Comparative study of the comet assay and the micronucleus test in amphibian larvae (Xenopus laevis) using benzo(a)pyrene, ethyl methanesulfonate, and methyl methanesulfonate: establishment of a positive control in the amphibian comet assay

Response of hepatic biomarkers in Physalaemus nattereri (Anura) to different benzo(α)pyrene exposure routes

For over 40 years, anurans have been used as a study model to assess the adverse effects of benzo(α)pyrene (BαP), which include genotoxic, hepatotoxic, and immunotoxic effects. In these studies, BαP is administered cutaneously or by injection, with no comparison between two or more routes. The purpose of this study is to assess whether the effect of BαP is influenced by its route of administration, using the response of hepatic biomarkers of Physalaemus nattereri. Specimens (n = 108) were collected and divided into three experimental treatments (cutaneous, injection, and oral) and three experimental times (one, three, and seven days). Specimens received 0.02 ml of pure mineral oil (control) or mineral oil containing 2 mg/kg of BαP. The BαP causes changes in morphological (melanin, hemosiderin, lipofuscin, and mast cells) and biochemical (superoxide dismutase and glutathione S-transferase) hepatic biomarkers. Compared to biochemical, morphological biomarkers underwent a greater number of significant changes due to the treatment with BαP. The route of exposure alters the effects of BαP, mainly seen in morphological biomarkers, especially the pigments melanin, hemosiderin, and lipofuscin. In these pigments, the effect of the exposure pathway changes according to the analyzed biomarker, and the exposure time modulates the exposure pathway effect. These results are unprecedented for anurans and contribute to the field of herpetology and ecotoxicology.

Enzymatic, morphological, and genotoxic effects of benzo[a]pyrene in rainbow trout (Oncorhynchus mykiss)

Fish have defense systems that are capable of repairing damages caused by xenobiotics like benzo[a]pyrene (BaP), so the aims of this study were to identify BaP toxicity in melanomacrophages (MMs) cytoskeleton, evaluate the melanin area in MMs, and analyze genotoxicity. Rainbow trout juveniles (n = 24) were split in 48h and 7d treatments that received 2 mg/kg of BaP. After the experiment, blood samples were collected and liver was removed, to proceed with the analysis: EROD activity, MMs melanin area quantification, melanosomes movements, and a genotoxicity test. The results revealed increased in EROD activity after 48-h and 7-day BaP exposure. The group 7d displayed a reduction in MMs pigmented area, melanosomes aggregation, in addition to an increased frequency of micronucleus. By means of the EROD assay, it was possible to confirm the activation of BaP biotransformation system. The impairment of the melanosomes' movements possibly by an inactivation of the protein responsible for the pigment dispersion consequently affects the melanin area and thus might negatively impact the MMs detoxification capacity. In addition to this cytotoxicity, the increased frequency of micronucleus might also indicate the genotoxicity of BaP in this important fish species.

Cytogenotoxicity evaluation of a heavy metal mixture, detected in a polluted urban wetland: Micronucleus and comet induction in the Indian green frog (Euphlyctis hexadactylus) erythrocytes and the Allium cepa bioassay

