Detection by fluorescence analysis of DNA unwinding and unscheduled DNA synthesis, of DNA damage and repair induced in vitro by direct-acting mutagens on human lymphocytes

Cytotoxic and mutagenic effects of anti- and syn-benzo[a]pyrene diol epoxide in human lymphocytes

Cytotoxicity and mutagenicity were measured in human lymphocytes after treatment in vitro with anti- or syn-benzo[a]pyrene diolepoxide, two diastereoisomer metabolites of benzo[a]pyrene. These compounds were incubated with resting and cycling lymphocytes to determine the inhibition of cell proliferation induced by phytohemoagglutinin and interleukin2 at different times after treatment. Anti-benzo[a]pyrene diolepoxide was more cytotoxic than the syn-adduct under all conditions, and its effect on cell growth was more marked in cycling lymphocytes. In contrast, neither of the compounds induced alteration of the ATP intracellular pool. Cytotoxic effects of anti- and syn-benzo[a]pyrene diolepoxide were also assessed by determining the cloning efficiency. Both compounds affected the cloning efficiency in human lymphocytes and the effect of anti-benzo[a]pyrene was particularly marked. Mutagenic potency of anti- and syn-benzo[a]pyrene diolepoxide at the hgprt locus was measured both in the V79 cell line and in human lymphocytes by selection of mutant cells in medium containing 6-thioguanine. Both compounds increased the mutant frequency in comparison with the control and anti-benzo[a]pyrene diolepoxide was more active than the syn-metabolite.

Assays to determine DNA repair ability

DNA repair is crucial to the integrity of the human genome since mammalian cells are continuously exposed to different chemical and physical genotoxic agents. To counteract the lesions induced by these agents, organisms have developed a number of highly conserved repair mechanisms involving numerous protein complexes grouped in several different repair pathways. The importance of studying the individual capacity to repair DNA damage lies in the observation that deficient repair mechanisms of the genome have been linked to the presence of large number of diseases and cancer, and alterations in these mechanisms may also alter the susceptibility of individuals exposed to a particular mutagen. This review focused on the current knowledge of different assays developed to evaluate DNA repair capacity (DRC). These assays, which are grouped into five major categories, have been successfully applied in (1) in vitro studies, (2) epidemiological studies in patients with cancer or other different pathologies, and (3) environmentally or occupationally exposed populations. Nevertheless, some of the limitations include high interlaboratory variability and difficulty to implement the assays on a large scale. The selection of an adequate DRC assay needs to be made on the basis of the objective raised for its application and taking into account a number of determining factors, namely, (1) speed and cost, (2) type of DNA repair to be evaluated, and (3) sample availability.

Results from the "Technical workshop on genotoxicity biosensing" on the micro-scale fluorometric assay of deoxyribonucleic acid unwinding

The fluorometric analysis of DNA unwinding (FADU assay) was originally designed for rapid detection of X-ray-induced DNA damage in mammalian cells. This cellular bioassay is based on time-dependent alkaline denaturation of DNA under moderate denaturing conditions (pH 12.2-12.4) starting from ends as well as from all DNA break points (single-strand breaks, SSB; double-strand breaks, DSB; alkali-labile sites, ALS). DNA which remained double-stranded after 30 min of alkaline treatment was detected after neutralisation and immediate fragmentation followed by binding to the Hoechst 33258 dye (bisbenzimide) and fluorescence measures. In the current paper, a modified method was used which allows cell cultivation and chemical treatment in the same microplate (micro-FADU) followed by analysis of 96 samples in a microplate fluorescence reader. Exposure of mammalian cells to chemicals was performed for 60 min on ice thus allowing identification of direct acting substances capable of inducing DNA-strand breaks. As an inter-assay standard the action of hydrogen peroxide was tested in every test plate. The results demonstrate that the micro-FADU assay is suitable to detect the presence of chemically induced strand breaks within 3 h.

