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INTRODUCTION: Alzheimer's disease (AD) and vascular dementia (VaD) account for most dementia cases. AD biomarkers remain costly and invasive, and no specific biomarkers exist for VaD.METHODS: We analyzed plasma and brain proteomics in the UK Biobank (N=53,000) and ROSMAP (N=512) to identify shared and distinct proteomic signatures of AD and VaD and assess the influence of the APOE ε4 variant.RESULTS: We identified 55 AD-associated and 49 VaD-associated proteins, with 13 shared. AD proteins were enriched in glycosaminoglycan binding and cholesterol metabolism; VaD proteins in virus receptor activity, cytokine activity and metalloproteinases. Both showed IGF pathway dysregulation. APOE ε4 stratification revealed distinct AD proteomic signatures beyond GFAP and NeFL. Mendelian randomization suggested causal links for SNAP25 in AD, EDA2R and TIMP4 in VaD, and PVR in both.DISCUSSION: Findings underscore the importance of APOE genotype and highlight SNAP25, EDA2R, TIMP4, and PVR as potential biomarkers and therapeutic targets.