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Spinocerebellar Ataxia Type 3 (SCA3)


Machado-Joseph Disease, MJD

Klinische Symptomatik

The various forms of autosomal dominant spinocerebellar ataxia (SCA) normally manifest in adulthood. Due to the fact that they show a similar phenotype, these forms cannot always be clearly differentiated from one another clinically. They present with cerebellar ataxia of stance and gait, dysarthria, oculomotor disorders, hypotonia, dysdiadochokinesis, intention tremor, and extrapyramidal symptoms. Pyramidal tract symptoms and involvement of the peripheral nervous system (polyneuropathy) and autonomic systems (imperative urinary incontinence, impotence, orthostatic dysregulation) may also occur.  

Spinocerebellar ataxia type 3 (SCA3) or Machado-Joseph disease (MJD) is characterized by cerebellar ataxia and pyramidal tract symptoms (type II), variably associated with a dystonic-rigid extrapyramidal syndrome (type I) or peripheral amyotrophy (type III). Pronounced progressive eye movement disorders are relatively specific to SCA3. Age of onset varies from childhood to adulthood (from 10 to 70 years of age) but is typically in a patient’s 30s.


Machado-Joseph disease (MJD) is caused by an expansion of CAG repeats in the gene ATX3 (ataxin-3)  located on the long arm of chromosome 14q24.3-qter. The longer glutamine stretch leads to the formation of a structurally altered protein. Ataxin-3 is a ubiquitin-specific protease that binds and cleaves ubiquitin chains. As with other CAG repeat expansion disorders, a direct relationship exists between the age of onset and the number of CAG repeats in the expanded allele. However, other genetic or non-genetic factors may also contribute. In addition, a loose correlation between the repeat number and the clinical phenotype has been described.

MJD is an autosomal dominantly inherited disease. Instability of the CAG repeat has been documented in transmission of the repeat from parent to child. In general, repeat expansion (up to 8 triplets) is more frequent than contraction (up to 5 triplets); anticipation (earlier age of onset and more severe disease manifestation in offspring) occurs. The probability of expansion may be greater with paternal transmission.

Normal allele  < 44
Pathological allele, reduced penetrance 45-51
Pathological allele > 51
  • > 1:20 000
  • SCA3 is the most frequent form (25-35 %) of autosomal dominantly inherited SCA.
  • Differential diagnosis: atactic movement disorder
  • Predictive testing for at-risk individuals (after genetic counseling)
Methodik Next Generation Sequencing (NGS)

Parallele Sequenzierung mehrerer Gene

Material 2-4 ml EDTA-Blut
3-6 Wochen


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