Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant disorder that results in a cone-rod dystrophy form of retinal degeneration. The mutation inherited by SCA7 patients is a CAG/polyglutamine (polyQ) repeat expansion in the ataxin-7 gene. The SCA7 mutation results in production of a toxic polyQ-expanded ataxin-7 protein. Since the toxic gene product drives all subsequent disease pathology in SCA7, the most attractive therapeutic paradigm is to terminate expression of the mutant gene product. Here we propose continuation of a translational research program intended to yield therapeutic agents for SCA7 retinal disease, and potentially for related SCAs caused by CAG repeat expansion mutations. We are pursuing ataxin-7 "gene silencing" using a strategy that has already been successfully applied to reduce the expression of a toxic protein: oligonucleotide-based knock-down. To achieve the goals of this translational research program, we have created an academic-industrial partnership. In close collaboration with ISIS Pharmaceuticals, a company that specializes in anti-sense oligonucleotide (ASO) production and single-stranded siRNA (ss-siRNA) creation, we identified leads that markedly knock-down ataxin-7 expression. Based upon these in vitro results, we selected leads for in vivo validation, and performed pilot intra-vitreal injections in wild-type mice. These pilot intra-vitreal injections yielded robust (>0%) knock-down of ataxin-7 expression for eyes injected with anti-ataxin-7 ASO, in comparison to eyes injected with diluent. Further pilot studies in SCA7 knock-in mice resulted in significant reductions in ataxin-7 protein aggregates in retinal photoreceptor cells in treated eyes. We also identified CAG-repeat targeting oligonucleotides for allele-selective targeting of polyQ-expanded ataxin-7 in fibroblast cell lines from patients. We therefore propose two aims: 1) Based upon efficacy of knock-down and ocular toxicity parameters, we will select one ataxin-7 ASO and perform intra-vitreal injections of ataxin-7 ASO in the right eye and control ASO/diluent in the let eye of SCA7 knock-in mice to determine if ASO delivery can prevent or ameliorate SCA7 retinal degeneration. Read-outs will include ataxin-7 aggregation, retinal histology, and visual function. Once we identify an efficacious ataxin-7 ASO, we will repeat the trial to confirm efficacy, compare gene expression alterations in ASO-treated and control-treated eyes, determine if ASO delivery in symptomatic SCA7 mice can stop or reverse disease progression, and evaluate the utility of the ataxin-7 ASO as a therapy for SCA7 cerebellar degeneration. 2) After we select a CAG-repeat targeting oligonucleotide, we will test if the oligonucleotide can prevent or ameliorate retinal degeneration in SCA7 knock-in mice, and then advance the oligonucleotide for confirmatory studies, transcriptome expression analysis, symptomatic intervention trials, and evaluation as a therapy for SCA7 neurodegeneration, applying similar experimental strategies as with the ataxin-7 ASO. At the conclusion of this project, we will be well positioned to proceed with IND-enabling studies with our industrial collaborator, as a prelude to a clinical trial in humn SCA7 patients.
Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant disorder in which patients develop a cone-rod dystrophy form of retinal degeneration, caused by CAG / polyglutamine repeat expansions in the ataxin-7 gene, resulting in the production of a toxic polyglutamine- expanded ataxin-7 protein. As it is the toxic gene product that drives all subsequent disease pathology in SCA7 and in related polyglutamine disorders, an accepted therapeutic paradigm for treating these diseases is to terminate the expression of the mutant gene product;hence, we will pursue ataxin-7 knock-down therapy using an antisense oligonucleotide (ASO) approach and a CAG-repeat targeting ASO or single-stranded short-interfering RNA (ss-siRNA) approach by continuing ongoing studies, being performed in collaboration with ISIS Pharmaceuticals. In this project, we will evaluate ONE ataxin-7 ASO and ONE CAG-targeting oligonucleotide as a treatment for retinal degeneration in preclinical trials in SCA7 knock-in mice, we will determine if efficacious oligonucleotides can reverse existing retinal disease and work as a therapy for SCA7 cerebellar degeneration, and we will perform transcriptome analysis to delineate the molecular basis of SCA7 neurodegeneration - all to establish if one (or both) of these oligonucleotide leads represents a viable therapy for use in human SCA7 patients.
|Guyenet, Stephan J; Mookerjee, Shona S; Lin, Amy et al. (2015) Proteolytic cleavage of ataxin-7 promotes SCA7 retinal degeneration and neurological dysfunction. Hum Mol Genet 24:3908-17|