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

Levels of complement factor H-related 4 protein do not influence susceptibility to age-related macular degeneration or its course of progression

Dysregulation of the alternative pathway (AP) of the complement system is a significant contributor to age-related macular degeneration (AMD), a primary cause of irreversible vision loss worldwide. Here, we assess the contribution of the liver-produced complement factor H-related 4 protein (FHR-4) to AMD initiation and course of progression. We show that FHR-4 variation in plasma and at the primary location of AMD-associated pathology, the retinal pigment epithelium/Bruch's membrane/choroid interface, is entirely explained by three independent quantitative trait loci (QTL). Using two distinct cohorts composed of a combined 14,965 controls and 20,741 cases, we ascertain that independent QTLs for FHR-4 are distinct from variants causally associated with AMD, and that FHR-4 variation is not independently associated with disease. Additionally, FHR-4 does not appear to influence AMD progression course among patients with disease driven predominantly by AP dysregulation. Modulation of FHR-4 is therefore unlikely to be an effective therapeutic strategy for AMD.

The echinoid complement system inferred from genome sequence searches

The vertebrate complement cascade is an essential host protection system that functions at the intersection of adaptive and innate immunity. However, it was originally assumed that complement was present only in vertebrates because it was activated by antibodies and functioned with adaptive immunity. Subsequently, the identification of the key component, SpC3, in sea urchins plus a wide range of other invertebrates significantly expanded the concepts of how complement functions. Because there are few reports on the echinoid complement system, an alternative approach to identify complement components in echinoderms is to search the deduced proteins encoded in the genomes. This approach identified known and putative members of the lectin and alternative activation pathways, but members of the terminal pathway are absent. Several types of complement receptors are encoded in the genomes. Complement regulatory proteins composed of complement control protein (CCP) modules are identified that may control the activation pathways and the convertases. Other regulatory proteins without CCP modules are also identified, however regulators of the terminal pathway are absent. The expansion of genes encoding proteins with Macpf domains is noteworthy because this domain is a signature of perforin and proteins in the terminal pathway. The results suggest that the major functions of the echinoid complement system are detection of foreign targets by the proteins that initiate the activation pathways resulting in opsonization by SpC3b fragments to augment phagocytosis and destruction of the foreign targets by the immune cells.

The Factor H protein family: The switchers of the complement alternative pathway

The factor H (FH) protein family is emerging as a complex network of proteins controlling the fate of the complement alternative pathway (AP) and dictating susceptibility to a wide range of diseases including infectious, inflammatory, autoimmune, and degenerative diseases and cancer. Composed, in man, of seven highly related proteins, FH, factor H-like 1, and 5 factor H-related proteins, some of the FH family proteins are devoted to down-regulating the AP, while others exert an opposite function by promoting AP activation. Recent findings have provided insights into the molecular mechanisms defining their biological roles and their pathogenicity, illustrating the relevance that the balance between the regulators and the activators within this protein family has in defining the outcome of complement activation on cell surfaces. In this review we will discuss the emerging roles of the factor H protein family, their impact in the complement cascade, and their involvement in the pathogenesis of complement-mediated diseases associated with the AP dysregulation.

Identification of CFHR4 associated with poor prognosis of hepatocellular carcinoma

Background

Hepatocellular carcinoma (HCC) is one of the most leading causes of cancer death worldwide. The 5-year survival rate of HCC patients remains low due to the lack of early-stage symptoms. Human complement factor H-related protein 4 (CFHR4) is a critical gene that belongs to the factor H family of plasma glycoproteins, which has not been linked to HCC development. The correlations between CFHR4 and prognosis and tumor-infiltrating lymphocytes in HCC are yet unknown. The present study demonstrated the involvement of CFHR4 in HCC via data mining approaches.

Results

A total of 18 upregulated and 67 down-regulated differentially expressed genes (DEGs) were identified. Importantly, CFHR4, which was screened from DEGs, was shown to express at a lower level in HCC tumor tissue than normal tissues. Western blotting (WB), immunohistochemical (IHC) and quantitative reverse transcription PCR (qRT-PCR) experiments of clinical samples further validated CFHR4 was aberrantly expressed in HCC patients; Data from TCGA showed that CFHR4 was inversely correlated with a cancer family history, histological grade, tumor node metastasis (TNM) stage, and serum AFP level of HCC patients; Univariate and multivariate analyses revealed that low expression of CFHR4 was an independent predictive marker in patients with HCC; Kaplan-Meier analysis showed that the lower expression of CFHR4 was significantly associated with the progression of HCC and poor prognosis rates. Furthermore, TIMER analysis indicated that CFHR4 expression levels had correlations with infiltrating levels of immune cells in HCC.

