The SCA8 transcript is an antisense RNA to a brain-specific transcript encoding a novel actin-binding protein (KLHL1)

Spinocerebellar ataxia type 8 (SCAB) is a neurodegenerative disorder caused by the expansion of a CTG trinucleotide repeat that is transcribed as part of an untranslated RNA. As a step towards understanding the molecular path ology of SCA8, we have defined the genomic organization of the SCA8 RNA tra nscripts and assembled a 166 kb segment of genomic sequence containing the repeat. The most striking feature of the SCA8 transcripts is that the most 5' exon is transcribed through the first exon of another gene that is trans cribed in the opposite orientation. This gene arrangement suggests that the SCA8 transcript is an endogenous antisense RNA that overlaps the transcrip tion and translation start sites as well as the first splice donor sequence of the sense gene. The sense transcript encodes a 748 amino acid protein w ith a predicted domain structure typical of a family of actin-organizing pr oteins related to the Drosophila Kelch gene, and so has been given the name Kelch-like 1(KLHL1). We have identified the full-length cDNA sequence for both the human and mouse KLHLIgenes, and have elucidated the general genomi c organization of the human gene. The predicted open reading frame and prom oter region are highly conserved, and both genes are primarily expressed in specific brain tissues, including the cerebellum, the tissue most affected by SCA8. Transfection studies with epitope-tagged KLHL1 demonstrate that t he protein localizes to the cytoplasm, suggesting that it may play a role i n organizing the actin cytoskeleton of the brain cells in which it is expre ssed.