Thiopurine S-methyltransferase pharmacogenetics: functional characterization of a novel rapidly degraded variant allozyme.
A novel human thiopurine S-methyltransferase (TPMT) variant allele, (319 T>G, 107Tyr>Asp, *27), was identified in a Thai renal transplantation recipient with reduced erythrocyte TPMT activity. The TPMT*27 variant allozyme showed a striking decrease in both immunoreactive protein level and enzyme activity after transient expression in a mammalian cell line. We set out to explore the mechanism(s) responsible for decreased expression of this novel variant of an important drug-metabolizing enzyme. We observed accelerated degradation of TPMT*27 protein in a rabbit reticulocyte lysate. TPMT*27 degradation was slowed by proteasome inhibition and involved chaperone proteins-similar to observations with regard to the degradation of the common TPMT*3A variant allozyme. TPMT*27 aggresome formation was also observed in transfected mammalian cells after proteasome inhibition. Inhibition of autophagy also decreased TPMT*27 degradation. Finally, structural analysis and molecular dynamics simulation indicated that TPMT*27 was less stable than was the wild type TPMT allozyme. In summary, TPMT*27 serves to illustrate the potential importance of protein degradation - both proteasome and autophagy-mediated degradation - for the pharmacogenetic effects of nonsynonymous SNPs.
Feng
Qiping
Q
Division of Clinical Pharmacology, Department of Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic-Mayo Medical School, 200 First Street SW, Rochester, MN 55905, United States.
Vannaprasaht
Suda
S
Peng
Yi
Y
Angsuthum
Susothorn
S
Avihingsanon
Yingyos
Y
Yee
Vivien C
VC
Tassaneeyakul
Wichittra
W
Weinshilboum
Richard M
RM
D000483:Alleles; D000818:Animals; D001343:Autophagy; D019556:COS Cells; D002522:Chlorocebus aethiops; D006801:Humans; D007527:Isoenzymes / Q000502:physiology*; D008780:Methyltransferases / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D010597:Pharmacogenetics; D046988:Proteasome Endopeptidase Complex / Q000502:physiology; D011817:Rabbits; D012156:Reticulocytes / Q000378:metabolism
D007527:Isoenzymes; D008780:Methyltransferases; C022745:thiopurine methyltransferase; D046988:Proteasome Endopeptidase Complex