Human thiopurine methyltransferase pharmacogenetics: Identification of a novel variant allele

Thiopurine methyltransferase (TPMT, EC 2.1.1.67) catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine. The activity of this enzyme in human tissue is regulated by a common genetic polymorphism that is a major risk factor for large individual variations in thiopurine toxicity and therapeutic effect. We have previously cloned the functional gene for human TPMT. The allele for high enzyme activity has been designated TPMT*, while two variant alleles for low activity, TPMT*2 and TPMT*, have been reported. The most common of these two variants, TPMT', has G → A and A → G transitions at nucleotides 460 and 719, respectively, both of which alter amino acid sequence. These mutations, either alone or together, result in decreased expression of TPMT enzymatic activity. We have recently identified a proband with low TPMT activity who is heterozygous at both nucleotides 460 and 719. However, this subject has a new allele, TPMT*, with a G → A transition at the intron 9-exon 10 splice junction. If expressed, this new mutation would result in an aberrently spliced mRNA, and, therefore, a protein with an altered C-terminus. JPMT*4 cosegregated with reduced TPMT activity in an extended pedigree that included two subjects phenotypically homozygous for low TPMT activity who genetically were compound hétérozygotes for alleles *3/*4. These observations should help make it possible to develop DNA-based diagnostic tests for the TPMT genetic polymorphism.