APOE and obesity due to melanocortin 4 receptor deficiency: Of the three isoforms of ApoE, ApoE4 is a prevalent risk factor that is synergistic with obesity and age for AD (Butler, 1994; Riedel et al., 2016; Jones and Rebeck, 2018; O’Donoghue et al., 2018; Glorioso et al., 2019).