The human factor H-related gene 2 (FHR2): structure and linkage to the coagulation factor XIIIb gene

Case report: Novel FHR2 variants in atypical Hemolytic Uremic Syndrome: A case study of a translational medicine approach in renal transplantation

Atypical hemolytic–uremic syndrome (aHUS) is a severe thrombotic microangiopathy in which kidney involvement is common. aHUS can be due to either genetic or acquired abnormalities, with most abnormalities affecting the alternative complement pathway. Several genetic factors/alterations can drive the clinical presentation, therapeutic response, and risk of recurrence, especially recurrence following kidney transplantation. We report here the case of a 22-year-old man who developed a severe form of aHUS. Renal biopsy revealed thrombotic microangiopathy and features of chronic renal damage. Despite two eculizumab infusions, the patient remained dialysis dependent. Two novel rare variants, c.109G>A (p.E37K) and c.159 C>A (p.Y53*), were identified in the factor H-related 2 (FHR2) gene, and western blot analysis revealed a significant reduction in the level of FHR2 protein in the patient’s serum. Although FHR2 involvement in complement 3 glomerulopathy has been reported previously, a role for FRH2 as a complement modulator has not yet been definitively shown. In addition, no cases of aHUS in individuals with FHR2 variants have been reported. Given the role of FHRs in the complement system and the fact that this patient was a candidate for a kidney transplant, we studied the relevance of low FHR2 plasma levels through a set of functional in vitro assays. The aim of our work was to determine if low FHR2 plasma levels could influence complement control at the endothelial surface with a view to identifying a therapeutic approach tailored to this specific patient. Interestingly, we observed that low FHR2 levels in the patient’s serum could induce complement activation, as well as C5b–9 deposition on human endothelial cells, and affected cell morphology. As C5b–9 deposition is a prerequisite for endothelial cell damage, these results suggest that extremely low FHR2 plasma levels increase the risk of aHUS. Given their ability to reduce C5b–9 deposition, recombinant FHR2 and eculizumab were tested in vitro and found to inhibit hemolysis and endothelial cell surface damage. Both molecules showed effective and comparable profiles. Based on these results, the patient underwent a kidney transplant, and received eculizumab as induction and maintenance therapy. Five years after transplantation, the patient remains in good general health, with stable graft function and no evidence of disease recurrence. To our knowledge, this is first reported case of an aHUS patient carrying FHR2 mutations and provides an example of a translational therapeutic approach in kidney transplantation.

A Family Affair: Addressing the Challenges of Factor H and the Related Proteins

Inflammation is a common denominator of diseases. The complement system, an intrinsic part of the innate immune system, is a key driver of inflammation in numerous disorders. Recently, a family of proteins has been suggested to be of vital importance in conditions characterized by complement dysregulation: the human Factor H (FH) family. This group of proteins consists of FH, Factor H-like protein 1 and five Factor H-related proteins. The FH family has been linked to infectious, vascular, eye, kidney and autoimmune diseases. In contrast to FH, the functions of the other highly homologous proteins are largely unknown and, hence, their role in the different disease-specific pathogenic mechanisms remains elusive. In this perspective review, we address the major challenges ahead in this emerging area, including 1) the controversies about the functional roles of the FH protein family, 2) the discrepancies in quantification of the FH protein family, 3) the unmet needs for validated tools and 4) limitations of animal models. Next, we also discuss the opportunities that exist for the immunology community. A strong multidisciplinary approach is required to solve these obstacles and is only possible through interdisciplinary collaboration between biologists, chemists, geneticists and physicians. We position this review in light of our own perspective, as principal investigators of the SciFiMed Consortium, a consortium aiming to create a comprehensive analytical system for the quantitative and functional assessment of the entire FH protein family.

Large-scale modelling as a route to multiple surface comparisons of the CCP module family

Numerous mammalian proteins are constructed from a limited repertoire of module-types. Proteins belonging to the regulators of complement activation family--crucial for ensuring a complement-mediated immune response is targeted against infectious agents--are composed solely of complement control protein (CCP) modules. In the current study, CCP module sequences were grouped to allow selection of the most appropriate experimentally determined structures to serve as templates in an automated large-scale structure modelling procedure. The resulting 135 individual CCP module models, valuable in their own right, are available at the online database http://www.bru.ed.ac.uk/~dinesh/ccp-db.html. Comparisons of surface properties within a particular family of modules should be more informative than sequence alignments alone. A comparison of surface electrostatic features was undertaken for the first 28 CCP modules of complement receptor type 1 (CR1). Assignments to clusters based on surface properties differ from assignments to clusters based on sequences. This observation might reflect adaptive evolution of surface-exposed residues involved in protein-protein interactions. This illustrative example of a multiple surface-comparison was indeed able to pinpoint functional sites in CR1.

