APOE and Cognitive impairment: For example, some studies of Alzheimer’s disease showed a greater rate of hippocampal atrophy and cortical thinning in the presence of APOE ε4 allele in both individuals progressing from mild cognitive impairment to AD and those with demented AD (20, 21), highlighting the role of APOE ε4 in brain atrophy regardless of the stages of AD.