# Approach to the Neurogenetics Patient: Genetic Testing Principles

## Introduction

Advances in genomic technology have transformed neurology, enabling molecular diagnosis for an expanding number of neurological disorders. The neurologist must understand the principles of genetic testing, interpret results accurately, and navigate the complex ethical and psychosocial dimensions of genetic diagnosis. This lecture provides a framework for approaching the neurogenetics patient, from test selection to counseling and follow-up.

## When to Suspect a Genetic Neurological Disorder

### Clinical Red Flags

Several features should raise suspicion for a genetic etiology. A family history of similar neurological symptoms, particularly with a recognizable inheritance pattern, is the most obvious clue. Early age of onset for a given condition (such as dementia before age 60 or stroke before age 50), bilateral or symmetric involvement in conditions that are typically unilateral, and multisystem involvement spanning neurological, cardiac, hepatic, renal, dermatological, or ophthalmological systems all warrant genetic consideration. Consanguinity in the family increases the likelihood of autosomal recessive conditions. Specific ethnic predispositions (such as Tay-Sachs disease in Ashkenazi Jewish populations or sickle cell disease in individuals of African descent) provide further guidance. Progressive deterioration without an acquired explanation, dysmorphic features or congenital anomalies, and failure to respond to standard treatments are additional red flags.

### Inheritance Patterns

Autosomal dominant inheritance produces affected individuals in every generation with a 50% recurrence risk, though variable expressivity and incomplete penetrance may obscure the pattern. Autosomal recessive inheritance typically affects siblings with unaffected carrier parents and carries a 25% recurrence risk, with increased likelihood in the setting of consanguinity. X-linked inheritance predominantly affects males with no male-to-male transmission; carrier females may be mildly affected, and X-linked dominant forms affect both sexes. Mitochondrial inheritance is exclusively maternal, with variable expression due to heteroplasmy. De novo mutations occur without family history as new mutations in the proband and can arise in any gene. Trinucleotide repeat disorders may show anticipation, with earlier onset and increased severity in successive generations.

![Pedigree diagrams illustrating autosomal dominant, autosomal recessive, X-linked, and mitochondrial inheritance patterns](images/inheritance-patterns.jpg)

## Types of Genetic Tests

| Test | What It Detects | Strengths | Limitations |
|---|---|---|---|
| Single-gene testing | Point mutations, small indels in one gene | Fast, cheap, specific | Misses other causes; requires clinical suspicion |
| Gene panel | Variants in curated set of phenotype-relevant genes | Efficient; interpretable | Panel composition varies; may miss new genes |
| Chromosomal microarray (CMA) | Copy number variants (deletions/duplications) | First-tier for ID/ASD; genome-wide | Misses point mutations, balanced translocations, repeats |
| Whole exome sequencing (WES) | Variants in all protein-coding regions (~1-2% of genome) | Unbiased; 25-40% yield | Misses non-coding, repeats, structural variants; VUS burden |
| Whole genome sequencing (WGS) | All genomic variants including structural and non-coding | Most comprehensive | Higher cost; interpretive complexity |
| Repeat expansion testing | Trinucleotide/other repeat expansions | Specific for repeat disorders | Must be ordered specifically; not detected by WES |

### Targeted Single-Gene Testing

This approach is appropriate when clinical features strongly suggest a specific disorder, such as HTT testing for Huntington disease, SMN1 for spinal muscular atrophy, or DMPK for myotonic dystrophy type 1. It is fast, cost-effective, and highly specific, but it will miss the diagnosis if the wrong gene is tested.

### Gene Panels

Gene panels involve simultaneous sequencing of a curated set of genes associated with a specific phenotype, such as hereditary neuropathy, epilepsy, or ataxia panels. They are efficient, interpretable, and focused on clinically relevant genes. However, panel composition varies by laboratory, recently discovered genes may not be included, and structural variants or repeat expansions may not be detected depending on the methodology.

### Chromosomal Microarray (CMA)

CMA detects copy number variants (deletions and duplications) across the genome. It is the recommended first-tier test for intellectual disability, autism spectrum disorder, and multiple congenital anomalies. It does not detect balanced translocations, point mutations, or small insertions and deletions.

### Whole Exome Sequencing (WES)

WES sequences all protein-coding regions (exons) of the genome, which represent approximately 1-2% of the genome but contain approximately 85% of known disease-causing mutations. The diagnostic yield is 25-40% for undiagnosed neurological disorders. Its unbiased approach can identify novel gene-disease associations. However, it does not cover non-coding regions, repeat expansions, mitochondrial DNA (unless specifically included), or structural variants, and it generates variants of uncertain significance (VUS).

### Whole Genome Sequencing (WGS)

WGS sequences the entire genome including non-coding regions, and can detect structural variants, repeat expansions, and mitochondrial DNA variants. It comes with higher cost and greater interpretive complexity but is increasingly used as a first-tier test in some clinical and research settings.

