APOE and early-onset autosomal dominant Alzheimer disease: Although there are limited studies that provide a direct link between the APOE ε2/ε4 genotype or APOE ε4 allele and exacerbated gray matter atrophy or loss of dopaminergic neurons in PD, there is already enough evidence from other degenerative disorders, such as Alzheimer’s disease, to underscore this relationship.