As part of a general program of screening islet expression libraries we have identified a clone from a λgt11 human islet expression library that reacts with human diabetic sera and, upon sequencing, was determined to be the neuroendocrine islet autoantigen ICA512 (islet cell antigen 512). In the current communication, we describe the development of a radioassay for autoantibodies to ICA-512 (ICA512AA) using in vitro transcribed and translated protein for production of labeled antigen. Our initial results indicate that this radioassay is significantly more sensitive than the enzyme-linked immunosorbant assay, which uses a COOH-terminal fragment of ICA512. The ICA512AA radioassay uses a 96-well format with membrane separation of antibody bound from free antigen and should be readily adaptable to automated large-scale screening. Only 7 μl of serum is required for triplicate determinations. In order to determine the specificity and sensitivity of this assay and estimate its positive predictive value, we have studied 42 new-onset diabetic patients, 33 first-degree relatives of diabetic patients followed to diabetes, 694 islet cell antibody-negative (ICA–) relatives, and 205 normal control subjects. Thirty-eight percent of new-onset patients 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 for ICA512 autoantibodies. By using three radioassays for islet autoantibodies against insulin, glutamic acid decarboxylase 65 (GAD65), and ICA512, 100% of the prediabetic sera we have studied have been shown to express antibodies to at least one antigen, and the majority (88%, 27 of 31) express two or more. ICA512 autoantibodies provide a specific marker for type I diabetes and, in combination with antibodies to GAD65 and insulin, should facilitate the prediction of type I diabetes.

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