A GENOMIC POINT MUTATION IN THE EXTRACELLULAR DOMAIN OF THE THYROTROPIN RECEPTOR IN PATIENTS WITH GRAVES OPHTHALMOPATHY

Orbital and pretibial fibroblasts are targets of autoimmune attack in Graves' ophthalmopathy (GO) and pretibial dermopathy (PTD). The fibrob last autoantigen involved in these peripheral manifestations of Graves ' disease and the reason for the association of GO and PTD with hypert hyroidism are unknown. RNA encoding the full-length extracellular doma in of the TSH receptor has been demonstrated in orbital and dermal fib roblasts from patients with GO and normal subjects, suggesting a possi ble antigenic link between fibroblasts and thyrocytes. RNA was isolate d from cultured orbital, pretibial, and abdominal fibroblasts obtained from patients with severe GO (n = 22) and normal subjects (n = 5). RN A was reverse transcribed, and the resulting cDNA was amplified by the polymerase chain reaction, using primers spanning overlapping regions of the entire extracellular domain of the TSH receptor. Nucleotide se quence analysis showed an A for C substitution in the first position o f codon 52 in 2 of the patients, both of whom had GO, PTD, and acropac hy. Genomic DNA isolated from the 2 affected patients, and not from an additional 12 normal subjects, revealed the codon 52 mutation by dire ct sequencing and AciI restriction enzyme digestions. In conclusion, w e have demonstrated the presence of a genomic point mutation, leading to a threonine for proline amino acid shift in the predicted peptide, in the extracellular domain of the TSH receptor in two patients with s evere GO, PTD, acropachy, and high thyroid-stimulating immunoglobulin levels. RNA encoding this mutant product was demonstrated in the fibro blasts of these patients. We suggest that the TSH receptor may be an i mportant fibroblast autoantigen in GO and PTD, and that this mutant fo rm of the receptor may have unique immunogenic properties.