The comet assay in male reproductive toxicology

Salvia officinalis restores semen quality and testicular functionality in cadmium-intoxicated male rats

The present study investigated the potential ability of Salvia officinalis, one of the oldest medicinal plants, to protect male rats against cadmium reproductive toxicity. Twenty-eight healthy male rats were randomly allocated into four groups (n = 7); control, Salvia-extract treated group, cadmium treated group and a group treated with both Cd and Salvia. Administration of cadmium reduced the relative testis to body weight and significantly affected sperm parameters by decreasing motility, viability, count and increasing morphological aberrations. Comet assay was used to detect DNA fragmentation in sperms of the rats exposed to Cd. Serum levels of testosterone T, follicle stimulating hormone FSH, and luteinizing hormone LH were significantly decreased. The biochemical analysis of testicular tissue showed a significant rise in Malondialdehyde MDA level coupled with a decrease in the activity of antioxidant enzymes (superoxide dismutase SOD, glutathione peroxidase GPx and catalase CAT). The histological examination of testis sections after Cd administration revealed severe degeneration of spermatogenic cells. Seminiferous tubules were filled with homogenous eosinophilic fluid associated with atrophy of other seminiferous tubules. Co-treatment with the Salvia officinalis extract restored the oxidative enzymes activities and decreased the formation of lipid peroxidation byproduct, which in turn ameliorated the effect of Cd on sperm parameters, DNA damage and testis histopathology. Taken together, it can be concluded that the synergistic antioxidant and radical savaging activities of Salvia officinalis prevented the effect of Cd on semen quality, sperm DNA damage, along with the oxidative stress and histological abnormalities in the testis tissues.

In Vitro Co-Exposure to CeO2 Nanomaterials from Diesel Engine Exhaust and Benzo(a)Pyrene Induces Additive DNA Damage in Sperm and Cumulus Cells but Not in Oocytes

Benzo(a)pyrene (BaP) is a recognized reprotoxic compound and the most widely investigated polycyclic aromatic hydrocarbon in ambient air; it is widespread by the incomplete combustion of fossil fuels along with cerium dioxide nanomaterials (CeO₂ NMs), which are used in nano-based diesel additives to decrease the emission of toxic compounds and to increase fuel economy. The toxicity of CeO₂ NMs on reproductive organs and cells has also been shown. However, the effect of the combined interactions of BaP and CeO₂ NMs on reproduction has not been investigated. Herein, human and rat gametes were exposed in vitro to combusted CeO₂ NMs or BaP or CeO₂ NMs and BaP in combination. CeO₂ NMs were burned at 850 °C prior to mimicking their release after combustion in a diesel engine. We demonstrated significantly higher amounts of DNA damage after exposure to combusted CeO₂ NMs (1 µg·L^-1) or BaP (1.13 µmol·L^-1) in all cell types considered compared to unexposed cells. Co-exposure to the CeO₂ NMs-BaP mixture induced additive DNA damage in sperm and cumulus cells, whereas no additive effect was observed in rat oocytes. This result could be related to the structural protection of the oocyte by cumulus cells and to the oocyte's efficient system to repair DNA damage compared to that of cumulus and sperm cells.

DNA of crayfish spermatozoa as a target of waterborne pesticides - An ex vivo approach as a tool to short-term spermiotoxicity screening

The spermiotoxic properties of aquatic contaminants might be the cause of low fertilization rate and decreased prolificacy, affecting the success of the impacted populations. The genotoxic potential of pesticides in spermatozoa as an undesirable effect on non-target organisms, namely aquatic invertebrates with external fertilization, emerges as a key question in ecogenotoxicological research. Thus, this study aimed to clarify if DNA integrity of red swamp crayfish (Procambarus clarkii) spermatozoa is affected by waterborne pesticides at environmentally relevant concentrations. By adopting an ex vivo approach, six pesticides were addressed in a short-term assay: herbicides glyphosate (9 and 90 μg L^-1) and penoxsulam (2.3 and 23 μg L^-1); insecticides dimethoate (2.4 and 24 μg L^-1) and imidacloprid (13.1 and 131 μg L^-1); fungicides pyrimethanil (2.2 and 22 μg L^-1) and imazalil (16 and 160 μg L^-1). Genotoxicity was observed in higher concentrations of glyphosate, penoxsulam, dimethoate, pyrimethanil, and imazalil. Imidacloprid was the only pesticide that did not cause non-specific DNA damage, although displaying pro-oxidant properties. Overall, the present study demonstrated the suitability of the ex vivo approach on spermiotoxicity screening, highlighting the potential ecological impact of pesticides on non-target species, such as P. clarkii, compromising sperm DNA integrity and, subsequently, the population success.

