GAD, diabetes, and Stiff-Man syndrome: some progress and more questions

Antiglutamic acid decarboxylase 65 (GAD65) antibody-associated epilepsy

Glutamic acid decarboxylase (GAD) antibody-associated encephalitis causes both acute seizures and chronic epilepsy with predominantly temporal lobe onset. This condition is challenging in diagnosis and management, and the incidence of GAD antibody (Ab)-related epilepsy could be much higher than commonly believed. Imaging and CSF evidence of inflammation along with typical clinical presentations, such as adult onset temporal lobe epilepsy (TLE) with unexplained etiology, should prompt testing for the diagnostic antibodies. High serum GAD Ab titer (≥2000U/mL or ≥20nmol/L) and evidence of intrathecal anti-GAD Ab synthesis support the diagnosis. Unlike other immune-mediated epilepsies, antiglutamic acid decarboxylase 65 (GAD65) antibody-mediated epilepsy is often poorly responsive to antiepileptic drugs (AEDs) and only moderately responsive to immune therapy with steroids, intravenous immunoglobulin (IVIG), or plasma exchange (PLEX). Long-term treatment with more aggressive immunosuppressants such as rituximab (RTX) and/or cyclophosphamide is often necessary and may be more effective than current immunosuppressive approaches. The aim of this review is to review the physiology, pathology, clinical presentation, related ancillary tests, and management of GAD Ab-associated autoimmune epilepsy by searching the keywords and to promote the recognition and the initiation of proper therapy for this condition.

GAD65 epitope mapping and search for novel autoantibodies in GAD-associated neurological disorders

Antibodies against Glutamic-acid-decarboxylase (GAD65) are seen in various CNS excitability disorders including stiff-person syndrome, cerebellar ataxia, encephalitis and epilepsy. To explore pathogenicity, we examined whether distinct epitope specificities or other co-existing antibodies may account for each disorder. The epitope recognized by all 27 tested patients, irrespective of clinical phenotype, corresponded to the catalytic core of GAD. No autoantibodies against known GABAergic antigens were found. In a screen for novel specificities using live hippocampal neurons, three epilepsy patients, but no other, were positive. We conclude that no GAD-specific epitope defines any neurological syndrome but other antibody specificities may account for certain phenotypes.

Autoimmune stiff person syndrome and related myelopathies: understanding of electrophysiological and immunological processes

Stiff person syndrome (SPS) is a disabling autoimmune central nervous system disorder characterized by progressive muscle rigidity and gait impairment with superimposed painful spasms that involve axial and limb musculature, triggered by heightened sensitivity to external stimuli. Impaired synaptic GABAergic inhibition resulting from intrathecal B-cell-mediated clonal synthesis of autoantibodies against various presynaptic and synaptic proteins in the inhibitory neurons of the brain and spinal cord is believed to be an underlying pathogenic mechanism. SPS is most often idiopathic, but it can occur as a paraneoplastic condition. Despite evidence that anti-GAD and related autoantibodies impair GABA synthesis, the exact pathogenic mechanism of SPS is not fully elucidated. The strong association with several MHC-II alleles and improvement of symptoms with immune-modulating therapies support an autoimmune etiology of SPS. In this review, we discuss the clinical spectrum, neurophysiological mechanisms, and therapeutic options, including a rationale for agents that modulate B-cell function in SPS.

GAD65 autoantibodies and its role as biomarker of Type 1 diabetes and Latent Autoimmune Diabetes in Adults (LADA)

