Allelic variants of the thiopurine methyltransferase (TPMT) gene in the Colombian population

Population-Specific Distribution of TPMT Deficiency Variants and Ancestry Proportions in Ecuadorian Ethnic Groups: Towards Personalized Medicine

Purpose

Thiopurine S-methyltransferase (TPMT) is an enzyme that metabolizes purine analogs, agents used in the treatment of acute lymphoblastic leukemia. Improper drug metabolism leads to toxicity in chemotherapy patients and reduces treatment effectiveness. TPMT variants associated with reduced enzymatic activity vary across populations. Therefore, studying these variants in heterogeneous populations, such as Ecuadorians, can help identify molecular causes of deficiency for this enzyme.

Methods

We sequenced the entire TPMT coding region in 550 Ecuadorian individuals from Afro-Ecuadorian, Indigenous, Mestizo, and Montubio ethnicities. Moreover, we conducted an ancestry analysis using 46 informative ancestry markers.

Results

We identified 8 single nucleotide variants in the coding region of TPMT. The most prevalent alleles were TPMT*3A, TPMT*3B, and TPMT*3C, with frequencies of 0.055, 0.012, and 0.015, respectively. Additionally, we found rare alleles TPMT*4 and TPMT*8 with frequencies of 0.005 and 0.003. Correlating the ancestry proportions with TPMT-deficient genotypes, we observed that the Native American ancestry proportion influenced the distribution of the TPMT*1/TPMT*3A genotype (OR = 5.977, p = 0.002), while the contribution of African ancestral populations was associated with the TPMT*1/TPMT*3C genotype (OR = 9.769, p = 0.003). The rates of TPMT-deficient genotypes observed in Mestizo (f = 0.121) and Indigenous (f = 0.273) groups provide evidence for the influence of Native American ancestry and the prevalence of the TPMT*3A allele. In contrast, although Afro-Ecuadorian groups demonstrate similar deficiency rates (f = 0.160), the genetic factors involved are associated with contributions from African ancestral populations, specifically the prevalent TPMT*3C allele.

Conclusion

The distribution of TPMT-deficient variants offers valuable insights into the populations under study, underscoring the necessity for genetic screening strategies to prevent thiopurine toxicity events among Latin American minority groups.

TPMT gene polymorphisms (c.238G>C, c.460G>A and c.719A>G) in a healthy Venezuelan population

Background: The presence of polymorphisms in the TPMT gene is associated with adverse effects in patients treated with standard doses of thiopurine drugs. Scientific evidence recognizes significant ethnic differences in their frequencies and how their early identification can prevent clinical complications. Methods: 150 healthy residents of Aragua, Venezuela were enrolled. The SNPs c.460G>A and c.719A>G were detected by PCR-restriction fragment length polymorphism assay and c.238G>C by allele-specific PCR. Results: All genotype polymorphisms were heterozygous. TPMT*1/*3A, TPMT*1/*3C and TPMT*1/*2 genotypes were found in 4.0, 2.0 and 0.7%, respectively. Conclusion: 6.7% of individuals have an intermediate TPMT activity. These findings support the importance of prior genotyping of TPMT in Venezuelan patients who require thiopurine drug therapy.

Population pharmacogenomics: an update on ethnogeographic differences and opportunities for precision public health

Both safety and efficacy of medical treatment can vary depending on the ethnogeographic background of the patient. One of the reasons underlying this variability is differences in pharmacogenetic polymorphisms in genes involved in drug disposition, as well as in drug targets. Knowledge and appreciation of these differences is thus essential to optimize population-stratified care. Here, we provide an extensive updated analysis of population pharmacogenomics in ten pharmacokinetic genes (CYP2D6, CYP2C19, DPYD, TPMT, NUDT15 and SLC22A1), drug targets (CFTR) and genes involved in drug hypersensitivity (HLA-A, HLA-B) or drug-induced acute hemolytic anemia (G6PD). Combined, polymorphisms in the analyzed genes affect the pharmacology, efficacy or safety of 141 different drugs and therapeutic regimens. The data reveal pronounced differences in the genetic landscape, complexity and variant frequencies between ethnogeographic groups. Reduced function alleles of CYP2D6, SLC22A1 and CFTR were most prevalent in individuals of European descent, whereas DPYD and TPMT deficiencies were most common in Sub-Saharan Africa. Oceanian populations showed the highest frequencies of CYP2C19 loss-of-function alleles while their inferred CYP2D6 activity was among the highest worldwide. Frequencies of HLA-B*15:02 and HLA-B*58:01 were highest across Asia, which has important implications for the risk of severe cutaneous adverse reactions upon treatment with carbamazepine and allopurinol. G6PD deficiencies were most frequent in Africa, the Middle East and Southeast Asia with pronounced differences in variant composition. These variability data provide an important resource to inform cost-effectiveness modeling and guide population-specific genotyping strategies with the goal of optimizing the implementation of precision public health.

