Diet is not responsible for the presence of several oxidatively damaged DNA lesions in mouse urine

Nucleic acid adductomics - The next generation of adductomics towards assessing environmental health risks

This Discussion article aims to explore the potential for a new generation of assay to emerge from cellular and urinary DNA adductomics which brings together DNA-RNA- and, to some extent, protein adductomics, to better understand the role of the exposome in environmental health. Components of the exposome have been linked to an increased risk of various, major diseases, and to identify the precise nature, and size, of risk, in this complex mixture of exposures, powerful tools are needed. Modification of nucleic acids (NA) is a key consequence of environmental exposures, and a goal of cellular DNA adductomics is to evaluate the totality of DNA modifications in the genome, on the basis that this will be most informative. Consequently, an approach which encompasses modifications of all nucleic acids (NA) would be potentially yet more informative. This article focuses on NA adductomics, which brings together the assessment of both DNA and RNA modifications, including modified (2'-deoxy)ribonucleosides (2'-dN/rN), modified nucleobases (nB), plus: DNA-DNA, RNA-RNA, DNA-RNA, DNA-protein, and RNA-protein crosslinks (DDCL, RRCL, DRCL, DPCL, and RPCL, respectively). We discuss the need for NA adductomics, plus the pros and cons of cellular vs. urinary NA adductomics, and present some evidence for the feasibility of this approach. We propose that NA adductomics provides a more comprehensive approach to the study of nucleic acid modifications, which will facilitate a range of advances, including the identification of novel, unexpected modifications e.g., RNA-RNA, and DNA-RNA crosslinks; key modifications associated with mutagenesis; agent-specific mechanisms; and adductome signatures of key environmental agents, leading to the dissection of the exposome, and its role in human health/disease, across the life course.

DNA Damage as a Potential Non-Invasive Indicator of Welfare: A Preliminary Study in Zoo-Housed Grizzly Bears (Ursus arctos horribilis)

Measures of oxidative stress have potential for integrating positive and negative life experiences into comprehensive cellular indicators of animal welfare. We explored this possibility when three adult grizzly bear brothers at the Detroit Zoo were temporarily moved to a smaller habitat while their primary home was expanded. We expected that the spatial compression and construction activity might be sources of stress. We observed increased social play and other affiliative behavior in the smaller habitat, and we used daily fecal samples (17 to 24 per bear) to examine whether concentrations of fecal glucocorticoid metabolites (FGM) and 8-hydroxy-2′-deoxyguanosine (8-OHdG, a by-product of DNA damage) were correlated with social behavior. Our overall aim was to explore 8-OHdG as a potential indicator of welfare based on the prediction that 8-OHdG would be lower when more positive social interactions occurred. Concentrations of fecal 8-OHdG increased significantly with higher FGM concentrations, supporting a potential relationship between adrenal activity and rates of DNA damage. However, we found that on days when they engaged in higher rates of affiliative interactions, there were trends for 8-OHdG concentrations to increase for one bear and decrease for another, and no relationship for the third bear. These preliminary results should be interpreted with caution, but suggest a potential relationship between social behavior and 8-OHdG that is modulated by health, personality, or other individual factors. Further validation research is needed, but 8-OHdG may have promise as a non-invasive, cumulative indicator of animal welfare.

Biomarkers of nucleic acid oxidation - A summary state-of-the-art

Oxidatively generated damage to DNA has been implicated in the pathogenesis of a wide variety of diseases. Increasingly, interest is also focusing upon the effects of damage to the other nucleic acids, RNA and the (2′-deoxy-)ribonucleotide pools, and evidence is growing that these too may have an important role in disease. LC-MS/MS has the ability to provide absolute quantification of specific biomarkers, such as 8-oxo-7,8-dihydro-2′-deoxyGuo (8-oxodG), in both nuclear and mitochondrial DNA, and 8-oxoGuo in RNA. However, significant quantities of tissue are needed, limiting its use in human biomonitoring studies. In contrast, the comet assay requires much less material, and as little as 5 μL of blood may be used, offering a minimally invasive means of assessing oxidative stress in vivo, but this is restricted to nuclear DNA damage only. Urine is an ideal matrix in which to non-invasively study nucleic acid-derived biomarkers of oxidative stress, and considerable progress has been made towards robustly validating these measurements, not least through the efforts of the European Standards Committee on Urinary (DNA) Lesion Analysis. For urine, LC-MS/MS is considered the gold standard approach, and although there have been improvements to the ELISA methodology, this is largely limited to 8-oxodG. Emerging DNA adductomics approaches, which either comprehensively assess the totality of adducts in DNA, or map DNA damage across the nuclear and mitochondrial genomes, offer the potential to considerably advance our understanding of the mechanistic role of oxidatively damaged nucleic acids in disease.

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Oxidatively damaged nucleic acids are implicated in the pathogenesis of disease.

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LC-MS/MS, comet assay and ELISA are often used to study oxidatively damaged DNA.

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Urinary oxidatively damaged nucleic acids non-invasively reflect oxidative stress.

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DNA adductomics will aid understanding the role of ROS damaged DNA in disease.

