PROJECT SUMMARY (APOE U19 Project 5) The most important Alzheimer’s disease (AD) risk gene is apolipoprotein E (APOE). APOE4 is associated with a dose dependent increase in risk, while APOE2 is associated with a dose dependent decrease in risk and delayed age at onset (AAO), relative to APOE3/3. Population-based studies have demonstrated that APOE4/4 homozygotes have ~60% life-time risk for AD by age 85 yrs compared to ~10% in APOE3/3 homozygotes. Despite this strong effect on AD susceptibility, there is huge variation (several decades) in AAO even within a single APOE genotype. We hypothesize that in human populations there are both risk and protective alleles in genes that modify AAO within a single APOE genotype by acting within the same cascade of events that modify AD risk and AAO downstream of APOE (see Overall section for detailed description of the ApoE Cascade Hypothesis). In this project we will analyze publicly available data to identify genes that modify AD risk in APOE4 carriers and APOE3/3 homozygotes. Preliminary analyses have identified rare TREM2 variants as modifiers of AAO in APOE3/3 homozygotes and rare EIF2B3 variants as modifiers of AAO in APOE4 carriers. ApoE is a secreted apolipoprotein that is primarily expressed in astrocytes. However, APOE expression is highly upregulated in subpopulations of microglia and other macrophages, particularly in response to tissue damage and lipid overload in the aged or diseased brain, in a manner that is at least in part dependent on TREM2. This is of particular relevance to AD because genetic studies have demonstrated that common risk alleles are specifically enriched in myeloid/microglial enhancers, suggesting that the effects of APOE genotype and its genetic modifiers on AD risk and AAO may be mediated by their effects on microglial cell function. We hypothesize that APOE genotype and its genetic modifiers alter AD risk/AAO through modulation of microglial cell function, particularly in response to brain tissue damage in aging and disease. To address this hypothesis we will use isogenic hiPSC-derived microglia (iMGL) to assess the impact of APOE genotype and its genetic modifiers (TREM2 R47H & EIF2B3 S404A) on microglial cell function in vitro and transplantation of isogenic hiPSC-derived hematopoietic progenitor cells (iHPCs) into mice to assess the impact of APOE genotype and its genetic modifiers (TREM2 R47H & EIF2B3 S404A) on microglial cell function in AD mouse brains. This project will be performed in collaboration with Cores C, E and G and addresses aims 2, 4 and 6 of the overall U19 project. Project 5 will identify novel modifiers of AAO of AD in the context of different APOE risk backgrounds and determine the molecular consequences of these novel risk genes on microglial function in vitro and in vivo. In so doing, Project 5, together with Projects 2-4, will provide insight into ApoE function in microglia leading to the identification of novel therapeutic targets for AD and gene...