Thiopurine S-methyltransferase genetic polymorphism in the Thai population

AIMS: To determine the incidence of the thiopurine S-methyltransferase (TPMT) genetic polymorphism in the Thai population. METHODS: Genomic DNAs were isolated from peripheral blood leucocytes of 200 healthy Thais. The frequencies of five allelic variants of the TPMT gene, TPMT*2, *3A, *3B, *3C and *6 were determined using allele specific polymerase chain reaction (PCR) or PCR-Restriction fragment length polymorphism technique. RESULTS: Of the 200 Thai subjects participating in this study, 181 subjects (90.5%) were homozygous for TPMT*1, 18 subjects (9.0%) were heterozygous for TPMT*1/*3C. Only one subject (0.5%) was homozygous for TPMT*3C. The frequency of TPMT*3C mutant allele was 0.050. CONCLUSIONS: Although the TPMT*3C is the most prevalent mutant allele in Asian populations, the frequency of this defective allele is significantly higher in Thais than has been reported in other Asian populations.     

The frequency and distribution of thiopurine methyltransferase alleles in Caucasian and Asian populations

Thiopurine methyltransferase metabolizes 6-mercaptopurine, thioguanine and azathioprine, thereby regulating cytotoxicity and clinical response to these thiopurine drugs. In healthy Caucasian populations, 89-94% of individuals have high thiopurine methyltransferase activity, 6-11% intermediate and 0.3% low, resulting from genetic polymorphism. Four variant thiopurine methyltransferase alleles were detected in over 80% of individuals with low or intermediate thiopurine methyltransferase activity. The wild-type allele is defined as TPMT*1 and the mutant alleles are TPMT*2 (G238C), TPMT*3A (G460A and A719G), TPMT*3B (G460A) and TPMT*3B (A719G). The frequency of these alleles in different ethnic groups is not well defined. In this study, DNA from 199 British Caucasian, 99 British South West Asian and 192 Chinese individuals was analysed for the presence of these variant alleles using polymerase chain reaction-restriction fragment length polymorphism and allele-specific polymerase chain reaction based assays. The frequency of individuals with a variant thiopurine methyltransferase genotype was: Caucasians 10.1% (20/199), South West Asians 2.0% (2/99) and Chinese 4.7% (9/192). Two TPMT*2 heterozygotes were identified in the Caucasian population, but this allele was not found in the two Asian populations. TPMT*3A was the only mutant allele found in the South West Asians (two heterozygotes). This was also the most common mutant allele in the Caucasians (16 heterozygotes and one homozygote) but was not found in the Chinese. All mutant alleles identified in the Chinese population were TPMT*3C (nine heterozygotes). This allele was found at a low frequency in the Caucasians (one heterozygote). This suggests that A719G is the oldest mutation, with G460A being acquired later to form the TPMT*3A allele in the Caucasian and South West Asian populations. TPMT*2 appears to be a more recent allele, which has only been detected in Caucasians to date. These ethnic differences may be important in the clinical use of thiopurine drugs.     

Ethnic differences in thiopurine methyltransferase pharmacogenetics: evidence for allele specificity in Caucasian and Kenyan individuals

Thiopurine methyltransferase (TPMT) degrades 6-mercaptopurine, azathioprine and 6-thioguanine which are commonly used in the treatment of autoimmune diseases, leukaemia and organ transplantation. TPMT activity is polymorphic as a result of gene mutations. Heterozygous individuals have an increased risk of haematological toxicity after thiopurine medication, while homozygous mutant individuals suffer life threatening complications. Previous population studies have identified ethnic variations in both phenotype and genotype, but limited information is available within African populations. This study determined the frequency of common TPMT variant alleles in 101 Kenyan individuals and 199 Caucasians. The frequency of mutant alleles was similar between the Caucasian (10.1%) and Kenyan (10.9%) populations. However, all mutant alleles in the Kenyan population were TPMT*3C compared with 4.8% in Caucasians. In contrast TPMT*3A was the most common mutant allele in the Caucasian individuals. This study confirms ethnic differences in the predominant mutant TPMT allele and the findings will be useful for the development of polymerase chain reaction-based strategies to prevent toxicity with thiopurine medications.     

