Nick translation--a new assay for monitoring DNA damage and repair in cultured human fibroblasts

Human Health Effects of Biphenyl: Key Findings and Scientific Issues

BACKGROUND

In support of the Integrated Risk Information System (IRIS), the U.S. Environmental Protection Agency (EPA) has evaluated the human health hazards of biphenyl exposure.

OBJECTIVES

We review key findings and scientific issues regarding expected human health effects of biphenyl.

METHODS

Scientific literature from 1926 through September 2012 was critically evaluated to identify potential human health hazards associated with biphenyl exposure. Key issues related to the carcinogenicity and noncancer health hazards of biphenyl were examined based on evidence from experimental animal bioassays and mechanistic studies.

DISCUSSION

Systematic consideration of experimental animal studies of oral biphenyl exposure took into account the variety of study designs (e.g., study sizes, exposure levels, and exposure durations) to reconcile differing reported results. The available mechanistic and toxicokinetic evidence supports the hypothesis that male rat urinary bladder tumors arise through urinary bladder calculi formation but is insufficient to hypothesize a mode of action for liver tumors in female mice. Biphenyl and its metabolites may induce genetic damage, but a role for genotoxicity in biphenyl-induced carcinogenicity has not been established.

CONCLUSIONS

The available health effects data for biphenyl provides suggestive evidence for carcinogenicity in humans, based on increased incidences of male rat urinary bladder tumors at high exposure levels and on female mouse liver tumors. Kidney toxicity is also a potential human health hazard of biphenyl exposure.

CITATION

Li Z, Hogan KA, Cai C, Rieth S. 2016. Human health effects of biphenyl: key findings and scientific issues. Environ Health Perspect 124:703-712; http://dx.doi.org/10.1289/ehp.1509730.

Osmotic Stress and DNA Damage

Mammalian renal inner medullary cells are normally exposed to extremely high NaCl concentrations. The interstitial NaCl concentration in parts of a normal renal medulla can be 500 mM or more, depending on the species. Remarkably, under these normal conditions, the high NaCl causes DNA damage, yet the cells survive and function both in cell culture and in vivo. Both in cell culture and in vivo the breaks are repaired rapidly if the NaCl concentration is lowered. This chapter describes two methods used to detect and study the DNA damage induced by osmotic stress: comet assay or single cell electrophoresis and TUNEL assay or in situ labeling of 3'-OH ends of DNA strands. This chapter also discusses how specifics of the protocols influence the conclusions about types of DNA damage and what the limitations of these methods are for detecting different types of DNA damage.

A fluorescence-based screening assay for DNA damage induced by genotoxic industrial chemicals

A rapid screening assay to detect chemically-induced DNA damage resulting from exposure of surrogate DNA to genotoxic compounds is reported. This assay is based on changes in the melting and annealing behavior observed for damaged DNA. Exposure of calf thymus DNA to genotoxic industrial chemicals reduced the extent to which the DNA annealed as measured using a double strand DNA selective fluorescent indicator dye. Formaldehyde, acrolein, crotonaldehyde and bromoethane showed the most prominent effects, chloroacetone and allylamine exhibited lesser effects, and acryrlonitrile showed no statistically significant assay response. The assay response for formaldehyde and crotonaldehyde were measured over the concentration range of 10-100 mM and 50-300 mM, respectively. This assay showed little response for the cytotoxic compounds phenol, cyclohexane and toluene but was sensitive to the effects of DNA damaging compounds such as mitomycin C and glutaraldehyde.

