“Down syndrome, which arises in individuals carrying an extra copy of chromosome 21, is associated with a greatly increased risk of early-onset Alzheimer disease. It is thought that this risk is conferred by the presence of three copies of the gene encoding amyloid precursor protein (APP) – an Alzheimer disease risk factor – although the possession of extra copies of other chromosome 21 genes may also play a part.” This paper discusses the development of Alzheimer disease in Down syndrome, the mechanisms of which could provide insights into the causes of dementia in the general population.
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Wiseman FK, Al-Janabi T, Hardy J, Karmiloff-Smith A, Nizetic D, Tybulewicz VL, Fisher EM, Strydom A: A genetic cause of Alzheimer disease: mechanistic insights from Down syndrome. Nature Review Neuroscience [Epub ahead of print; August 5, 2015; doi: 10.1038/nrn3983].
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http://www.ncbi.nlm.nih.gov/pubmed/26243569