Thiopurine S-methyltransferase deficiency : Two nucleotide transitions define the most prevalent mutant allele associated with loss of catalytic activity in Caucasians

• Published in: American journal of human genetics Vol. 58; no. 4; pp. 694 - 702
• Main Authors: HUNG-LIANG TAI, KRYNETSKI, E. Y, YATES, C. R, LOENNECHEN, T, FESSING, M. Y, KRYNETSKAIA, N. F, EVANS, W. E
• Format: Journal Article
• Language: English
• Published: Chicago, IL University of Chicago Press 01.04.1996
• Subjects: Amino Acid Sequence; Base Sequence; Biological and medical sciences; Child, Preschool; Cloning, Molecular; DNA, Complementary - genetics; DNA, Neoplasm - analysis; Enzyme Stability; Errors of metabolism; Erythrocytes - enzymology; European Continental Ancestry Group - genetics; Gene Frequency; Humans; Kinetics; Male; Medical sciences; Mercaptopurine - metabolism; Metabolic diseases; Methylation; Methyltransferases - deficiency; Methyltransferases - genetics; Methyltransferases - metabolism; Miscellaneous hereditary metabolic disorders; Molecular Sequence Data; Point Mutation - genetics; Polymorphism, Restriction Fragment Length; Precursor Cell Lymphoblastic Leukemia-Lymphoma; Recombinant Proteins - biosynthesis; Recombinant Proteins - metabolism; RNA, Messenger - analysis; S-Adenosylmethionine - metabolism; Yeasts - genetics; Human; Pharmacogenetics; Enzyme; Transferases; Deficiency; Metabolic diseases; Enzymatic catalysis; Genetic disease; Phenotype; Methyltransferases; Thiopurine S-methyltransferase; Mutation; Caucasoid; Polymorphism
• ISSN: 0002-9297; 1537-6605

The autosomal recessive trait of thiopurine S-methytransferase (TPMT) deficiency is associated with severe hematopoietic toxicity when patients are treated with standard doses of mercaptopurine, azathioprine, or thioguanine. To define the molecular mechanism of this genetic polymorphism, we cloned and characterized the cDNA of a TPMT-deficient patient, which revealed a novel mutant allele (TPMT*3) containing two nucleotide transitions (G460-->A and A719-->G) producing amino acid changes at codons 154 (Ala-->Thr) and 240 (Tyr--> Cys), differing from the rare mutant TPMT allele we previously identified (i.e., TPMT*2 with only G238-->C). Site-directed mutagenesis and heterologous expression established that either TPMT*3 mutation alone leads to a reduction in catalytic activity (G460-->A, ninefold reduction; A719-->G, 1.4-fold reduction), while the presence of both mutations leads to complete loss of activity. Using mutation specific PCR-RFLP analysis, the TPMT*3 allele was detected in genomic DNA from approximately 75 percent of unrelated white subjects with heterozygous phenotypes, indicating that TPMT*3 is the most prevalent mutant allele associated with TPMT-deficiency in Caucasians.