Comprehensive study of thiopurine methyltransferase genotype, phenotype, and genotype-phenotype discrepancies in Sweden

Abstract

Thiopurines are widely used in the treatment of leukemia and inflammatory bowel diseases. Thiopurine metabolism varies among individuals because of differences in the polymorphic enzyme thiopurine methyltransferase (TPMT, EC 2.1.1.67), and to avoid severe adverse reactions caused by incorrect dosing it is recommended that the patient's TPMT status be determined before the start of thiopurine treatment. This study describes the concordance between genotyping for common TPMT alleles and phenotyping in a Swedish cohort of 12,663 patients sampled before or during thiopurine treatment. The concordance between TPMT genotype and enzyme activity was 94.5%. Compared to the genotype, the first measurement of TPMT enzyme activity was lower than expected for 4.6% of the patients. Sequencing of all coding regions of the TPMT gene in genotype/phenotype discrepant individuals led to the identification of rare and novel TPMT alleles. Fifteen individuals (0.1%) with rare or novel genotypes were identified, and three TPMT alleles (TPMT*42, *43, and *44) are characterized here for the first time. These 15 patients would not have been detected as carrying a deviating TPMT genotype if only genotyping of the most common TPMT variants had been performed. This study highlights the benefit of combining TPMT genotype and phenotype determination in routine testing. More accurate dose recommendations can be made, which might decrease the number of adverse reactions and treatment failures during thiopurine treatment.

<p>The Identification of a Novel Thiopurine S-Methyltransferase Allele, <em>TPMT*45</em>, in Korean Patient with Crohn’s Disease</p>

“…4,5 Thiopurine drugs, such as azathioprine, 6-mercaptopurine, and thioguanine, are widely used to treat IBD, as well as leukemia, rheumatic diseases, and solid organ transplant rejection. 6,7 Thiopurines are S-methylated by the enzyme thiopurine S-methyltransferase (TPMT) and are thereby converted to metabolites, including thioguanine nucleotides (TGNs) and methylated mercaptopurine (MMPN). 7,8 TGNs are involved in the inhibition of denovo purine synthesis in hematopoietic stem cells, [8][9][10] while MMPN is associated with hepatotoxicity.…”

“…6,7 Thiopurines are S-methylated by the enzyme thiopurine S-methyltransferase (TPMT) and are thereby converted to metabolites, including thioguanine nucleotides (TGNs) and methylated mercaptopurine (MMPN). 7,8 TGNs are involved in the inhibition of denovo purine synthesis in hematopoietic stem cells, [8][9][10] while MMPN is associated with hepatotoxicity. 9 In patients being treated with thiopurines, measuring these metabolites in washed red blood cells (RBCs) can be used for drug monitoring in association with TPMT activity.…”

“…10 Most of these alleles have a single nucleotide polymorphism that causes amino acid changes that reduce TPMT activity. 7 According to the Clinical Pharmacogenetics Implementation Consortium (CPIC) Guidelines, patients with TPMT variants demonstrate moderate to severe myelosuppression; therefore, 30-70% of the standard dose is recommended for heterozygous TPMT-deficient patients (ie, TPMT intermediate metabolizer phenotype), while 10% of the standard dose is typically prescribed for homozygous TPMT deficient patients (ie, TPMT poor metabolizer phenotype). 13 Previous studies have shown that three TPMT alleles (TPMT*2, TPMT*3A, and TPMT*3C) contribute to approximately 95% of TPMT deficiencies.…”

“…8,10,15 Ethnic variability in the alleles exists, with TPMT*3A being most commonly deficient in Caucasians and Southwest Asians, whereas TPMT*3C is most likely to be deficient in Africans and Southeast Asians. 6,7,15 Among the Korean population, TPMT*3C and TPMT*6 are the two most common variant alleles; [15][16][17] the allele frequencies of TPMT*1 (wild-type), *3C, and *6 are 98.1%, 1.4%, and 0.2%, respectively. 15 Since pediatric CD involves a genetic susceptibility, pharmacogenetic tests carried out both before and during treatment can contribute to personalized therapy through dose adjustments and attempts to reduce adverse effects.…”

Pharmacogenetic studies of thiopurine methyltransferase genotype‐phenotype concordance and effect of methotrexate on thiopurine metabolism

“…Furthermore, MTX is also known to inhibit the enzyme xanthine oxidase (XO), which deactivates 6-MP to thiouric acid (TUA); cotreatment with MTX therefore increases the bioavailability of 6-MP. 19,20 Despite the long and successful use of the combination of 6-MP and MTX during ALL treatment, the mechanism of this beneficial combination is still not fully understood.…”