Recent progress in the research field on triplet repeat diseases]

At present, 8 inherited neurodegenerative disorders (e.g. Huntington disease, Machado-Joseph disease, etc.) are identified to be caused by the polyglutamine-coding CAG triplet expansions in the responsible genes. These disorders commonly demonstrate dominant inheritance, if autosomal, and late onset of their manifestations. Interestingly, the longer expansions result in earlier onset and more severe clinical manifestations. Proteins containing expanded polyglutamine repeats appear to precipitate by self-aggregation, and as a result produce a core disease-related phenotype: neuronal cell death or degeneration. Given that polyglutamine aggregation might be central in neurodegeneration, the parameters that determine the feasibility of the polyglutamines to aggregate would determine the age of onset and the clinical severity. These parameters are postulated to be the concentration and the length of polyglutamines, which is supported by clinical and experimental observations. The stronger neuronal degenerations are always accompanied by the longer polyglutamine stretches and by the higher concentration of the expanded polyglutamines. In other neurodegenerative disorders, such as Alzheimer disease, prion disease, Parkinson disease and amyotrophic lateral sclerosis, precipitation of abnormal proteins is also now considered to play a key role. These observations might lead to the elucidation of universal mechanisms for neurodegeneration and to treatments effective for many neurodegenerative disorders.