Frequency distribution of thiopurine S-methyltransferase alleles in a polish population

Thiopurine S-methyltransferase (TPMT) is an enzyme that catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathioprine. TPMT activity exhibits an interindividual variability mainly a result of genetic polymorphism. Patients with intermediate or deficient TPMT activity are at risk for toxicity after receiving standard doses of thiopurine drugs. It has previously been reported that 3 variant alleles:TPMT*2, *3A, and *3C are responsible for over 95% cases of lower enzyme activity. The purpose of this study was to determine the frequency of TPMT variant alleles in a Polish population. DNA samples were obtained from 358 unrelated healthy Polish subjects of white origin, and TPMT genetic polymorphism was determined using PCR-RFLP and allele-specific PCR methods. The results showed that allelic frequencies were 0.4% for TPMT*2, 2.7% for TPMT*3A, and 0.1% for TPMT*3C, respectively. A TPMT*3B allele was not found in the studied population. The general pattern of TPMT allele disposition in the Polish population is similar to those determined for other white populations, but the frequency of total variant alleles is lower than in other European populations studied to date.     

Phenotypic and genotypic analysis of thiopurine s-methyltransferase polymorphism in the bulgarian population

Genetic polymorphism of TPMT activity is an important factor responsible for large individual differences in thiopurine toxicity and therapeutic efficacy. The aim of this study was to determine the distribution of TPMT activity as well as the types and frequencies of mutant alleles in a Bulgarian population sample. TPMT activity was measured in 313 Bulgarians, using an established HPLC procedure. All individuals with TPMT activity less than 12.0 nmol/(mL Ery.h) (n = 76) were additionally genotyped using a color multiplex hybridization assay. The samples were tested for TPMT*2, *3A, *3B, *3C, *3D, *4, and *6 mutant alleles. TPMT activities varied from 1.1 to 24.0 nmol/(mL Ery.h) [mean 14.2 +/- 3.2 nmol/(mL Ery.h)]: 92.3% of the individuals investigated had high TPMT activity [>10 nmol/(mL Ery. h)], whereas 7.4% were intermediate [2.8-10 nmol/(mL Ery.h)], and 0.3% were low metabolizers [< 2.8 nmol/(mL Ery.h)]. A significant gender-related difference in TPMT activity (P = 0.02) was observed with 6.2% higher values in men than in women. There was no significant correlation between age and enzyme activity (r = 0.06, P = 0.27). Genotype analysis revealed three mutant TPMT alleles: 2, 3A, and 3C. The frequency of these alleles among the TPMT-deficient individuals was 2.17%, 30.4%, and 2.17%, respectively. These data show a similar distribution of TPMT activity among the Bulgarian population investigated as in most other white populations with the frequency of intermediate metabolizers being somewhat lower (7.4% versus approximately 11%) in the Bulgarians. The most common variant allele was TPMT-3A, as in other white populations.     

中国新疆维吾尔族硫嘌呤甲基转移酶基因突变研究

目的 研究硫嘌呤甲基转移酶(thiopurine S-methyltransferase，TPMT)在新疆维吾尔族中的基因突变频率。方法 用等位基因特异性的PCR方法和限制性片断长度多态性的方法检测4种常见的导致酶活性降低的突变类型：TPMT*2、TPMT*3A、TPMT*3B和TPMT*3C。结果 在160名维吾尔族中发现了1例TPMT*3A(A719G／G460A)杂合子、5例TPMT*3C(A719G)杂合子，TPMT*3A和TPMT*3C的等位基因频率分别是0．3％和1．6％。结论 维吾尔族总的TPMT突变等位基因频率(1．9％)同中国其他民族相近；TPMT*3C是维吾尔族最主要的突变类型。     

