Residency · Residency · Medical Genetics & Genomics

Huntington Disease: Predictive Testing and Management

Introduction

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the HTT gene on chromosome 4p16.3. With a prevalence of approximately 5-10 per 100,000 in populations of European ancestry, HD is characterized by progressive motor dysfunction, cognitive decline, and psychiatric disturbance. HD serves as a paradigm for predictive genetic testing and raises critical ethical considerations in clinical genetics practice.

Molecular Genetics

The CAG Repeat

Normal alleles contain 10-26 CAG repeats. Intermediate alleles (27-35 repeats) are not associated with disease but may expand in future generations. The reduced penetrance range (36-39 repeats) shows variable expression, with some individuals remaining unaffected. Full penetrance occurs at 40 or more repeats, with virtually certain disease development if lifespan is sufficient. Juvenile HD typically involves more than 60 repeats and is often paternally inherited.

Anticipation

CAG repeat length tends to increase during paternal transmission due to spermatogenesis instability, explaining the phenomenon of earlier onset in successive generations. Maternal transmission is more stable but expansion can still occur.

Genotype-Phenotype Correlation

CAG repeat length accounts for approximately 50-70% of variance in age of onset. The remaining variance is attributed to genetic modifiers (including DNA mismatch repair genes such as MSH3, PMS1, and MLH1) and environmental factors. Repeat length does not predict rate of progression or specific symptoms.

Clinical Features

Motor Symptoms

Chorea (involuntary, irregular, flowing movements) is the hallmark of adult-onset HD. Dystonia (sustained abnormal postures) is more prominent in juvenile-onset disease. Progressive bradykinesia and rigidity dominate late-stage disease. Oculomotor dysfunction with impaired saccades is often an early sign. Dysarthria, dysphagia, and gait instability develop progressively.

Cognitive Decline

Executive dysfunction (impaired planning, judgment, and cognitive flexibility) characterizes the cognitive decline, which follows a progressive subcortical dementia pattern. Slowed processing speed and difficulty with multitasking are typical.

Psychiatric Manifestations

Depression is the most common psychiatric feature, affecting up to 50% of patients. Irritability, apathy, anxiety, and obsessive-compulsive behaviors are frequent. Suicidal ideation carries highest risk around the time of diagnosis and during the early symptomatic phase. Psychosis occurs in approximately 5-10% of patients.

Juvenile-Onset HD (Westphal Variant)

Onset before age 20 with typically more than 60 CAG repeats produces a rigid-akinetic presentation rather than choreiform. Seizures and rapid cognitive decline characterize a more aggressive course.

Neuropathology

Selective striatal degeneration primarily affects the caudate nucleus and putamen, with medium spiny neurons expressing DARPP-32 being most vulnerable. Progressive cortical atrophy follows in later stages. Vonsattel grading (0-4) is based on degree of striatal pathology.

Predictive and Diagnostic Testing

Diagnostic Testing

Diagnostic testing is indicated for symptomatic individuals with suspected HD. CAG repeat sizing by PCR-based fragment analysis is the standard method, with careful attention to repeat ranges and the reduced penetrance zone in result interpretation.

Predictive Testing Protocol

For asymptomatic at-risk individuals, predictive testing follows international guidelines (HDSA/IHA). A multidisciplinary team includes a geneticist, genetic counselor, neurologist, and psychologist/psychiatrist. A minimum of two pre-test counseling sessions with psychological assessment is required, followed by a mandatory waiting period (typically one month) between the decision and blood draw. In-person result disclosure with a support person present is standard, with post-test follow-up regardless of result.

Ethical Considerations

Nondirective counseling ensures that the decision to test remains autonomous. The right not to know must be respected, including from family pressure. Testing of minors is generally not recommended for adult-onset conditions. Exclusion testing in prenatal/preimplantation settings allows determination of fetal risk without revealing the parent's status. GINA provides limited protections that do not cover life, disability, or long-term care insurance.

Prenatal and Preimplantation Testing

Preimplantation genetic testing (PGT) is available for at-risk couples. Prenatal diagnosis via CVS or amniocentesis with CAG repeat analysis is an option. Exclusion testing using linkage analysis is possible when a parent does not wish to know their own status.

