Pharmacogenetic analysis of variable drug metabolism among inbred mouse strains.
Creators: name: Casley, William L.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1999
Subjects: subject: Health Sciences, Pharmacology.

description: Susceptibility to acetaminophen-induced hepatotoxicity was found to vary widely in an outbred colony of Swiss Webster mice based on differences in serum levels of the hepatic enzyme alanine aminotransferase among males. A selective breeding program produced inbred mouse strains which were either susceptible (APS strain) or nonsusceptible (APN strain) to the hepatotoxic effects of acetaminophen. Hepatic enzyme activities associated with the cytochrome P450 isoform CYP1A2 showed a statistically significant increase in APS versus APN mice. Further examination of hepatic Cyp1a1 and Cyp1a2 gene expression revealed that mRNA and specific protein levels were significantly elevated in animals from the susceptible group. The cosegregation of elevated basal gene expression for both Cyp1a subfamily genes in animals selected for susceptibility to acetaminophen-induced hepatotoxicity suggested a common heritable basis for regulation of basal expression. Caffeine 3-demethylation can be used as an index of CYP1A2 activity in vivo and was found to co-segregate with acetaminophen susceptibility in APS mice. Serum levels of caffeine and the 3 demethylated metabolite were compared among six common inbred strains (A/J, P/J, BALB/cJ, C3H/HeJ, AKR/J and SWR/J) and the APN strain. Significant variations were found between a number of different strains, including the APN strain, indicating a genetic basis for variation in this trait. Hepatic Cypla2 gene expression levels were significantly higher in C3H/HeJ, relative to APN male mice. The striking differences observed between the APN and C3H/HeJ mice suggested that these strains would be suitable for a genetic analysis of the factors affecting caffeine 3-demethylation and, by extension, its use as an index of CYP1A2 expression. Conflicting data from caffeine assays have suggested a uni-, bi- or tri-modal distribution of CYP1A2 activity in human populations, although no linkage of a genetic polymorphism affecting basal expression to the CYP1A2 locus has been demonstrated. In order to investigate the genetic determinants of variable caffeine 3-demethylation in the mouse, quantitative phenotypic data, in the form of caffeine 3-demethylation indices, were obtained for the progeny of an F2 intercross of C3H/HeJ X APN. The phenotypically extreme animals were genotyped at marker loci which achieved complete coverage of the genome. Interval mapping was employed to search for statistically significant linkages between trait data and marker genotypes across the genome. Two statistically significant quantitative trait loci (QTLs) were mapped to chromosomes 1 and 9, and statistically suggestive linkage was found for a QTL on chromosome 4. The QTL on chromosome 9 showed highly significant linkage to the Cyp1a2 gene, while no obvious candidate genes known to be involved in caffeine metabolism have been mapped to chromosomes 1 or 4. Pharmacogenetic studies of variable xenobiotic metabolism in humans and experimental animal models have historically focused on monogenic polymorphisms of the enzymes of metabolism and detoxification. Methods to permit genetic analysis of complex quantitative traits have not previously been applied to the study of variations in drug metabolism.