The recycling of the amyloid precursor protein (APP) from the cell surface via the endocytic pathways plays a key role in the generation of amyloid β-peptide (Aβ) in Alzheimer's Disease (AD). We report here that inherited variants in the SORL1 neuronal sorting receptor are associated with late-onset AD. These variants, which occur in at least two different clusters of intronic sequences may regulate tissue-specific expression of SORL1. We also show that SORL1 directs trafficking of APP into recycling pathways, and that when SORL1 is under-expressed, APP is sorted into Aβ-generating compartments. These data suggest that inherited or acquired changes in SORL1 expression or function are mechanistically involved in causing AD.

The presenilin proteins (PS1 and PS2) and their interacting partners nicastrin, aph-1 (refs 4, 5) and pen-2 (ref. 5) form a series of high-molecular-mass, membrane-bound protein complexes that are necessary for gamma-secretase and epsilon-secretase cleavage of selected type 1 transmembrane proteins, including the amyloid precursor protein, Notch and cadherins. Modest cleavage activity can be generated by reconstituting these four proteins in yeast and Spodoptera frugiperda (sf9) cells. However, a critical but unanswered question about the biology of the presenilin complexes is how their activity is modulated in terms of substrate specificity and/or relative activities at the gamma and epsilon sites. A corollary to this question is whether additional proteins in the presenilin complexes might subsume these putative regulatory functions. The hypothesis that additional proteins might exist in the presenilin complexes is supported by the fact that enzymatically active complexes have a mass that is much greater than predicted for a 1:1:1:1 stoichiometric complex (at least 650 kDa observed, compared with about 220 kDa predicted). To address these questions we undertook a search for presenilin-interacting proteins that differentially affected gamma- and epsilon-site cleavage events. Here we report that TMP21, a member of the p24 cargo protein family, is a component of presenilin complexes and differentially regulates gamma-secretase cleavage without affecting epsilon-secretase activity.

Importance Common single nucleotide polymorphisms in the SORL1 gene have been associated with late onset Alzheimer’s disease (LOAD) but causal variants have not been fully characterized nor has the mechanism been established. Objective To identify functional SORL1 mutations in patients with LOAD. Design and Participants This was a family- and cohort-based genetic association study. Caribbean Hispanics with familial and sporadic LOAD and similarly aged controls recruited from the United States and the Dominican Republic, and patients with sporadic disease of Northern European origin recruited from Canada. Main Outcome Measure(s) Prioritized coding variants in SORL1 detected by targeted re-sequencing and validated by genotyping in additional family members and unrelated healthy controls. Variants transfected into human embryonic kidney 293 (HEK) cell lines were tested for Aβ40 and Aβ42 secretion and the amount of the amyloid precursor protein (APP) secreted at the cell surface was determined. Results 17 coding exonic variants were significantly associated with disease. Two rare variants (rs117260922-E270K and rs143571823-T947M) with MAF<1% and one common variant (rs2298813-A528T) with MAF=14.9% segregated within families and were deemed deleterious to the coding protein. Transfected cell lines showed increased Aβ40 and Aβ42 secretion for the rare variants (E270K and T947M) and increased Aβ42 secretion for the common variant (A528T). All mutants increased the amount of APP at the cell surface, though in slightly different ways, thereby failing to direct full-length APP into the retromer-recycling endosome pathway. Conclusions and Relevance Common and rare variants in SORL1 elevate the risk of LOAD by directly affecting APP processing which, in turn can result in increased Aβ40 and Aβ42 secretion.