One of the hallmarks of autoimmune diabetes is the presence of adaptive responses directed to neuroendocrine proteins. One of these proteins is glutamic acid decarboxylase (GAD). While GAD is widely distributed in neuroendocrine tissues, its specific significance in diabetes has paralleled the advances in understanding humoral and cellular immunity in Type 1 diabetes (T1D) and in a subset of Type 2 diabetes (T2D), going from the seminal discoveries of islet autoantibodies to the development and standardization of bioassays as diagnostic tools, to studies on the structure of GAD and its antigenic determinants. GAD65 autoantibodies can accurately predict T1D development in combination with other surrogate humoral biomarkers and they are considered the most sensitive and specific biomarker which identifies a subset of clinically diagnosed T2D termed Latent Autoimmune Diabetes in Adults (LADA). We and others provided evidence indicating that GAD65 autoantibody detection should be part of the diagnostic assessment for clinically diagnosed T2DM mainly because it predicts the rate of progression to insulin requirement in patients affected by LADA. More recently GAD has been used as a "tolerogenic vaccine" to preserve beta cell function in autoimmune diabetes. While the results of Phase III clinical trials did not substantiate the earlier promise of Phase I and II trials, there are still many unanswered questions and approaches that need to be investigated in the applications of GAD in the therapy of T1D and LADA.

A critical update on the immunopathogenesis of Stiff Person Syndrome

BACKGROUND

Stiff Person Syndrome (SPS) is a relatively rare but often overlooked autoimmune neurological disorder that targets antigens within the brain's inhibitory pathways resulting in incapacitating stiffness and spasms that impact on the patients' quality of life. Although a number of immunomodulating therapies significantly improve the patients' symptoms, the exact pathogenic mechanisms remain unclear.

MATERIALS AND METHODS

The current literature on SPS was reviewed and combined with the authors' experience with many patients and various laboratory studies. The majority of the patients have high-titre anti-GAD (Glutamic Acid Decarboxylase) antibodies in the sera and CSF suggesting dysfunction of the GABAergic neurotransmission. These antibodies are excellent disease markers but their pathogenic role remains uncertain.

CONCLUSIONS

This review provides a critical assessment on the immunobiology of SPS, describes the identification of anti-GABARAP antibodies as a new antigenic target in the GABAergic synapse and identifies the areas for future research.

Homozygosity for a missense mutation in the 67 kDa isoform of glutamate decarboxylase in a family with autosomal recessive spastic cerebral palsy: parallels with Stiff-Person Syndrome and other movement disorders

BACKGROUND

Cerebral palsy (CP) is an heterogeneous group of neurological disorders of movement and/or posture, with an estimated incidence of 1 in 1000 live births. Non-progressive forms of symmetrical, spastic CP have been identified, which show a Mendelian autosomal recessive pattern of inheritance. We recently described the mapping of a recessive spastic CP locus to a 5 cM chromosomal region located at 2q24-31.1, in rare consanguineous families.

METHODS

Here we present data that refine this locus to a 0.5 cM region, flanked by the microsatellite markers D2S2345 and D2S326. The minimal region contains the candidate gene GAD1, which encodes a glutamate decarboxylase isoform (GAD67), involved in conversion of the amino acid and excitatory neurotransmitter glutamate to the inhibitory neurotransmitter gamma-aminobutyric acid (GABA).

RESULTS

A novel amino acid mis-sense mutation in GAD67 was detected, which segregated with CP in affected individuals.

CONCLUSIONS

This result is interesting because auto-antibodies to GAD67 and the more widely studied GAD65 homologue encoded by the GAD2 gene, are described in patients with Stiff-Person Syndrome (SPS), epilepsy, cerebellar ataxia and Batten disease. Further investigation seems merited of the possibility that variation in the GAD1 sequence, potentially affecting glutamate/GABA ratios, may underlie this form of spastic CP, given the presence of anti-GAD antibodies in SPS and the recognised excitotoxicity of glutamate in various contexts.

Autoantibody testing

Recent advances in neuroimmunology have led to improvements in the pathogenesis, diagnosis, prognosis, and treatment of many neuromuscular disorders. The value of autoantibody testing is increasing steadily in neurologic practice. Not all antibodies have a high yield in diagnosis. In some disorders, such as generalized adult onset of myasthenia gravis, Lambert-Eaton myasthenic syndrome,Miller Fisher syndrome, and multifocal motor neuropathy,autoantibody tests provide accurate diagnosis and can be considered biologic markers of these disorders.

Is innervation an early target in autoimmune diabetes?

