Identification and characterization of Spinocerebellar Ataxia Type 7 (SCA7) isoform SCA7b in mice

Spinocerebellar ataxia type 7 (SCA7) is a neurodegenerative disease primarily affecting the cerebellum, brainstem and retina. The disease is caused by a polyglutamine expansion in ataxin-7, a protein with largely unknown function. To improve our knowledge of the expression and function of wild-type and expanded ataxin-7, we looked for alternative SCA7 transcripts in mice. We identified a murine SCA7 isoform (SCA7b) containing an uncharacterized exon homologous to the newly identified human exon 12b. Northern blot analysis revealed three exon 12b containing transcripts with molecular sizes of 7.5, 4.4 and 3.0 kb in mice. This contrasted with the situation in humans, where only one exon 12b-containing transcript was observed. Furthermore, Northern blot of the human 4.4 kb SCA7b isoform predominantly showed expression in the brain, while expression of both the murine 7.5-kb and the 4.4-kb transcripts were observed in several tissues including brain, heart, liver, kidney and testis. Quantitative real-time RT-PCR analysis revealed that in muscle and heart SCA7b is the predominant SCA7 isoform, while in brain equal levels of SCA7a and SCA7b was observed. Insertion of exon 12b into the murine SCA7 ORF resulted in a frame-shift that gave rise to an alternative ataxin-7 protein (ataxin-7b). The novel 58-amino acid C-terminus in ataxin-7b directed the protein to a more cytoplasmic location.