RNA interference therapy for the Spinocerebellar ataxias

The spinocerebellar ataxias are a group of diseases characterized by loss of motor coordination. Spinocerebellar ataxia types 2 and 7 are monogenic, autosomal dominant, late-onset neurodegenerative diseases characterized by ataxia with no effective treatments in the clinic. The most striking feature of these diseases is the degeneration of Purkinje neurons of the cerebellum. Spinocerebellar ataxia type 7 is also characterized by vision loss due to degeneration of the retinal photoreceptors. In this work, we tested the hypothesis that reducing mutant gene expression by RNAi would alleviate disease phenotypes in these two spinocerebellar ataxias.

For spinocerebellar ataxia type 7 (SCA7), we designed and tested RNAi sequences that could reduce the expression of both wildtype and mutant ataxin-7, an approach that would be applicable to all SCA7 patients. We found that AAV1-mediated delivery of a candidate RNAi sequence to the Purkinje neurons of SCA7 mice resulted in long-term sustained reduction of both wildtype and mutant ataxin-7 and resulted in significant improvements in ataxic and neuropathological phenotypes. We also delivered the RNAi sequence (AAV1-mediated) to reduce the expression of both mutant and wildtype ataxin-7 in the SCA7 mouse retina and evaluated retinal function long-term. We observed a preservation of normal retinal function and no adverse toxicity with reduction of wildtype and mutant ataxin-7 alleles. These studies address an important safety concern regarding non-allele specific silencing of ataxin-7 for SCA7 therapy.

To identify therapies for spinocerebellar ataxia type 2 (SCA2), we designed and tested several RNAi sequences to reduce the expression of both wildtype and mutant ataxin-2 in vitro and in vivo. We found that reduction of wildtype ataxin-2 expression in the mouse cerebellum was tolerated 4 months post injection without inducing behavioral deficits or cerebellar pathology. Additionally, we tested other sequences for improved silencing efficacy, and identified a potent RNAi sequence that significantly reduced the expression of both mutant and wildtype ataxin-2 in the cerebellum of a SCA2 mouse model. Ongoing work will establish if long-term reduction of both mutant and wildtype ataxin-2 will provide therapeutic benefit in the SCA2 mouse setting, and the safety of this sequence in normal cerebella.