Oxidatively generated modifications to nucleic acids in vivo: Measurement in urine and plasma

BACKGROUND

The oxidized guanine nucleosides, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) and 8-oxo-7,8-dihydroguanosine (8-oxoGuo), derived from DNA and RNA, respectively, were used to investigate the importance of oxidative stress to nucleic acids in vivo. High urinary excretion of 8-oxodG is associated with cancer development, whereas high urinary excretion of 8-oxoGuo is associated with mortality in type 2 diabetes. Like creatinine, these small water-soluble molecules are not reabsorbed in the kidney. Therefore, 8-oxo nucleoside/creatinine reciprocal concentration ratios are identical in plasma and urine. The total amount of 8-oxo guanine nucleosides excreted by the kidneys is the product of plasma concentration and glomerular filtration rate.

METHODS

With relevant equations and an estimated glomerular filtration rate, the 24-h urinary excretion of 8-oxodG and 8-oxoGuo was calculated in 2679 subjects with type 2 diabetes, displaying good correlation with the measured urinary 8-oxo nucleoside/creatinine ratio: DNA oxidation r = 0.86 and RNA oxidation r = 0.84 (p < 0.05 for both).

RESULTS

Survival analyses based on the quartiles of the 8-oxodG/creatinine ratio and the quartiles of calculated 24-h urinary excretion rate of the 2679 subjects gave similar hazard ratio estimates for death due to all causes. This finding was similar for the 8-oxoGuo hazard ratio estimates.

CONCLUSIONS

This study shows that oxidatively generated modifications to DNA and RNA in vivo can be measured using 1) a spot urine sample, normalized to urinary creatinine, 2) 24-h urine, or 3) a single plasma sample based on concentrations of 8-oxo nucleoside and creatinine and glomerular filtration rate.

Urinary DNA adductomics - A novel approach for exposomics

The exposome is a concept that encompasses the totality of internal and external environmental exposures, from conception onwards. Evaluation of the exposome, across the lifecourse represents a significant challenge, e.g., methods/technology may simply not exist to comprehensively assess all exposures, or they may not be applicable to human populations, or may have insufficient sensitivity. Cellular DNA adductomics aims to determine the totality of DNA adducts in the cellular genome. However, application to human populations requires the necessarily invasive sampling of tissue, to obtain sufficient DNA for sensitive analysis, which can represent a logistical and IRB challenge, particularly when investigating vulnerable populations. To circumvent this, we recently applied DNA adductomics to urine, detecting a range of expected and unexpected 2'-deoxyribonucleoside DNA adducts. However, base excision repair, the main DNA repair pathway for non-bulky DNA adducts, and processes such as spontaneous depurination, generate nucleobase adducts. Herein we propose a strategy to simultaneously assess 2'-deoxyribonucleoside and nucleobase adducts, using a widely used mass spectrometic platform (i.e., triple quadrupole tandem mass spectrometry). This will provide a much needed DNA adductomic approach for non-invasively, biomonitoring environmental exposures, through assessing the totality of DNA adducts; contributing to the evaluation of the exposome, across the life-course.

Iron induced RNA-oxidation in the general population and in mouse tissue

Iron promotes formation of hydroxyl radicals by the Fenton reaction, subsequently leading to potential oxidatively generated damage of nucleic acids. Oxidatively generated damage to RNA, measured as 8-oxo-7,8-dihydroguanosine (8-oxoGuo) in urine, is increased in patients with genetic iron overload, which have led us to test the hypothesis that high iron status, assessed by iron biomarkers and genetic disposition, increases urinary excretion of 8-oxoGuo. In a general Danish population study we used a Mendelian randomization design with HFE genotypes as a proxy for iron status and supplemented with ex vivo experiments in mice muscle tissue exposed to iron(II) sulfate to attempt to clarify this hypothesis. The biomarkers ferritin, transferrin, and transferrin saturation (TS) were associated with 8-oxoGuo (in linear univariable and multivariable regression analyses: P < 0.001). Mendelian randomization indicated a causal pathway between genetically elevated iron biomarkers (assessed by ferritin and TS) and high levels of 8-oxoGuo. The ex vivo experiments showed a monotonically increase in 8-oxoGuo with increased iron concentration (ANOVA: P = 0.0008) that was prevented with iron chelation (P = 0.01). Our results indicate a causal relationship between iron biomarkers and 8-oxoGuo. Furthermore, the ex vivo experiment shows a mechanistic link between iron and 8-oxoGuo formation. Both iron overload and the biomarker 8-oxoGuo have been linked to e.g. diabetes, which merits future studies to investigate if iron induced 8-oxoGuo is involved in disease development.

