The Role of PICALM Mutations in Alzheimer’s Disease

The search for biological understanding of Alzheimer’s disease (AD) has expanded to include new risk factors, particularly genes. Thanks to breakthroughs in human genetics and high-throughput genome sequencing, PICALM, the gene encoding phosphatidylinositol binding clathrin assembly protein, has been identified as a new risk gene for AD. As a key player in clathrin-mediated endocytosis and intracellular trafficking, PICALM critically regulates amyloid beta brain metabolism and neuronal toxicity. Our group recently reported that reduction of PICALM in cerebral vasculature was strongly associated with AD progression in patients, and with accelerated disease progression in an animal model of AD. However, it still is unclear whether genetic variations or mutations of the PICALM gene directly contribute to the disease. Fortunately, the Alzheimer’s Genome Project™, led by Dr. Rudy Tanzi at Massachusetts General Hospital, discovered three novel PICALM genetic mutations strongly associated with AD through whole genome sequencing (WGS). We have expanded our work for our renewal grant to investigate an additional 14 variants supplied by the Tanzi lab. The biological functions of these rare mutations need to be examined urgently, especially in a cell type-specific manner, in order to determine their direct contributions to the pathogenesis of AD.

Our recent study, using both transgenic models and CRISPR-mediated genomic editing in human induced pluripotent stem cells (iPSCs), has demonstrated that PICALM plays a key role in mediating the clearance of amyloid beta at the blood-brain barrier (BBB), as well as mitigating amyloid beta toxicity in neurons, providing molecular and cellular bases to investigate the amyloid beta-dependent and independent functions of novel PICALM mutations. We hypothesize that these mutations may represent loss-of-function (LOF) or gain-of-function (GOF) mutations that affect amyloid beta metabolism and toxicity in different brain cell types, particularly the brain endothelial cells and neurons. In this proposed study, we will investigate the functions of the novel PICALM mutations using both endothelial cells and neurons derived from CRISPR-engineered iPSCs, as well as in transgenic animal models, and examine their impact on trans-vascular clearance and neuronal toxicity of amyloid beta. We expect to generate unique new insights into AD-related PICALM mutations that will have important impact on our understanding of the disease and implications to the development of new PICALM-targeted therapies for AD.