Alternative titles; symbols
HGNC Approved Gene Symbol: GAD2
Cytogenetic location: 10p12.1 Genomic coordinates (GRCh38): 10:26,216,372-26,304,558 (from NCBI)
Glutamate decarboxylase (GAD; L-glutamate 1-carboxy-lyase; EC 4.1.1.15), which catalyzes formation of gamma-aminobutyric acid (GABA) from L-glutamic acid, is detectable in different isoforms with distinct electrophoretic and kinetic characteristics. The enzyme has also been implicated as an autoantigen in the autoimmune disease stiff person syndrome (SPS; 184850) and in insulin-dependent diabetes mellitus (IDDM; 222100) (Karlsen et al., 1991).
See also GAD1 (605363), which maps to chromosome 2q31.
Using sequence information from GAD1 to screen a human pancreatic islet cDNA library, Karlsen et al. (1991) isolated a second GAD cDNA (GAD2). GAD2 recognized a 5.6-kb transcript in both islets and brain; in contrast, GAD1 recognized a 3.7-kb transcript in brain only. The deduced 585-amino acid sequence encoded by GAD2 shows less than 65% identity to previously published, highly conserved GAD1 brain sequences, which show more than 96% deduced amino acid sequence homology among rat, mouse, and cat.
Bu et al. (1992) isolated a cDNA corresponding to the GAD2 gene from a human brain cDNA library. The deduced 585-amino acid protein has a molecular mass of 65 kD. The protein showed 95% identity to the rat protein; the human GAD65 and GAD67 proteins showed 65% identity. Northern blot analysis detected a 5.7-kb mRNA transcript for GAD65.
Erlander et al. (1991) determined that the brain contains 2 forms of GAD which differ in molecular size, amino acid sequence, antigenicity, cellular and subcellular location, and interaction with the GAD cofactor pyridoxal phosphate (PLP). They reported the cloning of rat Gad65 cDNA and demonstrated the different nucleotide sequences of Gad65 and Gad67. The 2 cDNAs hybridized to genomic fragments of different sizes, implying that they are encoded by 2 distinct genes. Erlander et al. (1991) showed that the enzymatic activity of Gad65 was more responsive to PLP than that of Gad67.
Karlsen et al. (1991) mapped the GAD2 gene to chromosome 10p13-p11.2 by in situ hybridization.
Bu et al. (1992) mapped the GAD2 gene to 10p11.23 by in situ hybridization of fluorescently labeled GAD probes to human chromosomes. By in situ hybridization, Edelhoff et al. (1993) mapped the GAD2 gene to human 10p12-p11.2 and to mouse 2A2-B, thereby identifying a new region of conservation between human and mouse chromosomes.
Cram et al. (1991) concluded that there are 45 nucleotide differences between the brain and islet GAD sequences; at the translational level, this would result in 7 amino acid substitutions. The differences in the isomeric forms may account for the fact that the stiff man syndrome and insulin-dependent diabetes mellitus are clinically distinct.
Both GAD65, encoded by the GAD2 gene, and GAD67, encoded by the GAD1 gene, are targets of autoantibodies in people who later develop insulin-dependent diabetes mellitus (IDDM; 222100) (Baekkeskov et al., 1990; Kaufman et al., 1992; Richter et al., 1992). A 24-amino acid residue segment of GAD65 shares 10 identities and 9 similarities with the P2-C protein of coxsackievirus, an agent often suggested as an environmental triggering agent of insulin-dependent diabetes mellitus. Autoimmunity in IDDM may thus arise by 'molecular mimicry' (Albert and Inman, 1999) between GAD and a viral polypeptide. De Aizpurua et al. (1992) demonstrated that autoantibody against GAD was present in most subjects defined as having preclinical IDDM and that pancreatic islet and brain GAD are probably cross-reactive. The low frequency of GAD antibodies in recent-onset IDDM subjects was thought to indicate either that immunoreactivity is lost with near-total beta-cell destruction or that GAD antibodies denote a low risk of progression to clinical disease.
Although cross-reactivity between coxsackievirus P2-C and GAD65 has been demonstrated in mice, and an immune response to the homologous peptides is generated by immunization of mice with full-length proteins (Tian et al., 1994), cross-reactivity between GAD and coxsackievirus P2-C has not been found consistently in studies of T cells or serum antibodies from patients with diabetes. Furthermore, a search of databases identified 17 viruses with some homology to various fragments of GAD65 (Jones and Armstrong, 1995), indicating that cross-reactivity between GAD65 and coxsackieviruses is not unique.
Kobayashi et al. (2003) studied disease-specific epitope profiles of GAD65 autoantibodies (GAD65Ab) in slowly progressive type 1 (insulin-dependent) diabetes mellitus (SPIDDM) and acute onset type 1 (insulin-dependent) diabetes mellitus (AIDDM) using 7 kinds of GAD65/67 chimeric molecules. GAD65Ab in all SPIDDM samples reacted specifically with an N-terminal linear epitope located on the membrane-anchoring domain between amino acids 17 and 51 and a C-terminal conformational epitope between amino acids 443 and 585 of GAD65. The binding of GAD65Ab with the N-terminal 83 residues in SPIDDM inversely correlated with the period in which insulin was not required. GAD65Ab in AIDDM did not react with this N-terminal epitope, irrespective of the titer of GAD65Ab. A novel epitope of GAD65Ab in AIDDM residing between amino acids 244 to 360 was identified in 17% (8 of 46) of patients whose age of onset was younger than other AIDDM patients. The authors concluded that an association is suggested between GAD65Ab targeted to this region and slowly progressive beta-cell failure in SPIDDM.
Xiang et al. (2007) reported that an excitatory rather than inhibitory GABAergic system exists in airway epithelial cells. Both GABA-A receptors and the GABA synthetic enzyme glutamic acid decarboxylase are expressed in pulmonary epithelial cells. Activation of GABA-A receptors depolarized these cells. The expression of GAD65/67 in the cytosol and GABA-A receptors in the apical membranes of airway epithelial cells increased markedly when mice were sensitized and then challenged with ovalbumin, an approach for inducing allergic asthmatic reactions. Similarly, GAD and GABA-A receptors in airway epithelial cells of humans with asthma increased after allergen inhalation challenge. Intranasal application of selective GABA-A receptor inhibitors suppressed the hyperplasia of goblet cells and the overproduction of mucus induced by ovalbumin or interleukin-13 (147683) in mice. Xiang et al. (2007) concluded that the airway epithelial GABAergic system has an essential role in asthma.
Fink et al. (2014) used Gad2 as a genetic entry point to manipulate the spinal GABAergic interneurons that contact sensory terminals, and showed that activation of these interneurons in mice elicits the defining physiologic characteristics of presynaptic inhibition. Selective genetic ablation of Gad2-expressing interneurons severely perturbs goal-directed reaching movements, uncovering a pronounced and stereotypic forelimb motor oscillation, the core features of which are captured by modeling the consequences of sensory feedback at high gain. Fink et al. (2014) concluded that their findings defined the neural substrate of a genetically hardwired gain-control system crucial for the smooth execution of movement.
Among 1,122 patients with type 2 diabetes (T2D; 125853), Tuomi et al. (1999) found GAD antibody in 9.3%, a significantly higher prevalence than that found in patients with impaired glucose tolerance or in controls. The GADab+ patients had lower fasting C-peptide concentration, lower insulin response to oral glucose, and higher frequency of the high-risk HLA-DQB1*0201/0302 (see 604305) genotype (though significantly lower than in patients with type 1 diabetes) when compared with GADab- patients. Tuomi et al. (1999) suggested the designation latent autoimmune diabetes in adults (LADA) to define the subgroup of type 2 diabetes patients with GADab positivity (greater than 5 relative units) and age at onset greater than 35 years.
Lohmann et al. (2000) compared T-cell responses in 14 stiff man syndrome (184850) patients with axial disease and 17 patients with type 1 diabetes (T1D; 222100). Peripheral blood T cells of 8 SMS patients recognized different immunodominant epitopes of GAD65 compared with T cells from 17 patients with type 1 diabetes. GAD regions 81-171 and 313-403 induced a dominant T-cell response in 6 of 8 patients with SMS but in only 1 of 17 patients with type 1 diabetes (P = 0.001). No SMS patients responded dominantly to GAD fragments 161-243 and 473-555 compared with 10 patients with type 1 diabetes (P = 0.008). GAD antibodies were detected in 11 of 14 SMS patients (7 of whom had diabetes) and in 11 of 17 patients with type 1 diabetes; IgG1 was dominant in both groups. SMS patients, however, were more likely than patients with diabetes to have isotypes other than IgG1, in particular, IgG4 or IgE isotypes, which were not detected in patients with type 1 diabetes. Lohmann et al. (2000) concluded that their data indicate differences between patients with SMS and type 1 diabetes in cellular (epitope recognition) and humoral (isotype pattern) responses to GAD65.
Drost et al. (2004) detected GAD antibodies in a patient with Satoyoshi syndrome (600705).
Meyre et al. (2005) studied the potential effects of the functional -243A-G polymorphism in the 5-prime promoter region of the GAD2 gene on fetal growth, insulin secretion, food intake, and risk of obesity in 635 severely obese French Caucasian children from 3 different medical centers. The case-control study confirmed the association between the GAD2 single-nucleotide polymorphism (SNP) -243A-G and obesity (odds ratio, 1.25; P = 0.04). In addition, SNP -243 GG children carriers showed a 270-gram lower birth weight and a 1.5-cm lower birth height compared with AA carriers (P = 0.009 and P = 0.013, respectively). The relation between birth weight and Z score of body mass index (BMI) was linear in AA carrier children (P = 0.00001) and quadratic. Eighteen percent of GG obese carriers versus 5.7% of AA carriers reported binge eating phenotype (P = 0.04). The authors concluded that these results confirmed the association between the GAD2 -243 promoter SNP and the risk for obesity and suggested that GAD2 may be a polygenic component of the complex mechanisms linking birth weight to further risk for metabolic diseases, possibly involving the pleiotropic effect of insulin on fetal growth and later on feeding behavior.
