Fusion proteins for combined analysis of autoantibodies to the 65-kDa isoform of glutamic acid decarboxylase and islet antigen-2 in insulin-dependentdiabetes mellitus

Background: Prediction, risk assessment, and diagnosis of autoimmune diseas es often rely on detection of autoantibodies directed to multiple target an tigens, such as the 65-kDa isoform of glutamic acid decarboxylase (GAD65-ab s) and the tyrosine phosphatase-like protein islet antigen-2 (IA2-abs), the two major subspecificities of islet cell antibodies (ICAs) associated with insulin-dependent diabetes mellitus, We hypothesized that a combination of autoantigens in a fusion protein unifying the important immunodominant epi topes could provide an efficient target for cost-effective, one-step screen ing of sera. Methods: Chimeric proteins composed of GAD65 and IA2 residues were construc ted, analyzed for their immune reactivity with monoclonal antibodies and se ra, and used in a diagnostic assay with S-35-labeled protein as antigen, Results: Length and order of GAD65 and IA2 sequences were critical for cons ervation of the conformational epitopes in the fusion protein, among four c himera tested, only IA2((606-979))/GAD65((1-585)) retained wild-type-like f olding of GAD65 and IA2 domains and yielded a stable protein after baculovi rus expression. Reactivity of GAD65 antibody- and IA2 antibody-positive ser a from patients newly diagnosed with insulin-dependent diabetes mellitus, f rom ICA-positive prediabetics, and from ICA-positive first-degree relatives demonstrated conservation of the relevant autoreactive epitopes, The assay based on the in vitro translated fusion antigen had a sensitivity and spec ificity identical to those for detection of GAD65- and IA2-abs based on the two separate GAD65 and IA2 proteins, Conclusions: Autoantigens such as GAD65 and IA2 can be combined successfull y in a fusion protein of similar immune reactivity. This allows simultaneou s detection of GAD65- and IA2-abs in a one-step screening assay and cost-ef fective identification of positive individuals at risk of diabetes or at on set of disease. (C) 2001 American Association for Clinical Chemistry.