Nucleotide excision repair of oxidised genomic DNA is not a source of urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine

Urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) is a widely measured biomarker of oxidative stress. It has been commonly assumed to be a product of DNA repair, and therefore reflective of DNA oxidation. However, the source of urinary 8-oxodGuo is not understood, although potential confounding contributions from cell turnover and diet have been ruled out. Clearly it is critical to understand the precise biological origins of this important biomarker, so that the target molecule that is oxidised can be identified, and the significance of its excretion can be interpreted fully. In the present study we aimed to assess the contributions of nucleotide excision repair (NER), by both the global genome NER (GG-NER) and transcription-coupled NER (TC-NER) pathways, and sanitisation of the dGTP pool (e.g. via the activity of the MTH1 protein), on the production of 8-oxodGuo, using selected genetically-modified mice. In xeroderma pigmentosum A (XPA) mice, in which GG-NER and TC-NER are both defective, the urinary 8-oxodGuo data were unequivocal in ruling out a contribution from NER. In line with the XPA data, the production of urinary 8-oxodGuo was not affected in the xeroderma pigmentosum C mice, specifically excluding a role of the GG-NER pathway. The bulk of the literature supports the mechanism that the NER proteins are responsible for removing damage to the transcribed strand of DNA via TC-NER, and on this basis we also examined Cockayne Syndrome mice, which have a functional loss of TC-NER. These mice showed no difference in urinary 8-oxodGuo excretion, compared to wild type, demonstrating that TC-NER does not contribute to urinary 8-oxodGuo levels. These findings call into question whether genomic DNA is the primary source of urinary 8-oxodGuo, which would largely exclude it as a biomarker of DNA oxidation. The urinary 8-oxodGuo levels from the MTH1 mice (both knock-out and hMTH1-Tg) were not significantly different to the wild-type mice. We suggest that these findings are due to redundancy in the process, and that other enzymes substitute for the lack of MTH1, however the present study cannot determine whether or not the 2'-deoxyribonucleotide pool is the source of urinary 8-oxodGuo. On the basis of the above, urinary 8-oxodGuo is most accurately defined as a non-invasive biomarker of oxidative stress, derived from oxidatively generated damage to 2'-deoxyguanosine.

Comparison of oxidative stress/DNA damage in semen and blood of fertile and infertile men

Abnormal spermatozoa frequently display typical features of oxidative stress, i.e. excessive level of reactive oxygen species (ROS) and depleted antioxidant capacity. Moreover, it has been found that a high level of oxidatively damaged DNA is associated with abnormal spermatozoa and male infertility. Therefore, the aim of our study was the comparison of oxidative stress/DNA damage in semen and blood of fertile and infertile men. The broad range of parameters which describe oxidative stress and oxidatively damaged DNA and repair were analyzed in the blood plasma and seminal plasma of groups of fertile and infertile subjects. These parameters include: (i) 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG) and 8-oxo-7,8-dihydroguanine (8-oxoGua) levels in urine; (ii) 8-oxodG level in DNA isolated from leukocytes and spermatozoa; (iii) antioxidant vitamins (A, C and E) and uric acid. Urinary excretion of 8-oxodG and 8-oxoGua and the level of oxidatively damaged DNA in leukocytes as well as the level of antioxidant vitamins were analyzed using HPLC and HPLC/GC/MS methods.

The results of our study demonstrate that 8-oxodG level significantly correlated with every parameter which describe sperm quality: sperm count, motility and morphology. Moreover, the data indicate a higher level of 8-oxodG in sperm DNA compared with DNA of surrogate tissue (leukocytes) in infertile men as well as in healthy control group. For the whole study population the median values of 8-oxodG/10⁶ dG were respectively 7.85 and 5.87 (p = 0.000000002). Since 8-oxodG level in sperm DNA is inversely correlated with urinary excretion rate of 8-oxoGua, which is the product of OGG1 activity, we hypothesize that integrity of spermatozoa DNA may be highly dependent on OGG1 activity. No relationship between the whole body oxidative stress and that of sperm plasma was found, which suggests that the redox status of semen may be rather independent on this characteristic for other tissues.

8-Oxo-2'-deoxyguanosine as a biomarker of tobacco-smoking-induced oxidative stress

7,8-Dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dGuo) is a useful biomarker of oxidative stress. However, its analysis can be challenging because 8-oxo-dGuo must be quantified in the presence of dGuo, without artifactual conversion to 8-oxo-dGuo. Urine is the ideal biological fluid for population studies, because it can be obtained noninvasively and it is less likely that artifactual oxidation of dGuo can occur because of the relatively low amounts that are present compared with hydrolyzed DNA. Stable isotope dilution liquid chromatography-selected reaction monitoring/mass spectrometry (LC-SRM/MS) with 8-oxo-[(15)N(5)]dGuo as internal standard provided the highest possible specificity for 8-oxo-dGuo analysis. Furthermore, artifact formation was determined by addition of [(13)C(10)(15)N(5)]dGuo and monitoring of its conversion to 8-oxo-[(13)C(10)(15)N(5)]dGuo during the analytical procedure. 8-Oxo-dGuo concentrations were normalized for interindividual differences in urine flow by analysis of creatinine using stable isotope dilution LC-SRM/MS. A significant increase in urinary 8-oxo-dGuo was observed in tobacco smokers compared with nonsmokers either using simple urinary concentrations or after normalization for creatinine excretion. The mean levels of 8-oxo-dGuo were 1.65ng/ml and the levels normalized to creatinine were 1.72μg/g creatinine. Therefore, stable isotope dilution LC-SRM/MS analysis of urinary 8-oxo-dGuo complements urinary isoprostane (isoP) analysis for assessing tobacco-smoking-induced oxidative stress. This method will be particularly useful for studies that employ polyunsaturated fatty acids, in which a reduction in arachidonic acid precursor could confound isoP measurements.

