Catherine M. Gedik

Oxidative DNA damage measured in human lymphocytes: large differences between sexes and between countries, and correlations with heart disease mortality rates

The ‘antioxidant hypothesis’ proposes that vitamin C, vitamin E, carotenoids, and other antioxidants occurring in fruit and vegetables afford protection against heart disease and cancer by preventing oxidative damage to lipids and to DNA, respectively. To test elements of this hypothesis, we have measured blood levels of dietary antioxidants, and 8‐oxodeoxyguanosine (8‐oxo‐dG) concentrations in lymphocyte DNA, in healthy men and women from five European countries: France, Ireland, The Netherlands, Spain, and the U.K. Volunteers, aged 25–45, all nonsmokers, gave blood samples before and after a 12‐wk carotenoid supplementation regime. Vitamin C was measured in plasma and vitamin E and carotenoids were measured in serum by high‐performance liquid chromatography (HPLC). 8‐oxo‐dG was assayed by HPLC (with coulometric detection) in DNA isolated from lymphocytes from the same blood samples. Mean values were calculated for groups of volunteers at each sampling time according to country, sex, and supplementation (between 9 and 24 individual samples contributing to each mean). We found that 8‐oxo‐dG levels in lymphocyte DNA vary significantly according to sex and country. A low mean 8‐oxo‐dG concentration is seen in DNA of women from all five countries, and of men from France and Spain. 8‐oxo‐dG is significantly higher (up to about threefold) in lymphocyte DNA from men in Ireland and the U.K. Oxidative DNA damage is not significantly affected by carotenoid supplementation; nor is there any association with mean baseline levels of antioxidants, which are generally similar in the five countries. The five countries sampled lie on an axis from northern to southern Europe with a steep gradient in terms of premature heart disease. There is a strong association between premature coronary heart disease mortality in men and the mean levels of 8‐oxo‐dG for the five countries (r = 0.95, P < 0.01). Women have low coronary heart disease mortality rates, which do not correlate with 8‐oxo‐dG. In terms of cancer deaths, only colorectal cancer in men shows a significant positive correlation (r = 0.91, P < 0.05), and stomach cancer in women is negatively correlated with DNA oxidation (r = −0.92, P = 0.01).—Collins, A. R., Gedik, C. M., Olmedilla, B., Southon, S., Bellizzi, M. Oxidative DNA damage measured in human lymphocytes: large differences between sexes and between countries, and correlations with heart disease mortality rates. FASEB J. 12, 1397–1400 (1998)

MEASURING OXIDATIVE DAMAGE TO DNA; HPLC AND THE COMET ASSAY COMPARED

Depending on the analytical method employed estimates of background levels of base oxidation in human DNA vary over orders of magnitude. It is now realised that oxidation of guanine in vitro can result in serious overestimation of the nucleoside by HPLC (with electrochemical detection). We have modified procedures of isolation, hydrolysis and storage of DNA with the aim of eliminating this artefact. Vacuum- or freeze-drying, and dialysis, tend to encourage oxidation. We compare results obtained with HPLC and with the comet assay, which employs lesion-specific enzymes to introduce breaks in DNA at sites of oxidative damage. Although estimates of background levels of DNA oxidation using the comet assay are several-fold lower than the estimates by HPLC, both approaches have been used successfully to detect differences between human subjects or population groups that seem to relate to human disease and nutritional factors.

Inter-laboratory validation of procedures for measuring 8-oxo-7,8-dihydroguanine/8-oxo-7,8-dihydro-2 '-deoxyguanosine in DNA

The aim of ESCODD, a European Commission funded Concerted Action, is to improve the precision and accuracy of methods for measuring 8-oxo-7,8-dihydroguanine (8-oxoGua) or the nucleoside (8-oxodG). On two occasions, participating laboratories received samples of different concentrations of 8-oxodG for analysis. About half the results returned (for 8-oxodG) were within 20% of the median values. Coefficients of variation (for three identical samples) were commonly around 10%. A sample of calf thymus DNA was sent, dry, to all laboratories. Analysis of 8-oxoGua/8-oxodG in this sample was a test of hydrolysis methods. Almost half the reported results were within 20% of the median value, and half obtained a CV of less than 10%. In order to test sensitivity, as well as precision, DNA was treated with photosensitiser and light to introduce increasing amounts of 8-oxoGua and samples were sent to members. Median values calculated from all returned results were 45.6 (untreated), 53.9, 60.4 and 65.6 8-oxoGua/10 6 Gua; only seven laboratories detected the increase over the whole range, while all but one detected a dose response over two concentration intervals. Results in this trial reflect a continuing improvement in precision and accuracy. The next challenge will be the analysis of 8-oxodG in DNA isolated from cells or tissue, where the concentration is much lower than in calf thymus DNA.

