Highlights d Transcriptomes of SCA7 mice revealed downregulation of genes controlling Ca 2+ flux d Altered membrane excitability in SCA7 involves NAD + depletion and PARP1 activation d Increased Sirt1 rescued cerebellar degeneration and calcium defects in SCA7 mice d NAD + repletion prevented disease in SCA7 mice and patient stem cell-derived neurons

SUMMARYSpinocerebellar ataxia type 7 (SCA7) is a retinal-cerebellar degenerative disorder caused by CAG-polyglutamine (polyQ) repeat expansions in the ataxin-7 gene. As many SCA7 clinical phenotypes occur in mitochondrial disorders, and magnetic resonance spectroscopy of patients revealed altered energy metabolism, we considered a role for mitochondrial dysfunction. Studies of SCA7 mice uncovered marked impairments in oxygen consumption and respiratory exchange. When we examined cerebellar Purkinje cells in mice, we observed mitochondrial network abnormalities, with enlarged mitochondria upon ultrastructural analysis. We developed stem cell models from patients and created stem cell knockout rescue systems, documenting mitochondrial morphology defects, impaired oxidative metabolism, and reduced expression of nicotinamide adenine dinucleotide (NAD+) production enzymes in SCA7 models. We observed NAD+ reductions in mitochondria of SCA7 patient NPCs using ratiometric fluorescent sensors and documented alterations in tryptophan-kynurenine metabolism in patients. Our results indicate that mitochondrial dysfunction, stemming from decreased NAD+, is a defining feature of SCA7.

Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant neurodegenerative disorder characterized by cerebellar and retinal degeneration, and is caused by a CAG-polyglutamine repeat expansion in the ATAXIN-7 gene. SCA7 patients develop progressive cone-rod dystrophy, typically resulting in blindness. Antisense oligonucleotides (ASOs) are single-stranded chemically modified nucleic acids designed to mediate the destruction, prevent the translation or modify the processing of targeted RNAs. Here we evaluated ASOs as treatments for SCA7 retinal degeneration in representative mouse models via injection into the vitreous humor of the eye. Using Ataxin-7 aggregation, visual function, retinal histopathology, gene expression, and epigenetic dysregulation as outcome measures, we found that ASO-mediated Ataxin-7 knockdown yielded significant improvements in treated SCA7 mice. In SCA7 mice with significant retinal disease, intravitreal injection of Ataxin-7 ASO also improved visual function despite initiating treatment after symptom onset. By using color fundus photography and autofluoresence imaging, we also determined the nature of retinal degeneration in human SCA7 patients; we observed variable disease severity, and catalogued rapidly progressive degeneration. Given the accessibility of neural retina, availability of objective, quantitative read-outs for monitoring therapeutic response, and rapid disease progression, ASOs targeting ATAXIN-7 might represent a viable treatment for SCA7 retinal degeneration.

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