lay the foundation for future studies to improve risk assessment of VAM.

Effects of acetaldehyde-induced DNA lesions on DNA metabolism

BACKGROUND

Acetaldehyde, produced upon exposure to alcohol, cigarette smoke, polluted air and sugar, is a highly reactive compound that is carcinogenic to humans and causes a variety of DNA lesions in living human cells. Previously, we reported that acetaldehyde reacts with adjacent deoxyguanosine residues on oligonucleotides, but not with single deoxyguanosine residues or other deoxyadenosine, deoxycytosine, or thymidine residues, and revealed that it forms reversible intrastrand crosslinks with the dGpdG sequence (GG dimer).

RESULTS

Here, we show that restriction enzymes that recognize a GG sequence digested acetaldehyde-treated plasmid DNA with low but significant efficiencies, whereas restriction enzymes that recognize other sequences were able to digest such DNA. This suggested that acetaldehyde produced GG dimers in plasmid DNA. Additionally, acetaldehyde-treated oligonucleotides were efficient in preventing digestion by the exonuclease function of T4 DNA polymerase compared to non-treated oligonucleotides, suggesting structural distortions of DNA caused by acetaldehyde-treatment. Neither in vitro DNA synthesis reactions of phi29 DNA polymerase nor in vitro RNA synthesis reactions of T7 RNA polymerase were observed when acetaldehyde-treated plasmid DNA was used, compared to when non-treated plasmid DNA was used, suggesting that acetaldehyde-induced DNA lesions inhibited replication and transcription in DNA metabolism.

CONCLUSIONS

Acetaldehyde-induced DNA lesions could affect the relative resistance to endo- and exo-nucleolytic activity and also inhibit in vitro replication and in vitro transcription. Thus, investigating the effects of acetaldehyde-induced DNA lesions may enable a better understanding of the toxicity and carcinogenicity of acetaldehyde.

Preparation of Cyclic-1,N2-propano-2'-deoxyguanosine-d7 as an Internal Standard for ESI-MS/MS Determination of DNA Damage from Acetaldehyde

Cyclic-1,N²-propano-2'-deoxyguanosine-d₇ (CPr-dG-d₇) was prepared as an isotopic internal standard (IS) for electrospray ionization tandem mass spectrometry (ESI-MS/MS) quantification of CPr-dG in DNA as a candidate cancer risk marker of acetaldehyde intake, mainly from drinking. The deuterated compound was reasonably synthesized from acetaldehyde-d₄ and 2'-deoxyguanosine in deuterium oxide (D₂O), preventing the deuterium atoms of acetaldehyde-d₄ from being substituted by hydrogen atoms, which occurred seriously in aqueous synthesis media via keto-enol tautomerism. Furthermore, another deuterium atom was added from D₂O to form CPr-dG-d₇. After four weeks of storage in H₂O at 10°C, CPr-dG-d₇ was found to be sufficiently stable for practical use. The calibration curve of CPr-dG by using a hydrophilic interaction chromatography-ESI-MS/MS system with CPr-dG-d₇ as the IS showed sufficient linearity from 1.0 × 10^-10 to 4.0 × 10^-9 M with r² = 0.998.

Acetaldehyde forms covalent GG intrastrand crosslinks in DNA

Carcinogens often generate mutable DNA lesions that contribute to cancer and aging. However, the chemical structure of tumorigenic DNA lesions formed by acetaldehyde remains unknown, although it has long been considered an environmental mutagen in alcohol, tobacco, and food. Here, we identify an aldehyde-induced DNA lesion, forming an intrastrand crosslink between adjacent guanine bases, but not in single guanine bases or in other combinations of nucleotides. The GG intrastrand crosslink exists in equilibrium in the presence of aldehyde, and therefore it has not been detected or analyzed in the previous investigations. The newly identified GG intrastrand crosslinks might explain the toxicity and mutagenicity of acetaldehyde in DNA metabolism.