Comparative evaluation of genotoxicity of captan in amphibian larvae (Xenopus laevis and Pleurodeles waltl) using the comet assay and the micronucleus test

Captan (N-trichloromethylthio-4-cyclohexene-1,2-dicarboximide) is a fungicide used to inhibit the growth of many types of fungi on plants used as foodstuffs. The toxic and genotoxic potentials of captan were evaluated with the micronucleus test (MNT; AFNOR,2000) and the comet assay (CA) using amphibian larvae (Xenopus laevis and Pleurodeles waltl). Acute toxicity results showed that captan was toxic (1) to Xenopus larvae exposed to from 2 mg/L to 125 or 62.5 microg/L, depending on the nature of the water [reconstituted water containing mineral salts or mineral water (MW; Volvic, Danone, France)] and (2) to Pleurodeles exposed to from 2 mg/L to 125 microg/L in both types of water. The MNT results obtained in MW showed that captan (62.5 microg/L) was genotoxic to Xenopus but not genotoxic to Pleurodeles at all concentrations tested. CA established that the genotoxicity of captan to Xenopus and Pleurodeles larvae depended on the concentration, the exposure times, and the comet parameters (tail DNA, TEM, OTM, and TL). The CA and MNT results were compared for their ability to detect DNA damage at the concentrations of captan and the exposure times applied. CA showed captan to be genotoxic from the first day of exposure. In amphibians, CA appears to be a sensitive and suitable method for detecting genotoxicity such as that caused by captan.

The mutagenic hazards of aquatic sediments: a review

Sediments are the sink for particle-sorbed contaminants in aquatic systems and can serve as a reservoir of toxic contaminants that continually threaten the health and viability of aquatic biota. This work is a comprehensive review of published studies that investigated the genotoxicity of sediments in rivers, lakes and marine habitats. The Salmonella mutagenicity test is the most frequently used assay and accounts for 41.1% of the available data. The Salmonella data revealed mutagenic potency values for sediment extracts (in revertants per gram dry weight) that spans over seven orders of magnitude from not detectable to highly potent (10(5) rev/g). Analyses of the Salmonella data (n=510) showed significant differences between rural, urban/industrial, and heavily contaminated (e.g., dump) sites assessed using TA98 and TA100 with S9 activation. Additional analyses showed a significant positive correlation between Salmonella mutagenic potency (TA98 and TA100 with S9) and PAH contamination (r2=0.19-0.68). The second and third most commonly used assays for the analysis of sediments and sediment extracts are the SOS Chromotest (9.2%) and the Mutatox assays (7.8%), respectively. These assays are frequently used for rapid initial screening of collected samples. A variety of other in vitro endpoints employing cultured fish and mammalian cells have been used to investigate sediment genotoxic activity. Endpoints investigated include sister chromatid exchange frequency, micronucleus frequency, chromosome aberration frequency, gene mutation at tk and hprt loci, unscheduled DNA synthesis, DNA adduct frequency, and DNA strand break frequency. More complex in vivo assays have documented a wide range of effects including neoplasms and preneoplastic lesions in fish and invertebrate exposed ex situ. Although costly and time consuming, these assays have provided definitive evidence linking sediment contamination and a variety of genotoxic and carcinogenic effects observed in situ.

Simple radioactive assay for the estimation of DNA breaks

The intactness of DNA is an important part of the normal cellular structure. Any change to the DNA in the form of breaks leads to a change in the integrity, which in turn leads to abnormality in the cellular activity. Many discrepancies have been reported among the various methods of detecting DNA damage. Here, a simple, sensitive and reproducible method has been developed for the detection of DNA breaks by radioactive labelling of 5' broken ends. The method was evaluated by studying chemically induced DNA damage by using both organochloride (2,4-dichlorophenoxyacetic acid and lindane) and organophosphorus (sevin and phosphamidon) compounds at different concentrations. Phosphamidon, one of the organophosphorus compounds studied, showed complete degradation of the DNA after treatment. Radioactive analysis of phosphamidon showed higher counts at the lowest concentration (20 microg) of the chemical when compared with the control (2752 scintillation counts per minute, scm). Studies on the chemically induced DNA breaks by radiolabelling revealed that the cumulative effect of the organophosphorus and organochloride compounds showed maximum counts in all the samples (the highest being 2904 scm) when compared with the organophosphorus and organochloride compounds studied separately (the highest being 1881 and 2260 scm, respectively). Radiolabelling studies on the blood samples of 23 pesticide workers by the newly developed assay showed a significant positive correlation (0.893) between the number of years of exposure and the scintillation counts. A maximum of 11,702 scm (for 18 years of exposure) and a minimum of 1682 scm (for 4 years of exposure) were recorded compared with 1253 scm for the negative control. This method can be used effectively for estimation of the DNA breaks, irrespective of its nature.