Conclusion

CFHR4 expression was low in HCC and was significantly related to the poor prognosis of HCC and the level of immune infiltration. CFHR4 played important roles in regulating the initiation and progression of HCC and could be a potential biomarker for the diagnosis and prognosis of HCC.

Methods

The expression of CFHR4 was analyzed by GEO and TCGA-LIHC database and verified by WB and IHC assay. The biological function of CFHR4 was performed by GO and KEGG enrichment analysis, and the genomic alteration of CFHR4 was investigated by cBioPortal database.The correlation between CFHR4 expression and clinical relevance was evaluated through Cox proportional hazards model, and the correlation between CFHR4 expression and tumor immune infiltrates were studied by TIMER database.

Pathogenic Neisseria Bind the Complement Protein CFHR5 via Outer Membrane Porins

Neisseria meningitidis and Neisseria gonorrhoeae are important human pathogens that have evolved to bind the major negative regulator of the complement system, complement factor H (CFH). However, little is known about the interaction of pathogens with CFH-related proteins (CFHRs) which are structurally similar to CFH but lack the main complement regulatory domains found in CFH. Insights into the role of CFHRs have been hampered by a lack of specific reagents. We generated a panel of CFHR-specific monoclonal antibodies and demonstrated that CFHR5 was bound by both pathogenic Neisseria spp. We showed that CFHR5 bound to PorB expressed by both pathogens in the presence of sialylated lipopolysaccharide and enhanced complement activation on the surface of N. gonorrhoeae. Our study furthered our understanding of the interactions of CFHRs with bacterial pathogens and revealed that CFHR5 bound the meningococcus and gonococcus via similar mechanisms.

Contribution of functional and quantitative genetic variants of Complement Factor H and Factor H-Related (FHR) proteins on renal pathology

The complement system is a first line of defence against infectious, tumoral or autoimmune processes, and it is constitutively regulated to avoid excessive or unspecific activation. Factor H (FH), a most relevant complement regulator, controls complement activation in plasma and on the cellular surfaces of autologous tissues. FH shares evolutionary origin and structural features with a group of plasma proteins known as FH-Related Proteins (FHRs), which could act as FH functional antagonists. Studies in patient cohorts of atypical Haemolytic-Uraemic Syndrome (aHUS), C3 Glomerulopathy (C3G), and IgA nephropathy (IgAN), have identified rare genetic variants that give rise to severe FH and FHRs dysfunctions, and are major genetic predisposing factors. These patients also have a higher frequency of a few polymorphisms whose relevance as disease risk factors is incompletely understood. In the last years, the availability of specific reagents has allowed a more precise quantitation of FH and FHRs in plasma samples from patients and controls. These studies have revealed that some aHUS, C3G or IgAN risk polymorphisms determine mild changes in FH or FHRs levels that could somehow perturb complement regulation and favour disease pathogenesis.

Contribution of functional and quantitative genetic variants of Complement Factor H and Factor H-Related (FHR) proteins on renal pathology

The complement system is a first line of defence against infectious, tumoral or autoimmune processes, and it is constitutively regulated to avoid excessive or unspecific activation. Factor H (FH), a most relevant complement regulator, controls complement activation in plasma and on the cellular surfaces of autologous tissues. FH shares evolutionary origin and structural features with a group of plasma proteins known as FH-Related Proteins (FHRs), which could act as FH functional antagonists. Studies in patient cohorts of atypical Haemolytic-Uraemic Syndrome (aHUS), C3 Glomerulopathy (C3G), and IgA nephropathy (IgAN), have identified rare genetic variants that give rise to severe FH and FHRs dysfunctions, and are major genetic predisposing factors. These patients also have a higher frequency of a few polymorphisms whose relevance as disease risk factors is incompletely understood. In the last years, the availability of specific reagents has allowed a more precise quantitation of FH and FHRs in plasma samples from patients and controls. These studies have revealed that some aHUS, C3G or IgAN risk polymorphisms determine mild changes in FH or FHRs levels that could somehow perturb complement regulation and favour disease pathogenesis.

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.

Regulation of regulators: Role of the complement factor H-related proteins

The complement system, while being an essential and very efficient effector component of innate immunity, may cause damage to the host and result in various inflammatory, autoimmune and infectious diseases or cancer, when it is improperly activated or regulated. Factor H is a serum glycoprotein and the main regulator of the activity of the alternative complement pathway. Factor H, together with its splice variant factor H-like protein 1 (FHL-1), inhibits complement activation at the level of the central complement component C3 and beyond. In humans, there are also five factor H-related (FHR) proteins, whose function is poorly characterized. While data indicate complement inhibiting activity for some of the FHRs, there is increasing evidence that FHRs have an opposite role compared with factor H and FHL-1, namely, they enhance complement activation directly and also by competing with the regulators FH and FHL-1. This review summarizes the current stand and recent data on the roles of factor H family proteins in health and disease, with focus on the function of FHR proteins.

Recombinant Production of MFHR1, A Novel Synthetic Multitarget Complement Inhibitor, in Moss Bioreactors

The human complement system is an important part of the immune system responsible for lysis and elimination of invading microorganisms and apoptotic body cells. Improper activation of the system due to deficiency, mutations, or autoantibodies of complement regulators, mainly factor H (FH) and FH-related proteins (FHRs), causes severe kidney and eye diseases. However, there is no recombinant FH therapeutic available on the market. The first successful recombinant production of FH was accomplished with the moss bioreactor, Physcomitrella patens. Recently, a synthetic regulator, MFHR1, was designed to generate a multitarget complement inhibitor that combines the activities of FH and the FH-related protein 1 (FHR1). The potential of MFHR1 was demonstrated in a proof-of-concept study with transiently transfected insect cells. Here, we present the stable production of recombinant glyco-engineered MFHR1 in the moss bioreactor. The key features of this system are precise genome engineering via homologous recombination, Good Manufacturing Practice-compliant production in photobioreactors, high batch-to-batch reproducibility, and product stability. Several potential biopharmaceuticals are being produced in this system. In some cases, these are even biobetters, i.e., the recombinant proteins produced in moss have a superior quality compared to their counterparts from mammalian systems as for example moss-made aGal, which successfully passed phase I clinical trials. Via mass spectrometry-based analysis of moss-produced MFHR1, we now prove the correct synthesis and modification of this glycoprotein with predominantly complex-type N-glycan attachment. Moss-produced MFHR1 exhibits cofactor and decay acceleration activities comparable to FH, and its mechanism of action on multiple levels within the alternative pathway of complement activation led to a strong inhibitory activity on the whole alternative pathway, which was higher than with the physiological regulator FH.

Self-Damage Caused by Dysregulation of the Complement Alternative Pathway: Relevance of the Factor H Protein Family

The alternative pathway is a continuously active surveillance arm of the complement system, and it can also enhance complement activation initiated by the classical and the lectin pathways. Various membrane-bound and plasma regulatory proteins control the activation of the potentially deleterious complement system. Among the regulators, the plasma glycoprotein factor H (FH) is the main inhibitor of the alternative pathway and its powerful amplification loop. FH belongs to a protein family that also includes FH-like protein 1 and five factor H-related (FHR-1 to FHR-5) proteins. Genetic variants and abnormal rearrangements involving the FH protein family have been linked to numerous systemic and organ-specific diseases, including age-related macular degeneration, and the renal pathologies atypical hemolytic uremic syndrome, C3 glomerulopathies, and IgA nephropathy. This review covers the known and recently emerged ligands and interactions of the human FH family proteins associated with disease and discuss the very recent experimental data that suggest FH-antagonistic and complement-activating functions for the FHR proteins.

Complement Factor H-Related Protein 4A Is the Dominant Circulating Splice Variant of CFHR4

Recent research has elucidated circulating levels of almost all factor H-related (FHR) proteins. Some of these proteins are hypothesized to act as antagonists of the important complement regulator factor H (FH), fine-tuning complement regulation on human surfaces. For the CFHR4 splice variants FHR-4A and FHR-4B, the individual circulating levels are unknown, with only total levels being described. Specific reagents for FHR-4A or FHR-4B are lacking due to the fact that the unique domains in FHR-4A show high sequence similarity with FHR-4B, making it challenging to distinguish them. We developed an assay that specifically measures FHR-4A using novel, well-characterized monoclonal antibodies (mAbs) that target unique domains in FHR-4A only. Using various FHR-4A/FHR-4B-specific mAbs, no FHR-4B was identified in any of the serum samples tested. The results demonstrate that FHR-4A is the dominant splice variant of CFHR4 in the circulation, while casting doubt on the presence of FHR-4B. FHR-4A levels (avg. 2.55 ± 1.46 µg/mL) were within the range of most of the previously reported levels for all other FHRs. FHR-4A was found to be highly variable among the population, suggesting a strong genetic regulation. These results shed light on the physiological relevance of the previously proposed role of FHR-4A and FHR-4B as antagonists of FH in the circulation.

