The comet assay or single cell gel electrophoresis, is the most common method used to measure strand breaks and a variety of other DNA lesions in human populations. To estimate the risk of overall mortality, mortality by cause, and cancer incidence associated to DNA damage, a cohort of 2,403 healthy individuals (25,978 person-years) screened in 16 laboratories using the comet assay between 1996 and 2016 was followed-up. Kaplan–Meier analysis indicated a worse overall survival in the medium and high tertile of DNA damage (p < 0.001). The effect of DNA damage on survival was modelled according to Cox proportional hazard regression model. The adjusted hazard ratio (HR) was 1.42 (1.06–1.90) for overall mortality, and 1.94 (1.04–3.59) for diseases of the circulatory system in subjects with the highest tertile of DNA damage. The findings of this study provide epidemiological evidence encouraging the implementation of the comet assay in preventive strategies for non-communicable diseases.
DNA integrity is considered an important parameter of semen quality and is of significant value as a predictor of male fertility. Currently, there are several methods that can assess sperm DNA integrity. One such assay is the comet assay, or single-cell gel electrophoresis, which is a simple, sensitive, reliable, quick and low-cost technique that is used for measuring DNA strand breaks and repair at the level of individual cells. Although the comet assay is usually performed with somatic cells from different organs, the assay has the ability to detect genotoxicity in germ cells at different stages of spermatogenesis. Since the ability of sperm to remove DNA damage differs between the stages, interpretation of the results is dependent on the cells used. In this paper we give an overview on the use and applications of the comet assay on mature sperm and its ability to detect sperm DNA damage in both animals and humans. Overall, it can be concluded that the presence in sperm of significantly damaged DNA, assessed by the comet assay, is related to male infertility and seems to reduce live births. Although there is some evidence that sperm DNA damage also has a long-term impact on offspring’s health, this aspect of DNA damage in sperm is understudied and deserves further attention. In summary, the comet assay can be applied as a useful tool to study effects of genotoxic exposures on sperm DNA integrity in animals and humans.
This optimized protocol (including links to instruction videos) describes a comet-based in vitro DNA repair assay that is relatively simple, versatile, and inexpensive, enabling the detection of base and nucleotide excision repair activity. Protein extracts from samples are incubated with agarose-embedded substrate nucleoids (‘naked’ supercoiled DNA) containing specifically induced DNA lesions (e.g., resulting from oxidation, UVC radiation or benzo[a]pyrene-diol epoxide treatment). DNA incisions produced during the incubation reaction are quantified as strand breaks after electrophoresis, reflecting the extract’s incision activity. The method has been applied in cell culture model systems, human biomonitoring and clinical investigations, and animal studies, using isolated blood cells and various solid tissues. Once extracts and substrates are prepared, the assay can be completed within 2 d. This protocol describes a comet-based in vitro assay for detecting base and nucleotide excision repair activity for use in cell culture model systems, human biomonitoring and clinical investigations, and animal studies, using isolated blood cells and various solid tissues.
This study proposes the application of the comet assay for the evaluation of DNA damage from frozen human whole blood samples that could be readily used in human biomonitoring and epidemiological studies. It was done on simply frozen whole blood samples collected from male volunteers (N = 60) aliquoted in small volumes and stored at -80 °C without the addition of cryopreservatives for a period of 5 years. To test the applicability of the alkaline comet assay for the evaluation of DNA damage in frozen whole blood, samples were quickly thawed at 37 °C and immediately embedded in an agarose matrix followed by an alkaline comet assay procedure. We concluded that the whole blood freezing and prolonged storage do not severely affect comet assay values, although background values were higher compared to our historical control data from the fresh whole blood. Even the influence of the variables tested, such as age, body mass index, smoking habit and alcohol consumption were in agreement with our previous data using fresh blood. The obtained results suggest that the comet assay could be applied to frozen blood samples, if properly stored, even for decades, which would certainly facilitate large-scale human biomonitoring and long-term epidemiological studies.
