Detection of crosslinks with the comet assay in relationship to genotoxicity and cytotoxicity

The alkaline comet assay is a sensitive test for the detection of a variety of DNA lesions. However, crosslinks are not readily detected under standard test conditions. Recently, modifications have been introduced measuring crosslinks by determining the reduction of induced DNA migration. We used the comet assay to comparatively investigate in V79 cells the effect of three different crosslinkers: formaldehyde (FA), which predominantly induces DNA-protein crosslinks, cisplatin (DDP), which mainly produces DNA-DNA-intrastrand crosslinks, and mitomycin C (MMC), which mainly leads to DNA-DNA-interstrand crosslinks. In the standard alkaline comet assay, only MMC induced a slight increase in DNA migration at high toxic concentrations. FA and DDP did not induce any DNA migration under the test conditions used. In the modified comet assay, all three crosslinkers led to a clear reduction of gamma-ray-induced DNA migration. This reduction was seen in the case of FA parallel to the induction of cytotoxicity and SCE, while for MMC and DDP induction of cytotoxicity, SCE and HPRT gene mutations occurred at much lower concentrations than the effects in the comet assay. The DNA-DNA crosslinkers caused a reduction of induced DNA migration only at cytotoxic concentrations. Our results indicate that the modified comet assay protocol is a sensitive test for the detection of DNA-protein crosslinks. However, the results for MMC and DDP suggest that the modified protocol is not well suited for the evaluation of DNA-DNA crosslinkers. Furthermore, the relationship between crosslinking and genotoxicity seems to be very different for the three different types of crosslinking substances.