APP and Alzheimer disease: Indeed, AD and PD can be seen as “gene-dosage effect” disorders: AD could be caused by gene duplication of Aβ precursor protein (APP; Podlisny et al., 1987; Rovelet-Lecrux et al., 2006), likewise α-synuclein locus duplication or triplication causes PD (Singleton et al., 2003; Chartier-Harlin et al., 2004).