Voltammetric behaviour of DNA bases at a screen-printed carbon electrode and its application to a simple and rapid voltammetric method for the determination of oxidative damage in double stranded DNA

Screen-printed carbon electrodes (SPCEs) have been investigated as possible sensors to identify gamma-irradiation induced oxidative damage in double stranded (ds) DNA. Studies were undertaken to explore the possibility of using both cyclic voltammetry and differential pulse voltammetry to identify changes due to oxidative damage. Initially, guanine, adenine and 8-oxoguanosine were examined and it was found possible to differentiate them from their voltammetric responses. The voltammetric response of 8-oxoguanosine was found to be linear over the concentration range 1-400 microM, with a slope of 0.0296 microA microM(-1) (R2 value of 0.9984), in the presence of 2mM concentrations of guanine and adenine. Investigations were made into harnessing these findings to identify oxidative damage in gamma-irradiated dsDNA. The presence of oxidative damage in these samples was readily identifiable, and the magnitude of the voltammetric response was found to be dose dependant (R2=0.9919). A simple sample preparation step involving only the dissolution of double stranded DNA sample in the optimised electrolyte (0.1M acetate buffer pH 4.5) was required. This report appears to be first describing the use of a SPCE to detect DNA damage which can be related to the dose of gamma-radiation used.

Spontaneous DNA repair in human peripheral blood mononuclear cells

DNA molecules are constantly damaged during mitosis and by oxygen-free radicals produced by either cellular metabolism or by external factors. Populations at risk include patients with cancer-prone disease, patients under enhanced oxidative stress, and those treated with immunosuppressive/cytotoxic therapy. The DNA repair process is crucial in maintaining the genomal DNA integrity. The aim of this study was to evaluate spontaneous DNA repair capacity of peripheral blood mononuclear cells (PBMC) from normal blood donors. PBMC DNA repair ability represents DNA repair by other tissues as well. It is shown in the present study that in vitro incorporation of [3H]thymidine in non-stimulated PBMC expresses the ability of the cells to repair DNA damage. This method was validated by double-stranded DNA measurements. Both catalase and Fe2+ increased DNA repair, the former by preventing re-breakage of newly repaired DNA and the latter by introducing additional DNA damage, which enhanced DNA repair. Better understanding of DNA repair processes will enable to minimize DNA damage induced by oxidative stress.

Does reduced DNA repair capacity play a role in HIV infection and progression in the lymphocytes of opiate addicts?

Opiate use in vivo and in vitro reduces the ability of human peripheral lymphocytes to repair DNA damage caused by both the physical and chemical mutagens that produce single-strand adducts. This decrease in repair leads to increased genetic damage to the individual cell as measured by cytogenetic damage, including sister chromatid exchanges and formation of micronuclei. The expected consequences of this increase in damage can also be established by increases in host cell mutation rate and rate of apoptosis. The effect of this increase in genetic damage can be expected to have significant consequences for HIV-1 or simian immunodeficiency virus infecting those lymphocytes. For example, DNA damaging agents have long been known to induce lentiviral growth and propagation, and this has been found to be true for HIV-1 following ultraviolet light treatment of lymphocytes. However, to date, no one has fully explored the consequences of increased host mutation rate on HIV growth and maintenance. Recent reports have demonstrated the role of viral mutation in such key physiologic processes as resistance to highly active antiretroviral therapy (HAART). Beyond the effects of random mutations in the viral genome, specific mutations in the HIV-1 transcriptase and protease lead to increased accumulation of mutant viruses and the gradual failure of HAART. It therefore remains to be tested whether changes in host cell mutation rate will also predict changes in susceptibility to drug therapy. This also leads to questions about whether the higher rate of viral mutation in HIV-infected drug addicts might be the basis for higher rates of neuroAIDS in this population. It would be attractive to speculate that the increase in the heterogeneity of the virus in addicts produces mutants with a greater capacity to attack neuronal tissue and a high affinity to replicate there.

