Relationship between cellular radiosensitivity and DNA damage measured by comet assay in human normal, NBS and AT fibroblasts

Purpose: To study the relationship between cellular radiosensitivity and DN A damage measured by the comet assay. Materials and methods: Experiments were performed with nine human fibroblas t lines (six normal, one NBS, and two AT). Cellular radiosensitivity was de termined by colony assay and DNA damage was assessed by the comet assay. Results: The cellular radiosensitivity of the fibroblast lines used covered a broad range with SF2 values varying, between 1.3% and 53%. The comets an alysed immediately after irradiation with doses up to 5 Gy showed marked di fferences among the cell lines; the relative initial tail moment at a dose of 5 Gy, ITM5, varied From 2.7 +/- 0.2 to 5.0 +/- 0.3. This variation was c onsidered not to result from different numbers of DNA strand breaks induced but front differences in chromatin Structure. There was an inverse correla tion between SF2 and ITM5, i.e. radiosensitive cell lines exhibited a highe r initial tail moment than radioresistant cell lines. In contrast, the repa ir kinetics measured with the comet assay for a dose of 2 Gy followed by an incubation of up to 2 h showed tittle variation and crc found not to corre late with SF2. Repair kinetic as well as the amount of residual damage meas ured by this version of the comet assay were fairly similar to those measur ed the alkaline unwinding technique and unlike that measured by neutral gel electrophoresis, indicating that this comet assay detects primarily single -strand breaks and alkali-labile sites, not double-strand breaks. Conclusions: The correlation between SF2 and the initial tail moment at 5 G y found here suggests that the cellular radiosensitivity of human fibroblas ts also depends on the chromatin structure.