Very early onset Huntington's disease: genetic mechanism and risk to siblings.

A study of very early onset Huntington's disease (VEOHD) has shown that at least 38% of gene-carrying sibs also develop symptoms before the age of 10, thus improving the genetic risk for those sibs who remain healthy. The prevalence of VEOHD among sibs shows that mutation during spermatogenesis is most unlikely to account for these uncommon cases. The data suggest that two mechanisms contribute to VEOHD: modification by many genes (individually of small effect), and an epigenetic mechanism occurring when transmission is through a series of males.