ICA512 AUTOANTIBODY RADIOASSAY

As part of a general program of screening islet expression Libraries w e have identified a clone from a lambda gt11 human islet expression Li brary that reacts with human diabetic sera and, upon sequencing, was d etermined to be the neuroendocrine islet autoantigen ICA512 (islet cel l antigen 512), In the current communication, we describe the developm ent of a radioassay for autoantibodies to ICA512 (ICA512AA) using in v itro transcribed and translated protein for production of labeled anti gen, Our initial results indicate that this radioassay is significantl y more sensitive than the enzyme-linked immunosorbant assay, which use s a COOH-terminal fragment of ICA512, The ICA512AA radioassay uses a 9 6-well format with membrane separation of antibody bound from free ant igen and should be readily adaptable to automated large-scale screenin g, Only 7 mu I of serum is required for triplicate determinations, In order to determine the specificity and sensitivity of this assay and e stimate its positive predictive value, we have studied 42 new-onset di abetic patients, 33 first-degree relatives of diabetic patients follow ed to diabetes, 694 islet cell antibody-negative (ICA-) relatives, and 205 normal control subjects, Thirty-eight percent of new-onset patien ts and 48% of relatives followed to diabetes express autoantibodies to ICA512 exceeding the 99th percentile of the normal control subjects, In contrast, only 1.4% of ICA- first-degree relatives were positive fo r ICA512 autoantibodies, By using three radioassays for islet autoanti bodies against insulin, glutamic acid decarboxylase 65 (GAD(65)), and ICA512, 100% of the prediabetic sera we have studied have been shown t o express antibodies to at least one antigen, and the majority (88%, 2 7 of 31) express two or more, ICA512 autoantibodies provide a specific marker for type I diabetes and, in combination with antibodies to GAD ,, and insulin, should facilitate the prediction of type I diabetes.