Validation of methods for CYP2C9 genotyping: frequencies of mutant alleles in a Swedish population

Cytochrome P450 2C9 (CYP2C9) catalysis the metabolism of important drugs such as phenytoin, S-warfarin, tolbutamide, losartan, torasemide, and nonsteroidal anti-inflammatory drugs. A functional polymorphism of the CYP2C9 gene has been described. The variant alleles include CYP2C9*2 having a point mutation in exon 3 causing an Arg144Cys exchange, and CYP2C9*3 with a point mutation in exon 7 resulting in an Ile359Leu exchange. Genotyping of these variant forms was carried out in 430 Swedish healthy volunteers and three different methods were compared. Sequence analysis of the different PCR products revealed that other genes in the CYP2C locus were co-amplified in one of the methods applied, whereas the other two methods were specific for CYP2C9. The frequencies of the CYP2C9*1, CYP2C9*2 and CYP2C9*3 alleles in the population examined were found to be 0.819, 0.107, and 0.074, respectively. The need for careful evaluation of the genotyping procedure by sequence analysis of PCR products is emphasised.