717. RNAi Therapy for Dominant Neurodegenerative Diseases

Abstract

In this study we investigated gene silencing by RNA interference (RNAi) as a potential therapy for Spinocerebellar ataxia type 1 (SCA1), a dominant neurodegenerative disease caused by expansion of a polyglutamine tract in ataxin-1. Therapies for SCA1 do not exist, and studies in mouse models of other polyglutamine repeat diseases support that silencing of the disease allele can improve disease manifestations. Short hairpin RNAs (shRNAs) specific to SCA1 were screened for effective silencing against a mutant human ataxin (Q = 82) in vitro prior to testing in a transgenic mouse model of SCA1. SCA1 trangenic mice express a mutant human ataxin-1 (Q = 82) from the Purkinje cell specific promoter, and recapitulate many aspects of human SCA1, including progressive ataxia, shrinkage of the molecular layer, Purkinje cell degeneration and nuclear inclusion formation. AAV1 vectors expressing the most effective shRNA and an hrGFP reporter (AAVshSCA1.hrGFP) were injected into the cerebellar lobules of 7-week old SCA1 or wildtype mice. Viruses expressing an irrelevant hairpin (AAVshlacZ.hrGFP), or saline, were included as hairpin and injection controls. Rotarod tests of motor coordination were done 2-week prior and every two weeks following injections, until sacrifice at 16 weeks of age. Beginning at 9 weeks of age and continuing until sacrifice, SCA1 mice treated with shSCA1-expressing viruses showed significantly improved performance on the rotarod relative to saline- or shlacZ-treated SCA1 mice. There was no effect of the therapy on the performance of wildtype mice. The efficiency of transduction, quantified by hrGFP expression, ranged from ~ 5–10% of all Purkinje cells. We took advantage of hrGFP reporter expression to directly compare transduced and untransduced lobules for ataxin-1 nuclear inclusions, Purkinje cell morphology, and the width of the cerebellar molecular layer. In shSCA-treated SCA1 mice, Purkinje cell dendritic arbors remained robust, and transduced lobules (hrGFP+) had molecular layer widths indistinguishable from those of wild-type mice. Moreover, ataxin-1 inclusions were notably absent from all hrGFP-positive Purkinje cells in shSCA1-treated SCA1 mice (0 inclusions in 400 cells). In contrast, 49% of Purkinje cells from shlacZ-treated mice had inclusions, similar to untreated 16-wk old SCA1 mice. These data provide the first in vivo demonstration of efficacy of RNAi for dominant neurodegenerative disease therapy.