Preferential accumulation of single-stranded regions in telomeres of human fibroblasts

We have demonstrated recently that chronic hyperoxic treatment accelerates the rate of aging of fibroblasts and the rate of telomere shortening in parallel. It was hypothesized that accelerated telomere shortening is due to preferential accumulation of oxidative damage in telomeres. To test this hypothesis, we measured the accumulation of sites sensitive to S1 nuclease treatment in telomeres, in minisatellites, and in the bulk of the genome of fibroblasts under different models of oxidative stress as well as after treatment with the alkylating agent, N-methyl-N'-nitro-N-nitrosoguanidine. A comparison with qualitative data obtained by alkaline electrophoresis reveals that the sites transferred to double-strand breaks by treatment with low concentrations of S1 nuclease are, in fact, single-stranded regions in the DNA. These regions may resemble single-stranded overhangs, gaps, or conventional single-strand breaks. The frequency of single-stranded regions is significantly higher in telomeres than in minisatellites or in the bulk of the genome under all conditions tested. Those regions induced in minisatellites or in the overall genome by a bolus dose of hydrogen peroxide are completely repaired within 24 h. On the other hand, 50 +/- 12% of H2O2-induced single-stranded regions remain unrepaired for at least 19 days in telomeres of human fibroblasts, leading to a significant increase of the telomeric steady-state level of these lesions. This preferential accumulation might significantly contribute to telomere shortening.

Genotoxicity of sediment extracts obtained in the vicinity of a creosote-treated wharf to rainbow trout hepatocytes

Genotoxicity and cytotoxicity were evaluated in rainbow trout hepatocytes exposed to sediment extracts obtained in the vicinity of a creosote-treated wharf. Sediment cores were collected at the intertidal and subtidal sections of the wharf at distances of 1, 5, 40 and 50 m. Moreover, subsamples were also taken at different depths of the cores ranging from 2 to 10 cm below the sediment/water interface. Sediment samples were air-dried and extracted in dichloromethane followed with an exchange into dimethylsulfoxide (DMSO). Rainbow trout hepatocytes were exposed for 24 h at 15 degrees C to several concentrations of the sediment extract. Afterwards, the cells were collected, and cell viability was assayed along with genotoxicity using the nick translation and the alkaline precipitation assays. Results showed that the wharf contained high concentration of polycyclic aromatic hydrocarbons (PAHs), displayed genotoxicity and cytotoxicity to hepatocytes. In addition, PAHs, cytotoxicity and sometimes genotoxicity were detected in all sediment samples and tended to decrease with distance. Chemical contamination and (geno)toxic effects were greater in sediment extracts from the intertidal section than from the subtidal section. However, no evident change in chemical or toxicological characteristics was noted between samples obtained at different depths. Spearman rank-correlation analysis revealed some trends between levels of some PAHs and (geno)toxicity in hepatocytes exposed to sediment extracts.

Detection of single-strand breaks and formamidopyrimidine-DNA glycosylase-sensitive sites in DNA of cultured human fibroblasts

Under oxidative stress 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG), a damaged base with mutagenic potential, and single-strand breaks (SSB) are formed in DNA. Both lesions are frequently used as a parameter for oxidative damage of DNA. Here we report on results from the evaluation of a modified nick translation assay, where 8-oxodG and SSB formation in cellular DNA of cultured human fibroblasts were simultaneously detected. The assay is based on a method previously described by others, with several modifications in reaction conditions and type of substrate. We used formamidopyrimidine-DNA glycosylase (FPG) in our assay in order to measure the formation of FPG-sensitive lesions (which include 8-oxodG) in DNA of human fibroblasts in response to ionising radiation. The quantification of the DNA damage was based on calibration experiments with plasmid DNA pUC19. Dose-response curves of SSB and FPG-sensitive lesion formation in human fibroblasts VH-10 were established. A very low background level of 8-oxodG was detected in unirradiated fibroblasts (approx. 500 residues per cell).

