Increased Expression of α_1ACa²⁺Channel Currents Arising from Expanded Trinucleotide Repeats in Spinocerebellar Ataxia Type 6

Abstract

The expansion of polyglutamine tracts encoded by CAG trinucleotide repeats is a common mutational mechanism in inherited neurodegenerative diseases. Spinocerebellar ataxia type 6 (SCA6), an autosomal dominant, progressive disease, arises from trinucleotide repeat expansions present in the coding region of CACNA1A (chromosome 19p13). This gene encodes 1A , the principal subunit of P/Q-type Ca 2 channels, which are abundant in the CNS, particularly in cerebellar Purkinje and granule neurons. We assayed ion channel function by introduction of human 1A cDNAs in human embryonic kidney 293 cells that stably coexpressed 1 and 2 subunits. Immunocytochemical analysis showed a rise in intracellular and surface expression of 1A protein when CAG repeat lengths reached or exceeded the pathogenic range for SCA6. This gain at the protein level was not a consequence of changes in RNA stability, as indicated by Northern blot analysis. The electrophysiological behavior of 1A subunits containing expanded (EXP) numbers of CAG repeats (23, 27, and 72) was compared against that of wild-type subunits (WT) (4 and 11 repeats) using standard whole-cell patch-clamp recording conditions. The EXP 1A subunits yielded functional ion channels that sup- ported inward Ca 2 channel currents, with a sharp increase in P/Q Ca 2 channel current density relative to WT. Our results showed that Ca 2 channels from SCA6 patients display nearnormal biophysical properties but increased current density attributable to elevated protein expression at the cell surface.