AUTOANTIBODIES AGAINST GAD(65) RATHER THAN GAD(67) PRECEDE THE ONSET OF TYPE-1 DIABETES

The enzyme glutamate decarboxylase (GAD) is considered one of the majo r Beta cell antigens in Type 1 diabetes mellitus. The GAD autoantibody (GAD-AAb) prevalence in newly diagnosed Type 1 diabetic patients has been described up to 80%, depending on the detection method used. The aim of this study was to evaluate a simple, specific, and sensitive ra dioimmunoassay (RIA) method for detection of AAb against both isoforms of the enzyme, GAD(65) and GAD(67), in a cross-sectional study using sera from newly diagnosed Type 1 diabetic patients and in a longitudin al study using sera from prediabetic patients and individuals at risk of developing the disease. The I-125-labelled full-length human recomb inant proteins of GAD(65) and GAD(67) expressed in SF9 cells were used as the antigen source. The prevalence of GAD(65)-AAb in newly diagnos ed Type 1 diabetic patients was found to be 73% (112/153), in contrast to 19% (14/72) of GAD(67)-AAb. Only one patient produced AAb restrict ed to GAD(67). Furthermore, GAD(65)-AAb could also be detected in 73% (11/15) of prediabetic patients (up to 122 months before clinical mani festation of the disease), whereas only 27% (4/15) of them were positi ve for GAD(67)-AAb. In the group at risk of developing Type 1 diabetes , these prevalences were 77% (10/13) and 46% (6/13), respectively. In all GAD(67)-AAb-positive patients investigated in the longitudinal stu dy, AAb to GAD(65) were detectable. In 47% of patients positive for bo th GAD(65)-AAb and ICA, the GAD(65)-AAb appeared by up to 46 months be fore the occurrence of ICA was detected. The data illustrated that GAD (65) is the main immunogenic isoform of the enzyme in the preclinical and clinical stages. The RIA detecting AAb against this isoform may fa cilitate the screening for individuals at risk of developing the disea se.