The Murine Factor H-Related Protein FHR-B Promotes Complement Activation

Factor H-related (FHR) proteins consist of varying number of complement control protein domains that display various degrees of sequence identity to respective domains of the alternative pathway complement inhibitor factor H (FH). While such FHR proteins are described in several species, only human FHRs were functionally investigated. Their biological role is still poorly understood and in part controversial. Recent studies on some of the human FHRs strongly suggest a role for FHRs in enhancing complement activation via competing with FH for binding to certain ligands and surfaces. The aim of the current study was the functional characterization of a murine FHR, FHR-B. To this end, FHR-B was expressed in recombinant form. Recombinant FHR-B bound to human C3b and was able to compete with human FH for C3b binding. FHR-B supported the assembly of functionally active C3bBb alternative pathway C3 convertase via its interaction with C3b. This activity was confirmed by demonstrating C3 activation in murine serum. In addition, FHR-B bound to murine pentraxin 3 (PTX3), and this interaction resulted in murine C3 fragment deposition due to enhanced complement activation in mouse serum. FHR-B also induced C3 deposition on C-reactive protein, the extracellular matrix (ECM) extract Matrigel, and endothelial cell-derived ECM when exposed to mouse serum. Moreover, mouse C3 deposition was strongly enhanced on necrotic Jurkat T cells and the mouse B cell line A20 by FHR-B. FHR-B also induced lysis of sheep erythrocytes when incubated in mouse serum with FHR-B added in excess. Altogether, these data demonstrate that, similar to human FHR-1 and FHR-5, mouse FHR-B modulates complement activity by promoting complement activation via interaction with C3b and via competition with murine FH.

The complement factor H-related proteins

The role of the complement factor H-related (FHR) proteins in homeostasis, pathogen defense, and autoimmune disease has recently attracted considerable interest. We highlight the exciting research that has contributed to our understanding of the FHR protein family. Unlike factor H, a potent negative regulator of complement C3 activation, the FHR proteins appear to promote C3 activation. These data have important implications for understanding complement-mediated diseases because, depending on the context, the balance between the actions of factor H and the FHR proteins determines the degree of complement activation.

Factor H Family Proteins in Complement Evasion of Microorganisms

Human-pathogenic microbes possess various means to avoid destruction by our immune system. These include interactions with the host complement system that may facilitate pathogen entry into cells and tissues, expression of molecules that defuse the effector complement components and complexes, and acquisition of host complement inhibitors to downregulate complement activity on the surface of the pathogen. A growing number of pathogenic microorganisms have acquired the ability to bind the complement inhibitor factor H (FH) from body fluids and thus hijack its host protecting function. In addition to FH, binding of FH-related (FHR) proteins was also demonstrated for several microbes. Initial studies assumed that these proteins are complement inhibitors similar to FH. However, recent evidence suggests that FHR proteins may rather enhance complement activation both directly and also by competing with the inhibitor FH for binding to certain ligands and surfaces. This mini review focuses on the role of the main alternative pathway regulator FH in host–pathogen interactions, as well as on the emerging role of the FHR proteins as enhancers of complement activation.

Successful therapy of C3Nef-positive C3 glomerulopathy with plasma therapy and immunosuppression

BACKGROUND

C3 glomerulopathies (C3G) are characterized by uncontrolled activation of the alternative pathway of complement. In most patients these diseases progress towards end-stage renal disease, and the risk of recurrence after renal transplantation is high. In the majority of patients, only antibodies against the C3 convertase, termed C3Nef, can be found as a potential pathogenic factor. Although a large variety of therapeutic approaches have been used, no generally accepted therapy exists.

METHODS

In four consecutive patients with C3G in whom all known complement factor mutations were excluded and only C3Nef could be identified as a potential cause of disease, a multimodal therapeutic regimen with plasma therapy, corticosteroids and mycophenolate mofetil was used.

RESULTS

The multimodal regimen achieved normalization of renal function in all four patients, with complete remission in two patients and a distinct reduction of proteinuria in the other two patients. The single patient with C3 glomerulonephritis (C3GN) and marked terminal complement complex elevation only showed partial remission; further improvement was achieved following the addition of eculizumab to the therapeutic regimen. Repeatedly measured C3Nef levels did not correlate with disease course or therapeutic response in any of the patients.

CONCLUSIONS

As this multimodal therapeutic approach was effective in all four treated patients with suspected autoimmune etiology of C3G, it offers a treatment option for severely affected patients with this rare disease until more specific regimens are available.

