PSEN2 and Alzheimer disease: In fact, most of the cases of early-onset AD are due to genetic causes (mutations in the amyloid precursor protein, APP, and the presenilins 1 and 2, PSEN1 and PSEN2 genes) (Cacace et al., 2016), while a minority has been categorized as non-Mendelian (Reitz et al., 2020).