Analysis of genetic polymorphisms in human drug-metabolizing enzyme genes and functions of enzymatic proteins

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The denaturing HPLC assay and the sequence analysis for novel SNPs were performed in 200 individual samples from Japanese subjects. The first SNP was identified as 2556C>T in exon 5 resulting in an amino acid change of Thr261Ile. Haplotype analysis indicated that 100C>T, 1039C>T, 1661G>C, and 4180G>C existed in the same allele of the CYP2D6 gene. Of the 200 individuals, one was heterozygous for the 2556C>T SNP, suggesting that the allele frequency was 0.002 in the Japanese population. The second SNP was identified as 3835A>C in exon 8 resulting in an amino acid change of Lys404G1n. Haplotype analysis indicated that 1661G>C, 2850C>T, and 4180G>C existed in the same allele of the CYP2D6 gene. Of the 200 individuals, one was heterozygous for the 3835A>C SNP, suggesting that the allele frequency was 0.002 in the Japanese population. These novel SNPs were located in the exons of the CYP2D6 gene and result in amino acid substitutions. The Thr261 and Lys404 in CYP2D6 are located in G-helix and K"-helix, respectively. These amino acid residues are not mapped in substrate recognition sited, but are conserved in the CYP2D subfamily in mammals. We succeeded in expressing 12 variant CYP2D6 proteins, which have been detected in the Japanese population, in COS-7 cells. We demonstrated their catalytic activities for dextromethorphan. The metabolic activities with the substrates catalyzed by 11 variants of the CYP2D6 protein were reduced to between 0% and 64% of the activity observed with CYP2D6.1. On the other hand, CYP2D6.53 exhibited a 2-fold higher activity. Two of the allelic variants (CYP2D6.27 and CYP2D6.54) had altered expression levels of the CYP2D6 protein. These results should assist us in gaining an understanding of the extensive individual variations associated with CYP2D6 function.