In the non-obese diabetic (NOD) mouse, a spontaneous model of type 1 diabetes (T1D), recent evidence suggests that Schwann cells (Scs) and neurons surrounding insulin-producing beta cells of the islets of Langerhans are destroyed before beta cells. During normal perinatal development, macrophages (MPhi) are involved in phagocytosis of apoptotic neurons. Pertinently, MPhi are already present at birth in NOD pancreata. Their possible abnormal control of nerve phagocytosis, together with transient beta-cell hyperactivity and lymphocyte anomalies, might conjointly participate in T1D pathogenesis.

Autoantibodies to glutamic acid decarboxylase in patients with therapy-resistant epilepsy

BACKGROUND

Autoantibodies to glutamic acid decarboxylase (GAD-A) are present in type 1 diabetes and stiff man syndrome (SMS), and have also been reported in cerebellar ataxia. Epilepsy was present in 4 of 19 patients with SMS and GAD-A, implying that epilepsy sometimes is associated with anti-GAD autoimmunity.

METHODS

The authors investigated the prevalence of GAD-A in patients with therapy-resistant localization-related epilepsy (n = 51) and generalized epilepsy (n = 49) by a radiobinding assay. The positive samples were confirmed by immunohistochemistry and immunoblotting of recombinant human GAD65.

RESULTS

GAD-A were found in eight patients with localization-related epilepsy, whereas none of the patients with generalized epilepsy, other neurologic disorders (n = 38), or the control subjects (n = 48) had GAD-A. Two patients had high levels of GAD-A, similar to SMS, whereas six patients had significantly lower titers, characteristic of type 1 diabetes. The two patients with high levels of GAD-A had GAD-A both in serum and CSF by immunohistochemistry and immunoblotting. Both of them had longstanding therapy-resistant temporal lobe epilepsy but did not have diabetes. One had a history of autoimmune disease, whereas the other had serologic evidence of multiple autoantibodies without any clinical signs of autoimmune disease.

CONCLUSIONS

GAD autoimmunity may be associated with refractory localization-related epilepsy.

The stiff-person syndrome: an autoimmune disorder affecting neurotransmission of gamma-aminobutyric acid

The stiff-person syndrome, a rare and disabling disorder, is characterized by muscle rigidity and episodic spasms that involve axial and limb musculature. Continuous contraction of agonist and antagonist muscles caused by involuntary motor-unit firing at rest are the hallmark clinical and electrophysiologic signs of the disease. Except for global muscle stiffness, results of neurologic examination are usually normal. Results of conventional computed tomography and magnetic resonance imaging of the brain are also normal. The cause of the stiff-person syndrome is unknown; however, an autoimmune pathogenesis is suspected because of 1) the presence of antibodies against glutamic acid decarboxylase (GAD), the rate-limiting enzyme for the synthesis of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA); 2) the association of the disease with other autoimmune conditions; 3) the presence of various autoantibodies; and 4) a strong immunogenetic association. Anti-GAD antibodies, which are found in high titers in most patients, seem to be directed against conformational forms of GAD. New evidence suggests that these antibodies may be pathogenic because they interfere with the synthesis of GABA. In addition, a reduction in brain levels of GABA, which is prominent in the motor cortex, has been demonstrated with magnetic resonance spectroscopy in patients with the stiff-person syndrome. The stiff-person syndrome is clinically elusive but potentially treatable and should be considered in patients with unexplained stiffness and spasms. Drugs that enhance GABA neurotransmission, such as diazepam, vigabatrin, and baclofen, provide mild to modest relief of clinical symptoms. Immunomodulatory agents, such as steroids, plasmapheresis, and intravenous immunoglobulin, seem to offer substantial improvement. Results of an ongoing controlled trial will elucidate the role of these agents in the treatment of the disease.