Genotype and allele frequencies of TPMT,NAT2, GST,SULT1A1 and MDR-1 in the Egyptian population

AIMS: The goal of this study was to determine the frequencies of important allelic variants in the TPMT, NAT2, GST, SULT1A1 and MDR-1 genes in the Egyptian population and compare them with the frequencies in other ethnic populations. METHODS: Genotyping was carried out in a total of 200 unrelated Egyptian subjects. TPMT*2 was detected using an allele-specific polymerase chain reaction (PCR) assay. TPMT*3C and NAT2 variants (*5,*6 and *7) were detected using an allele-specific real-time PCR assay. Detection of GSTM1 and GSTT1 null alleles was performed simultaneously using a multiplex PCR assay. Finally, a PCR-restriction fragment length polymorphism assay was applied for the determination of TPMT*3A (*3B), SULT1A1*2 and MDR-1 (3435T) variants. RESULTS: Genotyping of TPMT revealed frequencies of 0.003 and 0.013 for TPMT*3A and TPMT*3C, respectively. No TPMT*2 or *3B was detected in the analysed samples. The frequencies of specific NAT2 alleles were 0.215, 0.497, 0.260 and 0.028 for *4 (wild-type), *5 (341C), *6 (590A) and *7 (857A), respectively. GSTM1 and GSTT1 null alleles were detected in 55.5% and 29.5% of the subjects, respectively. SULT1A1*2 was detected at a frequency of 0.135. Finally, the frequencies of the wild-type allele (3435C) and the 3435T variant in the MDR-1 gene were found to be 0.6 and 0.4, respectively. CONCLUSIONS: We found that Egyptians resemble other Caucasians with regard to allelic frequencies of the tested variants of NAT2, GST and MDR-1. By contrast, this Egyptian population more closely resemble Africans with respect to the TPMT*3C allele, and shows a distinctly different frequency with regard to the SULT1A1*2 variant. The predominance of the slow acetylator genotype in the present study (60.50%) could not confirm a previously reported higher frequency of the slow acetylator phenotype in Egyptians (92.00%), indicating the possibility of the presence of other mutations not detectable as T341C, G590A and G857A. The purpose of our future studies is to investigate for new polymorphisms, which could be relatively unique to the Egyptian population.     

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

Thiopurine methyltransferase (TPMT) catalyzes the inactivation of thiopurine drugs (mercaptopurine, thioguanine and azathioprine) used to treat acute lymphoblastic leukemia, autoimmune diseases and recipients of transplanted organs. No endogenous substrates for this enzyme are known. The TPMT polymorphism is a major determinant of individual differences in the toxicity or therapeutic efficacy of these drugs. The molecular basis of this polymorphism has been established in Caucasians, Africans, African-Americans and Asians, but not yet in the heterogeneous Latin American groups, including the Colombian population. 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 140 Colombian volunteers of Mestizo origin, using allele-specific PCR and PCR-RFLP assays. The *3A allele was found in 10 samples and the *2 allele in one, all heterozygotes; neither homozygous mutant genotypes nor the *3B and *3C alleles were detected. In agreement with these results, 92.1% and 7.9% of the Colombian population correspond to the phenotypes high and intermediate methylators, respectively. These results show that the frequency of mutations and the allelic distribution of the TPMT gene in the Colombian population are similar to the genetic profile found among US and European Caucasian populations, where the *3A allele is prevalent and the *2 allele is currently present.     

