One out of eight people age 65 and older (13%) has Alzheimer’s disease. Women, who on average live longer than men, are more likely than men to have Alzheimer’s disease.
The greatest risk factor for Alzheimer’s disease is advancing age, but Alzheimer’s is not a normal part of aging.
A small percentage of Alzheimer’s disease cases, probably less than 1 percent, are caused by rare genetic variations found in a small number of families worldwide. These variations involve chromosome 21 on the gene for the amyloid precursor protein, chromosome 14 on the gene for the presenilin 1 protein and chromosome 1 on the gene for presenilin 2. In these inherited forms of Alzheimer’s, the disease tends to develop before age 65, sometimes in individuals as young as 30.
A genetic factor in late-onset Alzheimer’s disease (Alzheimer’s disease developing at age 65 or older) is apolipoprotein E-e4 (ApoE-e4). ApoE-e4 is one of three common forms of the ApoE gene, which provides the blueprint for a protein that carries cholesterol in the bloodstream. Everyone inherits one form of the ApoE gene from each of his or her parents. Those who inherit one ApoE-e4 gene have increased risk of developing Alzheimer’s disease. Those who inherit two ApoE-e4 genes have an even higher risk. However, inheriting one or two copies of the gene does not guarantee that the individual will develop Alzheimer’s.