Minimum Information for Reporting on the Comet Assay (MIRCA): recommendations for describing comet assay procedures and results.

The comet assay is a widely used test for the detection of DNA damage and repair activity. However, there are interlaboratory differences in reported levels of baseline and induced damage in the same experimental systems. These differences may be attributed to protocol differences, although it is difficult to identify the relevant conditions because detailed comet assay procedures are not always published. Here, we present a Consensus Statement for the Minimum Information for Reporting Comet Assay (MIRCA) providing recommendations for describing comet assay conditions and results. These recommendations differentiate between 'desirable' and 'essential' information: 'essential' information refers to the precise details that are necessary to assess the quality of the experimental work, whereas 'desirable' information relates to technical issues that might be encountered when repeating the experiments. Adherence to MIRCA recommendations should ensure that comet assay results can be easily interpreted and independently verified by other researchers.

Potassium bromate as positive assay control for the Fpg-modified comet assay.

The comet assay is a popular assay in biomonitoring studies. DNA strand breaks (or unspecific DNA lesions) are measured using the standard comet assay. Oxidative stress-generated DNA lesions can be measured by employing DNA repair enzymes to recognise oxidatively damaged DNA. Unfortunately, there has been a tendency to fail to report results from assay controls (or maybe even not to employ assay controls). We believe this might have been due to uncertainty as to what really constitutes a positive control. It should go without saying that a biomonitoring study cannot have a positive control group as it is unethical to expose healthy humans to DNA damaging (and thus potentially carcinogenic) agents. However, it is possible to include assay controls in the analysis (here meant as a cryopreserved sample of cells i.e. included in each experiment as a reference sample). In the present report we tested potassium bromate (KBrO3) as a positive comet assay control for the formamidopyrimidine DNA glycosylase (Fpg)-modified comet assay. Ten laboratories used the same procedure for treatment of monocytic THP-1 cells with KBrO3 (0.5, 1.5 and 4.5 mM for 1 h at 37°C) and subsequent cryopreservation. Results from one laboratory were excluded in the statistical analysis because of technical issues in the Fpg-modified comet assay. All other laboratories found a concentration-response relationship in cryopreserved samples (regression coefficients from 0.80 to 0.98), although with different slopes ranging from 1.25 to 11.9 Fpg-sensitive sites (%DNA in tail) per 1 mM KBrO3. Our results demonstrate that KBrO3 is a suitable positive comet assay control.

Residential exposure to radon and levels of histone γH2AX and DNA damage in peripheral blood lymphocytes of residents of Kowary city regions (Poland).

INTRODUCTION: Radon-induced biological effects have been studied mainly through epidemiological investigations, and well-controlled in vitro and in vivo experiments. To provide data explaining radon exposure-induced harmful effects in natural environment, exposure assessment under these conditions is needed. The objective of the study was to examine the level of genetic damage assessed with biomarkers of DNA single- and double-strand breaks (SSBs and DSBs) in peripheral blood mononuclear cells obtained from individuals continuously exposed to Rn in homes. Naturally elevated Rn concentrations in homes can be found in the South of Poland, in Kowary city. METHODS: Measurements of expression of phosphorylated histone γH2AX was used as a marker of DNA double strand breaks. To detect DNA single and double-strand breaks and alkali labile sites, the alkaline comet assay was used. Oxidative damage of DNA was evaluated by formamidopyrimidyne (FPG)-modified comet assay. The blood was collected from 94 volunteers living in Kowary. Subjects were grouped according to their status of living in radon concentration ≥100 Bq/m3 (n = 67), and <100 Bq/m3 (n = 27). RESULTS: The statistically significant differences in levels of DNA damage in peripheral lymphocytes assessed with comet assay were found to be associated with levels of radon exposure in indoor air (p = 0.034). DNA damage in the comet assay was significantly correlated with DNA damage assessed with γH2AX staining. CONCLUSIONS: Results of the present study indicate the suitability of alkaline comet assay for the detection of DNA damage in peripheral blood lymphocytes of people environmentally exposed to radon.

