Detection assay of rare variants of the thiopurine methyltransferase gene by PCR-RFLP using a mismatch primer in a Japanese population

Thiopurine methyltransferase (TPMT) catalyzes the metabolism of important d rugs such as 6-mercaptopurine, 6-thioguanine, and azathioprine. The identif ication and frequency distributions of several Variant TPMT alleles (TPMT*2 -*8) have been described recently in many ethnic groups. We have recently d emonstrated that TPMT*3C is the most common allele in Japanese subjects; ho wever, it remains to be elucidated whether TPMT*4-*8 variants also exist in Japanese subjects. To detect polymorphisms in the TPMT gene (TPMT*4-*8), w e have developed a mismatch polymerase chain reaction and restriction fragm ent length polymorphism method and conducted a population study of Japanese subjects, Genotyping of these variant forms was carried out in 192 Japanes e healthy volunteers. The TPMT*4, TPMT*5, TPMT*6, TPMT*7, and TPMT*8 varian ts were not detected in any of the samples analyzed. This study provides th e first analysis of the TPMT*4-*8 variants in a sample of the Japanese popu lation and indicates that TPMT*4-*8 variants do not occur or are rare allel es in this population.