Spinocerebellar Ataxia Type 1 Expansion Analysis

Clinical Information

Spinocerebellar ataxia type 1 is a progressive form of cerebellar ataxia with features that include lack of coordination and balance and dysarthria. Additional symptoms include vision problems such as nystagmus, dysphagia, hypotonia, peripheral neuropathy, and dysmetria. Bulbar signs such as facial muscle atrophy, difficulty chewing and swallowing, and perioral fasciculations may develop in the latter stages. Cognitive impairment may occur over time. Average age of onset is in the 20s-30s although childhood and late-adult onset have been reported.

SCA1 is inherited in an autosomal dominant pattern, and it is caused by a CAG trinucleotide repeat expansion in the ATXN1 gene. One to three CAT interruptions are normally found within the CAG repeat stretch, and the presence of CAT interruptions helps to stabilize the repeat region. In the absence of CAT interruptions, the repeat region is less stable and susceptible to expansion. Repeat ranges are as follows: Normal (CAT absent 6-35; CAT present 6-44), Intermediate (CAT absent 36-38), Pathogenic (CAT absent ≥ 39; CAT present ≥ 45). Anticipation is more likely to occur with paternal transmission.

Technical Information

• This test is performed using triplet repeat-primed PCR and restriction enzyme digest for determination of CAT interruption status.
• Genomic DNA is analyzed for trinucleotide (CAG) repeat expansion in the ATXN1 gene. The triplet repeat-primed polymerase chain reaction (TP-PCR) assay used for analysis is based on a method described by Cagnoli et al. (J Mol Diagn. 2018 May;20(3):289-297).
• The number of trinucleotide repeats in the ATXN1 gene is briefly analyzed using TP-PCR and capillary electrophoresis followed by data analysis using GeneMarker software.
• To confirm the size of the trinucleotide expansion in the ATXN1 gene and determine the CAT interruption status, this sample is also subjected to PCR and SfaNI restriction enzyme digestion. PCR amplicons are specifically digested and separated using capillary gel electrophoresis, and run in parallel with control samples for which the number of CAG repeats have been previously determined. Alleles are interpreted to have a CAT interruption if the corresponding PCR product is digested by the SfaNI enzyme (Chung et al. (1993) Nat Genet. 5(3):254-258).

Specimen Requirements

• The preferred sample type is 3-4 ml of peripheral blood collected in an EDTA (purple top) tube. Extracted DNA and saliva are also accepted for this test. Blood and saliva kits are available by request.
• Send approximately 5µg of extracted DNA at a requested concentration of 90-130 ng/µl.
• Saliva samples must be submitted in an approved saliva kit, and sample kits are available upon provider request.

Transport Instructions

• The blood specimen should be kept at room temperature and delivered via overnight shipping. If shipment is delayed by one or two days, the specimen should be refrigerated and shipped at room temperature. Do not freeze the specimen. Samples collected on Friday can be safely designated for Monday delivery.
• Extracted DNA should be sent at room temperature via overnight delivery.
• Saliva is stable at room temperature and can be delivered via overnight or ground shipping.