Dissecting the role of the PICALM/EED locus on myeloid cells in Alzheimer's disease. - Project Summary Alzheimer’s disease (AD) is a progressive neurodegenerative disease affecting more than 50 million people worldwide, but the mechanisms involved in disease development remain poorly understood. Genome-wide association studies have uncovered numerous loci associated with disease, but identifying causal variants and genes remains a challenge. Studies have revealed that AD risk variants are enriched in active enhancers of human monocytes, macrophages, and microglia, suggesting many of these variants act by disrupting gene expression specifically in myeloid cells. In addition, many genes implicated in AD risk are highly expressed in myeloid cells, strongly implicating these cell types in the etiology of AD. By integrating human genetics with epigenomic and transcriptomic datasets from human myeloid cells, we identified a myeloid cell enhancer containing an AD-associated functional variant on chromosome 11, and two putative target genes of this enhancer, embryonic ectoderm development (EED) and phosphatidylinositol binding clathrin assembly protein (PICALM). EED is an essential subunit of the polycomb repressive complex 2 (PRC2) known to function in regulation of gene expression and clearance behavior in mouse microglia, but it remains relatively unstudied in the context of AD in human cells. PICALM is an adaptor protein known to function in endolysosomal pathway and autophagy, but its role in microglia remains unknown. The overall goal of this proposal is to understand how our nominated AD risk enhancer influences the expression of its two target genes PICALM and EED, and to test the hypothesis that these two potential causal genes regulate microglia functions downstream of TREM2. In Aim 1, I will determine the role of the candidate AD risk enhancer in regulating the expression of PICALM and EED and human microglial cell function by combining CRISPR gene editing in human induced pluripotent stem cells (iPSCs), microglial differentiation protocols, and xenotransplantation methods involving direct injection of microglia precursor cells into the mouse brain. I will delete the candidate enhancer region and perform transcriptomic and epigenetic profile of edited microglia via RNAseq and ATACseq in addition to microglia functional assays. To assess the in vivo consequences of deleting the risk enhancer in microglia, I will transplant the edited microglia into the brains of wildtype and 5xFAD humanized mice and characterize transcriptomic profile of these microglia via snRNAseq analysis and perform immunohistochemistry. In Aim 2, I will investigate how PICALM and EED regulate human microglia function, namely TREM2 mediated efferocytosis, or phagocytic pathway. I will determine the subcellular localization of PICALM and EED using biochemical and imaging techniques and test the role of these two genes in microglial functions downstream of TREM2 signaling such as actin rearrangement, phagosome formation, and downstream kinase signaling. My proposed aims will help understand the mechanisms in which the causal variants and target genes drive the AD risk, and enhance our understanding of PICALM and EED in microglia and in AD.
