Characterization of an Alzheimer disease-associated deletion in SORL1.

Abstract

Greater than 20 genetic loci have been consistently associated with Alzheimer disease (AD). Accumulating evidence has implicated the endolysosomal pathway in disease pathology. The sortilin-related receptor 1 (SORL1) gene has been associated with AD through multiple studies and has been suggested to function as a guide for amyloid precursor protein (APP) to the endocytic pathway. We identified a family with multiple AD affected individuals that have a single base pair deletion (g.4292delG) resulting in a frameshift alteration and premature termination of the protein (p.Cys143Cys*1). Although previous studies have implicated SORL1 in AD pathogenesis, it remains unclear how this deletion impacts SORL1 protein function in AD development. Patient-specific induced pluripotent stem cells (iPSC) lines were derived to examine the molecular consequences of this SORL1 deletion in neuronal and glial cells (astrocytes and microglia), which can otherwise only be collected from autopsy samples. Each iPSC line generated was validated for pluripotency through immunocytochemical staining and karyotypic stability. Patient iPSC lines and control lines were differentiated into cortical neurons and glia for functional characteristics, including amyloid beta production and uptake, vesicular trafficking, and neurite outgrowth. We have derived iPSC lines bearing the SORL1 deletion and have confirmed pluripotency by immunocytochemistry. These iPSC lines were differentiated into cortical neurons whose identity was confirmed by immunocytochemical staining for neuronal markers. The SORL1 variant bearing cortical neurons showed enlarged endosomes, as measured by RAB5 staining and fluorescent microscopy, compared to the neurons derived from control iPSCs. In addition, the SORL1 deletion bearing neurons showed an upregulation in the ratio of amyloid beta 42:amyloid beta 40 compared to control neurons as measured by ELISA. Finally, SORL1 deletion bearing neurons had reduced neurite outgrowth and branch point numbers compared to controls. In summary, our results support that the p.Cys143Ser*1 variant is pathogenic and alters the functionality of cells from the central nervous system. iPSC-derived neurons and glia cells bearing the deletion in SORL1 p.Cys143Ser*1 variant shows cellular deficits associated with AD pathology, particularly in endolysomal trafficking defects. Studies are continuing to determine if this effect is due to a gain or loss of function.