Thiopurine S-Methyltransferase Polymorphism in Iraqi Paediatric Patients with Acute Lymphoblastic Leukaemia

Introduction: Acute Lymphoblastic Leukaemia (ALL) treatment protocols widely used thiopurine drugs as an anti-cancer agent, which over the course of time results in drug toxicity. Thiopurine S-Methyltransferase Enzyme (TPMT) is responsible for the activation of 6-Mercaptopurine (6-MP) to Thioguanine Nucleotides (TGNs) that are incorporated into DNA and trigger cell death. Low TPMT activity is strongly correlated to TPMT genetic polymorphism. Aim: To identify the level of TPMT activity and the most common TPMT polymorphism (TPMT*3A, TPMT*3B and TPMT*3C) and its frequencies in a sample of Iraqi ALL paediatric patients. Materials and Methods: Eighty-one Iraqi ALL paediatric patients receiving 6-MP in the maintenance phase of treatment were enrolled in the study. TPMT activity in the serum was measured by using Enzyme-Linked Immunosorbent Assay Technique (ELISA) in the serum and TPMT genetic polymorphism (TPMT*3A, TPMT*3B and TPMT*3C) was detected by allele-specific multiplex-PCR analysis. Statistical analysis was performed by using a two-sample t-test to evaluate the difference in allele frequencies proportion in TPMT polymorphism. Pearson's correlation analysis was done to determine the correlation between TPMT enzyme genotype and phenotype. Results: There were 51 paediatric ALL patients carrying the wildtype allele with allele frequencies of (62.96%), 30 paediatric ALL patients carrying the mutant alleles either TPMT*3A or TPMT*3C with allele frequencies of 29.62% and 7.4% respectively. The mutant allele TPMT*3B was not detected in the patients under study. The difference in mean of the TPMT enzyme activity between the ALL patients carrying the wild-type allele and the mutant allele was highly significant with p-value <= 0.001. A highly significant positive co-relation (r=0.939) was found between TPMT low activity and presence of genetic mutation across the TPMT gene (p-value <= 0.001). Conclusion: TMPT genotyping and phenotyping is an essential tool to reduce the cytotoxic effects of the anti-cancer drug 6-MP in Iraqi paediatric patients with ALL for a successful recovery.