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Validation of a flow cytometric in vitro DNA repair (UDS) assay in rat hepatocytes

An in vitro flow cytometric (FCM) DNA repair assay has been developed and validated by comparison to conventional autoradiography (ARG). Both assays measure unscheduled DNA synthesis (UDS). Cultures of hepatocytes from young male Sprague-Dawley rats were exposed to a battery of 26 chemicals plus bromodeoxyuridine (BrdUrd) or 3H-thymidine (3H-dT) for 18-20 h before harvest. Selection of test chemicals was based upon both their genotoxicity classifications and carcinogenicity bioassay results in male rats. DNA repair in chemically treated cultures was detected flow cytometrically by measuring the uptake of BrdUrd in non-replicating (G1, G2, mitotic and 4C) cells. Intracellular levels of incorporated BrdUrd were visualized by immunochemical labeling with fluorescein isothiocyanate (FITC), and total cellular DNA content was simultaneously estimated by counterstaining samples with the nucleic acid intercalator, propidium iodide (PI). Information was obtained from 10(4) cells/sample. Since repairing cells incorporate significantly less BrdUrd per unit of time than replicating cells, low intensity BrdUrd-FITC fluorescent signals from repairing cells are readily discriminated from high intensity signals from replicating cells when displayed on linear univariate histograms. Further distinction between repairing and replicating cells was achieved by displaying the DNA contents of all cells on linear bivariate histograms. Thus, repairing cells were resolved without subjecting these cultures to agents which suppress replicative synthesis (e.g., hydroxyurea). Results from these concurrent FCM and ARG investigations include the following: (1) conclusions (DNA repair positive or negative) were in agreement, with one exception, cinnamyl anthranilate, for which cytotoxic doses produced a positive FCM response, but lack of intact hepatocytes in parallel ARG preparations prevented analysis; (2) similar sensitivities for most of the positive chemicals were reported; (3) a high correlation (85%) exists between the reported genotoxicity classification and these DNA repair results in the absence of overt cytotoxicity; (4) a poor correlation exists between these DNA repair results and hepatocarcinogenesis (only 4/11 liver carcinogens tested positive) or overall carcinogenesis in the male rat (only 9/21 carcinogens tested positive). This FCM assay provides a rapid, sensitive, safe and reliable means of identifying agents which induce DNA repair in mammalian cells.

A novel technique for the detection of DNA single-strand breaks in human white blood cells and its combination with the unscheduled DNA synthesis assay

A modified assay for the detection of DNA single-strand breaks (SSBs) in human mononucleated white blood cells (MWBCs) based on the nick translation (NT) reaction was developed and combined with the test for unscheduled DNA synthesis (UDS). Both assays were performed on disposable 96-well filtration plates and therefore allowed rapid and sensitive examination of SSBs and UDS. Only 5-8 ml of heparinized blood is required for an eightfold determination in both assays. The uptake of radioactive nucleotide precursors was demonstrated to depend linearly upon the NT reaction time and in both assay systems on the number of investigated cells. The best results and the lowest signal to noise ratio were obtained when the NT assay was performed at 25 degrees C for 20 min. The test was standardized for 150,000 MWBCs/well and a polymerase I concentration of 20 U/ml. The same number of cells were used to measure UDS during a 4-h incubation at 37 degrees C. We observed a dose-dependent increase in SSBs after in vitro incubation with N-methyl-N-nitrosoguanidine (MNNG), with a detection limit of 50 microM when MNNG was present for 1 h and of 5 microM after 20-h incubation period. UDS in MWBCs was increased after treatment for 1 h with MNNG (200 microM) only if poly(ADP)ribose synthesis was inhibited by 3-aminobenzamide. UDS was induced by 320 microM methyl methanesulfonate, but SSBs could only be detected after inhibition of UDS by 100 microM hydroxyurea.(ABSTRACT TRUNCATED AT 250 WORDS)

Statistical confirmation that immunofluorescent detection of DNA repair in human fibroblasts by measurement of bromodeoxyuridine incorporation is stoichiometric and sensitive

