Evidence for an indirect effect of radiation on mammalian chromosomes. III. UV- and x-ray-induced sister chromatid exchanges in heterokaryons.

The hypothesis that chromosomes may be damaged indirectly by radiation was examined by assaying sister chromatid exchange, (SCE) frequency in heterokaryons between irradiated and unirradiated mouse and Chinese hamster cells. One cell line was UV or x irradiated, then fused to unirradiated BrdU-labeled cells of the other line; SCEs in the unirradiated set were scored in heterokaryons. A dose-dependent increase was consistently observed; the magnitude of which suggested that 25% of UV-induced and up to 60% of x-ray-induced SCEs are indirectly induced. Medium transfer experiments, cell mixing, and fusion with irradiated chick erythrocyte ghosts suggested that unirradiated chromosomes in heterokaryons are damaged by a stable, nondiffusible cytoplasmic component contributed by the irradiated cell.