Delayed appearance of lethal and specific gene mutations in irradiated mammalian cells.

We have examined the occurrence of lethal and 6-thioguanine resistant (hprt locus) mutants among the progeny of irradiated CHO and BALB/3T3 cells. The expression of lethal mutations, as measured by a reduced cloning efficiency in the progeny cell population, was detected up to 30 mean population doublings after x-radiation. Preliminary evidence indicates that the expression of mutations at the hprt locus may also be delayed for at least 6-7 population doublings. These results suggest that radiation can induce genetic instability in cells, resulting in an increased rate of spontaneous mutations which persists for many generations of cell division. These findings are discussed in terms of their possible influence on the response of irradiated tumor cell populations in vivo.