Fluorometric analysis of DNA unwinding (FADU) as a method for detecting repair-induced DNA strand breaks in UV-irradiated mammalian cells

Fluorometric analysis of DNA unwinding (FADU assay) was originally designed to detect X-ray-induced DNA damage in repair-proficient and repair-deficient mammalian cell lines. The method was modified and applied to detect DNA strand breaks in Chinese hamster ovary (CHO) cells exposed to ionizing radiation as well as to UV light. Exposed cells were allowed to repair damaged DNA by incubation for up to 1 h after exposure under standard growth conditions in the presence and in the absence of the DNA synthesis inhibitor aphidicolin. Thereafter, cell lysates were mixed with 0.15 M sodium hydroxide, and DNA unwinding took place at pH 12.1 for 30 min at 20 degrees C. The amount of DNA remaining double-stranded after alkaline reaction was detected by binding to the Hoechst 33258 dye (bisbenzimide) and measuring the fluorescence. After exposure to X-rays DNA strand breaks were observed in all cell lines immediately after exposure with subsequent restitution of high molecular weight DNA during postexposure incubation. In contrast, after UV exposure delayed production of DNA strand break was observed only in cell lines proficient for nucleotide excision repair of DNA photoproducts. Here strand break production was enhanced when the polymerization step was inhibited by adding the repair inhibitor aphidicolin during repair incubation. These results demonstrate that the FADU approach is suitable to distinguish between different DNA lesions (strand breaks versus base alterations) preferentially induced by different environmental radiations (X-rays versus UV) and to distinguish between the different biochemical processes during damage repair (incision versus polymerization and ligation).

Immunoperoxidase detection of 4-aminobiphenyl- and polycyclic aromatic hydrocarbons-DNA adducts in induced sputum of smokers and non-smokers

Tobacco smoke constituents, 4-aminobiphenyl (4-ABP) and polycyclic aromatic hydrocarbons (PAH) possess carcinogenic properties as their reactive metabolites form DNA adducts. We studied the formation of 4-ABP- and PAH-DNA adducts in induced sputum, a non-invasively obtainable matrix from the lower respiratory tract, of smokers (n=20) and non-smokers (n=24) utilizing a semi-quantitative immunohistochemical peroxidase assay. Smokers had significantly higher levels of 4-ABP-DNA adducts as compared to non-smokers (0. 08+/-0.02 versus 0.04+/-0.01, P=0.001, density of immunohistochemical staining), and the levels of adducts were related to current smoking indices (cigarettes/day: r=0.3, P=0.04 and tar/day: r=0.4, P=0.02). Likewise, smokers had elevated levels of PAH-DNA adducts as compared to non-smokers, however, the differences was not statistically significant (0.13+/-0.02 versus 0. 08+/-0.02, P=0.07). The levels of PAH-DNA adducts were only significantly related to the amount of tar consumed per day (r=0.4, P=0.04) but not to the number of cigarettes smoked per day. Neither the levels of 4-ABP-DNA adducts nor those of PAH-DNA adducts were related to smoking history index (pack years). Further, the levels of 4-ABP-DNA adducts were correlated with those of PAH-DNA adducts (r=0.4, P=0.02). We conclude that immunohistochemistry of 4-ABP-DNA adducts in induced sputum is a specific approach to assess current exposure to tobacco smoke in the lower respiratory tract, however, in the case of PAH-DNA adducts, such analysis is less specific as it does not explicitly reflect the magnitude of the exposure.

Thymidine in the micromolar range promotes rejoining of UVC-induced DNA strand breaks and prevents azidothymidine from inhibiting the rejoining in quiescent human lymphocytes

