The first 142 amino acids of glutamate decarboxylase do not contribute to epitopes recognized by autoantibodies associated with Type 1 diabetes. Issue 7 (19th April 2018)

• Record Type: Journal Article
• Authors: Wyatt, R. C.; Brigatti, C.; Liberati, D.; Grace, S. L.; Gillard, B. T.; Long, A. E.; Marzinotto, I.; Shoemark, D. K.; Chandler, K. A.; Achenbach, P.; Gillespie, K. M.; Piemonti, L.; Lampasona, V.; Williams, A. J. K.
• Is Part Of: Diabetic medicine. Volume 35:Issue 7(2018)
• Page Start: 954
• Page End: 963
• Publication Date: 2018-04-19
• Subjects: Diabetes -- Periodicals; 616.462
• DOI: 10.1111/dme.13628
• Languages: English
• ISSNs: 0742-3071

Abstract

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 35 S‐GAD65 (96–585) offer better disease specificity with similar sensitivity to full‐length 35 S‐GAD65 (1–585). To determine whether assay performance could be improved further, we evaluated a more radically truncated 35 S‐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 35 S‐labelled GAD65 (143–585). These were screened previously using a local radioimmunoassay with 35 S‐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 35 S‐GAD65 (1–585) and 35 S‐GAD65 (96–585). Results: Sensitivity of GAD antibody measurement was maintained using 35 S‐GAD65 (143–585) compared with 35 S‐GAD65 (1–585) and 35 S‐GAD65 (96–585). Specificity for Type 1 diabetes was improved compared with 35 S‐GAD65 (1–585), but was similar to 35 S‐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 35 S‐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. What's new?: The N‐terminus of glutamate decarboxylase (GAD65 ) contributes little to epitopes recognized by Type 1 diabetes associated GAD antibodies. Radioimmunoassays using N‐terminally truncated 35 S‐GAD65 (96–585) improve the specificity of GAD antibody measurement. We show that using a more radically truncated 35 S‐GAD65 (143–585) radiolabel to measure GAD antibodies does not impact assay sensitivity and improves diabetes specificity compared with full‐length 35 S‐GAD65 (1–585). Relatives of people with Type 1 diabetes who were GAD antibody‐positive using truncated radiolabels had increased risk of progression to diabetes within 15 years compared with those positive for GAD(1–585) antibody alone, and were at similar risk to those found positive using a high‐performing commercial GAD antibody enzyme‐linked immunosorbent assay.