Binding of thyrotrophin to low molecular weight fragments of human thyroid membranes

A Modifying Autoantigen in Graves' Disease

The TSH receptor (TSHR) is the major autoantigen in Graves' disease (GD). Bioinformatic analyses predict the existence of several human TSHR isoforms from alternative splicing, which can lead to the coexpression of multiple receptor forms. The most abundant of these is TSHRv1.3. In silico modeling of TSHRv1.3 demonstrated the structural integrity of this truncated receptor isoform and its potential binding of TSH. Tissue profiling revealed wide expression of TSHRv1.3, with a predominant presence in thyroid, bone marrow, thymus, and adipose tissue. To gain insight into the role of this v1.3 receptor isoform in thyroid pathophysiology, we cloned the entire open reading frame into a mammalian expression vector. Immunoprecipitation studies demonstrated that both TSHR-stimulating antibody and human TSH could bind v1.3. Furthermore, TSHRv1.3 inhibited the stimulatory effect of TSH and TSHR-Ab MS-1 antibody on TSHR-induced cAMP generation in a dose-dependent manner. To confirm the antigenicity of v1.3, we used a peptide ELISA against two different epitopes. Of 13 GD samples, 11 (84.6%) were positive for a carboxy terminal peptide and 10 (76.9%) were positive with a junction region peptide. To demonstrate that intracellular v1.3 could serve as an autoantigen and modulate disease, we used double-transfected Chinese hamster ovary cells that expressed both green fluorescent protein (GFP)-tagged TSHRv1.3 and full-length TSHR. We then induced cell stress and apoptosis using a TSHR monoclonal antibody and observed the culture supernatant contained v1.3-GFP protein, demonstrating the release of the intracellular receptor variant by this mechanism.

Cloning and sequencing of a 1.3 KB variant of human thyrotropin receptor mRNA lacking the transmembrane domain

We amplified human thyroidal cDNA using oligonucleotide primers designed to reveal putative human thyrotropin receptor (hTSHR) mRNA variants encoding the extracellular, ligand-binding domain but lacking the transmembrane domain. Whereas the major 4.3 kb hTSHR mRNA species was not amplified to detectable levels, several shorter products were detected. A strongly amplified 1 kb product was cloned and sequenced. It contained coding sequences at the 5' end which were colinear with exons 1-8 of the hTSHR gene, encoding most of the extracellular domain. This was followed at the 3' end by additional coding and noncoding information not present in the 4.3 kb transcript. A probe specific for the 5' end recognized polyadenylated thyroidal transcripts of 4.3, 1.7, and 1.3 kb, indicating the presence of several hTSHR mRNA variants. A probe specific for the 3' end recognized only the 1.3 kb transcript. The level of the 1.3 kb variant (hTSHR-v 1.3 mRNA) was about half that of the 4.3 kb hTSHR mRNA and twice that of the 1.7 kb variant. The presence of a thyroidal mRNA encoding both the signal peptide and ligand-binding region of the hTSHR, but not the seven transmembrane helices, provides the potential to produce soluble receptors which could play important roles in thyroid physiology and/or autoimmune thyroid disease.

The pathogenesis of Graves' disease

The abnormally increased thyroid activity that is characteristic of Graves' disease is caused by immunoglobulins which specifically interact with the thyroid cell and stimulate it. Increases and decreases in thyroid activity in Graves' disease can be clearly related to rise and fall of these immunoglobulin-mediated activities. The level of immunoglobulin stimulatory activity can be used for prediction of the likelihood of neonatal Graves' disease and of recurrence of disease after cessation of treatment with antithyroid drugs. Investigation of patients with Graves' disease and their families has led to identification of particular human leukocyte antigens and genetically linked markers on immunoglobulins which both appear to incur increased susceptibility to certain autoimmune diseases. Differences in immune function, when compared with control populations, have been found in patients with these genetically linked markers. Protection against autoimmune disease is maintained by purposeful inhibition of any self-directed activity within each function of the immune system and by the controlling interaction of other immune functions. No single deficiency of immune function can be selected as giving the major risk of autoimmune disease, but rather a sum of relative defects resulting in an increased risk. In some patients with Graves' disease the self-protection mechanisms regain sufficient control of the immune functions to reduce the activity of the autoimmune disease, and the patient may achieve clinical remission. Often, however, there is evidence that abnormal immune activity directed against thyroid tissue has persisted with liability to recurrence of the Graves' disease.