Heavy metal contamination in wetland ecosystems is a serious environmental and health concern. This study evaluated the cytogenotoxicity of a previously evidenced heavy metal contamination (Cd, Cr, Cu, Pb and Zn ∼5 ppm each) in a polluted urban wetland, the Bellanwila-Attidiya sanctuary (BAS) in Sri Lanka, using a battery of cytogenotoxic assays. Micronucleus and comet assays evaluated the genotoxicity in erythrocytes of a common amphibian, the Indian green frog (Euphlyctis hexadactylus), under natural metal exposure in the wetland, and in vitro exposure, respectively.The Allium cepa bioassay assessed the cytogenotoxicity of the heavy metal mixture and of the individual metals, under laboratory exposure. Although in vivo natural exposure showed no significant induction of micronuclei in frog erythrocytes (P > 0.1), a significant and dose dependent elevation of comets was evident with in vitro exposure to the metal mixture (P < 0.001). Field controls did not show significant impacts in the A. cepa bioassay, whereas individual exposure to heavy metals reported lower effects than their combined exposure under laboratory conditions; Pb²⁺was the most toxic metal, with the highest mitotic inhibition (Pb²⁺>Cd²⁺>Zn²⁺>Cr⁶ >Cu²⁺), mutagenic potential as evaluated in the percentage incidence of chromosomal aberrations (Pb²⁺> Zn²⁺> Cu²⁺> Cr⁶⁺> Cd²⁺) and cytotoxicity evaluated by the incidence of cell apoptosis and necrosis (Pb²⁺>Cr⁶⁺>Cu²⁺>Cd²⁺>Zn²⁺). Thus, the test battery of micronucleus, comet and A. cepa assays that reveal differential aspects of cytogenotoxicity may serve as a valuable tool in environmental monitoring, primarily to screen for complex environmental mixtures of heavy metals that may impact ecological health.

Micronucleus Assays in Amphibians

Induction of micronucleus (MN) expression is a well-validated biomarker of genotoxic exposure in eukaryotic cells and is widely used in biomonitoring programs. The presence of permeable skin, which facilitates toxicant exposure and local abundance, is among the characteristics that make amphibians good indicators of ecosystem health. The presence of large nucleated erythrocytes, which divide in the circulation, makes amphibian erythrocytes an ideal target tissue to detect genotoxin exposure using the MN assay. Published literature have highlighted the promising prospects of using the amphibian MN assay as a sensitive biomonitoring tool for water quality assessment to detect potential genotoxins. The present chapter provides the basic outline of the amphibian MN assay and highlights its use in genotoxicity testing in experimental and biomonitoring studies.

The comet assay in animal models: From bugs to whales - (Part 2 Vertebrates)

The comet assay has become one of the methods of choice for the evaluation and measurement of DNA damage. It is sensitive, quick to perform and relatively affordable for the evaluation of DNA damage and repair at the level of individual cells. The comet assay can be applied to virtually any cell type derived from different organs and tissues. Even though the comet assay is predominantly used on human cells, the application of the assay for the evaluation of DNA damage in yeast, plant and animal cells is also quite high, especially in terms of biomonitoring. The present extensive overview on the usage of the comet assay in animal models will cover both terrestrial and water environments. The first part of the review was focused on studies describing the comet assay applied in invertebrates. The second part of the review, (Part 2) will discuss the application of the comet assay in vertebrates covering cyclostomata, fishes, amphibians, reptiles, birds and mammals, in addition to chordates that are regarded as a transitional form towards vertebrates. Besides numerous vertebrate species, the assay is also performed on a range of cells, which includes blood, liver, kidney, brain, gill, bone marrow and sperm cells. These cells are readily used for the evaluation of a wide spectrum of genotoxic agents both in vitro and in vivo. Moreover, the use of vertebrate models and their role in environmental biomonitoring will also be discussed as well as the comparison of the use of the comet assay in vertebrate and human models in line with ethical principles. Although the comet assay in vertebrates is most commonly used in laboratory animals such as mice, rats and lately zebrafish, this paper will only briefly review its use regarding laboratory animal models and rather give special emphasis to the increasing usage of the assay in domestic and wildlife animals as well as in various ecotoxicological studies.

Thermal Reduction of Graphene Oxide Mitigates Its In Vivo Genotoxicity Toward Xenopus laevis Tadpoles