Myasthenia gravis and azathioprine treatment: Adverse events related to thiopurine S-methyl-transferase (TPMT) polymorphisms

Azathioprine (AZA) is the most common immunosuppressive drug used to treat myasthenia gravis (MG). To analyses the prevalence of thiopurine S-methyl-transferase (TPMT) genotypes and their association with adverse events due to azathioprine therapy in MG patients. Allele-specific polymerase chain reaction (PCR) and PCR with restriction fragment length polymorphism (RFLP) analysis were carried out to determine the prevalence of the most common TPMT genotypes (*2, *3A, *3B and *3C) in 50 MG patients from Southern Brazilian. The frequency of adverse reactions due to azathioprine therapy was analysed and correlated with different genotypes groups. The prevalence of TPMT gene variants was 12%. The allelic frequency of variant TPMT*2 (C238G), TPMT*3A (G460A/A719G), TPMT*3B (G460A), and TPMT*3C (A719G) genotypes was 1, 3, 2 and 1%, respectively. Adverse events occurred in 44%, of MG patients, of which 86% were minor and 14% were major. One patient, who presented a major adverse event (bone marrow suppression), was homozygous for the TPMT*3A genotype. Our study estimated the prevalence of TPMT genotypes for Brazilian MG patients. The profile of TPMT genotypes was different from other Brazilian populations. Hardy-Weinberg equilibrium and allelic frequencies of TPMT*3A and TPMT*3B, respectively, were different than expected, a finding that suggests a possible founder effect. Major adverse events were statistically significant for TPMT genotypes compared to wild-type. Although TPMT genotype has been associated with AZA-related adverse events, since no statistically significant difference among wild-type and other TPMT genotypes for minor adverse events, our study supports the view that TPMT genotype alone is not enough to adequately personalise the AZA therapy in MG patients. In conclusion, these results were important to characterise the prevalence of TPMT gene variants in MG patients treated with AZA and correlate the adverse events of this therapy in a real-world outpatient clinic from Southern Brazil.

Thiopurine Methyltransferase Genetic Polymorphisms and Activity and Metabolic Products of Azathioprine in Patients with Inflammatory Bowel Disease

BACKGROUND

Thiopurine S-methyltransferase (TPMT) is a cytoplasmic enzyme that catalyzes thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathioprine. There is a correlation between thiopurine drug metabolism, response, and toxicity and genetic polymorphism of TPMT. The aim of this study is to assess TPMT genetic polymorphisms activity and metabolic products of AZA in patients with IBD.

METHODS

Blood samples were obtained from 50 IBD unrelated patients from a private laboratory. We used polymerase chain reaction-restriction length polymorphism (PCR-RFLP) and allele-specific PCRbased assays to determine the TPMT gene for the different variants. A high-performance liquid chromatography system (HPLC) was carried out to determine the whole blood 6-TGN concentration. Determination of serum TMPT activity was done by ELISA kit.

RESULTS

In IBD patients, 46/50 (92%) subjects were homozygous for the wild-type allele (TPMT*1/*1). Mutant TPMT*1/*2 and TPMT*1/*3C alleles were found in 4/46 (8%) and 3/47 (6%) of IBD patients, respectively. TPMT*1/*3B variant was not detected in any of the IBD patients. TPMT enzyme activity was higher in wild-type than that mutant variants TPMT*1/*2 and TPMT*1/*3C, suggesting that there are statistically significant differences between 6-TG levels and polymorphisms of TMPT enzyme. 6-TG levels significantly increased in IBD patients mutant variants TPMT*1/*2 and TPMT*1/*3C.

CONCLUSIONS

Our results showed that TPMT polymorphisms are associated with 6-TGN levels in patients using AZA. This study suggests that AZA dosage may be determined according to the high or low prevalence of a TPMT genotype. Moreover, the results present the determination of metabolite for assessing possible safe effective dosage of the drug.