Evaluation of Type-A Endonucleases for the Quantitative Analysis of DNA Damage due to Exposure to Acetaldehyde Using Capillary Electrophoresis

The substrate selectivities of three endonucleases were studied quantitatively using capillary zone electrophoresis to find one giving N²-ethyl(Et)-2'-deoxyguanosine-5'-monophosphate (5'-dGMP) and cyclic 1,N²-propano(CPr)-5'-dGMP from DNAs damaged by acetaldehyde (AA). Six 2'-deoxyribonucleoside-5'-monophosphates to be quantified in the hydrolysis solutions of DNAs, namely, Et-5'-dGMP, CPr-5'-dGMP, and four authentic ones, were completely separated using a 100 mM borate running buffer solution having an optimized pH of 9.67. Using the present method, nuclease reactions of nuclease S1 (NS1), nuclease P1 (NP1), and nuclease Bal 31 to 2'-deoxyribonucleoside-5'-monophosphates from damaged Calf thymus (CT-) DNAs were monitored. The CT-DNAs were prepared by treatment with AA to generate Et-guanine or CPr-guanine internally. Bal 31 hydrolyzed the damaged CT-DNAs to yield Et-5'-dGMP and CPr-5'-dGMP quantitatively. The two 5'-dGMP adducts were not detected in the hydrolysis solutions using NS1 or NP1. Bal 31 can be a suitable nuclease for analyzing DNA damages caused by AA.

Detection of 1,N(2)-propano-2'-deoxyguanosine adducts in genomic DNA by ultrahigh performance liquid chromatography-electrospray ionization-tandem mass spectrometry in combination with stable isotope dilution

Crotonaldehyde (Cro) is one of widespread and genotoxic α,β-unsaturated aldehydes and can react with the exocyclic amino group of 2'-deoxyguanosine (dG) in genomic DNA to form 1,N(2)-propano-2'-deoxyguanosine (ProdG) adducts. In this study, two diastereomers of high purity were prepared, including non-isotope and stable isotope labeled ProdG adducts, and exploited stable isotope dilution-based calibration method. By taking advantage of synthesized ProdG standards, we developed a sensitive ultrahigh performance liquid chromatography-electrospray ionization-tandem mass spectrometry (UHPLC-ESI-MS/MS) method for accurate quantification of two diastereomers of ProdG adducts. In addition to optimization of the UHPLC separation, ammonium bicarbonate (NH4HCO3) was used as additive in the mobile phase for enhancing the ionization efficiency to ProdG adducts and facilitating MS detection. The limits of detection (LODs, S/N=3) and the limits of quantification (LOQs, S/N=10) are estimated about 50 amol and 150 amol, respectively. By the use of the developed method, both diastereomers of ProdG adducts can be detected in untreated human MRC5 cells with a frequency of 2.4-3.5 adducts per 10(8) nucleotides. Crotonaldehyde treatment dramatically increases the levels of ProdG adducts in human MRC5 in a concentration-dependent manner.

Functional preconcentration tip of total volume injection for ESI/MS analysis of DNA adducts

We have developed a simple method to significantly improve the sensitivity in the LC/MS analysis of DNA adducts. A preconcentration tip for the selective recovery of DNA adducts was prepared. Using this tip, the total amount of DNA adducts in a treated DNA sample was injected in a one-shot manner into an LC/MS system. We were able to improve the sensitivity by more than one order of magnitude in concentration. This method will be a useful tool for the quantitative determination of trace DNA adducts.

Cross-link dimer formation of the acetaldehyde-derived cyclic 1,N(2)-Propano-2'-deoxyguanosine adduct using electrochemical oxidation

The electrochemically oxidative lesion of the acetaldehyde-derived cyclic propano adduct 2 of 2'-deoxyguanosine 1 was identified as the cross-linked dimer 4 of adduct 2. Cross-link formation is explained by the nucleophilic preference of the exocyclic amino group in 2 to the carbocation 3 electrogenerated by 1-proton and 2-electron transfers. Dimer formation was also detected in duplex DNA during exposure to acetaldehyde followed by electrochemical oxidation. The dimer has been deduced to be an intrastrand cross-link generated specifically in the G-G sequence in duplex DNA, which is expected to contribute to acetaldehyde-mediated genotoxicity.

