Spinocerebellar Ataxia Type 7 Repeat Analysis

Casandra

Casandra Test Code GE89937Version 1 (DRAFT)

Performing Lab

Lab GeneDxLab Test ID TH88

Spinocerebellar Ataxia Type 7 Repeat Analysis

Clinical Use

Use

SCA7 is a neurodegenerative disorder typically presenting with adult-onset vision concerns or poor coordination, and infantile-onset with hypotonia, developmental delay, and retinal degeneration. Expanded CAG repeats in ATXN7 (≥37 repeats) are fully penetrant and pathogenic; 34–36 repeats are associated with incomplete penetrance and milder phenotype with anticipation across generations. Alleles with >100 repeats require Southern blot and are not resolved by this assay. These features establish the clinical utility of identifying CAG repeat expansions in ATXN7 for diagnostic confirmation.

Special Instructions

Analysis uses genomic DNA from submitted specimen. Standard PCR fragment analysis detects alleles up to ~100 repeats; repeat‑primed PCR identifies alleles >100 repeats and quantifies repeats ≤100 with specified accuracy. Southern blot needed for precise sizing of alleles >100 repeats. Internal standards used to ensure assay performance.

Limitations

Accuracy of repeat sizing is ±2 repeats for ≤50 repeats and ±5 repeats for 51–100 repeats. Alleles with >100 repeats cannot be precisely sized by this test and require Southern blot analysis. No ability to detect other variant types beyond CAG repeat expansions. Results must be interpreted in clinical context; rare diagnostic errors may occur due to primer/probe binding site mutations or polymorphisms.

Test Details