FHR-4A: a new factor H-related protein is encoded by the human FHR-4 gene

We describe a new member of the human Factor H protein family, termed Factor H-related protein 4A (FHR-4A). The corresponding cDNA sequence was isolated and encodes a secreted protein of 559 amino acids, with a predicted molecular weight of 63.2 kDa. Apparently, this novel cDNA is derived from the human FHR-4 gene. Genetic analysis shows that the human FHR-4 gene is composed of 10 coding exons, and two distinct mRNA transcripts are derived from this gene by alternative splicing. The short FHR-4B form represents a truncated variant and encodes a secreted protein of five domains (previously termed FHR-4). The long transcript encodes the novel FHR-4A protein that is composed of nine complement control protein (CCP) domains. A unique feature of FHR-4A is the tandem arrangement of four CCP domains forming a 'natural dimer' of the short isoform. The FHR-4A protein is identified in human plasma as a 86 kDa protein. The difference between the predicted and observed molecular masses is explained by glycosylation. Comparison of the deduced protein sequence of FHR-4A with peptides from a 86 kDa apolipoprotein described by us earlier suggests that the long form, FHR-4A, represents this apoprotein. In summary, FHR-4A is a new Factor H-related protein with a unique domain composition, that is, an internal duplication of four CCP domains. To our knowledge, FHR-4A provides the first evidence for alternative splicing among Factor H-related genes.

Evolutionary relationships among proteins encoded by the regulator of complement activation gene cluster

Evolutionary relationships among members of the regulator of complement activation (RCA) gene cluster were analyzed using neighbor-joining and parsimony methods of phylogenetic tree inference. We investigated the structural and functional similarities among short consensus repeats (SCRs) of the following human proteins: the alpha chain of the C4b-binding protein (C4bpalpha), factor H (FH), factor H-related proteins (FHR-1 through FHR-4), complement receptors type 1 (CR1) and type 2 (CR2), the CR1-like protein (CR1L), membrane cofactor protein (MCP), decay accelerating factor (DAF), and the sand bass proteins, the cofactor protein (SBP1) and its homolog, the cofactor-related protein (SBCRP-1). Also included are the beta chain of the human C4b-binding protein (C4bpbeta) and the b subunit of human blood-clotting factor XIII (FXIIIb). Our results indicate that the human plasma complement regulators, FH and C4bpalpha, fall into two distinct groups on the basis of their sequence divergence. Homology among RCA proteins is in agreement with their chromosomal location, with the exception of C4bpbeta. The evolutionary relationships among individual short consensus repeats are confirmed by the exon/intron structure of the RCA members. Structural similarities among repeats of the RCA proteins correlate with their functional activities and demonstrate the importance of the N-terminal SCRs.

Complement factor H: sequence analysis of 221 kb of human genomic DNA containing the entire fH, fHR-1 and fHR-3 genes

Complement factor H (fH) is a member of a family of proteins involved in the regulation of complement activation (RCA). These proteins share a common structural motif, the Short Consensus Repeat (SCR), which is structurally conserved among related genes and between phylogenetically divergent species. fH is composed of 20 such SCRs and a variety of biological functions have been localised to specific SCR domains. The majority of individual SCRs identified are encoded by single exons, and processes such as gene conversion, duplication and exon shuffling have been implicated in the evolution and genomic radiation of SCR-encoding genes. We have analysed two GenBank sequence entries relating to two overlapping PAC clones sequenced at the Sanger Centre which contain the entire human fH gene and two adjacent fH-related (fHR) genes, fHR-1 and fHR-3. Here, we report the detailed analysis of the assembled 221 kb of contiguous, ungapped genomic sequence from human chromosome 1q32, in part employing the RUMMAGE-DP automated annotation tool. Genomic duplications involving fH and fHR exons were identified and Alu/L1 repeat dating established that the duplications occurred after the separation of rodent and primate lineages. The analysis indicates that retrotransposition as well as single and multiple exon duplication events are likely to have been involved in SCR radiation and RCA gene evolution, facilitated by conservation of splice-phasing and the single-exon, single-SCR nature of the encoded domains.

Complement factor H gene mutation associated with autosomal recessive atypical hemolytic uremic syndrome

Atypical hemolytic uremic syndrome (HUS) presents with the clinical features of hypertension, microangiopathic hemolytic anemia, and acute renal failure. Both dominant and recessive modes of inheritance have been reported. This study describes the genetic and functional analysis of a large Bedouin kindred with autosomal recessive HUS. The kindred consists of several related nuclear families in which all parent unions of affected children are consanguineous. A previous report demonstrated that a dominant form of HUS maps to chromosome 1q and that complement factor H (CFH), a regulatory component of the complement system, lies within the region and is involved in the dominant disorder. Early-onset and persistent hypocomplementemia in this Bedouin kindred prompted us to evaluate the CFH gene. Linkage analysis was performed, demonstrating linkage between the disorder and the markers near the CFH gene. Mutation analysis of the CFH coding region revealed a single missense mutation. Functional analyses demonstrate that the mutant CFH is properly expressed and synthesized but that it is not transported normally from the cell. This is the first study reporting that a recessive, atypical, early-onset, and relapsing HUS is associated with the CFH protein and that a CFH mutation affects intracellular trafficking and secretion.

