Examining the impact of a rare protein‐truncating SORL1 variant on AD pathology

Abstract

AbstractBackgroundRecent analyses of rare variants using whole exome sequencing have found an enrichment of SORL1 loss‐of‐function (LoF) variants in early onset Alzheimer’s disease (EOAD). This makes SORL1 one of the highest risk factors for AD, along with other candidate EOAD genes such as APP, PS1, and PS2. SORL1 encodes an endocytic receptor involved in the trafficking of amyloid‐β precursor protein (APP) and the secretion of amyloid‐β. We identified a family with multiple individuals affected with EOAD carrying a single base pair deletion (c.4293del) in SORL1, resulting in a frameshift and premature termination of the protein (p.Cys1431fs; Kunkle et al. JAMA Neurol. 2017). Several recent studies have demonstrated that haploinsufficiency of SORL1 can induce AD‐related phenotypes in cultured neurons. However, the functional consequences of specific variants remain largely undefined. In this study, we used patient‐specific induced pluripotent stem cell (iPSC)‐derived neurons to evaluate the effect of a rare LoF SORL1 variant on AD‐related pathology.MethodPatient‐specific iPSC lines were derived from two related heterozygous p.Cys1431fs carriers with EOAD. Each line was validated for pluripotency through immunocytochemical staining and RT‐PCR. Karyotypic stability was assessed by G‐banding. To evaluate the functional consequences of the SORL1 deletion, patient and control iPSC lines were differentiated into cortical neurons and assessed for AD‐related phenotypes.ResultThe iPSC lines were successfully differentiated into cortical neurons, characterized by immunostaining for various neuronal markers. Our data show that neurons bearing the p.Cys1431fs SORL1 mutation have increased levels of APP accumulated in EEA1+ endosomes compared to neurons from cognitively intact individuals. Additionally, analysis of synaptic density revealed that neurons carrying the p.Cys1431fs variant show a significant reduction of SYN1+ puncta compared to cognitively intact individuals.ConclusionOur results indicate that patient‐derived neurons carrying the p.Cys1431fs variant have cellular defects associated with AD pathology while replicating, at least in part, previous in vitro findings on SORL1 haploinsufficiency.