Objective

Ataxia channelopathies share common features such as slow motor progression and variable degrees of cognitive dysfunction. Mutations in KCND3, encoding the K⁺ channel, Kv4.3, are associated with spinocerebellar ataxia 19 (SCA19), allelic with spinocerebellar ataxia 22 (SCA22). Mutations in KCNC3, encoding another K⁺ channel, Kv3.3, cause spinocerebellar ataxia 13 (SCA13). First, a comprehensive phenotype assessment was carried out in a family with autosomal dominant ataxia harboring two genetic variants in KCNC3 and KCND3. In order to evaluate the physiological impact of these variants on channel currents, in vitro studies were performed.

Methods

Clinical and psychometric evaluations, neuroimaging, and genotyping of a family (mother and son) affected by ataxia were carried out. Heterozygous and homozygous Kv3.3 A671V andKv4.3 V374A variants were evaluated in Xenopus laevis oocytes using two-electrode voltage-clamp. The influence of Kv4 conductance on neuronal activity was investigated computationally using a Purkinje neuron model.

Results

The main clinical findings were consistent with adult-onset ataxia with cognitive dysfunction, and acetazolamide (ACZ)-responsive paroxysmal motor exacerbations in the index case. Despite cognitive deficits, FDG-PET displayed hypometabolism mainly in the severely atrophic cerebellum. Genetic analyses revealed the new variant c.1121T>C (V374A), in KCND3 and c.2012T>C (A671V) in KCNC3. In vitro electrophysiology experiments on Xenopus oocytes demonstrated that the V374A mutant was non-functional when expressed on its own. Upon equal co-expression of WT and V374A channel subunits, Kv4.3 currents were significantly reduced in a dominant negative manner, without alterations of the gating properties of the channel. In contrast, Kv3.3 A671V, when expressed alone, exhibited moderately reduced currents compared to WT, with no effects on channel activation or inactivation. Immunohistochemistry demonstrated adequate cell membrane translocation of the Kv4.3 V374A variant, thus suggesting an impairment of channel function, rather than of expression. Computational modeling predicted an increased Purkinje neuron firing frequency upon reduced Kv4.3 conductance.

Conclusions Our findings suggest that Kv4.3 V374A, is likely pathogenic and associated with paroxysmal ataxia exacerbations, a new trait for SCA19/22. The present FDG PET findings contrast with a previous study demonstrating widespread brain hypometabolism in SCA19/22.