A combination of genetic (hereditary factors) and environmental factors may contribute to Alzheimer’s disease. Not everyone who develops Alzheimer’s disease has a family history. However, individuals with a parent or sibling affected by Alzheimer’s are more likely to develop the disease. The risk further increases for those with multiple first-degree relatives with Alzheimer’s.

At this time, three Alzheimer’s mutations have been identified, including the amyloid precursor protein (APP) discovered in 1987 and Presenilin-1 (PS-1) identified in 1992, which is the most common cause of inherited Alzheimer’s. Presenilin-2 (PS-2), was identified in 1993, and is the third gene with mutations found to cause inherited Alzheimer’s. Those are named “autosomal dominant” mutations which means that if a person has it, 100% chance that he/she will develop the disease and 50% chance that any of his/her offspring will develop it. These mutations are very rare, occurring in 1-2% of all Alzheimer’s patients. Those carrying the mutations have early Alzheimer’s, typically before the age of 60.

In addition, the apolipoprotein E gene was identified in 1993 as the first risk gene for Alzheimer’s disease. Currently, it is the gene with the greatest known impact on the Alzheimer’s risk. The gene has three alleles (three forms): the 2, 3 and 4. Each person receives one of these forms from their mother and another from their father. A person that carries an APOE 4 allele is at higher risk of developing Alzheimer’s disease. In fact, 40-65% of Alzheimer’s patients carry an APOE4 allele. Having two copies may lead to earlier symptom onset. In contrast to the three mutations discussed above, the APOE gene does not cause the disease but increases the risk of developing it. That means that it is possible for a person who does not carry an APOE4 allele to develop Alzheimer’s and for a person carrying an APOE4 allele not to develop the disease.