Frequencies of Defective CYP2C19 Alleles in a Hong Kong Chinese Population: Detection of the Rare Allele CYP2C19*4

A genetic polymorphism in the metabolism of the anticonvulsant drug mephenytoin in humans is a prime example of interracial and individual variability in drug metabolism. This polymorphism affects the metabolism of several other clinically used drugs, some of which are of interest in psychopharmacotherapy (1). The polymorphic enzyme that 4′-hydroxylates S-mephenytoin is known as CYP2C19 (2). In subjects deficient for CYP2C19, drugs metabolized by this enzyme may be associated with toxic concentrations or therapeutic failure when administered in dosages within the normal dosage range. Depending on the activity of CYP2C19, individuals can be characterized phenotypically as extensive metabolizers or poor metabolizers (PMs) of S-mephenytoin. CYP2C19 is encoded by the CYP2C19 gene, and its defective alleles account for the PM phenotype, which is inherited as an autosomal recessive trait (3). There is a marked interethnic difference in the distribution of this polymorphism, with the PM phenotype found in 2–5% of Caucasians but in 13–23% of Orientals (4)(5). The main defective allele, CYP2C192, accounts for ∼75–85% of alleles responsible for the PM phenotype in both Orientals and Caucasians (4). CYP2C193 is extremely rare in the Caucasian population but accounts for almost all of the remaining defec- tive alleles in Orientals (3)(4)(5)(6)(7)(8)(9). Subsequent studies have revealed additional defective CYP2C19 alleles in Caucasian PMs, identifying ≥93% of the defective alleles: CYP2C192B (10), CYP2C194 (9), CYP2C195B (11), and CYP2C196 (10). Only one new allele, CYP2C195A (7), has been found in the Chinese Bai population at a very low frequency (0.0025). In addition to the interracial differences in the genetic polymorphism of the CYP2C19 enzyme, Xiao et al. (7) showed a different incidence of the CYP2C192 and CYP2C191 alleles in two Chinese ethnic groups. These differences in allele frequencies were consistent with phenotypic data suggesting that different Chinese ethnic groups may exhibit somewhat different sensitivities to drugs metabolized by CYP2C19. Therefore, it was of interest to us to investigate the CYP2C19 genotype in a Hong Kong Chinese population. One hundred nineteen unrelated Hong Kong Chinese individuals participated in the genotyping tests. Forty-five patients diagnosed with depressive disorders according to Diagnostic and Statistical Manual IV but without impairment in liver or kidney function, and 74 healthy subjects were included in this study after giving their informed consent. This research project has been approved by the ethics committee of the Chinese University of Hong Kong. Blood samples (12 mL) were collected in Vacutte® (Greiner) tubes containing EDTA as anticoagulant. DNA was extracted using the Qiamp Blood kit (Qiagen). PCR followed by restriction enzyme analysis of the amplified products was used to identify CYP2C19 alleles as described previously (7)(8)(9)(11). Binomial probabilities were used to compute the exact confidence intervals for the frequencies. Allele frequencies in different populations were compared with the χ2 test. The allele frequencies are listed in Table 1. In general, our data were in agreement with the incidence of CYP2C19 alleles reported previously for other Chinese Han populations (Table 1). The frequency of the wild-type allele (CYP2C191) appears marginally higher than that found by Xiao et al. (7) for the Chinese Han population studied (χ2 = 3.872; P = 0.051). Frequencies of CYP2C191 alleles and PM in Chinese. Alleles designated according to the current nomenclature (14). Number of individuals per study. PM genotypes: *2/*2, *3/*3, *2/*3, *2/*4, *2/*5. 95% confidence limits. No data available. Incidence of PM based on phenotyping data. Frequencies of CYP2C191 alleles and PM in Chinese. Alleles designated according to the current nomenclature (14). Number of individuals per study. PM genotypes: *2/*2, *3/*3, *2/*3, *2/*4, *2/*5. 95% confidence limits. No data available. Incidence of PM based on phenotyping data. CYP2C192, CYP2C193, and CYP2C195 have been shown to account for >99% of the defective CYP2C19 alleles in the three main Oriental races, indicating a complete concordance between phenotype and genotype (5)(8). This suggests that other defective alleles are unlikely to contribute to the PM phenotype in these populations. Surprisingly however, one subject (C33) harbored a CYP2C194 allele (Fig. 1). To our knowledge, this is the first CYP2C194 allele found in a Chinese individual. CYP2C194 accounted for the predicted phenotype of one PM individual, with a CYP2C192/CYP2C194 genotype, and represented 2.38% of the defective alleles in the PMs. PCR-restriction fragment length polymorphism mismatch test for CYP2C19*4. A 195-bp fragment of exon 1 was amplified by PCR, followed by PstI digestion and electrophoresis. The 195-bp product was not digested in individuals (C31, C32, and C34; lanes 1, 2, and 4 from the left) presenting alleles other than CYP2C19*4. The restriction enzyme PstI completely digested the 195-bp PCR-amplified product from CYP2C19*4 allele to yield 167- and 28-bp restriction fragments. The presence of 195- and 167-bp fragments after PstI digestion, as shown in C33 (lane 3 from the left), indicated that this sample was heterozygous for CYP2C19*4. Twenty-one subjects (17.65%) were genotypically identified as PMs. The incidence of PM genotypes is comparable to the incidences reported for other Chinese groups (Table 1) (5)(7)(12)(13). CYP2C192 and CYP2C193 accounted for 80.95% and 16.66%, respectively, of the defective alleles in the 21 PMs. We did not detect any occurrence of the alleles CYP2C195 (5A, 5B) (7)(11). The incidence of PMs found in this study differs only from that found in Chinese Dong subjects, which was estimated as 11.1% (17.65%; χ2 = 3.02; P = 0.01). The different incidences of PM for different Chinese ethnic groups may reflect differences in sample size and/or in methods. However, further studies on larger samples should be carried out. In summary, our data were generally in agreement with the prevalence of PMs in the Chinese populations previously genotyped and phenotyped except for the detection of the CYP2C194 allele. We thank R. J. Morgan for reading this article and for helpful comments and criticisms.