The first 142 amino acids of glutamate decarboxylase do not contribute to epitopes recognized by autoantibodies associated with Type 1 diabetes.

Aims: Glutamate decarboxylase (GAD) antibodies are the most widely used predictive marker for Type 1 diabetes, but many individuals currently found to be GAD antibody-positive are unlikely to develop diabetes. We have shown previously that radioimmunoassays using N-terminally truncated S-GAD (96-585) offer better disease specificity with similar sensitivity to full-length S-GAD (1-585). To determine whether assay performance could be improved further, we evaluated a more radically truncated S-GAD (143-585) radiolabel.

Methods: Samples from people with recent-onset Type 1 diabetes (n = 157) and their first-degree relatives (n = 745) from the Bart's-Oxford family study of childhood diabetes were measured for GAD antibodies using S-labelled GAD (143-585). These were screened previously using a local radioimmunoassay with S-GAD (1-585). A subset was also tested by enzyme-linked immunosorbent assay (ELISA), which performs well in international workshops, but requires 10 times more serum. Results were compared with GAD antibody measurements using S-GAD (1-585) and S-GAD (96-585).

Results: Sensitivity of GAD antibody measurement was maintained using S-GAD (143-585) compared with S-GAD (1-585) and S-GAD (96-585). Specificity for Type 1 diabetes was improved compared with S-GAD (1-585), but was similar to S-GAD (96-585). Relatives found to be GAD antibody-positive using these truncated labels were at increased risk of diabetes progression within 15 years, compared with those positive for GAD(1-585) antibody only, and at similar risk to those found GAD antibody-positive by ELISA.

Conclusions: The first 142 amino acids of GAD do not contribute to epitopes recognized by Type 1 diabetes-associated GAD antibodies. Low-volume radioimmunoassays using N-terminally truncated S-GAD are more specific than those using full-length GAD and offer practical alternatives to the GAD antibody ELISA for identifying children at increased risk of Type 1 diabetes.