Plasma levels of apolipoprotein E, APOE genotype and risk of dementia and ischemic heart disease: A review.

Dementia is one of the major causes of disability in later life, and ischemic heart disease is a leading cause of morbidity and mortality. Apolipoprotein E (apoE) plays a pivotal role in lipoprotein metabolism in the brain and in the periphery, and is implicated in both dementia and ischemic heart disease. Peripherally, liver-derived apoE is the main source of plasma apoE. Approximately half of plasma apoE is associated with triglyceride-rich lipoproteins, where apoE serves as the main ligand for the low density lipoprotein (LDL) receptor and the LDL receptor Related Protein (LRP). In the brain, apoE is produced mainly by astrocytes. Astrocyte-derived apoE is pivotal for cerebral cholesterol metabolism and clearance of β-amyloid, a major pathological hallmark of Alzheimer disease. Plasma levels of apoE and other lipids and lipoproteins are under strong genetic influence by the APOE polymorphism, and the ε4 allele is a strong genetic risk factor for Alzheimer disease. The characteristics of the APOE polymorphism thus suggest the qualitative importance of apoE, whereas studies of familial absolute apoE deficiency suggest a quantitative importance of plasma apoE levels in lipid metabolism. Whether plasma levels of apoE are associated with increased risk of dementia and ischemic heart disease, and whether these associations are independent of the APOE polymorphism and of lipids and lipoproteins has only recently been established. The aim of this review is to summarize and discuss the current epidemiological and biological evidence for an association of plasma levels of apoE with risk of dementia and ischemic heart disease.