One autoantigen commonly recognised early in diabetes is the widely distributed neuroendocrine protein termed GAD (glutamate decarboxylase). There are two forms of GAD with molecular masses of 65 and 67kD, encoded by different genes located on different chromosomes. Both forms of GAD have been identified in the brain as well as the islet cells. GAD catalyses the formation of the inhibitory neurotransmitter GABA from glutamine. Within the islet, GAD may have a role in the inhibition of somatostatin and glucagon secretion and proinsulin synthesis. Some sera from type 1 diabetes patients recognise both forms of GAD but isoform specific antibodies also exist, anti-GAD 67 are not found in IDDM.
Anti-GAD65 is now the preferred serological test for IDDM. Autoantibodies recognising the same autoantigen but with different specificity are found in Stiff man (person) syndrome. In Stiff person syndrome the autoantibody, is often paraneoplastic.
Type 1 Diabetes: Stiff person syndrome
Anti-GAD65 autoantibodies have been reported in up to 78% of IDDM sera compared with only 1.7% of the healthy control sera. GAD 65 autoantibodies are found in >90% patients with Stiff Person Syndrome (SMS) exhibiting rigidity and spasm of axial musculature. Only 20% of patients exhibiting rigidity and spasm of distal musculature have anti- GAD. Some but not all of these patients (around 30-60%) also have diabetes and related serology.
Serum Separator Tube (SST)
Assay range notes
Positive >10 IU/ml.
Assay range up to 2000 IU/ml