TPMT deficiency

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... 

Deficiency of thiopurine S-methyl transferase (TPMT) is another phenotype that exhibits inter-ethnic differences in frequency. TPMT is an enzyme that catalyzes methylation of therapeutic agents used in the treatment of acute lymphoblastic leukemia, rheumatoid arthritis, and autoimmune/inflammatory diseases, as well as in organ transplantation. Patients who have TPMT deficiency experience less efficient methylation and are at greater risk of fatal toxicity when treated with standard doses of fhiopurines. TPMT phenotype is defined by erythrocyte 6-mercapto-purine methylation. African American populations exhibit a 20% lower erythrocyte TPMT than Caucasian Americans, and persons of Chinese descent tend to exhibit greater activity than either of these other American subpopulations. 

Tests for the TPMT genotype and phenotype are commercially available. Attention should be paid for those patients who test negative for TPMT status. Patients with poor or intermediate TPMT activity may tolerate only 1/10 to 1/2 of the average 6-MP dose. A pharmacoeconomic model has been developed to analyze the potential cost of screening to prevent azathioprine toxicity. In this model, it was assumed that TPMT deficiency is present in 0.3% of the population, that intermediate activity is present in 11%, and that both groups have an increased risk of developing myelosuppression. Under these circumstances, the model predicted that the costs per Caucasian patient for the first 6 mo of therapy with screening are lower... 

Other stndies have examined the association between the activity of TPMT and other enzymes in the pnrine pathway and AZA toxicity. In one stndy, TPMT, HPRT, 5 -nncleotidase, and pnrine nncleoside phosphorylase activity in the RBCs of 33 RA patients on AZA (dose of approximately 2mg/kg/d) and 66 controls was meas-nred. Compared to patients with normal TPMT activity, 14 RA patients with low (p = 0.004) and 7 patients with intermediate TPMT activity (RR 3.1) developed AZA toxicity(4d). None of the patients were genotyped. Another study measured TPMT activity in 3 RA patients who had experienced AZA-induced hematologic toxicity and 16 RA patients withont AZA toxicity. In this study, 2 patients with AZA-indnced hematologic toxicity were TPMT deficient, one partial and the other complete (50). Patients were not genotyped in either of these studies. 

Thiopurine methyltransferase methylates 6-mercaptopurine, a commonly used treatment for childhood acute lymphocytic leukemia, reducing its conversion to the active form of the drug. Approximately 10% of patients have intermediate enzyme activity, and 0.3% are deficient for TPMT activity. Intermediate activity patients have a greater incidence of thiopurine toxicity, whereas TPMT-deficient patients have severe or fatal hematological toxicity from 6-mercaptopurine therapy. In one study, patients deficient for TPMT tolerated only 7% of a 2.5-yr mercaptopurine treatment regimen. Patients with intermediate TPMT activity tolerated 65% of total weeks of therapy and patients with normal TPMT activity tolerated 84% of total weeks of therapy (3). 

In this study, 21 patients had a heterozygous TPMT phenotype. With a frequency of 85%, TPMT 3A was the most prevalent variant allele, followed by TPMT 2 and TPMT 3C with about 5% each. All 6 patients who phenotypically displayed TPMT deficiency had two mutant alleles 20 of the 21 patients with intermediate TPMT activity had one variant allele and all of the selected 21 patients with high activity did not carry one of the tested JPMT variant alleles. Thus, the major inactivating TPMT variants can be detected reliably by a PCR-based method and demonstrated an excellent concordance with TPMT phenotype. 

The most comprehensive analysis of TPMT phenotype versus genotype published to date was conducted by Schaeffeler et al. (140). In their study, RBC TPMT activity and genotype TPMT 2 and 3 alleles) were analyzed in 1214 healthy Caucasian blood donors. Discordant cases between phenotype and genotype were systematically sequenced. The frequencies of the mutant alleles were 4.4% for TPMT 3A, 0.4% for TPMT 3C, and 0.2% for TPMT 2. All seven TPMT-deficient subjects identified by Schaeffeler and colleagues were homozygous or compound heterozygous carriers for these alleles. 

Schaelfeler E, Stanulla M, Greil J et al. A novel TPMT missense mutation associated with TPMT deficiency in a 5-year-old boy with ALL. Leukemia 2003 17 1422-1424. 

The TPMT gene is located on chromosome 6 and includes 10 exons (68). TPMT" 3A, the most common mutated allele, contains two point mutations in exons 7 (G460A and Alal54Thr) and 10 (A719G and Tyr240Lys). Two other alleles contain a single mutation, the first SNP (TPMT SB) and the second SNP (TPMT 3C) (69). Aarbakke et al. (70) have reviewed the variant alleles of the TPMT gene and the relationship to TPMT deficiency. In Caucasians, TPMT 3A accounts for about 85% of mutated alleles, and in such populations the analysis of the known alleles may... 

Of 78 patients treated with azathioprine for systemic lupus erjdhematosus, 10 developed azathioprine-associated reversible neutropenia (93). Only one of these patients was homozygous for TPMT deficiency, but he had the most severe episode (aplastic anemia). 

Numerous studies have shown that TPMT-deficient patients are at high risk for severe, and sometimes fatal, hematological toxicity. 

Genotyping tests, however, may not detect TPMT deficiency in all patients, particularly those with a rare unknown but clinically significant variant. Compound 1 erozygotes also present a challenge to PCR-based ge ... 

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-... 

TPMT 6-MP Dose-limiting hematopoietic More in TPMT deficiency 39-41... 

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