Application of the comet assay for monitoring DNA damage in workers exposed to chronic low-dose irradiation. I. Strand breakage

DNA damage evaluated by the comet assay in lymphocytes of children with 137Cs internal contamination caused by the Chernobyl accident

The comet assay is one of the most versatile and popular tools for evaluating DNA damage. Its sensitivity to low dose radiation has been tested in vitro, but there are limited data showing its application and sensitivity in chronic exposure situations. The influence of the internal contamination caused by the Chernobyl accident on the level of DNA damage was evaluated by the comet assay on lymphocytes of 56 Ukrainian children. The study was performed during 2003 on children with demonstrable 137Cs internal contamination caused by food consumption. The children were selected for the study immediately after a 137Cs whole body counter measurement of internal contamination. The minimal detectable amount of 137Cs was 75 Bq. The control group included 29 children without detectable internal contamination, while in the exposed group 27 children with measured activity between 80 and 4037 Bq and committed effective dose between 54 and 3155 microSv were included. Blood samples were taken by a finger prick. The alkaline version of the comet assay was used, in combination with silver stained comets and arbitrary units (AU), for comet measurement. Factors such as disease, medical treatment, surface contamination of children's living location, etc., were considered in the study. Non-significant differences (p > 0.05) in DNA damage in control (9.0 +/- 5.7 AU) versus exposed (8.5 +/- 4.8 AU) groups were found. These results suggest that low doses of 137Cs internal contamination are not able to produce detectable DNA damage under the conditions used for the comet assay in this study. Further studies considering effects of high exposure should be performed on chronically exposed people using this assay.

DNA damage in peripheral blood lymphocytes of individuals residing near a wastewater drain and using underground water resources

Mahal is a linear village settlement situated about 0.5 km from an open waste-water drain, the Tung Dhab drain, which carries effluents from local industrial sites. Villagers generally have a low-to-middle socio-economic status and use ground water or a combination of ground water and tap water for drinking and for their other daily activities. The land in and around Mahal is used for agriculture and is irrigated by water from the Tung Dhab. The drain water contains heavy metals, and there is a possibility that these and other contaminants may reach the ground water table of Mahal and thereby compromise the health of the residents. The comet assay was performed on peripheral blood lymphocytes from Mahal villagers and revealed statistically significant increases in DNA damage as compared to a control group that does not use ground water. DNA damage was also significantly related to the age of the villagers and to the length of residence in the village. In the absence of other environmental exposures, it is concluded that the elevated DNA damage in the villagers is a consequence of continuous utilization of contaminated ground water.

Differential toxic effect of cis-platinum(II) and palladium(II) chlorides complexed with methyl 3,4-diamine-2,3,4,6-tetradeoxy-alpha-L-lyxo-hexopyranoside in mouse lymphoma cell lines differing in DSB and NER repair ability

The aim of this work was to test the cytotoxicity of newly synthesized cis-type complexes of platinum(II) and palladium(II) dichloride with methyl 3,4-diamine-2,3,4,6-tetradeoxy-alpha-L-lyxohexopyranoside, [M(C(7)H(16)N(2)O(2))Cl(2)].H(2)O, against two mouse lymphoma cell lines (L5178Y) differing in their double strand breaks and nucleotide excision repair ability. cis- Diaminedichloroplatinum (CDDP) was used as a reference compound. The toxicity of Pt(C(7)H(16)N(2)O(2))Cl(2) appeared to be similar for both cell lines: IC(50) is 8 microM for L5178Y-R cells and 12 microM for L5178Y-S cells, respectively. In contrast, the palladium complex was found to be more toxic for the LY-R cells than for the LY-S cells. The cytotoxicity of both compounds was compared with their ability to induce DNA crosslinks, as measured by the modified comet assay. CDDP caused retardation of the DNA migration induced by 2 Gy of the X-irradiation in a dose-dependent manner. The ability of Pd(C(7)H(16)N(2)O(2))Cl(2) to retard X-ray induced DNA migration was more pronounced than its platinum analogue and CDDP (see Fig. 6). However, this was not reflected in the toxicity of the compound. Such results indicate that these two compounds may cause a different type of DNA damage and/or that the DNA damage caused by the palladium(II) compound was dealt with in a different manner from that induced by the platinum(II) complex.