A novel approach to increase robustness, precision and high-throughput capacity of single cell gel electrophoresis.

The routine use of single cell gel electrophoresis assay in medical diagnostics and biomonitoring is prevented by its high variability. Several factors have been identified and can be grouped into four main categories: 1) the biological sample, 2) the assay protocol, 3) the physical parameters during electrophoresis and 4) the analysis. Even though the scientific knowledge on assay variability is available, not much has been done so far to tackle the issues from the technological side. Therefore, this study addresses the question in how far the precise and accurate control over the physical parameters of electrophoresis is able to reduce variability of single cell gel electrophoresis assay results. All four above mentioned categories make up the overall assay variability. To resolve the contribution from a single category, the remaining three have to be kept as constant as possible. To achieve this we generated a set of x-ray treated control cells, worked according to a well-defined standard operating procedure and one single operator performed the analysis. Thereby variability resulting from the electrophoresis tank could be elucidated. We compared assay performance in two such tank systems: a newly developed electrophoresis tank that accurately controls voltage, temperature during the electrophoretic run and the homogeneity of the electric field, and a widely used commercially available standard platform tank. In summary, our results demonstrate that, irrespective of the cellular sample and its intrinsic biological variability, accurate control over physical parameters considerably increases repeatability, reproducibility and precision of single cell gel electrophoresis.

The comet assay applied to HepG2 liver spheroids.

In accordance with the 3 Rs to reduce in vivo testing, more advanced in vitro models, moving from 2D monolayer to 3D cultures, should be developed for prediction of human toxicity of industrial chemicals and environmental pollutants. In this study we compared cytotoxic and genotoxic responses induced by chemicals in 2D and 3D spheroidal cultures of the human liver cancer cell line HepG2. HepG2 spheroids were prepared by hanging drop technology. Both 3D spheroids and 2D monolayer cultures were exposed to different chemicals (colchicine, chlorpromazine hydrochloride or methyl methanesulfonate) for geno- and cytotoxicity studies. Cytotoxicity was investigated by alamarBlue assay, flow cytometry and confocal imaging. DNA damage was investigated by the comet assay with and without Fpg enzyme for detection of DNA strand breaks and oxidized or alkylated base lesions. The results from the cyto- and genotoxicity tests showed differences in sensitivity comparing the 2D and 3D HepG2 models. This study shows that human 3D spheroidal hepatocellular cultures can be successfully applied for genotoxicity testing by the comet assay and represent a promising advanced in vitro model for toxicity testing.

Assessment of DNA damages in lymphocytes of agricultural workers exposed to pesticides by comet assay in a cross-sectional study.

PURPOSE: To assess the predictive power of the comet assay in the context of occupational exposure to pesticides. MATERIALS AND METHODS: The recruited subjects completed a structured questionnaire and gave a blood sample. Exposure to pesticides was measured by means of an algorithm based on Dosemeci's work (Agricultural Health Study). Approximately 50 images were analyzed for each sample via fluorescence microscopy. The extent of DNA damage was estimated by tail moment (TM) and is the product of tail DNA (%) and tail Length. RESULTS: Crude significant risks (odds ratios, ORs) for values higher than the 75th percentile of TM were observed among the exposed subjects (score > 1). The frequency of some confounding factors (sex, age and smoking) was significantly higher among the exposed workers. A significant dose-effect relationship was observed between TM and exposure score. Significant high-risk estimates (ORs), adjusted by the studied confounding factors, among exposure to pesticides and TM, % tail DNA and tail length were confirmed using unconditional logistic regression models. CONCLUSIONS: The adjusted associations (ORs) between the comet parameters and exposure to pesticides were significant. The sensitivity of the comet test was low (41%), the specificity (89%) and the predictive positive value (0.77) were found acceptable.

