PROJECT SUMMARY ABSTRACT By the age of forty, every person with Down syndrome has Alzheimer's disease brain pathology, and most will go on to develop Alzheimer's disease dementia, due to triplication of the amyloid precursor protein gene (APP) that resides on chromosome 21. As the strongest genetic risk factor and greatest overall risk factor for Alzheimer's disease in the typical population, other than increasing age itself, inheritance of the ε4 allele of the apolipoprotein E gene (APOE) also significantly increases the risk and severity of Alzheimer's disease in people with Down syndrome. We have found a key mechanism by which apoE promotes Alzheimer's disease: it binds to the Aβ peptide and converts it into a toxic species that kills neurons and causes neurodegeneration, with the apoE4 form being the most effective amyloid catalyst. In view of the essential contributions of apoE to Alzheimer's disease, it is critical that the mechanisms underlying the enhanced Alzheimer's disease risk for people with Down syndrome be elucidated and that new therapies be developed to effectively target its pathogenic activity. We have developed an in vitro assay to screen the NIH Clinical Collection (NCC) small molecule library for inhibitors of apoE4-catalyzed Aβ oligomer/fibril formation. We have identified eight hit compounds, each of which has been tested previously in Phase I-III clinical trials for other indications and thus have known safety profiles. In a secondary screen, we found that three out of the eight initial compounds were non-neurotoxic inhibitors of apoE that significantly reduced Aβ and tau pathology and cell death in neurons from two rodent models of Alzheimer's disease. Furthermore, an analysis of the National Alzheimer's Coordinating Center (NACC) database showed use of either one of two drugs we identified as apoE inhibitors by Alzheimer's patients was associated with improved cognition over time and increased odds of reverting to a better clinical diagnosis from Alzheimer's disease to mild cognitive impairment (MCI) or from MCI to normal cognition, providing translational support for their further study. Herein, we will use a panel of human induced pluripotent stem cell (iPSC)-derived cerebral organoid (CO) models of Down syndrome and Alzheimer's disease to study the mechanisms of apoE-induced Alzheimer's disease phenotypes and to evaluate whether our top candidate apoE inhibitors can block the development of Alzheimer's disease phenotypes in Down syndrome. We will also develop the first mouse model of Down syndrome expressing human APOE4 and assess the ability of our top candidate apoE inhibitors to prevent the development of cognitive deficits, cerebrovascular damage, synaptic dysfunction, neurodegeneration, and/or neuroinflammation in this novel model of Down syndrome-associated Alzheimer's disease in order to inform future clinical trials. Our proposed approach should result in highly targeted Alzheimer's disease therapies for people with Down syndrome with few side effects, because the drugs that inhibit the interaction of apoE4 and Aβ should not, or can be selected to not, affect the normal functions of apoE.

PROJECT NARRATIVE We have discovered that apolipoprotein E (apoE) promotes Alzheimer's disease by binding to the amyloid-beta peptide (Aβ) and converting into a toxic species that kills neurons and causes neurodegeneration, with the apoE4 form being the most effective catalyst of Aβ. To help inform future clinical trials, we have carried out a high- throughput screen and identified a set of small molecule drugs that prevent apoE4 from promoting the formation of Aβ oligomers and fibrils that lead to Alzheimer's disease. Here, we propose to further evaluate our most promising apoE-Aβ inhibitors for their ability to prevent the pathological functions of apoE4 in human induced pluripotent stem cell (iPSC)-derived cerebral organoid models of Down syndrome, and we will also evaluate their ability to improve cognition and shift Alzheimer's disease-associated biomarker profiles towards normal in Dp16 Down syndrome mice that express human APOE4.