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We investigated whether characterization of full-length GAD (f-GADA) antibody (GADA) responses could identify early insulin requirement in adult-onset diabetes. In 179 f-GADA–positive participants diagnosed with type 2 diabetes, we assessed associations of truncated GADA (t-GADA) positivity, f-GADA IgG subclasses, and f-GADA affinity with early insulin requirement (<5 years), type 1 diabetes genetic risk score (T1D GRS), and C-peptide. t-GADA positivity was lower in f-GADA–positive without early insulin in comparison with f-GADA–positive type 2 diabetes requiring insulin within 5 years, and T1D (75% vs. 91% and 95% respectively, P < 0.0001). t-GADA positivity (in those f-GADA positive) identified a group with a higher T1D genetic susceptibility (mean T1D GRS 0.248 vs. 0.225, P = 0.003), lower C-peptide (1,156 pmol/L vs. 4,289 pmol/L, P = 1 × 10−7), and increased IA-2 antigen positivity (23% vs. 6%, P = 0.03). In survival analysis, t-GADA positivity was associated with early insulin requirement compared with those only positive for f-GADA, independently from age of diagnosis, f-GADA titer, and duration of diabetes (adjusted hazard ratio 5.7 [95% CI 1.4, 23.5], P = 0.017). The testing of t-GADA in f-GADA–positive individuals with type 2 diabetes identifies those who have genetic and clinical characteristics comparable to T1D and stratifies those at higher risk of early insulin requirement. Article Highlights: Progression to insulin therapy is highly variable in adult-onset GAD antibody (GADA)–positive diabetes. We further characterized GADA in adult-onset diabetes and assessed whether these are associated with early insulin requirement. Truncated GADA positivity was associated with a type 1 diabetes–like phenotype and stratified risk of early insulin requirement. Those GADA positive who were negative for truncated GADA had the characteristics and progression of classic type 2 diabetes. Assessing full-length GADA IgG subclass and affinity did not further stratify risk of progression. Truncated GADA assessment remains underused in clinical practice but could assist correct therapy allocation in adult-onset diabetes. [ABSTRACT FROM AUTHOR]

Autoantibodies to glutamate decarboxylase (GADA) are widely used in the prediction and classification of type 1 diabetes. GADA radiobinding assays (RBAs) using N-terminally truncated antigens offer improved specificity, but radioisotopes limit the high-throughput potential for population screening. Luciferase-based immunoprecipitation system (LIPS) assays are sensitive and specific alternatives to RBAs with the potential to improve risk stratification. The performance of assays using the Nanoluc luciferase (Nluc)-conjugated GAD65 constructs, Nluc-GAD65(96-585) and full length Nluc-GAD65(1-585), were evaluated in 434 well-characterized serum samples from patients with recent-onset type 1 diabetes and first-degree relatives. Nonradioactive, high-throughput LIPS assays are quicker and require less serum than RBAs. Of 171 relatives previously tested single autoantibody positive for autoantibodies to full-length GAD65 by RBA but had not progressed to diabetes, fewer retested positive by LIPS using either truncated (n = 72) or full-length (n = 111) antigen. The Nluc-GAD65(96-585) truncation demonstrated the highest specificity in LIPS assays overall, but in contrast to RBA, N-terminus truncations did not result in a significant increase in disease-specificity compared with the full-length antigen. This suggests that binding of nonspecific antibodies is affected by the conformational changes resulting fromaddition of the Nluc antigen. Nluc-GAD65(96-585) LIPS assays offer low-blood-volume, high-specificity GADA tests for screening and diagnostics. [ABSTRACT FROM AUTHOR]