Autoantibodies to the IA-2 Extracellular Domain Refine the Definition of “A+” Subtypes of Ketosis-Prone Diabetes

Abstract

Autoantibodies directed against tyrosine phosphatase IA-2 antibody (IA-2 Ab) are diagnostic for autoimmune type 1 diabetes. Conventional assays target the intracellular domain of IA-2. Among patients with ketosis-prone diabetes (KPD), characterized by presentation with diabetic ketoacidosis (DKA), >60% of adults lack three classic islet autoantibodiesdIA-2, GAD65, and ZnT8 Absdassociated with type 1 diabetes. We aimed to determine whether apparently autoantibodynegative ("A2") KPD patients possess occult IA-2 Ab directed against fulllength IA-2 (IA-2FL) or its extracellular domain (IA-2EC). RESEARCH DESIGN AND METHODS We developed an assay that targets IA-2FL and IA-2EC and used it to analyze 288 subjects with A2 KPD. RESULTS Ten A2 KPD patients were positive for IA-2EC Ab (3.5%), and three were also positive for IA-2FL Ab (1.0%), similar to frequencies in type 1 and type 2 diabetes. CONCLUSIONS Measurement of IA-2FL Ab and IA-2EC Ab improves the accuracy of the Ab classification of KPD patients. Ketosis-prone diabetes (KPD) is a heterogenous syndrome characterized by presentation with diabetic ketoacidosis (DKA) and classified by the presence or absence of islet autoantibodies ("A+" or "A2") and presence or absence of b-cell functional reserve ("b+" or "b2") (1,2). Distinct from patients with type 1 diabetes, patients with KPD often present when older, have fewer recurrences of DKA, and can often discontinue insulin treatment while maintaining glycemic control (3). More than 60% of KPD adult patients lack evidence of islet autoimmunity (i.e., are A2) by testing for the presence of autoantibodies against the 65-kDa isoform of glutamate decarboxylase (GAD65), zinc transporter T8 (ZnT8), and the neuroendocrine autoantigen IA-2 (or ICA512) (1,2,4). Constructs used in conventional IA-2 autoantibody assays include intracellular fragments, but not the extracellular domain (IA-2EC), which has recently been investigated as a target for IA-2-specific autoantibodies (5). We reported that 1% of patients with autoimmune type 1 diabetes are positive only for the IA-2EC antibody (Ab), as were 4.7% of 258 patients with type 2 diabetes (5,6). Furthermore, we reported that full-length IA-2 (IA-2FL

Comment on Mulukutla et al. Autoantibodies to the IA-2 Extracellular Domain Refine the Definition of “A+” Subtypes of Ketosis-Prone Diabetes. Diabetes Care 2018;41:2637–2640

“…Diabetes Care 2019;42:e81 | https://doi.org/10.2337/dc19-0022 I read with great interest the article by Mulukutla et al (1) demonstrating that antibodies against the luminal (extracellular) domain of phosphatase IA-2, termed IA-2EC (amino acids 26-577), are expressed by patients with ketosisprone diabetes. Among patients that were negative for other aIA-2 autoantibodies, the aIA-2EC antibodies were reported in 2% of patients with ketosisprone diabetes without the presence of autoantibodies (1), 6% (2) or 7% (3) of patients with type 1 diabetes, and 5% of patients diagnosed with type 2 diabetes (3).…”

“…Some patients who reacted with the IA-2EC polypeptide did not react with the full-length (IA-2FL) polypeptide (1) or with its truncated variant IA-2BDC, which lacks the transmembrane domain (D557-629) and also lacks 255 amino acids of the N-terminus (2,3). Such observation is surprising and points attention toward the engineering of previously used aIA-2 assays.…”

“…Even these illustrative figures show that the constructs used were cleaved as the sera reacted with both the full-length and cleaved products (although the existence of such products was not discussed). The recent article by Mulukutla et al (1) suffers from the same issues as it completely adopted the methodology that was used in the abovementioned studies (2,3).…”

Response to Comment on Mulukutla et al. Autoantibodies to the IA-2 Extracellular Domain Refine the Definition of “A+” Subtypes of Ketosis-Prone Diabetes. Diabetes Care 2018;41:2637–2640

“…We thank Dr. Heneberg ( 1 ) for the comments regarding our article ( 2 ) describing autoantibody responses to the IA-2 extracellular domain in ketosis-prone diabetes (KPD).…”

“…Nonetheless, we identified autoantibody and T-cell responses specifically directed to the IA-2 extracellular domain (IA-2EC) in individuals with or without antibodies against IA-2IC ( 4 ). Our recent studies demonstrate that IA-2EC is a target of humoral immune responses in a subgroup of patients with KPD ( 2 ). To demonstrate specificity of IA-2EC autoantibodies, we performed competitive autoantibody binding studies according to previously published protocols ( 4 ).…”