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The alkaline single cell gel test (SCG test or comet assay) was used to study the contribution of excision repair activity to the observed DNA effect after mutagen treatment. The cytotoxicity and genotoxicity of UV-irradiation and the chemical mutagens 4-nitroquinoline-1-oxide (4NQO), benzoapyrene (BP) and 7,12-dimethyl-benzaanthracene (DMBA) were compared in a normal human cell line (MRC5CV1) and an excision-deficient xeroderma pigmentosum (XP) cell line (XP12ROSV). The XP cells showed increased cell killing after treatment with all mutagens tested, but did not show a clear increase in DNA migration in the comet assay. DNA effects in MRC5 cells were strongly enhanced by the repair inhibitor aphidicolin (APC), while under the same experimental conditions, APC had no effect on the XP cell line. The enhancing effect of APC on DNA migration in MRC5 cells and the lack of effects in XP cells indicate that the induced DNA effects of 4NQO, BP and DMBA in the comet assay mainly represent the activity of an excision repair process.
Speit et al. (Sun,) studied this question.