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

Abstract: 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 ³⁵S‐GAD₆₅(96–585) offer better disease specificity with similar sensitivity to full‐length ³⁵S‐GAD₆₅(1–585). To determine whether assay performance could be improved further, we evaluated a more radically truncated ³⁵S‐GAD₆₅(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 ³⁵S‐labelled GAD₆₅(143–585). These were screened previously using a local radioimmunoassay with ³⁵S‐GAD₆₅(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 ³⁵S‐GAD₆₅(1–585) and ³⁵S‐GAD₆₅(96–585). Results: Sensitivity of GAD antibody measurement was maintained using ³⁵S‐GAD₆₅(143–585) compared with ³⁵S‐GAD₆₅(1–585) and ³⁵S‐GAD₆₅(96–585). Specificity for Type 1 diabetes was improved compared with ³⁵S‐GAD₆₅(1–585), but was similar to ³⁵S‐GAD₆₅(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 GAD₆₅ do not contribute to epitopes recognized by Type 1 diabetes‐associated GAD antibodies. Low‐volume radioimmunoassays using N‐terminally truncated ³⁵S‐GAD₆₅ are more specific than those using full‐length GAD₆₅ and offer practical alternatives to the GAD antibody ELISA for identifying children at increased risk of Type 1 diabetes. [ABSTRACT FROM AUTHOR]