Longitudinal Early-onset Alzheimer's Disease Study Protocol

The LEADS study is a non-randomized, natural history, non-treatment study. Enrolled participants must be 40 - 64 (inclusive) years of age, with MCI due to AD or probable AD dementia (cognitively impaired participants) or have no significant memory impairment (cognitively normal \[CN\] participants). Approximately 850 participants with cognitive impairment (650 with early onset Alzheimer's Disease \[EOAD\] and 200 with early onset non-Alzheimer's Disease \[EOnonAD\]) and 100 CN participants will be enrolled at approximately 20 sites in the United States. At approximately 5 sites outside of the United States, approximately 400 cognitively impaired participants and 10 CN participants will be enrolled. Cognitively impaired participants will take part in the study for 48+ months; CN participants will take part in the study for 24+ months. Participants will undergo longitudinal clinical and cognitive assessments, computerized cognitive tests, biomarker and genetic tests, PET (FDG, amyloid and tau) and MRI brain scans, and optional cerebrospinal fluid (CSF) collection. Participants will be invited to consider autopsy brain donation The primary objectives of the LEADS study are to: * collect longitudinal assessments and biomarker data in individuals with early onset cognitive impairment (EOAD / EOnonAD) and cognitively normal (CN) controls; * to compare baseline and longitudinal cognitive and functional characteristics, between EOAD and CN, and EOAD and Late Onset Alzheimer's Disease (LOAD) from the Alzheimer's Disease Neuroimaging Initiative (ADNI); and * to study the associations of longitudinal clinical and cognitive assessments with multimodal imaging and biofluid markers that capture different elements of the AD pathophysiological cascade