What is the underlying mechanism behind why gene mutations that lead to Huntington's disease act as dominant mutations?

The mutations that lead to Huntington's disease are examples of expanded microsatellite repeats.In the case of the gene responsible for Huntington's disease,there is a triplet CAG repeat in the first exon.Expansion of this repeat results in synthesis of long stretches of polyglutamine.Over time,the protein products that contain long stretches of polyglutamine aggregate.Protein aggregation leads to neuronal cell death,which in turn gives rise to the symptoms of Huntington's disease.These microsatellite mutations are dominant because the presence of aggregated proteins causes symptoms,even though some normal proteins are produced from the normal allele.