CeO2 Nanomaterials from Diesel Engine Exhaust Induce DNA Damage and Oxidative Stress in Human and Rat Sperm In Vitro

Cerium dioxide nanomaterials (CeO₂ NMs) are widely used in nano-based diesel additives to decrease the emission of toxic compounds, but they have been shown to increase the emission of ultrafine particles as well as the amount of released Ce. The Organization for Economic Cooperation and Development included CeO₂ NMs in the priority list of nanomaterials that require urgent evaluation, and the potential hazard of aged CeO₂ NM exposure remains unexplored. Herein, human and rat sperm cells were exposed in vitro to a CeO₂ NM-based diesel additive (called Envirox^TM), burned at 850 °C to mimic its release after combustion in a diesel engine. We demonstrated significant DNA damage after in vitro exposure to the lowest tested concentration (1 µg·L⁻¹) using the alkaline comet assay (ACA). We also showed a significant increase in oxidative stress in human sperm after in vitro exposure to 1 µg·L⁻¹ aged CeO₂ NMs evaluated by the H₂DCF-DA probe. Electron microscopy showed no internalization of aged CeO₂ NMs in human sperm but an affinity for the head plasma membrane. The results obtained in this study provide some insight on the complex cellular mechanisms by which aged CeO₂ NMs could exert in vitro biological effects on human spermatozoa and generate ROS.

DNA fragmentation of human spermatozoa: Simple assessment of single- and double-strand DNA breaks and their respective dynamic behavioral response

BACKGROUND

Procedures to detect sperm DNA fragmentation (SDF), like the sperm chromatin dispersion (SCD) test, determine the "global" SDF without discriminating between spermatozoa with single-strand DNA breaks only (SDF-SSBs) and those containing double-strand DNA breaks (SDF-DSBs).

OBJECTIVES

(a) To validate a test to distinguish human spermatozoa with massive DSBs (DSB-SCD assay), (b) to study the baseline SDF-SSBs and SDF-DSBs, and (c) to assess their dynamics in vitro.

MATERIALS AND METHODS

(a) SDF-DSBs were determined by visualization of diffused DNA fragments from spermatozoa lysed under non-denaturing conditions. This was validated by in vitro incubation with DNase I and the comet assay. (b) Baseline SDF-DSBs and SDF-SSBs were determined in ejaculates from 95 males. (c) Their dynamic appearance was studied in samples untreated or exposed to hyperthermia, acidic pH, nitric oxide released by sodium nitroprusside (SNP), and the metabolic energy inhibitors 2-deoxy-D-glucose and antimycin A.

RESULTS

(a) DNase I and comet assay experiments confirmed that the assay successfully determined SDF-DSBs. (b) The higher the SDF of the semen sample, the higher the frequency of SSBs, whereas DSBs behaved independently. Abnormal samples showed higher SDF than normozoospermic, the difference being only significant for SDF-SSBs. (c) During the first hours of incubation, the linear rate of increase in SDF-SSBs was 3.7 X higher than that of SDF-DSBs. All hazardous agents accelerated the SDF rate when compared to untreated spermatozoa, primarily being associated with SDF-SSBs. SNP treatment was the most damaging, rapidly inducing spermatozoa with SSBs which progressively evolved to DSBs. Remarkably, this phenomenon was also evidenced after acute SNP exposure, revealing cryptic sperm damage.

CONCLUSION

The DSBs-SCD is an easy complement for SDF assessment. The dynamic study of SSBs and DSBs may improve the evaluation of sperm quality in clinical settings, particularly "unmasking" the presence of non-specific cryptic sperm damage that might otherwise go undetected.

Comet assay on one- and two-cell mouse embryos

DNA damage quantified as the comet tail length was assessed using in vitro and in vivo comet assay on one- and two-cell mouse embryos obtained by natural mating. The use of a protocol with three layers of agarose reduces the embryo loss and makes it possible to study a small number of embryos. A significantly lower level of basal, but not induced DNA damage was found in embryos with cleaved zona pellucida compared to embryos with intact zona pellucida. There were no significant differences in the length of the comet's tail between embryos lysed in different lysis solutions, both in cases of basal and induced DNA damage. A significant increase in the comet tail length was detected in one-cell embryos of mice treated with methyl methanesulfonate and etoposide compared to the control. The data show that DNA damage induced in maternal germ cells persists, which can be detected in embryos using the comet assay.

Influence of polycyclic aromatic hydrocarbon exposure on IVF: now is the time to focus on women

RESEARCH QUESTION

Is polycyclic aromatic hydrocarbon (PAH) exposure associated with the reproductive outcomes of IVF treatment?

DESIGN

A prospective, small-scale monocentric cohort study of couples who underwent IVF treatment between January 2018 and June 2019. Both members of each couple answered a questionnaire on PAH exposure and provided urine samples to measure urinary 1-hydroxypyrene (1-OHP) the day before oocyte retrieval and semen collection for fertilization. To assess the specific PAH exposure of gamete cells, immunostaining was conducted on both spermatozoa and granulosa cells obtained during IVF with an anti-benzo(a)pyrene diol epoxide (BPDE) monoclonal antibody that recognizes BDPE-DNA adducts. To assess DNA damage, a comet assay on spermatozoa was conducted. The PAH exposure was compared between couples who had positive HCG and couples who had negative HCG on day 14 after embryo transfer.

