Functional characterization of 23 allelic variants of thiopurine S-methyltransferase gene (TPMT*2 - *24)

Objective Thiopurine S-methyltransferase (TPMT) is an enzyme responsible for the detoxification of the widely used thiopurine drugs. TPMT is genetically polymorphic and is associated with large interindividual variations in thiopurine drug toxicity and therapeutic efficacy. In this study, we performed an in-vitro analysis of TPMT variant alleles, namely, TPMT*2, *3A, *3B, *3C, *5, *6, *7, *8, *9, *10, *11, *12, *13, *14, *16, *17, *18, *19, *20, *21, *22, *23, and *24. Methods The wild-type TPMT proteins, TPMT.1 and 23 variants were heterologously expressed in COS-7 cells, and the kinetic parameters K_m, V _max, and intrinsic clearance (V_max/K_m) of 6-thioguanine S-methylation were determined. Results The expression levels of TPMT.2, TPMT.3A, TPMT.5, TPMT.12, TPMT.14, and TPMT.22 were considerably lower than that of TPMT.1 (P<0.005), and that of TPMT.18 was slightly reduced (P<0.05). The kinetic parameters of TPMT.3A, TPMT.3B, TPMT.5, TPMT.14, TPMT.18, TPMT.21, and TPMT.22 could not be accurately established because of no activity in 6-thioguanine S-methylation. The V_max/K_m values of TPMT.2, TPMT.7, TPMT.17, and TPMT.24 were displayed less than 10% of the wild-type. Conclusion This functional analysis with respect to TPMT variants could provide useful information for individualization of thiopurine drugs therapy.