Analysis of crotonaldehyde- and acetaldehyde-derived 1,n(2)-propanodeoxyguanosine adducts in DNA from human tissues using liquid chromatography electrospray ionization tandem mass spectrometry

Crotonaldehyde, a mutagen and carcinogen, reacts with deoxyguanosine (dGuo) in DNA to generate a pair of diastereomeric 1,N(2)()-propanodeoxyguanosine adducts (Cro-dGuo, 2), which occur in (6S,8S) and (6R,8R) configurations. They can also be formed through the consecutive reaction of two acetaldehyde molecules with dGuo. Cro-dGuo adducts inhibit DNA synthesis and induce miscoding in human cells. Considering their potential role in carcinogenesis, we have developed a sensitive and specific liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) method to explore the presence of Cro-dGuo adducts in DNA from various human tissues, such as liver, lung, and blood. DNA was isolated from human tissues and enzymatically hydrolyzed to deoxyribonucleosides. [(15)N(5)]Cro-dGuo was synthesized and used as an internal standard. The Cro-dGuo adducts were enriched from the hydrolysate by solid-phase extraction and analyzed by LC-ESI-MS/MS using selected reaction monitoring (SRM). This method allows the quantitation of the Cro-dGuo adducts at a concentration of 4 fmol/micromol dGuo, corresponding to about 1 adduct per 10(9) normal nucleosides starting with 1 mg of DNA, with high accuracy and precision. DNA from human liver, lung, and blood was analyzed. The Cro-dGuo adducts were detected more frequently in human lung DNA than in liver DNA but were not detected in DNA from blood. The results of this study provide quantified data on Cro-dGuo adducts in human tissues. The higher frequency of Cro-dGuo in lung DNA than in the other tissues investigated is potentially important and deserves further study.

Identification of the adduct between a 4-Aza-3-ene-1,6-diyne and DNA using electrospray ionization mass spectrometry

The interactions between a novel enediyne [1-methyl-2-(phenylethynyl)-3-(3-phenylprop-2-ynyl)-3H-benzimidazolium] (1) and various cytosine-containing oligonucleotides were studied using electrospray ionization mass spectrometry (ESI-MS) in a flow injection analysis mode useful for small volumes. This enediyne ligand, developed as a potential alternative to the highly cytotoxic natural enediynes, some of which have been successfully used as anti-tumor agents, has previously been shown to interact with DNA through frank strand scission as well as via the formation of adducts that lead to 2'-deoxycytidine-specific cleavage. Through ESI-MS, the structures of these adducts were examined and a sequence dependence of the 2'-deoxycytidine-specific cleavage was noted. Collisionally activated dissociation of the observed adducts confirmed the strength of the interactions between the enediyne and DNA and supports a direct linkage between the enediyne and the cytosine nucleobase, likely the result of a nucleophilic attack of the phenylethynyl group by the cytosine amine.

LC/MS/MS method for the quantitation of trans-2-hexenal-derived exocyclic 1,N(2)-propanodeoxyguanosine in DNA

