IMPAIRED INTRACELLULAR-TRANSPORT CONTRIBUTES TO PARTIAL THYROXINE-BINDING GLOBULIN DEFICIENCY IN A JAPANESE FAMILY

We have previously reported a Japanese family manifesting partial TBG deficiency (TBG-PDJ). This variant was characterized by a decreased le vel of serum TBG concentration, heat lability, and normal isoelectric focussing pattern, but the affinity to iodothyronine is unknown. The T BG-PDJ gene possesses a single nucleotide substitution replacing the n ormal Pro363(CCT) with Leu(CTT); however, the precise mechanism that r esults in the reduction of the TBG concentration in the serum of the s ubjects harboring this mutation remains unknown. This was investigated in the current communication by expressing the complementary DNAs of TBG-PDJ and the common type TBG (TBG-C) in COS-1 cells. Pulse-chase ex periments revealed impaired secretion of TBG-PDJ. TBG-C secretion into the medium was evident during 60 min of the pulse period and was almo st completed by 12 h. On the other hand, TBG-PDJ was secreted slowly a nd continued to accumulate between 12-24 h of the chase period. The mo lecular mass of TBG-PDJ in the cell lysate was identical to that of TB G-C when estimated by gel electrophoresis (54 kilodaltons). The conten t of TBG-PDJ in the cell lysate decreased less rapidly than that of TB G-C, indicating that impaired TBG-PDJ secretion accounts for the parti al TBG deficiency. Oligosaccharide units of intracellular TBG-C were r esistant to endoglycosidase-H, but half of those of TBG-PDJ were sensi tive to the enzyme digestion, suggesting partial retention of TBG-PDJ within the rough endoplasmic reticulum. Northern blot analysis reveale d abundant messenger ribonucleic acid for the glucose-regulated protei n-78, the level of which was 3.54-fold greater in the cells transfecte d with TBG-PDJ than in nontransfected COS-1 cells, whereas that in TBG -C-transfected cells was same as that in the nontransfected cells. Inc reased expression of glucose-regulated protein-78 together with the se nsitivity to endoglycosidase-a suggests impairment of intracellular pr ocessing of TBG-PDJ. Our results indicate that the impaired intracellu lar transport of the TBG-PDJ molecule is the main cause of the reduced concentration of immunoreactive TBG in the serum of subjects harborin g this TBG variant.