Apolipoprotein E gene allele 4 (APOE4) is a major genetic risk factor for late-onset Alzheimer’s disease (AD). Individuals carrying one copy of the APOE4 allele are known to be at 3-4-fold increased risk of developing AD compared with those carrying the more common APOE3 allele. The risk of developing AD is increased by 8-15-fold if an individual is carrying two copies of the APOE4 gene. The proportion of APOE genotypes and alleles differ between populations of different ethnic groups and gender . A meta-analysis of clinical and autopsy-based studies on five ethnic groups (Caucasian, African American, Hispanic, and Japanese) revealed that the risk of developing AD was increased among Caucasian subjects with one APOE4 copy compared to individuals homozygous for APOE3 . With regards to the gender, evidence indicate that the APOE4 risk for AD is greater in women, especially in heterozygous individuals carrying one APOE4 allele .
What is the role of APOE in brain? Like other apolipoproteins, APOE is involved in lipid binding, and it is the predominant cholesterol transporter in the brain. While it is mainly generated by astrocytes, during stress, it has also shown to secrete by other cell types such as microglia and neurons. This multicellular expression and transport between cells often complicate our understanding of APOE4-mediated pathogenesis. To investigate this complexity, it is important to investigate cell-type-specific molecular processes influenced by APOE4.
APOE works to reduce lipid and cholesterol levels by binding to the low-density lipoprotein receptor allowing for cellular lipid uptake. The APOE isoforms impact how lipoproteins are cleared and the extent in which it is accomplished. APOE3 is considered as the control phenotype when comparing the function of APOE2 and APOE4 alleles. APOE2 has a decreased affinity to the receptor and decreases the risk of late-onset AD compared with APOE3 through both amyloid-β (Aβ)-dependent and independent mechanisms . However, APOE2 carriers could develop type III hyperlipoproteinemia and exhibit increased risk of cerebrovascular diseases and neurological disorders.
APOE4 has an increased lipid binding ability and shows increased cellular accumulation of cholesterol and cholesterol esters. Recent work by Blanchard et al. showed that aberrant cholesterol deposition in APOE4 oligodendrocytes affects cellular functions including myelination . This observation agrees with reduced myelination found in APOE4 brains. By facilitating cholesterol transport pharmacologically, Blanchard et al. observed improved axonal myelination and learning and memory in APOE4 mice. This work highlighted possible therapeutic opportunities for AD by modulating cholesterol transport.
Are there any non-pharmacological interventions? Around 40% of dementia is suggested to attribute to a combination of twelve risk factors: education to a maximum of age 11–12 years, midlife hypertension, midlife obesity, hearing loss, late life depression, diabetes, physical inactivity, smoking, social isolation, excessive alcohol consumption, head injury, and air pollution . These highlight the importance of managing lifestyle factors in preventing or delaying dementia. Fernandez-Matarrubia showed an active lifestyle favorably associated with improving cognitive function among APOE4 non-carriers but not with APOE4 carriers . The authors suggest low number of APOE4 carriers is one of the limitations for this study, which may be overcome with a bigger sample size. There is still a lot of work to be done to better understand how environmental factors interact with genetics to help individuals prevent or delay cognitive decline at an early stage.
APOE also known to interact with Aβ, tau tangles, and pathogenic molecules such as lipopolysaccharides. Hence, APOE is believed to facilitate the clearance of inflammatory and pathogenic molecules suggesting its role in neuroinflammation. Prolonged neuroinflammation could induce neuronal damage and cell death by activating reactive oxygen species that can directly damage neurons or by sustaining chronic stress to induce a battery of inflammatory pathways that can damage or kill neurons. Carotenoids, antioxidant micronutrients that could accumulate in APOE containing lipid particles, have been suggested as another approach to reduce neuroinflammation . Future research would benefit from understanding the relationship between APOE genotype and how we could use micronutrients to accumulate in brain to help relive oxidative stress. Such research could help clinicians to decide on precision medicine.
However, this opens new sets of questions. Should we test for the APOE4 gene? What information is out there for the public if they find out they carry one or two alleles of APOE4 genotype? What should they do?