Assays for urinary biomarkers of oxidatively damaged nucleic acids

The analysis of oxidized nucleic acid metabolites can be performed by a variety of methodologies: liquid chromatography coupled with electrochemical or mass-spectrometry detection, gas chromatography coupled with mass spectrometry, capillary electrophoresis and ELISA (Enzyme-linked immunosorbent assay). The major analytical challenge is specificity. The best combination of selectivity and speed of analysis can be obtained by liquid chromatography coupled with tandem mass spectrometric detection. This, however, is also the most demanding technique with regard to price, complexity and skills requirement. The available ELISA methods present considerable specificity problems and cannot be recommended at present. The oxidized nucleic acid metabolites in urine are assumed to originate from the DNA and RNA. However, direct evidence is not available. A possible contribution from the nucleotide pools is most probably minimal, if existing. Recent investigation on RNA oxidation has shown conditions where RNA oxidation but not DNA oxidation is prominent, and while investigation on DNA is of huge interest, RNA oxidation may be overlooked. The methods for analyzing oxidized deoxynucleosides can easily be expanded to analyze the oxidized ribonucleosides. The urinary measurement of oxidized nucleic acid metabolites provides a non-invasive measurement of oxidative stress to DNA and RNA.

Association between 8-oxo-7,8-dihydroguanine excretion and risk of lung cancer in a prospective study

Oxidative damage to guanine (8-oxoGua) is one of the most abundant lesions induced by oxidative stress and documented mutagenic. 8-Oxoguanine DNA glycosylase 1 (OGG1) removes 8-oxoGua from DNA by excision. The urinary excretion of 8-oxoGua is a biomarker of exposure, reflecting the rate of damage in the steady state. The aim of this study was to investigate urinary 8-oxoGua as a risk factor for lung cancer. In a nested case-cohort design we examined associations between urinary excretion of 8-oxoGua and risk of lung cancer as well as potential interaction with the OGG1 Ser326Cys polymorphism in a population-based cohort of 25,717 men and 27,972 women aged 50-64 years with 3-7 years follow-up. We included 260 cases with lung cancer and a subcohort of 263 individuals matched on sex, age, and smoking duration for comparison. Urine collected at entry was analysed for 8-oxoGua by HPLC with electrochemical detection. There was no significant effect of smoking or OGG1 genotype on the excretion of 8-oxoGua. Overall the incidence rate ratio (IRR) (95% confidence interval) of lung cancer was 1.06 (0.97-1.15) per doubling of 8-oxoGua excretion. The association between lung cancer risk and 8-oxoGua excretion was significant among men [IRR: 1.17 (1.03-1.31)], never-smokers [IRR: 9.94 (1.04-94.7)], and former smokers [IRR: 1.19 (1.07-1.33)]. There was no significant interaction with the OGG1 genotype, although the IRR was 1.14 (0.98-1.34) among subjects homozygous for Cys326. The association between urinary 8-oxoGua excretion and lung cancer risk among former and never-smokers suggests that oxidative stress with damage to DNA is important in this group.

Non-invasive assessment of oxidatively damaged DNA: liquid chromatography-tandem mass spectrometry analysis of urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine

The ability to non-invasively assess DNA oxidation and its repair, has significant utility in large-scale, population-based studies. Such studies could include the assessments of: the efficacy of antioxidant intervention strategies, pathological roles of DNA oxidation in various disease states and population or interindividual differences in antioxidant defence and DNA repair. The most popular method, to non-invasively assess oxidative insult to the genome is by the analysis of urine for 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), using chromatographic techniques or immunoassay procedures. The provenance of extracellular 8-oxodG remains a subject for debate. However, previous studies have shown that factors, such as diet and cell death, do not appear to contribute to extracellular 8-oxodG, leaving processes, such as the repair of DNA and/or the 2'-deoxyribonucleotide pool, as the sole source of endogenous 8-oxodG. The method in this chapter describes a non-invasive approach for assessing oxidative stress, via the efficient extraction of urinary 8-oxodG using a validated solid-phase extraction procedure. Subsequent analysis by liquid chromatography-tandem mass spectrometry provides the advantages of sensitivity, internal standardisation, and robust peak identification, and is widely considered to be the "gold standard".

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

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

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.

