APOE and Alzheimer disease: And though other reports show that MMP-9 may be elevated at the blood brain barrier with APOE4 or aggressive mouse models of AD (Halliday et al., 2016; Weekman and Wilcock, 2016; Montagne et al., 2020), potential confounds include elevated amyloid levels in aggressive murine models and/or select APOE4 patient populations (Deb and Gottschall, 1996).