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Spinocerebellar Ataxia Type 17.


GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2018.
2005 Mar 29 [updated 2012 May 17].

Author information

Associate Professor, Department of Pathology, Brain Research Institute, Niigata University, Niigata, Japan
Professor, Department of Molecular Neuroscience, Brain Research Institute, Niigata University, Niigata, Japan
Department of Clinical Research, Saigata National Hospital, National Hospital Organization, Niigata, Japan
Professor, Department of Neurology, University of Tokyo Graduate School of Medicine, Tokyo, Japan
Professor, Department of Pathology, Brain Research Institute, Niigata University, Niigata, Japan



Spinocerebellar ataxia type 17 (SCA17) is characterized by ataxia, dementia, and involuntary movements, including chorea and dystonia. Psychiatric symptoms, pyramidal signs, and rigidity are common. The age of onset ranges from three to 55 years. Individuals with full-penetrance alleles develop neurologic and/or psychiatric symptoms by age 50 years. Ataxia and psychiatric abnormalities are frequently the initial findings, followed by involuntary movement, parkinsonism, dementia, and pyramidal signs. Brain MRI shows variable atrophy of the cerebrum, brain stem, and cerebellum. The clinical features correlate with the length of the polyglutamine expansion but are not absolutely predictive of the clinical course.


The diagnosis of SCA17 relies on molecular genetic testing to detect an abnormal CAG/CAA repeat expansion in TBP, the only gene in which mutation is known to cause SCA17. Affected individuals usually have more than 42 repeats. Such testing detects 100% of affected individuals. The CAA and CAG codons both encode glutamine residues resulting in a pathogenic polyglutamine expansion.


Treatment of manifestations: psychotropic medications for psychiatric problems, antiepileptic drugs for seizures (AEDs); botulinum toxin injections for dystonia; and adaptation of the environment to accommodate dementia. Prevention of secondary complications: Side effects of psychotropic medications and AEDs may require total or intermittent discontinuation of the treatment or reduction in dose. Surveillance: Annual or semiannual evaluation by a neurologist or more frequently if symptoms are progressing rapidly. Agents/circumstances to avoid: Sedative/hypnotic agents, such as ethanol or certain medications, may exacerbate incoordination.


SCA17 is inherited in an autosomal dominant manner. Offspring of affected individuals are at a 50% risk of inheriting the expanded TBP allele. The age of onset, severity, specific symptoms, and progression of the disease are variable and cannot be precisely predicted by family history or size of expansion. Prenatal diagnosis is possible for fetuses at 50% risk if the diagnosis has been confirmed in at least one relative; requests for prenatal testing of typically adult-onset diseases are uncommon.

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