Abstracts

Rationale: The human CLCN1 gene encodes the ClC-1 voltage-gated chloride channel that regulates excitability in skeletal muscle, where missense and nonsense mutations cause non-dystrophic myotonia congenita. Unlike family members CLCN2 (ClC-2) and CLCN3 (ClC-3), its presence and pathogenic potential in human brain have not been reported. We analyzed ion channel gene variant profiles in individuals with complex idiopathic epilepsy syndromes and identify a novel de novo CLCN1 human truncation mutation, demonstrate regional brain expression, and explore CLCN1 as a candidate gene for comorbid brain phenotypes in a mouse model of myotonia. Methods: We used parallel exomic sequencing of 237 ion channel subunit genes to screen individuals with a clinical diagnosis of epilepsy including those with a muscle/movement disorder co-morbidity. We examined regional expression of CLCN1 in human and mouse brain using RT-PCR, in situ hybridization, and Western immunoblotting. The truncated Clcn1-adr^mto2J mouse model of myotonia was evaluated for epilepsy by videoEEG. Results: We report a novel de novo CLCN1 truncation mutation in a patient with pharmacoresistant convulsive and absence seizures and a dystonic writer's cramp without evidence of other known genomic channelopathy for epilepsy. Molecular localization of CLCN1 and its mouse homologue Clcn1 revealed wide expression of mRNA and subunit protein in human and murine brain. Cortical EEG recordings in the myotonic Clcn1 truncation mouse demonstrated generalized synchronous discharges associated with absence-like behavioral arrest. Epileptiform discharges in littermate controls did not display behavioral arrest. Conclusions: Disruption of the ‘muscle' voltage-gated chloride channel gene reveals a candidate role for CLCN1 in human CNS excitability disorders. This is the first membrane ion channel gene linking a muscle and brain excitability phenotype. Our study illustrates how genetic screening can lead to the discovery of shared CNS expression of voltage-gated ion channels with disparate tissues, pointing to interesting and sometimes occult co-morbid neurological syndromes. Elucidating molecular links for these co-morbid syndromes may enhance their clinical detection as well as provide a more efficient therapeutic target in the treatment of multiple pathologies caused by single ion channel gene defects. Supported by Epilepsy Foundation Postdoctoral Fellowship (TLK), HHMI Medical Student Fellowship (MJK), and NINDS (JLN).