The worldwide increase of graphene family materials raises the question of the potential consequences resulting from their release in the environment and future consequences on ecosystem health, especially in the aquatic environment in which they are likely to accumulate. Thus, there is a need to evaluate the biological and ecological risk but also to find innovative solutions leading to the production of safer materials. This work focuses on the evaluation of functional group-safety relationships regarding to graphene oxide (GO) in vivo genotoxic potential toward X. laevis tadpoles. For this purpose, thermal treatments in H₂ atmosphere were applied to produce reduced graphene oxide (rGOs) with different surface group compositions. Analysis performed indicated that GO induced disturbances in erythrocyte cell cycle leading to accumulation of cells in G0/G1 phase. Significant genotoxicity due to oxidative stress was observed in larvae exposed to low GO concentration (0.1 mg·L⁻¹). Reduction of GO at 200 °C and 1000 °C produced a material that was no longer genotoxic at low concentrations. X-ray photoelectron spectroscopy (XPS) analysis indicated that epoxide groups may constitute a good candidate to explain the genotoxic potential of the most oxidized form of the material. Thermal reduction of GO may constitute an appropriate “safer-by-design” strategy for the development of a safer material for environment.

Effects of benzo[a]pyrene on the blood and liver of Physalaemus cuvieri and Leptodactylus fuscus (Anura: Leptodactylidae)

Benzo[a]pyrene (BaP) is a bio-accumulative toxic compound found in the atmosphere, water, and soil that may affect the life cycle of amphibians. In this study, a few contamination biomarkers, such as hepatic melanomacrophages (MMs), mast cells, erythrocyte micronuclei (MN) and white blood cells were used to determine how BaP acts in these cells in the anurans Physalaemus cuvieri and Leptodactylus fuscus. Animals of both species were divided into three treatment groups: 1 day, 7 days and 13 days, subcutaneously injected 2 mg/kg BaP diluted in mineral oil and control group with only mineral oil. After 7 days, BaP caused the frequency of MN to increase in both species while reducing melanin area. The micronucleus frequency increased due to the genotoxicity of BaP, while the decreasing melanin area may be related to the inhibition of tyrosinase activity, an enzyme responsible for regulating melanogenesis, decreasing the synthesis of melanin. The mast cell density increased in all groups and in both species as a response to the inflammatory action of BaP. These cells respond to nonspecific inflammatory effects leading, therefore, to this response in all treatments. The percentage of leukocytes remained unchanged probably due to great intraspecific variability. Additionally, the leukocyte profiles of both species were characterized and the differences were attributed to extrinsic factors. In short, BaP can affect the integrity of several organs and tissues, and cell functions leading to the conclusion that this compound is hepatotoxic, genotoxic and immunotoxic for anurans.

The Comet Assay and its applications in the field of ecotoxicology: a mature tool that continues to expand its perspectives

Since Singh and colleagues, in 1988, launched to the scientific community the alkaline Single Cell Gel Electrophoresis (SCGE) protocol, or Comet Assay, its uses and applications has been increasing. The thematic areas of its current employment in the evaluation of genetic toxicity are vast, either in vitro or in vivo, both in the laboratory and in the environment, terrestrial or aquatic. It has been applied to a wide range of experimental models: bacteria, fungi, cells culture, arthropods, fishes, amphibians, reptiles, mammals, and humans. This document is intended to be a comprehensive review of what has been published to date on the field of ecotoxicology, aiming at the following main aspects: (i) to show the most relevant experimental models used as bioindicators both in the laboratory and in the field. Fishes are clearly the most adopted group, reflecting their popularity as bioindicator models, as well as a primary concern over the aquatic environment health. Amphibians are among the most sensitive organisms to environmental changes, mainly due to an early aquatic-dependent development stage and a highly permeable skin. Moreover, in the terrestrial approach, earthworms, plants or mammalians are excellent organisms to be used as experimental models for genotoxic evaluation of pollutants, complex mix of pollutants and chemicals, in both laboratory and natural environment. (ii) To review the development and modifications of the protocols used and the cell types (or tissues) used. The most recent developments concern the adoption of the enzyme linked assay (digestion with lesion-specific repair endonucleases) and prediction of the ability to repair of oxidative DNA damage, which is becoming a widespread approach, albeit challenging. For practical/technical reasons, blood is the most common choice but tissues/cells like gills, sperm cells, early larval stages, coelomocytes, liver or kidney have been also used. (iii) To highlight correlations with other biomarkers. (iv) To build a constructive criticism and summarize the needs for protocol improvements for future test applications within the field of ecotoxicology. The Comet Assay is still developing and its potential is yet underexploited in experimental models, mesocosmos or natural ecosystems.

