A comparison of molecular and enzyme-based assays for the detection of thiopurine methyltransferase mutations

S-Methylation by thiopurine methyltransferase (TPMT) is an important route of metabolism for the thiopurine drugs. About one in 300 individuals are ho mozygous for a TPMT mutation associated with very low enzyme activity and s evere myelosuppression if treated with standard doses of drug. To validate the use of molecular genetic techniques for the detection of TPMT deficienc y, we have determined red blood cell TPMT activity in 240 adult blood donor s and 55 normal children. Genotype was determined by restriction fragment l ength analysis of polymerase chain reaction products in a cohort of 79 of t he blood donors and five cases of azathioprine-induced myelosupression, and this confirmed a close relationship between genotype and phenotype. In 17 of the 24 cases in which mutations were found, DNA was also available from remission bone marrow. In one of these cases, DNA from the remission marrow sample indicated the presence of a non-mutated allele that had not been se en in the blast DNA sample obtained at presentation. These results indicate that polymerase chain reaction-based assays give reliable and robust resul ts for the: detection of TPMT deficiency, but that caution should be exerci sed in relying exclusively on DNA obtained from lymphoblasts in childhood l eukaemia.