GAB2 alleles modify Alzheimer's risk in APOE epsilon4 carriers

Abstract

The apolipoprotein E (APOE) epsilon4 allele is the best established genetic risk factor for late-onset Alzheimer's disease (LOAD). We conducted genome-wide surveys of 502,627 single-nucleotide polymorphisms (SNPs) to characterize and confirm other LOAD susceptibility genes. In epsilon4 carriers from neuropathologically verified discovery, neuropathologically verified replication, and clinically characterized replication cohorts of 1411 cases and controls, LOAD was associated with six SNPs from the GRB-associated binding protein 2 (GAB2) gene and a common haplotype encompassing the entire GAB2 gene. SNP rs2373115 (p = 9 x 10(-11)) was associated with an odds ratio of 4.06 (confidence interval 2.81-14.69), which interacts with APOE epsilon4 to further modify risk. GAB2 was overexpressed in pathologically vulnerable neurons; the Gab2 protein was detected in neurons, tangle-bearing neurons, and dystrophic neuritis; and interference with GAB2 gene expression increased tau phosphorylation. Our findings suggest that GAB2 modifies LOAD risk in APOE epsilon4 carriers and influences Alzheimer's neuropathology.

Figure 1. Linkage Disequilibrium Structure and Haplotype Significance Levels for the Region Encompassing GAB2

Plots follow the Haploview GOLD heatmap color scheme. SNP numbers correspond to the following dbSNP ID identification numbers: 1:rs579711, 2:rs977978, 3:rs637149, 4:rs977977, 5:rs901104, 6:rs1385600, 7:rs11237419, 8:rs1007837, 9:rs2450130, 10:rs2510054, 11:rs11237429, 12:rs2510038, 13:rs2511170, 14:rs4945261, 15:rs7101429, 16:rs10793294, 17:rs4291702, 18:rs11602622, 19:rs10899467, 20:rs2458640, 21:rs10793302, 22:rs2373115, 23:rs12280198, 24:rs12287010, 25:rs17136630, 26:rs4945276, 27:rs1996172, 28:rs7395344, 29:rs11237522, and 30:rs7950813. In all three cohorts, there was a GAB2 risk haplotype, a protective haplotype, and a haplotype unrelated to LOAD risk in APOE ε4 carriers.

Figure 2. siRNA Knockdown of GAB2 Increases Tau Phosphorylation

In comparison with vehicle treatment (red, upper left), GAB2 siRNA treatment resulted in a 1.70-fold increase in tau phosphorylation at the serene-262 residue (red, lower left), which is phosphorylated in LOAD neurofibrillary tangle-bearing neurons. This fold-change was not attributable to an increase in total tau (green, upper and lower right).

Figure 3. Gab2 Colocalizes with Dystrophic Neurons in the AD Brain

(A) LOAD hippocampus (neutral red counterstain) (40× objective). The arrow indicates a highly dystrophic cell with the size and morphology of a cortical pyramidal neuron. Arrowheads point to one of many structures in the sections that resemble dystrophic neurites or neuropil threads. (B) LOAD hippocampus (neutral red counterstain) (40×). The arrow denotes a putative neurofibrillary tangle-containing neuron. Arrowheads again indicate a dystrophic neurite. (C) LOAD posterior cingulate gyrus (40×). Filled arrows point to normal-appearing putative neurons. The open arrow points to a cell with the features of a neurofibrillary tangle-bearing neuron. Immunoreactive structures clearly resembling pyramidal cell apical dendrites were also observed ascending through the cortical layers (arrowheads). (D) LOAD posterior cingulate cortex (100× objective). Shown is a Gab2 immunoreactive cell with the flame-shaped cytoplasmic inclusion typical of the neurofibrillary tangle.