[Significance of thiopurine s-methyltransferase gene test in a clinical case].

Thiopurine s-methyltransferase enzyme is responsible for the metabolism of immunosuppressant thiopurines, which are used in inflammatory bowel diseases, acute lymphoblastic leukemia and autoimmune diseases. Because of the relative narrow therapeutic index of thiopurines serious or life threatening side effects can occur. A total of 28 variant alleles of the gene coding for the thiopurine s-methyltransferase enzyme are responsible for altered catalytic activity of the enzyme. Patients with one non-functional (heterozygous) allele have intermediate, while those with two non-functional (homozygous) alleles have low enzyme activity. Using polymerase chain reaction/restriction fragment length polymorphism and direct DNA sequencing the authors determined the G238C, G460A and A719G polymorphisms of the thiopurine s-methyltransferase gene in a child with Crohn's disease who developed thiopurine-induced severe agranulocytosis. The presence of the G460A and A719G polymorphic alleles in homozygous forms were detected which corresponded to the *3A variant allele. This variant has been shown to be associated with lower enzyme activity and low amount of the enzyme resulting in thiopurine toxicity and agranulocytosis. These findings underline the need for genotyping of the thiopurine s-methyltransferase variants prior to thiopurine treatment.