Identification of Thiopurine S-Methyltransferase genotypes

Thiopurine S-Methyltransferase is a cytoplasmic enzyme that catalyzes the S-methylation of aromatic and heterocyclic sulfhydryl compounds. TPMT enzyme activity varies considerably from one individual to another and this variation is genetically determined. About 89% of people have relatively high TPMT activity. The remaining ~11% have partial or complete TPMT deficiency. TPMT deficient individuals can not tolerate thiopurine drugs such as 6-mercaptopurine or azathioprine, resulting in haematopoeitic toxicity or sometimes death. Several studies among different populations have demonstrated a >98% concordance between TPMT enzyme activity and genotype. Three point mutations in the TPMT gene are responsible for low TPMT activity. The wild-type allele is named as TPMT*1 and the common mutant alleles include TPMT*2 (G238C), TPMT*3A (G460A and A719G), TPMT*3B (G460A) and TPMT*3C (A719G). In this study, DNA was isolated from white blood cell. TPMT*2, TPMT*3B and TPMT*3C point mutations were analyzed by PCR based ARMS and RFLP. DNA sequencing was used for verification of G460A and A719G mutations. A total of 64 Pemfigus patients were analyzed and two different mutant TPMT alleles was identifed. One of the patients was heterozygote for TPMT*3A and all the family members were screened for TPMT3A. The proband had the mutant allele from her mother and inherited it to her two daughters. The other patient was identified as heterozgotes for TPMT*2.