thiopurine S-methyltransferase; thiopurine methyltransferase (TPMT)
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Function
- catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine
Structure
- monomer
- belongs to the methyltransferase superfamily, TPMT family
Compartment
Pathology
- defects in TPMT are the cause of:
- thiopurine S-methyltransferase deficiency
- disruption of normal metabolic inactivation of thiopurine drugs
Polymorphism
- individual variation in the toxicity & therapeutic efficacy of thiopurine drugs is associated with a common genetic polymorphism that controls levels of TPMT activity
- genetic polymorphism in the TPMT gene is such that ~90% of Caucasians have high TPMT activity, 10% have intermediate activity & 1 in 300 individuals has low activity
- TPMT activity varies among ethnic groups
- TPMT*3A is the only mutant allele found in South West Asians; this is also the most common mutant allele in Caucasians, but is not found in Chinese
- all mutant alleles identified in the Chinese population were TPMT*3C; this allele is found at a low frequency in Caucasians; this suggests that TPMT*3C is the oldest mutation, with TPMT*3B being acquired later to form the TPMT*3A allele in the Caucasian & South West Asian populations
- TPMT*2 appears to be a more recent allele, which has only been detected in Caucasians to date
- ethnic differences may be important in the clinical use of thiopurine drugs