APOE and early-onset autosomal dominant Alzheimer disease: The APOE ε4 allele variant is the best-known genetic risk factor associated with Alzheimer disease (Farrer et al., 1997, Poirier et al., 1993, Strittmatter et al., 1993), and infants carrying this variant show delayed myelination and gray matter development in brain regions typically affected by Alzheimer disease (Dean et al., 2014).