Characterization of a variable number tandem repeat region in the thiopurine S-methyltransferase gene promoter

Characterization of the genetic polymorphism of thiopurine S-methyltransfer ase enzyme (TPMT; EC 2.1.1.67) is required because of its clinical importan ce for patients exposed to to thiopurine drugs. A number of point mutations have already been characterized in exons and introns of the TPMT gene. Her e we report the identification of a polymorphic locus within the promoter r egion of the gene. This polymorphism was detected by polymerase chain react ion-single strand conformation polymorphism analysis of DNA samples from 54 unrelated European individuals, A total of five alleles with length variat ions were distinguished through the 5'-flanking region involved in the TPMT gene expression. Sequence analysis revealed that these variations were due to a variable number of tandem repeats (VNTR), ranging from four to eight repeats. Each repeat consists of 17 or 18 bp units and contains putative bi nding sites for transcription factors. The most frequent alleles harbour fo ur or five tandem repeats, a heter ozygosity rate of 0.44 was calculated, a nd a stable Mendelian inheritance of alleles was demonstrated Analysis of t he effect of each VNTR allele on promoter activity of a reporter gene was f urther performed in various cell lines by transient transfection assay. A m odulatory effect off VNTR alleles was observed in vitro, but the repeat pol ymorphism did not display a significative role in TPMT gene regulation in v ivo. Further studies need to be carried out to support the hypothesis that VNTR may contribute to the large interindividual variations of TPMT activit y. Pharmacogenetics 9:189-198 (C) 1999 Lippincott Williams & Wilkins.