RESULTS

Eighteen couples were included. The mean 1-OHP level in women whose HCG tests were positive (n = 6) was significantly lower than that in women with negative HCG tests (0.098 [0.042-0.170] versus 0.177 [0.067-0.812] μg/g creatinine; P = 0.048). The presence of BPDE-DNA adducts in granulosa cells of women with a negative (29.7 [16.2-57.5] arbitrary units) or positive HCG test (20.3 [9.3-23.3] arbitrary units) were not significantly different (P = 0.092). The urinary 1-OHP levels of men and BPDE-DNA adducts in spermatozoa showed no differences between groups.

CONCLUSIONS

This exploratory research should encourage further studies to determine the effect of women's exposure to PAHs on reproductive outcomes of IVF treatment.

Evaluation of the Major Steps in the Conventional Protocol for the Alkaline Comet Assay

Single cell gel electrophoresis, also known as the comet assay, has become a widespread DNA damage assessment tool due to its sensitivity, adaptability, low cost, ease of use, and reliability. Despite these benefits, this assay has shortcomings, such as long assay running time, the manipulation of multiple slides, individually, through numerous process steps, the challenge of working in a darkened environment, and reportedly considerable inter- and intra-laboratory variation. All researchers typically perform the comet assay based upon a common core approach; however, it appears that some steps in this core have little proven basis, and may exist, partly, out of convenience, or dogma. The aim of this study was to critically re-evaluate key steps in the comet assay, using our laboratory's protocol as a model, firstly to understand the scientific basis for why certain steps in the protocol are performed in a particular manner, and secondly to simplify the assay, and decrease the cost and run time. Here, the shelf life of the lysis and neutralization buffers, the effect of temperature and incubation period during the lysis step, the necessity for drying the slides between the electrophoresis and staining step, and the need to perform the sample workup and electrophoresis steps under subdued light were all evaluated.

Signification of DNA integrity in sperm of Palaemon serratus (Pennant 1777): Kinetic responses and reproduction impairment

The study of the effects of contamination on sperm quality not only provides an early, specific and integrative response to the fraction of bioavailable pollutants, but also has been shown to predict the potential of this fraction to modify an organism's capacity to reproduce. In addition, fertility damage in invertebrates has been addressed as a major problem that may pose a threat to the maintenance of populations. In this context, the present study proposes a methodology based on the measurement of sperm DNA integrity to evaluate the impact of paternal damaged DNA on the reproductive success of Palaemon serratus. A preliminary methodological optimization step was carried out to assess the kinetics of response of spermatozoa as well as the sensitivity of the spermatozoa according to their location in the genital tract. Spermatozoa appeared to be sensitive to a short in vivo exposure to the direct acting agent methyl methanesulfonate (i.e. MMS; 2 days), with a persistence of damage even after a 30 days' recovery in a clean environment, suggesting a probable lack of DNA repair machinery. Moreover, our results revealed no difference in the level of DNA damage in mature spermatozoa whatever the exposure in spermatophore located in the terminal ampulla or in the proximal and distal part of the vas deferens. Finally, a significant decrease in the percentage of naturally bred prawns has been observed at the highest concentration of MMS (i.e. 100 μM). Nevertheless, no reproduction impairment (i.e. fertilization rate and early embryo development) following a paternal exposure has been shown in spite of very high levels of sperm DNA damage. In regard to the literature, this result raises questions concerning the kinetics of expression of genotoxic damage on progeny in the Palaemon model and future work will be led in this way.

Very low concentration of cerium dioxide nanoparticles induce DNA damage, but no loss of vitality, in human spermatozoa

Cerium dioxide nanoparticles (CeO₂NP) are widely used for industrial purposes, as in diesel, paint, wood stain and as potential therapeutic applications. The Organization for Economic Cooperation and Development included CeO₂NP in the priority list of nanomaterials requiring urgent evaluation. As metal nanoparticles can cross the blood-testis barrier, CeO₂NP could interact with spermatozoa. The genotoxicity of CeO₂NP was demonstrated in vitro on human cell lines and mouse gametes. However, the effects of CeO₂NP on human spermatozoa DNA remain unknown. We showed significant DNA damage induced in vitro by CeO₂NP on human spermatozoa using Comet assay. The genotoxicity was inversely proportional to the concentration (0.01 to 10 mg·L^-1). TEM showed no internalization of CeO₂NP into the spermatozoa. This study shows for the first time that in vitro exposure to very low concentrations of cerium dioxide nanoparticles can induce significant DNA damage in human spermatozoa. These results add new and important insights regarding the reproductive toxicity of priority nanomaterials, which require urgent evaluation.

