Analysis of the Putative Role of CR1 in Alzheimer’s Disease: Genetic Association, Expression and Function

Abstract

Chronic activation of the complement system and induced inflammation are associated with neuropathology in Alzheimer’s disease (AD). Recent large genome wide association studies (GWAS) have identified single nucleotide polymorphisms (SNPs) in the C3b/C4b receptor (CR1 or CD35) that are associated with late onset AD. Here, anti-CR1 antibodies (Abs) directed against different epitopes of the receptor, were used to localize CR1 in brain, and relative binding affinities of the CR1 ligands, C1q and C3b, were assessed by ELISA. Most Abs tested stained red blood cells in blood vessels but showed no staining in brain parenchyma. However, two monoclonal anti-CR1 Abs labeled astrocytes in all of the cases tested, and this reactivity was preabsorbed by purified recombinant human CR1. Human brain-derived astrocyte cultures were also reactive with both mAbs. The amount of astrocyte staining varied among the samples, but no consistent difference was conferred by diagnosis or the GWAS-identified SNPs rs4844609 or rs6656401. Plasma levels of soluble CR1 did not correlate with diagnosis but a slight increase was observed with rs4844609 and rs6656401 SNP. There was also a modest but statistically significant increase in relative binding activity of C1q to CR1 with the rs4844609 SNP compared to CR1 without the SNP, and of C3b to CR1 in the CR1 genotypes containing the rs6656401 SNP (also associated with the larger isoform of CR1) regardless of clinical diagnosis. These results suggest that it is unlikely that astrocyte CR1 expression levels or C1q or C3b binding activity are the cause of the GWAS identified association of CR1 variants with AD. Further careful functional studies are needed to determine if the variant-dictated number of CR1 expressed on red blood cells contributes to the role of this receptor in the progression of AD, or if another mechanism is involved.

Genome-wide significant, replicated and functional risk variants for Alzheimer’s disease

“…The protein-coding risk gene CR1 (i.e., CD35 ) was expressed much lower than the threshold in brains in our four independent brain cohorts (data not shown), consistent with previous reports (Fonseca et al 2016). However, this very low expression was also significantly correlated with APOE expression in multiple brain regions in our cohorts.…”

High complement levels in astrocyte‐derived exosomes of Alzheimer disease

“…In the same brain tissues, the complement regulatory proteins CD59, CR1, DAF and MCP (CD46) were not detected raising the possibility of lack of control of activation of complement pathways. CR1 antigens were detected in astrocytes in culture and in brain tissues subsequently, but neither CR1 levels nor binding activity accounted for the associations of sporadic AD with some genetic variants of CR1 31 . More complete analyses may be required however, as results of genome-wide association studies of AD have implicated single nucleotide polymorphism variants of CR1 as possible risk factors 32, 33 .…”

Whole genome sequencing of Caribbean Hispanic families with late‐onset Alzheimer's disease

“…It is a receptor for the complement components C3b and C4b. It has been consistently identified as a risk factor in Alzheimer's disease 3, 4, 464, 47…”