Validation of the 3D Skin Comet assay using full thickness skin models: Transferability and reproducibility.

Recently revised OECD Testing Guidelines highlight the importance of considering the first site-of-contact when investigating the genotoxic hazard. Thus far, only in vivo approaches are available to address the dermal route of exposure. The 3D Skin Comet and Reconstructed Skin Micronucleus (RSMN) assays intend to close this gap in the in vitro genotoxicity toolbox by investigating DNA damage after topical application. This represents the most relevant route of exposure for a variety of compounds found in household products, cosmetics, and industrial chemicals. The comet assay methodology is able to detect both chromosomal damage and DNA lesions that may give rise to gene mutations, thereby complementing the RSMN which detects only chromosomal damage. Here, the comet assay was adapted to two reconstructed full thickness human skin models: the EpiDerm™- and Phenion® Full-Thickness Skin Models. First, tissue-specific protocols for the isolation of single cells and the general comet assay were transferred to European and US-American laboratories. After establishment of the assay, the protocol was then further optimized with appropriate cytotoxicity measurements and the use of aphidicolin, a DNA repair inhibitor, to improve the assay's sensitivity. In the first phase of an ongoing validation study eight chemicals were tested in three laboratories each using the Phenion® Full-Thickness Skin Model, informing several validation modules. Ultimately, the 3D Skin Comet assay demonstrated a high predictive capacity and good intra- and inter-laboratory reproducibility with four laboratories reaching a 100% predictivity and the fifth yielding 70%. The data are intended to demonstrate the use of the 3D Skin Comet assay as a new in vitro tool for following up on positive findings from the standard in vitro genotoxicity test battery for dermally applied chemicals, ultimately helping to drive the regulatory acceptance of the assay. To expand the database, the validation will continue by testing an additional 22 chemicals.

Assessment of the predictive capacity of the optimized in vitro comet assay using HepG2 cells.

Evaluation of DNA damage is critical during the development of new drugs because it is closely associated with genotoxicity and carcinogenicity. The in vivo comet assay to assess DNA damage is globally harmonized as OECD TG 489. However, a comet test guideline that evaluates DNA damage without sacrificing animals does not yet exist. The goal of this study was to select an appropriate cell line for optimization of the in vitro comet assay to assess DNA damage. We then evaluated the predictivity of the in vitro comet assay using the selected cell line. In addition, the effect of adding S9 was evaluated using 12 test chemicals. For cell line selection, HepG2, Chinese hamster lung (CHL/IU), and TK6 cell lines were evaluated. We employed a method for the in vitro comet assay based on that for the in vivo comet assay. The most appropriate cell line was determined by% tail DNA increase after performing in vitro comet assays with 6 test chemicals. The predictivity of the in vitro comet assay using the selected cell line was measured with 10 test chemicals (8 genotoxins and 2 non-genotoxic chemicals). The HepG2 cell line was found to be the most appropriate, and in vitro comet assays using HepG2 cells exhibited a high accuracy of 90% (9/10). This study suggests that HepG2 is an optimal cell line for the in vitro comet assay to assess DNA damage.

Standardisation of the in vitro comet assay: influence of lysis time and lysis solution composition on the detection of DNA damage induced by X-rays.

The alkaline comet assay, in vivo and in vitro, is currently used in several areas of research and in regulatory genotoxicity testing. Several efforts have been made in order to decrease the inter-experimental and inter-laboratory variability and increase the reliability of the assay. In this regard, lysis conditions are considered as one of the critical variables and need to be further studied. Here, we tested different times of lysis (from no lysis to 1 week) and two different lysis solutions in human lymphoblast (TK6) cells unexposed or exposed to X-rays. Similar % tail DNA values were obtained independently of the time of lysis employed for every X-ray dose tested and both lysis solutions. These results, taken together with our previous ones with methyl methanesulfonate and H2O2, which showed clear lysis-time dependence, support that the influence of the lysis time in the comet assay results depends on the type of lesion being detected; some DNA lesions may spontaneously give rise to apurinic or apyrimidinic (AP) sites during the lysis period, which can be converted into strand breaks detectable with the comet assay. Testing different times of lysis would be useful to increase the sensitivity of the comet assay and to ensure the detection of DNA lesions of an unknown compound, thereby providing some insight into the chemical nature of the lesions induced. However, the same lysis conditions (i.e. lysis time and lysis solution) should be used when comparing results between different experiments or laboratories.

