Alternative Selection of BACE1 Cleavage Sites in APP Harboring Protective And Pathogenic Mutations Within the Aβ Sequence — Alternative Selection of β-Site APP-Cleaving Enzyme 1 (BACE1) Cleavage Sites in Amyloid β-Protein Precursor (APP) Harboring Protective and Pathogenic Mutations within the Aβ Sequence — Mechanism and Role of Alternative APP Cleavage by BACE1