Increase in Radiosensitivity to Ionizing Radiation Related to Replacement of Thymidine in Mammalian Cells with 5-Bromodeoxyuridine

Abstract

Chinese hamster cells were treated with 5-bromodeoxyuridine, BUdR, (labeled with H3) under conditions to obtain either single-or double-strand labeling of the DNA. The cells were irradiated with cobalt-60 gamma-rays and were fixed 6 hours later for analysis of chromosomal damage in meta phase cells; the 6-hour time interval selected metaphase cells that were primarily in the latter part of the S phase at the time or irradiation. The amount of BUdR incorporated into these metaphase cells were determined by counting the grains in autoradiographic film covering the cells. Both potentiation and synergistic effects were observed between the radiation and BUdR treatments, and the amount of chromosomal damage increased with the amount of BUdR incorporated into the cells. Even for variations in incorporation that occurred within a given sample, which was treated with one concentration of BUdR for a certain period of time, the cells with more incorporation sustained more chromosomal damage. Also, the long arm of the X chromosome both incorporated more BUdR and was sensitized more than the short arm. The aberration frequency in unirradiated cells (0.08) did not increase until 25% of the thymine was replaced with BU. At this level of incorporation, cells irradiated with 250 rads sustained 1.3 aberrations /cell, whereas 250 rads to normal cells induced only 0.4 aberration /cell. At the same incorporation levels double-strand labeling was no more effective than single-strand labeling. Evidence is presented relating to the hypothesis that radio-sensitization after BUdR treatment is attributable to replacement of T with BU.