Role of Factor H and Related Proteins in Regulating Complement Activation in the Macula, and Relevance to Age-Related Macular Degeneration

The recent revolution in age-related macular degeneration (AMD) genetics has demonstrated that genetic alterations affecting the alternative pathway of the complement cascade have a major influence on AMD risk. One of the two most important genetic loci is on chromosome 1 and contains genes encoding complement factor H (FH) and the factor H related proteins (FHR proteins). In macular tissue, especially Bruch’s membrane, relatively high levels of a truncated splice variant of FH called factor H-like protein 1 (FHL-1) are present. Here we discuss how genetic variations may alter the amounts, or by altering their protein sequences, the functions of these proteins. In particular, the common Y402H polymorphism affects the ability of FHL-1 and FH to localize to Bruch’s membrane and the inner choroid because it alters the ability of these complement regulators to bind heparan sulphate (HS) in these structures. In addition, there is an age-related loss of HS from Bruch’s membrane. We hypothesize that a combination of poor binding of the 402H variants of FHL-1 and FH to Bruch’s membrane, combined with a decrease in binding due to age-related HS loss, eventually results in insufficient FHL-1 and FH binding to Bruch’s membrane. This could result in complement activation, inflammation and thereby predispose to AMD.

Of mice and men: The factor H protein family and complement regulation

For decades immunological research has relied, with variable success, on mouse models to investigate diseases and possible therapeutic interventions. With the approval of the first therapeutic antibody targeting complement, called eculizumab, as therapy in paroxysmal nocturnal hemoglobinuria (PNH) and more recently atypical hemolytic uremic syndrome (aHUS), the viability of targeting the complement system was demonstrated. The potent, endogenous complement regulators have become of increasing interest as templates for designing and developing new therapeutics. Recently, complement inhibitors based on (parts of) the human complement regulator factor H (FH) are being examined for therapeutic intervention in inflammatory conditions. The first step to evaluate the potency of a new drug is often testing it in a mouse model for the target disease. However, translating results to human conditions requires a good understanding of similarities and, more importantly, differences between the human and mouse complement system and particularly regulation. This review will provide a comprehensive overview of the complement regulator FH and its closely related proteins and current views on their role in mice and men.

Factor H-related proteins

Factor H-related proteins (CFHRs) are plasma glycoproteins related in structure and antigenicity to each other and to the complement inhibitory protein factor H. Such proteins are found in most mammals but their number and domain composition vary. This chapter summarizes our current knowledge on the human factor H-related proteins. In contrast to factor H, they have no strong complement inhibitory activity, although for some of them regulatory or complement modulatory activity has been reported. A common feature of CFHRs is that they bind to the C3b component of complement. Novel links between CFHRs and various diseases (C3 glomerulopathies, atypical hemolytic uremic syndrome and age-related macular degeneration) have been revealed in recent years, but we are still far from understanding their biological function.

Complement factor H related proteins (CFHRs)

Factor H related proteins comprise a group of five plasma proteins: CFHR1, CFHR2, CFHR3, CFHR4 and CFHR5, and each member of this group binds to the central complement component C3b. Mutations, genetic deletions, duplications or rearrangements in the individual CFHR genes are associated with a number of diseases including atypical hemolytic uremic syndrome (aHUS), C3 glomerulopathies (C3 glomerulonephritis (C3GN), dense deposit disease (DDD) and CFHR5 nephropathy), IgA nephropathy, age related macular degeneration (AMD) and systemic lupus erythematosus (SLE). Although complement regulatory functions were attributed to most of the members of the CFHR protein family, the precise role of each CFHR protein in complement activation and the exact contribution to disease pathology is still unclear. Recent publications show that CFHR proteins form homo- as well as heterodimers. Genetic abnormalities within the CFHR gene locus can result in hybrid proteins with affected dimerization or recognition domains which cause defective functions. Here we summarize the recent data about CFHR genes and proteins in order to better understand the role of CFHR proteins in complement activation and in complement associated diseases.

Function of alternative splicing

Almost all polymerase II transcripts undergo alternative pre-mRNA splicing. Here, we review the functions of alternative splicing events that have been experimentally determined. The overall function of alternative splicing is to increase the diversity of mRNAs expressed from the genome. Alternative splicing changes proteins encoded by mRNAs, which has profound functional effects. Experimental analysis of these protein isoforms showed that alternative splicing regulates binding between proteins, between proteins and nucleic acids as well as between proteins and membranes. Alternative splicing regulates the localization of proteins, their enzymatic properties and their interaction with ligands. In most cases, changes caused by individual splicing isoforms are small. However, cells typically coordinate numerous changes in 'splicing programs', which can have strong effects on cell proliferation, cell survival and properties of the nervous system. Due to its widespread usage and molecular versatility, alternative splicing emerges as a central element in gene regulation that interferes with almost every biological function analyzed.

