What is Spinocerebellar Ataxia?
 

Spinocerebellar Ataxia (SCA) is a group of neurodegenerative diseases caused by mutations in different genes. The underlying causes are diverse, typically involving mutations in various genes, with many subtypes linked to dynamic mutations like CAG trinucleotide repeat expansions. Core clinical symptoms include progressive gait instability, limb incoordination, dysarthria, and abnormal eye movements. Different subtypes may also present with other neurological manifestations such as peripheral neuropathy, cognitive decline, and extrapyramidal symptoms. The global incidence is approximately 1-5 per 100,000. Among Chinese SCA patients, SCA3 is the most prevalent, accounting for 51.1% to 72.5%.
 

To date, based on distinct causative genes, over 40 SCA subtypes have been identified and named (e.g., SCA1-40). The primary genetic mechanisms fall into two categories: 1) diseases caused by CAG repeat expansions in coding regions, known as polyglutamine (polyQ) diseases (e.g., SCA1, 2, 3, 6, 7); 2) non-polyQ SCAs caused by repeat expansions or point mutations in non-coding regions (e.g., SCA8, 10, 12, 31).
 

Pathogenesis
 

The pathogenic mechanisms of Spinocerebellar Ataxia can be categorized into three major classes based on their core molecular pathways and corresponding key causative genes:
 

1.PolyQ Protein Toxicity Pathway:​ Driven by CAG repeat expansions in coding regions (causative genes: ATXN1, ATXN2, ATXN3, CACNA1A, ATXN7, etc.). Mutant genes encode proteins with abnormally extended polyglutamine tracts. These proteins are prone to misfolding and aggregation, forming nuclear inclusions. Their toxicity directly damages neurons and broadly disrupts the intracellular protein quality control (PQC) system, such as interfering with the ubiquitin-proteasome pathway, depleting chaperone proteins, and impairing autophagy-lysosomal clearance. This cascade leads to transcriptional dysregulation, mitochondrial dysfunction, and ultimately, neurodegeneration, particularly affecting cerebellar Purkinje cells.
 

2.RNA Toxicity and RAN Translation Pathway:​ Driven by repeat sequence expansions in non-coding regions (causative genes/loci: ATXN8OS/ATXN8, ATXN10, PPP2R2B(SCA12), BEAN1/TK2(SCA31), NOP56(SCA36), DAB1(SCA37), etc.). The core mechanism involves pathogenic RNA foci formed in the nucleus by transcripts containing expanded repeat sequences (e.g., CTG, ATTCT, TGGAA, GGCCTG, ATTTC). These abnormal RNA foci can sequester or recruit various key RNA-binding proteins, broadly interfering with their normal functions and disrupting cellular RNA metabolism homeostasis, including splicing, transport, and stability regulation. Additionally, some repeat sequences can undergo repeat-associated non-AUG (RAN) translation, producing neurotoxic abnormal polypeptides (e.g., polyglutamine, polyalanine), causing secondary damage at the protein level. This pathway is the key pathogenic mechanism for subtypes like SCA8, 10, 12, 31, 36, and 37.
 

3.Protein Function Abnormality Pathway:​ Primarily caused by point mutations or small deletions/insertions in causative genes (e.g., SPTBN2, TTBK2, PRKCG, TRPC3, KCND3, KCNC3, FGF14, etc.), with missense mutations being the vast majority. The pathogenic mechanisms are complex, primarily manifesting in two directions: 1) Toxic gain-of-function, such as mutations causing abnormal protein aggregation, abnormally enhanced enzyme activity, or ion channel dysfunction (e.g., subunits related to TRPC3, CACNA1A); 2) Loss-of-function or haploinsufficiency, leading to the inability to maintain normal physiological functions, specifically manifested as abnormal signal transduction, loss of kinase activity, and impaired cytoskeletal stability.
 

These three pathways ultimately converge on the collapse of neuronal internal homeostasis, selective neuronal (most critically Purkinje cells) functional failure and death, clinically presenting as progressive cerebellar ataxia and other related neurological symptoms. Widespread disruption of the proteostasis network is a common final pathological hub across many subtypes.
 

Image source: Spinocerebellar ataxias: from pathogenesis to recent therapeutic advances
 

Image source: Spinocerebellar ataxias: from pathogenesis to recent therapeutic advances
 

Mouse Models
 

SCA1 Knock-in Mice:​ Generated by endogenously knocking in an expanded CAG repeat, accurately modeling the natural disease progression of human SCA1. Used to study nuclear inclusion formation and specific functional impairments like respiratory neural circuit dysfunction.
 

SCA6 Knock-in Mice:​ Generated by amplifying the CAG repeat in the endogenous mouse Cacna1agene. They exhibit significant motor incoordination and are an important tool for studying SCA6 pathogenesis and long-term drug efficacy evaluation.
 

SCA7 Knock-in Mice:​ Generated by amplifying the CAG repeat in the endogenous mouse Atxn7gene. They display cerebellar degeneration accompanied by retinal degeneration, suitable for studying the multisystem involvement mechanism specific to SCA7.
 

SCA3 Humanized Mice:​ Replace the mouse Atxn3gene with the human ATXN3gene containing the pathogenic CAG repeat, modeling human SCA3 pathology. Suitable for screening nucleic acid drugs targeting the human gene.
 

SCA8 Mice:​ Carry the human ATXN8OS/ATXN8locus with expanded CTG•CAG repeats, recapitulating cerebellar dysfunction and polyglutamine/alanine aggregation. Primarily used to study the pathogenicity of non-coding repeat expansions.
 

SCA41 Point Mutation Mice:​ Model a TRPC3gene missense mutation, recapitulating gait ataxia and Purkinje cell abnormalities. An important model for studying SCA caused by ion channel dysfunction.
 

Supporting Gene Therapy
 

Gene therapy offers hope for rare diseases, but its development and validation rely heavily on animal model support. MingCeler Biotech, leveraging its self-developed TurboMice™ technology, has developed multiple rare disease mouse models. TurboMice™ technology overcomes the technical challenges of long mouse model generation cycles and low success rates for complex models, enabling editing at almost any target gene locus. Complete homozygous gene-edited mouse models can be prepared directly from embryonic stem cells in as little as two months.
 

MingCeler Biotech​ can customize various SCA mouse models according to client needs, such as SCA1 knock-in mice, SCA6 knock-in mice, SCA7 knock-in mice, SCA3 humanized mice, SCA8 mice, and SCA41 point mutation mice. Inquiries are welcome.
 

References:

1.Cui ZT, Mao ZT, Yang R, Li JJ, Jia SS, Zhao JL, Zhong FT, Yu P, Dong M. Spinocerebellar ataxias: from pathogenesis to recent therapeutic advances. Front Neurosci. 2024 Jun 4;18:1422442. doi: 10.3389/fnins.2024.1422442. PMID: 38894941; PMCID: PMC11185097.

2.Garden GA, La Spada AR. Molecular pathogenesis and cellular pathology of spinocerebellar ataxia type 7 neurodegeneration. Cerebellum. 2008;7(2):138-49. doi: 10.1007/s12311-008-0027-y. PMID: 18418675; PMCID: PMC4195584.