In vivo protective effect of Uridine, a pyrimidine nucleoside, on genotoxicity induced by Levodopa/Carbidopa in mice

Parkinson's disease (PD) is a common neurodegenerative disorder that affects millions of people all over the world. Motor symptoms of PD are most commonly controlled by L-3,4-dihydroxyphenylalanine (Levodopa, L-DOPA), a precursor of dopamine, plus a peripherally-acting aromatic-L-amino-acid decarboxylase (dopa decarboxylase) inhibitor, such as carbidopa. However, chronic treatment with a combination of Levodopa plus carbidopa has been demonstrated to cause a major complication, namely abnormal involuntary movements. On the other hand, the effect of this treatment on bone marrow cells is unknown. Therefore, in this study, we aimed to investigate possible genotoxic effects of Levodopa and Carbidopa using male Balb/C mice. Our results showed that Levodopa alone or in combination with carbidopa caused genotoxicity in in vivo micronucleus test (mouse bone marrow) and Comet assay (blood cells). Furthermore, we showed that simultaneous administration of uridine, a pyrimidine nucleoside, reversed the genotoxic effect of Levodopa and Carbidopa in both assays. Our data show for the first time that Levodopa plus carbidopa combination causes genotoxicity which is reversed by uridine treatment. These findings might enhance our understanding for the complications of a common Parkinson's treatment and confer benefit in terms of reducing a possible genotoxic effect of this treatment.

Sodium arsenite induced changes in survival, growth, metamorphosis and genotoxicity in the Indian cricket frog (Rana limnocharis)

Arsenic contamination of the environment is a matter of great concern. Understanding the effects of arsenic on aquatic life will act as biological early warning system to assess how arsenic could shape the biodiversity in the affected areas. Rapid decline in amphibian population in recent decades is a cause of major concern. Over the years, amphibians have been recognized as excellent bio-indicators of environmental related stress. In the present study, we examined the toxic and genotoxic effects of sodium arsenite in the tadpoles of the Indian cricket frog (Rana limnocharis). Sodium arsenite at different concentrations (0, 50, 100, 200 and 400 μg L(-1)) neither induced lethality nor significantly altered body weight at metamorphosis. However, it accelerated the rate of metamorphosis at higher concentrations, reduced body size (snout-vent length) and induced developmental deformities such as loss of limbs. Besides, at concentration ranges between 100 and 400 μg L(-1), sodium arsenite induced statistically significant genotoxicity at 24, 48, 72 and 96 h of the exposure in a concentration-dependent manner. However, it did not show time effects as the highest frequency was found between 48 and 72 h which remained steady subsequently. The genotoxicity was confirmed by comet assay in the whole blood cells. These findings suggest that arsenic at environmentally relevant concentrations has significant sub-lethal effects on R.limnocharis, which may have long-term fitness consequence to the species and may have similar implications in other aquatic life too.

Short term exposure to multi-walled carbon nanotubes induce oxidative stress and DNA damage in Xenopus laevis tadpoles

The potential impact of Multiwalled Carbon NanoTubes (MWCNTs) was investigated on Xenopus laevis tadpoles exposed to 0.1, 1 and 10mg/L. Oxidative stress was measured in entire larvae exposed and DNA damage (Comet assay) was carried out in erythrocytes of circulating blood from 2h to 24h according to standardized recommendations. Results showed significant H2O2 production when larvae were exposed to 1mg/L and 10mg/L of MWCNTs after 4h and 2h of exposure, respectively. Antioxidant enzyme activities showed significant induction of catalase (CAT), glutathione reductase (GR) and superoxide dismutase (SOD) from only 2h of exposure to 10mg/L of MWCNTs. In presence of 1mg/L of MWCNTs, only GR and CAT activities were significantly induced at 4h. Enzyme activities do not follow a simple dose-effect relation, but the time of induction is shortened in relation with the tested concentration. The Comet assay results showed significant DNA damages with a dose dependent response. The profiles of DNA damages show fluctuations, in course of time, which are characteristics of oxidative stress response in relation with the continuous balance between damage and compensation process.

