Distribution of genetic polymorphisms of genes implicated in thiopurine drugs metabolism

Introduction:Thiopurine-S-methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA) are crucial enzymes involved in the metabolism of thiopurine drugs: azathioprine and 6-mercaptopurine, used in the treatment of... [ view full abstract ]

Introduction:

Thiopurine-S-methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA) are crucial enzymes involved in the metabolism of thiopurine drugs: azathioprine and 6-mercaptopurine, used in the treatment of several diseases, such as inflammatory bowel diseases and rheumatoid arthritis. Significant interethnic variation in the expression of TPMT and ITPA is caused by Single Nucleotide Polymorphisms (SNPs) of genes encoding these proteins.

Purpose

The aim of this study was to describe the distribution of TPMT and ITPA polymorphisms in Tunisian healthy subjects and establish accordingly, the metabolizer status of thiopurines drugs in this population.

Methods

A total of 309 healthy Tunisian subjects were recruited among blood donors. Informed written consent was obtained from all subjects. The genomic DNA was extracted using a salting-out procedure. TPMT (c.238G>C, c.460G>A and c.719A>G) and ITPA (c.94C>A and IVS2+21A>C) mutations were genotyping using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP).

Results

The observed frequencies of TPMT*3A and TPMT*3C alleles were 0.8% each. 299 subjects were homozygous for wild type gene and 10 were heterozygous. The phenotype distribution of TPMT was bimodal: 96.8% of subjects were extensive metabolizers (EX) and 3.2% were intermediate metabolizers (IM). Genotyping of ITPA revealed frequencies of 9% and 3% for IVS2+21A>C and c.94C>A mutations respectively. 233 subjects were homozygous for wild type gene, 73 were heterozygous and 3 were homozygous for variant alleles. Accordingly, a trimodal phenotype distribution was found: 75.4% of subjects were EM, 23.4% were IM and 1.2% were SM (slow metabolizers).

Conclusions

We have shown that TPMT allele frequencies distribution of our population are similar to that observed in African and Asian populations, while that of ITPA was similar to Caucasians. Combination of TPMT and ITPA has revealed that a quarter of Tunisian population have a reduced metabolic activity of thiopurines. Our results help to optimize thiopurine therapy by a pharmacogenetic approach in the context of personalized medicine.