Background: Thiopurine methyltransferase (TPMT) is a genetically polymorphic enzyme that catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine. Recently, a variable number tandem repeat (VNTR) within the TPMT promoter has been reported to "modulate" levels of this enzyme activity.
Methods: We set out to perform genotype-phenotype correlation analysis for the polymorphic TPMT tandem repeat in 1211 clinical laboratory samples in which red blood cell (RBC) TPMT activity had been measured and to compare those results with data for 279 control DNA samples.
Results: TPMT VNTR length varied from three to nine repeats ( *V3 to *V9), but the most common alleles were *V4 and *V5, with frequencies in the control samples of 0.54 and 0.36, respectively. The clinical laboratory samples were then stratified into those with "low," "intermediate," or "high" levels of RBC TPMT activity; that is, samples presumed to be homozygous for open reading frame (ORF)-based variant alleles, heterozygous for those alleles, or homozygous for the "wild-type" ORF sequence, respectively. TPMT VNTR genotype *V4/*V5 was associated with significantly higher RBC TPMT activity than were *V4/*V4 or *V5/*V5. Lowest activity levels were associated with genotypes that included an allele with more than 5 repeat elements. However, all of these effects were quantitatively small. Finally, there was linkage disequilibrium between VNTR allele *V5 and TPMT*3A, the most common ORF-based polymorphism associated with very low TPMT activity in white persons.
Conclusions: These observations suggest that, in addition to the striking effects of ORF-based single nucleotide polymorphisms on TPMT activity, the VNTR within the 5'-flanking region of the TPMT gene also may modulate levels of RBC TPMT activity.