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One of the major susceptibility factors to the clinical expression of either early-onset or late-onset Alzheimer disease (AD) is the presence of the ε4 allelic variant of apolipoprotein E (APOE). Two other forms of APOE exist, APOE ε2 and APOE ε3, because of single base changes in the APOE gene. Individuals with the ε4 allele are more likely to present with senile plaques, a pathological hallmark of AD, and the primary proteinaceous component of these plaques is the amyloid-β protein (Aβ). Moreover, the APOE ε4 allele has been shown to bind to Aβ in cerebrospinal fluid and promote toxic, soluble Aβ oligomer formation.1 On the other hand, APOE ε4 also has a key role in Aβ-independent mechanisms that are involved in AD pathogenesis, and APOE ε4 mouse models display impairments in long-term potentiation2 and have synaptic deficits.3
Mc Donald J, Krainc D. Alzheimer Gene APOE ε4 Linked to Brain Development in Infants. JAMA. 2014;311(3):298-299. doi:10.1001/jama.2013.285400