The genetics of Alzheimer's disease.

Alzheimer's disease (AD) is the major cause of dementia in the U.K. The clinical diagnosis of the specific disease resulting in dementia is unreliable and thus a definitive diagnosis of AD is best made in conjunction with post-mortem findings of amyloid plaques and neurofibrillary tangles. Alzheimer's disease is neuropathologically indistinguishable in the young and old, but has been divided arbitrarily into early- and late-onset disease using age cut-offs of 60 or 65 years. Twin and family studies suggest that genetic factors play a major role in its aetiology. This review considers the three loci which have been shown to be associated with early-onset AD: amyloid precursor protein, presenilin (PS)-1 and PS-2. Mutations in these genes seem to be associated with overproduction of the 42-amino acid form of beta-amyloid, suggesting that this may be a central pathological process in AD. The impact of the different apo E alleles on the risks for late- and early-onset AD is discussed and compared with other dementing conditions. Recent analyses suggest that there are likely to be other genes besides apo E which impact on late-onset AD risk. The possible roles in AD of the mitochondrial mutation at position 4336, the PS intron 8 polymorphism, and variants in the alpha 1-antichymotrypsin and VLDL-receptor genes, are considered.