Alternative Names
GAD2, GAD-65, MGC161605, MGC161607
Purity
> 90% as determined by SDS-PAGE
Buffer
25mM Tirs-HCl, 0.02%SDS, pH8.5
Storage
Short Term Storage +4°C, Long Term Storage -20°C. After reconstitution, prepare aliquots and store at -20°C. Avoid freeze/thaw cycles.
Introduction
This gene encodes one of several forms of glutamic acid decarboxylase, identified as a major autoantigen in insulin-dependent diabetes. The enzyme encoded is responsible for catalyzing the production of gamma-aminobutyric acid from L-glutamic acid. A pathogenic role for this enzyme has been identified in the human pancreas since it has been identified as an autoantibody and an autoreactive T cell target in insulin-dependent diabetes. This gene may also play a role in the stiff man syndrome. Alternative splicing results in multiple transcript variants that encode the same protein.
Keywords
GAD2; GAD65; Glutamic Acid Decarboxylase 65 kDa; Type 1 Diabetes; T1D
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Glutamate decarboxylase (GAD) is a key factor in the dynamic regulation of excitability in neuronal networks and is widely distributed in the central nervous system, pancreas, and other organs. It contains two isoenzymes, GAD65 and GAD67, which convert glutamate to gamma-aminobutyric acid (GABA). Anti-GAD65 antibodies can be detected in patients with neurological disorders, including stiff-person syndrome (SPS), cerebellar ataxia (CA), and myelitis. Anti-GAD65 antibodies have also been found in patients with chronic epilepsy syndrome and seizures with symptoms of encephalitis.
Figure 1. Autoantigens and related processes associated with GABAergic and glycinergic synaptic transmission in patients with SPS
(Source: McKeon A, et al. 2017)
The diagnosis of diseases associated with high levels of GAD65 antibodies is based on clinical presentation and the detection of high titers of anti-GAD65 antibodies or intrathecal synthesis (IS) in cerebrospinal fluid (CSF). It is generally believed that the results of judging the high expression of anti-GAD65 are: 1) radioimmunoassay (RIA) value > 20 nmol/L or 2000 U/mL; 2) ELISA detection > 20 nmol/L or 1000 IU/mL;3) Immunohistochemical screening, RIA or WB confirmed positive cases. The IS index is calculated from the titers of anti-GAD65 antibodies in CSF and serum, and the results of tests for albumin in CSF and serum. Values above the IgG index, especially those above 1, are strong indicators of antibody-specific IgG IS. The IS metrics can indicate which patients have an active phase of CNS inflammation that will persist for a while, thus identifying the appropriate patients for immunosuppressive therapy.
However, there are some researchers who believe that anti-GAD65 antibody testing has no clinical significance and they believe that anti-GAD65 antibodies are non-specific and can be detected in a wide range of diseases. On the other hand, if the cause of the disease is due to anti-GAD65 antibodies, then the antibody titer should increase as the disease progresses, and in fact there is no correlation between anti-GAD65 antibody titer and clinical manifestations. Second, GAD65 is localized inside the cell and anti-GAD antibodies do not bind directly to it. Furthermore, the use of ventricular, intrathecal, and intraperitoneal preparations of CSF IgG did not affect cerebellar function. However, subsequent experiments found that cerebellar-mediated modulation was impaired after intrathecal application of CSF immunoglobulin, as evidenced by excitability, gait, behavioral tasks, and blink reflexes in the spinal cord or motor cortex. The pathogenic principles associated with anti-GAD65 antibodies have also been progressively elucidated, all of which point to the clinical value of anti-GAD65 antibodies.
Alternative Names
Glutamate decarboxylase 65
References
1. McKeon A, et al. GAD65 neurological autoimmunity. Muscle Nerve. 2017 Jul;56(1):15-27.
2. Li X, et al. Immune-mediated epilepsy with GAD65 antibodies. J Neuroimmunol. 2020 Apr 15;341:577189.
Q & A
Q: Can you provide some indication on how to best aliquot: such as minimum concentration, carrier protein etc..
A: Here are some recommendations for aliquot and storage:
Concentration: Store at 0.5-2 mg/mL for long-term storage and 1–100 µg/mL for working stocks.
Carrier Protein: Add 0.1-1% BSA for stabilization, unless interfering with experiments.
Aliquot Size: Use 10-50 µL aliquots to minimize freeze-thaw cycles. The size of the aliquot should depend on the amount of protein you typically use in your experiments.
Storage: Store at -80°C or liquid nitrogen for long-term storage, and 4°C for short-term storage.
