SEGREGATION ANALYSES OF 4 URINARY CAFFEINE METABOLITE RATIOS IMPLICATED IN THE DETERMINATION OF HUMAN ACETYLATION PHENOTYPES

Human acetylation phenotypes were determined with caffeine (137X) as t he test substance, improved by measuring urinary caffeine metabolites with a previously described HPLC method. Caffeine, 5-acetylamino-6-for mylamino-3-methyluracil (AFMU), 1-methylxanthine (1X), 1-methyluric ac id (IU), 1,7-dimethylxanthine (17X), and 1,7-dimethyluric acid (17U) w ere quantified. This study tested the hypothesis, suggested by previou s studies, that the acetylation polymorphism is strongly influenced by a major gene. Phenotypes were assessed by using four urinary caffeine metabolite ratios: AFMU/1X, AFMU/[1X + 1U + 17U], AFMU/[AFMU + 1X + 1 U], and AFMU/[1X + 1U + 17X + 17U] in a population included 281 nuclea r family members who were healthy volunteer subjects. Each urinary rat io revealed strong familial aggregation with correlations between pare nts and offspring varying from 0.340 to 0.486 as a function of the rat io considered, and between sibs from 0.410 to 0.512 whereas correlatio ns among spouses were not significant, excluding an effect of environm ental factors. Segregation analyses were conducted upon these four rat ios testing a series of specific models of inheritance and gave eviden ce for single locus control of N-acetyltransferase (NAT) activity, wit h Mendelian codominant transmission using the AFMU/1X, AFMU/[1X + 1U 17U], and AFMU/[1X + 1U + 17X + 17U] ratios. The slow allelic frequen cies were 0.739, 0.753, and 0.724, respectively, and the phenotypic co ncordance was 90 to 92% with the AFMU/1X ratio. The familial aggregati on observed in using the AFMU/[AFMU + 1X + 1U] ratio was consistent wi th a recessive transmission for the allele controlling the homozygous slow phenotype. This last ratio is not convenient to differentiate rap id heterozygous and homozygous phenotype. Considering the number of mi sclassified subjects, this study would be completed by genotyping the same families as demonstrated by some authors who predicted 97.5% of a cetylation phenotype when using PCR-based DNA amplification test. (C) 1994 Wiley-Liss, Inc.