The SORL1 p.Y1816C variant causes impaired endosomal dimerization and autosomal dominant Alzheimer’s disease

Truncating genetic variants of SORL1 , encoding the endosome recycling receptor SORLA, have been accepted as causal of Alzheimer’s disease (AD). However, most genetic variants observed in SORL1 are missense variants, for which it is complicated to determine the pathogenicity level because carriers come from pedigrees too small to be informative for penetrance estimations. Here, we describe three unrelated families in which the SORL1 coding missense variant rs772677709, that leads to a p.Y1816C substitution, segregates with Alzheimer’s disease. Further, we investigate the effect of SORLA p.Y1816C on receptor maturation, cellular localization and trafficking in cell-based assays. Under physiological circumstances, SORLA dimerizes within the endosome, allowing retromer-dependent trafficking from the endosome to the cell surface, where the luminal part is shed into the extracellular space (sSORLA). Our results showed that the p.Y1816C mutant impairs SORLA dimerization in the endosome leading to a strong decrease in trafficking to the cell surface, resulting in decreased sSORLA shedding. Furthermore, we find that iPSC-derived neurons with engineered p.Y1816C mutation have enlarged endosomes, a defining cytopathology of AD. Our studies provide genetic as well as functional evidence that the SORL1 p.Y1816C variant is causal for AD. The high penetrance of the mutation suggests this mutation should be considered in clinical genetic screening of multiplex early-onset AD families. ### Competing Interest Statement O.M.A. is a consultant for Retromer Therapeutics and has equity. ### Funding Statement H.H., O.M.A., and D.B. are a part of the EU Joint Programme-Neurodegenerative Disease Research (JPND) Working Group SORLA-FIX under the 2019 'Personalized Medicine' call (JPND2019-466-197, ZonMW 733051110, Danish Innovation Foundation and the Velux Foundation Denmark, and the Ministry of Education, Youth and Sports of the Czech Republic no. 8F20009). H.H., is a recipient of ABOARD, a public-private partnership receiving funding from ZonMW (#73305095007) and Health~Holland, Topsector Life Sciences & Health (PPP-allowance; #LSHM20106). H.H. was supported by the Hans und Ilse Breuer Stiftung (2020) and the HorstingStuit Foundation (2018). O.M.A is supported by Novo Nordisk Foundation (#NNF20OC0064162), the Alzheimer's Association (ADSF-21-831378-C), the Independent Research Fund Denmark (#3101-00065B) and the Danish Alzheimer's Research Foundation (recipient of the 2022 Basic Research Science Award). Research of K.S. is supported in part by the SAO and UA special research fund. SvdL received funding from ZonMW (#733050512). J.R. is supported by the Czech Health Research Council (AZV project No. NU22J-08-00075). D.B. is spuported by project nr. LX22NPO5107 (MEYS): Financed by European Union - Next Generation EU. L.B. and R.G. were supported by the Italian Ministry of Health (Ricerca Corrente). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The Medical Ethics Committee of the Amsterdam University Medical Center gave ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors