SUCLG2 identified as both a determinator of CSF Aβ1-42 levels and an attenuator of cognitive decline in Alzheimer's disease

Abstract

Cerebrospinal fluid amyloid-beta 1-42 (Aβ1-42) and phosphorylated Tau at position 181 (pTau181) are biomarkers of Alzheimer's disease (AD). We performed an analysis and meta-analysis of genome-wide association study data on Aβ1-42 and pTau181 in AD dementia patients followed by independent replication. An association was found between Aβ1-42 level and a single-nucleotide polymorphism in SUCLG2 (rs62256378) (P = 2.5×10(-12)). An interaction between APOE genotype and rs62256378 was detected (P = 9.5 × 10(-5)), with the strongest effect being observed in APOE-ε4 noncarriers. Clinically, rs62256378 was associated with rate of cognitive decline in AD dementia patients (P = 3.1 × 10(-3)). Functional microglia experiments showed that SUCLG2 was involved in clearance of Aβ1-42.

Figure 1.

Genome-wide signal intensity (Manhattan) plots. (A) Results for the association with CSF Aβ_1–42. (B) Results for the association with CSF pTau₁₈₁. Both plots show the individual P-values against genomic position. Chromosome number is shown on the x-axis. The results within each chromosome are plotted left to right, with left being the P-terminal end of the corresponding chromosome. Horizontal dashed lines indicate a P-value threshold of 5 × 10⁻⁸ (i.e. genome-wide significance). Green dots depict single-nucleotide polymorphisms with genome-wide significance in the genome-wide association study step.

Figure 2.

Functional studies of SUCLG2. (A and B) SUCLG2 localized to microglia in AD brains. (A) Representative DAB immunostaining with SUCLG2 and HLA-DR (as a human microglial marker) in brain cortex of AD subjects. DAB-positive cells with characteristic romboidal nuclei (typically microglial cells) are indicated by arrows. Scale bars are 100 and 20 µm on the main and small images, respectively. (B) High-resolution confocal image of an area of frontal cortex from an AD dementia patient immune labelled with HLA-DR (green) and SUCLG2 (red). Plaque autofluorescence is visible in the blue channel and it is highlighted with a discontinuous line. Most of the SUCLG2 is enclosed by the HLA-DR (arrow). The top panel shows several viewpoints of a 3D reconstruction of the same area, which is shown in the lower panel. (Scale is 10 μm.) (C) Increased levels of SUCLG2 in AD ApoE 3/3 brains. Soluble extracts from postmortem frontal cortex of AD dementia patients with ApoE 3/4 and ApoE 3/3 status plus age-matched healthy controls (CN) were probed with an antibody to SUCLG2. Expression levels were normalized to Hsp75 (CN, n = 6; AD, 3/4, n = 8; AD, 3/3, n = 4). Data are presented as mean ± SEM. Asterisk denotes P < 0.05. (D) SUCLG2 silencing is detrimental to extracellular Aβ_1–42. FAM-Aβ_1–42 phagocytosis was impaired in BV2 cells in which SUCLG2 was silenced by two distinct siRNA compared with negative control siRNA (n = 6). FAM-Aβ_1–42 was added at a 500 nm final concentration and the amount of internalized Aβ_1–42 was assessed at the indicated time points, showing that cells contain less Aβ_1–42 in the absence of SUCLG2. Data are presented as mean ± SEM. Asterisk denotes P < 0.05.