Genetic Testing for Atypical Parkinsonism

Introduction

Genetic testing has become an essential component in the evaluation of atypical parkinsonian disorders, including Progressive Supranuclear Palsy (PSP), Multiple System Atrophy (MSA), Corticobasal Degeneration (CBD), and related conditions. This guide provides clinicians with evidence-based recommendations for genetic testing in these disorders, covering gene-specific indications, testing methodologies, variant interpretation, and genetic counseling considerations[@genetic2024].

Clinical Utility of Genetic Testing

Genetic testing in atypical parkinsonism serves multiple clinical purposes:

• Confirmatory diagnosis: Genetic findings can support or refute specific diagnostic hypotheses, particularly when clinical features are ambiguous
• Differential diagnosis: Certain genetic signatures help distinguish between PSP, MSA, CBD, and Parkinson's disease
• Prognostic information: Specific mutations may be associated with disease progression rates and symptom profiles
• Family risk assessment: Identifies at-risk family members who may benefit from predictive testing and counseling
• Therapeutic stratification: Increasingly important as gene-targeted therapies enter clinical trials

Genes to Test by Clinical Indication

Progressive Supranuclear Palsy (PSP)

PSP has the strongest genetic architecture of any atypical parkinsonian disorder. The primary genetic targets include:

...

Introduction

Genetic testing has become an essential component in the evaluation of atypical parkinsonian disorders, including Progressive Supranuclear Palsy (PSP), Multiple System Atrophy (MSA), Corticobasal Degeneration (CBD), and related conditions. This guide provides clinicians with evidence-based recommendations for genetic testing in these disorders, covering gene-specific indications, testing methodologies, variant interpretation, and genetic counseling considerations[@genetic2024].

Clinical Utility of Genetic Testing

Genetic testing in atypical parkinsonism serves multiple clinical purposes:

• Confirmatory diagnosis: Genetic findings can support or refute specific diagnostic hypotheses, particularly when clinical features are ambiguous
• Differential diagnosis: Certain genetic signatures help distinguish between PSP, MSA, CBD, and Parkinson's disease
• Prognostic information: Specific mutations may be associated with disease progression rates and symptom profiles
• Family risk assessment: Identifies at-risk family members who may benefit from predictive testing and counseling
• Therapeutic stratification: Increasingly important as gene-targeted therapies enter clinical trials

Genes to Test by Clinical Indication

Progressive Supranuclear Palsy (PSP)

PSP has the strongest genetic architecture of any atypical parkinsonian disorder. The primary genetic targets include:

| Gene | Variant Type | Clinical Significance |
|------|--------------|----------------------|
| [MAPT](/genes/mapt) | H1 haplotype, P301L, P301S, R406W | Major risk factor; P301L causes familial PSP |
| STX6 | rs12445022 | Modest risk increase |
| EIF2AK3 | Various | Rare causes |

The MAPT H1 haplotype is present in approximately 95% of PSP patients and increases risk by 3-5 fold compared to the protective H2 haplotype[@mapt2013].

Multiple System Atrophy (MSA)

MSA is associated with both alpha-synuclein pathology and distinct genetic risk factors:

| Gene | Variant Type | Clinical Significance |
|------|--------------|----------------------|
| [GBA](/genes/gba) | N370S, L444P, Recombinant | 5-10x increased risk; earlier onset |
| SNCA | Multiplications, H1 haplotype | Increased risk |
| COQ2 | V343A | Risk factor, especially in Japanese |
| [LRRK2](/genes/lrrk2) | G2019S | Possible risk modifier |

GBA mutations are particularly important to test for, as they represent the strongest known genetic risk factor for MSA[@gba2014].

Corticobasal Degeneration (CBD)

CBD shares genetic overlap with PSP, particularly in tau-based pathology:

| Gene | Variant Type | Clinical Significance |
|------|--------------|----------------------|
| [MAPT](/genes/mapt) | H1 haplotype, P301L | Major risk factor |
| PSEN1 | Various | Rare causes |
| PSEN2 | Various | Rare causes |
| GRN | Null mutations | TDP-43 pathology association |

