APOE and Alzheimer disease: The association between APOE ε4 and cognitive decline has been demonstrated in AD, MCI, and normal aging in many studies (Whitehair et al., 2010; Wisdom et al., 2011; Prada et al., 2014; Suarez et al., 2014), and the protective outcome of APOE ε2 on cognitive decline has been observed in AD patients (Corder et al., 1994; Suri et al., 2013).