Impaired liver function in Xenopus tropicalis exposed to benzo[a]pyrene: transcriptomic and metabolic evidence

Background

Despite numerous studies suggesting that amphibians are highly sensitive to cumulative anthropogenic stresses, the role pollutants play in the decline of amphibian populations remains unclear. Amongst the most common aquatic contaminants, polycyclic aromatic hydrocarbons (PAHs) have been shown to induce several adverse effects on amphibian species in the larval stages. Conversely, adults exposed to high concentrations of the ubiquitous PAH, benzo[a]pyrene (BaP), tolerate the compound thanks to their highly efficient hepatic detoxification mechanisms. Due to this apparent lack of toxic effect on adults, no studies have examined in depth the potential toxicological impact of PAH on the physiology of adult amphibian livers. This study sheds light on the hepatic responses of Xenopus tropicalis when exposed to high environmentally relevant concentrations of BaP, by combining a high throughput transcriptomic approach (mRNA deep sequencing) and a characterization of cellular and physiological modifications to the amphibian liver.

Results

Transcriptomic changes observed in BaP-exposed Xenopus were further characterized using a time-dependent enrichment analysis, which revealed the pollutant-dependent gene regulation of important biochemical pathways, such as cholesterol biosynthesis, insulin signaling, adipocytokines signaling, glycolysis/gluconeogenesis and MAPK signaling. These results were substantiated at the physiological level with the detection of a pronounced metabolic disorder resulting in a possible insulin resistance-like syndrome phenotype. Hepatotoxicity induced by lipid and cholesterol metabolism impairments was clearly identified in BaP-exposed individuals.

Conclusions

Our data suggested that BaP may disrupt overall liver physiology, and carbohydrate and cholesterol metabolism in particular, even after short-term exposure. These results are further discussed in terms of how this deregulation of liver physiology can lead to general metabolic impairment in amphibians chronically exposed to contaminants, thereby illustrating the role xenobiotics might play in the global decline in amphibian populations.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-666) contains supplementary material, which is available to authorized users.

DNA strand breaks detected in embryos of the adult snails, Potamopyrgus antipodarum, and in neonates exposed to genotoxic chemicals

We tested the freshwater mudsnail Potamopyrgus antipodarum, which is a species that has already been used for endocrine-disrupting compounds (EDCs) to determine whether early life stages of aquatic organisms are sensitive to genotoxic chemicals. For this purpose, we first developed the alkaline comet assay on adults, embryos, and neonates. The comet assay protocol was validated on both embryonic cells exposed in vitro to hydrogen peroxide and adult snails in the reproducing stage exposed to methyl methane sulfonate. During the latter experiment, DNA strand breaks were investigated on both embryonic cells and on adult gill cells. The second part of this study investigated the stability of DNA strand breaks in adult reproducing snails and neonates exposed to cadmium (Cd) and bisphenol A for 8 days. Hydrogen peroxide-induced DNA strand breaks in vitro in isolated embryonic cells. Exposure of adult reproducing snails to methyl methane sulfonate for 24h induced DNA strand breaks in embryos. Bisphenol A induced a significant increase in the DNA strand-break level in whole embryonic cells and whole neonate cells. Cd was genotoxic for both embryos and neonates during the exposure time and also after 7 days of depuration, suggesting that Cd could inhibit DNA repair enzymes. These preliminary results on this original model have encouraged us to consider the impact of genotoxic environmental contaminants on the F1 generation.

