Cat Red Blood Cell Thiopurine S-Methyltransferase: Companion Animal Pharmacogenetics

A common genetic polymorphism for thiopurine S-methyltransferase (TPMT) is a major factor responsible for individual variation in the toxicity and therapeutic efficacy of thiopurine drugs in humans. We set out to determine whether inheritance might also influence the level of TPMT activity in the domestic cat, Felis domesticus. As a first step, red blood cell (RBC) TPMT activity was measured in blood samples from 104 cats. The average level of cat RBC TPMT activity was lower than that observed in humans and was not related to either age or sex of the animal. We then cloned and characterized the F. domesticus TPMT cDNA and gene. Genotype-phenotype correlation analysis was performed by resequencing the cat TPMT gene using DNA samples from 12 animals with high and 12 with low levels of RBC TPMT activity. Thirty-one single nucleotide polymorphisms (SNPs) were observed in these 24 DNA samples, including five that altered the encoded amino acid, resulting in nine allozymes (six observed and three inferred). Twelve of the 31 feline TPMT SNPs were associated, collectively, with 56% of the variation in level of RBC TPMT activity in these 24 animals. When those 12 SNPs were assayed in all 89 cats for which DNA was available, 30% of the variation in level of RBC TPMT activity was associated with these 12 polymorphisms. After expression in COS-1 cells, five of the eight variant cat allozymes displayed decreased levels of both TPMT activity and immunoreactive protein compared with the wild-type allozyme. These observations are compatible with the conclusion that inheritance is an important factor responsible for variation in levels of RBC TPMT activity in the cat. They also represent a step toward the application of pharmacogenetic principles to companion animal thiopurine drug therapy. Thiopurine drugs such as 6-mercaptopurine (6-MP) and azathioprine, which is converted to 6-MP in vivo, are used to treat both humans and companion animals, including the domestic cat, Felis domesticus (Paterson and Tidd, 1975; Lennard, 1992; White et al., 2000). In humans, these drugs are used as cytotoxic agents to treat neoplasia and as immune suppressants (Paterson and Tidd, 1975; Lennard, 1992), with a similar spectrum of therapeutic applications in companion animals (Beale, 1988). However, thiopurine drugs have a relatively narrow therapeutic index and are capable of causing life-threatening toxicity, most often myelosuppression (Paterson et al., 1975; Lennard, 1992). Thiopurines also provide an excellent example of the successful use of pharmacogenetics to individualize human drug therapy (Weinshilboum et al., 1999; Weinshilboum, 2001). That is true because a genetic polymorphism for thiopurine S-methyltransferase (TPMT) (Weinshilboum and Sladek, 1980), an enzyme that catalyzes the S-methylation of these drugs (Remy, 1963; Woodson and Weinshilboum, 1983), plays an important role in individual variation in their toxicity and therapeutic efficacy (Weinshilboum et al., 1999; Wein-