Autoantibodies in IDDM Primarily Recognize the 65,000-M_r Rather Than 67,000-M_r Isoform of Glutamic Acid Decarboxylase

Glutamic acid decarboxylase autoantibodies may aid in rapid screening strategies predicting IDDM before clinical onset. Rat islets contain GAD₆₅ and GAD₆₇ autoantibody targets, but human islets express only GAD₆₅, now confirmed by direct immunoprecipitation from radiolabeled rat and human islets. Because human IDDM involves β-cell-specific autoimmunity, we tested 190 new IDDM patients and 51 healthy control subjects for antibodies to recombinant human islet GAD₆₅, rat islet GAD₆₇, or human insulinoma/cerebellum GAD₆₇, each expressed separately in hamster fibroblasts. By using immunoprecipitation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and densitometric fluorogram scanning, 132 of 190 (70%) of new IDDM patients had GAD₆₅ autoantibodies, whereas only 17 of 190 (9%) had antibodies to rat GAD₆₇ (P < 0.001). Of healthy control subjects, 2 of 51 (3.9%) and 1 of 51 (1.9%) had antibodies to GAD₆₅ and GAD₆₇, respectively. All 17 GAD₆₇ antibody-positive patients also had GAD₆₅ antibodies; 14 of 17 with greater GAD₆₅ than GAD₆₇ index. Control studies showed comparable reactivity between recombinant rat and human GAD₆₇ and between different subcellular preparations of recombinant GAD₆₇ of either species. In conclusion, only GAD₆₅ is expressed in human islets, the autoantibody response is primarily to this isoform, and GAD₆₇ antibodies add little to IDDM detection.