Dependence of the Mutation Spectrum in a Shuttle Plasmid Replicated in Human Lymphoblasts on Dose of Gamma Radiation

The frequencies and types of mutations induced in the target gene, supF-tRNA, of the shuttle vector pZ189 were analysed following the replication of the gamma-irradiated plasmid in the human lymphoblastoid cell line, GM606. The mutation frequency measured in progeny of unirradiated pZ189 was 1.02 x 10(-4), increasing to 17.5 x 10(-4) at 1000 cGy, and to 63.4 x 10(-4) at 5000 cGy, approximately 17- and 62-fold over background levels, respectively. Simultaneously, the number of plasmids capable of replicating in Escherichia coli decreased with increasing radiation dose to 4% of the control value at 5000 cGy. Electrophoresis of the irradiated DNA showed a correlation between increases in mutation frequency and decreases in plasmid survival, and the formation of open-circular and linear DNA. The majority of the spontaneous (69.8%) and induced mutations (85.7%) at 1000 and 79.4% at 5000 cGy) were base substitutions and were generally of similar types among all groups. However, changes at 2500 (12.7%) and 5000 cGy (13.2%) involving A:T base pairs were greater than those in unirradiated controls (3.4%) or those at 1000 cGy (2.0%). This increase in A:T base pair mutations could be a result of reduced repair fidelity when the DNA is extensively damaged by high doses of ionizing radiation.