DNA repair and the origins of urinary oxidized 2'-deoxyribonucleosides

Monitoring oxidative stress in vivo is made easier by the ability to use samples obtained non-invasively, such as urine. The analysis of DNA oxidation, by measurement of oxidized 2'-deoxyribonucleosides in urine, particularly 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), has been reported extensively in the literature in many situations relating to various pathologies, populations and environmental exposures. Understanding the origins of urinary 8-oxodG, other than it simply being a marker of DNA oxidation or its synthetic precursors, is important to being able to effectively interpret differences in baseline urinary 8-oxodG levels between subject groups and changes in excretion. Diet and cell turnover play negligible roles in contributing to urinary 8-oxodG levels, leaving DNA repair as a primary source of this lesion. However, which repair processes contribute, and to what extent, to urinary 8-oxodG is still open to question. The most rational source would be the activity of selected members of the Nudix hydrolase family of enzymes, sanitizing the deoxyribonucleotide pool via the degradation of 8-oxo-7,8-dihydro-2'-deoxyguanosine-5'-triphosphate and 8-oxo-7,8-dihydro-2'-deoxyguanosine-5'-diphosphate, yielding mononucleotide products that can then be dephosphorylated to 8-oxodG and excreted. However, nucleotide excision repair (NER), transcription-coupled repair, nucleotide incision repair (NIR), mismatch repair and various exonuclease activities, such as proofreading function associated with DNA polymerases, can all feasibly generate initial products that could yield 8-oxodG after further metabolism. A recent study implying that a significant proportion of genomic 8-oxodG exists in the context of tandem lesions, refractory to repair by glycosylases, suggests the roles of NER and/or NIR remain to be further examined and defined as a source of 8-oxodG. 8-OxodG has been the primary focus of investigation, but other oxidized 2'-deoxyribonucleosides have been detected in urine, 2'-deoxythymidine glycol and 5-hydroxymethyl-2'-deoxyuridine; the origins of these compounds in urine, however, are presently even more speculative than for 8-oxodG.

Analysis of urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine by liquid chromatography-tandem mass spectrometry

The ability to non-invasively monitor DNA oxidation and its repair has significant utility in large-scale, population-based studies. Such studies could include assessments of the efficacy of antioxidant intervention strategies, pathological roles of DNA oxidation in various disease states and population or inter-individual differences in antioxidant defence and DNA repair. The analysis of urine, or indeed any extracellular matrix, for 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), using chromatographic or immunoassay procedures, is by far the most popular method to non-invasively assess oxidative insult to the genome. The actual biological significance of the presence of extracellular 8-oxodG is still a subject for debate however. Studies are slowly ruling out confounding factors such as diet and cell turnover, which would leave endogenous processes, notably repair, as the sole source of extracellular 8-oxodG. The method described herein exploits the non-invasive properties of urine sampling, coupled with efficient extraction of 8-oxodG by a validated solid-phase extraction procedure. Subsequent analysis by liquid chromatography-tandem mass spectrometry has the advantages of sensitivity, internal standardisation and robust peak identification.

Sources of extracellular, oxidatively-modified DNA lesions: implications for their measurement in urine

There is a robust mechanistic basis for the role of oxidation damage to DNA in the aetiology of various major diseases (cardiovascular, neurodegenerative, cancer). Robust, validated biomarkers are needed to measure oxidative damage in the context of molecular epidemiology, to clarify risks associated with oxidative stress, to improve our understanding of its role in health and disease and to test intervention strategies to ameliorate it. Of the urinary biomarkers for DNA oxidation, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) is the most studied. However, there are a number of factors which hamper our complete understanding of what meausrement of this lesion in urine actually represents. DNA repair is thought to be a major contributor to urinary 8-oxodG levels, although the precise pathway(s) has not been proven, plus possible contribution from cell turnover and diet are possible confounders. Most recently, evidence has arisen which suggests that nucleotide salvage of 8-oxodG and 8-oxoGua can contribute substantially to 8-oxoG levels in DNA and RNA, at least in rapidly dividing cells. This new observation may add an further confounder to the conclusion that 8-oxoGua or 8-oxodG, and its nucleobase equivalent 8-oxoguanine, concentrations in urine are simply a consequence of DNA repair. Further studies are required to define the relative contributions of metabolism, disease and diet to oxidised nucleic acids and their metabolites in urine in order to develop urinalyis as a better tool for understanding human disease.

Impact of radiotherapy and chemotherapy on biomarkers of oxidative DNA damage in lung cancer patients

OBJECTIVES

To assess oxidative damage to DNA during lung cancer (LC) treatments.

DESIGN AND METHODS

Urinary levels of 8-oxoguanine (8-oxoGua) and levels of 8-oxo-2'-deoxyguanosine (8-oxodG) from urine and whole blood were determined in 36 non-cancer controls and 65 LC patients before any treatments. Samples were also obtained of LC patients during and after radiotherapy (RT, n=33) and chemotherapy (CT, n=16).

RESULTS

Stage IV LC patients had higher urinary 8-oxoGua and 8-oxodG levels than patients with stage I-III disease (p=0.044 and p=0.034, respectively). Urinary 8-oxodG levels increased during the first week of RT (p<0.001). Nuclear 8-oxodG increased during RT and 3 months after start of RT. Nuclear 8-oxodG levels also rose between the first two CT cycles (p=0.043), and urinary 8-oxodG levels during the sixth CT cycle (p=0.009).

CONCLUSIONS

Urinary DNA damage biomarker levels may be associated with LC stage. Both RT and CT increase the parameters of DNA oxidation.