Analysis of oxidative DNA damage and HPRT mutations in humans after hyperbaric oxygen treatment

DNA damage induced by reactive oxygen species (ROS) seems to play an important role in the induction of mutations and cancer. We have recently shown that hyperbaric oxygen (HBO) treatment of volunteers (i.e., exposure to 100% oxygen at a pressure of 2.5 ATA) induces DNA damage detected in leukocytes with the comet assay. Using formamidopyrimidine-DNA glycosylase (FPG protein) we provided indirect evidence for the induction of oxidative DNA base damage. We now comparatively evaluated FPG-sensitive sites with the comet assay and 7,8-dihydro-8-oxo-deoxyguanosine (8-OHdG) with HPLC analysis after a single HBO. As 8-OHguanine (8-OHgua) is one of the major DNA modifications induced by ROS and a pre-mutagenic lesion, we looked for HBO-induced mutations at the HPRT locus with the T cell cloning test. We also determined the genotypes for glutathione transferases (GST) and tested a possible influence of the GSTM1 and GSTT1 genotypes on the sensitivity of subjects against HBO-induced genotoxicity. Our results indicate that despite a clear induction of FPG-sensitive sites no increased levels of 8-OHdG and no induction of HPRT mutations was detected in lymphocytes after HBO. Furthermore, the DNA effects in the comet assay and the mutant frequencies in the HPRT test seem to be unrelated to the GST genotypes of the test subjects.

In vitro repair of oxidative and ultraviolet-induced DNA damage in supercoiled nucleoid DNA by human cell extract.

A human cell extract is capable of carrying out damage-specific incision on supercoiled nucleoid DNA. The DNA breaks are detected by single cell gel electrophoresis. Incubation of the incised DNA with extract and DNA precursors results in resealing of the breaks, with retention of the supercoiled state of the DNA.

Measurement of 8-Oxo-deoxyguanosine in Lymphocytes, Cultured Cells, and Tissue Samples by HPLC with Electrochemical Detection

8-Oxoguanine is one of the most studied base oxidation products found in DNA. It has potential biological significance, because if present in DNA that is replicating, it can lead to incorporation of adenine rather than cytosine in the daughter strand. Thus it is considered as a premutagenic lesion. It occurs as a result of attack by reactive oxygen species released during the inflammatory response, and in small but significant amounts during normal respiration. The hydroxyl (OH) radical (arising from H(2)O(2) by the transition metal ion-catalyzed Fenton reaction within the nucleus) is most likely responsible for the formation of 8-oxoguanine. Analytical methods-gas chromatography with mass spectrometric detection (GC-MS) and high-performance liquid chromatography (HPLC) -were developed for quantitation of oxidized bases produced in experimental studies of radiation and chemical damage to DNA, and these methods were naturally also applied to the measurement of background levels of oxidized bases in cellular DNA (1). With GC-MS, very high levels of 8-oxoguanine have been reported, typically between 10 and 100 for every 10(5) normal guanines. It has recently been recognized that spurious oxidation of DNA readily occurs during isolation and hydrolysis of DNA, and derivatization of the bases for analysis. HPLC, normally applied to measurement of the nucleoside, 8-oxo-deoxyguanosine (8-oxo-dG), has generally given values below those obtained with GC-MS; but with HPLC, too, oxidation artefacts have been identified.

Measurement of DNA oxidation in human cells by chromatographic and enzymic methods

The European Standards Committee on Oxidative DNA Damage (ESCODD) was set up to resolve problems in the measurement of DNA oxidation that have resulted in varying estimates of the extent of this damage in humans. HeLa cells, sent to members for analysis, were either untreated, or treated with light in the presence of a photosensitizer to induce different amounts of 8-oxo-7,8-dihydroguanine (8-oxoGua) in DNA. Laboratories employing HPLC with electrochemical detection were able to measure the induced damage with similar efficiency; dose response gradients for seven of the eight sets of results were almost identical. GC-MS and HPLC-MS/MS, employed in three laboratories, did not convincingly detect the dose response. An alternative approach to measuring base oxidation employs the enzyme formamidopyrimidine DNA N-glycosylase (FPG) to convert 8-oxoGua to strand breaks, which are then measured by alkaline unwinding, alkaline elution, or the comet assay. Ten laboratories used this approach; five were able to detect the dose response in cells treated with photosensitizer plus light (at lower doses than for chromatographic methods, because the enzymic methods are more sensitive and less prone to spurious oxidation). Median values for 8-oxoGua (or FPG-sensitive sites) in untreated cells were 4.01 per 106 guanines for chromatographic methods, and 0.53 per 106 guanines for techniques based on FPG.The European Standards Committee on Oxidative DNA Damage (ESCODD) was set up to resolve problems in the measurement of DNA oxidation that have resulted in varying estimates of the extent of this damage in humans. HeLa cells, sent to members for analysis, were either untreated, or treated with light in the presence of a photosensitizer to induce different amounts of 8-oxo-7,8-dihydroguanine (8-oxoGua) in DNA. Laboratories employing HPLC with electrochemical detection were able to measure the induced damage with similar efficiency; dose response gradients for seven of the eight sets of results were almost identical. GC-MS and HPLC-MS/MS, employed in three laboratories, did not convincingly detect the dose response. An alternative approach to measuring base oxidation employs the enzyme formamidopyrimidine DNA N-glycosylase (FPG) to convert 8-oxoGua to strand breaks, which are then measured by alkaline unwinding, alkaline elution, or the comet assay. Ten laboratories used this approach; five were able to detect the dose response in cells treated with photosensitizer plus light (at lower doses than for chromatographic methods, because the enzymic methods are more sensitive and less prone to spurious oxidation). Median values for 8-oxoGua (or FPG-sensitive sites) in untreated cells were 4.01 per 10(6) guanines for chromatographic methods, and 0.53 per 10(6) guanines for techniques based on FPG.