Thyroxine-binding globulin: organization of the gene and variants.

Thyroxine-binding globulin (TBG), the principal thyroid hormone transport protein in human serum, is synthesized by the liver and secreted into the bloodstream as a 54-kD acidic glycoprotein made up of a single polypeptide chain of 395 amino acids and four heterosaccharide units. The carbohydrate chains are important for the correct posttranslational folding, secretion and degradation of the molecule but are not required for hormone binding. TBG, encoded by a single gene copy located on Xq22, consists of five exons spanning 5.5 kbp. An upstream sequence of 218 nucleotides containing a hepatocyte nuclear factor 1 binding motif imparts to the gene a strong liver-specific transcriptional activity. Inherited TBG defects produce three phenotypes based on the level of TBG in serum of affected hemizygotes: complete TBG deficiency (TBG-CD), partial TBG deficiency (TBG-PD) and TBG excess (TBG-E). The molecular basis of the TBG defect has been identified in 12 of 16 known TBG variants. TBG-CD is caused by either premature termination of translation or an amino acid substitution resulting in failure of secretion. Point mutations resulting in single amino acid substitutions are responsible for the alteration of the properties and/or concentration of TBG-PD variants. Gene duplication and triplication has been recently identified in subjects with TBG-E.