Clinical and Molecular Studies of Thiopurine Methyltransferase Pharmacogenetics: Genetic SNPs of Thiopurine Methyltransferase in Chinese childhood with acute lymphoblastic leukemia

MA XL ¹, WU MY ¹,ZHU P², HU YM ¹

¹ Beijing children's Hospital, Capital University of Medical sciences, Beijing, China

² The First Hospital, Beijing Medical University, Beijing, China

Objective: Thiopurine methyltransferase (TPMT) catalyses the S-methylation of thiopurine drugs such as 6-mercaptopurine(6MP), 6-thioguanine and azathioprine. The efficacy of 6MP in the treatment of childhood acute lymphoblastic leukaemia (ALL) is significantly influenced by TPMT. The molecular basis for the TPMT polymorphism has been determined. And there are about 30 single-nucleotide polymorphisms (SNPs) in the entire genomic structure of TPMT gene. A series of SNPs result in low levels of enzyme activity. This study aim is to gain an insight into the SNPs of TPMT activity and further elucidate its biological function and regulation.

Methods: The erythrocyte TPMT activity in 1000 healthy Chinese volunteers and 98 children with ALL were measured by means of HPLC. Genomic DNA samples were performed by PCR-based methods to detect various SNPs of the human TPMT gene. All fragments were examined by direct DNA sequencing.

Results: Enzyme activity in erythrocytes has shown that TPMT expression displays genetic polymorphism. 9.1% of individuals have intermediate and low activity. The most frequent mutant TPMT alleles are TPMT *2 (G238C) and TPMT *3A (G460A and A719C). Patients with biallelic mutations and low enzyme activity have an increased risk of drug-associated toxicity when treated with conventional doses of 6-MP.

Conclusion: TPMT genotype correlates well with in vivo enzyme activity within erythrocytes and is clearly associated with risk of toxicity. Prospective identification of SNPs in the TPMT genomic structure may improve the safety and efficacy of thiopurine therapy.

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