Genes implicated in the pathogenesis of spinocerebellar ataxias.

The degenerative ataxias comprise a number of heterogeneous diseases, many of which are genetically determined. Loss of cerebellar Purkinje and brainstem neurons as well as degeneration of spinal pathways are the major morphological findings of most ataxias, but neuronal loss may also affect the basal ganglia and the retina. While the degenerative ataxias initially were classified on a neuropathological basis, more recent classifications focused on clinical hallmarks and the mode of inheritance, separating inherited, sporadic and symptomatic ataxias. Genetic linkage analysis and molecular genetic studies identified various genotypes and revealed genetic heterogeneity of the autosomal dominant ataxias (ADCA), which on the basis of the genotypes are now classified as spinocerebellar ataxias (SCA1-22). Based on pathogenesis these disorders fall into three discrete groups: the polyglutamine disorders, SCA1-3, 7 and 17; the channelopathies, SCA6 and episodic ataxia types 1 and 2 (EA1-2); and SCA8, 10 and 12, which result from repeat expansions outside the coding regions and reduce gene expression. The etiologies of SCAs 4, 5, 9, 11, 13-16, 19, 21 and 22 remain unknown as of today. The recent advances in the identification of the underlying gene defects of most of the inherited ataxias have opened new avenues to a better understanding of the molecular mechanisms leading to cellular dysfunction and cell death.