Factor H-related protein 4 activates complement by serving as a platform for the assembly of alternative pathway C3 convertase via its interaction with C3b protein

Human complement factor H-related protein (CFHR) 4 belongs to the factor H family of plasma glycoproteins that are composed of short consensus repeat (SCR) domains. Although factor H is a well known inhibitor of the alternative complement pathway, the functions of the CFHR proteins are poorly understood. CFHR4 lacks SCRs homologous to the complement inhibitory domains of factor H and, accordingly, has no significant complement regulatory activities. We have previously shown that CFHR4 binds C-reactive protein via its most N-terminal SCR, which leads to classical complement pathway activation. CFHR4 binds C3b via its C terminus, but the significance of this interaction is unclear. Therefore, we set out to clarify the functional relevance of C3b binding by CFHR4. Here, we report a novel role for CFHR4 in the complement system. CFHR4 serves as a platform for the assembly of an alternative pathway C3 convertase by binding C3b. This is based on the sustained ability of CFHR4-bound C3b to bind factor B and properdin, leading to an active convertase that generates C3a and C3b from C3. The CFHR4-C3bBb convertase is less sensitive to the factor H-mediated decay compared with the C3bBb convertase. CFHR4 mutants containing exchanges of conserved residues within the C-terminal C3b-binding site showed significantly reduced C3b binding and alternative pathway complement activation. In conclusion, our results suggest that, in contrast to the complement inhibitor factor H, CFHR4 acts as an enhancer of opsonization by promoting complement activation.

Age-related macular degeneration: genetic and clinical findings

Age-related macular degeneration (AMD) is a sight threatening eye disease that affects millions of humans over the age of 65 years. It is considered to be the major cause of irreversible blindness in the elderly population in the developed world. The disease is prevalent in Europe and the United States, which has a large number of individuals of European descent. AMD is characterized by a progressive loss of central vision attributable to degenerative and neovascular changes that occur in the interface between the neural retina and the underlying choroid. This location contains the retinal photoreceptors, the retinal pigmented epithelium, a basement membrane complex known as Bruch's membrane and a network of choroidal capillaries. AMD is increasingly recognized as a complex genetic disorder where one or more genes contribute to an individual's susceptibility to development of the condition, while the prevailing view is that the disease stems from the interaction of multiple genetic and environmental factors. Although it has been proposed that a threshold event occurs during normal aging, the sequelae of biochemical, cellular, and molecular events leading to AMD are not fully understood. Here, we review the clinical aspects of AMD and summarize the genes which have been reported to have a positive association with the disease.

Interactions between Bordetella pertussis and the complement inhibitor factor H

Bordetella pertussis causes whooping cough in humans, a highly contagious disease of the upper respiratory tract. An increase in cases of whooping cough in adolescents and adults in many countries has been reported, despite high immunization rates in children. To efficiently colonize the host the bacteria have to resist complement, the first defence line of innate immunity. B. pertussis has previously been shown to bind the classical pathway inhibitors C4b-binding protein and C1-inhibitor being thereby able to escape the classical pathway of complement. In this study recent clinical isolates of B. pertussis and B. parapertussis were found to survive alternative pathway attack in fresh non-immune serum better than the reference B. pertussis strain, Tohama I. By using adsorption assays, flow cytometry and a radioligand binding assay we observed that both B. pertussis and B. parapertussis bound the alternative pathway inhibitor factor H (FH) from normal human serum. The surface attached FH maintained its complement regulatory activity and promoted factor I-mediated cleavage of C3b. The main binding region was located to the C-terminal part of FH, into short consensus repeat domains 19-20. In contrast, the avian pathogen B. avium did not bind FH and was sensitive to the alternative pathway of human complement. In conclusion, the human pathogens B. pertussis and B. parapertussis are able to evade the alternative complement pathway by surface acquisition of the host complement regulator FH.

DEAP-HUS: deficiency of CFHR plasma proteins and autoantibody-positive form of hemolytic uremic syndrome

DEAP-HUS [Deficiency of CFHR (complement factor H-related) plasma proteins and Autoantibody Positive form of Hemolytic Uremic Syndrome] represents a novel subtype of hemolytic uremic syndrome (HUS) with unique characteristics. It affects children and requires special clinical attention in terms of diagnosis and therapy. DEAP-HUS and other atypical forms of HUS share common features, such as microangiopathic hemolytic anemia, acute renal failure, and thrombocytopenia. However, DEAP-HUS has the unique combination of an acquired factor in the form of autoantibodies to the complement inhibitor Factor H and a genetic factor which, in most cases, is the chromosomal deletion of a 84-kbp fragment within human chromosome 1 that results in the absence of the CFHR1 and CFHR3 proteins in plasma. Special attention is required to diagnose and treat DEAP-HUS patients. Most patients show a favorable response to the reduction of autoantibody titers by either plasma therapy, steroid treatment, and/or immunosuppression. In addition, in those DEAP-HUS patients with end-stage renal disease, the reduction of autoantibody titers prior to transplantation is expected to prevent post-transplant disease recurrence by aiming for full complement control at the endothelial cell surface in order to minimize adverse complement and immune reactions.

