The goal of this observational study is to learn about the efficacy of Lecanemab treatment for early-onset familial Alzheimer's disease (AD) in patients under 65 years of age with a family history of AD. Participants will receive Lecanemab at a dosage of 10 mg/kg every two weeks for a total of 18 months and will undergo cognitive assessments, PET and MRI scans, blood/fluid tests and whole genome sequencing. The study will explore the effects of genetic and hereditary factors on the efficacy of Lecanemab treatment in early-onset familial AD patients.