Trinucleotide repeat variants in the promoter of the thiopurine S-methyltransferase gene of patients exhibiting ultra-high enzyme activity

Abstract

Thiopurine S-methyl transferase (TPMT) is a cytosolic enzyme that catalyses the S-methylation of the thiopurine immunosuppressants. To date, 22 variants have been identified that are predictive of decreased TPMT activity. In contrast, no molecular explanation has been found for the 1–2% of Caucasians who exhibit ultra-high TPMT activity. Here, we report the characterization of polymorphisms within a trinucleotide (GCC) repeat element of the TPMT promoter in two patients with inflammatory bowel disease exhibiting the highest TPMT activity from two testing centres. The first patient was heterozygous for a variant allele carrying seven GCC repeats [(GCC)₇], whereas the second patient was heterozygous for a variant allele containing five GCC repeats [(GCC)₅]. Fifty patients with inflammatory bowel disease with normal TPMT activity were all homozygous for six GCC repeats [(GCC)₆]. Of 200 healthy controls, five were found to be heterozygous for the (GCC)₇ variant. Within in vitro reporter gene assays, the mean luciferase activities of the (GCC)₆, (GCC)₇, and (GCC)₅ constructs were 8.0±0.26, 13.2±0.10 and 12.3±0.12, respectively. The significant increase in activity observed for (GCC)₅ and (GCC)₇ compared with (GCC)₆ (P-value ≤0.001) strongly suggests that alteration in the number of trinucleotide repeats is responsible for the ultra-high TPMT activity observed in these patients.