Characterization of complement factor H-related (CFHR) proteins in plasma reveals novel genetic variations of CFHR1 associated with atypical hemolytic uremic syndrome

The factor H-related protein family (CFHR) is a group of minor plasma proteins genetically and structurally related to complement factor H (fH). Notably, deficiency of CFHR1/CFHR3 associates with protection against age-related macular degeneration and with the presence of anti-fH autoantibodies in atypical hemolytic uremic syndrome (aHUS). We have developed a proteomics strategy to analyze the CFHR proteins in plasma samples from controls, patients with aHUS, and patients with type II membranoproliferative glomerulonephritis. Here, we report on the identification of persons carrying novel deficiencies of CFHR1, CFHR3, and CFHR1/CFHR4A, resulting from point mutations in CFHR1 and CFHR3 or from a rearrangement involving CFHR1 and CFHR4. Remarkably, patients with aHUS lacking CFHR1, but not those lacking CFHR3, present anti-fH autoantibodies, suggesting that generation of these antibodies is specifically related to CFHR1 deficiency. We also report the characterization of a novel CFHR1 polymorphism, resulting from a gene conversion event between CFH and CFHR1, which strongly associates with aHUS. The risk allotype CFHR1*B, with greater sequence similarity to fH, may compete with fH, decreasing protection of cellular surfaces against complement damage. In summary, our comprehensive analyses of the CFHR proteins have improved our understanding of these proteins and provided further insights into aHUS pathogenesis.

Factor H family proteins and human diseases

Complement is a major defense system of innate immunity and aimed to destroy microbes. One of the central complement regulators is factor H, which belongs to a protein family that includes CFHL1 and five factor H-related (CFHR) proteins. Recent evidence shows that factor H family proteins (factor H and CFHRs) are associated with diverse and severe human diseases and are also used by human pathogenic microbes for complement evasion. Therefore, dissecting the exact functions of the individual CFHR proteins will provide insights into the pathophysiology of such inflammatory and infectious diseases and will define the therapeutic potential of these proteins.

Hemolytic Uremic Syndrome: A Factor H Mutation (E1172Stop) Causes Defective Complement Control at the Surface of Endothelial Cells

Defective complement regulation results in hemolytic uremic syndrome (HUS), a disease that is characterized by microangiopathy, thrombocytopenia, and acute renal failure and that causes endothelial cell damage. For characterization of how defective complement regulation relates to the pathophysiology, the role of the complement regulator factor H and also of a mutant factor H protein was studied on the surface of human umbilical vein endothelial cells. The mutant 145-kD factor H protein was purified to homogeneity, from plasma of a patient with HUS, who is heterozygous for a factor H gene mutation G3587T, which introduces a stop codon at position 1172. Functional analyses show that the lack of the most C-terminal domain short consensus repeats 20 severely affected recognition functions (i.e., binding to heparin, C3b, C3d, and the surface of endothelial cells). Wild-type factor H as well as the mutant protein formed dimers in solution as shown by cross-linking studies and mass spectroscopy. When assayed in fluid phase, the complement regulatory activity of the mutant protein was normal and comparable to wild-type factor H. However, on the surface of endothelial cells, the mutant factor H protein showed severely reduced regulatory activities and lacked protective functions. Similarly, with the use of sheep erythrocytes, the mutant protein lacked the protective activity and caused increased hemolysis when it was added to factor H-depleted plasma. This study shows how a mutation that affects the C-terminal region of the factor H protein leads to defective complement control on cell surfaces and damage to endothelial cells in patients with HUS. These effects explain how mutant factor H causes defective complement control and in HUS-particularly under condition of inflammation and complement activation-causes endothelial cell damage.

