Apolipoprotein E: A type of lipoprotein (a protein connected to a fat). Apolipoprotein E is abbreviated ApoE and the gene that encodes it is known as APOE. APOE is located on chromosome 19 in band 19q13.2.
Lipoproteins are responsible for carrying cholesterol and other fats through the bloodstream as little packages and are essential for the normal breakdown of these molecules. In particular, apolipoprotein E is a major component of specific lipoproteins called very low-density lipoproteins (VLDL). A major function of VLDLs is to remove excess cholesterol from the blood and carry it to the liver for processing. Maintaining normal levels of cholesterol is essential for the prevention of cardiovascular diseases, including heart attacks and strokes.
There are three isoforms (slightly different forms) of the ApoE lipoprotein. These three isoforms are known as ApoE2, E3 and E4. They are encoded by slightly different versions, or alleles, of the APOE gene. These three alleles are called APOE e2, e3, and e4 (e stands for epsilon). The most common allele is APOE e3, which is present in more than half of the population.
People with one copy of the e4 allele have an increased risk of developing type 2 Alzheimer disease, a familial late-onset form of the disease. People who inherit two copies of the e4 allele have a still higher chance of developing type 2 Alzheimer disease. However, the relationship between APOE e4 and Alzheimer disease is not a simple direct one. APOE e4 is clearly neither necessary nor sufficient by itself to cause Alzheimer disease. It may modify the preclinical progression of the disease and accelerate the clinical onset of it in people who are already predisposed to develop Alzheimer disease.
Apolipoprotein E is also associated with several cardiovascular disorders. Most people with familial hypercholesterolemia, a condition that causes very high levels of cholesterol and an increased risk of heart attacks and strokes, have two copies of the e2 allele. This allele seems to be one of several genetic factors that play a part in this disorder. Another version of apolipoprotein E, the e4 allele, is a risk factor for coronary artery disease.