Thiopurine S-methyltransferase (TPMT) pharmacogenetics: variant allele functional genomics

Abstract

TPMT genetic polymorphisms influence thiopurine drug toxicity and efficacy. We set out to perform functional genomic studies of 7 human TPMT variant alleles, TPMT*5: to *11:, that had not previously been studied by expression in mammalian cells. After expression in COS-1 cells, allozymes encoded by these alleles displayed from 2.3% (*5:) to 97.5% (*7:) of the level of TPMT activity–corrected for transfection efficiency–found with the wild type (WT) allele. TPMT*5:, *6: and *11: all displayed less than 50% of WT activity. Quantitative Western analysis showed that level of immunoreactive protein correlated closely with level of activity for all allozymes except TPMT*6:. However, substrate kinetic studies of all of the recombinant allozymes showed that *6: had significantly elevated apparent Km values for both cosubstrates for the reaction, 6-mercaptopurine and S-adenosyl-L-methionine, when compared with WT. These results indicate that some of the TPMT alleles that have been reported to be associated with clinical consequences do not appear to be functionally impaired after expression in mammalian cells. Clinical Pharmacology & Therapeutics (2004) 75, P19–P19; doi: 10.1016/j.clpt.2003.11.072