High frequency of mutant thiopurine S-methyltransferase genotypes in Mexican patients with systemic lupus erythematosus and rheumatoid arthritis

Systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) are treated with immunosuppressive purine analogs, 6-mercaptopurine/6-thioguanine/azathiopurine, which are inactivated by thiopurine S-methyltransferase (TPMT). Non-synonymous polymorphisms in TPMT are associated with increased risk of adverse effects in patients treated with thiopurines. This study aimed to determine the frequency of the most common mutant TPMT alleles in Mexican patients with SLE (a prototype autoimmune disease) and RA (one of the most common autoimmune diseases in Mexico). Five hundred fifty-three consecutive patients from Central Mexico with SLE (178) and RA (375) were included. Subjects were genotyped to identify TPMT*2 (rs1800462), TPMT*3A (rs1800460 and rs1142345), TPMT*3B (rs1800460), and TPMT*3C (rs1142345) mutant alleles. DNA samples were assayed with the 5' exonuclease technique and TaqMan probes. Mutant alleles were detected in 6.2 and 5.2% of SLE and RA cases, respectively. Of note, 12.4% of SLE cases and 10.1% of RA cases carried mutant genotypes. Among those, the null genotype (TPMT*2/*3A, 0.3%) and the TPMT*3B (0.5%) and TPMT*3C (1.0%) alleles were found in RA, but not SLE cases. Mexican SLE cases displayed the highest frequency of mutant TPMT genotypes worldwide. TPMT genotyping should be performed for Mexican patients with SLE and RA before prescribing purine analogs.

Prevalence of thiopurine S-methyltransferase gene polymorphisms in patients with inflammatory bowel disease from the island of Crete, Greece

BACKGROUND

There is evidence that genotyping for the thiopurine S-methyltransferase (TPMT) gene variants is useful for the prediction of response to thiopurine analogs (azathioprine and 6-mercaptopurine) in patients with inflammatory bowel disease (IBD). The aim of the present study was to determine the prevalence of TPMT gene polymorphisms in a genetic homogenous population of IBD patients in Crete and to correlate the results with adverse reactions to thiopurine drugs.

PATIENTS AND METHODS

Genotyping for the most common TPMT variants TPMT*2, TPMT*3A, TPMT3*C, and TPMT*3B was performed using the PCR-restriction fragment length polymorphism method in 223 consecutive IBD patients and 119 age-matched and sex-matched healthy controls. The hospital medical records were reviewed for thiopurine use in these patients and related adverse events.

RESULTS

The prevalence of TPMT variants TPMT*2, TPMT*3A, TPMT*3B, and TPMT*3C was 1.8, 2.7, 1.3, and 1.8%, respectively. The G238C mutation was detected in four (1.8%) out of 223 patients, three (1.3%) patients were carriers of the G460A mutation, four (1.8%) of the A719G mutation, and six (2.7%) of both G460A and A719G mutations. In healthy controls, only one (0.8%) carried both the G460A and the A719G mutation, whereas TPMT*2, TPMT*3C, and TPMT*3B were not detected. None of the genotypes was homozygous. A statistically significant correlation between the presence of the G460A mutation and the development of leucopenia after the administration of thiopurines was observed (P=0.048).

CONCLUSION

This study showed a lower frequency of total TPMT variants and a higher frequency of TPMT*3B in Cretan IBD patients compared with other Caucasian populations. The presence of the G460A mutation is associated with the development of leukopenia.

Comparison of Direct Sequencing, Real-Time PCR-High Resolution Melt (PCR-HRM) and PCR-Restriction Fragment Length Polymorphism (PCR-RFLP) Analysis for Genotyping of Common Thiopurine Intolerant Variant Alleles NUDT15 c.415C>T and TPMT c.719A>G (TPMT*3C)