DNA damage and repair activity are often assessed in blood s#38les from humans in different types of molecular epidemiology studies. However, it is not always feasible to analyse the s#38les on the day of collection without any type of storage. For instance, certain studies use repeated s#38ling of cells from the same subject or s#38les from different subjects collected at different time-points, and it is desirable to analyse all these s#38les in the same comet assay experiment. In addition, flawless comet assay analyses on frozen s#38les opens up for the possibility of using this technique on biobank material. In this article we discuss the use of cryopreserved peripheral blood mononuclear cells (PBMCs), buffy coat (BC) and whole blood (WB) for analysis of DNA damage and repair using the comet assay. The published literature and the authors’ experiences indicate that various types of blood s#38les can be cryopreserved with only minor effect on the basal level of DNA damage. There is evidence to suggest that WB and PBMCs can be cryopreserved for several years without much effect on the level of DNA damage. However, care should be taken when cryopreserving WB and BCs. It is possible to use either fresh or frozen s#38les of blood cells, but results from fresh and frozen cells should not be used in the same dataset. The article outlines detailed protocols for the cryopreservation of PBMCs, BCs and WB s#38les.
The comet assay is a well-accepted biomonitoring tool to examine the effect of dietary, lifestyle, environmental and occupational exposure on levels of DNA damage in human cells. With such a wide range of determinants for DNA damage levels, it becomes challenging to deal with confounding and certain factors are inter-related (e.g. poor nutritional intake may correlate with smoking status). This review describes the effect of intrinsic (i.e. sex, age, tobacco smoking, occupational exposure and obesity) and extrinsic (season, environmental exposures, diet, physical activity and alcohol consumption) factors on the level of DNA damage measured by the standard or enzyme-modified comet assay. Although each factor influences at least one comet assay endpoint, the collective evidence does not indicate single factors have a large impact. Thus, controlling for confounding may be necessary in a biomonitoring study, but none of the factors is strong enough to be regarded a priori as a confounder. Controlling for confounding in the comet assay requires a case-by-case approach. Inter-laboratory variation in levels of DNA damage and to some extent also reproducibility in biomonitoring studies are issues that have haunted the users of the comet assay for years. Procedures to collect specimens, and their storage, are not standardized. Likewise, statistical issues related to both sample-size calculation (before sampling of specimens) and statistical analysis of the results vary between studies. This review gives guidance to statistical analysis of the typically complex exposure, co-variate, and effect relationships in human biomonitoring studies.
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.
The comet assay, also called single cell gel electrophoresis, is a sensitive, rapid and low-cost technique for quantifying and analysing DNA damage and repair at the level of individual cells. The assay itself can be applied on virtually any cell type derived from different organs and tissues of eukaryotic organisms. Although it is mainly used on human cells, the assay has applications also in the evaluation of DNA damage in yeast, plant and animal cells. Therefore, the purpose of this review is to give an extensive overview on the usage of the comet assay in animal models from invertebrates to vertebrates, covering both terrestrial and water biota. The comet assay is used in a variety of invertebrate species since they are regarded as interesting subjects in ecotoxicological research due to their significance in ecosystems. Hence, the first part of the review (Part 1) will discuss the application of the comet assay in invertebrates covering protozoans, platyhelminthes, planarians, cnidarians, molluscs, annelids, arthropods and echinoderms. Besides a large number of animal species, the assay is also performed on a variety of cells, which includes haemolymph, gills, digestive gland, sperm and embryo cells. The mentioned cells have been used for the evaluation of a broad spectrum of genotoxic agents both in vitro and in vivo. Moreover, the use of invertebrate models and their role from an ecotoxicological point of view will also be discussed as well as the comparison of the use of the comet assay in invertebrate and human models. Since the comet assay is still developing, its increasing potential in assessing DNA damage in animal models is crucial especially in the field of ecotoxicology and biomonitoring at the level of different species, not only humans.