Autosomal Dominant Cerebellar Ataxia Type I Manual Deep-Research Note

Scope

• Parent disease term: MONDO:0019792 autosomal dominant cerebellar ataxia type I.
• Curation frame: historical Harding clinical class for dominantly inherited cerebellar ataxia with extracerebellar features, not a single-gene disorder.
• PR scope: SCA1, SCA2, SCA4, SCA8, SCA17, and SCA36, matching issue #1675.
• Out of scope for this scaffold: SCA3/Machado-Joseph disease and ADCA-II/SCA7, which are handled separately in the repo issue set and are not listed in the requested has_subtypes for PR #2755.

Search Strategy

Manual PubMed searches checked the following literature groups:

• Harding classification and genetic counseling: Harding autosomal dominant cerebellar ataxias, late onset autosomal dominant cerebellar ataxias.
• Shared repeat-expansion mechanism: CAG polyglutamine repeat diseases, spinocerebellar ataxia transcriptional dysregulation, polyglutamine autosomal dominant spinocerebellar ataxias.
• Subtype anchors: Spinocerebellar Ataxia Type 1 GeneReviews, Spinocerebellar Ataxia Type 2 GeneReviews, SCA4 ZFHX3 GGC repeat, SCA8 ATXN8OS CTG CAG, SCA17 TBP CAG repeat, and SCA36 NOP56 GGCCTG repeat.

Two issue/review-suggested PMIDs were corrected during verification:

• PMID:7181479 is not Harding 1982; it is an antibiotic susceptibility paper. The Harding classification article is PMID:7066668.
• The review suggestion PMID:20301489 is not SCA1 GeneReviews; it is glycogen storage disease type I. SCA1 GeneReviews is PMID:20301363.

Core Conclusion

ADCA-I is best modeled as a parent-level clinical/mechanistic class. The existing YAML scaffold is conceptually aligned with the literature: it centers the class on autosomal dominant repeat-expansion SCAs with progressive cerebellar degeneration plus extracerebellar involvement. It should remain a parent scaffold until subtype-level entries can carry detailed repeat thresholds, phenotype frequencies, diagnostic nuances, and treatment/surveillance detail.

The current PR's minimal shared pathophysiology - repeat expansion, misfolded protein/proteostatic stress, transcriptional dysregulation, Purkinje-cell degeneration, and extracerebellar neurodegeneration - is supported by the literature. SCA4 should be treated as a recently solved ZFHX3 GGC-repeat / poly-glycine disease, not as an unresolved 16q locus, because multiple 2024 papers and the 2024 GeneReviews entry now support the assignment.

Curation-Relevant Findings

Nosology and inheritance

Harding's 1982 clinical classification remains the source of the ADCA-I/II/III clinical grouping. The Harding paper proposed "a simple classification of the autosomal dominant cerebellar ataxias" and described extracerebellar associated features including "dementia, supranuclear ophthalmoplegia, extrapyramidal dysfunction" (PMID:7066668). A companion genetic paper supports autosomal dominant segregation and counseling logic (PMID:7334501).

For the PR scaffold, the parent should not imply one founder mutation or one uniform phenotype. It should state autosomal dominant inheritance, repeat instability/anticipation where applicable, and reduced penetrance in specific subtypes such as SCA8 and lower-range SCA17 alleles.

Shared repeat-expansion mechanism

The broader polyglutamine/repeat-expansion literature supports a convergent mechanism across SCA1, SCA2, SCA17, and related SCAs. Stoyas and La Spada list SCA1, SCA2, and SCA17 among the CAG-polyglutamine disease family and identify "protein aggregation" and "transcription dysregulation" among recurring pathologic processes (PMID:29325609). Orr and Zoghbi similarly frame unstable repeat expansion as a cause of inherited ataxias and emphasize altered protein conformation in neuronal survival (PMID:17417937).

More explicit SCA1 work supports transcriptional dysregulation as a strong follow-up evidence target: PMID:20628574 reports shared gene-expression changes in SCA1 model cerebella, PMID:32964235 links regional Purkinje-neuron vulnerability to ion-channel gene dysregulation, and PMID:32306062 summarizes SCA mechanisms around "ion channel dysfunction and transcriptional dysregulation."

Subtype source map

Phenotypes

The parent-level phenotype block should stay conservative. Progressive cerebellar ataxia and dysarthria are safe parent-level features. Additional parent-level candidates for later curation include cerebellar atrophy, abnormal saccadic eye movements, pyramidal signs/spasticity, dysphagia, peripheral neuropathy, extrapyramidal signs, cognitive impairment, psychiatric symptoms, and sensorineural hearing loss. These should preferably be subtype-scoped rather than asserted uniformly at the parent level.

Diagnosis

The literature consistently supports a clinical-neurologic suspicion followed by molecular repeat-expansion testing:

• SCA1 GeneReviews: diagnosis by characteristic clinical findings and abnormal ATXN1 CAG expansion (PMID:20301363).
• SCA2 GeneReviews: molecular genetic testing for abnormal ATXN2 CAG expansion (PMID:20301452).
• SCA4 GeneReviews: heterozygous abnormal ZFHX3 GGC expansion (PMID:39666847).
• SCA8 GeneReviews: heterozygous abnormal ATXN8OS/ATXN8 repeat expansion (PMID:20301445).
• SCA17/SCA36: repeat-primed PCR, Southern blot, long-read sequencing, or targeted repeat analysis may be needed depending on the locus and local test availability (PMID:30532692, PMID:22492559, PMID:37810464).

Parent YAML can mention molecular repeat-expansion testing in a future diagnosis section, but subtype entries should carry locus-specific thresholds and methodology.

