APOE and major depressive disorder: Depression, rapid eye movement sleep behaviour disorder (Aarsland et al., 2003; Zhu et al., 2014), gait dysfunction, cerebrovascular diseases associated with white matter lesions (Ma et al., 2015), olfactory dysfunction, APOE genotype, and CSF amyloid-β:tau ratio (Schrag et al., 2017) have been suggested as predictors for the development of cognitive impairment in Parkinson’s disease (Aarsland et al., 2003); however, several of these have been disputed (Zhu et al., 2014).