Asada et al. (1996) found that Gad65 -/- mice showed no change in brain GABA content or animal behavior, except for a slight increase in susceptibility to seizures. Kash et al. (1997) stated that in contrast to Gad67-/- animals, which are born with developmental abnormalities and die shortly after birth, Gad65-/- mice appear normal at birth; however, they develop spontaneous seizures that result in increased mortality. Seizures were precipitated by fear or mild stress. Kash et al. (1997) found that seizure susceptibility was dramatically increased in Gad65-/- mice backcrossed into a second genetic background, the nonobese diabetic (NOD/LtJ) strain of mice. The generally higher basal brain GABA levels in this backcross were significantly decreased by the Gad65-/- mutation, suggesting to Kash et al. (1997) that the relative contribution of GABA synthesized by Gad65 to total brain GABA levels is genetically determined. The data suggested that GABA synthesized by Gad65 is important in the dynamic regulation of neural network excitability, implicated at least 1 modifier locus in the NOD/LtJ strain, and presented Gad65-/- animals as a model of epilepsy involving GABA-ergic pathways.
Yoon et al. (1999) made transgenic mice expressing an antisense construct against Gad65 and Gad67. The various lines of mice expressed different amounts of the antisense GAD. Beta cell-specific suppression of GAD expression in 2 lines of antisense GAD transgenic NOD mice prevented autoimmune diabetes, whereas persistent GAD expression in the beta cells in the other 4 lines of antisense GAD-transgenic NOD mice resulted in diabetes similar to that seen in transgene-negative NOD mice. Complete suppression of beta-cell GAD expression blocked the generation of diabetogenic T cells and protected islet grafts from autoimmune injury. Thus, beta cell-specific GAD expression is required for the development of autoimmune diabetes in NOD mice, and modulation of GAD might, therefore, have therapeutic value in type 1 diabetes (222100). The expression of GAD was blocked only in beta islet cells and not in brain. In addition to the absence of T cells directed against GAD, there were also fewer T cells reactive to other beta islet-specific autoantigens, such as insulin, in the antisense NOD mice, but not in the nontransgenic control animals. In an accompanying article, von Boehmer and Sarukhan (1999) suggested that GAD is the initiating autoantigen in human type 1 diabetes because GAD-specific autoantibodies are among the first to appear in the prediabetic phase in human patients.
In rats, Zhang et al. (2011) showed that persistent inflammatory pain was associated with epigenetic changes in the brainstem nucleus raphe magnus, namely global histone hyperacetylation. However, transcription of Gad65 was suppressed via histone deacetylase (HDAC)-mediated histone hypoacetylation, resulting in impaired GABA synaptic inhibition. Gad65 knockout mice showed sensitized pain behavior and impaired GABA synaptic function in brainstem neurons. In wildtype but not Gad-knockout mice, HDAC inhibitors strongly increased GAD65 activity, restored GABA synaptic function, and relieved sensitized pain behavior. These findings suggested GAD65 and HDACs as potential therapeutic targets in an epigenetic approach to the treatment of chronic pain.
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Kaufman, D. L., Erlander, M. G., Clare-Salzler, M., Atkinson, M. A., Maclaren, N. K., Tobin, A. J. Autoimmunity to two forms of glutamate decarboxylase in insulin-dependent diabetes mellitus. J. Clin. Invest. 89: 283-292, 1992. [PubMed: 1370298] [Full Text: https://doi.org/10.1172/JCI115573]
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Tuomi, T., Carlsson, A., Li, H., Isomaa, B., Miettinen, A., Nilsson, A., Nissen, M., Ehrnstrom, B.-O., Forsen, B., Snickars, B., Lahti, K., Forsblom, C., Saloranta, C., Taskinen, M.-R., Groop, L. C. Clinical and genetic characteristics of type 2 diabetes with and without GAD antibodies. Diabetes 48: 150-157, 1999. [PubMed: 9892237] [Full Text: https://doi.org/10.2337/diabetes.48.1.150]
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Xiang, Y.-Y., Wang, S., Liu, M., Hirota, J. A., Li, J., Ju, W., Fan, Y., Kelly, M. M., Ye, B., Orser, B., O'Byrne, P. M., Inman, M. D., Yang, X., Lu, W.-Y. A GABAergic system in airway epithelium is essential for mucus overproduction in asthma. Nature Med. 13: 862-867, 2007. [PubMed: 17589520] [Full Text: https://doi.org/10.1038/nm1604]
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Zhang, Z., Cai, Y. Q., Zou, F., Bie, B., Pan, Z. Z. Epigenetic suppression of GAD65 expression mediates persistent pain. Nature Med. 17: 1448-1455, 2011. [PubMed: 21983856] [Full Text: https://doi.org/10.1038/nm.2442]