Molecular and genetic characterization of spinocerebellar ataxia type 5 (SCA5)

Abstract

Spinocerebellar ataxia type 5 (SCA5) is a progressive neurodegenerative disorder, which primarily affects the cerebellum. The disease is inherited in an autosomal dominant pattern, with onset typically occurring in the 3rd or 4th decade of life. In 1994, SCA5 was mapped to the centromeric region of chromosome 11 using an 11-generation American kindred, and was later refined to 11q13. Using a multifaceted mapping approach, which involved screening for expansion mutations, searching for haplotype conservation and sequencing, we discovered β-III spectrin (SPTBN2) mutations cause SCA5 in the American family and two additional ataxia families from France and Germany. In-frame microdeletions were identified in the American (39 bp) and French (15 bp) families within the third of 17 homologous, triple-helical, spectrin repeat motifs. In the German SCA5 family, a point mutation (Leu253Pro) was found within the C-terminal calponin homology (CH) domain, which is known to bind F-actin. Through expression studies in tissue culture and circular dichroism analyses, I characterized the effects of the German SCA5 mutation on protein folding, stability and function. These experiments demonstrate that the leucine to proline substitution causes a temperature sensitive misfolding of the protein, rendering it insoluble and subject to degradation. Additionally, the mutation likely disrupts the tightly controlled binding between spectrin and actin, resulting in an enhanced interaction of the mutant protein with both G- and F-actin. Spectrin mutations are a novel cause of ataxia and the continued characterization of these mutations will lead to a better understanding of the pathogenic mechanisms underlying SCA5 and neurodegenerative disease in general.