Customer Reviews
Autoimmune Encephalitis Misdiagnosis in Adults
JAMA Neurol
Authors: Flanagan EP, Geschwind MD, Lopez-Chiriboga AS, Blackburn KM, Turaga S, Binks S, Zitser J, Gelfand JM, Day GS, Dunham SR, Rodenbeck SJ, Clardy SL, Solomon AJ, Pittock SJ, McKeon A, Dubey D, Zekeridou A, Toledano M, Turner LE, Vernino S, Irani SR.
Abstract
Importance: Autoimmune encephalitis misdiagnosis can lead to harm.
Objective: To determine the diseases misdiagnosed as autoimmune encephalitis and potential reasons for misdiagnosis.
Design, setting, and participants: This retrospective multicenter study took place from January 1, 2014, to December 31, 2020, at autoimmune encephalitis subspecialty outpatient clinics including Mayo Clinic (n = 44), University of Oxford (n = 18), University of Texas Southwestern (n = 18), University of California, San Francisco (n = 17), University of Washington in St Louis (n = 6), and University of Utah (n = 4). Inclusion criteria were adults (age ≥18 years) with a prior autoimmune encephalitis diagnosis at a participating center or other medical facility and a subsequent alternative diagnosis at a participating center. A total of 393 patients were referred with an autoimmune encephalitis diagnosis, and of those, 286 patients with true autoimmune encephalitis were excluded.
Main outcomes and measures: Data were collected on clinical features, investigations, fulfillment of autoimmune encephalitis criteria, alternative diagnoses, potential contributors to misdiagnosis, and immunotherapy adverse reactions.
Results: A total of 107 patients were misdiagnosed with autoimmune encephalitis, and 77 (72%) did not fulfill diagnostic criteria for autoimmune encephalitis. The median (IQR) age was 48 (35.5-60.5) years and 65 (61%) were female. Correct diagnoses included functional neurologic disorder (27 [25%]), neurodegenerative disease (22 [20.5%]), primary psychiatric disease (19 [18%]), cognitive deficits from comorbidities (11 [10%]), cerebral neoplasm (10 [9.5%]), and other (18 [17%]). Onset was acute/subacute in 56 (52%) or insidious (>3 months) in 51 (48%). Magnetic resonance imaging of the brain was suggestive of encephalitis in 19 of 104 patients (18%) and cerebrospinal fluid (CSF) pleocytosis occurred in 16 of 84 patients (19%). Thyroid peroxidase antibodies were elevated in 24 of 62 patients (39%). Positive neural autoantibodies were more frequent in serum than CSF (48 of 105 [46%] vs 7 of 91 [8%]) and included 1 or more of GAD65 (n = 14), voltage-gated potassium channel complex (LGI1 and CASPR2 negative) (n = 10), N-methyl-d-aspartate receptor by cell-based assay only (n = 10; 6 negative in CSF), and other (n = 18). Adverse reactions from immunotherapies occurred in 17 of 84 patients (20%). Potential contributors to misdiagnosis included overinterpretation of positive serum antibodies (53 [50%]), misinterpretation of functional/psychiatric, or nonspecific cognitive dysfunction as encephalopathy (41 [38%]).
Conclusions and relevance: When evaluating for autoimmune encephalitis, a broad differential diagnosis should be considered and misdiagnosis occurs in many settings including at specialized centers. In this study, red flags suggesting alternative diagnoses included an insidious onset, positive nonspecific serum antibody, and failure to fulfill autoimmune encephalitis diagnostic criteria. Autoimmune encephalitis misdiagnosis leads to morbidity from unnecessary immunotherapies and delayed treatment of the correct diagnosis.
GAD65 Autoimmune Encephalitis: A Cause of Rapidly Evolving Frontotemporal Atrophy
Alzheimer Dis Assoc Disord
Authors: Vanhoorne A, Van Langenhove T, Miatton M, Laureys G.
Abstract
We describe a patient who presented with subacute onset of short-memory impairment, disorientation, and gait instability, with progressive deterioration. Workup demonstrated glutamic acid decarboxylase antibody-related encephalitis. Aggressive immunotherapy with high-dose intravenous corticoids, followed by slow oral taper, plasmapheresis, rituximab, and cyclophosphamide did not halt disease progression. During follow-up, she developed a frontotemporal dementia phenotype. Serial imaging showed the appearance of marked atrophy of the frontal and anterior temporal regions. We conclude that glutamic acid decarboxylase antibody-related encephalitis may rarely present with a treatment-refractory frontotemporal phenotype.
COA
Certificate of Analysis
