TPMT

In vivo azathioprine is rapidly converted into its active metabolite 6-mercaptopurine by the enzyme thiopurine methyltransferase (TPMT). The active agent inhibits IMPDH function. Furthermore, it also acts as antimetabolite of the RNA and DNA synthesis particularly in T-lymphocytes leading to cell death. Due to genetic polymorphism of TPMT, therapy may fail, thus it is currently discussed whether individual patients should be monitored before the use of azathioprine. 

Mercaptopurine (6-MP) is an oral purine analog that is converted to a ribonucleotide to inhibit purine synthesis. Mercaptopurine is converted into thiopurine nucleotides, which are catabolized by thiopurine S-methyltransferase (TPMT), which is subject to genetic polymorphisms and may cause severe myelosuppression. TPMT status may be assessed prior to therapy to reduce drug-induced morbidity and the costs of hospitalizations for neutropenic events. Mercaptopurine is poorly absorbed, with a time to peak concentration of 1 to 2 hours after an oral dose. The half-life is 21 minutes in pediatric patients and 47 minutes in adults. Mercaptopurine is used in the treatment of acute lymphocytic leukemia and chronic myelogenous leukemia. Significant side effects include myelosuppression, mild nausea, skin rash, and cholestasis. When allopurinol is used in combination with 6-MP, the dose of 6-MP must be reduced by 66% to 75% of the usual dose because allopurinol blocks the metabolism of 6-MP. 

Seki T, Tanaka T, Nakamura Y. Genomic stmcture and multiple single-nucleotide polymorphisms (SNPs) of the thio-purine S-methyltransferase (TPMT) gene. J Hum Genet 2000 45 299-302. 

Reduced Tolerance to 6-MP in Patients with Genetic Impairment of TPMT Activity... 

TMPT activity in human erythrocytes is transmitted as an autosomic codominant trait [15] and is trimodally distributed, with 89-94% of the individuals having high, 6-11% intermediate, and 0.3% low activity [7, 15-17] (Figure 14.2). The measurement of TPMT activity in erythrocytes closely reflects the ability of bone marrow to inactivate 6-MP. TPMT activity is inversely related to erythrocyte 6-TGN levels [7, 13, 18, 19], and children with low TPMT activity and very high 6-TGN levels experienced profound myelotoxicity [20, 21]. Moreover, TPMT phenotype in erythrocyte reflects that in leukemic blasts [22]. Patients with intermediate TPMT activity had a 5-fold greater cumulative incidence of dose reductions than subjects with high activity [13], and TPMT activity has been inversely related to the time of treatment withdrawal due to cytopenia [21]. 

Ten TPMT variants associated with low enzyme activity have been described, and 7 PM7 2, TPMT 3A, and TPMT 3C account for about 80-95% of the TPMT-defi-... 

Krynetski EY, Fessing MY, Yates CR et al. Promoter and intronic sequences of the human thiopurine S-methyltransfer-ase (TPMT) gene isolated from a human PAC1 genomic library. Pharm Res 1997 14 1672-1678. 

Spire-Vayron de la Moureyre C, De-buysere H, Mastain B et al. Genotypic and phenotypic analysis of the polymorphic thiopurine S-methyltransferase gene (TPMT) in a European population. Br J Pharmacol 1998 125 879-887. 

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of thiopurine dmgs, such as 6-mercaptopurine (6-MP), 6-thioguanine and azathioprine, to inactive metabolites [29-32]. Thiopurines form part of the routine treatment for patients with acute lymphoblastic leukemia, rheumatoid arthritis, and autoimmune diseases such as SLE and Crohn s disease, and are used as an immunosuppressant following organ transplantation. 

Interethnic variability in RBC TPMT activity has been reported in several populations. RBC TPMT was 29% higher in Saami subjects in Northern Norway compared to white subjects from the same geographic region [41]. African American subjects have 17-33% lower RBC TPMT activity than American white subjects [34, 42]. The TPMT activity in African and white Americans was substantially lower than that reported in 119 Chinese subjects [43]. 

The association between low TPMT activity and excessive hematological toxicity has been recognized [31, 35, 37]. Molecular analysis of the TPMT genotype is able to identify patients at risk for acute toxicity from thiopurines. A recent study involving 180 children identified that the TPMT genotype plays an important role in a patients tolerance to 6-MP therapy [51]. Two of the patients, who were TPMT-de-... 

Fig. 24.1 Variant alleles at the human TPMT locus. Grey boxes are exons containing mutations. White boxes are untranslated regions and black boxes represent exons in the ORF. Dashed box represents exon 2, which was detected in one of 16 human liver cDNAs (adapted from [30]).

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... 

Fig. 24.2 TPMT 3C allele frequency (%) in different ethnic groups. 

TPMT 3C accounted for 100% of the mutant alleles observed in the Ghanaian subjects. This is similarly found in 101 Kenyans and 192 Chinese subjects, as well as in Sudanese and Filipino subjects (Table 24.1 Figure 24.2) [52, 54]. This contrasts with the Caucasian (British, American, French) subjects, where 5.7, 5.5 and 11.4% of variant alleles were TPMT 3C, respectively (Table 24.1). TPMT 3 A was not detected in the African or Asian populations, but accounted for 84.9, 81.4 and 88.9% of variant alleles in British, American, and French Caucasians, respectively. Therefore, mutations at nucleotide 719 (TPMT 3C) is common in all populations studied to date, but occurs most often in the presence of a simultaneous mutation at nucleotide 460 (TPMT 3A) in Caucasian subjects (Table 24.1). 

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-... 

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