Engineering a Potential Antagonist of Human Thyrotropin and Thyroid-stimulating Antibody

Thyrotropin (TSH) and the gonadotropins (FSH, LH, hCG) are a family of heterodimeric glycoprotein hormones composed of two noncovalently linked subunits, α and β. We have recently converted the hTSH heterodimer to a biologically active single chain (hTSHβ·CTPα) by fusing the common α-subunit to the C-terminal end of the hTSH β-subunit in the presence of a ∼30-amino acid peptide from hCGβ (CTP) as a linker. The hTSHβ·CTPα single chain was used to investigate the role of the N-linked oligosaccharides of α- and β-subunits in the secretion and function of hTSH. Using overlapping PCR mutagenesis, two deglycosylated variants were prepared: one lacking both oligosaccharide chains on the α-subunit (hTSHβ·CTPα₁₊₂) and the other lacking the oligosaccharide chain on the β-subunit (hTSHβ·CTPα(deg)). The single chain variants were expressed in CHO cells and were secreted into the medium. hTSH variants lacking the oligosaccharide chains were less potent than hTSHβ·CTPα wild-type with respect to cAMP formation and thyroid hormone secretion in cultured human thyroid follicles. Both deglycosylated variants competed with hTSH in a dose-dependent manner. The hTSHβ·CTPα₁₊₂ variant blocked cAMP formation and thyroid hormone secretion stimulated by hTSH as well as by the antibody, thyroid-stimulating immunoglobulins, responsible for the most common cause of hyperthyroidism, Graves disease. Thus, this variant behaves as a potential antagonist, offering a novel therapeutic strategy in the treatment of thyrotoxicosis caused by Graves' disease and TSH-secreting pituitary adenoma.