The role of the enzyme-modified comet assay in in vivo studies

The in vivo comet assay is an established genotoxicity test, with an OECD test guideline, but in its standard form it measures only DNA strand breaks. Including in the assay an additional step, in which the DNA is incubated with a lesion-specific enzyme, can provide important information about the nature of the DNA damage. Formamidopyrimidine DNA glycosylase, 8-oxoguanine DNA glycosylase or endonuclease III are commonly used in the in vitro genotoxicity test and in human biomonitoring to detect oxidised bases, but in vivo applications are rarer. A systematic literature search has identified a total of 60 papers that report such in vivo experiments, testing a variety of agents. In many cases, strand breaks were not seen, but significant levels of enzyme-sensitive sites were induced - indicating a mechanism of action involving oxidative stress. Compounds such as methyl methanesulfonate (MMS) or ethyl methanesulfonate (EMS) could be used as positive controls in both the standard and the enzyme-modified in vivo comet assays.