Project Summary Recent efforts to uncover genetic factors responsible for Alzheimer's disease (AD) have identified dozens of associated loci. Among these, our research group previously reported ATXN1 in a family-based AD genome-wide association study. ATXN1 has been otherwise known for to harbor mutations causing spinocerebellar ataxia type 1, a neurodegenerative disease that primarily impairs coordinated movement. In a cell-based study, knockdown of Ataxin-1 gene (ATXN1) expression increased amyloidogenic processing of the amyloid precursor protein (APP) and the secretion of A, the main component of senile plaques in AD brains. In the preliminary examination of Ataxin-1 KO mice, we found Ataxin-1 regulates BACE1 expression, selectively in AD-vulnerable brain regions. In AD mice, depletion of Ataxin-1 increased -secretase-cleavage of APP, A deposition and gliosis in the cerebrum. Furthermore, Ataxin-1 KO impaired hippocampal neurogenesis and axonal targeting, which are regulated by BACE1. To validate and expand upon these findings, here, we propose to 1. Elucidate the molecular mechanism by which Ataxin-1 regulates BACE1 expression in the brain; 2. Assess the impact of Ataxin-1 loss of function on AD pathogenesis; and 3. Identify and characterize novel AD-associated Ataxin-1 gene variants and/or mutations from AD DNA sample sets. Specifically, for Aim 1, we will first determine whether Ataxin-1 regulates BACE1 mRNA level by affecting its stability or by its transcription. To test this, we will employ acute brain slice cultures of Ataxin-1 KO and WT mice, and examine the differential effects of transcriptional inhibitors on the steady-state level of BACE1 mRNA and also measure newly synthesized BACE1 mRNA incorporating nucleotide analogs in the cultured slices. We will then examine if Ataxin-1-interacting transcriptional factors bind to and regulate BACE1 promoter by ChIP analysis. With regard to Aim 2, we will examine if the increased A plaque load and gliosis are maintained in APP-PS1/ATXN1-KO mice at older age (9 month). To determine if impaired hippocampal neurogenesis and axonal targeting are caused by increased BACE1, we will generate ATXN1 −/−:BACE1 +/− mice and examine if the two deficits are rescued, as compared to ATXN1 −/− mice. Finally, in Aim 3, we will identify ATXN1 mutations/variants that either increase risk or confer protection for AD by analysis of WGS/WES data of NIMH, NIA and ADNI AD DNA sample sets. For the most associated mutations/variants, we will examine their effects on BACE1 expression and APP processing by incorporating them into the genome of human neuronal cells via CRISPR/Cas9 technology. At the completion of the proposed study, we believe we will provide critically needed data addressing the role of Ataxin-1 in regulating BACE1 expression while also facilitating novel therapies aimed at targeting BACE1 to prevent and treat AD.