Increased expression of alpha(1A) Ca2+ channel currents arising from expanded trinucleotide repeats in spinocerebellar ataxia type 6

The expansion of polyglutamine tracts encoded by CAG trinucleotide repeats is a common mutational mechanism in inherited neurodegenerative diseases. S pinocerebellar ataxia type 6 (SCA6), an autosomal dominant, progressive dis ease, arises from trinucleotide repeat expansions present in the coding reg ion of CACNA1A (chromosome 19p13). This gene encodes alpha (1A), the princi pal subunit of P/Q-type Ca2+ channels, which are abundant in the CNS, parti cularly in cerebellar Purkinje and granule neurons. We assayed ion channel function by introduction of human alpha 1A cDNAs in human embryonic kidney 293 cells that stably coexpressed beta (1) and alpha2 delta subunits. Immun ocytochemical analysis showed a rise in intracellular and surface expressio n of alpha (1A) protein when CAG repeat lengths reached or exceeded the pat hogenic range for SCA6. This gain at the protein level was not a consequenc e of changes in RNA stability, as indicated by Northern blot analysis. The electrophysiological behavior of alpha (1A) subunits containing expanded (E XP) numbers of CAG repeats (23, 27, and 72) was compared against that of wi ld-type subunits (WT) (4 and 11 repeats) using standard whole-cell patch-cl amp recording conditions. The EXP alpha (1A) subunits yielded functional io n channels that supported inward Ca2+ channel currents, with a sharp increa se in P/Q Ca2+ channel current density relative to WT. Our results showed t hat Ca2+ channels from SCA6 patients display near-normal biophysical proper ties but increased current density attributable to elevated protein express ion at the cell surface.