The human epilepsy mutation GABRG2(Q390X) causes chronic subunit accumulation and neurodegeneration

Abstract

Genetic epilepsy and neurodegenerative diseases are two common neurological disorders conventionally viewed as being unrelated. A subset of patients with severe genetic epilepsies with impaired development and often death respond poorly to anticonvulsant drug therapy, suggesting a need for new therapeutic targets. Previously, we reported that multiple GABAA receptor epilepsy mutations caused protein misfolding and abnormal receptor trafficking. Here we establish in a novel model of a severe human genetic epileptic encephalopathy, the Gabrg2+/Q390X knock-in mouse, that in addition to impairing inhibitory neurotransmission, mutant GABAA receptor γ2(Q390X) subunits accumulated and aggregated intracellularly, activated caspase 3 and caused widespread, age-dependent neurodegeneration. These novel findings suggest that the fundamental protein metabolism and cellular consequences of the epilepsy-associated mutant γ2(Q390X) ion channel subunit are not fundamentally different from those associated with neurodegeneration. The study has far-reaching significance for identification of conserved pathological cascades and mechanism-based therapies that overlap genetic epilepsies and neurodegenerative diseases.

Biallelic mutations in the gene encoding eEF1A2 cause seizures and sudden death in F0 mice

“…It is difficult to establish the effects of loss of eEF1A2 on the brain as mutant animals die as a result of severe neurodegeneration in the spinal cord, but since motor neurons lacking eEF1A2 degenerate and die (presumably due to a failure to carry out de novo protein synthesis) it is reasonable to assume that neurodegeneration would also occur in the brain if the mice survived longer. Similarly, the strain of Gabrg2 mutant mice described above show widespread neurodegeneration in addition to their epilepsy and SUDEP phenotype, in this case caused by accumulation and intracellular aggregation of mutant GABAA receptor subunits15. Whilst there is no evidence that intracellular aggregates are forming in the eEF1A2-mutant mice, one of the disease causing mutations in humans, E122K, has been shown to be associated with translational infidelity in yeast17, a process that would be predicted to lead to accumulation of misfolded proteins.…”

“…SUDEP is a known, extreme, consequence of epilepsy responsible for a significant proportion of deaths in individuals with childhood-onset epilepsy. SUDEP has also been seen in a number of mouse models of epilepsy, for example mice with a knock-in of the Q390X mutation in the Gabrg2 gene, which can suffer SUDEP at any stage15 and those with a heterozygous knock-in of N1768D in the Scn8a gene; these mice also exhibit seizures and SUDEP16. An alternative explanation for the sudden deaths in the eEF1A2-null mice is neurodegeneration of brainstem nuclei leading to respiratory arrest.…”

Beyond Epilepsy and Autism: Disruption of GABRB3 Causes Ocular Hypopigmentation

“…Sectioned brain slices were mounted on Superfrost Plus slides for immunohistochemical studies. The H&E, Fontana Masson staining, and immunohistochemistry were performed with standard procedures in the Vanderbilt Translational Pathology Shared Resource Core (Kang et al, 2015). …”

Altered GABAA receptor expression in brainstem nuclei and SUDEP in Gabrg2+/Q390X mice associated with epileptic encephalopathy

“…Here we compared GABA A receptor expression and mortality of two genetically modified mice, Gabrg2 +/Q390X knockin (KI)(Kang et al, 2015) and Gabrg2 +/ − KO mice associated with different human epilepsy syndromes. The KI mouse is associated with a severe epileptic encephalopathy(Kang et al, 2015), while the KO mouse has been reported to be associated with a mild epilepsy only in mice with a seizure-prone genetic background(Crestani et al, 1999;Reid et al, 2013).…”

“…The KI mouse is associated with a severe epileptic encephalopathy(Kang et al, 2015), while the KO mouse has been reported to be associated with a mild epilepsy only in mice with a seizure-prone genetic background(Crestani et al, 1999;Reid et al, 2013). We have recently demonstrated that the heterozygous KO mice in a seizure-resistant background (C57BL/6J) also had increased frequency and duration of spike (Warner et al, in revision) wave discharges (SWDs), suggesting that the KO mice could serve as a mild epilepsy absence model (Warner et al, in revision).…”