Diploid human fibroblasts (IMR-90 cells), grown to confluency and growth-arrested by serum starvation, were irradiated with a variety of doses of UV light (0.025-40 J/m2) or incubated with broad dose ranges of four direct-acting mutagens [ethyl methanesulfonate (EMS), ICR-170, methyl methanesulfonate (MMS), and 4-nitroquinoline oxide (4-NQO)] and pulsed with a thymidine analog, 5-bromodeoxyuridine (BrdUrd) to detect evidence of DNA repair. These cells and appropriate controls were immunochemically stained and subjected to flow cytometric analysis to quantify BrdUrd incorporation into DNA and simultaneously measure cellular DNA content. Since the maximal quantity of BrdUrd incorporated with repairing cells is profoundly less than the amount incorporated during replicative synthesis and flow cytometric analysis collects information on a cell-to-cell basis, data collection using linear histograms succeeded both in revealing repairing cellular populations and eliminating replicative cells from the analysis. Technical modifications necessary to achieve stoichiometry with UV-irradiated IMR-90 fibroblasts included a 48h repair (and pulse) period, followed by denaturing cellular DNA for 15 min at 90 degrees C. The limit of detection was equal to or below the lowest dose investigated (0.025 J/m2). DNA repair was also detected with cultures incubated with low doses of all chemicals and pulsed with BrdUrd for a 24 h period. The limits of detection were near or below 500 microM EMS, 5 microM MMS, 0.25 microM 4-NQO, and 0.1 microM ICR-170.

Mutagenesis and comutagenesis by lead compounds

We have previously reported that lead(II) is weakly mutagenic to Chinese hamster V79 cells. A transgenic cell line G12 containing a single copy of the E. coli gpt gene was developed in this laboratory from Chinese hamster V79 cells. The gpt locus in the G12 cells is more mutable by radiation and oxidative agents compared with the endogenous hprt locus of wild-type V79 cells. We have investigated the mutagenicity of two lead compounds at the gpt locus in G12 cells. Only at a toxic dose is lead acetate significantly mutagenic to G12 cells. Lead nitrate is not significantly mutagenic at any dose. Although both compounds are water-soluble, lead acetate, but not lead nitrate, forms a fine white insoluble precipitate upon addition to growth medium. A nick translation assay on cells treated with lead compounds and then permeabilized indicated that lead nitrate and, to a greater extent, lead acetate causes the appearance of nicks in chromosomal DNA. Lead ions in the presence of hydrogen peroxide, but not alone, introduced nicks into supercoiled plasmid DNA in vitro, suggesting that lead ions can partake in a Fenton reaction and thereby damage DNA. At lower nonmutagenic concentrations, lead acetate enhances the mutagenicity of MNNG and ultraviolet light. DNA damage by ultraviolet light is not enhanced by lead ions in vitro. Our data support the concept that non-toxic concentrations of lead(II) can inhibit DNA repair. Thus, at biologically relevant doses, lead(II) could act as a comutagen and possibly a cocarcinogen, but is not likely to act as an initiating genotoxic carcinogen.

The genetic toxicology of acridines

Acridine and its derivatives are planar polycyclic aromatic molecules which bind tightly but reversibly to DNA by intercalation, but do not usually covalently interact with it. Acridines have a broad spectrum of biological activities, and a number of derivatives are widely used as antibacterial, antiprotozoal and anticancer drugs. Simple acridines show activity as frameshift mutagens, especially in bacteriophage and bacterial assays, by virtue of their intercalative DNA-binding ability. Acridines bearing additional fused aromatic rings (benzacridines) show little activity as frameshift mutagens, but interact covalently with DNA following metabolic activation (forming predominantly base-pair substitution mutations). Compounds where the acridine acts as a carrier to target alkylating agents to DNA (e.g. the ICR compounds) cause predominantly frameshift as well as base-pair substitution mutations in both bacterial and mammalian cells. Nitroacridines may act as simple acridines or (following nitro group reduction) as alkylating agents, depending upon the position of the nitro group. Acridine-based topoisomerase II inhibitors, although frameshift mutagens in bacteria and bacteriophage systems, are primarily chromosomal mutagens in mammalian cells. These mutagenic activities are important, since the compounds have considerable potential as clinical antitumour drugs. Although evidence suggests that simple acridines are not animal or human carcinogens, a number of the derived compounds are highly active in this capacity.

