Thiopurine S-methyltransferase (TPMT), a well-studied enzyme in clinically setting, is responsible for inactivation of anti-tumor and imunosuppresive group of drugs, called thiopurines. Due to the narrow therapeutic window of thiopurines and significant population differences in TPMT activity, studying genetic polymorphisms in the TPMT gene is increasingly important. Clinically significant polymorphisms include TPMT*3C and TPMT*3B. However, these do not account for all variations that affect enzyme activity, provoking the search for new genetic factors that could affect thiopurine therapy. Previously, new genetic polymorphisms, including rs114611199, rs17839843, and rs2518469, were identified that could alter TPMT activity. In this study, we analyzed the frequency of these polymorphisms in 161 healthy Slovenian donnors. DNA was isolated from blood samples, diluted, and genotyped using hydrolyzing probes. We compared the allele and genotype frequencies with European and global populations, finding deviations in the allele frequencies of rs114611199 and rs17839843 in Slovenian population. Next, we measured TPMT enzyme activity via production of 6-methylmercaptopurine in hemolysates from the same individuals using high-performance liquid chromatography (HPLC). Results indicated that TPMT activity levels in the Slovenian population were comparable to other studies and they were showing the expected bimodal distribution. We confirmed the correlation between TPMT*3 genotypes and enzyme activity. Additionally, we evaluated the connection of the less-studied polymorphisms rs114611199, rs17839843, and rs2518469 with TPMT activity. We found that individuals who are variant homozygotes for rs2518469 have significantly reduced TPMT activity. No statistically significant correlations were found for the other two polymorphisms. Our findings pave the way for further research from both scientific and clinical perspectives.
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