An interethnic comparison of polymorphisms of the genes encoding drug-metabolizing enzymes and drug transporters: Experience in Singapore

Much of the interindividual variability in drug response is attributable to the presence of single nucleotide polymorphisms ( SNPs) in genes encoding drug- metabolizing enzymes and drug transporters. In recent years, we have investigated the polymorphisms in a number of genes encoding phase I and II drug- metabolizing enzymes including CYP1A1, CYP3A4, CYP3A5, GSTM1, NAT2, UGT1A1, and TPMT and drug transporter ( MDR1) in three distinct Asian populations in Singapore, namely the Chinese, Malays, and Indians. Significant differences in the frequencies of common alleles encoding these proteins have been observed among these three ethnic groups. For example, the frequency of the variant A2455G polymorphism of CYP1A1 was 28% in Chinese and 31% in Malays, but only 18% in Indians. CYP3A4* 4 was detected in two of 110 Chinese subjects, but absent in Indians and Malays. Many Chinese and Malays ( 61 - 63%) were homozygous for the GSTM1* 0 null genotype compared with 33% of Indians. The frequency of the UGT1A1* 28 allele was highest in the Indian population ( 35%) compared to similar frequencies that were found in the Chinese ( 16%) and Malay ( 19%) populations. More importantly, our experience over the years has shown that the pharmacogenetics of these drug- metabolizing enzymes and MDR1 in the Asian populations are different from these in the Caucasian and African populations. For example, the CYP3A4* 1B allele, which contains an A- 290G substitution in the promoter region of CYP3A4, is absent in all three Asian populations of Singapore studied, but occurs in more than 54% of Africans and 5% of Caucasians. There were no difference in genotype and allelic variant frequencies in exon 12 of MDR1 between the Chinese, Malay, and Indian populations. When compared with other ethnic groups, the distribution of the wild- type C allele in exon 12 in the Malays ( 34.2%) and Indians ( 32.8%) was relatively high and similar to the Japanese ( 38.5%) and Caucasians ( 41%) but different from African Americans ( 15%). The frequency of wild- type TT genotype in Asians ( 43.5% to 52.1%) and Japanese ( 61.5%) was much higher than those found in Caucasians ( 13.3%) All the proteins we studied represent the primary hepatic or extrahepatic enzymes, and their polymorphic expression may be implicated in disease risk and the disposition of drugs or endogenous substances. As such, dose requirements of certain drugs may not be optimal for Asian populations, and a second look at the factors responsible for this difference is necessary.