Abstract
The 4′-hydroxylation of the S-enantiomer of the anticonvulsant drug mephenytoin exhibits a genetic polymorphism in humans. This polymorphism shows marked interracial heterogeneity, with the poor metabolizer (PM) phenotype representing 2 to 5% of Caucasian and 13 to 23% of Asian populations. Two defective CYP2C19alleles, CYP2C19*2 and CYP2C19*3, have been described which account for ∼87% of Caucasian and >99% of Oriental PM alleles. The present study identifies a new allele (CYP2C19*4) in Caucasian PMs which contains an A → G mutation in the initiation codon. A new polymerase chain reaction-restriction fragment length polymorphism genotyping test was developed, and the incidence of this allele was examined in a European Caucasian population which had been phenotyped for mephenytoin metabolism. One of nine putative PMs was heterozygous forCYP2C19*2/CYP2C19*4, which suggests thatCYP2C19*4 represents a defective allele. Six of the seven remaining putative PMs available for genotyping were explained byCYP2C19*2. The frequency of the CYP2C19*4 allele in Caucasians was 0.6%. An additional Caucasian PM from a separate study was also heterozygous for CYP2C19*2 andCYP2C19*4. To verify that CYP2C19*4 represented a defective CYP2C19 allele, the initiation codon of the normal CYP2C19*1 cDNA was mutated to a GTG, and both cDNAs were expressed in yeast. Recombinant CYP2C19 protein was detected by Western blot analysis of colonies transformed with CYP2C19*1 cDNA, but not in those transformed with CYP2C19*4 cDNA. The two cDNAs were also used in an in vitro coupled transcription/translation assay. CYP2C19 protein was translated only from the CYP2C19*1 allele. These data indicate that CYP2C19*4 represents a new PM allele.
Footnotes
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Send reprint requests to: Dr. Joyce Goldstein, NIEHS, PO Box 12233, Research Triangle Park, NC 27709.
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↵1 This work was supported in part (S.M., C.B., P.D.) by the Swiss Cancer League, Switzerland (FOR063); League against Cancer of Fribourg, Switzerland (FOR381.88); Cancer Research, Switzerland (AKT617); and Fund for Clinical Research against Cancer, Gustave-Roussy Institute, Villejuif, France (88D28), by USPHS grants GM31304 (G.R.W.); and by Program Project Grant 32165, National Institutes of Health (T.C.S., J.M.W.).
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↵2 Current affiliation: University of Florida Shands Cancer Center, Health Science Center, PO Box 100286, Gainesville, FL 32610-0286.
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↵3 Current affiliation: Boehringer Ingelheim Pharmaceuticals, Inc., Ridgefield, CT.
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↵4 The present paper uses a nomenclature for theCYP2C19 alleles based on the recommendations of Dalyet al. (1996). We denote the wild-type allele reported byRomkes et al. (1991) as CYP2C19*1A. A second wild-type allele (C99T, A991 G, Ile331 Val) (Richardson et al., 1997; S.M.F. de Morais, J. Blaisdell and J. A. Goldstein, unpublished data) is designated CYP2C19*1B. The mutant alleles previously reported by our laboratory (deMorais et al., 1994a, b) are designated as follows: CYP2C19m1 is designatedCYP2C19*2, CYP2C19m2 is designatedCYP2C19*3 and CYP2C19m3 is designatedCYP2C19*4.
- Abbreviations:
- PM
- poor metabolizer
- EM
- extensive metabolizer
- PCR
- polymerase chain reaction
- HI
- hydroxylation index
- CYP
- cytochrome P450
- RFLP
- restriction fragment length polymorphism
- Received March 25, 1997.
- Accepted August 21, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
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