Toxicokinetic of benzo[a]pyrene and fipronil in female green frogs (Pelophylax kl. esculentus)

A general consensus that an increased logK(ow) led to an increase in xenobiotic uptake and bioaccumulation is accepted. In this study we compared the toxicokinetics of two chemically different xenobiotics, i.e. benzo[a]pyrene and fipronil in female green frogs. Surprisingly, the uptake rates and the bioconcentration factors (BCF) of the two contaminants were not predicted by their logK(ow). The uptake rates obtained were of the same order of magnitude for the two contaminants and the BCFs measured for fipronil were about 3-fold higher than those obtained for benzo[a]pyrene. Fipronil appeared to be more recalcitrant than benzo[a]pyrene to detoxification processes leading to the accumulation of sulfone-fipronil especially in the ovaries. This phenomenon may explain reproductive influence of this contaminant described in other studies. Detoxification processes, including metabolism and the excretion of pollutants, are of importance when considering their persistence in aquatic organisms and trying to quantify their risks.

Frogs, sentinels of DNA damage induced by pollution in Naples and the neighbouring Provinces

Many DNA mutation-based diseases recognised in Campania have recently been related to toxic substances in illegal dumping areas. We performed a comet assay on edible frog erythrocytes to evaluate DNA damage. Differences in genotoxic parameters were observed among populations. We show that severe DNA damage occurred in the north Campania where the emergence of environmental waste exploded recently. Although a similar magnitude of genotoxic damage was observed in some southern populations, it is attributable to a massive pesticide pollution related to intensive farming. The frog species analysed seems to be a good bioindicator for detecting genotoxic effects of chemical environmental hazards.

Carbon nanotube ecotoxicity in amphibians: assessment of multiwalled carbon nanotubes and comparison with double-walled carbon nanotubes

The potential impact of industrial multiwalled carbon nanotubes (MWNTs) was investigated under normalized laboratory conditions according to the International Standard micronucleus assay ISO 21427–1 for 12 days of half-static exposure to 0.1, 1, 10 and 50 mg/l of MWNTs in water. Three different end points were carried out for 12 days of exposure: mortality, growth inhibition and micronuclei induction in erythrocytes of the circulating blood of larvae. Raman spectroscopy analysis was used to study the presence of carbon nanotubes in the biological samples. Considering the high diversity of carbon nanotubes according to their different characteristics, MWNTs were analyzed in Xenopus larvae, comparatively to double-walled carbon nanotubes used in a previous study in similar conditions. Growth inhibition in larvae exposed to 50 mg/l of MWNTs was evidenced; however, no genetoxicity (micronucleus assay) was noticed, at any concentration. Carbon nanotube localization in the larvae leads to different possible hypothesis of mechanisms explaining toxicity in Xenopus.

Genotoxicity of two pathogenic strains of zoosporic fungi (Achlya klebsiana and Aphanomyces laevis) on erythrocytes of Nile tilapia Oreochromis niloticus niloticus

In the present work we have described the genotoxic potential of two pathogenic strains of zoosporic fungi (Achlya klebsiana and Aphanomyces laevis) on erythrocytes of Nile tilapia Oreochromis niloticus niloticus for the first time by three complementary tests: micronucleus test (MN), nuclear lesions (NL) and comet assay (CA). The groups exposed to the zoosporic fungi subjected to the MN and NL test showed statistically significant differences in MN and NL frequencies with respect to the control one. Also, a significant increase (p<0.001) in micronuclei and nuclear lesions frequencies were recorded with the increase in exposure time. A correlation was observed between the frequencies of MN and NL, suggesting the importance for recording this anomaly in order to improve the information obtained with the MN test. Therefore, our results suggest that the nuclear lesions found here should be considered indicators of genotoxicity, in addition to the typical micronuclei forms. The result of the comet assay showed a significant difference in the percentages of the damaged DNA in the comet tail (%TDNA) of the treated groups comparing to the control. A significant (p<0.001) increase in the DNA damage of the treated groups with the increase in exposure time was recorded, confirming the results of the MN and the NL tests. The results of the current study will be useful for future work involving the biomonitoring of regions where Nile tilapia survive. These data allow us to consider O. niloticus niloticus as a good bioindicator of the effects of genotoxic agents that might be present in the aquatic habitat.

