What can we learn from study of Alzheimer's disease in patients with Down syndrome for early-onset Alzheimer's disease in the general population?

The clinical and scientific study of dementia in adults with Down syndrome led to the development of the amyloid hypothesis as a fundamental concept in Alzheimer's disease pathogenesis. The journey started with the discovery of the structure and metabolic processing of β-amyloid brain deposits associated with Alzheimer's dementia in adults with Down syndrome, and then the prediction and confirmation of the amyloid precursor protein gene on chromosome 21. The processes and genes responsible for tau hyperphosphorylation contributing to toxic brain deposits were additionally identified. With increasing sophistication in genetic experimental techniques, additional mechanisms associated with excessive amyloid deposits were postulated and tested in brains of people with Down syndrome and Alzheimer's disease and in those with early-onset Alzheimer's disease. This in turn led to the proposal and testing for particular genetic defects associated with familial early-onset Alzheimer's disease. Nearly 200 genetic causes of early-onset types of Alzheimer's disease have since been identified. Only a minority of these causes are on chromosome 21, although the aetiology of excess amyloid production remains fundamental to their pathogenesis. Knowledge of the pathogenic mechanisms of Alzheimer's disease in predisposed families and in people with Down syndrome is a step closer to prevention or cure of this devastating disease.