The molecular biology of the autosomal-dominant cerebellar ataxias.

Autosomal-dominant cerebellar ataxias (ADCA) may present as progressive or paroxysmal disorders. While the progressive ataxias have been named spinocerebellar ataxias (SCA), the paroxysmal disorders are designated episodic ataxias (EA). Until now, three different mutational mechanisms resulting in distinctive pathogenesis have been identified. The first type of mutation present in SCA1, SCA2, SCA3, and SCA7 is an expanded CAG repeat in genes of unknown function that are translated into proteins with expanded polyglutamine tracts. A common ultrastructural feature of these disorders is the formation of neuronal intranuclear inclusions (NII) harboring the expanded disease proteins and a variety of other proteins. The pathogenic role of these inclusions has yet to be clarified. A second group of disorders is the result of mutations in genes that code for ion channels. In EA-1, a disorder characterized by episodes of ataxia provoked by movement and startle, missense mutations in a potassium channel gene, KCNA1, have been found. Patients with EA-2, another form of paroxysmal ataxia, carry nonsense mutations of the gene encoding the alpha1A voltage-dependent calcium channel subunit, CACNA1A, that are predicted to result in truncated channel proteins. In SCA6, a progressive ataxia, an expanded CAG repeat in the 3' translated region of the CACNA1A gene, has been found. The third type of mutation is an untranslated CTG expansion resembling the mutation found in myotonic dystrophy. It is associated with a progressive ataxia, SCA8.