Microsatellite expansions cause a number of dominantly-inherited neurological diseases. Expansions in coding-regions cause protein gain-of-function effects, while non-coding expansions produce toxic RNAs that alter RNA splicing activities of MBNL and CELF proteins. Bi-directional expression of the spinocerebellar ataxia type 8 (SCA8) CTG CAG expansion produces CUG expansion RNAs (CUG exp) from the ATXN8OS gene and a nearly pure polyglutamine expansion protein encoded by ATXN8 CAG exp transcripts expressed in the opposite direction. Here, we present three lines of evidence that RNA gain-of-function plays a significant role in SCA8: 1) CUG exp transcripts accumulate as ribonuclear inclusions that co-localize with MBNL1 in selected neurons in the brain; 2) loss of Mbnl1 enhances motor deficits in SCA8 mice; 3) SCA8 CUG exp transcripts trigger splicing changes and increased expression of the CUGBP1-MBNL1 regulated CNS target, GABA-A transporter 4 ( GAT4/Gabt4). In vivo optical imaging studies in SCA8 mice confirm that Gabt4 upregulation is associated with the predicted loss of GABAergic inhibition within the granular cell layer. These data demonstrate that CUG exp transcripts dysregulate MBNL/CELF regulated pathways in the brain and provide mechanistic insight into the CNS effects of other CUG exp disorders. Moreover, our demonstration that relatively short CUG exp transcripts cause RNA gain-of-function effects and the growing number of antisense transcripts recently reported in mammalian genomes suggest unrecognized toxic RNAs contribute to the pathophysiology of polyglutamine CAG CTG disorders.
We describe several lines of evidence that RNA gain-of-function effects play a significant role in spinocerebellar ataxia type 8 (SCA8) and has broader implications for understanding the CNS effects of other trinucleotide expansion disorders including myotonic dystrophy type 1, Huntington disease like-2, and spinocerebellar ataxia type 7. The SCA8 mutation is bidirectionally transcribed resulting in the expression of CUG exp transcripts from ATXN8OS and CAG exp transcripts and polyglutamine protein from the overlapping ATXN8 gene. These data suggest that SCA8 pathogenesis involves toxic gain-of-function effects at the RNA (CUG exp) and/or protein (PolyQ) levels. We present three lines of evidence that CUG exp transcripts play a significant role in SCA8: 1) CUG exp transcripts accumulate as ribonuclear inclusions that co-localize with MBNL1 in selected neurons; 2) loss of Mbnl1 enhances motor deficits in SCA8 mice; 3) SCA8 CUG exp transcripts trigger alternative splicing changes and increased expression of the CUGBP1-MBNL1 regulated CNS target, GABA-A transporter 4 ( GAT4/Gabt4) which is associated with the predicted loss of GABAergic inhibition within the granular cell layer in SCA8 mice. Additionally, alternative splicing changes and GAT4 upregulation are induced by CUG exp but not CAG exp transcripts. From a therapeutic viewpoint, it is promising that this change is reversed in cells overexpressing MBNL1.