Complete loss of P/Q calcium channel activity caused by a CACNA1A missensemutation carried by patients with episodic ataxia type 2

Familial hemiplegic migraine, episodic ataxia type 2 (EA2), and spinocerebe llar ataxia type 6 are allelic disorders of the CACNA1A gene (coding for th e alpha (1A) subunit of P/Q calcium channels), usually associated with diff erent types of mutations (missense, protein truncating, and expansion, resp ectively). However, the finding of expansion and missense mutations in pati ents with EA2 has blurred this genotype-phenotype correlation. We report th e first functional analysis of a new missense mutation, associated with an EA2 phenotype-that is, T-->C transition of nt 4747 in exon 28, predicted to change a highly conserved phenylalanine residue to a serine at codon 1491, located in the putative transmembrane segment S6 of domain III. Patch-clam p recording in HEK 293 cells, coexpressing the mutagenized human alpha (1A- 2) subunit, together with human beta (4) and alpha (2)delta subunits, showe d that channel activity was completely abolished, although the mutated prot ein is expressed in the cell. These results indicate that a complete loss o f P/Q channel function is the mechanism underlying EA2, whether due to trun cating or to missense mutations.