APOE and Alzheimer disease: Owing to the protein product of the MPO gene being involved in AD pathology, possibly through oxidation of Aβ or ApoE, promoting their aggregation into insoluble complexes, or directly through oxidation-induced damage to associated neurons [9], there is biologic evidence implicating MPO in the cognitive decline in patients with MS, but no definite data are currently available on the possible role of MPO polymorphism in the development of cognitive decline in MS.