Long-term effects of caloric restriction or exercise on DNA and RNA oxidation levels in white blood cells and urine in humans

Excessive adiposity is associated with increased oxidative stress and accelerated aging. Weight loss induced by negative energy balance reduces markers of oxidation in experimental animals and humans. The long-term effects of weight loss induced by calorie restriction or increased energy expenditure induced by exercise on measures of oxidative stress and damage have not been studied in humans. The objective of the present study was to compare the effects of 20% caloric restriction or 20% exercise alone over 1 year on oxidative damage to DNA and RNA, as assessed through white blood cell and urine analyses. Eighteen men and women aged 50 to 60 years with a body mass index (BMI) between 23.5 to 29.9 kg/m(2) were assigned to one of two conditions--20% CR (n = 9) or 20% EX (n = 9)--which was designed to produce an identical energy deficit through increased energy expenditure. Compared to baseline, both interventions significantly reduced oxidative damage to both DNA (48.5% and 49.6% reduction for the CR and EX groups, respectively) and RNA (35.7% and 52.1% reduction for the CR and EX groups, respectively) measured in white blood cells. However, urinary levels of DNA and RNA oxidation products did not differ from baseline values following either 12-month intervention program. Data from the present study provide evidence that negative energy balances induced through either CR or EX result in substantial and similar improvements in markers of DNA and RNA damage to white blood cells, potentially by reducing systemic oxidative stress.

Biomarkers of oxidative damage to DNA and repair

Oxidative-stress-induced damage to DNA includes a multitude of lesions, many of which are mutagenic and have multiple roles in cancer and aging. Many lesions have been characterized by MS-based methods after extraction and digestion of DNA. These preparation steps may cause spurious base oxidation, which is less likely to occur with methods such as the comet assay, which are based on nicking of the DNA strand at modified bases, but offer less specificity. The European Standards Committee on Oxidative DNA Damage has concluded that the true levels of the most widely studied lesion, 8-oxodG (8-oxo-7,8-dihydro-2′-deoxyguanosine), in cellular DNA is between 0.5 and 5 lesions per 106 dG bases. Base excision repair of oxidative damage to DNA can be assessed by nicking assays based on oligonucleotides with lesions or the comet assay, by mRNA expression levels or, in the case of, e.g., OGG1 (8-oxoguanine DNA glycosylase 1), responsible for repair of 8-oxodG, by genotyping. Products of repair in DNA or the nucleotide pool, such as 8-oxodG, excreted into the urine can be assessed by MS-based methods and generally reflects the rate of damage. Experimental and population-based studies indicate that many environmental factors, including particulate air pollution, cause oxidative damage to DNA, whereas diets rich in fruit and vegetables or antioxidant supplements may reduce the levels and enhance repair. Urinary excretion of 8-oxodG, genotype and expression of OGG1 have been associated with risk of cancer in cohort settings, whereas altered levels of damage, repair or urinary excretion in case-control settings may be a consequence rather than the cause of the disease.

Oxidatively damaged DNA and its repair after experimental exposure to wood smoke in healthy humans

Particulate matter from wood smoke may cause health effects through generation of oxidative stress with resulting damage to DNA. We investigated oxidatively damaged DNA and related repair capacity in peripheral blood mononuclear cells (PBMC) and measured the urinary excretion of repair products after controlled short-term exposure of human volunteers to wood smoke. Thirteen healthy adults were exposed first to clean air and then to wood smoke in a chamber during 4h sessions, 1 week apart. Blood samples were taken 3h after exposure and on the following morning, and urine was collected after exposure, from bedtime until the next morning. We measured the levels of DNA strand breaks (SB), oxidized purines as formamidopyrimidine-DNA-glycosylase (FPG) sites and activity of oxoguanine glycosylase 1 (hOGG1) in PBMC by the comet assay, whereas mRNA levels of hOGG1, nucleoside diphosphate linked moiety X-type motif 1 (hNUDT1) and heme oxygenase 1 (hHO1) were determined by real-time RT-PCR. The excretion of 8-oxo-7,8-dihydro-oxoguanine (8-oxoGua) and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) in urine was measured by high performance liquid chromatography purification followed by gas chromatography with mass spectrometry. The morning following exposure to wood smoke the PBMC levels of SB were significantly decreased and the mRNA levels of hOGG1 significantly increased. FPG sites, hOGG1 activity, expression of hNUDT1 and hHO1, urinary excretion of 8-oxodG and 8-oxoGua did not change significantly. Our findings support that exposure to wood smoke causes systemic effects, although we could not demonstrate genotoxic effects, possibly explained by enhanced repair and timing of sampling.

Effects of basal level of antioxidants on oxidative DNA damage in humans

BACKGROUND

Vitamins A, E and C, and uric acid, which can scavenge free radicals should also protect DNA from the damage. It is reasonable to assume that agents that decrease oxidative DNA damage should also decrease subsequent cancer development.

AIM OF THE STUDY

A relationship between basal level of antioxidants (vitamins A, C and E and uric acid) and oxidative DNA damage was assessed. For the first time, the broad spectrum of oxidative DNA damage biomarkers: urinary excretion of 8-oxodG, 8-oxoGua and 5HMUra as well as the level of oxidative DNA damage in leukocytes was analyzed in healthy subjects (n = 158).

