Role of the Eye Muscles in Thyroid Eye Disease: Identification of the Principal Autoantigens

Abstract

Thyroid-associated ophthalmopathy (TAO) is a progressive orbital disorder associated with Graves' hyperthyroidism and, less often, Hashimoto's thyroiditis in which autoantibodies react with orbital antigens and lead to exophthalmos and eye muscle inflammation. Eye muscle (EM) membrane proteins initially reported as 55 and 64 kd are the best markers of ophthalmopathy. The "64-kd protein" is now shown to be the flavoprotein subunit of mitochondrial succinate dehydrogenase and to have a correct molecular weight of 67 kd. We have cloned a fragment of a novel eye muscle protein, which we call G2s, and sequenced 1.4 kb of the full length cDNA. G2s does not share any significant homologies with other reported proteins. The 5.9 kb G2s mRNA, that corresponds to a protein of approximately 220 kd, is expressed in EM, other skeletal muscle and thyroid, but not in other tissues tested. We have also cloned and sequenced a 63-kd eye muscle protein identified as the calcium binding protein calsequestrin. Antibodies against calsequestrin were found in 40% of patients with active ophthalmopathy, but in 0% of normal subjects. Finally, we have sequenced a 19 amino acid fragment of a 55-kd porcine eye muscle membrane protein that exactly matched porcine and human sarcalumenin, a 160-kd glycoprotein localized in the lumen of the longitudinal sarcoplasmic reticulum of the skeletal muscle fiber where it binds calcium. A 53-kd glycoprotein fragment of the molecule corresponds to the 55-kd protein. In a preliminary study, serum antibodies against purified sarcalumenin were detected in 40% of patients with active TAO of less than 1 year duration, but in no controls tested. We porpose that the primary autoantigen in TAO is G2s, which would also explain the association of ophthalmopathy with thyroid autoimmunity, and that antibodies against the intracellular proteins flavoprotein, calsequestrin, and sarcalumenin are secondary markers of an immune-mediated reaction in eye muscle in patients with thyroid autoimmunity.

Management of Graves' Ophthalmopathy: Reality and Perspectives

“…Other eye muscle autoantigens possibly involved in GO include 1) a 63-kDa calcium-binding protein, called calsequestrin, expressed in extraocular muscle and skeletal muscle, but not in the thyroid (67); 2) a 53-kDa protein, identified as another calcium-binding glycoprotein, sarcalumenin, expressed in extraocular muscle and skeletal muscle, but not in the thyroid (68); 3) a different 63-to 64-kDa protein, called 1D protein, cloned from a thyroid cDNA expression library (62), which is expressed in extraocular muscles, skeletal muscles, thyroid, testis, and other tissues (69,70); and 4) a novel eye muscle protein, called G2s, with an estimated molecular mass of about 220 kDa, expressed in extraocular muscles, skeletal muscles, and thyroid (68). A higher prevalence of circulating antibodies directed against two porcine eye muscle antigens (64-kDa and 95-kDa) has been found by immunoblotting in GO patients compared with those without eye involvement or to normal controls (71).…”

Serum antibodies to collagen XIII: a further good marker of active Graves’ ophthalmopathy

“…[1][2][3] Graves' patients with mild ophthalmopathy do not usually require any specific or aggressive treatment, 4 while those with moderate to severe ocular involvement need immunosuppressive or orbital surgical therapy, depending on the activity of the disease. [14][15][16][17][18][19] Moreover, quantitative magnetic resonance imaging (MRI) has been used to assess disease in eye muscle; in particular, it has been shown that T2 relaxation times are longer in oedematous than in fibrotic tissue. 8 -10 The severity of the ophthalmopathy is graded on the basis of the degree of ocular alterations, 11 whereas the activity is assessed by using the clinical activity score (CAS), 12 reflecting inflammatory changes in the periorbital tissue and the orbital adipose connective tissue, but not the involvement of the extraocular muscles (EM) in GO.…”

Ocular myasthenia gravis associated with euthyroid ophthalmopathy

“…6 A thyroid and eye-muscle shared protein has been proposed as the primary autoantigen in thyroid-associated ophthalmopathy. 5,7 The primary autoantigens in ocular MG also may differ from those in generalized MG. In ocular MG, AChR antibodies tend to be present in low titer, and were not found in one-third of a patient population studied.…”