APOE and depressive disorder: For example, the epsilon 4 allele of the apolipoprotein E epsilon 4 gene (APOE-e4) is a known genetic risk factor for Alzheimer’s disease (AD) and has been linked to depression, with studies suggesting an additive risk for cognitive impairment and a synergistic effect in increasing the incidence of dementia and mild cognitive impairment [32,33,34,35,36,37,38].