Neuropathological changes consistent with a diagnosis of Alzheimer's disease (AD) have been found in virtually all individuals with Down's syndrome (DS) over the age of 40, including deposition of p-amyloid (Ap) in diffuse and neuritic plaques (Mann, 1988; Wisniewski et al., 1994). Most adults with DS will develop dementia by the time they are 70 years of age, and it has been suggested that the identification of processes which contribute to a high risk of AD can serve as a model for the role of genetics in the etiology of AD (Lai and Williams, 1989). The neuropathological manifestations of AD in DS have been attributed to triplication and overexpression of the gene for Ap precursor protein (pAPP), located on chromosome 21, and the increased risk of AD may be mediated by an increased substrate for cellular production of Ap peptides.
Before age 50, diffuse plaques are the most prevalent lesion seen in DS, while after age 50, neuritic plaques, containing fibrilized Ap peptides, predominate (Wisniewski et al., 1994). Examination of the age-specific prevalence of dementia in DS shows that risk of AD begins to increase after 50 years of age (range 38-70 years of age), supporting the hypothesis that dementia in DS is initiated by the transition from diffuse to fibrilized plaques (Lai and Williams, 1989; Visser et al., 1997; Holland et al., 1998; Lai et al., 1999) (Figure 3.1). The wide distribution of age at onset of dementia cannot be accounted for solely on the basis of the triplication of pAPP, since 95% of persons with DS have triplication of the entire chromosome. Rather, other factors must account for individual differences in susceptibility to the development of fibrilized plaques and for the range in age at onset. This paper will review factors that may influence onset of AD by accelerating formation of Ap, including (1) atypical karyotypes, (2) polymorphisms in apolipoprotein E (apoE), (3) estrogen deficiency, and (4) individual differences in Ap peptide levels.
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