A Single Amino Acid Deletion (ΔF1502) in the S6 Segment of Ca(V)2.1 Domain III Associated with Congenital Ataxia Increases Channel Activity and Promotes Ca(2+) Influx

Mutations in the CACNA1A gene, encoding the pore-forming Ca(V)2.1 (P/Q-type) channel α(1A) subunit, result in heterogeneous human neurological disorders, including familial and sporadic hemiplegic migraine along with episodic and progressive forms of ataxia. Hemiplegic Migraine (HM) mutations induce gain-of-channel function, mainly by shifting channel activation to lower voltages, whereas ataxia mutations mostly produce loss-of-channel function. However, some HM-linked gain-of-function mutations are also associated to congenital ataxia and/or cerebellar atrophy, including the deletion of a highly conserved phenylalanine located at the S6 pore region of α(1A) domain III (ΔF1502). Functional studies of ΔF1502 Ca(V)2.1 channels, expressed in Xenopus oocytes, using the non-physiological Ba(2+) as the charge carrier have only revealed discrete alterations in channel function of unclear pathophysiological relevance. Here, we report a second case of congenital ataxia linked to the ΔF1502 α(1A) mutation, detected by whole-exome sequencing, and analyze its functional consequences on Ca(V)2.1 human channels heterologously expressed in mammalian tsA-201 HEK cells, using the physiological permeant ion Ca(2+). ΔF1502 strongly decreases the voltage threshold for channel activation (by ~ 21 mV), allowing significantly higher Ca(2+) current densities in a range of depolarized voltages with physiological relevance in neurons, even though maximal Ca(2+) current density through ΔF1502 Ca(V)2.1 channels is 60% lower than through wild-type channels. ΔF1502 accelerates activation kinetics and slows deactivation kinetics of Ca(V)2.1 within a wide range of voltage depolarization. ΔF1502 also slowed Ca(V)2.1 inactivation kinetic and shifted the inactivation curve to hyperpolarized potentials (by ~ 28 mV). ΔF1502 effects on Ca(V)2.1 activation and deactivation properties seem to be of high physiological relevance. Thus, ΔF1502 strongly promotes Ca(2+) influx in response to either single or trains of action potential-like waveforms of different durations. Our observations support a causative role of gain-of-function Ca(V)2.1 mutations in congenital ataxia, a neurodevelopmental disorder at the severe-most end of CACNA1A-associated phenotypic spectrum.