Increased instability of intermediate alleles in families with sporadic Huntington disease compared to similar sized intermediate alleles in the general population

Abstract

We have directly compared intergenerational stability of intermediate alleles (IAs) derived from new muta tion families (IA_NM) for Huntington disease (HD) with IAs in the general population (IA_GP) which occur in ∼1 in 50 persons. Analysis of meiotic events in blood and sperm reveals that IA_NM are significantly more unstable than IA_GP despite similar size. However, for both IA_NM and IA_GP CAG changes were small and risks for inheriting an expansion into the HD affected range were low. Sequence analysis reveals that the CAG tract is generally interrupted by a penultimate CAA in IA_GP IA_NM and alleles in the affected range. In one new mutation family, however, two A→G mutations result In a pure CAG tract which is associated with very marked instability. These mutations alter the predicted DNA hairpin structure with a predicted increase in the likelihood of large expansion, supporting the model that hairpin loop formation plays an important role in trinucleotide instability.