Seasonal variations as predictive factors of the comet assay parameters: a retrospective study.

Since there are several predicting factors associated with the comet assay parameters, we have decided to assess the impact of seasonal variations on the comet assay results. A total of 162 volunteers were retrospectively studied, based on the date when blood donations were made. The groups (winter, spring, summer and autumn) were matched in terms of age, gender, smoking status, body mass index and medical diagnostic exposure in order to minimise the impact of other possible predictors. Means and medians of the comet assay parameters were higher when blood was sampled in the warmer period of the year, the values of parameters being the highest during summer. Correlation of meteorological data (air temperature, sun radiation and sun insolation) was observed when data were presented as the median per person. Using multivariate analysis, sampling season and exposure to medical radiation were proved to be the most influential predictors for the comet assay parameters. Taken together, seasonal variation is another variable that needs to be accounted for when conducting a cohort study. Further studies are needed in order to improve the statistical power of the results related to the impact of sun radiation, air temperature and sun insolation on the comet assay parameters.

Searching for assay controls for the Fpg- and hOGG1-modified comet assay.

The formamidopyrimidine DNA glycosylase (Fpg) and human 8-oxoguanine DNA glycosylase (hOGG1)-modified comet assays have been widely used in human biomonitoring studies. The purpose of this article is to assess differences in reported levels of Fpg- and hOGG1-sensitive sites in leukocytes and suggest suitable assay controls for the measurement of oxidatively damaged DNA. An assessment of the literature showed a large variation in the reported levels of Fpg-sensitive sites (range 0.05-1.31 lesions/106 bp). The levels of Fpg-sensitive sites are lower in studies where Fpg has been obtained from commercial suppliers or unknown sources as compared to Fpg from one particular non-commercial source (χ2 = 7.14, P = 0.028). The levels of hOGG1-sensitive sites are lower (range: 0.04-0.18 lesions/106 bp in leukocytes) compared to the Fpg-sensitive sites. Surprisingly, few publications have reported the use of oxidising agents as assay controls, with the exception of hydrogen peroxide. This may be due to a lack of consensus about suitable controls for the Fpg- and hOGG1-modified comet assay. A major challenge is to find an oxidising agent that only oxidises nucleobases and does not generate DNA strand breaks because this reduces the dynamic range of Fpg- and hOGG1-sensitive sites in the comet assay. Based on a literature search we selected the photosensitiser Ro19-8022 plus light, KBrO3, 4-nitroquinoline-1-oxide, Na2Cr2O7 and ferric nitrilotriacetate as possible assay controls. A subsequent assessment of these compounds for generating cryopreserved assay controls in mononuclear blood cells showed that Ro19-8022 plus light, KBrO3 and 4-nitroquinoline-1-oxide provided suitable assay controls. We recommend these compounds as comet assay controls for oxidatively damaged DNA.

The comet assay in human biomonitoring: cryopreservation of whole blood and comparison with isolated mononuclear cells.

