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

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 alpha(1A) (Ca(v)2.1) 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.