Do the distribution patterns of polymorphisms at the thiopurine S-methyltransferase locus in sub-Saharan populations need revision? Hints from Cabinda and Mozambique

Genetic data on the thiopurine S-methyltransferase (TPMT) polymorphism were obtained in population samples from Cabinda and Mozambique (located in the western and eastern coasts of sub-Saharan Africa, respectively). The overall frequency of TPMT-deficient alleles was 5.6% in Mozambique and 6.3% in Cabinda. Accordingly, one out of the 103 individuals from Cabinda tested had a genotype associated with TPMT deficiency, yielding a frequency that is threefold higher than heretofore reported in any population. In addition, in both Cabinda or Mozambique, TPMT*8 accounted for a significant proportion of non-functional alleles (nearly 40% in Cabinda). Since the substitution defining TPMT*8 seems to be highly specific of sub-Saharan Africa populations and given the fact it has not been integrated into the set of single nucleotide polymorphisms routinely tested for TPMT, a re-design of molecular screenings should be considered in the future in order to avoid serious underestimates of TPMT deficiency when the enzymatic profiles in populations are unknown.     

Genotype–phenotype correlation for thiopurine S-methyltransferase in healthy Italian subjects

OBJECTIVE: The aim of the present study was to estimate the concordance rate between erythrocyte thiopurine methyltransferase (TPMT) activity and genotype at the TPMT locus in an Italian population sample. METHODS: The TPMT phenotype and genotype were determined in an unrelated population of 103 Italian healthy blood donors. Erythrocyte TPMT activity was measured with a radiochemical assay using 12.5 microM S-adenosyl-L-(methyl-14C)-methionine and 4 mM 6-mercaptopurine. The genotyping assay was based on restriction fragment length polymorphism polymerase chain reaction (RFLP-PCR) and allele-specific oligonucleotide polymerase chain reaction (ASO-PCR) methods. RESULTS: All subjects had detectable TPMT activity. The activity of TPMT varied 2.8-fold between the 5th and 95th percentile. This variation was neither age (P = 0.63) nor gender (P = 0.44) regulated and the frequency distribution of TPMT activity is compatible with a polymorphic distribution. The presence of the four most common defective alleles, i.e. TPMT*2, TPMT*3A, TPMT*3B and TPMT*3C, was examined through the entire phenotyped population. Ninety-two subjects did not carry any of the tested mutations. Eleven individuals were heterozygous for one of the mutant alleles and had a TPMT activity lower than 30 pmol/min/mg. Eight subjects were TPMT*1/TPMT*3A, two TPMT*1/TPMT*3C and one was TPMT*1/TPMT*2. The TPMT*3B allele was not detected in the samples analysed. CONCLUSION: There was a concordance of 97% between genotype and phenotype. All the heterozygotes had an intermediate phenotype. However, the wide variation range in TPMT activity detected in the wild-type homozygotes indicates that other genetic or epigenetic factors influence the TPMT phenotype.     

河南地区汉族人群药物代谢酶CYP2C19，NAT2和TPMT基因多态性分析(英文)

目的：了解细胞色素P450(cytochromes P450,CYP)2C19,N-乙酰基转移酶2(arylamine N- acetyltransferase 2,NAT2)和硫嘌呤甲基转移酶(thiopurine S-methyltransferase,TPMT)基因常见的遗传多态性在河南地区汉族人群中的分布及其频率。方法：应用聚合酶链反应-限制性片段长度多态性分析(PCR-RFLP)对210名河南地区汉族人群的CYP2C19突变基因(*2和*3)、NAT2突变基因(*6和*7)和TPMT突变基因(*3A,*3B和*3C)进行检测。用聚合酶链反应-等位基因特异性扩增(PCR-ASA)对NAT2突变基因(*5)和TPMT突变基因(*2)进行检测。结果：CYP2C19*2和*3等位基因分布频率分别为34．76%和6．4%,同时携带2个等位突变基因的慢基因型频率占14．8%。NAT2*4(wt),*5(341C),*6(590A)和*7(857A)等位基因分布频率分别为59．1%,4．1%,26．4%和9．5%,慢基因型分布频率占19．5%。TPMT*3C等位基因分布频率为1．2%,未发现TPMT*2,TPMT*3A或TPMT*3B。结论：CYP2C19,NAT2和TPMT基因常见的遗传多态性在汉族人群中的分布及其频率与白人存在明显差异,这将有助于我国汉族人群临床药动学研究和给药剂量的确定。     