Variation in genotoxic stress tolerance among frog populations exposed to UV and pollutant gradients

Populations of widely distributed species can be subjected to unequal selection pressures, producing differences in rates of local adaptation. We report a laboratory experiment testing tolerance variation to UV-B and polycyclic aromatic hydrocarbons (PAHs) among common frog (Rana temporaria) populations according to their natural exposure level in the field. Studied populations were naturally distributed along two gradients, i.e. UV-B radiation with altitude and level of contamination by PAHs with the distance to emitting sources (road traffic). Tadpoles from eight populations were subjected to (1) no or high level of artificial UV-B; (2) four concentrations of benzo[a]pyrene (BaP) (0, 50, 250, 500microgL(-1)); (3) simultaneously to UV-B and BaP. Since both stressors are genotoxic, the number of micronucleated erythrocytes (MNE) in circulating red blood cells was used as a bioindicator of tadpole sensitivity. High-altitude populations appear to be locally adapted to better resist UV-B genotoxicity, as they showed the lowest MNE numbers. Conversely, no correlation was observed between levels of PAH contamination in the field and tadpole tolerance to BaP in the laboratory, indicating the absence of local adaptation for BaP tolerance in these populations. Nevertheless, the decrease of MNE formation due to BaP exposure with altitude suggests that high-altitude populations were intrinsically more resistant to BaP genotoxicity. We propose the hypothesis of a co-tolerance between UV-B and BaP in high-altitude common frog populations: local adaptation to prevent and/or repair DNA damage induced by UV-B could also protect these highland populations against DNA damage induced by BaP. The results of this study highlight the role of local adaptation along pollutant gradients leading to tolerance variation, which implies that is it necessary to take into account the history of exposure of each population and the existence of co-tolerance that can hide toxic effects of a new pollutant.

Comet assay: a reliable tool for the assessment of DNA damage in different models

New chemicals are being added each year to the existing burden of toxic substances in the environment. This has led to increased pollution of ecosystems as well as deterioration of the air, water, and soil quality. Excessive agricultural and industrial activities adversely affect biodiversity, threatening the survival of species in a particular habitat as well as posing disease risks to humans. Some of the chemicals, e.g., pesticides and heavy metals, may be genotoxic to the sentinel species and/or to non-target species, causing deleterious effects in somatic or germ cells. Test systems which help in hazard prediction and risk assessment are important to assess the genotoxic potential of chemicals before their release into the environment or commercial use as well as DNA damage in flora and fauna affected by contaminated/polluted habitats. The Comet assay has been widely accepted as a simple, sensitive, and rapid tool for assessing DNA damage and repair in individual eukaryotic as well as some prokaryotic cells, and has increasingly found application in diverse fields ranging from genetic toxicology to human epidemiology. This review is an attempt to comprehensively encase the use of Comet assay in different models from bacteria to man, employing diverse cell types to assess the DNA-damaging potential of chemicals and/or environmental conditions. Sentinel species are the first to be affected by adverse changes in their environment. Determination of DNA damage using the Comet assay in these indicator organisms would thus provide information about the genotoxic potential of their habitat at an early stage. This would allow for intervention strategies to be implemented for prevention or reduction of deleterious health effects in the sentinel species as well as in humans.

Toxic effects of carbendazim and n-butyl isocyanate, metabolites of the fungicide benomyl, on early development in the African clawed frog, Xenopus laevis