Two factor H-related proteins from the mouse: expression analysis and functional characterization

Complement factor H-related (FHR) proteins display structural and functional similarities to each other and to the complement regulator factor H (FH). FHRs have been identified in various species, including human, rat, and the fish barred sand bass. As mice provide a useful model system to study the physiological role of FHRs in vivo, we aimed at characterizing murine FHR proteins. Two putative FHRs of approximately 100 and 38 kDa were detected in mouse plasma using FH-specific antiserum. In a liver cDNA library, three murine FHR-encoding transcripts were identified. Two clones code for related FHR proteins termed FHR-C and FHR-C_v1, which in secreted form are composed of 14 and 13 short consensus repeat (SCR) domains, homologous to SCRs 6-17 and 19-20 of FH. The third transcript, FHR-B, is derived from a separate gene and codes for a secreted protein composed of five SCR domains. FHR-B displays homology to SCRs 5-7 and 19-20 of FH. Expression of FHR-B in various tissues was analyzed by real-time polymerase chain reaction and was identified at high levels in liver, kidney and heart. In liver, FHR-B transcript level was even higher than that of FH. In addition, FHR-B was expressed as a recombinant 37-kDa protein, and this recombinant FHR-B interacted with the ligands heparin and human C3b. Using mouse plasma, the native presumptive FHR proteins were also analyzed in binding assays. In summary, we identify two FHR proteins in mice and for the first time characterize a murine FHR as a heparin- and C3b-binding protein.

A common CFH haplotype, with deletion of CFHR1 and CFHR3, is associated with lower risk of age-related macular degeneration

Age-related macular degeneration (AMD; OMIM #603075) is the most frequent cause of visual impairment in the elderly population, with severe disease affecting nearly 10% of individuals of European descent over the age of 75 years. It is a complex disease in which genetic and environmental factors contribute to susceptibility. Complement factor H (CFH) has recently been identified as a major AMD susceptibility gene, and the Y402H polymorphism has been proposed as the likely causative factor. We genotyped polymorphisms spanning the cluster of CFH and five CFH-related genes on chromosome 1q23 in 173 individuals with severe neovascular AMD and 170 elderly controls with no signs of AMD. Detailed analysis showed a common haplotype associated with decreased risk of AMD that was present on 20% of chromosomes of controls and 8% of chromosomes of individuals with AMD. We found that this haplotype carried a deletion of CFHR1 and CFHR3, and the proteins encoded by these genes were absent in serum of homozygotes. The protective effect of the deletion haplotype cannot be attributed to linkage disequilibrium with Y402H and was replicated in an independent sample.

Human Factor H-Related Protein 5 Has Cofactor Activity, Inhibits C3 Convertase Activity, Binds Heparin and C-Reactive Protein, and Associates with Lipoprotein

Factor H-related protein 5 (FHR-5) is a recently discovered member of the factor H (fH)-related protein family. FHR proteins are structurally similar to the complement regulator fH, but their biological functions remain poorly defined. FHR-5 is synthesized in the liver and consists of 9 short consensus repeats (SCRs), which display various degrees of homology to those of fH and the other FHR proteins. FHR-5 colocalizes with complement deposits in vivo and binds C3b in vitro, suggesting a role in complement regulation or localization. The current study examined whether rFHR-5 exhibits properties similar to those of fH, including heparin binding, CRP binding, cofactor activity for the factor I-mediated degradation of C3b and decay acceleration of the C3 convertase. rFHR-5 bound heparin-BSA and heparin-agarose and a defined series of truncations expressed in Pichia pastoris localized the heparin-binding region to within SCRs 5-7. rFHR-5 bound CRP, and this binding was also localized to SCRs 5-7. FHR-5 inhibited alternative pathway C3 convertase activity in a fluid phase assay; however, dissociation of the convertase was not observed in a solid phase assay. rFHR-5 displayed factor I-dependent cofactor activity for C3b cleavage, although it was apparently less effective than fH. In addition, we demonstrate association of FHR-5 with high density lipid lipoprotein complexes in human plasma. These results demonstrate that FHR-5 shares properties of heparin and CRP binding and lipoprotein association with one or more of the other FHRs but is unique among this family of proteins in possessing independent complement-regulatory activity.

Membranoproliferative glomerulonephritis type II (dense deposit disease): an update

Membranoproliferative glomerulonephritis type II (MPGN II) is a rare disease characterized by the deposition of abnormal electron-dense material within the glomerular basement membrane of the kidney and often within Bruch's membrane in the eye. The diagnosis is made in most patients between the ages of 5 and 15 yr, and within 10 yr, approximately half progress to end-stage renal disease, occasionally with the late comorbidity of visual impairment. The pathophysiologic basis of MPGN II is associated with the uncontrolled systemic activation of the alternative pathway (AP) of the complement cascade. In most patients, loss of complement regulation is caused by C3 nephritic factor, an autoantibody directed against the C3 convertase of the AP, but in some patients, mutations in the factor H gene have been identified. For the latter patients, plasma replacement therapy prevents renal failure, but for the majority of patients, there is no proven effective treatment. The disease recurs in virtually all renal allografts, and a high percentage of these ultimately fail. The development of molecular diagnostic tools and new therapies directed at controlling the AP of the complement cascade either locally in the kidney or at the systemic level may lead to effective treatments for MPGN II.