Objective: To describe two patients with genetically confirmed spinocerebellar ataxia type 17 (SCA17) demonstrating markedly different clinical phenotypes, disease severity, and neuroimaging findings.

Background: SCA17 is a rare autosomal dominant neurodegenerative disease in the group of polyglutamate disorders caused by CAG/CAA repeat expansions in the TATA-box binding protein (TBP) gene.

Method: Retrospective chart and literature review.

Results: Case 1*:

A 63-year-old male presented with a history of focal epilepsy since age 4, and right upper extremity incoordination and tremor since age 55.  Examination revealed only mild asymmetric intention tremor.  EEG demonstrated left temporoparietal sharp waves.  Brain MRI showed right mesial temporal sclerosis, bilateral superior cerebellar hemisphere atrophy and focal right occipital volume loss.  Family history was negative for epilepsy or ataxia.

Over several years, he developed progressive tremor, gait ataxia, right upper extremity dystronia, and slowed saccades.  Dopamine transporter scan (DatSCAN) was normal and he had no response to an empiric trial of levodopa.  Genetic testing showed a 41 CAG/CAA repeat expansion in the TBP gene (STUB 1 gene negative).  His symptoms remain stable.

Case 2:

A 38 -year-old man presented with several years of progressive hand and head tremor, bradykinesia, abnormal gait and posture, cognitive decline, behavioral changes, and weight loss.  Examination revealed dysarthria, executive dysfunction, bradykinesia, nystagmus, stooped posture, postural/ action/intention tremor of the hands, hyperreflexia, and gait ataxia..Laboratory studies were notable for mildly elevated transaminases.  MRI brain demonstrated mild cerebellar atrophy.

DatSCAN was normal.  His father had adolescent onset seizures, similar neurological symptoms and death in his mid-40s.  Genetic testing revealed a 54 CAG/CAA repeat expansion in the TBP gene.

Conclusion: Theses cases illustrate the clinical and radiologic heterogeneity of SCA17 and support a correlation between repeat length and phenotypic severity.  Case 1 supports that 41 CAG/CAA trinucleotide expansion can be considered a critical threshold in SCA17.  We suggest that SCA17 should be suspected in patients with movement disorders associated with epilepsy.

References: *This case was presented as a poster at MDS Congress in 2025. (E. Levoir et all. Mov Disord. 2025, 40 (suppl 1) a).

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MDS Abstracts - https://www.mdsabstracts.org/abstract/variable-clinical-and-neuroimaging-features-in-spinocerebellar-ataxia-type-17/