Pharmacogenetic studies of thiopurines : focus on thiopurine methyltransferase

Abstract: Pharmacogenetics represents the study ofvariability in drug response due to genetic variations. The thiopurines (6-mercaptopurine, 6-thioguanine and azathioprine) are prodrugs which require metabolic transformation to exert effect. Thiopurines are used in inflammatory bowel disease, as maintenance treatment of childhood acute lymphoblastic leukaemia, and for immunosuppression after transplantation. The metabolism is complex and one important enzyme involved is thiopurine methyltransferase (TPMT). Inherited variation in TPMT activity is one factor responsible for individual differences in susceptibility to thiopurine-induced toxicity or in the therapeutic response to thiopurines. The enzyme activity is under controi of agenetic polymorphism. The frequency distribution of TPMT activity in Caucasians is trimodal, with 89% having high enzyme activity, 10% having intermediate activity and l of 300 having almost undetectable activity. The TPMT gene has been characterised and several single nucleotide polymorphisms (SNPs) identified causing decreased enzyme activity. The most common SNPs are TPMT'2, TPMT'3A and TPMT'3C.In the investigations for this thesis we have studied the pharmacogenetics of thiopurines with focus on TPMT. A real-time RT-PCR method was developed for quantification of TPMT gene expression, and a pyrosequencing method was developed for genotyping of TPMT SNPs. TPMT gene expression correlated to enzyme activity in individuals with high enzyme activity. The allele frequencies of TPMT'3A and TPMT'3C in samples from 800 Swedish individuals were in agreement with those in other Caucasian populations, although TPMT'3B was more common and TPMT'2 was rarer.We investigated the concordance between genotype and phenotype and found discordance between genotype and phenotype in two unrelated patients. In these patients, we detected two new sequence variants, TPMT'14 and TPMT'15, which lead to a non-functional TPMT enzyme. The TPMT genotype and phenotype were also determined in the parents of the two patients and the inheritance of these alleles was investigated.Sixty patients with inflammatory bowel disease following a standardised dose escalation schedule of azathioprine or 6-mercaptopurine were closely monitored over the course of 20 weeks. During treatment, the TPMT gene expression decreased. In contrast, TPMT enzyme activity did not change. TPMT heterozygous patients had a lower probability of remaining in the 20week study. Forty-five percent of the patients were withdrawn due to adverse events, but 67% of these tolerated a lower dose of thiopurines. The inosine triphosphate pyrophosphatase polymorphism (ITPA 94C>A) was not associated with occurrences of adverse events.