While the role of APOE ε4 has been well established in AD, previous studies have been conflicting regarding the potential role of APOE ε4 in PD susceptibility risk and onset age (Li et al., 2004; Federoff et al., 2012), disease severity and progression (Jo et al., 2021), as well as the extent of cognitive impairment and rate of cognitive decline (Aarsland et al., 2017; Fagan and Pihlstrøm, 2017) especially in the context of earlier development of PD Dementia (Pang et al., 2018). Here, APOE is linked to Alzheimer disease.