Putative Survival Advantages in Young Apolipoprotein ϵ 4 Carriers are Associated with Increased Neural Stress

Abstract

Inheritance of a single copy of the apolipoprotein E (APOE) ϵ4 allele increases risk of Alzheimer's disease (AD) by 3-4-fold, with homozygosity associated with a 12-16-fold increase in risk, relative to ϵ3 allele homozygosity. There is a decreased risk associated with the APOE ϵ2 allele. The pathological consequence of APOE genotype has led to intense efforts to understand the mechanistic basis of the interplay between APOE status and loss of synapses. Numerous ϵ4 allele-related associations have been reported with the potential relevance of these associations to the pathogenesis of AD unknown at this time. In primarily young subjects, we have reviewed a representative body of literature on ϵ4 allele-associations related to the following: cardiovascular responses; impacts on reproduction and fetal development; co-morbidities; resistance to infectious disease; responses to head injury; biochemical differences possibly related to neural stress; and brain structure-function differences. In addition, the literature on the association between the ϵ4 allele and cognitive performance has been reviewed comprehensively. The weight-of-the-evidence supports the hypothesis that possession of the ancestral ϵ4 allele in youth is associated with improved fitness during fetal development, infancy, and youth relative to the more recently appearing ϵ3 allele, at the expense of decreased fitness in old age, which is substantially improved by the ϵ3 allele. However, possession of the ϵ4 allele is also associated with higher levels of synaptic macromolecular turnover, which likely stresses basic cellular neuroplasticity mechanisms. Clinical trials of potential AD therapeutics should consider APOE status as an enrollment criterion.