Arylamine N-acetyltransferase-2 genotypes in the Thai population

AIMS: To determine the frequencies of the major arylamine- N-acetyltransferase-2 (NAT2) alleles in the Thai population. METHODS: DNA samples from 235 Thai individuals were analysed by polymerase chain reaction with restriction fragment length polymorphism assays. RESULTS: The frequency distribution of major NAT2 alleles, including NAT2*4, NAT2*5, NAT2*6 and NAT2*7 were 0.381 (95% CI 0.337, 0.426), 0.038 (0.023, 0.060), 0.326 (0.283, 0.370) and 0.204 (0.169, 0.244), respectively. When converted to phenotypes, the study population comprised 63.8% rapid acetylators and 36.2% slow acetylators. CONCLUSIONS: The pattern of NAT2 alleles of Thais is similar to those of many Asian populations, although the frequency of NAT2*4 is significantly lower and NAT2*7 is higher than that of Oriental populations.     

Allelotype frequency of the thiopurine methyltransferase (TPMT) gene in Japanese

Polymorphisms at three loci in the thiopurine methyltransferase (TPMT) gene are known to be responsible for azathioprine and 6-mercaptopurine (6MP) toxicity. Among them, only TPMT*3C variant allele with A719G mutation was found in 15/522 (2.9%; 17/1044 alleles; 1.6%) Japanese individuals including two homozygotes. The allele frequency was different from that in Caucasians, and investigation of TPMT polymorphisms with consideration of ethnic differences before administration of azathioprine or 6MP may provide clinically useful information.     

Detection assay of rare variants of the thiopurine methyltransferase gene by PCR-RFLP using a mismatch primer in a Japanese population

Thiopurine methyltransferase (TPMT) catalyzes the metabolism of important drugs such as 6-mercaptopurine, 6-thioguanine, and azathioprine. The identification and frequency distributions of several variant TPMT alleles (TPMT*2--*8) have been described recently in many ethnic groups. We have recently demonstrated that TPMT*3C is the most common allele in Japanese subjects; however, it remains to be elucidated whether TPMT*4--*8 variants also exist in Japanese subjects. To detect polymorphisms in the TPMT gene (TPMT*4--*8), we have developed a mismatch polymerase chain reaction and restriction fragment length polymorphism method and conducted a population study of Japanese subjects. Genotyping of these variant forms was carried out in 192 Japanese healthy volunteers. The TPMT*4, TPMT*5, TPMT*6, TPMT*7, and TPMT*8 variants were not detected in any of the samples analyzed. This study provides the first analysis of the TPMT*4--*8 variants in a sample of the Japanese population and indicates that TPMT*4--*8 variants do not occur or are rare alleles in this population.     

Frequencies of thiopurine S-methyltransferase mutant alleles (TPMT*2, *3A, *3B and *3C) in 151 healthy Japanese subjects and the inheritance of TPMT*3C in the family of a propositus

AIMS: To determine the frequencies of four thiopurine S-methyltransferase (TPMT) mutant alleles, TPMT*2, *3A, *3B and *3C in a normal Japanese population. METHODS: Genotypes were determined in 151 Japanese subjects and in six family members of a propositus using polymerase chain reaction (PCR)-restriction fragment length polymorphism and allele-specific PCR assays. RESULTS: Only one TPMT*3C heterozygote was identified (gene frequency 0.3%). TPMT*2, *3A and *3B were not detected. In addition, TPMT*3C was found to have been inherited from the mother and passed on to the son of the propositus. CONCLUSIONS: TPMT*3C appears to be most prevalent among the known mutant allele of TPMT in a Japanese population which may have some relevance for the treatment of Japanese patients with thiopurine drugs.     

