The Comet Assay in Human Biomonitoring

Cell survival after DNA damage in the comet assay

The comet assay is widely used in basic research, genotoxicity testing, and human biomonitoring. However, interpretation of the comet assay data might benefit from a better understanding of the future fate of a cell with DNA damage. DNA damage is in principle repairable, or if extensive, can lead to cell death. Here, we have correlated the maximally induced DNA damage with three test substances in TK6 cells with the survival of the cells. For this, we selected hydrogen peroxide (H2O2) as an oxidizing agent, methyl methanesulfonate (MMS) as an alkylating agent and etoposide as a topoisomerase II inhibitor. We measured cell viability, cell proliferation, apoptosis, and micronucleus frequency on the following day, in the same cell culture, which had been analyzed in the comet assay. After treatment, a concentration dependent increase in DNA damage and in the percentage of non-vital and apoptotic cells was found for each substance. Values greater than 20-30% DNA in tail caused the death of more than 50% of the cells, with etoposide causing slightly more cell death than H2O2 or MMS. Despite that, cells seemed to repair of at least some DNA damage within few hours after substance removal. Overall, the reduction of DNA damage over time is due to both DNA repair and death of heavily damaged cells. We recommend that in experiments with induction of DNA damage of more than 20% DNA in tail, survival data for the cells are provided.

CometChip enables parallel analysis of multiple DNA repair activities

DNA damage can be cytotoxic and mutagenic, and it is directly linked to aging, cancer, and other diseases. To counteract the deleterious effects of DNA damage, cells have evolved highly conserved DNA repair pathways. Many commonly used DNA repair assays are relatively low throughput and are limited to analysis of one protein or one pathway. Here, we have explored the capacity of the CometChip platform for parallel analysis of multiple DNA repair activities. Taking advantage of the versatility of the traditional comet assay and leveraging micropatterning techniques, the CometChip platform offers increased throughput and sensitivity compared to the traditional comet assay. By exposing cells to DNA damaging agents that create substrates of Base Excision Repair, Nucleotide Excision Repair, and Non-Homologous End Joining, we show that the CometChip is an effective method for assessing repair deficiencies in all three pathways. With these applications of the CometChip platform, we expand the utility of the comet assay for precise, high-throughput, parallel analysis of multiple DNA repair activities.

Impact of 12-month cryopreservation on endogenous DNA damage in whole blood and isolated mononuclear cells evaluated by the comet assay

The comet assay is an electrophoretic technique used to assess DNA damage, as a marker of genotoxicity and oxidative stress, in tissues and biological samples including peripheral blood mononuclear cells (PBMCs) and whole blood (WB). Although numerous studies are performed on stored samples, the impact of cryopreservation on artifactual formation of DNA damage is not widely considered. The present study aims to evaluate the impact of storage at different time-points on the levels of strand breaks (SBs) and formamidopyrimidine DNA glycosylase (Fpg)-sensitive sites in isolated PBMCs and WB. Samples were collected, aliquoted and stored at − 80 °C. DNA damage was analyzed on fresh samples, and subsequently on frozen samples every 2 months up to a year. Results have shown no changes in DNA damage in samples of PBMCs and WB stored for up to 4 months, while a significant increase in SBs and Fpg-sensitive sites was documented starting from 6-month up to 12-month storage of both the samples. In addition, fresh and frozen WB showed higher basal levels of DNA damage compared to PBMCs. In conclusion, WB samples show high levels of DNA damage compared to PBMCs. One-year of storage increased the levels of SBs and Fpg-sensitive sites especially in the WB samples. Based on these findings, the use of short storage times and PBMCs should be preferred because of low background level of DNA damage in the comet assay.

Genotoxic and antigenotoxic potential of amygdalin on isolated human lymphocytes by the comet assay

Amygdalin is a cyanogenic glycoside, mainly present in the seeds of the Rosaceae family such as apricots, peaches, and bitter almond. In this study, in vitro genotoxic and antigenotoxic effects of amygdalin have been investigated on human peripheral blood lymphocytes using the comet assay. The antigenotoxic effect of amygdalin was performed against hydrogen peroxide (H2O2) using three different treatment types (pre-, simultaneous, and post-treatment). The isolated lymphocytes were incubated with different concentrations of amygdalin (0.86-13.75 µg/ml) alone and in combination with H2O2 (100 µM). The results indicated that amygdalin exhibited an antigenotoxic effect against H2O2, but it did not induce the genotoxic effect alone in tested concentrations in vitro on human lymphocytes. PRACTICAL APPLICATIONS: Amygdalin is a natural compound used in alternative medicine as an anti-cancer, antipyretic, and cough suppressant. The comet assay which is relatively simple, rapid, sensitive, and economically efficient, measures the changes in genomic stability. Assessment of amygdalin alone has no genotoxic effect on human lymphocytes. Moreover, antigenotoxicity applications (pre-, simultaneous, and post-treatments) of amygdalin significantly reduced the DNA damage induced by H2O2 on isolated human lymphocytes. In conclusion, amygdalin is not genotoxic, also, it exhibited antigenotoxic activity against oxidatively damaged DNA due to its antioxidant properties on human lymphocytes.