Radioimmunoassay for glutamic acid decarboxylase (GAD65) autoantibodies as a diagnostic aid for stiff-man syndrome and a correlate of susceptibility to type 1 diabetes mellitus

Objective: To establish and validate a double-antibody radioimmunoassay (RIA) for detecting serum autoantibodies against glutamic acid decarboxylase (GAD65). This enzyme catalyzes synthesis of the neurotransmitter γ- aminobutyric acid in neurons and pancreatic islet cells. Material and Methods: We compared the frequency of GAD65 and other 'thyrogastric' autoantibodies in adult patients with stiff-man (Moersch-Woltman) syndrome, type 1 diabetes, or polyendocrine disorders and in healthy subjects. The frequency of pancreatic islet cell antibody (ICA) detection was also assessed. The GAD65 RIA was validated by testing blinded samples, by confirming the specificity of low-titered positive results by 'cold' antigen inhibition, and by comparing the RIA results with results of a kit assay incorporating staphylococcal protein A as immunoprecipitant. Recombinant GAD65 protein labeled with ¹²⁵I was used as antigen, and a combination of antihuman IgG and IgM was used as immunoprecipitant. Seropositivity was determined for ICA and gastric parietal cell antibodies by indirect immunofluorescence assays and for thyroid peroxidase (microsome) and thyroglobulin antibodies by agglutination assays. Results: We detected GAD65- specific antibodies in all but 1 of 46 local patients with stiff-man syndrome (98%); 16 had evidence of diabetes. Positive values exceeded 20 nmol/L in 96%, and 89% were ICA-positive; 76% had additional thyrogastric antibodies. Of 41 patients with type 1 diabetes (17 local and 24 workshop serum specimens), 33 were GAD65 antibody-positive (80%); 85% of these positive values were 20 nmol/L or lower. Only 18% of sera from patients with type 1 diabetes were ICA-positive, but 59% had other thyrogastric autoantibodies. Of 20 patients with autoimmune endocrinopathies without diabetes or stiff-man syndrome, 35% were GAD65 antibody-positive, 5% were ICA-positive, and 90 % were thyrogastric antibodypositive. Of 117 healthy control subjects, 8% were GAD65 antibody-positive, and a third of those had other thyrogastric antibodies (14% overall); none was ICA-positive. Conclusion: Seropositivity in the double-antibody RIA for GAD65 autoantibody is a sensitive and specific marker of predisposition to type 1 diabetes and related organ-specific autoimmune disorders. As such, this RIA is complemented by assays for thyroid and gastric parietal cell autoantibodies.