CYTOTOXICITY OF I-125 DECAY IN THE DNA DOUBLE STRAND BREAK REPAIR DEFICIENT MUTANT-CELL LINE, XRS-5

The survival of parental Chinese hamster ovary (CHO) K1 cells and the DNA double strand break (DSB) repair deficient mutant, xrs-5 was determined after accumulation of I-125 decays. Both CHO and xrs-5 cells were extremely sensitive to accumulated I-125 decays. The D(o) values for CH6 and xrs-5 cells were 40 and approximately 7 decays per cell, respectively. The difference in cell survival between CHO and xrs-5 cells was not due to differences in overall I-125UdR incorporation, differences in labelling index (LI) or differences in plating efficiency (PE). Relative biological effectiveness (RBE) values calculated relative to Cs-137 gamma radiation survival values (D(o) and D10) were higher in xrs-5 cells compared with CHO cells. Although both CHO and xrs-5 cells have high RBE values that correspond to a high sensitivity of CHO and xrs-5 cells to I-125 decay. The higher RBE observed for xrs-5 cells in combination with the known repair defect in xrs-5 cells support the idea that unrepaired DNA double strand breaks are lethal to the cell.