Assessment of sperm quality in palaemonid prawns using Comet assay: methodological optimization

The aim of this study was to adapt the Comet assay in spermatozoa of the marine prawn Palaemon serratus to use it as a marker of sperm quality. Indeed, due to the characteristics of their spermatozoa, the measurement of DNA integrity is one of the few markers which can be transferred to crustaceans to assess the quality of their semen. In the first step, the methods of collecting and maintaining spermatozoa were optimized. Cell survival was estimated during kinetics of preservation (i.e. 1, 2, 4 and 8 h) in various suspension media to define artificial seawater (ASW) as optimal. Several methods in the releasing of spermatozoa from the spermatophore of prawns were estimated with regard to their incidence both on the efficiency of extraction and the survival of cells. Pipetting up and down turned out to be the most successful and the least invasive technique. Secondly, the transfer of Comet assay was optimized by studying various times in both cell lysis (i.e. 1, 6, 18 h) and DNA denaturation (i.e. 15, 30 and 45 min), after in vitro exposure of spermatozoa to an H2O2 gradient as model genotoxicant. Results revealed that a minimum of 1 h in cell lysis and 15 min of DNA denaturation were sufficient to obtain valuable results, linked with a low compaction of DNA in spermatozoa of Palaemon sp. Finally, the sensitivity of P. serratus spermatozoa was assessed after in vitro exposures to model genotoxicants displaying various modes of interaction with DNA (i.e. UV-C, 13.3-79.5 J m^-2; H₂O₂, 5-10 μM and MMS, 0.5-5 mM) and some environmental contaminants known or suspected to be genotoxic (i.e. cadmium and diuron, 0.015-1.5 μg L^-1; carbamazepine, 0.1-10 μg L^-1) for invertebrates. The low variability of the baseline level of DNA strand breaks recorded in controls highlighted the robustness of the method. P. serratus spermatozoa displayed significant DNA damage from the lowest doses tested for all model genotoxicants, but conversely, no genotoxic effect of tested environmental contaminants was observed. These results, which are discussed according to the protocol tested in the present study and the comparison with literature data, could suggest a difference in the response or sensitivity of spermatozoa to environmental genotoxicity between invertebrate species, and therefore the interest of Palaemonidae prawns in ecogenotoxicology. In conclusion, the present study underlines the potential of the Comet assay as a marker to assess the contamination impact on the sperm quality in Palaemonidae prawns in view to a potential application for in situ biomonitoring surveys.

Comet assay on thawed embryos: An optimized technique to evaluate DNA damage in mouse embryos

Our objective was to optimize the CA technique on mammal embryos.

MATERIALS AND METHODS

1000 frozen 2-cell embryos from B6CBA mice were used. Based on a literature review, and after checking post-thaw embryo viability, the main outcome measures included: 1) comparison of the embryo recovery rate between 2 CA protocols (2 agarose layers and 3 agarose layers); 2) comparison of DNA damage by the CA on embryos with (ZP+) and without (ZP-) zona pellucida; and 3) comparison of DNA damage in embryos exposed to 2 genotoxic agents (H₂O₂ and simulated sunlight irradiation (SSI)). DNA damage was quantified by the % tail DNA.

RESULTS

1) The recovery rate was 3,3% (n=5/150) with the 2 agarose layers protocol and 71,3% (n=266/371) with the 3 agarose layers protocol. 2) DNA damage did not differ statistically significantly between ZP- and ZP+ embryos (12.60±2.53% Tail DNA vs 11.04±1.50 (p=0.583) for the control group and 49.23±4.16 vs 41.13±4.31 (p=0.182) for the H₂O₂ group); 3) H₂O₂ and SSI induced a statistically significant increase in DNA damage compared with the control group (41.13±4.31% Tail DNA, 36.33±3.02 and 11.04±1.50 (p<0.0001)). The CA on mammal embryos was optimized by using thawed embryos, by avoiding ZP removal and by the adjunction of a third agarose layer.

Spermatozoa: A relevant biological target for genotoxicity assessment of contaminants in the estuarine bivalve Scrobicularia plana

Evaluation of DNA quality of gametes is a relevant method to predict potential consequences of pollutants in the next generations, as it allows to define adverse outcome pathways implicated in pollutant-mediated toxicity for risk assessment. In the present study, a comet assay was developed for the spermatozoa of Scrobicularia plana exposed to 10 and 100μg/L of benzo[a]pyrene (B[a]P) for 24h and 5days. The induction of apoptosis and repair mechanisms was assessed by determining caspase-3 activity and polymerase cell nuclear antigen (PCNA) mRNA expression level. Results showed that B[a]P induced high levels of DNA breaks that were associated with apoptosis for all the conditions tested, indicating that the spermatozoa were sensitive to B[a]P. PCNA gene expression was induced in animals exposed to the highest concentrations of B[a]P, suggesting that defence mechanisms were enhanced in these animals. This preliminary study demonstrated the utility of spermatozoa as a relevant biological target for genotoxicity assessment of contaminants and will enable to predict the effect of contaminants on future generations.

