Autoantibodies Against Gad₆₅ Rather Than Gad₆₇ Precede the Onset of Type 1 Diabetes

Abstract

The enzyme glutamate decarboxylase (GAD) is considered one of the major 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 radioimmunoassay (RIA) method for detection of AAb against both isoforms of the enzyme, GAD₆₅ and GAD₆₇, in a cross-sectional study using sera from newly diagnosed Type 1 diabetic patients and in a longitudinal study using sera from prediabetic patients and individuals at risk of developing the disease. The ¹²⁵I-labelled full-length human recombinant proteins of GAD₆₅ and GAD₆₇ expressed in SF9 cells were used as the antigen source. The prevalence of GAD₆₅-AAb in newly diagnosed Type 1 diabetic patients was found to be 73% (112/153), in contrast to 19% (14/72) of GAD₆₇-AAb. Only one patient produced AAb restricted to GAD₆₇. Furthermore, GAD₆₅-AAb could also be detected in 73% (11/15) of prediabetic patients (up to 122 months before clinical manifestation of the disease), whereas only 27% (4/15) of them were positive for GAD₆₇-AAb. In the group at risk of developing Type 1 diabetes, these prevalences were 77% (10/13) and 46% (6/13), respectively. In all GAD₆₇-AAb-positive patients investigated in the longitudinal study, AAb to GAD₆₅ were detectable. In 47% of patients positive for both GAD₆₅-AAb and ICA, the GAD₆₅-AAb appeared by up to 46 months before the occurrence of ICA was detected. The data illustrated that GAD₆₅ is the main immunogenic isoform of the enzyme in the preclinical and clinical stages. The RIA detecting AAb against this isoform may facilitate the screening for individuals at risk of developing the disease.

Key Words:

• Glutamic acid decarboxylase
• autoantibodies
• radioimmunoassay
• Type 1 diabetes mellitus
• ICA