Thiopurine intolerance and treatment-related toxicity, such as fatal myelosuppression, is related to non-function genetic variants encoding thiopurine S-methyltransferase (TPMT) and Nudix hydrolase 15 (NUDT15). Genetic testing of the common variants NUDT15:NM_018283.2:c.415C>T (Arg139Cys, dbSNP rs116855232 T allele) and TPMT: NM_000367.4:c.719A>G (TPMT*3C, dbSNP rs1142345 G allele) in East Asians including Chinese can potentially prevent treatment-related complications. Two complementary genotyping approaches, real-time PCR-high resolution melt (PCR-HRM) and PCR-restriction fragment length morphism (PCR-RFLP) analysis were evaluated using conventional PCR and Sanger sequencing genotyping as the gold standard. Sixty patient samples were tested, revealing seven patients (11.7%) heterozygous for NUDT15 c.415C>T, one patient homozygous for the variant and one patient heterozygous for the TPMT*3C non-function allele. No patient was found to harbor both variants. In total, nine out of 60 (15%) patients tested had genotypic evidence of thiopurine intolerance, which may require dosage adjustment or alternative medication should they be started on azathioprine, mercaptopurine or thioguanine. The two newly developed assays were more efficient and showed complete concordance (60/60, 100%) compared to the Sanger sequencing results. Accurate and cost-effective genotyping assays by real-time PCR-HRM and PCR-RFLP for NUDT15 c.415C>T and TPMT*3C were successfully developed. Further studies may establish their roles in genotype-informed clinical decision-making in the prevention of morbidity and mortality due to thiopurine intolerance.

Pharmacogenomic assessment of Mexican and Peruvian populations

BACKGROUND

Clinically relevant polymorphisms often demonstrate population-specific allele frequencies. Central and South America remain largely uncategorized in the context of pharmacogenomics.

MATERIALS & METHODS

We assessed 15 polymorphisms from 12 genes (ABCB1 3435C>T, ABCG2 Q141K, CYP1B1*3, CYP2C19*2, CYP3A4*1B, CYP3A5*3C, ERCC1 N118N, ERCC2 K751Q, GSTP1 I105V, TPMT 238G>C, TPMT 460G>A, TPMT 719A>G, TYMS TSER, UGT1A1*28 and UGT1A1 -3156G>A) in 81 Peruvian and 95 Mexican individuals.

RESULTS

Six polymorphism frequencies differed significantly between the two populations: ABCB1 3435C>T, CYP1B1*3, GSTP1 I105V, TPMT 460G>A, UGT1A1*28 and UGT1A1 -3156G>A. The pattern of observed allele frequencies for all polymorphisms could not be accurately estimated from any single previously studied population.

CONCLUSION

This highlights the need to expand the scope of geographic data for use in pharmacogenomics studies.

Polymorphisms of the thiopurine S-methyltransferase gene among the Libyan population

BACKGROUND

Thiopurine S-methyltransferase (TPMT) is a cytosolic enzyme that catalyses the S-methylation of 6-mercaptopurine and azathioprine. Low activity phenotypes are correlated with polymorphism in the TPMT gene. Patients with low or undetectable TMPT activity could develop severe myelosuppression when they are treated with standard doses of thiopurine drugs. Since ethnic differences in the TPMT gene polymorphism have been demonstrated worldwide, assessing it in the Libyan population is worthwhile.

METHODS

We investigated TPMT gene polymorphism in a total of 246 Libyan healthy adult blood donors from three different Libyan regions (Tripoli, Yefren, and Tawargha) and 50 children with acute lymphoblastic leukaemia (ALL). We used polymerase chain reaction restriction length polymorphism (PCR-RFLP) and allele-specific PCR-based assays to analyse the TPMT gene for the variants *2 c.238 G>C, *3A (c.460 G>A and c.719 A>G), *3B (c.460 G>A), and *3C (c.719 A>G).

RESULTS

Our results show that the TPMT variants associated with low enzymatic activity were detected in 3.25% (8 in 246) of adult Libyan individuals and the frequency of total mutant alleles was 1.63%. Heterozygous genotypes were TPMT*3A in three subjects (0.61%) and TPMT*3C in five subjects (1.02%). No TPMT*2 and TPMT*3B allelic variants and no homozygous or compound heterozygous mutant alleles were detected. The normal allele (wild-type) was found in 98.4% of the adult individuals studied. No mutant alleles were detected among the 50 children who had ALL.

CONCLUSIONS

We report on the presence of the TPMT*3C and *3A mutant alleles in the Libyan population. Therefore, monitoring the patients to be treated with doses of thiopurine drugs for TPMT variants is worthwhile to avoid the development of severe myelosuppression.

