SORL1 and its involvement in Alzheimer's disease pathogenesis and pathophysiology - Project 4

PROJECT 4 SUMMARY Recent genetic studies have established that rare loss-of-function mutations in the Sortilin-related Receptor 1 (SORL1) gene confer extremely high risk for developing Alzheimer’s disease (AD). Currently, over 500 known coding variants in SORL1 have been described, although the pathogenicity of these variants and how they impact SORL1 biology and downstream biofluid profiles are largely unknown. As discussed in the Overview, this Program Project Grant (PPG) will initially tackle this question by relying on a recently developed prioritization framework designed to identify SORL1 variants deemed pathogenic. Towards this aim, the Projects and Cores will perform extensive in-depth studies across multiple models and analyses on a subset of 22 AD- associated SORL1 missense variants (see Overview, Table 1). Project 4 is one of the Projects tasked with investigating the pathogenicity and downstream biology of these variants using mouse biofluids, conditioned medium of human neural cells, and CSF and plasma from patients harboring SORL1 variants. This Project will examine the biomarker profiles downstream of each SORL1 variant, by not only focusing on canonical and novel biofluid markers (hypothesis-driven), but also performing an unbiased proteomic screen to identify novel and specific biomarkers for SORL1 variants (hypothesis-free). Studies from our group and others have provided evidence that SORL1-retromer dysfunction results in AD-associated abnormalities in secreted tau, amyloid, and n-APLP1 as well as shedding of sSORL1. Thus, we will rely on highly sensitive assays using Single-molecule-array and Mesoscale Discovery technologies to assess the downstream biomarker signatures of SORL1 missense variants. First, in a hypothesis-driven approach, we will test if AD-associated SORL1 missense variants do phenocopy canonical AD-related biomarkers, in both in vivo and in vitro systems (Aim 1). Second, we will investigate levels of novel AD-related biomarkers linked to endosomal trafficking (sSORL1 and n-APLP1), as well as microglia (complement C1q) and astrocyte (GFAP) immune responses (Aim 2). Third, we will perform an unbiased proteomic screen of SORL1 transgenic lines to identify unique biomarker signatures for each SORL1 missense variant (Aim 3) and will assess their translational potential in humans. Fourth, we will perform an exploratory study to determine n-APLP1 and sSORL1 levels in biofluids from SORL1-variant carriers and healthy controls, to assess their utility in detecting endosomal dysfunction in this population (Aim 4). By completing this study, Project 4 will contribute to the PPG’s goal of validating the pathogenicity and pathophysiology of SORL1 AD-associated variants. Our studies will elucidate how disease-associated variants in SORL1 affect AD’s biomarker profiles. Moreover, we predict the experiments proposed will identify a biomarker, or panel of biomarkers, that will aid in diagnosing patient-carriers and inform future clinical trials designed for this particular population.