A genome-wide association meta-analysis of plasma Aβ peptides concentrations in the elderly

Abstract

Amyloid beta (Aβ) peptides are the major components of senile plaques, one of the main pathological hallmarks of Alzheimer disease (AD). However, Aβ peptides' functions are not fully understood and seem to be highly pleiotropic. We hypothesized that plasma Aβ peptides concentrations could be a suitable endophenotype for a genome-wide association study (GWAS) designed to (i) identify novel genetic factors involved in amyloid precursor protein metabolism and (ii) highlight relevant Aβ-related physiological and pathophysiological processes. Hence, we performed a genome-wide association meta-analysis of four studies totaling 3 528 healthy individuals of European descent and for whom plasma Aβ1-40 and Aβ1-42 peptides levels had been quantified. Although we did not observe any genome-wide significant locus, we identified 18 suggestive loci (P<1 × 10(-)(5)). Enrichment-pathway analyses revealed canonical pathways mainly involved in neuronal functions, for example, axonal guidance signaling. We also assessed the biological impact of the gene most strongly associated with plasma Aβ1-42 levels (cortexin 3, CTXN3) on APP metabolism in vitro and found that the gene protein was able to modulate Aβ1-42 secretion. In conclusion, our study results suggest that plasma Aβ peptides levels are valid endophenotypes in GWASs and can be used to characterize the metabolism and functions of APP and its metabolites.

Figure 1

Results of a genome-wide meta-analysis of plasma Aβ levels. The plots represent the - log₁₀(P-values) of the SNPs on the y axis and chromosome positions on the x axis for (a) Aβ_1-42 and (b) Aβ_{1 - 40} plasma levels. The red line indicates the genome-wide significance threshold (5 × 10⁻⁸) and the blue line indicates the suggestive significance threshold (1 × 10⁻⁵). SNP, single-nucleotide polymorphism.

Figure 2

Overexpression of CTXN3-myc in a HEK293 cell line. (a) Representative western blots of extracts from HEK293-APP^695wt—stably transfected with APP^695wt—and transiently transfected with CTXN3-myc cDNA or a control empty plasmid (vector). Anti-APP (APPCter-C17), anti-myc (Invitrogen 46-0603) and anti-actin antibodies have been used for western blots. This experiment was repeated three times. (b) Measurement of secreted Aβ_1–40 and Aβ_1–42 by ELISA following transfection of the CTXN3-myc expression vector in APP-HEK293 cells. Variations in Aβ_1–42 (left) and Aβ_1–40 (right) secretion from three independent experiments (performed in duplicate) are shown. (c) Representative confocal images from immunofluorence staining of HEK293-APP^695wt cells (APP695) or HEK293 (endogenous APP) transfected with CTXN3-myc using anti-myc (Invitrogen 46-0603, green) and anti-APP (APPCter-C17, red) antibodies. (d) An in situ proximity ligation assay measuring the interaction between overexpressed CTXN3 and holoAPP in HEK293 cells transfected as previously described above (in panel c). *P<0.05 in a Mann–Whitney non-parametric test); AU: arbitrary units.