Management

Pharmacologic Treatment

Tetrabenazine and deutetrabenazine (VMAT2 inhibitors) are FDA-approved for chorea. Antipsychotics (olanzapine, risperidone) manage chorea, psychosis, and behavioral symptoms. SSRIs are first-line for depression and anxiety. Valproate and benzodiazepines serve as adjuncts for seizures in juvenile HD and myoclonus. No disease-modifying therapy is currently approved.

Multidisciplinary Care

Physical therapy addresses mobility and fall prevention. Speech and swallowing therapy manages dysphagia to prevent aspiration. Nutritional support addresses the high caloric needs from hyperkinetic movements. Cognitive rehabilitation and behavioral strategies assist with daily functioning. Psychiatric monitoring with active suicide risk assessment is ongoing. Palliative care is integrated as disease progresses.

Emerging Therapies

HTT-lowering strategies using antisense oligonucleotides (tominersen) showed promise but Phase 3 trials were halted due to adverse effects. Splicing modulators (branaplam and related compounds) are being explored. Somatic repeat expansion inhibitors targeting the mismatch repair pathway represent a novel approach. Gene therapy using AAV-delivered miRNA and zinc finger repressors is in preclinical development.

Clinical Pearls

HD predictive testing requires a structured, multistep protocol with psychological support -- it is not a routine genetic test. Approximately 5-25% of at-risk individuals elect to undergo predictive testing after counseling. The reduced penetrance zone (36-39 repeats) creates significant counseling challenges as disease may or may not manifest. Depression and suicidality are treatable and should be actively screened for throughout the disease course, particularly around the time of genetic testing and diagnosis.

References

1. McColgan P, Tabrizi SJ. Huntington's disease: a clinical review. Eur J Neurol. 2018;25(1):24-34.
2. MacLeod R, Tibben A, et al. Recommendations for the predictive genetic test in Huntington's disease. Clin Genet. 2013;83(3):221-231.
3. Genetic Modifiers of Huntington's Disease Consortium. CAG repeat not polyglutamine length determines timing of Huntington's disease onset. Cell. 2019;178(4):887-900.
4. Tabrizi SJ, Flower MD, Ross CA, Wild EJ. Huntington disease: new insights into molecular pathogenesis and therapeutic opportunities. Nat Rev Neurol. 2020;16(10):529-546.

For the intern

At 3am, your priorities are managing acute psychiatric crises—particularly suicidal ideation or severe agitation—and addressing aspiration events in patients with advanced dysphagia. Ensure safety, stabilize the patient, and involve psychiatry if needed for acute risk. Most other HD-related issues can wait until morning: predictive testing inquiries, chorea management adjustments, and routine multidisciplinary care coordination should be addressed during regular hours with the full genetics and neurology teams available. Never initiate predictive testing protocols overnight; these require structured pre-test counseling and psychological assessment that cannot be rushed.

Common pitfalls

• Treating predictive testing like a routine genetic test: HD predictive testing requires a mandatory multistep protocol with psychological assessment, pre-test counseling sessions, and a waiting period. Ordering this test without proper protocol violates ethical guidelines and may cause psychological harm.
• Forgetting the reduced penetrance zone: CAG repeats of 36-39 create significant uncertainty—some individuals develop disease, others don't. This requires nuanced counseling; don't present it as a simple positive/negative result.
• Underestimating suicide risk: Depression and suicidality peak around diagnosis and early symptomatic phases. Screen actively and don't assume psychiatric symptoms are "just part of HD"—they're treatable.
• Testing minors for adult-onset disease: Predictive testing should generally wait until adulthood when the individual can provide autonomous consent and there's no medical benefit to earlier knowledge.

<image>Simple flowchart showing decision algorithm for HD genetic testing. Panel 1: Diamond decision box "Is patient symptomatic with movement disorder or cognitive decline?" with YES arrow leading to rectangular box "DIAGNOSTIC testing—standard CAG repeat analysis, single session genetic counseling sufficient." Panel 2: NO arrow from Panel 1 leading to second diamond "Asymptomatic at-risk individual requesting predictive testing?" with YES arrow leading to rectangular box "PREDICTIVE testing protocol required: Minimum 2 pre-test counseling sessions, psychological assessment, mandatory waiting period, multidisciplinary team involvement." Bottom of Panel 2: NO arrow leading to rectangular box "Consider other indications: prenatal/PGT testing (special protocols), research participation, or defer testing."</image>

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