Thiopurine methyltransferase phenotypes and genotypes in Brazilians

The polymorphism of thiopurine methyltransferase (TPMT) was studied in 306 healthy Brazilians who were classed, on the basis of self-declared colour and ancestry, as Euro-derived (n = 81), Afro-derived (n = 18) or having interethnic admixture (n = 204). TPMT activity (range 0.17-25.93 U) displayed a trimodal distribution of high (> 11.3 U; 9% of individuals), intermediate (5-11.3 U; 9.8%) and low (0.17 U; 0.3%) phenotypes. The occurrence of the TPMT mutations 238G>C, 460G>A and 719A>G was investigated in all individuals with low or intermediate phenotype, and in 43 with high-activity phenotype. None and two mutant alleles were associated with high- or low-activity phenotypes, respectively, whereas one mutant allele was detected in 26 of the 30 intermediate phenotype individuals. The allele frequencies of TPMT*2, TPMT*3A and TPMT*3C did not differ between individuals classed as Euro-derived (0.76%, 2.03% and 2.54%, respectively) or having interethnic admixture (0.60%, 1.81% and 1.81%, respectively). Furthermore, within each of these groups, the frequencies of TPMT*3A and TPMT*3C were not significantly different.     

Genetic determinants of the thiopurine methyltransferase intermediate activity phenotype in British Asians and Caucasians

OBJECTIVE: Polymorphisms in the TPMT gene open reading frame (ORF) are associated with reduced TPMT activity. Variable number tandem repeats (VNTR*3 to VNTR*9) in the promoter region of the gene consisting of combinations of Type A, B and C repeat units, may modulate TPMT activity. Here we present the allele frequencies of genetic modifiers of TPMT activity in a British Asian population, as well as the concordance between intermediate TPMT activity and ORF and VNTR genotypes in a predominantly Caucasian population. METHODS: VNTR type and ORF mutations were determined in two selected TPMT activity ranges, intermediate activity (4-8 U, 108 patients), normal (12-15 U, 53 patients) and in 85 British Asians. RESULTS: In British Asians, TPMT*3C was the prevalent mutant allele (four heterozygotes). One patient was heterozygous for TPMT*3A. Overall VNTR frequencies did not differ from Caucasians. Three new VNTR alleles were designated VNTR*6c, VNTR*6d, and VNTR*7c. Forty-one percent of patients with intermediate activity were heterozygous for a TPMT ORF mutation (3A, 2B, 1C). Marked linkage disequilibrium was noted between VNTR*6b - TPMT*3A (D' = 1), VNTR*4b - TPMT*3C (D' = 0.67) and VNTR*6a - TPMT*1 (D' = 1) alleles. As a result, significant differences (P < 0.05) in the distribution of Type A, B or the total number of repeats summed for both alleles, were found between the ORF heterozygous intermediate activity group and the wild-type intermediate or normal activity groups. No significant difference was found between the two wild-type groups. CONCLUSION: Our results suggest that TPMT gene VNTRs do not significantly modulate enzyme activity.     

Polymorphism of N-acetyltransferase 1 and correlation between genotype and phenotype in a Thai population

Objectives To analyze the common allele frequencies of the arylamine-N-acetyltransferase 1 (NAT1) and examine the relationship between genotype and phenotype in a Thai population.Methods Peripheral blood samples from 233 Thai individuals were analyzed for genotype using polymerase chain reaction with restriction fragment-length polymorphism assays and for phenotype by determination of NAT1 enzyme kinetics in leukocytes using para-aminobenozic acid as a specific substrate.Results Of 466 NAT1 alleles assayed, the frequency of the NAT1*4 allele (wild-type) was 0.504 (95%CI 0.458–0.551) and those of the NAT1*10, *3 and *11 alleles were 0.438 (0.392–0.484), 0.034 (0.02–0.055) and 0.024 (0.012–0.042), respectively. Neither NAT1*14A nor *14B alleles were found in this studied population. The activity of NAT1 enzyme from peripheral blood leukocytes determined in 47 subjects was found to vary widely. The intrinsic clearance and V_max values of NAT1 enzymes with genotypes NAT1 *4/*4, *10/*10 and *4/*10 were not significantly different.Conclusion The frequency distribution of the major NAT1 alleles in the Thai population has a similar pattern to some Asian populations; however, racial differences among Asian populations need further clarification.     