Unpacking the mysteries of sperm DNA fragmentation

Although it has been thirty years since publication of one of the most influential papers on the value of assessing sperm DNA damage, andrologists have yet to reach a general consensus about how to apply this seminal parameter to improve or predict reproductive outcomes. Studies that have attempted to establish a causal relationship between sperm DNA damage and pregnancy success have often resulted in conflicting findings, eroding the practitioner’s confidence to incorporate this phenomenon into their appraisal of fertility. In this review we have identified and answered ten important unresolved questions commonly asked by andrologists with respect to the relationship between sperm DNA damage and fertility. We answer questions ranging from a basic comprehension of biological mechanisms and external factors that contribute to increased levels of sperm DNA damage in the ejaculate to what type of DNA lesions we might be expect to occur and what are some of the consequences of DNA damage on early embryonic development. We also address some of the fundamental technical issues associated with the most appropriate measurement of sperm DNA damage and the need to attenuate the confounding impacts of iatrogenic damage. We conclude by asking whether it is possible to reduce elevated levels of sperm DNA damage therapeutically.

Human sperm sex chromosome disomy and sperm DNA damage assessed by the neutral comet assay

STUDY QUESTION

Is there an association between human sperm sex chromosome disomy and sperm DNA damage?

SUMMARY ANSWER

An increase in human sperm XY disomy was associated with higher comet extent; however, there was no other consistent association of sex chromosome disomies with DNA damage.

WHAT IS KNOWN ALREADY

There is limited published research on the association between sex chromosome disomy and sperm DNA damage and the findings are not consistent across studies.

STUDY DESIGN, SIZE, AND DURATION

We conducted a cross-sectional study of 190 men (25% ever smoker, 75% never smoker) from subfertile couples presenting at the Massachusetts General Hospital Fertility Clinic from January 2000 to May 2003.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Multiprobe fluorescence in situ hybridization for chromosomes X, Y and 18 was used to determine XX, YY, XY and total sex chromosome disomy in sperm nuclei using an automated scoring method. The neutral comet assay was used to measure sperm DNA damage, as reflected by comet extent, percentage DNA in the comet tail, and tail distributed moment. Univariate and multiple linear regression models were constructed with sex chromosome disomy (separate models for each of the four disomic conditions) as the independent variable, and DNA damage parameters (separate models for each measure of DNA damage) as the dependent variable.

MAIN RESULTS AND THE ROLE OF CHANCE

Men with current or past smoking history had significantly greater comet extent (µm: regression coefficients with 95% CI) [XX18: 15.17 (1.98, 28.36); YY18: 14.68 (1.50, 27.86); XY18: 15.41 (2.37, 28.45); Total Sex Chromosome Disomy: 15.23 (2.09, 28.38)], and tail distributed moment [XX18: 3.01 (0.30, 5.72); YY18: 2.95 (0.24, 5.67); XY18: 3.04 (0.36, 5.72); Total Sex Chromosome Disomy: 3.10 (0.31, 5.71)] than men who had never smoked. In regression models adjusted for age and smoking, there was a positive association between XY disomy and comet extent. For an increase in XY disomy from 0.56 to 1.47% (representing the 25th to 75th percentile), there was a mean increase of 5.08 µm in comet extent. No other statistically significant findings were observed.

LIMITATIONS, REASONS FOR CAUTION

A potential limitation of this study is that it is cross-sectional. Cross-sectional analyses by nature do not lend themselves to inference about directionality for any observed associations; therefore we cannot determine which variable is the cause and which one is the effect. A small sample size may be a further limitation. Comparison of these findings to other studies is limited due to methodological differences.

WIDER IMPLICATIONS OF THE FINDINGS

Although consistent associations across sex chromosome disomies or DNA damage measures were not observed, this study highlights the need to explore etiologies of sperm DNA damage and sex chromosome disomy to better understand the potential mechanistic overlaps between the two.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by NIOSH Grant T42 OH008416, and NIH/NIEHS Grants ES 009718, ES 000002, and R01 ES017457. During the study M.E.M. was affiliated with the Department of Environmental Health at the Harvard School of Public Health.

TRIAL REGISTRATION NUMBER

N/A.

Impact of adrenalectomy and dexamethasone treatment on testicular morphology and sperm parameters in rats: insights into the adrenal control of male reproduction

