What is genetic testing for motor neuron disease (MND) and ataxia?

Genetic Testing for Motor Neuron Disease and Ataxia

Genetic testing for motor neuron disease (MND) and ataxia involves DNA analysis to identify pathogenic variants in genes known to cause these conditions, with testing now recommended for all patients with definite MND diagnosis and for ataxia patients following a stepwise diagnostic approach starting with targeted repeat expansion testing.

Motor Neuron Disease (MND/ALS) Genetic Testing

Who Should Be Tested

All patients with a definite diagnosis of MND should be offered genetic testing, particularly as gene-specific therapies emerge 1. This represents a shift from previous practice where testing was limited to familial cases.

• Affected individuals: Anyone with confirmed MND/ALS diagnosis
• Family history considerations: Extend beyond classic MND to include relatives with frontotemporal dementia (FTD) or psychiatric presentations, as C9ORF72 expansions can manifest as either condition within the same family 1, 2
• Unaffected relatives: Predictive testing available but requires extensive genetic counseling (median 4 appointments for predictive vs. 2 for diagnostic testing) 2

Testing Approach for MND

First-line testing should target the most common genetic causes:

1. C9ORF72 hexanucleotide repeat expansion - the most common hereditary cause (accounts for 36.1% of genetic cases in some cohorts) 1, 3
2. SOD1 gene sequencing - second most common, with specific variants showing founder effects (e.g., p.(Ile114Thr) variant) 3
3. Multigene panel if initial testing negative, covering 40+ MND-associated genes 3

Critical Caveats for MND Testing

• Incomplete penetrance: Only 10-15% of ALS cases are attributable to single gene variants; C9ORF72 carriers have low penetrance (0.67% in heterozygotes, >78% in homozygotes) 1, 4
• Variants of uncertain significance (VUS): Common finding that complicates interpretation and can cause harm through uncertainty 1, 2
• Psychosocial impact: Limited support structures exist post-testing; patients report difficulty adjusting regardless of result, with disruption to future planning 5, 6
• Family implications: 48.4% of pathogenic variants may be found in pre-symptomatic individuals, necessitating cascade testing and surveillance 3

Hereditary Ataxia Genetic Testing

Stepwise Diagnostic Approach

Begin with targeted testing for nucleotide repeat expansions, as these account for the majority of hereditary ataxias and are NOT detected by standard exome sequencing 7.

First-Line Testing (Repeat Expansion Analysis)

Test for the most common spinocerebellar ataxias (SCAs) and related conditions:

• SCA1, 2,3,6,7,8,12,17
• DRPLA (dentatorubral-pallidoluysian atrophy)
• Friedreich ataxia (FRDA) - most common (36.1% of genetic ataxias) 8
• RFC1-related ataxia/CANVAS (11.1% of cases) 8
• FXTAS (Fragile X-associated tremor/ataxia syndrome)
• Mitochondrial DNA-related ataxia

This first-line approach achieves diagnosis in approximately 30.4% of patients 8.

Second-Line Testing (NGS-Based)

If repeat expansion testing is negative, proceed to:

1. Targeted gene panel using next-generation sequencing (NGS)
2. Clinical exome sequencing (CES) if panel is negative

This sequential NGS approach provides diagnosis in approximately 20% of previously unsolved cases 8. Common genes identified include:

• SPG7 (most common in NGS-diagnosed cases)
• SACS, CACNA1A, CACNA1G, PRKCG, KCNC3, ADCK3, SYNE1, ITPR1 8

Predictive Factors for Positive Results

• Positive family history of ataxia: Significantly increases diagnostic yield 8
• Early symptom onset: Associated with higher likelihood of genetic diagnosis 8

Critical Pitfall to Avoid

Do NOT start with clinical exome sequencing alone - this will miss the majority of hereditary ataxias caused by nucleotide repeat expansions, which are the most common genetic causes but require specific assays for detection 7. Standard exome sequencing cannot detect these pathogenic variants, leading to false-negative results.

Genetic Counseling Requirements

Pre-Test Counseling Must Address:

• Penetrance issues: Many identified variants show incomplete penetrance
• VUS possibility: Uncertain results are common and require ongoing reinterpretation
• Family implications: Cascade testing considerations for relatives
• Insurance concerns: No legislation in some jurisdictions prevents insurers from using genetic test results 9
• Psychosocial support: Arrange follow-up regardless of result 5, 6

Post-Test Management

• Positive results: Minimum 2 genetic counseling appointments for affected individuals, 4 for predictive testing 2
• Negative results: Still require follow-up as testing sensitivity is not 100% and variant reclassification occurs over time
• VUS results: Periodic reinterpretation needed as databases evolve 10

When Genetic Testing Has Limited Utility

• Unaffected general population screening: Not recommended due to low penetrance 4
• Absence of clinical diagnosis: Testing in low-probability cases yields predominantly VUS results 10
• Without genetic counseling infrastructure: Predictive testing should not proceed without adequate support systems 5, 6