Spinocerebellar Ataxia 7 (SCA7) Cell Models for Research

Spinocerebellar ataxia 7 (SCA7) is a rare autosomal dominant neurodegenerative disorder with an estimated prevalence of 1-2 per 100,000 individuals worldwide (WHO, 2023). It is characterized by progressive cerebellar ataxia, retinal degeneration leading to blindness, and variable other neurological signs. The disease typically manifests in adulthood, with anticipation leading to earlier onset and increased severity in successive generations. There is no cure, and current treatments are symptomatic only. The median survival after onset is approximately 20 years, but varies widely. The disease imposes a significant burden on patients and families, with a profound impact on quality of life.

SCA7 is an ideal model for studying polyglutamine (polyQ) disorders, a family of neurodegenerative diseases caused by expanded CAG repeats. The disease is caused by a single gene (ATXN7), making it amenable to precise genetic manipulation. Research focuses on understanding the molecular mechanisms of neurodegeneration, including protein aggregation, transcriptional dysregulation, and mitochondrial dysfunction. Public datasets, such as those from the NCBI Gene and ClinVar, provide extensive information on ATXN7 mutations. Open questions include the role of the ataxin-7 protein in normal cellular function and the mechanisms of selective neuronal vulnerability.

Although SCA7 is not a cancer, the term 'carcinogenic' is not applicable. Instead, we focus on pathogenic pathways. The primary mechanism is a gain-of-function toxicity of the mutant ataxin-7 protein, which contains an expanded polyglutamine tract. This leads to:

• • Protein misfolding and aggregation into nuclear inclusions.
• • Disruption of the STAGA transcription coactivator complex, leading to transcriptional dysregulation.
• • Impairment of the ubiquitin-proteasome system and autophagy.
• • Mitochondrial dysfunction and oxidative stress.
• • Altered calcium homeostasis and excitotoxicity.

These pathways are interconnected and contribute to neuronal dysfunction and death, particularly in the cerebellum and retina.

Data from ClinVar and NCBI Gene. The CAG repeat expansion is the sole causative mutation, with normal alleles having 4-35 repeats and pathogenic alleles having 36 or more.

The mutant ataxin-7 protein disrupts several signaling networks:

• • Transcriptional regulation: Ataxin-7 is a subunit of the STAGA complex, which acetylates histones and regulates gene expression. Mutant ataxin-7 impairs this complex, leading to altered expression of genes involved in neuronal survival, including those for calcium-binding proteins and neurotrophic factors.
• • Protein quality control: Mutant ataxin-7 aggregates overwhelm the proteasome and autophagy pathways, leading to accumulation of damaged proteins and cellular stress.
• • Mitochondrial function: Mutant ataxin-7 interacts with mitochondria, causing mitochondrial fragmentation, reduced ATP production, and increased reactive oxygen species.
• • Calcium signaling: Dysregulation of calcium homeostasis contributes to excitotoxicity and neuronal death.

Key nodes include the STAGA complex, p53, and the mTOR pathway.

Organoids derived from patient iPSCs provide a three-dimensional model of the developing brain and can recapitulate disease phenotypes, offering a more physiologically relevant system for studying SCA7.

• • Genetically engineered mouse models (GEMMs): Knock-in mice with expanded CAG repeats in the endogenous Atxn7 gene recapitulate key features of SCA7, including ataxia and retinal degeneration.
• • Transgenic mice: Overexpression of mutant ATXN7 under a neuronal promoter leads to rapid onset of symptoms.
• • Induced models: Viral delivery of mutant ATXN7 to specific brain regions can induce localized pathology.
• • Patient-derived xenografts (PDX): Not applicable for SCA7 as it is not a cancer, but similar approaches using patient-derived cells in immunodeficient mice could be used for studying neurodegeneration.

CRISPR-Cas9 gene editing enables the creation of isogenic cell lines that differ only in the ATXN7 mutation, providing powerful tools to study disease mechanisms and test therapies. Examples include:

• • ATXN7 knockout cell lines: These lines have a complete loss of ATXN7 function, allowing study of the normal role of ataxin-7.
• • ATXN7 knock-in cell lines: These lines carry a pathogenic CAG repeat expansion in the endogenous ATXN7 gene, mimicking the patient genotype.
• • Reporter cell lines: These lines express a fluorescent protein tagged to ATXN7, enabling real-time tracking of protein aggregation.

Such sequence-verified, commercially available models accelerate research by providing consistent and reproducible systems. They are essential for high-throughput drug screening and functional genomics.

Gene-edited cell lines are used to validate the function of ATXN7 and identify genetic modifiers. For example, ATXN7 knockout lines can be used in CRISPR screens to identify genes that, when silenced, rescue the toxic effects of mutant ATXN7. Knock-in lines with different repeat lengths allow study of the threshold for pathogenicity and the effects of repeat instability.

Isogenic pairs of wild-type and mutant ATXN7 cell lines are used in high-throughput screens to identify compounds that reduce mutant ATXN7 toxicity or aggregation. These models can also be used to test drug resistance mechanisms, such as upregulation of heat shock proteins or autophagy enhancers.

CRISPR synthetic lethality screens can identify genes that are essential only in the presence of mutant ATXN7, revealing potential therapeutic targets. Gene-edited cells can also be used to discover biomarkers by comparing the transcriptome, proteome, and metabolome of mutant versus wild-type cells.