APOE4 Gene

The APOE4 gene is a variant of the apolipoprotein E (APOE) gene, which plays a critical role in lipid transport, cholesterol metabolism, and the maintenance of neuronal integrity in the human body. The APOE gene is located on chromosome 19 (19q13.2) and exists in three common allelic forms: APOE ε2, APOE ε3, and APOE ε4. Among these, the APOE ε4 allele is strongly associated with an increased risk of developing late-onset Alzheimer’s disease (AD), making it one of the most significant genetic risk factors for this condition.

Structure and Function of APOE

The APOE gene encodes apolipoprotein E, a 299-amino-acid protein primarily synthesised in the liver, brain, and macrophages. Its main physiological function is to facilitate the transport and redistribution of lipids, such as cholesterol and phospholipids, among cells through its interaction with lipoprotein receptors.
In the central nervous system (CNS), APOE is mainly produced by astrocytes and, to a lesser extent, by microglia. It transports cholesterol to neurons via receptor-mediated uptake, thereby supporting membrane repair, synaptogenesis, and neural plasticity. These processes are vital for normal cognitive functioning and neuronal health.
The three major APOE isoforms—APOE2, APOE3, and APOE4—differ by only two amino acid substitutions at positions 112 and 158, yet these minor changes significantly alter the protein’s structure and biological properties:

• APOE2: Cysteine at positions 112 and 158.
• APOE3: Cysteine at position 112 and arginine at 158.
• APOE4: Arginine at both positions 112 and 158.

This double arginine configuration in APOE4 alters its folding and stability, leading to distinct biochemical behaviour compared with other isoforms.

Role of APOE4 in Alzheimer’s Disease

The APOE4 variant contributes to Alzheimer’s disease through multiple interconnected mechanisms. Its effects are both biochemical and cellular, influencing amyloid beta metabolism, tau pathology, and neuroinflammation.

1. Amyloid Beta AccumulationOne of the most widely studied functions of APOE is its interaction with amyloid beta (Aβ), the peptide responsible for forming plaques in Alzheimer’s disease. APOE binds to amyloid beta and facilitates its clearance from the brain. However, APOE4 is less efficient at this process than APOE3 or APOE2, leading to reduced amyloid clearance and increased deposition of Aβ plaques. Additionally, APOE4 can promote amyloid aggregation, exacerbating plaque formation and accelerating neurodegeneration.
2. Tau PathologyIn addition to amyloid-related effects, APOE4 influences the abnormal hyperphosphorylation and aggregation of tau proteins, which form neurofibrillary tangles inside neurons. Evidence suggests that APOE4 enhances tau-mediated neurotoxicity, contributing to neuronal dysfunction and cell death.
3. Lipid DysregulationAPOE4 is less effective in lipid transport and neuronal membrane repair than other variants. This impairment compromises neuronal integrity and synaptic resilience. Neurons rely on cholesterol for membrane fluidity and synaptic vesicle formation; thus, APOE4 carriers may experience compromised synaptic function even before the onset of visible pathology.
4. NeuroinflammationAPOE4 is associated with an exaggerated inflammatory response in the brain. Microglia and astrocytes in APOE4 carriers exhibit higher reactivity and release pro-inflammatory cytokines, contributing to chronic neuroinflammation that damages neuronal structures.
5. Mitochondrial and Metabolic DysfunctionAPOE4 affects mitochondrial function by promoting oxidative stress and impairing energy metabolism. This dysfunction reduces neuronal energy availability, further aggravating neurodegenerative processes.

Genetic Risk and Inheritance

Humans inherit one APOE allele from each parent, resulting in six possible genotype combinations: ε2/ε2, ε2/ε3, ε2/ε4, ε3/ε3, ε3/ε4, and ε4/ε4. The distribution and risk implications are as follows:

• ε3/ε3 is the most common genotype (about 60% of the population) and is considered neutral in terms of Alzheimer’s risk.
• ε4/ε3 carriers have approximately a threefold to fourfold increased risk of developing late-onset Alzheimer’s disease.
• ε4/ε4 homozygotes have a tenfold to fifteenfold higher risk compared to non-carriers.
• ε2 appears to be protective, with ε2 carriers showing a reduced risk and delayed onset of the disease.

