PREVALENCE OF AUTOANTIBODIES TO THE 65-KD AND 67-KD ISOFORMS OF GLUTAMATE-DECARBOXYLASE IN INSULIN-DEPENDENT DIABETES-MELLITUS

We investigated the presence of autoantibodies to baculovirus-expresse d human recombinant 65- and 67-kD isoforms of glutamate decarboxylase (GAD65 and GAD67) in insulin-dependent diabetes mellitus (IDDM). In th e immunoprecipitation test using [S-35]methionine-labeled GADs antibod ies to GAD65 were detected in 13/15 (87%) islet cell antibody (ICA)-po sitive and in 1/35 (2.9%) ICA-negative first-degree relatives of patie nts with IDDM, in 6/11 (54.5%) ICA-positive nondiabetic schoolchildren , and in 35/50 (70%) patients with newly diagnosed IDDM. GAD67 antibod ies were positive only in five (33%) of the ICA-positive relatives (P < 0.05) and in nine (18%) IDDM patients at onset (P < 0.00001). After onset of IDDM antibodies to GAD65 and GAD67 declined but were still po sitive in 25 and 9.4% of subjects with long-standing IDDM (> 10 yr). I n all study groups antibodies to GAD67 were only detected in GAD, anti body-positive sera. An immunotrapping enzyme activity assay for GAD65 antibodies was positive in 64/75 (85.3%) of sera that were GAD antibod y positive in the immunoprecipitation test (r = 0.870, P < 0.0001). In two (2.7%) sera GAD, antibodies that block GAD enzyme activity were f ound. Our data suggest that antibodies to GAD65 but not to GAD67 repre sent sensitive markers for preclinical and overt IDDM. The immunotrapp ing assay here described represents a valuable technique for specific and sensitive screening for GAD antibodies.