trans-2-Hexenal is an alpha,beta-unsaturated aldehyde to which humans are exposed daily in small amounts. Hexenal has demonstrated mutagenicity and genotoxicity in vitro and reacts with deoxyguanosine to form diastereomeric hexenal-derived exocyclic 1,N(2)-propanodeoxyguanosine (Hex-PdG) adducts. A highly sensitive and specific method for the measurement of Hex-PdG in DNA has not previously been available. An LC/MS/MS assay for the quantitation of Hex-PdG, using [(13)C4(15)N2]Hex-PdG as an internal standard, was developed, to assess binding of hexenal to DNA. Samples were purified prior to analysis by centrifuge filtration and solid phase extraction and analyzed by LC/MS/MS in the selected reaction monitoring (SRM) mode (SRM m/z 366.2 --> 250.2 for Hex-PdG; SRM m/z 372.2 --> 256.2 for [(13)C4(15)N2]Hex-PdG). Recovery of standards was 89% or greater, and quantitation was unaffected by the addition of increasing concentrations of calf thymus DNA (ctDNA). The limit of quantitation, determined in samples of 200 microg of ctDNA spiked with analyte standard, was 0.015 fmol/microg DNA, which corresponds to approximately 5 Hex-PdG/10(9) unmodified nucleotides. Hex-PdG was detected in ctDNA treated with 0.021 microM, 0.21 microM, or 2.1 mM hexenal but not in untreated DNA. Furthermore, Hex-PdG was not detected in DNA exposed to reactive oxygen species-mediated deoxyribose attack and lipid peroxidation, which resulted in a significant increase in the malondialdehyde-derived pyrimido[1,2-a]purin-10(3H)one. Hex-PdG was not detected in DNA of untreated rat liver, but Hex-PdG in hexenal-treated calf thymus DNA was quantifiable when spiked into the rat liver DNA at 0.035 or 0.35 fmol/microg DNA. These data indicate that Hex-PdG is formed following hexenal treatment and that this method is suitable for in vitro or in vivo assessment of Hex-PdG formation.

Identification of an acetaldehyde adduct in human liver DNA and quantitation as N2-ethyldeoxyguanosine

Acetaldehyde, an ubiquitous mutagen and carcinogen, could be involved in human cancer etiology. Because DNA adducts are important in carcinogenesis, we have used liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) to explore the presence in human liver DNA of the major acetaldehyde DNA adduct, N2-ethylidenedeoxyguanosine (1). DNA was isolated and enzymatically hydrolyzed in the presence of NaBH3CN, which quantitatively converts adduct 1 to N2-ethyldeoxyguanosine (N2-ethyl-dGuo, 2). [15N5]N2-Ethyl-dGuo was synthesized and used as an internal standard. Adduct 2 was enriched from the hydrolysate by solid phase extraction and analyzed by LC-ESI-MS/MS. Clear peaks were observed for adduct 2 in analyses of human liver DNA, calf thymus DNA, and rat liver DNA. These peaks were not observed, or were much smaller, when the NaBH3CN step was omitted. When the DNA was subjected to neutral thermal hydrolysis prior to NaBH3CN treatment, adduct 2 was not observed. Control experiments using [13C2]acetaldehyde demonstrated that adducts 1 and 2 were not formed as artifacts during DNA isolation and analysis. These results strongly indicate that adduct 1 is present in human liver DNA and demonstrate that it can be quantified as adduct 2. Levels of adduct 2 measured in 12 human liver samples were 534 +/- 245 fmol/micromol dGuo (mean +/- SD). The results of this study establish the presence of an acetaldehyde adduct in human liver DNA and suggest that it is a commonly occurring endogenous DNA adduct.

Simultaneous determination of O6-methyl-2'-deoxyguanosine, 8-oxo-7,8-dihydro-2'-deoxyguanosine, and 1,N6-etheno-2'-deoxyadenosine in DNA using on-line sample preparation by HPLC column switching coupled to ESI-MS/MS

