GABAA receptor subunit gene mutations are frequently associated with epilepsy. Most such epilepsy syndromes are rather benign and are outgrown as patient age. However, truncation mutations in the GABAA receptor gamma2 subunit gene GABRgamma2 are often but not always associated with a more severe phenotype, Dravet syndrome (DS). To date, most DS patients are associated with SCN1A loss of function mutations that result in impaired GABAergic interneuron activity and action potential firing. Synaptic GABAA receptors mediate inhibitory GABAergic transmission and characterizations of DS mouse model harboring GABRgamma2 (Q351X) mutation would provide new insights into the pathogenesis of DS as well as epilepsy in general. Combination of understandings from both SCN1A and GABRgamma2 mouse models will help pinpoint the final common pathophysiologic pathway on which two distinct groups of mutations converge. GABRgamma2 (Q351X) mutation is associated with two DS pedigrees. The heterozygous GABRgamma2 (Q351X) knockin (KI) mice which are patient condition recapitulate the major features of DS. The mice displayed multiple neurodevelopmental abnormalities and multiple forms of epilepsy including generalized tonic clonic epilepsy, suggesting impaired synapse formation and function. Since simple GABRgamma2 gene knockout (KO) heterozygous mice do not have seizures, this suggests that presence of the mutant gamma2 (Q351X) subunit protein and its related pathology contribute to the severe DS phenotype. We have previously demonstrated that the mutant gamma2 (Q351X) subunits were loss of function. Additionally, the mutant protein accumulated intracellularly, formed aggregates, and imposed a dominant-negative effect on the wildtype subunits. The mutant aggregates were identified by mass spectrometry, and the components of the aggregates were similar to those identified in inclusions found in neurodegenerative diseases, such as the Lewy bodies characteristic of Parkinson's disease. Our pilot data demonstrated that the mutant protein also formed substantial aggregates in the heterozygous KI mice. The mutant subunits impaired GABAergic synaptogenesis and GABAergic transmission. The single quantum dot imaging indicated the synapse vs. extrasynapse distribution of GABAA receptors were altered in the mutant KI mice. We hypothesize that the accumulation and aggregation of the mutant gamma2 (Q351X) subunits impair synapse development and GABAergic transmission, thus resulting in the severe phenotype as DS. In this proposal, we will characterize in detail the mutant subunit accumulation, aggregation and its impact on synapse formation, stabilization, connectivity and transmission. We will also characterize adaptive changes of wildtype GABAA receptor expression, distribution, mobility and turnover as well as neurobehaviors in the mutant GABRG2 (Q351X) heterozygous KI mice. We found overexpression of heat shock protein (Hsp)70 and Hsp40 reduced the total mutant and aggregated gamma2 subunit protein. We thus propose to test a novel therapeutic strategy by upregulating chaperones like Hsp70 and Hsp40 in the mutant KI mice.

Public Health Relevance

Seizure disorders (epilepsies) are very common and Dravet syndrome (DS) is the most severe kind among seizure disorders. The patients suffering from DS have intractable seizures and mental impairment and with poor outcome. In this proposal, we will use a mouse model which harbors a mutation causing DS in humans and manifests the symptoms seen in humans with DS to study the detailed underlying mechanisms of DS and to develop a novel therapeutic strategy against this disorder.

Agency
National Institute of Health (NIH)
Institute
National Institute of Neurological Disorders and Stroke (NINDS)
Type
Research Project (R01)
Project #
5R01NS082635-05
Application #
9304342
Study Section
Clinical Neuroplasticity and Neurotransmitters Study Section (CNNT)
Program Officer
Whittemore, Vicky R
Project Start
2013-07-01
Project End
2019-06-30
Budget Start
2017-07-01
Budget End
2019-06-30
Support Year
5
Fiscal Year
2017
Total Cost
Indirect Cost
Name
Vanderbilt University Medical Center
Department
Type
DUNS #
079917897
City
Nashville
State
TN
Country
United States
Zip Code
37232
Mi, Deborah J; Dixit, Shilpy; Warner, Timothy A et al. (2018) Altered glutamate clearance in ascorbate deficient mice increases seizure susceptibility and contributes to cognitive impairment in APP/PSEN1 mice. Neurobiol Aging 71:241-254
Kang, Jing-Qiong (2017) Defects at the crossroads of GABAergic signaling in generalized genetic epilepsies. Epilepsy Res 137:9-18
Huang, Xuan; Zhou, Chengwen; Tian, Mengnan et al. (2017) Overexpressing wild-type ?2 subunits rescued the seizure phenotype in Gabrg2+/Q390X Dravet syndrome mice. Epilepsia 58:1451-1461
Warner, Timothy A; Liu, Zhong; Macdonald, Robert L et al. (2017) Heat induced temperature dysregulation and seizures in Dravet Syndrome/GEFS+ Gabrg2+/Q390X mice. Epilepsy Res 134:1-8
Wang, Juexin; Shen, Dingding; Xia, Geqing et al. (2016) Differential protein structural disturbances and suppression of assembly partners produced by nonsense GABRG2 epilepsy mutations: implications for disease phenotypic heterogeneity. Sci Rep 6:35294
Warner, Timothy A; Shen, Wangzhen; Huang, Xuan et al. (2016) Differential molecular and behavioural alterations in mouse models of GABRG2 haploinsufficiency versus dominant negative mutations associated with human epilepsy. Hum Mol Genet 25:3192-3207
Xia, Geqing; P Pourali, Sarah; Warner, Timothy A et al. (2016) Altered GABAA receptor expression in brainstem nuclei and SUDEP in Gabrg2(+/Q390X) mice associated with epileptic encephalopathy. Epilepsy Res 123:50-4
Delahanty, Ryan J; Zhang, Yanfeng; Bichell, Terry Jo et al. (2016) Beyond Epilepsy and Autism: Disruption of GABRB3 Causes Ocular Hypopigmentation. Cell Rep 17:3115-3124
Kang, Jing-Qiong; Macdonald, Robert L (2016) Molecular Pathogenic Basis for GABRG2 Mutations Associated With a Spectrum of Epilepsy Syndromes, From Generalized Absence Epilepsy to Dravet Syndrome. JAMA Neurol 73:1009-16
Kang, Jing-Qiong; Shen, Wangzhen; Zhou, Chengwen et al. (2015) The human epilepsy mutation GABRG2(Q390X) causes chronic subunit accumulation and neurodegeneration. Nat Neurosci 18:988-96

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