Metformin reduces RAN proteins and rescues molecular 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 disease-targeting treatments. RAN translation, which was discovered in SCA8, has previously been shown to occur across CAG and CUG expansion transcripts, making treatments that work for SCA8 potentially relevant to a broader group of diseases, including SCA1, 2, 3, 6, 7, 12, Huntington’s Disease (HD), Fuch’s endothelial corneal dystrophy (FECD), and myotonic dystrophy type 1 (DM1). Additionally, CUG and CAG expansion transcripts have been reported to cause RNA gain-of-function effects. In C9orf72 ALS/FTD mice, we previously showed that metformin reduced RAN protein levels and improved behavior and neurological phenotypes. 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 measures. At the molecular level, metformin-treated mice showed reduced RAN protein levels and improved alternative splicing abnormalities without changing the levels of the expanded RNAs. Metformin-treated mice also showed 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.