Metformin improves RAN protein pathology, alternative splicing, and behavioral phenotypes in SCA8 mice

Spinocerebellar ataxia type 8 (SCA8) is a member of a group of dominantly inherited, debilitating neurological diseases caused by CAG•CTG expansions for which there are no effective treatments. RAN translation, which was discovered in SCA8, has previously been shown to occur across CAG and CUG expansion transcripts, making treatments for SCA8 potentially relevant to a broad group of diseases, including SCA1, SCA2, SCA3, SCA6, SCA7, SCA12, Huntington's disease, and myotonic dystrophy type 1. In addition, CUG and CAG expansion transcripts have been reported to cause RNA gain-of-function effects. Using SCA8 BAC transgenic mice as a model for CAG•CTG expansion diseases, we now show that metformin improves ambulatory performance using rotarod, DigiGait, and open-field testing. At the molecular level, metformin-treated mice show reduced RAN protein levels and improved splicing, without altering sense or antisense RNA levels. Metformin-treated mice also show decreased neuroinflammation, with reduced astrogliosis and fewer activated microglia. These data provide strong preclinical support for testing metformin in clinical trials for SCA8 and potentially the broader group of CAG•CTG repeat expansion disorders.