CYP2A6 has been characterized as a coumarin 7-hydroxylase in humans. A large interindividual difference in the activity of coumarin 7-hydroxylation suggested an existence of genetic polymorphism of this enzyme. In fact, CYP2A6*2 variant allele which has T→A substitution, leading to amino acid change from Leu160 to His160, has been found in Caucasian population as the most frequent mutation in poor metabolizers (PM) of coumarin. Although several drugs used clinically or under development such as fadrozole, losigamone and methoxyflurane are recognized at present to be good substrates of CYP2A6, no specific substrate of this CYP isoform has been known until we found a drug, SM-12502. In the phase I trial, 3 out of 28 Japanese subjects were classified as PM of the drug. In vitro studies demonstrated that CYP2A6 played a major role on the metabolism of the drug. Genomic analysis revealed that the PM phenotype was caused by the presence of a novel CYP2A6 gene variant which lacks the entire region of open reading frame encoding the enzyme in the PM. Thus, we designated the variant as “deletion-type” allele. We examined the frequency of individuals carrying homozygous deletion by a genotyping method established in our laboratory. Thus, the frequency was estimated to be 3-4% in Japanese. We found another CYP2A6 gene variant whose 60 bp in the 3'-untranslated region was substituted by the corresponding region of the CYP2A7 pseudogene. This variant was designated as “conversion-type” allele. We found that the allele frequency of the conversiontype was comparable to that of wild-type, CYP2A6*1 allele in Japanese. We also compared the frequency of the CYP2A6*2 allele as well as the deletion and the conversion alleles between Japanese and Caucasian. Consequently, a marked interracial difference in the frequency of the genetic variants of the CYP2A6 gene was observed. These results give an interesting insight into racial difference in response to drugs and evolution of the CYP2A gene subfamily in humans.
