Mutational spectrum of X-ray induced TK− human cell mutants

Mutations induced by ionizing radiation have historically elicited significant public concern. However, only a limited database of ionizing radiation-induced point mutations is available, particularly at endogenous human cell loci. Here, we report the mutational spectrum for 184 X-ray induced TK− mutants derived from TK6 human lymphoblasts. This report represents the first large scale utilization of the tk locus for investigation of mutational specificity at the DNA sequence level. Rapid, single nucleotide sequencing assays at frameshift polymorphism sites in tk exons 4 and 7 were used to partition TK− mutants into two groups: 126 were attributed to either partial gene deletion or to loss of heterozygosity, and DNA sequence alterations were identified for 51. X-ray-induced point mutations included all classes of transitions and transversions, tandem base substitutions, frameshifts, small deletions and a small duplication. The distribution within tk was characterized by clustering at some sites. Twelve TK− point mutations, including five entirely within the coding sequence in exons 3 and 4, resulted in aberrant splicing of the tk transcript. The spectrum of X-ray-induced point mutations was found to be highly reproducible when TK− mutations were compared with HPRT− mutations in TK6. A statistically significant decrease in transitions (P = 0.04) was observed in the combined data set as compared to the spontaneous background. These findings suggest a reproducible pattern which may be utilized in recognizing radiation-induced mutations at other loci of interest.