Indels and imperfect duplication have driven the evolution of human Complement Receptor 1 (CR1) and CR1-like from their precursor CR1 alpha: importance of functional sets

Cloning and structural analysis of complement component 3d in wild birds provides insight into its functional evolution

Complement component 3 d (C3d) is the final cleavage product of the complement component C3 and serves as a crucial role in link innate and adaptive immunity, and increase B-cell sensitivity to an antigen by 1000-10000 fold. The crystal structure of human C3d revealed there are two distinct surfaces, a convex surface containing the thioester-constituting residues that mediate covalent binding to the target antigen, and a concave surface with an acidic pocket responsible for interaction with CR2. In this study, we cloned and sequenced cDNA fragment encoding C3d region from 15 wild bird species. Then, the C3d sequences from wild birds, chicken and mammals were aligned to construct phylogenetic trees. Phylogenetic tree displayed two main branches, indicating mammals and birds, but the bird C3d branch was divided into two main parts, with five wild birds (Ardeola bacchus, Zoothera, Bubo, Crossoptilon mantchuricum and Caprimulgus europaeus) clustering much closer to mammals. In addition, the C3d proteins of Ardeola bacchus, Bubo, Crossoptilon mantchuricum and Caprimulgus europaeus contained a Glu163 residue at the position at which Lys163 was found in other birds. However, Glu163 have the same charge polarity as Asp163, which is the key amino acid residue comprising the acidic pocket combined with CR2 found at this position in mammals, and Zoothera also possessed Asp163 at this position. Structure modeling analyses also verified that the C3ds of these five wild bird species exhibited the amino acid sequence and structure comprising the typical acidic pocket found in mammals that is required for combination with B cell surface receptors, which contribute electrostatic forces to interact with CR2. Our investigations indicate that some bird C3ds may already have the ability to bind with CR2 by electrostatic force, like mammals. As Ardeola bacchus, Zoothera, Bubo, Crossoptilon mantchuricum and Caprimulgus europaeus have more typical C3d concave acid pockets and thus a stronger ability to bind CR2, we speculate that these five wild birds may have a solider immunity against pathogens. Our phylogenetic and structural analyses of bird C3ds provide insights on the evolutionary divergence in the function of immune factors of avian and mammalian.

The Impact of Complement Genes on the Risk of Late-Onset Alzheimer's Disease

Late-onset Alzheimer's disease (LOAD), the most common cause of dementia, and a huge global health challenge, is a neurodegenerative disease of uncertain aetiology. To deliver effective diagnostics and therapeutics, understanding the molecular basis of the disease is essential. Contemporary large genome-wide association studies (GWAS) have identified over seventy novel genetic susceptibility loci for LOAD. Most are implicated in microglial or inflammatory pathways, bringing inflammation to the fore as a candidate pathological pathway. Among the most significant GWAS hits are three complement genes: CLU, encoding the fluid-phase complement inhibitor clusterin; CR1 encoding complement receptor 1 (CR1); and recently, C1S encoding the complement enzyme C1s. Complement activation is a critical driver of inflammation; changes in complement genes may impact risk by altering the inflammatory status in the brain. To assess complement gene association with LOAD risk, we manually created a comprehensive complement gene list and tested these in gene-set analysis with LOAD summary statistics. We confirmed associations of CLU and CR1 genes with LOAD but showed no significant associations for the complement gene-set when excluding CLU and CR1. No significant association with other complement genes, including C1S, was seen in the IGAP dataset; however, these may emerge from larger datasets.

Epistasis between the MHC and the RCA alpha block in primary Sjögren syndrome

OBJECTIVE

The RCA alpha block (Regulators of Complement Activation, 1q32) contains critical complement regulatory genes such as CR1 and MCP. This study examined RCA alpha block haplotype associations with both disease susceptibility and diversification of the anti-Ro/La autoantibody response in primary Sjögren syndrome (pSS).

METHODS

115 patients with pSS and 98 controls were included in the study. 93 of 109 (85%) of the patients with pSS were seropositive for Ro/La autoantibodies. The Genomic Matching Technique (GMT) was used to define RCA alpha block ancestral haplotypes (AH).

RESULTS

RCA alpha block haplotypes, AH1 and AH3, were both associated with autoantibody-positive pSS (p = 0.0003). Autoantibody associations with both HLA DR3 and DR15 have been previously defined. There was an epistatic interaction (p = 0.023) between RCA alpha AH1 and HLA DR3, and this genotypic combination was present in 48% of autoantibody-positive patients with pSS compared with 8% of controls. This epistasis is most simply attributable to an interaction between C4 and its receptor, CR1, encoded within the RCA alpha block. Both DR3 and a relative C4 deficiency are carried on the major histocompatibility complex 8.1 ancestral haplotype. Only four of 92 (4%) autoantibody-positive patients with pSS did not carry any risk RCA alpha or HLA haplotype, compared with 36 of 96 (38%) controls, and there were differences in haplotype frequencies within autoantibody subsets of pSS.

CONCLUSIONS

Normal population variation in the RCA alpha block, in addition to the major histocompatibility complex, contributes genetic susceptibility to systemic autoimmune disease and the autoantibody response. This finding provides evidence for the role of regulation of complement activation in disease pathogenesis.

Extensive genomic and functional polymorphism of the complement control proteins

Using combinations of genomic markers, we describe more than 20 distinct ancestral haplotypes (AH) of complement control proteins (CCPs), located within the regulators of complement activation (RCA) alpha block at 1q32. This extensive polymorphism, including functional sites, is important because CCPs are involved in the regulation of complement activation whilst also serving as self and viral receptors. To identify haplotypes, we used the genomic matching technique (GMT) based on the pragmatic observation that extreme nucleotide polymorphism is packaged with duplicated sequences as polymorphic frozen blocks (PFB). At each PFB, there are many alternative sequences (haplotypes) which are inherited faithfully from very remote ancestors. We have compared frequencies of RCA haplotypes and report differences in recurrent spontaneous abortion (RSA) and psoriasis vulgaris (PV).