Analysis of Thiopurine S-methyltransferase Polymorphism in the Population of Serbia and Montenegro and Mercaptopurine Therapy Tolerance in Childhood Acute Lymphoblastic Leukemia

Abstract

Thiopurine S-methyltransferase (TPMT) is an enzyme that converts thiopurine drugs into inactive metabolites. It is now well established that interindividual variation in sensitivity to thiopurines can be the result of the presence of genetic polymorphisms in the TPMT gene. The aim of this study was to determine the frequency and type of TPMT polymorphisms in the population of Serbia and Montenegro and to assess its relevance in the management of childhood acute lymphoblastic leukemia (ALL). Blood samples from 100 healthy adults and 100 children with ALL were analyzed for common mutations in the TPMT gene using polymerase chain reaction-based assays. The results revealed that allelic frequencies were 0.2% for TPMT*2, 3.2% for TPMT*3A, and 0.5% for TPMT*3B. A rare TPMT*3B allele was detected in 2 families. No TPMT*3C allele was found. The general pattern of TPMT-variant allele distribution as well as their frequencies in the population of Serbia and Montenegro, is similar to those determined for other Slavic and Mediterranean populations. The ability to tolerate 6-mercaptopurine (6-MP) -based maintenance therapy was used as a surrogate marker of hematologic toxicity. In the study of 50 patients with childhood ALL treated according to the BFM-like protocol, it was found that even TPMT-heterozygous patients are at greater risk of thiopurine drug-related leukopenia (mean duration of period when children missed therapy as a result of leukopenia for TPMT-heterozygous patients was 11.3 weeks vs 3.4 weeks for wild-type genotype patients, P < 0.01). In another group of 50 patients, the TPMT genotype was determined prospectively. The therapy protocol was modified considering their TPMT genotype. Administering reduced 6-MP dosages in the initial phase of maintenance allowed TPMT-heterozygous patients to later receive full protocol doses of both 6-MP and nonthiopurine therapy without omitting therapy resulting from myelotoxicity. These results justify performing TPMT genotyping before initiating thiopurine therapy in all children with ALL to minimize consequent toxicity.

An Epidemiological Study of Thiopurinemethyltransferase Variants in a Croatian Inflammatory Bowel Disease Patient Cohort

“…The results for the Croatian and Slovenian samples do not differ for TPMT*2 variant allele, as the Real-time PCR was applied on both sides, but differ for other alleles where the methods were different 12 . Our data are in accordance with the data obtained for Serbian population 14 . Since our laboratory is taking part in the external quality control schemes (DGKL, Germany) for TPMT genotyping, we are rather confident for the data presented in this study.…”

“…A few reports on Slavic populations have revealed differences in the frequency of variant alleles among Slavic populations [10][11][12][13][14][15] . This is the first study on TPMT polymorphisms in a Croatian population of IBD patients.…”

Individualized Therapy: Role of Thiopurine S-Methyltransferase Protein and Genetic Variants

“…We showed that genetic TPMT variants in exons account for 7.5% of all TPMT variants with decreased enzyme activity (11). Similar to the other Caucasian subjects, TPMT*3A is the most prevalent variant allele in Serbia (3.2%).…”

“…Consequently, thiopurine drug dose reduction is necessary to avoid toxicity (8). Patients with intermediate TPMT activity also require dose reduction (8,11). Therefore, knowledge of the level of TPMT enzyme activity is essential for the balance of therapeutic and toxic effects of thiopurine drug dose.…”

Molecular Genetic Markers as a Basis for Personalized Medicine / MOLEKULARNO-GENETIČKI MARKERI KAO OSNOV ZA PERSONALIZOVANU MEDICINU

“…Additionally, prior to application of irinotecan therapy, used for colorectal and pulmonary cancer treatment, pharmacogenomic markers in the UGT1A1 (uridine diphosphate glucuronosyltransferase 1 family, polypeptide A1) gene need to be analyzed (46). Variants in the TPMT gene (thiopurine S-methyltransferase) are tested in order to adjust immunosuppressive therapy (6-mercaptopurine, azathioprine and thioguanine), used in the treatment of acute leukemia, inflammatory and autoimmune diseases and in transplantation medicine (47,48).…”