CHARACTERIZATION OF INVIVO SOMATIC MUTATIONS AT THE HYPOXANTHINE PHOSPHORIBOSYLTRANSFERASE GENE OF A HUMAN CONTROL POPULATION

The ability to recognize a change in mutation spectrum after an exposure to a toxic substance and then relate that exposure to health risk depends on the knowledge of mutations that occur in the absence of exposure. Toward this end, we have been studying both the frequency and molecular nature of mutations of the hypoxanthine phosphoribosyltransferase (hprt) gene in peripheral blood lymphocytes as surrogate reporters of genetic damage. We have analyzed mutants, one per donor to ensure independence, from a control population in which the quantitative effects of smoking and age on mutant frequency have been well defined. Analyses of cDNA and genomic DNA by polymerase chain reaction and sequencing have identified the mutations in 63 mutants, 45 from males and 18 from females, of which 34 were smokers and 29 were nonsmokers. Slightly less than half of the mutations were base substitutions (28); they were predominantly at GC base pairs (19). Different mutations at the same site indicated that there are features of the hprt polypeptide that affect the mutation spectrum. Two pairs of identical mutations indicated that there may also bc hot spots. Mutations not previously reported have been detected, indicating that the mutation spectrum is only partly defined. The remainder of the mutations were deletions (32) or insertions/duplications (3); deletions ranged from one base pair to complete loss of the locus. Despite a small average increase in mutant frequency for smokers, an increased proportion of base substitutions at AT base pairs in smokers (p = 0.2) hinted at a smoking-associated shift in the mutation spectrum. Expansion of the study to include individuals with larger, smoking-associated increases of mutant frequency will determine the significance of this observation. This background mutation study provides insight into factors that determine the mutation spectra of the hprt locus and provides data for comparison with mutation spectra of other populations.