Here we investigated the hypothesis that normal levels of glucocorticoids, a class of adrenal steroid hormones, are required for normal testicular and epididymal functions. We examined the effects of the manipulation of glucocorticoid plasma levels by bilateral adrenalectomy (1, 2, 7 and 15 days) alone or in combination with daily treatment with the synthetic glucocorticoid dexamethasone (DEX; 5 μg/kg, i.p., 6 days) on the morphology of the testis and sperm parameters in rats. We showed that adrenalectomy led to a reduction in testicular sperm count and daily sperm production starting 2 days after surgery and a differential decrease in sperm count in the epididymis, according to the region and time post-adrenalectomy analysed. In parallel, testes from 7-day adrenalectomized (ADX) rats displayed a higher frequency of damaged seminiferous tubules and the presence of elongated spermatids retained in the basal epithelial compartment in stages IX-XVII, which is indicative of defective spermiation. The alkaline comet assay revealed a late effect of adrenalectomy on epididymal sperm DNA fragmentation, which was increased only 15 days after surgery. DEX treatment prevented the changes in testicular and epididymal sperm count observed in 7-day ADX rats, but failed to protect the testis from ADX-induced morphological abnormalities. Thus, our results indicated that glucocorticoids may be involved in events related to the maintenance of spermatogenesis and sperm maturation during adulthood. These findings provide new insights into the importance of adrenal steroids to male fertility.

In vitro tests aiding ecological risk assessment of ciprofloxacin, tamoxifen and cyclophosphamide in range of concentrations released in hospital wastewater and surface water

Ciprofloxacin (CIP), tamoxifen (TAM) and cyclophosphamide (CP) which are often used in anticancer treatment are released in hospital effluent and into the environment. Although the concentrations are low (from ng/L to μg/L), no data exist concerning their ecotoxicological impact. In this study two biomarkers of early effect were performed on hepatic cells (HepG2): cell viability and genotoxicity (DNA breaks) using cell proliferative assay and comet assay, respectively. These data were compared with two standardized ecotoxicological tests: algaltoxkit F™ and microtox®. Cells were exposed to an increasing amount of an individual drug or in a mixture for 24, 48 or 72h. The time-exposure of bacteria and algae ranged between 5 and 30min and 72h, respectively. A non-monotonic dose-response on cell viability was observed when HepG2 cells were exposed to TAM alone or in the presence of CIP. The same scheme was observed with microtox® when the bacteria were exposed to the mixtures. On the other side, an individual drug does not induce any DNA breaks on hepatic cells, whereas a mixture leads to a dose dependent increase of DNA breaks. Similarly a positive response was observed with algaltoxkit F™ only with mixtures. Synergistic effects observed when drugs are in a mixture highlight the importance of investigating the ecotoxicological effects of contaminants at low concentrations and in mixtures.

Analysis of DNA damage via single-cell electrophoresis

The comet assay or single-cell gel electrophoresis assay is a relatively simple and sensitive technique for quantitatively measuring DNA damage and repair at the single-cell level in all types of tissue where a single-cell suspension can be obtained. Isolated cells are mixed with agarose, positioned on a glass slide, and then lysed in a high-salt solution which removes all cell contents except the nuclear matrix and DNA, which is finally subjected to electrophoresis. Damaged DNA is electrophoresed from the nuclear matrix into the agarose gel, resembling the appearance of a comet, while undamaged DNA remains largely within the proximity of the nuclear matrix. By choosing different pH conditions for electrophoresis, different damage types and levels of sensitivity are produced: a neutral (pH 8-9) electrophoresis mainly detects DNA double-strand breaks, while alkaline (pH ≥ 13) conditions detect double- and single-strand breaks as well as alkali-labile sites. This protocol describes a standard comet assay study for the analysis of DNA damage and outlines important variations of this protocol.

The comet assay: a sensitive genotoxicity test for the detection of DNA damage and repair

The comet assay (single-cell gel electrophoresis) is a simple and sensitive method for studying DNA damage and repair. In this microgel electrophoresis technique, a small number of cells suspended in a thin agarose gel on a microscope slide is lysed, electrophoresed, and stained with a fluorescent DNA-binding dye. Cells with increased DNA damage display increased migration of chromosomal DNA from the nucleus towards the anode, which resembles the shape of a comet. The assay has manifold applications in fundamental research for DNA damage and repair, in genotoxicity testing of novel chemicals and pharmaceuticals, environmental biomonitoring, and human population monitoring. This chapter describes a standard protocol of the alkaline comet assay and points to some useful modifications.

Estimates of DNA damage by the comet assay in the direct-developing frog Eleutherodactylus johnstonei (Anura, Eleutherodactylidae)

The aim of this study was to use the Comet assay to assess genetic damage in the direct-developing frog Eleutherodactylus johnstonei. A DNA diffusion assay was used to evaluate the effectiveness of alkaline, enzymatic and alkaline/enzymatic treatments for lysing E. johnstonei blood cells and to determine the amount of DNA strand breakage associated with apoptosis and necrosis. Cell sensitivity to the mutagens bleomycin (BLM) and 4-nitro-quinoline-1-oxide (4NQO) was also assessed using the Comet assay, as was the assay reproducibility. Alkaline treatment did not lyse the cytoplasmic and nuclear membranes of E. johnstonei blood cells, whereas enzymatic digestion with proteinase K (40 μg/mL) yielded naked nuclei. The contribution of apoptosis and necrosis (assessed by the DNA diffusion assay) to DNA damage was estimated to range from 0% to 8%. BLM and 4NQO induced DNA damage in E. johnstonei blood cells at different concentrations and exposure times. Dose-effect curves with both mutagens were highly reproducible and showed consistently low coefficients of variation (CV ≤ 10%). The results are discussed with regard to the potential use of the modified Comet assay for assessing the exposure of E. johnstonei to herbicides in ecotoxicological studies.