The effect and inter-individual variation in the effect of exogenously added deoxynucleosides (2 x 10(-6) M) on rejoining of UVC-induced DNA strand breaks was examined in quiescent human lymphocytes from 25 healthy persons. Thymidine at concentrations below 2 x 10(-6) M, effectively and with statistically extreme significance, increased rejoining of UVC-induced DNA strand breaks in the lymphocytes of every one of the 25 persons tested (p < 0.0001, Wilcoxon's signed ranks test). The mean stimulation after 20 h of postirradiation repair was 48% (range 18-78%) with an inter-individual variation of 30% (coefficient of variation, CV). Deoxyguanosine stimulated rejoining in 16, but inhibited in three of 19 test persons (mean stimulation 28%, range -31 to 71%). The stimulating effect of deoxyguanosine was also extremely significant (p < 0.0004). Deoxycytidine and deoxyadenosine stimulated rejoining in some persons and inhibited it in others, and without statistical significance (p values above 0.5). The stimulating effect of thymidine was significantly inhibited by deoxycytidine (p < 0.05, n = 12) whereas deoxyguanosine neither promoted or inhibited the stimulation by thymidine (p = 1, n = 12). Rejoining of DNA strand breaks induced by methyl methanesulfonate did not appear significantly stimulated or inhibited by any of the four deoxynucleosides. Finally, the inhibiting effect of azidothymidine (AZT) on rejoining of UVC-induced DNA strand breaks was nullified by the addition of thymidine. In three donors examined, 10(-4) M AZT inhibited the rejoining by about 40-50%. The presence of less than 10(-5) M thymidine reduced the level of UVC-induced DNA strand breaks to below the level in control lymphocytes allowed to repair without AZT. These results indicate that among the four deoxynucleoside triphosphates, dTTP has a crucial role on the repair of UVC-induced DNA damage in quiescent lymphocytes. The results also indicate that an expansion of the dTTP pool may counteract the inhibiting effect of AZT on DNA repair in quiescent lymphocytes.

Correlation between age and DNA damage detected by FADU in human peripheral blood lymphocytes

Fluorometric analysis of DNA unwinding (FADU) is a fast and reliable method for detecting single strand DNA breaks as an index of DNA damage induced by clastogenic agents. A study of damage detected by FADU was conducted on DNA extracted from peripheral blood lymphocytes of 128 healthy nonsmoking regular donors (ranging in age from 19 to 67 years) and from 5 umbilical cord blood samples. DNA damage was measured as percentage of unwound DNA after alkalinization. Statistical analyses, both parametric (Pearson r correlation coefficient, b regression coefficient, ANOVA) and nonparametric (Kruskal-Wallis H test, Spearman rs rank correlation coefficient), support a significant correlation between age of donors and amount of DNA damage. The same results are found when adult donors are divided in four age classes and the ANOVA test performed among the mean percentages of unwound DNA of each class. Furthermore, donors of the same age belonging to different blood groups (A, B, AB and O) do not show any difference in DNA damage detected by FADU.

Measurement by 32P-postlabeling of (+/-)anti-benzo[a]pyrene-diolepoxide-N2-deoxyguanosine adduct persistence in unstimulated human peripheral blood lymphocytes

In order to study the relative importance of endogenous and environmental factors for the individual relation between DNA damage and DNA excision repair, a method was developed for measuring quantitatively the persistence of N2-deoxyguanosine adducts formed in non-stimulated isolated human peripheral blood lymphocytes after in vitro incubation with 0.2 microM (+/-)anti-BPDE, applying 32P-postlabeling. Total binding of radiolabeled (+/-)anti-BPDE to DNA and its removal has been studied previously in human peripheral blood lymphocytes, but the method presented here enables the direct investigation of repair of the main (+/-)anti-BPDE-DNA adduct, which is implicated in benzo[a]pyrene-induced mutagenesis. Using this method, it was found that in lymphocytes, obtained from 5 individuals, most (+/-)anti-BPDE-N2-dG adducts are removed within the first 24 h after treatment, while interindividual differences appear to exist in both adduct formation and rate and extent of removal.

DNA repair in human lymphocytes treated in vitro with (+)-anti- and (+/-)-syn-benzo[a]pyrene diolepoxide

Human PBL were treated in vitro with the ultimate reactive metabolites of BaP anti- and syn-BaPDE and DNA damage and repair were measured. The incorporation of radioactivity into DNA due to UDS was higher after treatment with anti-BaPDE. Radioactive DNA adduct dosimetry applied to PBL treated with tritiated syn- and anti-BaPDE demonstrated that anti-BaPDE gave more DNA adducts, which were more efficiently removed than syn adducts in the 24 h following the treatment. HPLC analysis of deoxynucleosides obtained from the enzymatic digestion of DNA showed that in treated PBL the major DNA adduct involved deoxyguanosine. DNA strand breaks, detected by FADU, were induced at comparable levels by anti- and syn-BaPDE (0.1-0.4 micrograms/ml), and persisted after 20 h of post-treatment incubation. Only in the case of syn-BaPDE did the percentage of double-stranded DNA tend to increase with time after the treatment.