PSEN2 and early-onset autosomal dominant Alzheimer disease: In familial forms of Alzheimer’s disease, mutations in Amyloid precursor protein (APP), Persenilin-1 (PSEN1), and Presenilin-2 (PSEN2) genes lead to excessive formation of neurotoxic peptide amyloid- β (Aβ) plaques which aggregate extracellularly.