Predictability of phenotype in Huntington's disease.

Huntington's disease (HD) is an autosomal dominant disorder with variable age at onset and variable symptoms. Results from an analysis of questionnaire data on ages at onset and death, sex of the affected parent, and motor disorder in 624 patients gave no evidence of discrete phenotypes, as suggested in the literature. The tendency for muscular rigidity (instead of chorea), an accelerated natural history and paternal transmission, each of which is often associated with the juvenile-onset form of HD, is inversely related to age at onset in the affected child. The most parsimonious explanation for clinical variability in HD is that all HD types are part of a continuum, although expression of the juvenile form may be partly determined by a maternally transmitted factor. Evidence for accelerated aging in HD and correlations between age at onset and death in patients with HD and longevity in their unaffected relatives warrant further investigation into the relationship between aging and phenotypic expression of HD.