The factor H protein family

The factor H gene family provides a prime example of a multidomain multifunctional protein family whose individual members are defined by conserved common structural elements and display diverse but often overlapping functions. The six identified members of this protein family represent secreted plasma proteins that are primarily synthesized in the liver. Here, we summarize the current understanding of the function of these proteins and suggest a common role in complement control. Factor H is the best characterized member and acts as a complement regulator. The protein displays cofactor activity for factor I in the degradation of the central complement component C3b, acts as a decay accelerating factor for the C3 convertase, C3bBb and is a competitor for factor B binding to C3b. Factor H is a multifunctional protein and displays functions outside the complement system: it binds to the cellular integrin receptor (CD11b/CD18), interacts with cell surface glycosaminoglycans and also binds to the surface of certain pathogenic microorganisms. In addition, factor H has several binding sites for the C3 protein. The factor H-like protein 1 (FHL-1) or reconectin shares the complement regulatory functions with factor H and interacts with heparin. The protein displays cell spreading activity and binds to the N-terminus of the streptococcal M protein. The function of the factor H-related proteins (FHR-1 to FHR-4) is currently under investigation. These proteins are differently distributed. Three proteins (FHR-1, FHR-2 and FHR-4) are constituents of lipoproteins, while FHR-3 interacts with heparin. Binding to C3b and C3d has been demonstrated for FHR-3 and FHR-4 and the two proteins display a cofactor related activity.

Human factor H deficiency. Mutations in framework cysteine residues and block in H protein secretion and intracellular catabolism

The synthesis and secretion of factor H, a regulatory protein of the complement system, were studied in skin fibroblasts from an H-deficient child who has chronic hypocomplementemic renal disease. In normal fibroblasts, factor H transcripts of 4.3 and 1.8 kilobase pairs (kb) encode a 155-kDa protein containing short consensus repeat (SCR) domains 1-20 and a 45-kDa protein which contains SCRs 1-7, respectively. The patient's fibroblasts expressed normal amounts of the 4.3- and 1.8-kb messages constitutively and after tumor necrosis factor-alpha/interferon-gamma stimulation. Lysates of [35S]methionine-labeled fibroblasts from the patient contained the 155- and 45-kDa H polypeptides, but secretion of the 155-kDa protein was blocked; the 45-kDa protein was secreted with normal kinetics. The patient's plasma lacked the 155-kDa protein but contained the small form of H. Moreover, in fibroblasts the retained 155-kDa factor H protein was not degraded, even after 12 h. Immunoflourescent staining and confocal microscopic imaging of the patient's fibroblasts indicated that factor H was retained in the endoplasmic reticulum. Sequence analysis of reverse transcription-polymerase chain reaction products (the entire coding region) and genomic DNA revealed a T1679C substitution on one allele and a G2949A substitution on the other (C518R mutation in SCR 9 and C991Y mutation in SCR 16, respectively). Both mutations affect conserved cysteine residues characteristic of SCR modules and therefore predict profound changes in the higher order structure of the 155-kDa factor H protein. These data provide the first description of a molecular mechanism for factor H deficiency and yield important insights into the normal secretory pathway for this and other plasma proteins with SCR motifs.

The human factor H-related protein 4 (FHR-4). A novel short consensus repeat-containing protein is associated with human triglyceride-rich lipoproteins

A novel apoprotein of an apparent molecular mass of 86 kDa in its unreduced form was identified in human triglyceride-rich lipoproteins. This protein was purified and the amino acid sequence of six proteolytic fragments was found to overlap with that of the factor H-related proteins. In parallel we identified the cDNA encoding a new complement factor H-related protein, termed FHR-4. The sequences of the new apoprotein overlapped with that of the FHR-4 protein. Similar to the previously described factor H-related proteins, FHR-4 contains a hydrophobic signal sequence followed by a stretch of five repetitive elements termed short consensus repeats. Recombinant FHR-4 protein was expressed in the baculovirus system and has an apparent molecular mass of 42 kDa. In addition a 84-kDa dimeric form of the recombinant FHR-4 was detected. Using an immunoaffinity column with antibodies raised against the recombinant FHR-4, we isolated a 86-kDa protein from human plasma. The different molecular mass of the recombinant FHR-4 and the dimeric FHR-4 in plasma is due to different carbohydrate moieties. The 86-kDa plasma protein and the novel apolipoprotein had identical mobility on SDS-polyacrylamide gel electrophoresis analysis and reacted with antisera raised against the reFHR-4 and the purified apoprotein. In conclusion, we have identified a novel factor H-related protein, FHR-4, in human plasma and demonstrate that this protein is present in triglyceride-rich lipoproteins in a dimeric form. This observation provides an intriguing new aspect on possible function(s) of this novel protein and the other factor H-related proteins.