Thiopurine S-methyltransferase polymorphism in Iranian kidney transplant recipients

Thiopurine S-methyltransferase is an enzyme that catalyzes S-methylation of azathioprine as an immunosuppressive drug. Genetic polymorphisms influence thiopurine S-methyltransferase activity. There are 3 variant alleles: thiopurine S-methyltransferase*2, *3A, and *3C are responsible for more than 95% cases of low-enzyme activity.

We studied these polymorphisms and the occurrence of azathioprine adverse effects in 50 renal transplant recipients undergoing triple immunosuppressive therapy including azathioprine, cyclosporine, and prednisone. Thiopurine S-methyltransferase genetic polymorphism was determined by polymerase chain reaction restriction fragment length polymorphism assay and allele-specific polymerase chain reaction methods. Azathioprine dosage; leukocyte, erythrocyte, and platelet counts; and graft rejection episodes were analyzed during hospitalization.

Two patients (2%) were heterozygous for thiopurine S-methyltransferase*3C, the remaining patients were thiopurine S-methyltransferase wild-type *1/*1 (98%). Thiopurine S-methyltransferase wild-type homozygous and heterozygous patients were administered similar azathioprine dosages at the beginning of treatment (2.42 ± 0.50 and 2.52 ± 0.40 mg/kg/24 h). During subsequent days, mean azathioprine dosage administered to thiopurine S-methyltransferase wild-type homozygous patients was similar to heterozygous patients, but with no statistical difference (P = .28). Three patients had an acute rejection episode during this time. Five patients (10%) had reduced azathioprine dosage owing to adverse effects. Adverse reactions consisted of hematotoxicity (n=2), hepatotoxicity (n=1), and gastrointestinal toxicity (n=2). All recipients were wild-type homozygotes.

The frequency of thiopurine S-methyltransferase gene mutations is low among our patients. The incidence of adverse reactions to azathioprine was also low, even in patients carrying a variant of thiopurine S-methyltransferase. We conclude that determining thiopurine S-methyltransferase genotype is not useful in our population to predict adverse reactions to azathioprine.