Reproductive hazards of fire fighting. II. Chemical hazards

Studies of the health of fire fighters have historically focused on non-malignant respiratory disease and cancer. More recently, concerns have surfaced about reproductive health effects in many areas of the workforce, including fire fighting. These concerns prompted this review of chemical exposures that may contribute to adverse reproductive health outcomes in male as well as female fire fighters. A review of the industrial hygiene literature was undertaken to identify agents commonly found in fire smoke. These agents were then examined for evidence of reproductive toxicity or mutagenicity/carcinogenicity. This profile of chemical agents and their reproductive toxicities permits a qualitative determination that fire fighters are exposed to potential reproductive toxicants as a part of their normal fire fighting duties. Considerations for mitigating these risks are also discussed.

Arabinosylcytosine-induced accumulation of DNA nicks in myotube nuclei detected by in situ nick translation

This laboratory has recently reported the occurrence of DNA nicking at the onset of terminal skeletal myogenesis by using the technique of in situ nick translation (Dawson and Lough: Dev. Biol., 127:362-367, 1988). Because 1-beta-D-arabinofuranosylcytosine (araC), a cytocidal agent that is routinely used to removed dividing fibroblasts from myogenic cultures, inhibits DNA repair, it was of interest to determine whether araC treatment resulted in an accumulation of the endogenously created nicks. Thus, we have assessed the accumulation of DNA nicks in myotube cells during a 20 hour araC treatment period at the onset of terminal myogenesis (44-64 hours in vitro) by using three techniques: alkaline sucrose gradient density centrifugation, kinetic in situ nick translation, and cellular in situ nick translation. Although alkaline sucrose gradient centrifugation revealed no detectable nicking after 20 hours, kinetic in situ nick translation analysis revealed subtle but significant increases in DNA nicks caused by araC within 7 hours of drug application, and a 1.5-fold increase in DNA repair sites after 20 hours of drug treatment. That these observations reflected nicking specifically in myotube nuclei was determined by immunocytochemical localization of nicked sites after repair with a biotinylated nucleotide analog (biotin-11-dUTP). The effects of araC were only incompletely reversible, whether or not the drug was removed from the cultures, within 2 days of the treatment period.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of comutagenesis of sodium arsenite with n-methyl-n-nitrosourea

Arsenic compounds are known carcinogens. Although many carcinogens are also mutagens, we have previously shown that sodium arsenite is not mutagenic at either the Na+/K+ ATPase or hprt locus in Chinese hamster V79 cells. It can, however, enhance UV-mutagenesis. We now confirm the nonmutagenicity of sodium arsenite in line G12, a pSV2gpt-transformed V79 (hprt-) cell line, which is able to detect multilocus deletions in addition to point mutations and small deletions. The lack of arsenic mutagenicity has led to studies emphasizing its comutagenicity. Sodium arsenite at relatively nontoxic concentrations (5 microM for 24 h or 10 microM for 3 h) is comutagenic with N-methyl-N-nitrosourea (MMU) at the hprt locus in V79 cells. Using a nick translation assay, which measures DNA strand breaks by incorporating radioactive deoxyribonucleoside monophosphate at their 3'OH ends in permeabilized cells, we found that much more incorporation was seen in cells treated with MNU (4 mM, 15 min) followed by 3-h incubation with 10 microM sodium arsenite compared with cells exposed to the same MNU treatment followed by 3-h incubation without sodium arsenite. This result shows that in the presence of arsenite, strand breaks resulting from MNU or its repair accumulate over a 3-h period. We suggest that the repair of MNU-induced DNA lesions may be inhibited by arsenite either by affecting the incorporation of dNMPs into the MNU-damaged DNA template or by interfering with the ligation step.

