Pyrosequencing of TPMT alleles in a general Swedish population and in patients with inflammatory bowel disease.

BACKGROUND

Interindividual differences in therapeutic efficacy in patients treated with thiopurines might be explained by the presence of thiopurine S-methyltransferase (TPMT) alleles that encode for reduced TPMT enzymatic activity. It is therefore of value to know an individual's inherent capacity to express TPMT.

METHOD

We developed a pyrosequencing method to detect 10 single-nucleotide polymorphisms (SNPs) in TPMT. A Swedish population (n = 800) was examined for TPMT3A, TPMT3B, TPMT3C, and TPMT2. Patients with inflammatory bowel disease (n = 24) and healthy volunteers (n = 6), selected on the basis of TPMT enzymatic activity, were investigated for all 10 SNPs to determine the relationship between TPMT genotype and phenotype.

RESULTS

In the general population we identified the following genotypes with nonfunctional alleles: TPMT*1/*3A (3A allelic frequency, 3.75%), TPMT1/*3C (3C allelic frequency, 0.44%), TPMT1/*3B (3B allelic frequency, 0.13%), and TPMT1/*2 (2 allelic frequency, 0.06%). All nine individuals with normal enzymatic activity were wild-type TPMT1/1. Thirteen individuals with intermediate activity were either TPMT1/3A (n = 12) or TPMT1/2 (n = 1). Eight individuals with low enzymatic activity were TPMT3A/3A (n = 4), TPMT3A/3C (n = 2), or TPMT1/*3A (n = 2).

CONCLUSION

Next to wild type, the most frequent alleles in Sweden are TPMT3A and TPMT3C. A previously established phenotypic cutoff for distinguishing normal from intermediate metabolizers was confirmed. To identify the majority of cases (90%) with low or intermediate TPMT activity, it was sufficient to analyze individuals for only 3 of the 10 SNPs investigated. Nevertheless, this investigation indicates that other mutations might be of relevance for decreased enzymatic activity.