It is important to note that possessing one or two APOE4 alleles does not guarantee the development of Alzheimer’s disease; rather, it increases susceptibility, often interacting with lifestyle, environmental, and other genetic factors.

Physiological and Neurological Effects Beyond Alzheimer’s Disease

The impact of APOE4 extends beyond Alzheimer’s pathology. Carriers of this allele have been found to exhibit differences in brain structure and function even in early adulthood, long before clinical symptoms appear. Studies have reported altered connectivity in memory-related brain regions, such as the hippocampus and default mode network, in young APOE4 carriers.
Furthermore, APOE4 has been associated with a higher risk of several neurological and cardiovascular conditions, including:

• Cerebral amyloid angiopathy, involving amyloid deposits in cerebral blood vessels.
• Traumatic brain injury (TBI) outcomes, where APOE4 carriers may experience more severe cognitive decline post-injury.
• Atherosclerosis, due to altered cholesterol metabolism and lipid accumulation in arterial walls.

Despite its negative connotations in modern contexts, some evolutionary theories suggest that APOE4 may have conferred advantages in ancestral environments. For instance, its association with a heightened immune response and efficient fat metabolism may have benefited populations facing infectious diseases or nutrient-scarce conditions.

Molecular Mechanisms

The distinct structural configuration of APOE4 affects its interactions with lipids and receptors. The arginine at position 112 causes domain-domain interactions within the protein, creating instability and increasing its tendency to adopt an altered conformation. This results in several molecular consequences:

• Reduced binding affinity for low-density lipoprotein (LDL) receptors, impairing lipid recycling in the brain.
• Altered lipidation state, where APOE4 is poorly lipidated compared to APOE3, affecting its ability to transport cholesterol.
• Increased proteolytic cleavage, leading to toxic APOE4 fragments that accumulate in neurons and contribute to mitochondrial damage.

These molecular dysfunctions collectively lead to impaired neuronal maintenance and heightened vulnerability to degenerative processes.

Detection and Genetic Testing

Genetic testing for APOE genotype is available through clinical and research settings. Such tests determine whether an individual carries the ε4 allele, providing insights into Alzheimer’s risk. However, because APOE4 status is not deterministic, professional counselling is recommended before and after testing to contextualise results. The presence of APOE4 should not be viewed as a diagnosis but as an indicator of susceptibility.
Biochemical and imaging studies can complement genetic testing. For example, APOE4 carriers often show earlier and more extensive amyloid accumulation in positron emission tomography (PET) scans, and reduced levels of soluble Aβ42 in cerebrospinal fluid analysis.

Therapeutic Implications

Understanding the role of APOE4 in Alzheimer’s disease has opened new avenues for targeted therapies. Current strategies include:

• Modulating APOE expression: Reducing the harmful effects of APOE4 by converting it into an APOE3-like form through gene editing or small molecules.
• Enhancing lipidation: Increasing APOE4 lipidation using agonists of ABCA1, a transporter that facilitates lipid loading, thereby improving amyloid clearance.
• Anti-inflammatory therapies: Targeting inflammatory pathways hyperactivated by APOE4.
• Mitochondrial stabilisers: Mitigating APOE4-induced oxidative damage through antioxidant or metabolic support therapies.

Lifestyle interventions such as regular physical exercise, balanced diet, and cognitive engagement may also reduce the expression of APOE4-related risk by supporting brain resilience and vascular health.

Significance

The APOE4 gene represents a cornerstone in the study of neurodegeneration and cognitive ageing. Its influence extends across molecular, cellular, and systemic levels, affecting not only amyloid beta metabolism but also lipid balance, inflammation, and energy regulation in the brain. While it significantly elevates Alzheimer’s risk, it does not predetermine disease onset, offering opportunities for preventive strategies and personalised interventions.