Investigation of cellular DNA repair capacity with an inhibitor-modified comet assay

DNA repair plays an important role in genomic integrity. Major DNA repair pathways for substance-induced DNA damage are base excision and nucleotide excision repair, as well as the repair of double strand breaks by homologous recombination and non-homologous end joining. However, the quantitative measurement of cellular DNA repair capacity remains a challenge. Here we applied a modified comet assay in which DNA repair capacity can be tracked using DNA repair inhibitors combined with selected DNA-damaging substances. The combinations of the alkylating agent methylmethansulfonate + olaparib, UV-C (causing pyrimidine dimers) + aphidicolin, and the topoisomerase II inhibitor etoposide + SCR7 were used to reflect base excision repair, nucleotide excision repair, and non-homologous end joining. Although neither the agents nor the inhibitors are by themselves completely selective for these DNA repair pathways, the combinations and also the missing reaction to other combinations of the same substances indicated a certain degree of preference. By using all of them within one experiment, a pattern of DNA repair activity of cell lines was established. A comparison of five cell lines/types differing in their sensitivity to the damaging agents demonstrated its applicability. Although each of the damaging agents and inhibitors had been used in the comet assay before, this combination within one experimental setup was described here for the first time and the resulting characterization of the phenotypic pattern of DNA repair capacity of cell lines is more extensive than previously achieved. Its applicability to human peripheral blood mononuclear cells (PBMCs) suggest that the assay may in the future be useful to investigate interindividual variability in DNA repair capacity in humans, for example in relation to disease or exposures.