Management and treatments

No disease-modifying therapy is established for the ADCA-I parent class. The most defensible treatment frame is supportive, multidisciplinary management: physical therapy, occupational therapy, speech-language therapy, fall prevention/adaptive devices, swallowing assessment for dysphagia, nutrition support, psychiatric/neuropsychologic support when needed, avoidance of alcohol or medications that worsen cerebellar function, and genetic counseling.

SCA1, SCA2, SCA4, and SCA8 GeneReviews each support supportive care and surveillance rather than curative treatment (PMID:20301363, PMID:20301452, PMID:39666847, PMID:20301445). SCA36 review literature discusses emerging mechanistic therapeutic directions but does not establish an accepted disease-modifying treatment (PMID:35309140).

Prevalence

Do not add a single global parent prevalence for ADCA-I. The term is a historical clinical class, and prevalence varies by subtype and founder population. SCA36 is enriched in specific Japanese, Galician/Spanish, and British cohorts (PMID:22492559, PMID:22753339, PMID:37810464); SCA17 fully penetrant alleles were rare in the UK ataxia cohort (PMID:30532692); and SCA8 frequency varies with testing strategy, repeat thresholds, and penetrance interpretation (PMID:20301445, PMID:31471687). Prevalence should be subtype-scoped.

YAML Integration Notes

• The PR's current has_subtypes list matches issue #1675 and should remain as SCA1, SCA2, SCA4, SCA8, SCA17, and SCA36.
• The SCA4 text should keep ZFHX3 as the causative gene and describe the variant as a GGC repeat/poly-glycine expansion. Older issue comments calling the gene unresolved are now obsolete.
• The current pathophysiology graph is directionally sound after prior review fixes: extracerebellar degeneration should remain parallel downstream of repeat/protein/transcriptional toxicity, not downstream of Purkinje-cell loss.
• Future evidence improvement: replace the current implicit TBP-based evidence for transcriptional dysregulation with explicit SCA1/SCA mechanism evidence from PMID:20628574, PMID:32964235, or PMID:32306062, after fetching caches with just fetch-reference.
• Future content expansion: add subtype-scoped phenotypes, a parent diagnosis section, and supportive treatment/genetic counseling sections only after the relevant PMIDs are cached and exact evidence snippets are verified.
• Do not add a parent prevalence claim unless it is explicitly framed as unknown/not estimable and supported by a suitable review; subtype/regional prevalence is safer.

Exact Short Quotes Checked

• PMID:7066668: "A simple classification of the autosomal dominant cerebellar ataxias"
• PMID:20301363: "SCA1 is characterized by progressive cerebellar ataxia"
• PMID:20301452: "slow saccadic eye movements"
• PMID:20301445: "SCA8 is inherited in an autosomal dominant manner with reduced penetrance."
• PMID:39666847: "SCA4 is inherited in an autosomal dominant manner."
• PMID:38035881: "SCA4 is caused by repeat expansions in ZFHX3."
• PMID:29325609: "protein aggregation, proteolytic cleavage, transcription dysregulation"
• PMID:32306062: "ion channel dysfunction and transcriptional dysregulation"
• PMID:22492559: "caused by a GGCCTG repeat expansion in intron 1 of NOP56"
• PMID:35309140: "motor neuron-related symptoms like muscular atrophy"
• PMID:30532692: "dementia, psychiatric symptoms"

PMID Set Used

• PMID:7066668 - Harding 1982 ADCA clinical classification
• PMID:7334501 - Harding 1981 genetic counseling/aspects of AD late-onset ataxia
• PMID:17417937 - trinucleotide repeat disorders review
• PMID:29325609 - CAG-polyglutamine disease nosology
• PMID:32306062 - SCA1/common SCA pathogenic mechanisms review
• PMID:20628574 - SCA1 transcriptional dysregulation / ATXN1 function
• PMID:32964235 - SCA1 Purkinje vulnerability and ion-channel gene dysregulation
• PMID:20301363 - SCA1 GeneReviews
• PMID:20301452 - SCA2 GeneReviews
• PMID:20301445 - SCA8 GeneReviews
• PMID:39666847 - SCA4 GeneReviews
• PMID:38035881 - ZFHX3 exonic repeat expansion in SCA4
• PMID:38197134 - long-read sequencing ZFHX3/SCA4
• PMID:38684900 - ZFHX3 GGC expansion impairs autophagy
• PMID:38973251 - SCA4 dysautonomia and motor-neuron signs
• PMID:31471687 - SCA8 cohort in mainland China
• PMID:36703300 - SCA8 neuropathology
• PMID:29916049 - SCA8 PSP/parkinsonism phenotype review
• PMID:30532692 - SCA17 UK cohort
• PMID:15989694 - SCA17 reduced penetrance / unstable TBP allele
• PMID:22492559 - SCA36 Costa da Morte / NOP56 expansion
• PMID:22753339 - SCA36 Japanese cohort
• PMID:37810464 - SCA36 British cohort
• PMID:35309140 - SCA36 review
• PMID:32375043 - SCA36 repeat toxicity model

Follow-Up Curation Checklist

• Fetch and cache any new PMIDs before adding them to YAML evidence.
• Add a diagnosis section only with subtype-aware repeat-testing language.
• Add treatments as supportive/multidisciplinary care, not disease-modifying therapy.
• Add phenotype expansions as subtype-scoped when possible.
• Revisit SCA17 consolidation as a content-modeling decision; this research note does not resolve whether the existing standalone SCA17 YAML should be duplicated into the ADCA-I parent or kept as a linked sibling.