Genetics

The Huntington's disease (HD) gene has been mapped to chromosome 4p16.3. The disease is inherited in an autosomal dominant manner.

A gene, IT15, contains a polymorphic trinucleotide repeat that is expanded and unstable on HD chromosomes.

The (CAG)n repeat is found within the coding sequence of a previously unknown, but widely-expressed, gene.

The larger the number of CAG repeats, the earlier the likely onset of HD:

• the normal number of copies is approximately 20
• 40 copies results in adult-onset HD
• > 60 copies results in juvenile-onset HD (JHD)

Early-onset HD is associated with paternal inheritance of the HD gene. It may be that the trinucleotide repeats are more prone to expansion during spermatogenesis than oogenesis.

The trinucleotide expansion which causes Huntington's disease can be detected. This test is used in two main contexts:

• in the relative of an affected individual
• where Huntington's disease is suspected clinically

In common with other autosomal dominant conditions there is a 50% chance that the child of a patient with Huntington's disease will inherit the gene.

Notes:

• a review of patients with HD included showed, as expected, cases with onset <20 years and >60 repeats but also JHD cases with repeats <60, even as low as 42 CAG repeats (2)
  • thus mechanisms other than the triplet repeat expansion must contribute to onset and, possibly, to the variability of the HD phenotype

Reference:

1. 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):971-83.
2. Squitieri F et al. Juvenile Huntington's disease: does a dosage-effect pathogenic mechanism differ from the classical adult disease? Mech Ageing Dev. 2006 Feb;127(2):208-12