A TRUNCATED THYROXINE-BINDING GLOBULIN DUE TO A FRAMESHIFT MUTATION IS RETAINED WITHIN THE ROUGH ENDOPLASMIC-RETICULUM - A POSSIBLE MECHANISM OF COMPLETE THYROXINE-BINDING GLOBULIN DEFICIENCY IN JAPANESE

We have previously reported six unrelated Japanese families having the same mutation in the TBG gene and manifesting complete TBG deficiency (TBG-CDJ). The deficiency consists of a single nucleotide deletion re sulting in the production of C-terminal truncation due to a frameshift and premature termination. However, the reason for the failure to det ect TBG in the serum of subjects harboring this mutation remains unkno wn. In this communication we investigated the mechanism of the TBG def iciency associated with TBG-CDJ. The complementary DNAs of TBG-CDJ and normal TBG (TBG-N) were expressed in COS-1 cells. Pulse-chase experim ents revealed a complete absence of secretion of TBG-CDJ, whereas TBG- N was already present in the medium at time 0 and was almost entirely secreted by 3 h. In cell lysates, TBG-CDJ had a smaller molecular mass [52 kilodalton (kDa)] than TBG-N (54 kDa) and gradually decreased dur ing the chase. Thus, failure of TBG-CDJ secretion accounts for the com plete TBG deficiency. The molecular mass of TBG-CDJ (42 kDa) was also smaller than that of TBG-N (44 kDa) when synthesized in the presence o f tunicamycin. These findings are consistent with the premature termin ation of the TBG-CDJ molecule deduced from the nucleotide sequence ana lysis. Intracellular TBG-N was resistant to endoglycosidase H but not TBG-CDJ, suggesting the retention of TBG-CDJ within the rough endoplas mic reticulum. Indeed, subcellular fractionation revealed that most of TBG-CDJ was located in the rough endoplasmic reticulum compartment, a nd TBG-N was distributed in the Golgi fractions. Our results suggested that the lack of intracellular transport of the truncated TBG molecul e is the cause for the absence of immunoreactive TBG in the serum of s ubjects harboring the TBG-CDJ variant.