Phenotyping and genotyping study of thiopurine S-methyltransferase in healthy Chinese children: a comparison of Han and Yao ethnic groups

AIMS: Ethnicity is an important variable influencing drug response. Thiopurine S-methyltransferase (TPMT) plays an important role in the metabolism of thiopurine drugs. Previous population studies have identified ethnic variations in both phenotype and genotype of TPMT, but limited information is available within Chinese population that comprises at least 56 ethnic groups. The current study was conducted to compare both phenotype and genotype of TPMT in healthy Han and Yao Chinese children. METHODS: TPMT activity was measured in healthy Chinese children by a HPLC assay (n = 213, 87 Han Chinese and 126 Yao Chinese). Allele-specific polymerase chain reaction (PCR) and PCR-restriction fragment length polymorphism (RFLP) were used to determine the frequency of TPMT mutant alleles (TPMT*2, TPMT*3 A, TPMT*3B and TPMT*3C) in these children. RESULTS: There was no significant difference in the mean TPMT activity between Han and Yao Chinese children. A unimodal distribution of TPMT activity in Chinese children was found and the mean TPMT activity was 13.32 +/- 3.49 U ml(-1) RBC. TPMT activity was not found to differ with gender, but tended to increase with age in Yao Chinese children. TPMT*2, TPMT*3B and TPMT*3A were not detected, and only one TPMT*3C heterozygote (Han child) was identified in 213 Chinese children. Erythrocyte TPMT activity of this TPMT*3C heterozygote was 12.36 U ml(-1) RBC. The frequency of the known mutant TPMT alleles was 0.2%[1/426] in Chinese children. CONCLUSION: The frequency distribution of RBC TPMT activity was unimodal. The frequency of the known mutant TPMT alleles in Chinese Children is low and TPMT*3C appears to be the most prevalent among the tested mutant TPMT alleles in this population.     

Comprehensive analysis of thiopurine S-methyltransferase phenotype-genotype correlation in a large population of German-Caucasians and identification of novel TPMT variants

The thiopurine S-methyltransferase (TPMT) genetic polymorphism has a significant clinical impact on the toxicity of thiopurine drugs. It has been proposed that the identification of patients who are at high risk for developing toxicity on the basis of genotyping could be used to individualize drug treatment. In the present study, phenotype-genotype correlation of 1214 healthy blood donors was investigated to determine the accuracy of genotyping for correct prediction of different TPMT phenotypes. In addition, the influence of gender, age, nicotine and caffeine intake was examined. TPMT red blood cell activity was measured in all samples and genotype was determined for the TPMT alleles *2 and *3. Discordant cases between phenotype and genotype were systematically sequenced. A clearly defined trimodal frequency distribution of TPMT activity was found with 0.6% deficient, 9.9% intermediate and 89.5% normal to high methylators. The frequencies of the mutant alleles were 4.4% (*3A), 0.4% (*3C) and 0.2% (*2). All seven TPMT deficient subjects were homozygous or compound heterozygous carriers for these alleles. In 17 individuals with intermediate TPMT activity discordant to TPMT genotype, four novel variants were identified leading to amino acid changes (K119T, Q42E, R163H, G71R). Taking these new variants into consideration, the overall concordance rate between TPMT genetics and phenotypes was 98.4%. Specificity, sensitivity and the positive and negative predictive power of the genotyping test were estimated to be higher than 90%. Thus, the results of this study provide a solid basis to predict TPMT phenotype in a Northern European Caucasian population by molecular diagnostics.