V374A KCND3 pathogenic variant associated with paroxysmal ataxia exacerbations

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.