Alzheimer’s disease (AD) is characterized by the accumulation of amyloid-β (Aβ) and neurofibrillary tangles composed of the tau protein in the brain. More than 200 mutations have been identified in amyloid-β precursor protein (APP), presenilin 1 (PSEN1) and presenilin 2 (PSEN2) that cause autosomal dominant forms of AD (ADAD). PSEN1 and PSEN2 form the catalytic domain of the γ-secretase enzyme, which cleaves APP to generate many Aβ proteoforms which can be modified into variants including posttranslational modifications, truncations and sequence variations. Changes in the relative ratios of Aβ42/40 isoforms have been used to predict pathogenicity of ADAD variants. However, we know less about the contribution of other Aβ proteoforms to AD pathogenesis and the utility of the Aβ proteoform signature as a biomarker of mutation status and/or disease course. For example, what are the Aβ pathogenic cause(s) of ADAD? Several Aβ proteoforms support a causal role for AD, including Aβ42, Aβ43, Aβ37, Aβ39 and modifications including pyroglutamate, oxidation, isomerization, and N- and C-terminal truncation. The objective of this study is to define the effects of ADAD mutations and amyloidosis on Aβ proteoform and disease pathogenesis. To meet this objective, we will define mutation and gene-specific effects on Aβ proteoform signatures using novel mass spectrometry approaches in human plasma, CSF, stem cell derived neurons, and brain tissue. We will then determine how Aβ proteoform signatures relate to histologic amyloid plaque structure in human brains. We hypothesize that ADAD mutations produce a common pathogenic Aβ proteoform signature. The rationale for this proposal is that defining the effects of ADAD mutations and amyloidosis on Aβ proteoforms will be critical to define the common pathogenic Aβ signatures which cause AD. This target validation will guide clinical studies, therapeutic strategies and classify future novel ADAD mutations. This work will be performed in collaboration with the Genetics, Biomarker, Clinical, Neuropathology, Imaging, and Biostatistics Cores.