### Repeat Expansion Testing

Specific testing is required for trinucleotide and other repeat expansion disorders such as Huntington disease, fragile X, myotonic dystrophy, spinocerebellar ataxias, and C9orf72. Standard sequencing methods (WES/WGS) may miss repeat expansions, so specific PCR-based or Southern blot assays are needed. Repeat-specific testing should always be ordered when clinical suspicion is high, even if exome sequencing is negative.

## Interpreting Genetic Test Results

### Variant Classification (ACMG Guidelines)

The American College of Medical Genetics and Genomics classifies variants into five categories. Pathogenic variants have strong evidence that they cause disease. Likely pathogenic variants have a high probability of pathogenicity but are not definitive. Variants of uncertain significance (VUS) have insufficient evidence to classify and should not be used for clinical decision-making, though they may be reclassified over time. Likely benign variants have a low probability of causing disease, and benign variants have strong evidence against pathogenicity.

### Considerations in Interpretation

Several factors guide interpretation. Phenotype correlation asks whether the identified variant explains the clinical presentation. Segregation analysis examines whether the variant cosegregates with disease in the family. Population frequency considerations dictate that rare variants are more likely pathogenic while common variants are generally benign. In silico predictions from computational tools such as SIFT, PolyPhen, and CADD provide supportive but not definitive evidence. Functional studies, when available, provide the strongest evidence. VUS should be reported back to the laboratory periodically for potential reclassification.

![Flowchart for selecting the appropriate genetic test based on clinical presentation](images/genetic-testing-flowchart.jpg)

## Genetic Counseling

### Pre-Test Counseling

Pre-test counseling should address the purpose, scope, and limitations of the proposed test. Potential outcomes including positive results, negative results, VUS, and incidental or secondary findings should be explained. The psychological impact of results on the patient and family members must be discussed, along with implications for insurance and employment under the Genetic Information Nondiscrimination Act (GINA) and its limitations. Informed consent should be obtained and the discussion documented.

### Post-Test Counseling

Results should be explained in understandable terms, with discussion of recurrence risks and implications for family members. Predictive testing should be offered to at-risk relatives with appropriate counseling, especially for late-onset conditions like Huntington disease. Psychological support and resources should be provided, and reproductive options (preimplantation genetic testing, prenatal diagnosis) discussed if relevant.

### Predictive Testing for Late-Onset Neurological Disorders

Predictive testing is applicable when a pathogenic variant has been identified in a symptomatic family member. At-risk individuals may seek testing before symptom onset for conditions such as Huntington disease, familial ALS, or CADASIL. Strict protocols exist, originally developed for Huntington disease, involving pre-test genetic counseling sessions, psychological assessment, a waiting period, and post-test support. Testing of minors for adult-onset conditions is generally not recommended unless intervention during childhood is possible.

## Ethical and Practical Considerations

The ACMG recommends reporting pathogenic variants in approximately 80 actionable genes as incidental or secondary findings when WES or WGS is performed, and patients should be counseled about this before testing. GINA (2008) protects against discrimination in health insurance and employment but does not cover life insurance, disability insurance, or long-term care insurance. The duty to warn at-risk relatives presents an ethically complex situation in which patient confidentiality must be balanced against potential harm to family members. Genetic testing costs have decreased dramatically, but insurance coverage remains variable and prior authorization is often required.

![Diagram summarizing the approach to a neurogenetics consultation from phenotyping to test selection to counseling](images/neurogenetics-approach.jpg)

## Clinical Pearls

Whole exome sequencing does not reliably detect repeat expansions, so specific repeat testing (for Huntington disease, fragile X, SCAs, C9orf72, and myotonic dystrophy) should always be ordered when clinically indicated, even if WES is negative. A negative genetic test does not exclude a genetic disorder, as the causative gene or variant type may not have been covered by the test performed. Variants of uncertain significance should not be used to confirm a diagnosis or guide clinical decisions and require longitudinal follow-up and potential reclassification. Pre-test genetic counseling is essential, not optional, and patients must understand the implications of potential results before testing is performed. Chromosomal microarray is the recommended first-tier genetic test for intellectual disability and congenital anomalies, with WES or WGS as second-tier if CMA is non-diagnostic.

## References

1. Richards S, Aziz N, Bale S, et al. Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the ACMG and AMP. *Genet Med*. 2015;17(5):405-424.
2. Kalia SS, Adelman K, Bale SJ, et al. Recommendations for reporting of secondary findings in clinical exome and genome sequencing, 2016 update (ACMG SF v2.0). *Genet Med*. 2017;19(2):249-255.
3. Harding BN, Karaa A, Engel WK. Genetic testing strategies in neuromuscular disease. *Continuum (Minneap Minn)*. 2022;28(5):1474-1504.
4. MacArthur DG, Manolio TA, Dimmock DP, et al. Guidelines for investigating causality of sequence variants in human disease. *Nature*. 2014;508(7497):469-476.