Observation of DNA damage of human hepatoma cells irradiated by heavy ions using comet assay

AIM: Now many countries have developed cancer therapy with heavy ions, especially in GSI (Gesellschaft fürSchwerionenforschung mbH, Darmstadt, Germany), remarkable results have obtained, but due to the complexity of particle track structure, the basic theory still needs further researching. In this paper, the genotoxic effects of heavy ions irradiation on SMMC-7721 cells were measured using the single cell gel electrophoresis (comet assay). The information about the DNA damage made by other radiations such as X-ray, γ-ray, UV and fast neutron irradiation is very plentiful, while little work have been done on the heavy ions so far. Hereby we tried to detect the reaction of liver cancer cells to heavy ion using comet assay, meanwhile to establish a database for clinic therapy of cancer with the heavy ions.

METHODS: The human hepatoma cells were chosen as the test cell line irradiated by 80Mev/u ²⁰Ne¹⁰⁺ on HIRFL (China), the radiation-doses were 0, 0.5, 1, 2, 4 and 8 Gy, and then comet assay was used immediately to detect the DNA damages, 100-150 cells per dose-sample (30-50 cells were randomly observed at constant depth of the gel). The tail length and the quantity of the cells with the tail were put down. EXCEL was used for statistical analysis.

RESULTS: We obtained clear images by comet assay and found that SMMC-7721 cells were all damaged apparently from the dose 0.5 Gy to 8 Gy (t-test: P < 0.001, vs control). The tail length and tail moment increased as the doses increased, and the number of cells with tails increased with increasing doses. When doses were higher than 2 Gy, nearly 100% cells were damaged. Furthermore, both tail length and tail moment, showed linear equation.

CONCLUSION: From the clear comet assay images, our experiment proves comet assay can be used to measure DNA damages by heavy ions. Meanwhile DNA damages have a positive correlation with the dose changes of heavy ions and SMMC-7721 cells have a great radiosensitivity to ²⁰Ne¹⁰⁺. Different reactions to the change of doses indicate that comet assay is a useful tool to detect DNA damage induced by heavy ions.

Statistics of the Comet assay: a key to discriminate between genotoxic effects

The alkaline Comet assay is a widely used single cell gel electrophoresis technique for the quantification of DNA strand breaks, crosslinks and alkali-labile sites induced by a series of physical and chemical agents. DNA migration in an electric field, supposed proportional to strand breakage, is a proposed estimation of genotoxicity. Breaks are quantified from geometric and fluorescence measurements by image analysis of comet-shaped DNA, often reported parameters being tail DNA and tail moment. Although a variety of statistical approaches have been used in the literature, most of these do not take into account the distribution patterns of comet data. In order to investigate a methodology for statistically demonstrating a comet effect, two different experiments, a reproducibility study and a trend analysis, were undertaken on a murine lymphoma cell line (P388D1) photodynamically stressed after induction of porphyrins with delta-aminolaevulinic acid. This treatment results in significant heterogeneity of DNA damage, producing values ranging from 0 to 100% tail DNA in the same sample. The comparison of distribution curves for stressed and non-stressed samples shows that none of the application conditions are verified, either for parametric tests (which require normal distributions), or non-parametric tests (which assume essentially similar distributions). Meaningful statistics (median and 75th percentile) were consequently extracted from repeated experiments and found suitable for comparing stress conditions in an ANOVA and in a trend analysis; the 75th percentile is theoretically more sensitive but tends to more rapidly saturate at extensive stress levels. We conclude that a trend analysis of median comet metrics from repeated experiments at different stress levels is certainly an efficient way to statistically demonstrate a genotoxic effect. Whether the considered comet parameter is tail DNA or tail moment had no influence on the conclusions of our experiments, which were carried up to stress levels leading to a median 70% tail DNA.

A study on the effects of seasonal solar radiation on exposed populations

In the present study the effects of seasonal solar radiation (summer and winter) on exposed populations of two different age groups (20-25 and 40-55 years old) were investigated. In addition, the effects of external factors, such as hydrogen peroxide (H(2)O(2)) and gamma-irradiation, as well as the repair efficiency of human lymphocytes from these populations, was also evaluated. Our results show that the amount of DNA damage appears to be influenced by the exposure to solar radiation, with the summer exposure being the most damaging. Age was also found to be a significant factor, with the older population being more susceptible to solar radiation than the younger one. Season does not appear to affect the sensitivity to external DNA-damaging agents, while age does. Age was also found to have an effect on the DNA repair capacity of the examined populations.

A cross-platform public domain PC image-analysis program for the comet assay

The single-cell gel electrophoresis, also known as the comet assay, has gained wide-spread popularity as a simple and reliable method to measure genotoxic and cytotoxic effects of physical and chemical agents as well as kinetics of DNA repair. Cells are generally stained with fluorescent dyes. The analysis of comets--damaged cells which form a typical comet-shaped pattern--is greatly facilitated by the use of a computer image-analysis program. Although several image-analysis programs are available commercially, they are expensive and their source codes are not provided. For Macintosh computers a cost-free public domain macro is available on the Internet. No ready for use, cost-free program exists for the PC platform. We have, therefore, developed such a public domain program under the GNU license for PC computers. The program is called CASP and can be run on a variety of hardware and software platforms. Its practical merit was tested on human lymphocytes exposed to gamma-rays and found to yield reproducible results. The binaries for Windows 95 and Linux, together with the source code can be obtained from: http://www.casp.of.pl.

Cytogenetic biomonitoring of Brazilian dentists occupationally exposed to low doses of X-radiation

Exposure to X-rays (ionizing radiation) may cause chromosomal aberrations (CA) in somatic or germinative cells in exposed individuals, and may lead to manifestations of diseases such as cancer. This study was carried out to assess the CA frequency in lymphocytes obtained by means of temporary culture of peripheral blood from dentists in Londrina, Paraná, Brazil, who have worked for more than ten years with X-rays. The results obtained from the experimental group were compared with a matched negative control group, which had never been exposed to X-rays. All individuals, dentists and controls, answered a personal questionnaire, from which a profile of each group was obtained. Slides, prepared after the cultures, were stained with Giemsa, and 100 to 200 metaphase cells were analyzed per individual. CA frequencies and types were registered and statistical tests were not necessary to evaluate the obtained data. The analysis of mitotic index (MI) did not indicate significant differences (p < 0.05) between the group of individuals exposed to X-rays and the control group. The analyzed confounding factors did not influence the results of MI and CA frequencies.

Evaluation of DNA damage in flight personnel by Comet assay

There have been some suggestions that air-crew are at a higher-than-normal risk of developing cancer, since they are exposed to potential genotoxic factors. These include cosmic radiations, airborne pollutants such as the combustion products of jet propulsion, ozone, and electromagnetic fields. We used the Comet assay to investigate DNA damage in flight personnel with the aim of assessing potential health hazards in this occupational category. We studied 40 civil air-crew members who had been flying long-haul routes for at least 5 years, and compared them with a homogeneous control group of 40 healthy male ground staff. The Comet assay, or single-cell gel electrophoresis (SCGE), detects DNA single- and double-strand breaks (DSBs) and alkali-labile lesions in individual cells, and is a powerful and sensitive technique for detecting genetic damage induced by different genotoxic agents. Taking into consideration occupational risk and possible confounding factors, this assay showed a small increase, that did not reach statistical significance, of DNA damage in long-haul crew members compared to controls, indicating a lack of evident genotoxic effects. An association, although again not statistically significant, was found between reduced DNA damage and use of protective drugs (antioxidants).

Lead induced DNA strand breaks in lymphocytes of exposed workers: role of reactive oxygen species and protein kinase C

Lead and lead compounds play a significant role in modern industry; a wide variety of population is at risk of occupational exposure and lead is suspected to be a human carcinogen. The biochemical and molecular mechanisms of lead toxicity are poorly understood, but emerging data suggest that some of the effects of lead may be due to its interference with calcium in the activation of protein kinase C (PKC) and/or through production of reactive oxygen species (ROS). Many of these results are conducted in vitro on cell lines or ex vivo on human lymphocytes treated in vitro. We, therefore, performed a study on the induction of DNA damage, using the alkaline comet assay, in lymphocytes of battery plant workers. To elucidate in vivo the mechanism(s) responsible for this effect, we determined ROS production, and glutathione (GSH) levels in living cells using the fluorescent probe (2',7'-dichlorofluorescein and monochlorobimane, respectively). Subcellular fractions were obtained from sonicated lymphocytes; cytosolic and membrane expression of PKC isoforms (alpha, and zeta) was evaluated after electrophoresis by immunoblot analysis. The results indicate that lead-exposed workers have significantly elevated levels of DNA breaks compared to the unexposed group. A multivariate analysis of variance (ANOVA) shows that the most common confounding factors (smoking, drinking and age) have no synergistic effects with lead-exposure on the comet parameters or on GSH levels and ROS production. The logistic regression analysis distinguishing the exposed and non-exposed indicates that only GSH with tail moment are selected as significant risk factors. There is a significant positive correlation with ROS production and negative correlation with GSH levels. The content of PKC alpha in cytosol and membranes is decreased 40% (indicating a down-regulation of protein), whereas PKC zeta isoform is not modified in an evident manner. Our results suggest that lead-exposure induces an increase of DNA breakage with an alternate cellular redox state and a significant down-regulation of PKC alpha, suggesting that this metal may act as a tumor promoter.

Lymphocyte DNA damage in elevator manufacturing workers in Guangzhou, China

AIMS

To study the effect of smoking, passive smoking, alcohol drinking, and occupational exposure to low level of benzene on DNA strand breaks in elevator manufacturing workers in Guangzhou, China.

METHODS

Three hundred and fifty-nine workers (252 men and 107 women) of a modern elevator manufacturing factory, 205 were from production departments and 154 from managerial department. Information on the workers' health conditions, smoking, passive smoking, alcohol consumption and occupational exposure history was collected by personal interview. Lymphocyte DNA damage was measured by the Comet assay.

RESULTS

None of the women smoked and 20.6% of the men were daily smokers. In non-smokers, the prevalence of passive smoking at work was 25% for men and 11.2% for women, and at home, 37.8 and 48.6%, respectively. Smoking significantly increased tail moment (P<0.001). Daily smokers had the largest tail moment (geometric mean, 95% CI) (0.93 microm (0.81-0.94)), followed by occasional smokers (0.76 microm (0.59-0.95)), ex-smokers (0.70 microm (0.58-0.85)), and never smokers (0.56 microm (0.53-0.60)). Tail moment increased significantly with daily tobacco consumption (cigarettes per day) (r=0.26, P<0.001) after adjusting for age, gender, occupational exposure, passive smoking, and drinking. Analysis of covariance (ANCOVA) showed that smoking (P<0.001), passive smoking at home (P=0.026), occupational exposure (P<0.001), male gender (P<0.001), and age (P=0.001) had independent effects on tail moment, whereas passive smoking at work and alcohol drinking had no significant effect.

CONCLUSIONS

Smoking, passive smoking at home, male gender, age and occupational exposure independently increased lymphocyte DNA strand breaks. The presence of excess DNA damage under low level of occupational exposure to benzene or other solvents suggest that the current allowance concentrations may not be safe to prevent genotoxicity.

Tail moment versus tail length--application of an in vitro version of the comet assay in biomonitoring for genotoxicity in native surface waters using primary hepatocytes and gill cells from zebrafish (Danio rerio)

In order to investigate the suitability of an in vitro version of the comet assay with primary hepatocytes and gill cells from zebrafish (Danio rerio), cells were isolated by immersion in trypsin/EDTA solution after whole-body perfusion with phosphate-buffered saline. Within the scope of an 18-month biomonitoring study, primary cells were used to identify the genotoxic potential of native water samples from different sites along the major German rivers, Rhine and Elbe, and to evaluate the sensitivity and practicability of the chosen assay. Depending on the endpoint measured, considerable differences were detected with respect to the number of genotoxic surface water samples: Whereas no differences could be recorded for tail moment and relative DNA contents of head and tail, the number of positively tested native surface water samples significantly increased with tail length as endpoint.

Assessment of DNA damage in lymphocytes of workers exposed to X-radiation using the micronucleus test and the comet assay

The mutagenic and carcinogenic effects of genotoxic agents on exposed people have constituted an increasing concern. Therefore, the objective of this work was to assess DNA damage in lymphocytes of workers exposed to X-radiation using the cytokinesis-blocked micronucleus test and the comet assay (single-cell gel electrophoresis), and to compare these two techniques in the monitoring of exposed populations. The cytokinesis-blocked micronucleus test and the comet assay were employed in the monitoring of 22 workers occupationally exposed to X-radiation in a hospital in southern Brazil. The frequency of dicentric bridges was also measured. The results of both assays and the frequency of dicentric bridges revealed a significant increase in genetic effects on the cells of exposed individuals. Age was significantly correlated with micronucleus frequency and damage index in the comet assay. The concomitant analysis of dicentric bridges when determining micronucleus frequency does not require much extra work, and may serve as a reference to the type of mutagenic effect (clastogenic or aneugenic). The combination of the alkaline comet assay with the cytokinesis-blocked micronucleus test appears to be very informative for the monitoring of populations chronically exposed to genotoxic agents.

Single cell gel electrophoresis assay: a new technique for human biomonitoring studies

Human biomonitoring using the single cell gel electrophoresis (SCGE) or comet assay is a novel approach for the assessment of genetic damage in exposed populations. This assay enables the detection of various forms of DNA damage in individual cells with ease and speed and is, therefore, well suited to the analysis of a large group in a population. Here, application of SCGE assay in the identification of dietary protective factors, in clinical studies and in monitoring the risk of DNA damage resulting from occupational, environmental or lifestyle exposures is reviewed. Also, the comparative sensitivity of SCGE assay and conventional cytogenetic tests to detect genetic damage is discussed. Finally, strengths and shortcomings of the SCGE assay are addressed.

Laser scanning cytometry for comet assay analysis

BACKGROUND

The comet assay (single-cell gel electrophoresis) is a sensitive method for evaluating nuclear DNA damage. Previously used evaluation methods for the comet assay are time consuming and have an inherent risk of biased selection of comets due to manual selection and categorization of comet images. Laser scanning cytometry (LSC), the principle of which is equivalent to flow cytometry, enables quantification of fluorescence emitted from the cells on a microscope slide. In the present study, we explored whether LSC could be used to determine the degree of DNA damage demonstrated by the comet assay.

METHODS

DNA damage was induced by ultraviolet A irradiation of keratinocytes and visualized by the comet assay. The evaluation included (a) LSC determination of DNA-specific fluorescence in 1,000 comet heads (undamaged DNA), (b) image acquisition of comets by rescanning of the microscope slide, and (c) digital image analysis and computation of tail moment and DNA content in the comet tails.

RESULTS

Cells with damaged DNA were observed in a sub-G(1) area because the comet head loses DNA to the tail. We found a strong inverse correlation between tail moment and DNA content per nucleus.

CONCLUSIONS

LSC enables an automated method for cell recognition and evaluation of the comets, thus providing quantitative information about nuclear DNA damage without subjective selection of analyzed comets.

Use of the alkaline comet assay to monitor DNA damage in technicians exposed to low-dose radiation

The exposure of human beings to ionizing radiation is still of great concern in occupational and environmental medicine, and the widespread use of radiotherapy in the treatment of cancer has led to anxiety about the possible hazards to staff who are at risk of such occupational exposure. In this study, DNA damage in the peripheral lymphocytes of 30 technicians employed in radiation oncology departments for at least 1 year were examined by the alkaline single-cell gel electrophoresis "comet" technique. The results were compared with those of 30 controls with comparable age, sex, and smoking habits who were not working in radiation oncology or chemotherapy services. The DNA damage observed in the lymphocytes of the technicians was significantly higher than that in the controls (P < 0.001). Cigarette smoking was also related to increases in DNA damage, and a significant association was found between the duration of occupational exposure to low-dose ionizing radiation and the DNA damage.

Evaluation of DNA damage by the Comet assay in shoe workers exposed to toluene and other organic solvents

The alkaline single-cell gel electrophoresis (or Comet) assay was applied to evaluate DNA damage in cryopreserved peripheral blood mononuclear leukocytes from 34 female shoe workers exposed to organic solvents and a group of 19 non-exposed women. We also investigated whether the polymorphisms of glutathione S-transferase M1 (GSTM1) and T1 (GSTT1) genes affect individual level of DNA damage possibly induced by the solvent exposure. Chemical measurements of workplace air in the two factories studied showed that the workers were exposed to acetone, gasoline, and toluene in both factories and to ethylacetate and diisocyanate in one factory. In the exposed workers, the average level of blood hemoglobin was lower and that of urinary hippuric acid higher than in the non-exposed individuals. However, the occupational exposure to organic solvents did not affect the Comet values. Neither did age, smoking, or the GSTM1 genotype have any effect on the outcome of this assay. The low prevalence of the GSTT1-null genotype precluded conclusions on the influence of GSTT1 polymorphism.

Lack of adverse effect of smoking habit on DNA strand breakage and base damage, as revealed by the alkaline comet assay

In our preceding papers [M. Wojewódzka, M. Kruszewski, T. Iwanenko, A.R. Collins, I. Szumiel, Application of the comet assay for monitoring DNA damage in workers exposed to chronic low dose irradiation: I. Strand breakage, Mutat. Res., 416 (1998) 21-35; M. Kruszewski, M. Wojewódzka, T. Iwanenko, A.R. Collins, I. Szumiel, Application of the comet assay for monitoring DNA damage in workers exposed to chronic low dose irradiation: II. Base damage, Mutat. Res. , 416 (1998) 37-57.], we evaluated the DNA breakage and base damage with the use of comet assay in a group of 49 workers chronically exposed to low doses of ionizing radiation. There was a statistically significant difference in the damage levels between the hazard and control group. In this paper we describe a confounding lack of effect of the smoking habit on the DNA damage in the tested groups. The genotoxic effect of the smoking habit, as well as its modifying effect on genome damage inflicted by other agents, have been firmly established. However, no statistically significant effect of smoking was found in our study, neither in the control nor in the hazard group. This lack of effect was seen in all DNA damage determinations, both direct (DNA strand breakage and alkali-labile lesions) and enzyme-combined (base damage) and did not depend on the comet parameters, which were taken as damage indicators.

Molecular epidemiology in cancer research

The use of molecular biomarkers in epidemiological investigations brings clear advantages of economy, speed and precision. Epidemiology--the study of the factors that control the patterns of incidence of disease--normally requires large numbers of subjects and/or long periods of time, because what is measured (the occurrence of disease) is a rare event. Biomarkers are measurable biological parameters that reflect, in some way, an individual's risk of disease-because they indicate exposure to a causative (or protective) agent, or because they represent an early stage in development of the disease, or because they allow an assessment of individual susceptibility. Biomarkers must be usable on one of the few materials available for biomonitoring of humans, i.e. blood, urine, exfoliated epithelial cells and, with some difficulty, biopsies. The approach of molecular epidemiology has a great potential is several areas of cancer research: investigating the aetiology of the disease; monitoring cancer risk in people exposed to occupational or environmental carcinogens; studying factors that protect from cancer; and assessing intrinsic factors that might predispose to cancer. The biomarkers most commonly employed in cancer epidemiology include: measurements of DNA damage--DNA breaks, altered bases, bulky adducts--in lymphocytes; the surrogate marker of chemical modifications to blood proteins, caused by agents that also damage DNA; the presence of metabolites of DNA-damaging agents (or the products of DNA damage themselves) in urine; chromosome alterations, including translocations, micronuclei and sister chromatid exchange, resulting from DNA damage; mutations in marker genes; DNA repair; and the differential expression of a variety of enzymes, involved in both activation and detoxification of carcinogens, that help to determine individual susceptibility. The molecular approach has been enthusiastically employed in several studies of occupational/environmental exposure to carcinogens. While the estimation of biological markers of exposure has certainly shown the expected effects in terms of DNA damage and adducts, the detection of the biological effects of exposure (e.g. at the level of chromosome alterations) has not been so clear-cut. This is true also when smokers are examined as a group compared with non-smokers. Several markers (especially of chromosome damage and mutation) show a strong correlation with age-indicating either an increasing susceptibility to damage with age, or an accumulation of long-lived changes. DNA repair--a crucial player in the removal of damage before it can cause mutation--may vary between individuals, and may be modulated by intrinsic or extrinsic factors, but limited data are available because of the lack of a reliable assay. Information on other enzymes determining individual susceptibility does exist, and some significant effects of phenotypic or genotypic polymorphisms have emerged, although the interactions between various enzymes make the situation very complex. The important question of whether oxidative DNA damage in normal cells is decreased by dietary antioxidants (vitamin C, carotenoids etc., from fruit and vegetables) has been tackled in antioxidant supplementation experiments. The use of poorly validated assays for base oxidation has not helped us to reach a definitive answer; it seems that, in any case, the level of oxidative damage has been greatly exaggerated. DNA-damaging agents lead to characteristic kinds of base changes (transitions, transversions, deletions). The investigation of the spectrum of mutations in cancer-related genes studied in tumour tissue should lead to a better understanding of the agents ultimately responsible for inducing the tumour. Similarly, studying mutations in a neutral marker gene (not involved in tumorigenesis) can tell us about the origins of the 'background' level of mutations. So far, interpretation of the growing databases is largely speculative. (ABSTRACT

Application of the comet assay for monitoring DNA damage in workers exposed to chronic low-dose irradiation. II. Base damage

In the preceding paper [M. Wojewodzka, M. Kruszewski, T. Iwanenko, A.R. Collins, I. Szumiel, Application of the comet assay for monitoring DNA damage in workers exposed to chronic low dose irradiation. I. Strand breakage., Mutat. Res. 416 (1998) 21-35], we reported the results of DNA damage examination carried out for a group of people (49 individuals) professionally at risk of exposure to low doses of ionizing radiation as measured by the alkaline comet assay. Here, we used the method in combination with oxidative base damage-specific endonucleases to estimate base damage in the same individuals. These were endonuclease III (endoIII) and formamidopyrimidine glycosylase (FPG). In contrast to the previous investigations, we found no statistically significant difference in base damage between the control and hazard groups. Interestingly, the hazard group exhibited lower level of enzyme-sensitive sites than the control; however, this different was not significant. No correlation of base damage with age was found, similarly as in the case of DNA damage measured by the alkaline comet assay. Interindividual variability of base damage precluded exposure estimation for single individuals, since several members of the control group exhibited high comet parameters.