METHODS

Using HPLC prepurification/isotope dilution GC/MS methodology, we examined the amount of oxidative DNA damage products excreted into urine and the amount of 8-oxodG in leukocytes' DNA (with HPLC/EC technique). The level of antioxidant vitamins and uric acid was estimated by HPLC technique with fluorimetric and UV detection.

RESULTS

Analyses of relationship between the most common antioxidants (vitamins A, C, E and uric acid) and oxidative DNA damage products reveal weak, statistically significant negative correlation between retinol and all the measured parameters except 5HMUra. Vitamin C negatively correlates with urinary excretion of 8-oxodG and 8-oxoGua. Uric acid revealed statistically significant negative correlation with 8-oxodG in cellular DNA and urinary excretion of 5HMUra, while alpha-tocopherol correlates negatively only with 8-oxodG in cellular DNA. Good, significant (P < 0.0001), positive correlation (r = 0.61) was noted between urinary levels of the base, 8-oxoGua and the deoxynucleoside, 8-oxodG.

CONCLUSION

Our results suggest that oxidative DNA damage shows limited but significant response to antioxidants analyzed in this study and is more affected by many other cellular functions like antioxidant enzymes or DNA repair enzymes as well as genetics.

Evidence for attenuated cellular 8-oxo-7,8-dihydro-2'-deoxyguanosine removal in cancer patients

Measurement of the products of oxidatively damaged DNA in urine is a frequently used means by which oxidative stress may be assessed non-invasively. We believe that urinary DNA lesions, in addition to being biomarkers of oxidative stress, can potentially provide more specific information, for example, a reflection of repair activity. We used high-performance liquid chromatography prepurification, with gas chromatography-mass spectrometry (LC-GC-MS) and ELISA to the analysis of a number of oxidative [e.g., 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), 8-oxo-7,8-dihydro-guanine, 5-(hydroxymethyl)uracil], non-oxidative (cyclobutane thymine dimers) and oligomeric DNA products in urine. We analysed spot urine samples from 20 healthy subjects, and 20 age- and sex-matched cancer patients. Mononuclear cell DNA 8-oxodG levels were assessed by LC-EC. The data support our proposal that urinary DNA lesion products are predominantly derived from DNA repair. Furthermore, analysis of DNA and urinary 8-oxodG in cancer patients and controls suggested reduced repair activity towards this lesion marker in these patients.

8-Oxo-7,8-dihydro-2'-deoxyguanosine is not salvaged for DNA synthesis in human leukemic U937 cells

8-Oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG), the most common oxidatively modified nucleoside, is released from oxidized DNA and oxidized nucleotide pool. However, little information is available regarding the metabolic pathway of free 8-oxo-dG. In this study, we generated radiolabeled 8-oxo-dG to track its metabolic fate. We report that 8-oxo-dG is neither phosphorylated to 8-oxo-dGMP nor degraded to the free base, 8-oxo-7,8-dihydroguanine (8-oxo-Gua), indicating that 8-oxo-dG is not a substrate for nucleotide synthesis. This result was confirmed by the finding that no radioactivity was detected in the DNA of U937 cells after incubating the cells with radiolabeled 8-oxo-dG. These observations indicate that 8-oxo-dG produced by oxidative stress is not reutilized for DNA synthesis.

Oxidative DNA damage and human cancer: need for cohort studies

Research of the role of oxidative DNA damage is well established in experimental carcinogenesis. A large number of human studies on biomarkers of oxidative DNA damage, in particular related to guanine oxidation, have been published. The level of oxidative DNA damage and repair activity can be quite different between tumor and normal tissues; case-control studies have shown increased levels of oxidative DNA damage and decreased repair capacity in leukocytes from cases. Similarly, the urinary biomarkers of oxidative DNA damage may be elevated in patients with cancer. However, such studies are likely to be associated with reverse causality. Case-control studies of genetic polymorphisms in DNA repair enzymes suggest that the common variant Ser326Cys in OGG1 may be a risk factor for lung cancer, whereas a rare variant in OGG1 and germ line mutations in the corresponding mismatch repair gene MYH are risk factors for hereditary colon cancer. Cohort studies are required to provide evidence that a high level of oxidative DNA damage implies a high risk of cancer. However, this represents a real challenge considering the large number of subjects and long followup time required with likely spurious oxidation of DNA during collection, assay and/or storage of samples.

Urinary 8-hydroxyguanine may be a better marker of oxidative stress than 8-hydroxydeoxyguanosine in relation to the life spans of various species

Oxidative DNA damage is believed to be involved in the aging process. Species with shorter potential life spans generally have a higher specific metabolic rate (SMR), and would be expected to have increased levels of oxidative stress and DNA damage, as compared to long-lived species. An automatized HPLC method based on electrochemical detection was used to measure the levels of the oxidative DNA damage markers 8-hydroxydeoxyguanosine (8-OH-dG) and 8-hydroxyguanine (8-OH-Gua) in urinary samples from mammals with various potential life spans (mice, rats, guinea pigs, cats, chimpanzees, and humans). There was no significant linear correlation (r = -0.71, p = 0.11) between the species' potential life spans (log transformed) and the urinary levels of 8-OH-dG as normalized to creatinine (8-OH-dG/creatinine), although the species with longer life spans, such as chimpanzee and human, had among the lowest levels detected. In contrast, the negative linear correlation between the species' potential life span (log transformed) and the urinary levels of 8-OH-Gua as normalized to creatinine (8-OH-Gua/creatinine), was significant (r = -0.97, p = 0.002). In addition, there was a positive linear and significant correlation between SMR and 8-OH-dG/creatinine (r = 0.91, p = 0.01) or 8- OH-Gua/creatinine (r = 0.90, p = 0.01). These results suggest that 8-OH-Gua, rather than 8-OH-dG, may be a more general marker for oxidative damage.