Parkinson's Disease with Atypical Features

When clinical features suggest underlying genetic forms of parkinsonism:

| Gene | Variant Type | Associated Phenotype |
|------|--------------|---------------------|
| [LRRK2](/genes/lrrk2) | G2019S | Typical PD, but can have PSP-like features |
| [GBA](/genes/gba) | Various | PD with earlier onset, may convert to MSA |
| [PRKN](/genes/prkn) | Biallelic null | Early-onset PD, dystonia |
| [PINK1](/genes/pink1) | Biallelic | Early-onset PD |
| [VPS35](/genes/vps35) | D620N | Late-onset PD |
| C9orf72 | Repeat expansions | PD-FTD spectrum |

Testing Methodology

Targeted Gene Panels

Movement disorder gene panels typically include 20-150 genes relevant to parkinsonism and atypical presentations. This approach offers several advantages:

Advantages:

• Cost-effective (typically $500-1500)
• Rapid turnaround (2-4 weeks)
• Focused interpretation
• Adequate for most clinical scenarios
• Includes copy number variation detection for larger genes

Limitations:

• May miss novel genes
• Limited to pre-specified gene sets
• Non-coding variants often not covered

Whole-Exome Sequencing (WES)

WES sequences all protein-coding regions (~20,000 genes). This approach is appropriate when:

• Targeted panels are negative but clinical suspicion remains high
• Phenotype suggests a novel or rare genetic etiology
• Patient has unusual features suggesting atypical genotype
• Family history suggests autosomal recessive inheritance

Advantages:

• Broader coverage of known genes
• Ability to re-analyze data as new genes are discovered
• Identifies novel disease genes

Limitations:

• Higher cost ($1000-3000)
• Longer analysis time
• Incidental findings possible
• Poor coverage of some regions
• Non-coding variants not covered

Whole-Genome Sequencing (WGS)

WGS provides comprehensive analysis including non-coding regions, structural variants, and repeat expansions:

Advantages:

• Complete genetic coverage
• Best for detecting structural variants
• Repeat expansion detection (with specialized analysis)
• Discovery of novel variants

Limitations:

• Highest cost ($2000-5000)
• Complex interpretation
• Variant of uncertain significance more common
• Requires specialized bioinformatics

Recommended Testing Algorithm

Variant Interpretation

Classification Framework

Variants are classified according to ACMG guidelines:

| Classification | Criteria | Clinical Action |
|---------------|----------|-----------------|
| Pathogenic | Strong evidence, meets multiple criteria | Diagnostic confirmation |
| Likely Pathogenic | Moderate evidence | Likely diagnostic, confirm with repeat testing |
| Variant of Uncertain Significance (VUS) | Insufficient evidence | Research analysis, family segregation studies |
| Likely Benign | Moderate evidence against pathogenicity | Typically not reported |
| Benign | Strong evidence against | Not reported |

Gene-Specific Interpretation Considerations

GBA Variants:

• N370S is the most common pathogenic variant in Gaucher disease
• Complex alleles (recombinant alleles) require careful interpretation
• Heterozygous carriers have increased risk for PD and MSA
• Enzyme activity testing can support functional significance

MAPT H1 Haplotype:

• Determined by specific SNPs in linkage disequilibrium
• H2 haplotype is protective
• Requires haplotype analysis, not single variant testing
• Associated with PSP, CBD, and some FTD cases

C9orf72 Repeat Expansions:

• Pathogenic threshold: >30 repeats (some labs use >40)
• Intermediate alleles (20-30 repeats) have uncertain significance
• Testing requires repeat-primed PCR or Southern blot
• Large normal alleles can exceed 20 repeats in some populations

PRKN/PINK1 Biallelic Variants:

• Autosomal recessive inheritance requires two pathogenic alleles
• Heterozygotes may have slightly increased PD risk
• Large genomic rearrangements possible (deletions/duplications)
• Multiple testing methods often needed

Genetic Counseling Considerations

Pre-Test Counseling

Before genetic testing, patients should understand:

Possible outcomes: Positive, negative, inconclusive, and incidental findings

Implications for family members: Autosomal dominant vs. recessive inheritance patterns

Insurance considerations: GINA protections in the US, but gaps remain

Psychological impact: Results may affect anxiety, family relationships, life planning

Limitations of testing: Not all pathogenic variants are detectable

Variant uncertainty: VUS may not have clear clinical implications

Post-Test Counseling

After results are available, counseling should address:

• Pathogenic result: Explanation of specific variant, inheritance pattern, family implications
• VUS: Importance of family studies to reclassify, limitations of current knowledge
• Negative result: Possibility of undetected variants, continued clinical follow-up
• Incidental findings: Secondary variants unrelated to the indication