Frequency of thiopurine S-methyltransferase mutant alleles in indigenous and admixed Guatemalan patients with acute lymphoblastic leukemia

Thiopurine S-methyltransferase (TPMT) polymorphisms affect the enzyme's activity and are predictive for the efficacy and toxicity of thiopurine treatment of acute lymphoblastic leukemia (ALL), autoimmune diseases and organ transplants. Because inter-ethnic differences in the distribution of these polymorphisms have been documented, we sequenced the TMPT gene in 95 Guatemalans, yet identified no new alleles. We also determined the frequency of the TPMT 2, 3A, 3B and 3C alleles in 270 admixed and 177 indigenous pediatric patients with ALL and healthy subjects from Guatemala using TaqMan assays and DNA sequencing. Among the 447 subjects genotyped, 10.0 % of the ALL cases and 13.6 % of the healthy controls were heterozygous for one of the four TPMT variants screened. The genotype frequencies in ALL and control populations were 0.7 and 1.7 % for TPMT 1/ 2, 7.4 and 10 % for TPMT 1/3A, 0.3 and 0 % for TPMT 1/B, and 1.5 and 1.1 % for TPMT 1/C, respectively (p = 0.30). No statistically significant differences between admixed and indigenous ALL (p = 0.67) or controls (p = 0.41) groups were detected; however, 17 % of the admixed healthy group bore one TPMT mutant allele, and they have one of the highest reported frequencies of TPMT mutant allele carriers. Because of the clinical implications of these variants for therapeutic response, TPMT allele testing should be considered in all Guatemalan patients to reduce adverse side-effects from thiopurine drug treatments.

The frequency and distribution of thiopurine S-methyltransferase alleles in south Iranian population

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathiopurine. Variability in TPMT activity is mainly due to genetic polymorphism. The frequency of the four allelic variants of the TPMT gene, TPMT*2 (G238C), TPMT*3A (G460A and A719G), TPMT*3B (G460A) and TPMT*3C (A719G) were determined in an Iranian population from south of Iran (n = 500), using polymerase chain reaction (PCR)-RFLP and allele-specific PCR-based assays. Four hundred seventy four persons (94.8%) were homozygous for the wild type allele (TPMT*1/*1) and twenty five people were TPMT*1/*3C (5%). One patient was found to be heterozygous in terms TPMT*1 and *2 alleles with genotype of TPMT*1/*2 (0.2%). None of the participants had both defective alleles. The TPMT*3C and *2 were the only variant alleles observed in this population. The total frequency of variant alleles was 2.6% and the wild type allele frequency was 97.4%. The TPMT*3B and *3A alleles were not detected. Distributions of TPMT genotype and allele frequency in Iranian populations are different from the genetic profile found among Caucasian or Asian populations. Our findings also revealed inter-ethnic differences in TPMT frequencies between different parts of Iran. This view may help clinicians to choose an appropriate strategy for thiopurine drugs and reduce adverse drug reactions such as bone marrow suppression.

Genetic analysis of thiopurine methyltransferase polymorphism in the Jordanian population

This study provides the first analysis of the TPMT mutant allele frequency in a sample of the Jordanian population and indicates that TPMT*3A is the most common allele in Jordanian subjects.

PURPOSE

thiopurine methyltransferase TPMT catalyses the S-methylation of thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathiopurine. Thiopurine methyltransferase (TPMT) polymorphisms are the major determinants of interindividual differences in the severe haematological toxicity of 6-mercaptopurine. Several variants in the TPMT gene have been identified that correlate with a low activity phenotype. Four variant alleles, TPMT*2, TPMT*3A, TPMT*3B and TPMT*3C, are responsible for over 80% of the low or undetectable enzyme activity. The allelic frequency of TPMT variants has been established in many populations.

METHODS

In this study, the frequencies of four (TPMT*2, TPMT*3A, TPMT*3B and TPMT*3C) variants were investigated in 169 healthy Jordanian men (18-45 years of age). Single nucleotide polymorphisms (SNPs) were genotyped using the Sequenom MassARRAY technology (Sequenom; San Diego, CA, USA).

RESULTS

TPMT*3A and TPMT*3C were the only deficiency alleles detected in the Jordanian population with an allele frequency of 0.59% and 0.30% respectively. The TPMT*3A allele frequency is found to be lower than in the European Caucasian population.

CONCLUSION

TPMT*3A and TPMT*3C were the only deficiency alleles detected in the Jordanian population with an allele frequency of 0.59% and 0.30% respectively. The TPMT*3A allele frequency is found to be lower than in the European Caucasian population.

