Novel brain expression of ClC-1 chloride channels and enrichment of CLCN1 variants in epilepsy

From the Departments of Neurology (T.T.C., T.L.K., A.M.G., J.L.N.), Molecular and Human Genetics (J.L.N.), and Neuroscience (J.L.N.), Baylor College of Medicine, Houston, TX
Neurology (Impact Factor: 8.29). 02/2013; 80(12). DOI: 10.1212/WNL.0b013e31828868e7
Source: PubMed


To explore the potential contribution of genetic variation in voltage-gated chloride channels to epilepsy, we analyzed CLCN family (CLCN1-7) gene variant profiles in individuals with complex idiopathic epilepsy syndromes and determined the expression of these channels in human and murine brain.

We used parallel exomic sequencing of 237 ion channel subunit genes to screen individuals with a clinical diagnosis of idiopathic epilepsy and evaluate the distribution of missense variants in CLCN genes in cases and controls. We examined regional expression of CLCN1 in human and mouse brain using reverse transcriptase PCR, in situ hybridization, and Western immunoblotting.

We found that in 152 individuals with sporadic epilepsy of unknown origin, 96.7% had at least one missense variant in the CLCN genes compared with 28.2% of 139 controls. Nonsynonymous single nucleotide polymorphisms in the "skeletal" chloride channel gene CLCN1 and in CLCN2, a putative human epilepsy gene, were detected in threefold excess in cases relative to controls. Among these, we report a novel de novo CLCN1 truncation mutation in a patient with pharmacoresistant generalized seizures and a dystonic writer's cramp without evidence of variants in other channel genes linked to epilepsy. Molecular localization revealed the unexpectedly widespread presence of CLCN1 mRNA transcripts and the ClC-1 subunit protein in human and murine brain, previously believed absent in neurons.

Our findings support a possible comorbid contribution of the "skeletal" chloride channel ClC-1 to the regulation of brain excitability and the need for further elucidation of the roles of CLCN genes in neuronal network excitability disorders.

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Available from: Alica Goldman, Aug 08, 2014
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    • "Compared with the P-domain channels and pLGICs, other ion channels are less well understood. Nonetheless, recent studies suggest that the CLC chloride channels play a role in synaptic transmission and plasticity [54,55], as well as in neuronal excitability [56]. We found in Daphnia that the Clc-c2 mRNA expression was downregulated by midazolam, pentobarbital and ketamine, while the CLC-c1 mRNA expression was also downregulated by midazolam and ketamine, suggesting a new type of molecular targets possibly involved in drug addiction. "
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    • "Even the observation that the Gly544Arg mutation strongly impaired ClC-4 ion transport does not prove a role of CLCN4 in human epilepsy, especially because Clcn4 À/À mice do not display seizures or any other obvious phenotype (Rickheit et al., 2010). Interestingly, recent work has found an enrichment of missense mutations in CLCN1 and CLCN2, but not of CLCN4 in individuals with complex idiopathic epilepsy syndromes (Chen et al., 2013). However it is important to note that the ClC-1 Cl-channel is predominantly expressed in skeletal muscle (Steinmeyer et al., 1991) and mutations in its gene cause myotonia (Koch et al., 1992; Jentsch, 2008), while previous work claiming a causative role of CLCN2 in epilepsy has been retracted (Haug et al., 2009). "
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