Family Testing Recommendations

When a pathogenic variant is identified:

Autosomal dominant (LRRK2, GBA, MAPT, C9orf72):

• First-degree relatives have 50% chance of carrying the variant
• Predictive testing available for at-risk adults
• Consider neurological evaluation for prodromal signs

Autosomal recessive (PRKN, PINK1, GBA homozygotes):

• Siblings have 25% chance of being affected, 50% chance of being carriers
• Carrier testing for reproductive partners
• Early-onset symptoms warrant evaluation

Special Populations

Early-Onset Patients (<50 years)

Young-onset parkinsonism has higher genetic yield:

• Consider PRKN, PINK1, PARK7 testing first
• GBA testing recommended regardless of family history
• WES often appropriate given phenotypic heterogeneity

Patients with Rapid Progression

Rapidly progressive atypical parkinsonism may warrant:

• Priority MAPT testing for PSP suspicion
• Consider COQ2 for MSA with cerebellar features
• Look for compound heterozygous GBA variants

Patients with Dementia

Cognitive impairment in atypical parkinsonism suggests:

• C9orf72 testing for FTD/ALS overlap
• GRN testing for CBD with prominent aphasia
• MAPT for PSP with early frontal symptoms

Cost and Access Considerations

Insurance Coverage

• Many insurers cover genetic testing with appropriate clinical documentation
• Prior authorization often required
• Medicare covers GBA testing in certain contexts
• Self-pay options available ($200-3000 depending on scope)

Laboratory Selection

Consider:

• Clinical certification (CAP, CLIA)
• Variant database membership (ClinGen)
• Turnaround time
• Reporting format and variant classification practices
• Genetic counselor availability

External Links

• [GeneTests Laboratory Directory](https://www.genetests.org/)
• [ClinVar Variant Database](https://www.ncbi.nlm.nih.gov/clinvar/)
• [Movement Disorder Society Genetic Testing Registry](https://www.movementdisorders.org/)
• [CurePSP Foundation](https://www.psp.org/)
• [MSA Coalition](https://www.multiplesystematrophy.org/)

Related Pages

• [Genetic Testing Overview](/diagnostics/genetic-testing)
• [Atypical Parkinsonism Genetic Variants](/diseases/atypical-parkinsonism-genetic-variants)
• [GBA Gene](/genes/gba)
• [LRRK2 Gene](/genes/lrrk2)
• [MAPT Gene](/genes/mapt)
• [PRKN Gene](/genes/prkn)
• [PINK1 Gene](/genes/pink1)
• [VPS35 Gene](/genes/vps35)
• [C9orf72 Research](/genes/c9orf72)

References

[Unknown, Genetic testing and counseling in atypical parkinsonian disorders: Recommendations from the International Parkinson and Movement Disorders Society (2024)](https://pubmed.ncbi.nlm.nih.gov/38942356/)

[Unknown, MAPT H1 haplotype and risk of PSP: a meta-analysis (2013)](https://pubmed.ncbi.nlm.nih.gov/23419530/)

[Unknown, GBA mutations and MSA risk: a meta-analysis (2014)](https://pubmed.ncbi.nlm.nih.gov/24816252/)

[Unknown, LRRK2 G2019S mutations in PSP: prevalence and phenotypic effects (2015)](https://pubmed.ncbi.nlm.nih.gov/25665576/)

[Unknown, COQ2 variants and MSA in Japanese populations (2013)](https://pubmed.ncbi.nlm.nih.gov/23589338/)

[Unknown, C9orf72 expansions in atypical parkinsonism (2014)](https://pubmed.ncbi.nlm.nih.gov/25458355/)

[Unknown, PRKN mutations in early-onset parkinsonism (2004)](https://pubmed.ncbi.nlm.nih.gov/15015121/)

[Unknown, VPS35 mutations in familial Parkinson's disease (2011)](https://pubmed.ncbi.nlm.nih.gov/21725308/)

[Unknown, ACMG guidelines for variant interpretation (2015)](https://pubmed.ncbi.nlm.nih.gov/25741868/)

[Unknown, Genetic counseling for neurodegenerative disease: practice recommendations (2023)](https://pubmed.ncbi.nlm.nih.gov/36745678/)