Linking genotoxic responses in Gammarus fossarum germ cells with reproduction impairment, using the Comet assay

Germ cells perform a unique and critical biological function: they pass down DNA that will be used for the development of the next generation. Thus there is an increasing need to understand how the adult exposure to genotoxicants could show negative impact on the offspring of aquatic organisms. Hence this work addresses the question of the consequences of germ cell DNA damage resulting from parental exposure on reproduction quality in the freshwater crustacean Gammarus fossarum, a high ecologically relevant species. Initially, the sensitivity response of mature oocytes and spermatozoa to two model genotoxicants, MMS and K(2)Cr(2)O(7) was compared by implementing the Comet assay after the exposure of these gammarids in the laboratory and after the exposure of caged organisms in the field. Spermatozoa appeared significantly more susceptible than the oocytes to genotoxicants whatever were the exposure conditions. Secondly, a significant correlation between the level of damage to the sperm DNA of exposed parents and the abnormality rate in embryos that had developed in non-contaminated water were demonstrated. Interestingly, this relationship bridges the biomarker response measured in germ cells at molecular level and its consequences at individual level for the subsequent generation. Moreover, reproduction defects were observed for a level of DNA damage exceeding a minimal threshold, which could have significant consequences for the population dynamics of this high ecologically relevant species.

Chromosomal integrity and DNA damage in freeze-dried spermatozoa

Freeze-drying technology may one day be used to preserve mammalian spermatozoa indefinitely without cryopreservation. Freeze-dried mouse spermatozoa stored below 4°C for up to 1 year have maintained the ability to fertilize oocytes and support normal development. The maximum storage period for spermatozoa increases at lower storage temperatures. Freeze-drying, per se, may reduce the integrity of chromosomes in freeze-dried mouse spermatozoa, but induction of chromosomal damage is suppressed if spermatozoa are incubated with divalent cation chelating agents prior to freeze-drying. Nevertheless, chromosomal damage does accumulate in spermatozoa stored at temperatures above 4°C. Currently, no established methods or strategies can prevent or reduce damage accumulation, and damage accumulation during storage is a serious obstacle to advances in freeze-drying technology. Chromosomal integrity of freeze-dried human spermatozoa have roughly background levels of chromosomal damage after storage at 4°C for 1 month, but whether these spermatozoa can produce healthy newborns is unknown. The safety of using freeze-dried human spermatozoa must be evaluated based on the risks of heritable chromosome and DNA damage that accumulates during storage.

Characterization of chromosomal damage accumulated in freeze-dried mouse spermatozoa preserved under ambient and heat stress conditions

Structural chromosome aberrations and DNA damage generated in freeze-dried mouse spermatozoa were investigated. Freeze-dried sperm samples were preserved at 4, 25 and 50°C for short duration (1 day to 2 months) and at 25°C for long duration (2 years). The spermatozoa were injected into mouse oocytes to analyse the chromosomes of the zygotes at the first cleavage metaphase. Chromosome break of the chromosome-type aberrations was the most common type of structural chromosome aberrations observed in all freeze-dried samples. The frequency of chromatid exchanges rapidly increased in freeze-dried spermatozoa preserved at 50°C for 1-5 days. The frequency of chromatid-type aberrations (break and exchange) gradually increased in freeze-dried spermatozoa preserved at 25°C for up to 2 months. Alkaline comet assay revealed significant migration of damaged DNA accumulated in freeze-dried spermatozoa preserved at 50°C for 3 days and 25°C for 2 years. However, no DNA damage was detected using the same sperm samples by neutral comet assay, which can detect mostly DNA double-strand breaks in cellular DNA. These results suggest that DNA single-strand breaks were accumulated in freeze-dried spermatozoa preserved under ambient or heat conditions, and then chromatid-type aberrations, especially the chromatid exchanges, were formed via post-replication repair system in zygotes.

Shortening of alkaline DNA unwinding time does not interfere with detecting DNA damage to mouse and human spermatozoa in the comet assay

The comet assay was performed on mouse and human spermatozoa to examine the effect of alkaline DNA unwinding time. The spermatozoa were treated in vitro with the DNA-damaging agents, methyl methanesulfonate (MMS) or hydrogen peroxide (H₂O₂), and then embedded in agarose gel on glass slides. The slides were immersed in alkaline solution (> pH 13) for 1, 5, 10 and 20 min, and then subjected to the electrophoresis under neutral conditions. In mouse spermatozoa, comet tails seen in solvent controls became brighter and longer as the alkaline DNA unwinding time increased. However, in the MMS-treated mouse spermatozoa, a smaller difference in the damage from that in the solvent control was seen with time within a dose. DNA damage induced by H₂O₂ could also be detected accurately after alkali treatment for 1-20 min. In human spermatozoa, DNA damage induced by MMS and H₂O₂ could be detected in a dose-dependent manner after alkali treatment for 1 min. The ability of the comet assay to detect DNA damage was not adversely affected by the short period (1 min) of the alkaline DNA unwinding time.

