Project Summary/Abstract Alzheimer’s disease (AD) is the most common neurodegenerative disease that manifests a progressive decline in cognitive function. Advanced age is the greatest known risk factor for Alzheimer’s disease. The vast majority of AD cases (~95%) belong to the sporadic and late-onset subtype (sAD with onset >65 years). There is a significant knowledge gap between our current understanding of molecular and cellular pathways and how these contribute to the pathophysiology of Alzheimer’s disease, which is likely caused by a complex combination of multiple genetic, environmental, and behavioral factors. The e4 allele of the Apolipoprotein Epsilon (APOE) gene has been identified as the strongest genetic risk factor of sAD. p53, well-characterized as a tumor suppressor that responds to cellular stresses, also plays a critical role in aging and likely Alzheimer’s disease. Here, we will test the central hypothesis that the APOE e4 protein leads to p53 activation, which contributes to AD pathogenesis and the increased expression of AD biomarkers (e.g., phosphorylated Tau (p-Tau) and amyloid β (Aβ)) in human brain cells. This hypothesis is supported by our preliminary data and the literature including 1) the APOE e4 protein has a misfolded intermediate state associated with cellular stress; 2) p53 becomes stable and transcriptionally active under stressful conditions; 3) brain tissue from Alzheimer’s disease patients express elevated p53; 4) observations in our lab that the APOE e4/e4 B lymphocytes proliferate slower and more apoptotic than the e3/e3 counterparts; and 5) immunoblot analysis findings that APOE e4/e4 cells express more p53 than the e3/e3 cells, suggesting p53 activation may contribute to decreased cell growth kinetics. The rationale is that the activation of p53 by APOE4 would be a significant finding that could augment our understanding regarding the connection between genetic risk factors and patient pathophysiology, In turn, this would lead to the identification of new gene targets for disease prevention and intervention. Two specific aims are proposed. Aim 1 will assess how APOE e4 protein affects p53 abundance and transcriptional activity in iPSC-derived brain organoids. Aim 2 will assess how p53 activation affects Alzheimer’s disease-associated marker expression in iPSC-derived brain organoids. Upon successful completion, the proposed studies are expected to lead to a major discovery to deepen our understanding of the disease and identify new gene targets to treat and prevent Alzheimer’s disease.