Human and rodent cell lines showing no differences in the induction but differing in the repair kinetics of radiation-induced DNA base damage

Purpose: To compare the induction and repair of radiation-induced base damage in human and rodent cell lines. Material and methods: Experiments were performed with two human (normal fibroblasts HSF1 and tumour HeLa cells) and two rodent (mouse L929 and hamster CHO-K1) cell lines. Base damage was determined with the alkaline comet assay combined with the repair enzyme formamidopyrimidine-glycosylase (Fpg). Proteins were detected by Western blot. Results: The induction of Fpg-sensitive sites was measured in human and rodent cell lines for doses up to 8 or 5 Gy, respectively. Comets were analysed in terms of tail moments, which were transformed into Gy-equivalents. The amount of Fpg-sensitive sites increased linearly with doses up to 4 Gy, whereby the ratio of single-strand breaks (ssb) to Fpg-sensitive sites was nearly identical for human and rodent cells with ssb:Fpg-sensitive sites=1:0.41±0.07 and 1:0.45±0.05, respectively. For doses exceeding 4 Gy, the amount of Fpg-sensitive sites did not increase further, indicating a dose limit up to which the comet assay can be used to detect Fpg-sensitive sites. Repair of Fpg-sensitive sites was studied for an X-ray dose of 4 Gy. For all four cell lines, the repair was measured to be completed 24 h after irradiation, but with pronounced differences in the kinetics. In both rodent cell lines, 50% of Fpg-sensitive sites were removed after t½=25±10 min in contrast to t½=80±20 min in the two human cell lines. The two species also differed in the level of polymerase (beta) with, on average, a three- to fivefold higher level in rodent cells compared with human cells. Conclusions: Repair of radiation-induced Fpg-sensitive sites was much faster in rodent than in human cells, which might result from the higher level of polymerase (beta) found in rodent cells.