We investigated the toxic effects of carbendazim and n-butyl isocyanate (BIC), metabolites of the fungicide benomyl, on development in the African clawed frog, Xenopus laevis. To test the toxic effects, frog embryo teratogenesis assays using Xenopus were performed. Embryos were exposed to various concentrations of carbendazim (0-7 microM) and BIC (0-0.2 microM). LC(100) for carbendazim and BIC were 7 and 0.2 microM, respectively, and the corresponding LC(50), determined by probit analysis, were 5.606 and 0.135 microM. Exposure to carbendazim concentrations > or = 3 microM and BIC concentrations > or = 0.1 microM resulted in 10 different types of severe external malformation. Histological examinations revealed dysplasia of the brain, eyes, intestine, and somatic muscle, and swelling of the pronephric ducts. These phenomena were common in both test groups. The tissue-specific toxic effects were investigated with an animal cap assay. Neural tissues are normally induced at a high frequency by activin A, however, the induction of neural tissues was strongly inhibited by the addition of carbendazim. Conversely, the addition of BIC resulted in weak inhibition of neural tissues. Electron micrographs of animal cap explants revealed degeneration of cell junctions in the carbendazim-treated group, but not in the BIC-treated group. Numerous residual yolk platelets and mitochondrial degeneration were commonly observed in both test groups. The gene expression of cultivated animal cap explants was investigated by reverse transcriptase-polymerase chain reaction and revealed that expression of the neural-specific marker neural cell adhesion molecule was more strongly inhibited in the carbendazim-treated group than in the BIC-treated group.

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

The toxic and genotoxic potential of Cadmium (CdCl(2)) were evaluated by the micronucleus test (MNT) and comet assay (CA) using amphibian larvae (Xenopus laevis and Pleurodeles waltl). Acute toxicity results showed that Cd is toxic to Xenopus larvae exposed from 2 to 50 mg/L and to Pleurodeles from 5 to 50 mg/L, depending on the nature of the water (reconstituted water containing mineral salts or mineral water MW (Volvic)). The MNT results obtained in MW showed that Cd (2 mg/L) is genotoxic to Xenopus, whereas it was not genotoxic to Pleurodeles at all concentrations tested. The CA established that the genotoxicity of Cd to Xenopus and Pleurodeles larvae depends on the concentration, the exposure times, and the comet parameters (Tail DNA, ETM, OTM, and TL). The CA and MNT results were compared for their ability to detect genotoxic effects, considering the concentrations of Cd applied and the exposure time. The CA showed Cd to be genotoxic from the first day of exposure. In amphibians, the CA appears to be a sensitive and suitable method for detecting genotoxicity such as that caused by Cd.

Assessment of Lead Ecotoxicity in Water using the Amphibian Larvae (Xenopus laevis) and Preliminary Study of its Immobilization in Meat and Bone Meal Combustion Residues

Lead (Pb) is a major chemical pollutant of the environment. It has been associated with human activities for the last 6000 years. Quite rightly, it remains a public health concern today. The present investigation evaluates the toxic potential of Pb in larvae of the toad Xenopus laevis after 12 days exposure in lab conditions. Acute toxicity, genotoxicity and Pb bioaccumulation were analyzed. The genotoxic effects were analyzed in the circulating blood from the levels of micronucleus induction according to the French standard micronucleus assay (AFNOR 2000 Association française de normalization. Norme NFT 90-325. Qualité de l'Eau. Evaluation de la génotoxicité au moyen de larves d'amphibien (Xenopus laevis, Pleurodeles waltl)). Lead bioaccumulation was analyzed in the liver of larvae at the end of exposure. Moreover, the toxic potential of lead, in aquatic media, was investigated in the presence of meat and bone meal combustion residues (MBMCR) known to be rich in phosphates and a potential immobiliser of lead. Previously, acute toxicity and genotoxicity of MBMCR alone were evaluated using Xenopus larvae. The results obtained in the present study demonstrated: (i) that lead is acutely toxic and genotoxic to amphibian larvae from 1 mg Pb/l and its bioaccumulation is significant in the liver of larvae from the lowest concentration of exposure (1 microg Pb/l), (ii) MBMCR were not acutely toxic nor genotoxic in Xenopus larvae, (iii) lead in presence of MBMCR induced inhibition or reduction of the toxic and genotoxic potential of lead in water at concentrations that do not exceed the capacity of MBMCR of Pb-binding (iv) Pb accumulation in larvae exposed to lead with MBMCR in water was lower than Pb-accumulation in larvae exposed to lead alone except at the concentration of 0.01 mg Pb/l suggesting complex mechanisms of MBMCR interaction in organisms. The results confirm the high toxicity and genotoxicity of lead in the aquatic compartment and suggest the potential utility of MBMCR for use in remediation.

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.