Radioimmunoassay for glutamic acid decarboxylase-65

BACKGROUND

Glutamic acid decarboxylase-65 (GAD65), the enzyme that catalyzes the formation of gamma-aminobutyric acid (GABA), is the major autoantigen in both type 1 (insulin-dependent) diabetes and stiff-man syndrome (SMS). The observation that GAD65 autoantibodies may be present for years before the clinical onset of diabetes raises the question of when GAD65 is available to initiate an immune response to allow the formation of autoantibodies. In order to address this question it will be necessary to measure GAD65 in tissue, cells, and plasma.

METHODS

A radioimmunoassay (RIA) was developed for GAD65 based on the use of a polyclonal rabbit antiserum directed to the N-terminus of GAD65.

RESULTS

Using the GAD65 RIA, we have determined the GAD65 content in a human GAD65 gene transfected cell line and in beta-cell preparations from different species. The assay detects an increase of immunoreactive GAD65 after glucose-stimulation and GAD65 that is discharged from rat beta cells after their exposure to the diabetogenic agent streptozotocin. It also measures good recovery of GAD65 added to human plasma samples.

CONCLUSIONS

The GAD65 RIA makes it possible to determine both cellular and extracellular levels of GAD65; this might be useful in investigating the mechanisms leading to the formation of GAD65 autoantibodies in type 1 diabetes and SMS patients.

A monoclonal antibody-based characterization of autoantibodies against glutamic acid decarboxylase in adults with latent autoimmune diabetes

Autoantibodies to glutamic acid decarboxylase (GAD) are an important marker of the autoimmune-mediated beta-cell destruction in insulin-dependent (Type I) diabetes. However, these autoantibodies are also found in patients with Stiff-man syndrome (SMS) without onset of diabetes and some diabetic patients who initially present as non-insulin dependent (Type II) diabetes later becoming insulin-dependent, called as latent autoimmune diabetes in adults (LADA). To study the immune response to GAD in these LADA patients a competitive radiobinding assay based on murine monoclonal antibodies recognizing three different GAD regions was performed. The monoclonal antibodies against GAD recognize two different linear epitopes localized at the N- (amino acids 4-17) and C-terminus (amino acids 572-585) and one conformation-dependent epitope region (amino acids 221-442 IDDM-E1) known to be immunodominant for diabetes-associated autoantibodies. All LADA sera (20/20) reduced substantially the 125I-GAD binding of the monoclonal antibodies reactive with the conformation-dependent epitope region IDDM-E1 and only 20% of these sera additionally diminished the 125I-GAD65 binding by those monoclonals reactive with the both linear epitopes. The SMS sera completely abolished the GAD binding of all three monoclonals, reflecting a broader repertoire including an immune response against the IDDM-E1, a conformation-dependent GAD65 epitope region, also revealed if the SMS sera are diluted to equivalent antibody concentrations. In summary, our results show that diabetes-associated GAD autoantibodies even in adult patients with a late autoimmune process preferentially recognize a conformation-dependent middle GAD65 region. An immune response to all three GAD epitope regions is seldom in these LADA patients and only detectable in association with high antibody titres.

T cell recognition of human pre-proinsulin peptides depends on the polymorphism at HLA DQ locus: a study using HLA DQ8 and DQ6 transgenic mice

HLA DQ8 (DQ A1*0301/DQB1*0302) molecule is implicated in the susceptibility to insulin dependent diabetes mellitus whereas, HLA DQ6 (DQ A1*0103/DQB1*0601) molecule may have a protective effect. In this study we used mice transgenic to HLA DQ8 and HLA-DQ6 to elucidate the T cell determinants on a putative islet cell target antigen, insulin. These mice do not express endogenous mouse class II heterodimers on cell surface. Using overlapping synthetic peptides spanning the complete sequence of huma pre-proinsulin, we identified the sequences recognized by T cells in DQ8 transgenic mice and compared these to those in DQ6 transgenic mice. We observed a differential pattern of recognition of epitopes on human pre-proinsulin (HPI) polypeptide presented by the HLA DQ8 allele as compared to HLA DQ6. The sequences 1-24 and 44-63 were immunodominant in DQ8 transgenic mice while DQ6 transgenic mice primarily recognized sequences 14-33 and 74-93 of HPI. We found that the immune response generated in HLA DQ8 transgenic mice against HPI 1-24 cross-reacted to the mouse pre-proinsulin sequence 1-24. The T cell response were specifically inhibited using anti-CD4 and anti-DQ8 monoclonal antibodies. This cross-recognition of self sequences raises the possibility of modulation of experimental diabetes using this peptide.

