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Alzheimer disease(AD),the most common cause of dementia in the elderly, exists in both familial and sporadic forms.Genetic studies have led to the identification of 3 genes,B-amyloid precursor protein(APP),presenilin-1 (PS1),and presenilin-2(PS2),which,when mutated,can cause familial forms of AD.Mutations in each of these genes result in elevated levels of AB42/43,a proteolytic processing fragment of APP that is deposited in brains of AD patients.Because 50%of AD cases have no APOE e4 alleles and families showing mendelian inheritance of AD exist in whom there are no mutations in any of the APP,PS1,or PS2 genes,it is likely that there are additional AD risk factors,both genetic and environmental,still to be identified. |
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