The comet assay is often applied in human biomonitoring. Most of the time the assay is performed with isolated peripheral blood mononuclear cells (PBMC). However, using whole blood instead of isolated cells reduces processing time, and only 20 µl is sufficient for analysis. In this study, a cryopreservation protocol for human whole blood for application in the comet assay was optimised by removing excess plasma before adding freezing medium. Cryopreservation of whole blood samples (n = 30) did not increase the detected level of strand breaks and formamidopyrimidine DNA glycosylase (FPG)-sensitive sites. Although there was no significant correlation with breaks measured in fresh whole blood, strand breaks detected in frozen whole blood were significantly correlated with breaks measured in frozen PBMC (Pearson correlation r = 0.54, P < 0.01). This correlation was however not observed for FPG-sensitive sites. Since we do not yet know the full extent to which cryopreservation might influence the blood cell population, care should be taken to ensure a similar cell type and storage conditions for all samples in one study.

Inter-laboratory comparison of the in vivo comet assay including three image analysis systems.

To compare the extent of potential inter-laboratory variability and the influence of different comet image analysis systems, in vivo comet experiments were conducted using the genotoxicants ethyl methanesulfonate and methyl methanesulfonate. Tissue samples from the same animals were processed and analyzed-including independent slide evaluation by image analysis-in two laboratories with extensive experience in performing the comet assay. The analysis revealed low inter-laboratory experimental variability. Neither the use of different image analysis systems, nor the staining procedure of DNA (propidium iodide vs. SYBR® Gold), considerably impacted the results or sensitivity of the assay. In addition, relatively high stability of the staining intensity of propidium iodide-stained slides was found in slides that were refrigerated for over 3 months. In conclusion, following a thoroughly defined protocol and standardized routine procedures ensures that the comet assay is robust and generates comparable results between different laboratories.

JaCVAM-organized international validation study of the in vivo rodent alkaline comet assay for the detection of genotoxic carcinogens: I. Summary of pre-validation study results.

The in vivo rodent alkaline comet assay (comet assay) is used internationally to investigate the in vivo genotoxic potential of test chemicals. This assay, however, has not previously been formally validated. The Japanese Center for the Validation of Alternative Methods (JaCVAM), with the cooperation of the U.S. NTP Interagency Center for the Evaluation of Alternative Toxicological Methods (NICEATM)/the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM), the European Centre for the Validation of Alternative Methods (ECVAM), and the Japanese Environmental Mutagen Society/Mammalian Mutagenesis Study Group (JEMS/MMS), organized an international validation study to evaluate the reliability and relevance of the assay for identifying genotoxic carcinogens, using liver and stomach as target organs. The ultimate goal of this validation effort was to establish an Organisation for Economic Co-operation and Development (OECD) test guideline. The purpose of the pre-validation studies (i.e., Phase 1 through 3), conducted in four or five laboratories with extensive comet assay experience, was to optimize the protocol to be used during the definitive validation study.

The JaCVAM international validation study on the in vivo comet assay: Selection of test chemicals.

The Japanese Center for the Validation of Alternative Methods (JaCVAM) sponsored an international prevalidation and validation study of the in vivo rat alkaline pH comet assay. The main objective of the study was to assess the sensitivity and specificity of the assay for correctly identifying genotoxic carcinogens, as compared with the traditional rat liver unscheduled DNA synthesis assay. Based on existing carcinogenicity and genotoxicity data and chemical class information, 90 chemicals were identified as primary candidates for use in the validation study. From these 90 chemicals, 46 secondary candidates and then 40 final chemicals were selected based on a sufficiency of carcinogenic and genotoxic data, differences in chemical class or genotoxic or carcinogenic mode of action (MOA), availability, price, and ease of handling. These 40 chemicals included 19 genotoxic carcinogens, 6 genotoxic non-carcinogens, 7 non-genotoxic carcinogens and 8 non-genotoxic non-carcinogens. "Genotoxicity" was defined as positive in the Ames mutagenicity test or in one of the standard in vivo genotoxicity tests (primarily the erythrocyte micronucleus assay). These chemicals covered various chemicals classes, MOAs, and genotoxicity profiles and were considered to be suitable for the purpose of the validation study. General principles of chemical selection for validation studies are discussed.