Characterization of the dominant inheritance mechanism of Episodic Ataxia type 2.

Episodic Ataxia type 2 (EA2) is an autosomal dominant neuronal disorder linked to mutations in the Cav2.1 subunit of P/Q-type calcium channels. In vitro studies have established that EA2 mutations induce loss of channel activity and that EA2 mutants can exert a dominant negative effect, suppressing normal Cav2.1 activity through protein misfolding and trafficking defects. To date, the role of this mechanism in the disease pathogenesis is unknown because no animal model exists. To address this issue, we have generated a mouse bearing the R1497X nonsense mutation in Cav2.1 (Cav2.1R1497X). Phenotypic analysis of heterozygous Cav2.1R1497X mice revealed ataxia associated with muscle weakness and generalized absence epilepsy. Electrophysiological studies of the cerebellar circuits in heterozygous Cav2.1R1497X mice highlighted severe dysregulations in synaptic transmission of the two major excitatory inputs as well as alteration of the spontaneous activity of Purkinje cells. Moreover, these neuronal dysfunctions were associated with a strong suppression of Cav2.1 channel expression in the cerebellum of heterozygous Cav2.1R1497X mice. Finally, the presence of Cav2.1 in cerebellar lipid raft microdomains was strongly impaired in heterozygous Cav2.1R1497X mice. Altogether, these results reveal a pathogenic mechanism for EA2 based on a dominant negative activity of mutant channels.

RNAi silencing of P/Q-type calcium channels in Purkinje neurons of adult mouse leads to episodic ataxia type 2.

Episodic ataxia type-2 (EA2) is a dominantly inherited human neurological disorder caused by loss of function mutations in the CACNA1A gene, which encodes the CaV2.1 subunit of P/Q-type voltage-gated calcium channels. It remains however unknown whether the deficit of cerebellar CaV2.1 in adult is in direct link with the disease. To address this issue, we have used lentiviral based-vector RNA interference (RNAi) to knock-down CaV2.1 expression in the cerebellum of adult mice. We show that suppression of the P/Q-type channels in Purkinje neurons induced motor abnormalities, such as imbalance and ataxic gait. Interestingly, moderate channel suppression caused no basal ataxia, while ß-adrenergic activation and exercise mimicked stress induced motor disorders. Moreover, stress-induced ataxia was stable, non-progressive and totally abolished by acetazolamide, a carbonic anhydrase inhibitor used to treat EA2. Altogether, these data reveal that P/Q-type channel suppression in adult mice supports the episodic status of EA2 disease.