Identification of Alzheimer disease-associated variants in genes that regulate retromer function

The proteolytic processing of amyloid precursor protein (APP) to generate the neurotoxic amyloid β (Aβ) peptide is central to the pathogenesis of Alzheimer disease (AD). The endocytic system mediates the processing of APP by controlling its access to secretases that cleave APP. A key mediator of APP localization is SorL1-a membrane protein that has been genetically linked to AD. The retromer complex is a conserved protein complex required for endosome-to-Golgi retrieval of a number of physiologically important membrane proteins including SorL1. Based on the prior suggestion that endocytosis and retromer sorting pathways might be involved, we hypothesized that variants in other genes in this pathway might also modulate AD risk. Genetic association of AD with 451 polymorphisms in 15 genes encoding retromer or retromer-associated proteins was tested in a Caucasian sample of 8309 AD cases and 7366 cognitively normal elders using individual single nucleotide polymorphism (SNP)- and gene-based tests. We obtained significant evidence of association with KIAA1033 (VEGAS p = 0.025), SNX1 (VEGAS p = 0.035), SNX3 (p = 0.0057), and RAB7A (VEGAS p = 0.018). Ten KIAA1033 SNPs were also significantly associated with AD in a group of African Americans (513 AD cases, 504 control subjects). Findings with four significant SNX3 SNPs in the discovery sample were replicated in a community-based sample of Israeli-Arabs (124 AD cases, 142 control subjects). We show that Snx3 and Rab7A proteins interact with the cargo-selective retromer complex through independent mechanisms to regulate the membrane association of retromer and thereby are key mediators of retromer function. These data implicate additional AD risk genes in the retromer pathway and formally demonstrate a direct link between the activity of the retromer complex and the pathogenesis of AD.