The influence of chromatin structure on initial DNA damage and radiosensitivity in CHO-K1 and xrs1 cells at low doses of irradiation (1-10 Gy)

Mitotic compaction of chromatin was generated by treatment of cells with nocodazole. Alternatively, chromatin structure was altered by incubating cells in 500 mM NaCl. The irradiation response in the dose range of 1–10 Gy was measured by colony assay and by a modified fluorometric analysis of DNA unwinding (FADU) assay which measures the amount of undamaged DNA by EtBr fluorescence. Cell survival curves of irradiated CHO-K1 cells showed that treatment with nocodazole increases radiosensitivity as indicated by a decrease of the mean inactivation dose (\( \overline {\rm D} \) ) from 4.446 to 4.376. Nocodazole treatment increased the initial radiation-induced DNA damage detected by the FADU assay from 7% to 13%. In repair-defective xrs1 cells, the same conditions increased the radiosensitivity from \( \overline {\rm D} \) 1.209 to 0.7836 and the initial DNA damage from 43% to 57%. Alterations to chromatin structure by hypertonic medium increased radiosensitivity in CHO-K1 cells from \( \overline {\rm D} \) of 4.446 to 3.092 and the initial DNA damage from 7% to 15%. In xrs1 cells these conditions caused radiosensitivity to decrease from \( \overline {\rm D} \) 1.209 to 1.609 and the initial DNA damage to decrease from 43% to 36%. Disruption of chromatin structure by hypertonic treatment was found to be time-dependent. A threefold increase of exposure time to hypertonic medium from 40 to 120 min increased the initial DNA damage in CHO-K1 cells from 7% to 18% but decreased initial DNA damage in xrs1 cells from 43% to 21%. Perturbation of chromatin structure with hypertonic treatment has been shown to increase the radiosensitivity and the initial DNA damage in repair-competent CHO-K1 cells and decrease the radiosensitivity and DNA damage in repair-defective xrs1 cells. Hypertonic treatment thus abolishes differences in chromatin structure between cell lines and differences in initial DNA damage. Radiosensitivity and initial DNA damage are correlated (r2=0.92; p=0.0026) and this correlation also holds when chromatin compaction is altered. The experiments demonstrate that initial DNA damage and chromatin structure are major determinants of radiosensitivity.