Presymptomatic predictive testing in adults at 50% risk (parent with HD).
Clarifying allele status in borderline/intermediate CAG repeat range cases.
Prenatal and preimplantation genetic testing (PGT) in known carriers.
Endorsed by ACMG, ACHDG, and EHN as the definitive molecular diagnostic for Huntington Disease.
Critical Ethical Note
Predictive HD testing must ONLY be performed through a formal presymptomatic testing protocol with pre- and post-test genetic counselling. Testing must never be offered without psychological support infrastructure (ACMG, HDSA guidelines).
Section 2
Formula & Logic
CAG Repeat Classification
CAG Repeats
Category
Risk
≤ 26
Normal
No risk of HD; not transmissible in expanded form
27–35
Intermediate (mutable normal)
Not personally at risk; small risk of expansion to full penetrance in offspring
36–39
Reduced penetrance
May or may not develop HD; lifetime risk 50–90%
≥ 40
Full penetrance
Will develop HD if live long enough; 100% penetrant
Phenotypic Correlates
Age of Onset (Inverse correlation with CAG length): Earlier onset with longer repeats.
CAG 40–50: Adult onset HD (typically 30–50 years).
CAG > 60: Juvenile/childhood onset (< 20yr), often with rigidity-predominant presentation (Westphal variant).
CAG length explains ~60% of variance in age of onset — residual variance is explained by genetic modifiers (e.g., MSH3 CAA repeat, PMS2, FAN1).
Section 3
Pearls/Pitfalls
Key Pearls
Intermediate allele (27–35): Patient is NOT at personal risk of HD. However, the allele can expand during meiosis (especially paternal transmission), placing offspring at risk. Genetic counselling is essential to discuss this.
Reduced penetrance (36–39): Decision challenging — may or may not develop HD. Counselling must explore individual values and decision-making; surveillance for disease onset warranted.
Both alleles reported: Report includes normal allele (typically the shorter) and expanded allele.
HD is autosomal dominant — each child of an affected/carrier parent has 50% risk.
Section 4
Next Steps
Clinical Actions
01
CAG ≤ 26 (Normal): Reassurance; no HD surveillance required for self. If intermediate allele (27–35), counsel about offspring risk and offer family cascade.
02
CAG 36–39 (Reduced penetrance): Detailed counselling on uncertainty; annual neurological screening; referral to HD specialist centre; reproductive options discussion.
03
CAG ≥ 40 (Full penetrance): Post-test counselling protocol; HD specialist referral; baseline neuropsychological assessment; neuroimaging (MRI: caudate atrophy); Enroll in research registries (ENROLL-HD, REGISTRY).
A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's disease chromosomes
The Huntington's Disease Collaborative Research Group • Cell. 1993;72(6): 971–983
Section 6
Literature
Gene Discovery (1993)
The huntingtin gene (HTT, originally IT15) and its CAG trinucleotide repeat expansion were identified in 1993 by the Huntington's Disease Collaborative Research Group — a landmark discovery involving 58 authors across multiple international institutions after the HD locus was mapped to chromosome 4p16.3 a decade earlier (Gusella et al., 1983). The study ended a 10-year search, one of the longest gene-hunting campaigns in history.
Presymptomatic Testing Era
The identification of the HTT gene enabled predictive genetic testing, first implemented under protocols developed jointly by the World Federation of Neurology and International Huntington's Association (1994). These protocols established the ethical framework — mandatory pre-test counselling, psychological support, and follow-up — that remains central to presymptomatic HD testing globally. The CAG repeat classification thresholds (26/27/35/36/39/40) were refined through studies from multiple centres including the European HD consortium.
