Since there are existing studies that do observe a relationship between APOE4 and GMV alterations in regions relevant for cognitive function, it may be worthwhile to conduct more research in PD to further evaluate APOE4’s association with GMV (Spampinato et al., 2011; Haller et al., 2017; Cacciaglia et al., 2018). Here, APOE is linked to Parkinson disease.