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Huntington Disease-Like 2 Test

Genes:
JPH3

Syndrome Information

Clinical Description

Huntington Disease-like 2 (HDL2) is clinically similar to Juvenile onset Huntington Disease.  Patients experience adult onset (usually by the 4th decade) of symptoms that include progressive movement disorder (parkinsonism, chorea), cognitive and emotional decline (dementia, psychiatric disturbances). Unlike Juvenile onset HD, seizures and eye movement abnormalities are usually not described. Some cases may follow a pattern of symptom onset more like classic Huntington Disease. 

Inheritance Pattern

Autosomal Dominant

Genotype-Phenotype Correlation

None; known intra- and inter-familial variability; Reduced penetrance expansion alleles have been described.

Test Information

Test Method

PCR and fragment sizing of the CAG/CTG repeat region of the JPH3 gene

Clinical Utility

Identification of causative mutations in known or highly suspicious cases of a Huntington Disease-like phenotype, especially when HD testing is negative; Rule-out HDL2 in the presence of equivocal clinical presentation; Predictive testing in relatives of a proband with a JPH3 mutation.

Clinical Sensitivity

HDL2 is a rare disorder and the clinical significance of some repeat lengths is still being characterized.  Of Black African patients with symptoms of Huntington Disease who tested negative for HTT expansions, approximately 35% will have expansions of the JPH3 gene consistent with HDL2.   In North Americans, 3/374 patient (<1%) with symptoms of Huntington Disease had JPH3 expansions.  The penetrance of this disorder is not known.  

Analytic Sensitivity

The repeat size provided in the report is +/- 1 CAG/CTG repeat up to 40; +/- 2 CAG/CTG repeats 41-60; and, +/- 3 CAG/CTG 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

$437 for routine testing on a blood sample
CPT Code:  81479

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.