Genetic Polymorphism of Thiopurine S-Methyltransferase in Children with Acute Lymphoblastic Leukemia

Abstract

Background: 6-Mercaptopurine (6-MP), a widely used anti-metabolite for the maintenance phase of childhood acute lymphoblastic leukemia (ALL), has been observed to cause myelotoxicity due to genetic polymorphism of thiopurine methyl transferase (TPMT), one of the drug-metabolizing enzymes. This study aimed to determine the frequency of thiopurine S-methyl transferase (TPMT) polymorphic variants in a group of Pakistani children with acute lymphoblastic leukemia (ALL) using 6-mercaptopurine (6-MP).

Methods: Diagnosed cases (n=100) of ALL, either Pre-B or T Cell, between the ages of 2 to 18 years were randomly selected from OPD of Children Cancer Hospital and National Institute of Child Health (NICH), Karachi. The subjects were under BFM (Berlin Frankfurt Munster) protocol. Expired and relapsed patients (2/100) were excluded. Blood samples were drawn and DNA was extracted from serum to determine the genotype of TPMT (*2, *3a, *3B and *3C) using allele-specific PCR (AS-PCR) and RFLP. PCR assays were done to detect the G238C transversion in TPMT*2 and the G460A and A719G transition in TMPT*3 Alleles.

Results: Polymorphism in TMPT was found 100% (98/98) in ALL patients belonging to a heterozygous group. Out of them, 74.5% of ALL patients showed myelotoxicity. Furthermore, *1/*3B was the most prevalent variant TPMT allele observed with the highest frequency reported at 97%. The variant allele *1/*3A was found in only 3 patients, whereas, TMPT*2 and *3C Alleles were not found.

Conclusion: The majority of our patients displayed a distinct TPMT genetic polymorphism TPMT*1/*3B which is comparatively rare in other parts of the world.

Keywords: 6-Mercaptopurine; Precursor Cell Lymphoblastic Leukemia-Lymphoma; Genetic Polymorphism; Alleles; Pakistan.