Urinary measurement of 8-OxodG, 8-OxoGua, and 5HMUra: a noninvasive assessment of oxidative damage to DNA

Numerous DNA repair pathways exist to prevent the persistence of damage, and are integral to the maintenance of genome stability, and hence prevention of disease. Excised lesions arising from repair may ultimately appear in the urine where their measurement has been acknowledged to be reflective of overall oxidative stress. The development of reliable assays to measure urinary DNA lesions, such as HPLC prepurification followed by gas chromatography/mass spectrometry, offers the potential to assess whole body oxidative DNA damage. However, some studies suggest a possibility that confounding factors may contribute to urinary levels of 7,8-dihydro-8-oxoguanine (8-oxoGua) and 7,8-dihydro-8-oxo-2 -deoxyguanosine (8-oxodG). This article considers several possible sources of urinary lesions: (a) the repair of oxidatively damaged DNA; (b) a possible dietary influence; and (c) cell death. The authors conclude that data from their laboratories, along with a number of literature reports, form an argument against a contribution from cell death and diet. In the absence of these confounding factors, urinary measurements may be attributed entirely to the repair of DNA damage and suggests their possible use in studying associations between DNA repair and disease.

Helicobacter pylori infection is associated with oxidatively damaged DNA in human leukocytes and decreased level of urinary 8-oxo-7,8-dihydroguanine

Helicobacter pylori infection is responsible for inflammation, increased production of reactive oxygen species and oxidatively damaged DNA in the gastric mucosa. There is also evidence which suggests that H.pylori infection may lead to the development of several extragastroduodenal pathologies with reactive oxygen species involvement. In order to assess whether the infection may impose oxidatively damaged DNA not only in the target organ (stomach) but in other organs as well we decided, for the first time, to analyse the two kinds of oxidatively damaged DNA biomarkers: urinary excretion of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) and 8-oxo-7,8-dihydroguanine (8-oxoGua) as well as the level of oxidatively damaged DNA in leukocytes. Using high performance liquid chromatography prepurification/gas chromatography with isotope dilution mass detection methodology, we examined the amount of oxidatively damaged DNA products excreted into urine and the amount of 8-oxodG in the DNA of leukocytes' (with the the HPLC/EC technique) in three groups of children: (i) control group, (ii) H.pylori infected children and (iii) children with gastritis where H.pylori infection was excluded. The levels of 8-oxodG in DNA isolated from leukocytes of H.pylori infected patients and in the group with gastritis without H.pylori infection were significantly higher than in DNA isolated from the control group. The mean level of 8-oxoGua in urine samples of children infected with H.pylori was significantly lower than in the urine of the group with gastritis without H.pylori infection. The data suggest that inflammation itself, not just H.pylori infection, is responsible for the observed rise of 8-oxodG level in leukocytes. However, the observed decrease in the level of modified base in urine seems to be specific for H.pylori infection and possibly linked with nitric oxide mediated inhibition of a key base excision repair enzyme (human 8-oxo-7, 8-dihydroguanine glycosylase) responsible for the repair of 8-oxo-7,8-dihydroguanine.

Substantial decrease of urinary 8-oxo-7,8-dihydroguanine, a product of the base excision repair pathway, in DNA glycosylase defective mice

Genome integrity is maintained via removal (repair) of DNA lesions and an increased load of such DNA damage has been linked to numerous pathological conditions, including carcinogenesis and ageing. 8-Oxo-7,8-dihydroguanine is one of the most critical lesions of this type. The free 8-oxo-7,8-dihydroguanine produced by the action of a specific DNA glycosylase is a potential source of this compound in urine. To date, there has been no direct, experimental evidence demonstrating that urinary 8-oxo-7,8-dihydroguanine is produced by the base excision repair pathway. For clarification of this issue, we applied a recently developed methodology which involved high performance liquid chromatography pre-purification followed by gas chromatography with isotope dilution mass spectrometric detection to compare the urinary excretion rate of 8-oxo-7,8-dihydroguanine in wild type and OGG1 glycosylase knock out mice. Our study revealed a 26% reduction in urinary level of 8-oxo-7,8-dihydroguanine in OGG1 deficient mice in comparison with the wild type strain. This clearly indicates that the mouse OGG1 glycosylase contributes significantly to the generation of urinary 8-oxo-7,8-dihydroguanine. Therefore, urinary measurements of 8-oxo-7,8-dihydroguanine may be attributed to DNA damage and repair, which in turn suggests that they may be useful in studying associations between DNA repair and disease.