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
fibroblast autoantigen involved in these peripheral manifestations
of Graves' disease and the reason for the association of GO and PTD
with hyperthyroidism are unknown. RNA encoding the full-length
extracellular domain of the TSH receptor has been demonstrated in
orbital and dermal fibroblasts from patients with GO and normal
subjects, suggesting a possible antigenic link between fibroblasts
and thyrocytes. RNA was isolated from cultured orbital, pretibial,
and abdominal fibroblasts obtained from patients with severe GO (n
= 22) and normal subjects (n = 5). RNA 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 sequence
analysis showed an A for C substitution in the first position of
codon 52 in 2 of the patients, both of whom had GO, PTD, and
acropachy. Genomic DNA isolated from the 2 affected patients, and
not from an additional 12 normal subjects, revealed the codon 52
mutation by direct sequencing and AciI restriction enzyme
digestions. In conclusion, we 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 severe GO, PTD, acropachy,
and high thyroid-stimulating immunoglobulin levels. RNA encoding
this mutant product was demonstrated in the fibroblasts of these
patients. We suggest that the TSH receptor may be an important
fibroblast autoantigen in GO and PTD, and that this mutant form of
the receptor may have unique immunogenic properties.