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Huntington Disease Test

Genes:
HTT

Syndrome Information

Clinical Description

Huntington Disease (HD) features progressive motor, cognitive and psychiatric disturbances.  Patients may experience clumsiness, balance and gait disturbances as well as other issues with both voluntary and involuntary movement, dysarthria, oculomotor disturbances, weakness, weight loss, personality changes, and depression.  Age of onset is usually in the 4th decade. 

Patients with Juvenile HD have a similar set of symptoms, but experience a more rapid decline.  Motor and cerebellar symptoms are prominent.  Onset is usually before age 20.  Seizures occur in cases with onset before age 10.

Inheritance Pattern

Autosomal Dominant

Genotype-Phenotype Correlation

Repeat length is associated with age at symptom onset (Juvenile HD versus classic HD), faster progression of disease, and decreased variability in age at symptom onset.

Reduced penetrance alleles are described.   

Test Information

Test Method

PCR and fragment sizing of the CAG repeat region of the HTT gene

Clinical Utility

Identification of causative mutations in known or highly suspicious cases of a Huntington Disease; Rule-out HD in the presence of equivocal clinical presentation; Predictive testing in relatives of a proband with an HTT mutation.

Clinical Sensitivity

Of patients meeting clinical diagnostic criteria for Huntington Disease, 98-99% will have an expanded CAG repeat in exon 1 of the HTT gene.  Reduced penetrance (60% by age 65 and 70% by age 75) has been identified in individuals with expansions from  36 to 39 CAG repeats.

Analytic Sensitivity

The repeat size provided in the report is +/- 1 CAG repeat up to 40; +/- 2 CAG repeats 41-60; and, +/- 3 CAG repeats when >60 repeats.  The assay may not detect expansions larger than 101 repeats. 

Sample Requirements

3-6ml whole blood in EDTA (purple topped) tubes. (see Pediatric or Adult blood sample algorithms for additional information)

We prefer whole blood for all tests.

Turn Around Time

2-3 weeks

Fee and CPT Codes

$355 for routine testing on a blood sample
CPT Code:  81401

Please contact the lab to arrange testing for relatives of a proband or prenatal samples.  

Special Considerations

The Huntington Disease Society has guidelines for presymptomatic testing for Huntington Disease (HD) (and similar adult-onset neurodegenerative conditions) that outline a team approach over several in-person sessions.  These guidelines are summarized in our Presymptomatic HD checklist (sections 5, 8, and 9 for counseling recommendations).

INFORMED CONSENT from the patient is required prior to ordering a genetic test. The DNA Diagnostic Lab's consent is located on the second page of the requisition form. There is also a patient brochure, "Things Every Patient Should Know Before Consenting to a Genetic Test", available for download.