The causes of Alzheimer's disease are not fully understood, and many environmental conditions and genetic variations are likely to contribute:
- Health issues (e.g. diabetes, cardiovascular disease, high blood pressure and high cholesterol)
- Lifestyle choices (smoking and excess alcohol)
- Cognitive and physical inactivity
- Head trauma
- e4 form of the APOE gene
The APOE gene encodes Apolipoprotein E, a protein involved in injury repair and transporting fats between brain cells. The APOE gene is located on chromosome 19. There are three forms or alleles of APOE – e2, e3 and e4. One allele is inherited from each parent, resulting in six possible combinations: e2/e2, e3/e3, e4/e4, e2/e3, e2/e4 or e3/e4.
The presence of the e4 allele is linked to an increased risk of Alzheimer's disease, whereas the e2 allele has a protective affect. Individuals that have one copy of the e4 allele have a 3-fold increased risk of developing Alzheimer's, while e4 homozygotes (two copies of e4) have up to a 15-fold increased risk.