Thiopurine S-methyltransferase alleles, TPMT(*)2, (*)3B and (*)3C, and genotype frequencies in an Indian population

Thiopurine S-methyltransferase (TPMT) catalyzes the S-methylation of aromatic and heterocyclic sulfhydryl compounds including thiopurine drugs such as 6-mercaptopurine, 6-thioguanine and azathioprine. TPMT activity exhibits genetic variation and shows tri-modal distribution with 89-94% of individuals possessing high activity, 6-11% intermediate activity and approximately 0.3% low activity. Patients with intermediate or deficient TPMT activity exposed to thiopurine drugs show severe hematopoietic toxicity. Three single nucleotide polymorphisms (SNPs) in TPMT (NM_000367.2:c.238G>C, NM_000367.2:c.460G>A and NM_000367.2:c.719A>G) define the most prevalent mutant alleles associated with loss of catalytic activity reported in several populations. The present study investigated, for the first time, the frequency distribution of these three SNPs of TPMT, their alleles and genotypes in a Southern Indian population. Peripheral blood was obtained from 326 individuals of a Southern Indian population, and genomic DNA was isolated from total peripheral white blood cells. The genotypes at the polymorphic loci were determined by allele-specific polymerase chain reaction, restriction fragment length polymorphism and confirmatory DNA sequencing. The estimated genotype frequency for homozygous TPMT(*)1/(*)1 was 97.24%, for heterozygous TPMT(*)1/(*)2 and TPMT(*)1/(*)3B, 0.61% each, and for heterozygous TPMT(*)1/(*)3C, 1.53%. The frequency of heterozygous mutants in the studied Indian population was 2.76%. This study demonstrated significant variations in TPMT gene polymorphisms in an Indian population in relation to other human populations and may help to predict both clinical efficacy and drug toxicity of thiopurine drugs.

Clinical pharmacology and pharmacogenetics of thiopurines

The thiopurine drugs-azathioprine (AZA), 6-mercaptopurine (6-MP), and thioguanine-are widely used to treat malignancies, rheumatic diseases, dermatologic conditions, inflammatory bowel disease, and solid organ transplant rejection. However, thiopurine drugs have a relatively narrow therapeutic index and are capable of causing life-threatening toxicity, most often myelosuppression. Thiopurine S-methyltransferase (TPMT; EC 2.1.1.67), an enzyme that catalyzes S-methylation of these drugs, exhibits a genetic polymorphism in 10% of Caucasians, with 1/300 individuals having complete deficiency. Patients with intermediate or deficient TPMT activity are at risk for excessive toxicity after receiving standard doses of thiopurine medications. This report reviews the recent advances in the knowledge of the mechanism of action as well as the molecular basis and interethnic variations of TPMT and inosine triphosphate pyrophosphatase (ITPase; EC 3.6.1.19), another enzyme implicated in thiopurine toxicity. In addition, an update on pharmacokinetics, metabolism, drug-drug interactions, safety, and tolerability of thiopurine drugs is provided.

The low frequency of defective TPMT alleles in Turkish population: a study on pediatric patients with acute lymphoblastic leukemia

6-Mercaptopurine (6MP) is an essential anticancer drug used in the treatment of childhood acute lymphoblastic leukemia (ALL). Thiopurine methyltransferase (TPMT) polymorphisms are the major determinants of interindividual differences in the severe toxicity or efficacy of 6MP. Four variant alleles, TPMT*2, TPMT*3A, TPMT*3B, and TPMT*3C, are responsible over the 80% of low or undetectable enzyme activity. The frequencies of these variants were investigated among 106 children with ALL in Turkish population. TPMT*3A and TPMT*3C were the only deficiency alleles detected in Turkish population with an allele frequency of 0.9% for both. While *3C allele frequency in Turkish population was found to be very similar to Asian and other Caucasian populations, *3A allele frequency was significantly (P < 0.05) lower. So far, studies showed that the genetic polymorphisms of other drug metabolizing enzymes like CYP2E1, CYP1A1, GSTM1/ T1 in Turkish population were similar to Caucasian populations. However, we found that the distribution of TPMT polymorphisms in Turkish population was significantly lower than those in other Caucasians like British, French, and Italian whereas the distributions of TPMT variants were found to be very similar to Kazak population which is also Caucasian in ethnic origin. In this study, the clinical histories of the patients in the sample population were also examined, retrospectively. The patients with heterozygous or homozygous mutant genotypes had developed severe neutropenia and infection during 6MP therapy. The study provides the first data on the frequency of common TPMT variants in the Turkish population, based on analysis of pediatric patients with ALL.

