Most people with Alzheimer’s have the late-onset form of the disease, in which symptoms become apparent in their mid-60s. The apolipoprotein E (APOE) gene is involved in late-onset Alzheimer’s. This gene has several forms. One of them, APOE ε4, increases a person’s risk of developing the disease and is also associated with an earlier age of disease onset. However, carrying the APOE ε4 form of the gene does not mean that a person will definitely develop Alzheimer’s disease, and some people with no APOE ε4 may also develop the disease.
Also, scientists have identified a number of regions of interest in the genome (an organism’s complete set of DNA) that may increase a person’s risk for late-onset Alzheimer’s to varying degrees.
Early-onset Alzheimer’s occurs between a person’s 30s to mid-60s and represents less than 10 percent of all people with Alzheimer’s. Some cases are caused by an inherited change in one of three genes, resulting in a type known as early-onset familial Alzheimer’s disease, or FAD. For other cases of early-onset Alzheimer’s, research suggests there may be a genetic component related to factors other than these three genes.