O(6)-Methyl-2'-deoxyguanosine (O(6)-mdGuo), 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo), and 1,N(6)-etheno-2'-deoxyadenosine (epsilondAdo) are promutagenic DNA lesions originating from both endogenous and exogenous agents and actions (methylation, hydroxylation, lipid peroxidation products). A highly sensitive quantitative method was developed to measure these DNA adducts simultaneously, using liquid chromatography tandem mass spectrometry with column switching. Deuterated O(6)-[(2)H(3)]mdGuo was synthesized and used as internal standard. The limits of quantification for O(6)-mdGuo, 8-oxodGuo, and epsilondAdo were 24, 98, and 48 fmol on column, respectively. The method showed linearity in the range 0.24-125 pmol/ml, 0.98-125 pmol/ml, and 0.49-62.5 pmol/ml for the three adducts, respectively. The inter-day precision in the linear concentration range was between 1.7 and 9.3% for O(6)-mdGuo, 10.6 and 28.7% for 8-oxodGuo, and 6.2 and 10.4%, for epsilondAdo. In DNA isolated from liver of untreated 12-week-old female F344 rats, O(6)-mdGuo was above the limit of detection (37 adducts per 10(9) normal nucleosides) but could not be quantified. 8-oxodGuo and epsilondAdo showed background levels of 500 and 130 adducts per 10(9) normal nucleosides, respectively. DNA analyzed 1h after treatment of rats with dimethylnitrosamine by oral gavage of 50 microg/kg b.wt. did not affect the levels of 8-oxodGuo and epsilondAdo but resulted in 200 O(6)-mdGuo adducts per 10(9) normal nucleosides. The method developed will be of use to study the biological significance of exogenous DNA adducts as an increment to background DNA damage and the role of modulating factors, such as DNA repair.

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.

LC-MS Study on the Formation of Cyclic 1,N2-Propano Guanine Adduct in the Reactions of DNA with Acetaldehyde in the Presence of Histone

The formation of cyclic 1,N2-propano guanine (CPr-Gua) adduct is significantly accelerated by the addition of arginine or histone in the reaction of calf thymus DNA with acetaldehyde (AA) or crotonaldehyde (CA). Histone proteins, containing a large amount of basic amino acids such as arginine, are essential as nucleosome cores to package DNA. In the presence of 0.60% (w/v) histone in the reaction mixture, 8-times and 25-times larger amounts of the CPr-Gua adduct were formed in the reaction of the DNA with AA and CA, respectively, compared with those in the absence of histone. Furthermore, for the DNA incubated at 95 degrees C for 10 min and cooled on ice to make the single-stranded moieties, 72-times and 178-times larger amounts of the CPr-Gua adduct were formed by AA and CA, respectively, in the presence of 0.60% (w/v) histone. These results strongly suggest that DNA in vivo should be exposed to a much more dangerous situation, compared with DNA alone, from the viewpoint of reactivity with the aldehydes. During DNA replication and transcriptional events of cells, the danger will be further increased markedly because of opening of double-strands. Semi-micro HPLC-ESI-MS measurements following deprination of DNA samples were performed for quantification of the adduct as the corresponding base form, CPr-Gua, in evaluation of the reactivity of DNA with AA and CA.

Separation procedures capable of revealing DNA adducts

Detection and quantification of DNA adducts are very important in relation to diseases such as cancer. Both high sensitivity and high selectivity are required for the detection of DNA adducts because the content of adducts in DNA is very small compared with those of normal bases and only small amounts of DNA samples are available for analysis in general cases. In this paper are described separation procedures such as liquid chromatography, gas chromatography and capillary electrophoresis combined with a detection and identification method such as 32P-postlabeling, mass spectrometry, electrochemical detection, fluorescence detection and immunoassay. The merits and demerits of the procedures are also discussed.

Histones accelerate the cyclic 1, N 2 -propanoguanine adduct-formation of DNA by the primary metabolite of alcohol and carcinogenic crotonaldehyde

Chemical modification of 2'-deoxyguanosine and DNA by excessive acetaldehyde and crotonaldehyde were significantly accelerated by the presence of histones, which are nuclear proteins very rich in the basic amino acids such as L-arginine and L-lysine, resulting in the smooth and selective formation of the corresponding cyclic 1,N(2)-propanoguanine adducts under physiological conditions. Thus, histones have a very close connection with the genotoxic and carcinogenic effects of these aldehydes.