Thiopurine S-Methyltransferase Pharmacogenetics: Genotype to Phenotype Correlation in the Slovenian Population

The toxicity of thiopurine drugs has been correlated to the activity of thiopurine S-methyltransferase (TPMT), whose interindividual variation is a consequence of genetic polymorphisms. We have herein investigated the relevance of some genetic markers for the prediction of thiopurine-related toxicities and to determine the genotype to phenotype correlation in the Slovenian population. The most prevalent mutant allele in the Slovenian population is TPMT*3A (4.1%), followed by TPMT*3C (0.5) and TPMT*3B (0.3), while the TPMT*2 allele was not found in any of the examined samples. TPMT enzyme activity distribution in the subgroup sample was bimodal and as such correlated with genetic data. Using a cutoff value of 9.82 pmol/10(7) RBC per h, the genetic data correctly predicted TPMT enzyme activity in 91.6% of the examined individuals. Pharmacogenetic TPMT analyses have therefore proved to have significant clinical implications for prediction of individuals' responses to treatment with thiopurine drugs in order to avoid possible life-threatening therapy-related toxicities.

Frequency of the thiopurine S-methyltransferase alleles in the ancient genetic population isolate of Sardinia

BACKGROUND

Thiopurine S-methyltransferase (TPMT) is an enzyme involved in the normal metabolic inactivation of thiopurine drugs. Patients with intermediate or no TPMT activity are at risk of toxicity after receiving standard doses of thiopurine drugs and it was shown that inter-individual differences in response to these drugs is largely determined by genetic variation at the TPMT locus.

OBJECTIVE

This study was designed to investigate in the Sardinian population the frequency distribution of four of the most common variants accounting for TPMT deficiency and to conduct comparative analyses with other populations in order to obtain insights into the main factors that have shaped diversity at the TPMT locus in Sardinia.

METHODS

DNA was extracted in 259 Sardinians and the frequencies of allelic variants of TPMT were determined using polymerase chain reaction-restriction fragment length polymorphism technique.

RESULTS

Among the 259 Sardinians genotyped, 6.95% were found to be heterozygous for one of four TPMT variants screened; for each variant the frequency estimate was 1.74%, 0.58%, 0.39% and 0.77% for TPMT*2, TPMT*3A, TPMT*3B and TPMT*3C respectively.

CONCLUSIONS

Although Sardinia does not show reduced diversity at the TPMT locus, the spectrum of TPMT allele frequencies affords evidence of remarkable influence of genetic drift and founder effects throughout its population history. In the broad context of the European TPMT diversity, the Sardinians come out as outliers, an observation consistent with previous genetic inferences that Sardinia has features of a genetic isolate.

Azathioprine and 6-mercaptopurine pharmacogenetics and metabolite monitoring in inflammatory bowel disease

The thiopurine drugs azathioprine and 6-mercaptopurine (6-MP) are well-established in the treatment of inflammatory bowel disease (IBD). However, there is a wide inter- and intra-patient variation in the concentrations of active and toxic metabolites due to their complex metabolism and genetic polymorphisms in metabolizing enzymes. Serious drug toxicity leads to cessation of therapy in 9-25% of patients, and there is failure to achieve efficacy in approximately 15% of cases. Advances in the understanding of thiopurine drug metabolism have led to new genetic and metabolite tests to help clinicians optimize thiopurine use. Thiopurine methyltransferase (TPMT) enzyme activity can predict life-threatening myelotoxicity in the one in 300 patients who are TPMT-deficient. However, myelotoxicity can also occur in the presence of normal TPMT activity so blood count monitoring should remain standard practice. TPMT testing may also aid in dose individualization. 6-Thioguanine nucleotides (6-TGN) are thought to be the predominant active metabolites of the thiopurines. 6-thioguanine nucleotide concentration is correlated with bone marrow toxicity and may also correlate with efficacy in IBD. Measurement of 6-TGN and 6-methylmercaptopurine (6-MMP) concentration is most useful in determining why a patient is not responding to a standard dose of a thiopurine drug and may help in avoiding myelosuppression. The ratio of these metabolites can help distinguish non-compliance, under-dosing, thiopurine-resistant and thiopurine-refractory disease. Some of these investigations are entering routine clinical practice but more research is required to determine their optimal use in patients with IBD.