Germ cell mutagens: risk assessment challenges in the 21st century

Heritable mutations may result in a wide variety of detrimental outcomes, from embryonic lethality to genetic disease in the offspring. Despite this, today's commonly used test batteries do not include assays for germ cell mutation. Current challenges include a lack of practical assays and concrete evidence for human germline mutagens, and large data gaps that often impede risk assessment. Moreover, most regulatory assessments are based on the assumption that somatic cell mutation assays also protect the germline by default, which has not been adequately confirmed. The field is also faced with new challenges aimed at dramatically reducing animal testing, and attempts to rapidly classify thousands of chemicals using high throughput in vitro assays. These approaches may not adequately capture effects that may be particular to gametes, since many aspects of the germline are unique. In light of these challenges, an urgent need exists to develop new approaches to evaluate the potential of toxicants to cause germline mutation. The application of new technologies will greatly enhance our understanding of mutation in humans exposed to environmental mutagens. However, we must be poised to collect and interpret these data, and facilitate risk translation to regulators and the public. Genetic toxicologists must also become actively involved in the development of high-throughput tools to study germline mutation. Appropriate attention to these areas will result in the development of policies that prioritize the protection of the germline and future generations from DNA sequence mutations.

In vitro evaluation of baseline and induced DNA damage in human sperm exposed to benzo[a]pyrene or its metabolite benzo[a]pyrene-7,8-diol-9,10-epoxide, using the comet assay

Exposure to genotoxins may compromise DNA integrity in male reproductive cells, putting future progeny at risk for developmental defects and diseases. To study the usefulness of sperm DNA damage as a biomarker for genotoxic exposure, we have investigated cellular and molecular changes induced by benzo[a]pyrene (B[a]P) in human sperm in vitro, and results have been compared for smokers and non-smokers. Sperm DNA obtained from five smokers was indeed more fragmented than sperm of six non-smokers (mean % Tail DNA 26.5 and 48.8, respectively), as assessed by the alkaline comet assay (P < 0.05). B[a]P-related DNA adducts were detected at increased levels in smokers as determined by immunostaining. Direct exposure of mature sperm cells to B[a]P (10 or 25 μM) caused moderate increases in DNA fragmentation which was independent of addition of human liver S9 mix for enzymatic activation of B[a]P, suggesting some unknown metabolism of B[a]P in ejaculates. In vitro exposure of samples to various doses of B[a]P (with or without S9) did not reveal any significant differences in sensitivity to DNA fragmentation between smokers and non-smokers. Incubations with the proximate metabolite benzo[a]pyrene-r-7,t-8-dihydrodiol-t9,10-epoxide (BPDE) produced DNA fragmentation in a dose-dependent manner (20 or 50 μM), but only when formamidopyrimidine DNA glycosylase treatment was included in the comet assay. These levels of DNA fragmentation were, however, low in relation to very high amounts of BPDE–DNA adducts as measured with ³²P postlabelling. We conclude that sperm DNA damage may be useful as a biomarker of direct exposure of sperm using the comet assay adapted to sperm, and as such the method may be applicable to cohort studies. Although the sensitivity is relatively low, DNA damage induced in earlier stages of spermatogenesis may be detected with higher efficiencies.

Effects of borneol on the level of DNA damage induced in primary rat hepatocytes and testicular cells by hydrogen peroxide

The aim of this paper was to evaluate genotoxic effects of borneol and its ability to change DNA-damaging effects of H2O2 in rat hepatocytes and testicular cells. Both in vitro and ex vivo approaches were used in the case of hepatocytes. Testicular cells were tested only ex vivo, i.e. shortly after isolation from rats supplemented by borneol. Cytotoxicity of borneol increased in in vitro conditions in a concentration-dependent manner and it was associated with DNA-damaging effects at toxic concentrations. While non-toxic concentrations of borneol applied in vitro protected cells against H2O2-induced DNA damage and interfered only partly with rejoining of H2O2-induced DNA strand breaks, cytotoxic concentrations of borneol manifested synergy with H2O2, i.e. enhanced DNA-damaging effects of H2O2. On the other side, borneol given to rats in drinking water decreased the level of DNA damage induced by H2O2 in both hepatocytes and testicular cells. Our results show that though at higher concentrations (2-h treatment with >2 mM borneol >0.3084 mg/ml) borneol acts cytotoxically and genotoxically on primary hepatocytes cultured in vitro, if given to rats during 7 days in a daily concentration of 17.14 or 34.28 mg/kg it reduces genotoxicity of H2O2 in both hepatocytes and testicular cells.