Neurogenetics Reference Lab · Teaching Example
- Panel
- HTT CAG repeat sizing (PCR + capillary sizing; long-PCR for large alleles)
- Report ID
- TEACH-0011
- Indication
- Asymptomatic predictive testing — family history of Huntington disease (father affected, onset age 48).
- Specimen
- Peripheral blood, EDTA · Received 2026-02-05
- Reported
- 2026-02-15
- Methodology
- PCR amplification of HTT CAG repeat region followed by capillary electrophoresis sizing. Independent confirmation on duplicate sample.
Patient (anonymized teaching example)
Age 28 years · Male · Mixed European ancestry
Clinical: Asymptomatic. Pre-symptomatic predictive testing per HD genetic testing guidelines, including pre-test counseling and psychiatric evaluation.
Reportable Variants
| Gene | Variant | Zygosity | Condition (Mode) | Origin | Classification |
|---|---|---|---|---|---|
Interpretation
HTT CAG repeat sizing detected an and a normal allele of 19 repeats. An allele of ≥40 CAG repeats is fully penetrant for Huntington disease; alleles 36–39 are ; alleles ≤26 are normal; alleles 27–35 are (not personally at risk but unstable across generations). This result is diagnostic for Huntington disease. Symptom onset is typically inversely correlated with repeat size — 42 repeats corresponds to adult-onset disease, with mean onset in the late 40s. is possible: future paternal transmission may further expand the repeat.
Recommendations
- •Comprehensive post-test counseling — established HD predictive testing protocol.
- •Connect to the local HD center / Huntington's Disease Society chapter for support.
- •Consider reproductive options if relevant: PGT-M, prenatal testing, non-disclosure PGT.
- •Lifestyle, financial, and disability planning conversations as desired.
- •Annual clinical follow-up to monitor for prodromal/early motor symptoms.