Familial hemiplegic migraine (FHM) is a group of rare familial disorders caused, in most cases, by mutations in CACNA1A and ATP1A2.1 Heterozygous mutations in solute carrier family 1 member 3 (SLC1A3), encoding glial glutamate transporter, are associated with episodic ataxia type 6 (EA6).2,–5 In addition to episodic ataxia (EA), alternating hemiplegia and hemiplegic migraine have been reported twice in patients with EA6.2,4 Mutations in SLC1A3 are very rare; screenings in cohorts of EA and alternating hemiplegia have yielded either negative results3,6,–8 or variants of unclear significance.e-1--e-3 Herein, we characterize a patient featuring sporadic hemiplegic migraine (SHM) responsive to acetazolamide (ACZ) and mild cerebellar atrophy associated with a new variant in SLC1A3. We also report the utility of proton magnetic resonance spectroscopy (MRS) for this condition.

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.

Objective: To delineate the full phenotypic spectrum, discriminative features, piloting longitudinal progression data, and sample size calculations of RFC1-repeat expansions, recently identified as causing cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS). Methods: Multimodal RFC1 repeat screening (PCR, southern blot, whole-exome/genome (WES/WGS)-based approaches) combined with cross-sectional and longitudinal deep-phenotyping in (i) cross-European cohort A (70 families) with ≥2 features of CANVAS and/or ataxia-with-chronic-cough (ACC); and (ii) Turkish cohort B (105 families) with unselected late-onset ataxia. Results: Prevalence of RFC1-disease was 67% in cohort A, 14% in unselected cohort B, 68% in clinical CANVAS, and 100% in ACC. RFC1-disease was also identified in Western and Eastern Asians, and even by WES. Visual compensation, sensory symptoms, and cough were strong positive discriminative predictors (>90%) against RFC1-negative patients. The phenotype across 70 RFC1-positive patients was mostly multisystemic (69%), including dysautonomia (62%) and bradykinesia (28%) (=overlap with cerebellar-type multiple system atrophy [MSA-C]), postural instability (49%), slow vertical saccades (17%), and chorea and/or dystonia (11%). Ataxia progression was ~1.3 SARA points/year (32 cross-sectional, 17 longitudinal assessments, follow-up ≤9 years [mean 3.1]), but also included early falls, variable non-linear phases of MSA-C-like progression (SARA 2.5-5.5/year), and premature death. Treatment trials require 330 (1-year-trial) and 132 (2-year-trial) patients in total to detect 50% reduced progression. Conclusions: RFC1-disease is frequent and occurs across continents, with CANVAS and ACC as highly diagnostic phenotypes, yet as variable, overlapping clusters along a continuous multisystemic disease spectrum, including MSA-C-overlap. Our natural history data help to inform future RFC1-treatment trials.