TPMT allele

Based on the population genotype-phenotype studies performed to date, assays for the molecular diagnosis of TPMT deficiency have focussed on alleles TPMT 2, TPMT 3A and TPMT 3C, as these represent 80-95% of all mutant alleles of this gene in Caucasians [46, 50]. However, the frequency and pattern of mutant alleles of this gene is different among various ethnic populations. For example, Southwest Asians (Indian, Pakistani) have a lower frequency of mutant TPMT alleles and all mutant alleles identified to date are TPMT 3A (Table 24.1) [52]. This is in contrast to Kenyans and Ghanaians where the frequency of mutant alleles is similar to Caucasians, and all mutant alleles are TPMT 3C (Table 24.1) [53, 54]. Among African Americans, TPMT 3C is the most prevalent allele, but TPMT 2... 

The presence of TPMTk3A alleles in the African American population is consistent with the genetic mixing that has been identified through historical and molecular analysis [12, 27]. These data, compared with that of different ethnic populations, reveal that the pattern of variant TPMT alleles differs significantly be-... 

Eight TPMT alleles have been identified. Three of these alleles, TPMT 2 (45), 3A, and 3C (G460A), account for 80-95% of intermediate or low enzyme activity cases. High concentrations of variant TPMT (TPMT 3A, 3B, and 3C) are found in patients with decreased TPMT activity Patients with TMPT-3A have a complete loss of TPMT catalytic activity, patients with TPMT 3B have a 9-fold reduction, and those with TPMT 3C have a 1.4-fold reduction (43,46). The frequency of loss of TPMT activity (TPMT 3A) appears to vary with ethnicity but not with gender or age. In addition, haplotyping methods have been developed to discriminate the genotypes TPMT 1/ 3A (intermediate metabolizer) and TPMT 3B/ 3C (poor metabolizer) (47). 

In one study, 68 patients with rheumatic disease on AZA (2 to 3 mg/kg/d) were genotyped for TPMT and TPMT3A alleles. All patients were assessed for side effects from AZA, such as leukopenia, liver function abnormalities, and GI intolerance. Of these patients, 6 (9%) patients were heterozygous for TPMT3A, of whom 5 discontinued AZA within 4 weeks of starting the medication because of hematologic toxicity (48). In another study, 40 RA patients on AZA (0.7 to 1.4 mg/ kg/d) were genotyped for the TPMT alleles. Of the 40 patients, 6 discontinued... 

AZA because of toxicity. Of the 6 patients with severe GI toxicity, 3 were heterozygous for the TPMT3A allele the remainder possessed the wild-type TPMT allele. The correlation between the TPMT allele and AZA toxicity was significant, p =0.018. Based on the resnlts of this stndy, the positive predictive valne for a TPMT polymorphism carrier was 60% (49). 

There are several known polymorphisms in TPMT (4). Alleles TPMT 2, TPMT 3A, and TPMT 3C account for up to 95% of reduced TPMT activity. Patients heterozygous for these alleles have intermediate TPMT levels (5), and patients homozygous for the variant TPMT alleles are at high risk for severe, sometimes life-threatening, toxicity requiring significant reductions in mercaptopurine doses (5). 

PhenotypicaUy Relevant Variant TPMT Alleles Reported in the Literature ... 

With relevance to the administration of thiopurines in the early course of childhood ALL, the BFM Study Group reported on the association of TPMT genotype and minimal residual disease (MRD) in 810 children with childhood ALL enrolled into their trial ALL-BFM 2000 (206). In this trial, DNA-based MRD analysis after induction and after consolidation treatment was used for risk-adapted treatment stratification. A 4-week cycle of 6-MP was applied in-between these two MRD measurements. In patients homozygous for the TPMT 1 allele or those heterozygous for a variant TPMT allele, MRD levels on treatment day 33 were equally distributed between the groups. However, when MRD levels were assessed on treatment day 78, after administration of consolidation... 

In contrast, successful TPMT genotyping of 72 patients out of a total of 115 patients with subsequent secondary malignant neoplasms after treatment for childhood ALL on seven consecutive BFM protocols (ALL-BFM 79, 81, 83, 86, 90, 95, and 2000) did not reveal a higher frequency of TPMT alleles associated with lower TPMT activity among these patients (208). Also, in stratified analyses by entities of secondary malignant neoplasms, no significant associations with TPMT alleles conferring lower enzyme activity have been observed. 

Haglund S, Lindqvist M, Aimer S et al. Pyrosequencing of TPMT alleles in a general Swedish population and in patients with inflammatory bowel disease. Clin Chem 2004 50 288-295. Erratum in Clin Chem 2004 50 788. 

Figure 43-3 Thiopurine S-methyltransferase (TPMT) allele variants. Gray boxes represent mutations that result in amino acid changes.TPMT 4 is a 5 splice site mutation for exon 10 that does not alter an amino acid. White boxes represent untranslated regions. Black boxes represent exons in the open reading frame.The dashed box represents exon 2, which was detected in 6.25% of human liver cDNAs during initial evaluation. (From McLeod HL, Siva C. The thiopurine S-mefhy/transferose gene locus—implications for clinical pharmacogenomics. Pharmacogenomics 2002 3 89-98. Reproduced by permission from future Medicine Ltd [London].)...

Population studies have found that approximately ll Pbof Caucasians are heterozygous and 0.3% homozygous for TPMT deficiency (73). For the TPMT polymorphism, all patients who inherit two non-functional TPMT alleles will develop dose-limiting hematopoi-... 

In a retrospective cohort study in 71 Indian children with acute lymphoblastic leukemia, the median dose of mercaptopurine during the maintenance therapy was 31% lower among patients with heterozygous TPMT alleles, as clinicians were advised to adjust the dose to keep the total leukocyte count in a predefined range [171 ]. However, the incidence of common adverse reactions, including febrile episodes, vomiting, diarrhea, mucositis, and increased aminotransferases, were comparable in patients with the TPMT mutant and wild-type alleles. 

The frequencies of TPMT mutant alleles have been studied retrospectively in 147 Japanese patients with inflammatory bowel disease taking azathioprine, of whom 144 were wild-type for TPMT (TPMT 1/ 1) and three carried a mutant TPMT allele (TPMT 1/ 3C) [145 ]. The incidence of adverse reactions to azathioprine was 38/ 114 in the wild-type group. Leukopenia occurred in 16% of the patients with wild-type TPMT. The authors concluded that determination of TPMT genotype may not be useful in Japanese patients in predicting adverse reactions to azathioprine. 

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