Human thiopurine methyltransferase pharmacogenetics: Gene sequence polymorphisms*

Abstract

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of thiopurine drugs. TPMT activity is regulated by a common genetic polymorphism that is associated with large individual variations in thiopurine toxicity and efficacy. We previously cloned the functional gene for human TPMT and reported a common variant allele for low enzyme activity, TPMT*3A, that contains point mutations at cDNA nucleotides 460 and 719. In the present study, we set out to determine the number, types, and frequencies of TPMT variant alleles associated with low enzyme activity in clinical laboratory samples in the United States and to compare those results with data obtained from two different ethnic groups. We identified a total of six different variant alleles for low TPMT activity in the 283 clinical laboratory samples studied. The most common variant was *3A; the second most frequent variant allele, *3C, contained only the nucleotide 719 polymorphism; and four other variant alleles were detected. TPMT*3A also appeared to be the most common variant allele in a Norwegian white population sample, but it was not found in a population sample of Korean children. However, *3C was present in samples from the Korean children, as was novel allele, *6. Characterization of variant alleles for low TPMT enzyme activity will help make it possible to assess the potential clinical utility of deoxyribonucleic acid-based diagnostic tests for determining TPMT genotype.

Thiopurine S-methyltransferase pharmacogenetics: insights, challenges and future directions

“…Eighteen of those polymorphisms (*2, *3A, *3B, *3C, *5-*14 and *16-*19) involve nonsynonymous coding single-nucleotide polymorphisms (cSNPs) (Krynetski et al, 1995;Szumlanski et al, 1996;Otterness et al, 1997;Lindqvist et al, 2004;Schaeffeler et al, 2004;Hamdan-Khalil et al, 2005), that is, alterations in single DNA nucleotides that alter the encoded amino acid. The *14 SNP disrupts the translation initiation codon (Lindqvist et al, 2004) and prevents translation of the enzyme protein , while *4 and *15 involve alterations in canonical mRNA splice site sequences (Otterness et al, 1998;Lindqvist et al, 2004) and *3D contains a premature stop codon (Otterness et al, 1997). There is also a polymorphic GC-rich variable number of tandem repeats (VNTR) present in the 5 0 -flanking region of TPMT that can vary from 3 to 9 repeat elements (SpireVayron de la Moureyre et al, 1998Moureyre et al, , 1999Yan et al, 2000).…”

“…Population studies conducted among European, Caucasian, East and West African and East Asian populations have demonstrated significant ethnic differences in TPMT pharmacogenetics (Otterness et al, 1997;Collie-Duguid et al, 1998;Hon et al, 1999). TPMT *3A is the most common variant allele in Caucasians, with a frequency of approximately 5%, and -as a result -this allele accounts for the majority of variant alleles in white populations in the US and Northern Europe.…”

Human Erythrocyte Proteinl-Isoaspartyl Methyltransferase: Heritability of Basal Activity and Genetic Polymorphism for Thermal Stability

“…Blood was also donated by 20 healthy, genetically unrelated Korean children and processed as described above for the family studies on the levels of these methyltransferases Norwegian samples. The samples from the Norwegian and Korean as well as RBC histamine N-methyltransferase (29) donors were originally obtained for another, independent investigahave provided important insights into their patterns of tion of thiopurine methyltransferase activity (35). We found no siginheritance and have elucidated the molecular basis nificant correlation between PIMT and thiopurine methyltransferase activities or thermal stabilities (data not shown).…”

Azathioprine Pharmacogenetics: The Relationship between 6-Thioguanine Nucleotides and Thiopurine Methyltransferase in Patients after Heart and Kidney Transplantation

“…Brythrocyte thiopurine methyltransferase of randomly selected healthy Europeans typically shows a biphasic frequency distribution, consistent with a codominant inheritance (12,13). One single point mutation, which leads to a substantial reduction of catalytic activity has been described (18) and recently a predominant 2-point mutation, which is present in about 70% of thiopurine methyltransferase deficient subjects was discovered (19). Further factors complicating therapeutic management of thiopurines are their widely variable bioavailability (20,21) and an increase of thiopurine methyltransferase activity during therapy consistent with an induction phenomenon (10,22).…”