Human thiopurine S-methyltransferase pharmacogenetics: Variant allozyme misfolding and aggresome formation

Liewei Wang, Tien V. Nguyen, Richard W. McLaughlin, Laura A. Sikkink, Marina Ramirez-Alvarado, Richard M. Weinshilboum

Research output: Contribution to journalArticlepeer-review

83 Scopus citations


Thiopurine S-methyltransferase (TPMT) catalyzes the S-methylation of thiopurine drugs. TPMT genetic polymorphisms represent a striking example of the potential clinical value of pharmacogenetics. Subjects homozygous for TPMT*3A, the most common variant allele for low activity, an allele that encodes a protein with two changes in amino acid sequence, are at greatly increased risk for life-threatening toxicity when treated with standard doses of thiopurines. These subjects have virtually undetectable levels of TPMT protein. In this study, we tested the hypothesis that TPMT*3A might result in protein misfolding and aggregation. We observed that TPMT*3A forms aggresomes in cultured cells and that it aggregates in vitro, functional mechanisms not previously described in pharmacogenetics. Furthermore, there was a correlation among TPMT half-life values in rabbit reticulocyte lysate, aggresome formation in COS-1 cells, and protein aggregation in vitro for the three variant allozymes encoded by alleles that include the two TPMT*3A single-nucleotide polymorphisms. These observations were compatible with a common structural explanation for all of these effects, a conclusion supported by size-exclusion chromatography and CD spectroscopy. The results of these experiments provide insight into a unique pharmacogenetic mechanism by which common polymorphisms affect TPMT protein function and, as a result, therapeutic response to thiopurine drugs.

Original languageEnglish (US)
Pages (from-to)9394-9399
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number26
StatePublished - Jun 28 2005


  • Pharmacogenomics
  • Protein aggregation
  • Protein degradation
  • Thiopurine toxicity

ASJC Scopus subject areas

  • General


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