Functional implications of a novel EA2 mutation in the P/Q‐type calcium channel

Abstract

Episodic ataxia type 2 (EA2) is an autosomal dominant condition characterized by paroxysmal attacks of ataxia, vertigo, and nausea, typically lasting minutes to days in duration. These symptoms can be prevented or significantly attenuated by the oral administration of acetazolamide; however, the mechanism by which acetazolamide ameliorates EA2 symptoms is unknown. EA2 typically results from nonsense mutations in the CACNA1A gene that encodes the alpha1A (Cav2.1) subunit of the P/Q-type calcium (Ca2+) channel. We have identified a novel H1736L missense mutation in the CACNA1A gene associated with the EA2 phenotype. This mutation is localized near the pore-forming region of the P/Q-type Ca2+ channel. Functional analysis of P/Q-type channels containing the mutation show that the H1736L alteration affects several channel properties, including reduced current density, increased rate of inactivation, and a shift in the voltage dependence of activation to more positive values. Although these findings are consistent with an overall loss of P/Q-type channel function, the mutation also caused some biophysical changes consistent with a gain of function. We also tested the direct effect of acetazolamide on both wild-type and H1736L mutated P/Q-type channels and did not observe any direct action on channel properties of this pharmacological agent used to treat EA2 patients.

Episodic Ataxia Type 2

“…This suggests that ACTZ-induced metabolic acidosis probably has a therapeutic effect only by changing the steady-state properties of the mutants, and not their level of expression. Finally, Spacey et al 61 also demonstrated that ACTZ does not have a direct effect on either the wild-type or the mutated P/Q-channel. Thus, its mechanism of action most likely involves changes of pH that alter the transmembrane potential and excitability of neurons.…”

“…Thus, its mechanism of action most likely involves changes of pH that alter the transmembrane potential and excitability of neurons. 61 Possible adverse effects of ACTZ include nephrolithiasis, hyperhydrosis, paresthesia, muscle stiffness with easy fatigability, and gastrointestinal disturbances. Side effects are dose-related and can be partially reduced by potassium chloride supplementation.…”

Molecular Pathogenesis of Spinocerebellar Ataxia Type 6

“…CACNA1A mutations found in EA-2 are most commonly nonsense or splicing mutations that result in ␣1A subunits that are truncated within the repeat domains, or that predict skipped exons with loss of function of the mutant in expression studies. 4,46,47 Several studies suggest that the truncated subunits exhibit dominant inheritance by exerting a dominant negative effect on the wild-type P/Q current activity. 44,48,49 SCA6 is a progressive ataxic disorder caused by a trinucleotide repeat (CAG) expansion in exon 47 of the CACNA1A gene.…”

Novel missense variant of CACNA1A gene in a Slovak family with episodic ataxia type 2

“…12 ). Several missense variants in pore-loop regions have been described in the medical literature so far [12][13][14][15][16][17][18][19][20][21][22][23][24] . Structural changes in pore-loop regions can lead to functional changes disabling activation or inactivation of calcium flux 16 .…”