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 exposu re to a toxic substance and then relate that exposure to health risk d epends on the knowledge of mutations that occur in the absence of expo sure. Toward this end, we have been studying both the frequency and mo lecular nature of mutations of the hypoxanthine phosphoribosyltransfer ase (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 effe cts of smoking and age on mutant frequency have been well defined. Ana lyses of cDNA and genomic DNA by polymerase chain reaction and sequenc ing have identified the mutations in 63 mutants, 45 from males and 18 from females, of which 34 were smokers and 29 were nonsmokers. Slightl y less than half of the mutations were base substitutions (28); they w ere predominantly at GC base pairs (19). Different mutations at the sa me site indicated that there are features of the hprt polypeptide that affect the mutation spectrum. Two pairs of identical mutations indica ted that there may also bc hot spots. Mutations not previously reporte d have been detected, indicating that the mutation spectrum is only pa rtly defined. The remainder of the mutations were deletions (32) or in sertions/duplications (3); deletions ranged from one base pair to comp lete loss of the locus. Despite a small average increase in mutant fre quency for smokers, an increased proportion of base substitutions at A T base pairs in smokers (p = 0.2) hinted at a smoking-associated shift in the mutation spectrum. Expansion of the study to include individua ls with larger, smoking-associated increases of mutant frequency will determine the significance of this observation. This background mutati on study provides insight into factors that determine the mutation spe ctra of the hprt locus and provides data for comparison with mutation spectra of other populations.