Protection of DNA from radiation-induced double-strand breaks: influence of replication and nuclear proteins.

PURPOSE: To study the protective effect of histone and non-histone proteins on double-strand break (dsb) induction in replicating S-phase DNA as well as bulk DNA of plateau phase human tumour cells. MATERIALS AND METHODS: Induction of dsb was studied in two human adenocarcinoma cell lines: Colo320HSR and MCF-7. To assess the influence of chromatin structure on radiation-induced DNA dsb, different nuclear preparations of cells, either continuously labelled with 14C or pulse labelled with 3H, were assessed by pulsed-field gel electrophoresis (PFGE). RESULTS AND CONCLUSIONS: Stepwise removal of DNA-bound proteins from the chromatin increased the amount of radiation-induced dsb in both cell lines. However, the protective effect of DNA-associated proteins on dsb induction was significantly reduced in DNA of replicating S-phase cells compared with bulk DNA of plateau phase cells. These data show that proteins associated with the DNA have a different protective effect on radiation-induced dsb, rendering replicating DNA with open chromatin structure more sensitive to dsb induction by ionizing radiation.