Characterisation of novel defective thiopurine S-methyltransferase allelic variants

Mercaptopurine treatment should be considered in azathioprine intolerant patients with inflammatory bowel disease

“…[5][6][7][8][9] Studies have shown that patients with a very low enzyme activity are at risk for severe, and sometimes fatal, myelotoxicity and patients who are TPMT heterozygous for one allele with low activity have an intermediate risk for myelotoxicity. [10][11][12][13][14] Whether other types of adverse event (AE) in IBD patients can be attributed to the TPMT polymorphism is less clear.…”

Association of polymorphism of Enzyme Thiopurine Methyl Transferase in Head & Neck Squamous Cell Cancer and treatment response to Concurrent Chemo Radiotherapy

“…The best known recognized examples (TPMT, CYP2D6) are genetic polymorphisms of drug metabolizing enzymes which affect about 30% of all drugs [13]. TPMT activity in humans is inherited as an autosomal codominant trait [14] .…”

Thiopurine S-methyltransferase - characterization of variants and ligand binding

“…Although the precise circumstances have not been determined, deprotonization through water mediated hydrogen bonding has been proposed [9,27]. Despite the agreement between the calculated activation energy and observed reaction rates of the murine enzyme, the published kinetic parameters of human TPMT vary significantly, as does the source of protein, method of purification, choice of activity assay, reaction conditions (including choice of thiopurine substrate and reactant concentrations), use of units, and general presentation of results [24,28,29,30,31,32]. Both SAH and 6-Me MP coproducts inhibit the enzyme, albeit through different mechanisms; SAH is a potent competitive inhibitor with a K i of 0.75 µm with regard to SAM, and 6-Me MP is a non-competitive inhibitor with a K i of 560 µm with regard to 6-MP [10].…”

“…Most of the known protein variants exhibit decreased in vivo enzyme activity towards thiopurine substrates, but the molecular causes of reduced functionality have only been investigated for a handful of protein variants (to date), namely TPMT*2 (A80P) [33], TPMT*3A (A154T/Y240C) [34,35], TPMT*3B (A154T) [34,35], TPMT*3C (Y240C) [34,35], TPMT*5 (L49S) [33], TPMT*16 (R163H) [30], TPMT*21 (L69V) [29], TPMT*24 (Q179S) [29], TPMT*25 (C212R) [29], Introduction and TPMT*31 (I204T) [31]. The variant proteins are often found to be destabilized (due to amino acid substitution), making them susceptible to proteolytic degradation by the quality control system of the cell [34,35,36], although one exception is TPMT*5, which exhibits nearly complete loss of function rather than reduced structural stability [33].…”

“…In the crystal structure (PDB ID:2BZG), R215 packs against the side chains of L217 (β8) and L241 (β9), although these interactions could potentially be maintained in TPMT*8 since the Histidine side chain is of adequate length. Molecular modeling simulations have shown that TPMT has an inherent surface dipole moment [29], but substitution of Arginine to Histidine does not alter side chain polarity, and should therefore not affect the electrostatic properties of the protein surface. Although it cannot be completely ruled out that there might be perturbations to local structure, the stability and functionality of TPMT*8 are not affected by mutation, and the decreased in vivo activity of this variant cannot be explained by our findings.…”