Thiopurine S - Methyltransferase Gene (TMPT) Polymorphisms in Population of Polish Children Treated for Acute Leukemia.

Among drug-modifying elements age, sex, coexistent diseases, interactions with other drugs, as well as genetically determined altered drug response are included. Analysis of selected gene polymorphisms may lead to an increase treatment safety and efficacy in developing individual therapies. The objectives of the study: 1. Determination of the frequency of the allele variants: TPMT*2, *3A and *3C in population of Polish children treated for acute leukemia. 2. Analysis of the influence of TPMT genetic polymorphism on the occurrence of therapeutic and/or adverse effects such as hematological disorders, hepatotoxicity and nephrotoxicity after thiopurine administration. 3. Determination of the significance of pharmacogenetic profile for dosage optimizing of 6-mercaptopurine and 6-tioguanine. 4. Determination of the leukemia relapse risk in relation to TPMT polymorphism.

The study group consisted of 210 patients (121 boys, 89 girls) treated for acute leukemia (167 ALL, 43 AML), with median age 7 (range 1-18) yrs, at following time points: on diagnosis, during intensive chemotherapy and after therapy cessation. Adverse treatment effects during the entire observation period including 6 particular stages was classified according to WHO toxicity scale in each child. Either bone marrow or peripheral blood samples were used for the analyses. DHPLC and RFLP was used for genotype determination for TPMT on selected polymorphisms: TPMT*2,*3A,*3C. DNA fragment amplification was used with relevant starters, followed by HpyF10VI (MwoI) and XmiI (AccI) restricting enzymatic digestion. RFLP marking results were validated by DHPLC.

1. For TPMT IM phenotype (“medium” metabolizer) was detected in 7.65% patients (n=16) and PM phenotype (“slow” metabolizer) in 0.47% cases (n=1), the most frequent polymorphic variant being TPMT*3A (72%). 2. Analysis of changes in selected morphology indicated that in TPMT *2, *3A or *3C polymorphism carriers' group there is a significant decrease in white blood count (p=0.0025), platelet count (p=0.0014) and neutrophil count (p=0.019) in comparison with individuals with the wild variant (TPMT*1/*1). In patients with TPMT allelic variants, longer periods of thrombocytopenia in relation to other hematologic parameters were observed (re-induction period: p=0.041, maintenance therapy: p=0.03). In patients with IM phenotype, significantly lower haemoglobin levels were observed only in the end stage of chemotherapy (p=0.032). In patients with ALL and IM the observed risk of leukopenia, (p=0.006), neutropenia (p=0.015) and thrombocytopenia (p=0.011) as well as hepatotoxicity: GOT (p=0.009), LDH (p=0.0014) during the entire treatment period and particular study points (WBC: re-induction p=0.004; PLT: re-induction p=0.045 and maintenance therapy p=0.043) was significantly more frequent. 4. In TPMT polymorphic variant carriers, the relapse of primary disease was statistically less frequent (p=0.0341F). 5. Death risk was not relevant to TPMT genotype.

1. Frequency of detected TPMT single nucleotide polymorphism in children treated for acute leukemia is consistent with the published data for European population. 2. Thiopurine administration is related to the increase in hematological toxicity (leukopenia, neutropenia and thrombocytopenia) in TPMT heterozygotes. 3. Treatment with mercaptopurine or thioguanine should be regarded as an indication for genotyping. 4. TPMT single nucleotide mutation occurrence in children is related to lower risk of relapse in acute leukemia and might be included in the risk group stratification.

No relevant conflicts of interest to declare.