Techniques to measure DNA single-strand breaks in cells: a review

Alkaline sucrose sedimentation was for a number of years the standard procedure for the measurement of single-strand breaks. Some years ago a number of new techniques with improved sensitivity were introduced. The following techniques are presented and discussed: alkaline unwinding, alkaline filter elution, nucleoid sedimentation, viscoelastometry, microelectrophoresis of single cells, DNA precipitation, pulse field gel electrophoresis, fluctuation spectroscopy and nick translation.

Mutagenic potential of cordycepin (3'-deoxyadenosine) in salmonella and soybean tester strains

Cordycepin (3'-deoxyadenosine), which is known to inhibit the repair of potentially lethal damage in irradiated cells, induced mutations in both Salmonella and soybean test systems.

Metal-induced DNA damage and repair in human diploid fibroblasts and Chinese hamster ovary cells

Cloning efficiency and DNA strand breaks induction were compared in human diploid fibroblasts (HSBP) and chinese hamster ovary (CHO) cells treated with various metal salts. Cadmium (Cd2+), nickel (Ni2+) and chromate (Cr2O7) reduced the cloning efficiency of HSBP cells more than that of CHO cells whereas the reverse was true after treatment with mercury (Hg2+), manganese (Mn2+) and cobalt (Co2+). The effects on cloning efficiency did not consistently correlate with DNA strand breaking activity as all metals except Cr(VI) were more effective at producing DNA strand breaks in CHO cells than in human cells. The differential responses of the two cell types was shown to be only partially due to differences in cellular uptake of metals. DNA breaks induced in human cells by Hg2+ and Cr2O7 were shown most likely to be alkaline labile sites rather than true strand breaks since no damage was detected in a nick translation assay which measures the amount of free 3'-OH terminals. Damage induced by Mn2+ and Co2+, however, appeared to be comprised at least in part by true DNA strand breaks. DNA damage was also induced in HSBP cells following treatment with selenium but only in the presence of reduced glutathione. These studies indicate that DNA damage is not as major a consequence following some metal treatments in human cells as it appears to be in rodent cells. This suggests that rodent models for risk estimation of metal-induced tumorigenesis may not always be appropriate for extrapolation to humans.

Evaluation of putative inhibitors of DNA excision repair in cultured human cells by the rapid nick translation assay

A human fibroblast nick translation assay has been applied to an examination of 48 diverse chemical agents to assess their ability to specifically interfere with the DNA excision-repair process following ultraviolet irradiation. Certain inhibitors of DNA polymerase, ribonucleotide reductase and purine and pyrimidine biosynthesis are shown to inhibit the resynthesis step of repair while DNA intercalators and inhibitors of DNA topoisomerases appear to inhibit the incision step. A variety of other agents previously implicated as inhibitors of DNA repair was also examined and found to have no such effect. This type of analysis should prove useful in the rapid identification of new classes of compounds that antagonize normal cellular repair functions and that might, therefore, act as comutagens or cocarcinogens.

Genotoxicity of formaldehyde and an evaluation of its effects on the DNA repair process in human diploid fibroblasts

Formaldehyde treatment of human fibroblasts gave rise to DNA damage detected by a nick translation assay. This damage was not repaired by typical 'long-patch'-type excision repair as evidenced by the failure of DNA repair inhibitor post-treatment to elevate the amount of DNA strand breakage. In addition, the effects of formaldehyde on DNA repair were examined in light of a recent report suggesting that formaldehyde inhibited the repair of X-ray-induced strand breaks and UV- and benzo [a]pyrene diol epoxide-induced unscheduled DNA synthesis in human bronchial cells. We report that formaldehyde (1) was ineffective at inhibiting the sealing of X-ray- or bleomycin-induced DNA strand breaks, (2) did not inhibit the removal of pyrimidine dimers from cellular DNA at short treatment times, and (3) that the previously observed inhibition of unscheduled DNA synthesis was most likely due to the inhibition of uptake of labeled precursor into formaldehyde-treated cells. Thus, our findings are not consistent with the notion that formaldehyde inhibits the repair process in human fibroblasts. Finally, formaldehyde was shown to elevate the level of misincorporation of bases into synthetic polynucleotides catalyzed by E. coli DNA polymerase I, indicating that the mutagenicity of formaldehyde may be due to covalent alteration of DNA bases.