Murine monoclonal glutamic acid decarboxylase (GAD)65 antibodies recognize autoimmune-associated GAD epitope regions targeted in patients with type 1 diabetes mellitus and stiff-man syndrome

To study the immune response to glutamic acid decarboxylase (GAD) in insulin-dependent diabetes mellitus, monoclonal GAD antibodies after fusion of splenocytes from a nondiabetes-susceptible BALB/c mouse immunized with human recombinant GAD65 were generated. Of the 44 monoclonals, 35 are specific for the GAD65 isoform, whereas 9 also react with GAD67. Some 37 monoclonals, including all GAD65/67 reactive antibodies, react with GAD by Western blot analysis. The remaining 7 GAD65 monoclonals bind GAD only in an immunoprecipitation assay, which implies that they target epitopes dependent on the conformation of the GAD molecule. The 125I-GAD binding of the GAD65 monoclonals reactive on Western blotting was significantly diminished by all 3 sera from Stiff-man syndrome patients but only by 3/30 (10%) sera from type 1 diabetic patients. In contrast, the 7 monoclonal antibodies reactive with a conformation-dependent GAD epitope were competitive with 83% of GAD-autoantibody-positive sera from these diabetic patients. Using chimeric GAD65/67 proteins, the epitope region targeted by these monoclonals was mapped to the middle of GAD65 (amino acids 221-442). This central conformation-dependent GAD region was also targeted by sera from patients with type 1 diabetes. In conclusion, our data show that even after common immunization of a nondiabetes-susceptible mouse strain, monoclonal were obtained which preferentially react with the GAD65 linear amino-terminus (amino acids 4-17) and a conformation-dependent region located in the middle of GAD targeted by autoantibodies, indicating that this GAD region is not restricted to the autoimmune response associated with the Stiff-man syndrome and the beta-cell destruction in type 1 diabetes mellitus.

Comparison of the islet cell antibody pattern of monoclonal glutamic acid decarboxylase antibodies recognizing linear and conformational epitopes

In order to compare the reactivity of glutamic acid decarboxylase (GAD) antibodies recognizing linear and conformational epitopes as islet cell cytoplasmic antibodies (ICA), monoclonal antibodies were generated. An ELISA displacement test using two biotinylated monoclonals recognizing a linear (M61/7E11) or a conformational GAD65 epitope (M65/6B12) was performed to identify epitope regions recognized by monoclonal GAD antibodies. The GAD binding by monoclonal GAD antibodies was tested by immunofluorescence on fixed and unfixed pancreatic sections of human, rat, and mouse, and by Dot-blot experiments. 16/23 (69.6%) of the monoclonals were specifically reactive with GAD65 and 7/23 (30.4%) were reactive with both GAD isoforms. 8/16 (50%) of monoclonal GAD65 antibodies recognized a linear GAD epitope located at the N-terminus (pattern 1). 5/16 (31.3%) displaced M65/6B12, indicating the recognition of a conformational GAD epitope (pattern 2). Monoclonals belonging to patterns 1 and 2 showed strong ICA binding. 3/16 (18.8%) of monoclonals specific for GAD65 with weak or no immunostaining of pancreatic islets (pattern 3) did not inhibit the binding of both biotinylated antibodies in the displacement test, indicating other epitope specificities. In conclusion, GAD antibodies recognizing both conformational and linear epitopes of the GAD65 molecule are involved in ICA binding with strong reactivity. Furthermore, results obtained with monoclonals of pattern 3 suggest the occurrence of GAD65 epitopes partly inaccessible on cryosections, which may result in an ICA-negative test of GAD65 autoantibody positive sera.