SPECTRUM OF X-RAY-INDUCED MUTATIONS IN THE HUMAN HPRT GENE

We have characterized the molecular spectrum of mutations in 116 X-ray-induced and 78 spontaneous, HPRT(-) mutants derived from the human B lymphoblastoid cell line TK6. Multiplex PCR analysis demonstrated that the overall representation of large deletions was not significantly different in the two spectra. However, highly significant differences were observed for specific deletion types. Total gene deletions represented 41/78 (0.53) X-ray-induced, but only 7/43 (0.16) spontaneous deletions (P < 0.0001). In contrast, 5' terminal deletions were significantly more common among spontaneous (17/43, 0.40) than X-ray-induced (13/78, 0.17) large deletions (P = 0.0079). The types of point mutations induced by X-ray exposure were very diverse including all classes of transitions and transversions, tandem base substitutions, frameshifts, small deletions and a deletion/insertion compound mutation. Compared to spontaneous data, radiation-induced point mutations exhibited a reduced number of transitions and an increased representation of small deletions. Small deletions were uniformly surrounded by direct sequence repeats. The distribution of point mutations was characterized by a cluster within the 5' portion of exon 8. Thirteen HPRT(-) point mutations exhibited aberrant splicing. Four of these were attributable to coding sequence alterations in exons 4 and 8. These results suggest that it may be possible to identify hallmark mutations associated with X-ray exposure of human cells.