Thiopurine S-methyltransferase pharmacogenetics: autophagy as a mechanism for variant allozyme degradation

Pharmacogenet Genomics. 2008 Dec;18(12):1083-94. doi: 10.1097/FPC.0b013e328313e03f.


Objective: Thiopurine S-methyltransferase (TPMT)*3A is degraded much more rapidly than is the 'wild-type' enzyme through a ubiquitin-proteasome-dependent process. It also forms aggresomes, suggesting a possible dynamic balance between degradation and aggregation. We set out to identify genes encoding proteins participating in these processes.

Methods: Green fluorescent protein tagged TPMT*3A was expressed in a Saccharomyces cerevisiae gene deletion library, and flow cytometry was used to screen for cells with high fluorescence intensity, indicating the loss of a gene essential for TPMT*3A degradation.

Results: Twenty-four yeast genes were identified in functional categories that included ubiquitin-dependent protein degradation, vesicle trafficking, and vacuolar degradation. The presence of genes encoding proteins involved in vesicular transport and vacuolar degradation suggested a possible role in TPMT*3A degradation for autophagy--a process not previously identified as a pharmacogenomic mechanism. In support of that hypothesis, TPMT*3A aggregates increased dramatically in mutants for vacuolar protease and autophagy-related genes. Furthermore, TPMT*3A expression in human cells induced autophagy, and small interfering RNA-mediated knockdown of ATG7, an autophagy-related human protein, enhanced TPMT*3A aggregation but not that of TPMT*3C or wild-type TPMT, indicating that autophagy contributes to TPMT*3A degradation in mammalian cells. We also demonstrated that UBE2G2, the human homologue of the E2 ubiquitin-conjugating enzyme identified during the yeast genetic screen, was involved in TPMT*3A degradation in human cells.

Conclusion: These results indicate that autophagy should be considered among mechanisms responsible for the effects of pharmacogenetically significant polymorphisms that alter encoded amino acids.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Autophagy*
  • Cell Line
  • Dyneins / metabolism
  • Flow Cytometry
  • Gene Deletion
  • Genes, Fungal
  • Green Fluorescent Proteins / metabolism
  • Humans
  • Isoenzymes
  • Methyltransferases / chemistry
  • Methyltransferases / genetics*
  • Methyltransferases / metabolism*
  • Microtubules / metabolism
  • Molecular Chaperones / metabolism
  • Protein Processing, Post-Translational*
  • Protein Structure, Quaternary
  • Recombinant Fusion Proteins / metabolism
  • Saccharomyces cerevisiae / cytology
  • Saccharomyces cerevisiae / enzymology*
  • Saccharomyces cerevisiae / genetics


  • Isoenzymes
  • Molecular Chaperones
  • Recombinant Fusion Proteins
  • Green Fluorescent Proteins
  • Methyltransferases
  • thiopurine methyltransferase
  • Dyneins