DETECTION OF AUTOANTIBODIES TO THE 65-KD ISOFORM OF GLUTAMATE-DECARBOXYLASE BY RADIOIMMUNOASSAY

Autoantibodies (AAb) to glutamate decarboxylase (GAD) occur with a high prevalence in sera of newly diagnosed type I (insulin-dependent) diabetic patients. The aim of this study was to establish a GAD-AAb radioimmunoassay using I-125-labelled GAD(65) and to evaluate this assay in a cross-sectional study with newly diagnosed type I diabetic patients (diabetes duration < 6 weeks). Furthermore, subjects at high risk of developing type I diabetes and individuals suffering from other autoimmune diseases were examined in this assay. For GAD-AAb detection, I-125-labelled GAD(65) was incubated with 10 mu l of human serum overnight on ice. Thirty of 51 (59%) type I diabetic patients but none of the 54 healthy blood donors tested were found to be positive. A displacement step using 100000 g supernatant from rat brain containing or not containing GAD showed the specificity of the binding of I-125-GAD(65). Concerning the individuals at high risk of developing diabetes, 9/12 (75%) islet cell antibody (ICA)-positive non-diabetic and 4/34 (12%) ICA-negative subjects with metabolic abnormalities were GAD-AAb positive. These results show the association between type I (insulin-dependent) diabetes mellitus and the occurrence of GAD(65)-AAb, which possibly predicts a risk of developing the disease.