Human thiopurine S-methyltransferase (TPMT) pharmacogenetics: Variant allozyme aggresome formation

Abstract

Background/Aims TPMT*3A, the most common TPMT variant allozyme in Caucasians, has 2 alterations in amino acid sequence, resulting in striking decreases in TPMT protein levels as a result of rapid degradation. “Protein quality control” involves a dynamic balance among protein folding, degradation and aggregation with aggresome formation. We set out to test the hypothesis that, the presence of the proteasome inhibitor MG132, TPMT*3A can form aggresomes in cultured cells. Methods/Results TPMT*3A was highly ubiquinated and associated with hsp90 after transient expression in COS-1 cells. In the presence of MG132, aggresomes formed in cells transfected with TPMT*3A, and ubiquinated TPMT*3A redistributed from the cytosol into the pellet - compatible with aggresome formation. The aggresomes contained vimentin, dynein, γ-tubulin, ubiquitin, hsp70, hsp90 and the deacetylating enzyme HDAC6 that co-localized with TPMT*3A. Aggresome formation could be blocked by the microtubule destabilizing agent, vinblastine. Treatment with the HDAC inhibitor scriptaid inhibited aggresome formation and resulted in TPMT*3A microaggregate formation. Conclusions These results suggest that aggresome formation is one mechanism by which nonsynonymous cSNPs can alter function for proteins of pharmacogenetic importance such as TPMT. Clinical Pharmacology & Therapeutics (2005) 77, P95–P95; doi: 10.1016/j.clpt.2004.12.257