Alzheimer risk associated with a copy number variation in the complement receptor 1 increasing C3b/C4b binding sites

Two multicentre genome-wide association (GWA) studies provided substantial evidence, implicating the complement receptor 1 gene (CR1) in Alzheimer disease (AD) genetic etiology. CR1 encodes a large transmembrane receptor with a crucial role in the immune complement cascade. We performed a genetic follow-up of the GWA CR1 association in a Flanders-Belgian cohort (n=1883), and investigated the effect of single-nucleotide polymorphisms (SNPs) located in the CR1 locus on AD risk and cerebrospinal fluid (CSF) biomarker levels. We obtained significant association (P-adj < 0.03; odds ratio (OR) = 1.24 (95% confidence interval (Cl): 1.02-1.51)) for one CR1 risk haplotype, and haplotype association was strongest in individuals carrying apolipoprotein E (APOE) epsilon 4 alleles (P-adj < 0.006; OR = 1.50 (95% Cl: 1.08-2.09)). Also, four SNPs correlated with increased CSF amyloid A beta(1-42) levels, suggesting a role for the CR1 protein in A beta metabolism. Moreover, we quantified a low-copy repeat (LCR)-associated copy number variation (CNV) in CR1, producing different CR1 isoforms, CR1-F and CR1-S, and obtained significant association in carriers of CR1-S. We replicated the CR1 CNV association finding in a French cohort (n=2003) and calculated in the combined cohorts, an OR of 1.32; 95% CI: 1.10-1.59 (P=0.0025). Our data showed that the common AD risk association may well be explained by the presence of CR1-S increasing the number of C3b/C4b and cofactor activity sites and AD risk with 30% in CR1-S carriers. How precisely the different functional role of CR1-S in the immune complement cascade contributes to AD pathogenesis will need additional functional studies. Molecular Psychiatry (2012) 17, 223-233; doi:10.1038/mp.2011.24; published online 15 March 2011