Spinocerebellar Ataxia 1 (SCA1)

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Gene Symbol:  ATXN1   
     
Chromosomal Locus: 6p22.3
  
Protein:  ataxin 1 
 
Pseudonyms: Spinocerebellar ataxia type 1, Olivopontocerebellar ataxia, type 1 or 4
 
TURNAROUND TIME:     10 days
  
TESTING METHODOLOGY:
Polymerase chain reaction (PCR) that brackets the ATXN1 CAG repeat region using fluorescent capillary electrophoresis for fragment size analysis.
 
SPECIMEN REQUIREMENTS:
  • Collect: Prefer two 5ml whole blood EDTA (lavender top) tube. 
  • Min. Collection: 0.7 ml whole blood EDTA.
  • Transport: Blood EDTA at Room Temp shipped regular next day air (No Saturday delivery; store specimen refrigerated and ship Monday).
  • Stability: Ambient: up to 7 days; Refrigerated: 2 weeks. Frozen: unacceptable
  • Unacceptable Conditions: Serum. Frozen or severely hemolyzed blood. Clotted blood.       
  • Prenatal testing: Direct: 5ml direct unspun amniotic fluid or 15mgCVS tissue with a backup flask growing. Culture: confluent T25 flask.  Maternal blood sample is required for maternal cell contamination studies.
A Molecular Genetics Laboratory Test Requisition must accompany the specimen. Contact the Molecular Laboratory at 918-502-1721 to obtain further information.
 
Note: Counseling and informed consent are recommended for genetic testing. A consent form is available as a resource.
 
INTERPRETATIVE DATA:
 
Incidence: 1 in 100,000. About 6% of autosomal dominant ataxias.
 
Inheritance: Autosomal dominant. Variable age of onset but complete penetrance.
 
Disease Characteristics:
In individuals with molecularly confirmed ATXN1 there is progressive cerebellar ataxia, dysarthria, and eventual deterioration of bulbar functions. Early symptoms include gait disturbance, slurred speech, and difficulty with balance. Symptoms progress to include development of up-gaze palsy, dysdiadochokinesia, and hypotonia. In advanced stages, muscle atrophy, cognitive impairment, chorea, dystonia, and bulbar dysfunction are seen. Once the involuntary neurological symptoms start affecting autonomic muscle systems death occurs rapidly.  The size of the CAG repeat region dictates the age of onset and the speed of the progression of symptoms rather than the actual spectrum of symptoms.
 
Molecular Genetic Mechanism: A neurodegenerative disorder that demonstrates the genetic anticipation characteristic of an unstable expansion of CAG trinucleotide repeats within the coding region of the ATXN1 gene.  This glutamine repeat region is found to be polymorphic in the normal population with sizes ranging from 6 to 44 repeats.  Pathogenicity of alleles in the 36 to 44 repeat range depends on the presence of CAT repeat interruption; those without the CAT interruptions will be considered either mutable normal for 36 to 38 CAG repeats or fully penetrant for 39 to 44 CAG repeats. Alleles of more than 39 Uninterrupted CAG repeats and over 45 repeats are considered abnormal. The age of onset and increase of CAG size often increases each generation (Anticipation).
 
Reference Ranges (CAG repeats):
Normal alleles:  6 to 35
Mutable Normal Alleles : 36 to 38 (without CAT interruptions)
Abnormal Alleles : 39 to 44 CAG repeats without CAT interruptions and all alleles > 45  repeats
 
Related Tests:  SCA screen, HDDRPLA, FRDA and Neurological Panel 
 
Clinical Sensitivity: 99%
 
Analytical Sensitivity: 99%
 
Test Limitations:  This test examines the CAG repeat regions, exclusively. However, no other mechanism has been described for SCA1 and the test is considered diagnostic.  Test results should be interpreted in the context of clinical findings, family history, and other laboratory data. Errors in our interpretation of results may occur if information given is inaccurate or incomplete. Rare diagnostic errors can occur due to primer or probe site mutations or rare polymorphisms.
 
INDICATIONS FOR USE:
  • Individuals with a family history of SCA1 who want to determine their true risk.
  • Families considering future medical and disability insurance needs.
  • To differentiate individuals with SCA1 from other ataxias.
ADDITIONAL RESOURCES: