New alleles of C4-binding protein and factor H and further linkage data in the regulator of complement activation (RCA) gene cluster in man

The Chicken A and E Blood Systems Arise from Genetic Variation in and around the Regulators of Complement Activation Region

The tightly linked A and E blood alloantigen systems are 2 of 13 blood systems identified in chickens. Reported herein are studies showing that the genes encoding A and E alloantigens map within or near to the chicken regulator of complement activation (RCA) gene cluster, a region syntenic with the human RCA. Genome-wide association studies, sequence analysis, and sequence-derived single-nucleotide polymorphism information for known A and/or E system alleles show that the most likely candidate gene for the A blood system is C4BPM gene (complement component 4 binding protein, membrane). Cosegregation of single-nucleotide polymorphism–defined C4BPM haplotypes and blood system A alleles defined by alloantisera provide a link between chicken blood system A and C4BPM. The best match for the E blood system is the avian equivalent of FCAMR (Fc fragment of IgA and IgM receptor). C4BPM is located within the chicken RCA on chicken microchromosome 26 and is separated from FCAMR by 89 kbp. The genetic variation observed at C4BPM and FCAMR could affect the chicken complement system and differentially guide immune responses to infectious diseases.

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.

C3b and factor H: key components of the complement system

In all three complement pathways, the central molecule is C3, which, upon activation cleavage, forms the major opsonin C3b - the key component of complement. C3b is also essential for propagation of the complement cascade to the stage of the lytic terminal complement complexes. In order to prevent damage to self cells and tissues and restrict overconsumption of the complement components, C3b molecules need to be controlled by factor H. Defect in C3 functions leads to compromised microbial defence and increased susceptibility to certain autoimmune diseases. Deficiency of factor H, or a functional defect in its N terminus, often leads to membranoproliferative glomerulonephritis and complement depletion, owing to continuous overconsumption of C3. Defect in the factor H C terminus leads to a dramatically increased risk of atypical hemolytic uremic syndrome. In addition, recently, a polymorphism in the middle part of factor H (Y402H) has been shown to be the major risk factor for the most common cause of blindness in the industrialized world: age-related macular degeneration. In future, analysis of patient samples for defects in these key complement components may prove useful in diagnosis of these diseases and new therapeutic targets will certainly be the aim for use in the recently recognized factor H-related diseases.

Lack of association between polymorphisms in C4b-binding protein and atypical haemolytic uraemic syndrome in the Spanish population

Dysregulation of the alternative pathway of complement activation, caused by mutations or polymorphisms in the genes encoding factor H, membrane co-factor protein, factor I or factor B, is associated strongly with predisposition to atypical haemolytic uraemic syndrome (aHUS). C4b-binding protein (C4BP), a major regulator of the classical pathway of complement activation, also has capacity to regulate the alternative pathway. Interestingly, the C4BP polymorphism p.Arg240His has been associated recently with predisposition to aHUS and the risk allele His240 showed decreased capacity to regulate the alternative pathway. Identification of novel aHUS predisposition factors has important implications for diagnosis and treatment in a significant number of aHUS patients; thus, we sought to replicate these association studies in an independent cohort of aHUS patients. In this study we show that the C4BP His240 allele corresponds to the C4BP*2 allele identified previously by isoelectric focusing in heterozygosis in 1.9-3.7% of unrelated Caucasians. Crucially, we found no differences between 102 unrelated Spanish aHUS patients and 128 healthy age-matched Spanish controls for the frequency of carriers of the His240 C4BP allele. This did not support an association between the p.Arg240His C4BP polymorphism and predisposition to aHUS in the Spanish population. In a similar study, we also failed to sustain an association between C4BP polymorphisms and predisposition to age-related macular degeneration, another disorder which is associated strongly with polymorphisms in factor H, and is thought to involve alternative pathway dysregulation.

Genetic analysis of human complement factor H polymorphisms

Human complement factor H (factor H) is polymorphic, with five previously reported FH alleles and three previously reported HF alleles (HF*A, HF*B, and HF*Q0). The relationship between the FH and HF alleles is not clear, and the genetic basis of factor H phenotypes has not yet been identified. In this study, nucleotide sequence analysis of complementary DNA (cDNA) from individuals with each HF phenotype identified seven mutated sites in the factor H gene. However, in four cases, the same cDNA sequence was observed in individuals with two different HF phenotypes. Western blotting and 2-DE also showed that a 160 kDa protein corresponding to factor H was expressed in individuals with HF phenotypes. In addition, factor H cross-reacting 45 and 42 kDa polypeptides were detected in individuals with HF A, HF B, or HF AB phenotypes, but not in individuals with the HF Q0 (a null allele) phenotype. Thus, HF phenotype did not correlate well with factor H gene or protein structural variation. Evidence is provided to support the hypothesis that the HF phenotypes do not correspond to polymorphism in factor H, but instead correspond to polymorphism in factor H-related protein 1. A novel PCR-RFLP method was developed and used to detect four polymorphisms (G257A, G1492A, A2089G, and G2881T) in the factor H gene in 54 unrelated Japanese individuals. This method could be useful for studies on genetic disease associated with these mutations.

Phylogenetic aspects of the complement system

During evolution two general systems of immunity have emerged: innate or, natural immunity and adaptive (acquired), or specific immunity. The innate system is phylogenetically older and is found in some form in all multicellular organisms, whereas the adaptive system appeared about 450 million years ago and is found in all vertebrates except jawless fish. The complement system in higher vertebrates plays an important role as an effector of both the innate and the acquired immune response, and also participates in various immunoregulatory processes. In lower vertebrates complement is activated by the alternative and lectin pathways and is primarily involved in the opsonization of foreign material. The Agnatha (the most primitive vertebrate species) possess the alternative and lectin pathways while cartilaginous fish are the first species in which the classical pathway appears following the emergence of immunoglobulins. The rest of the poikilothermic species, ranging from teleosts to reptilians, appear to contain a well-developed complement system resembling that of the homeothermic vertebrates. It seems that most of the complement components have appeared after the duplication of primordial genes encoding C3/C4/C5, fB/C2, C1s/C1r/MASP-1/MASP-2, and C6/C7/C8/C9 molecules, in a process that led to the formation of distinct activation pathways. However, unlike homeotherms, several species of poikilotherms (e.g. trout) have recently been shown to possess multiple forms of complement components (C3, factor B) that are structurally and functionally more diverse than those of higher vertebrates. We hypothesize that this remarkable diversity has allowed these animals to expand their innate capacity for immune recognition and response. Recent studies have also indicated the possible presence of complement receptors in protochordates and lower vertebrates. In conclusion, there is considerable evidence suggesting that the complement system is present in the entire lineage of deuterostomes, and regulatory complement components have been identified in all species beyond the protochordates, indicating that the mechanisms of complement activation and regulation have developed in parallel.

Binding of a model regulator of complement activation (RCA) to a biomaterial surface: surface-bound factor H inhibits complement activation

The complement system is an important inflammatory mediator during procedures such as cardiopulmonary bypass and hemodialysis when blood is exposed to large areas of biomaterial surface. This contact between blood and the biomaterials of implants and extracorporeal circuits leads to an inflammatory response mediated by the complement system. The aim of this study was to assess the ability of a complement regulator (factor H) immobilised on a biomaterial surface to inhibit complement cascade mediated inflammatory responses. The cross-linker N-succinimidyl 3-(2-pyridyldithio) propionate was used to immobilise factor H on a model biomaterial surface without affecting the biological activity of the inhibitor. Binding of factor H was then characterised using quartz crystal microbalance-dissipation (QCM-D) and enzyme immunoassays for products of complement activation: bound C3 fragments and soluble C3a, sC5b-9, and C1s-C1INA. Immobilised factor H reduced the amount C3 fragments deposited on the biomaterial surface after incubation with serum, plasma. or whole blood. In addition, lower levels of soluble C3a and sC5b-9 were generated after incubation with whole blood. In summary, we have demonstrated that complement activation on a highly activating model surface can be inhibited by immobilised factor H and have defined prerequisites for the preparation of future biomaterial surfaces with immobilised regulators of complement activation.

FHL-1/reconectin: a human complement and immune regulator with cell-adhesive function

A novel regulator of the alternative complement pathway with functions similar to that of factor H has been identified in human plasma. The cDNA encoding this factor H-like protein 1 (FHL-1/reconectin) was isolated several years ago. Here, Peter Zipfel and Christine Skerka describe functional analyses revealing that this novel complement regulatory protein forms a molecular link between immune defense and cell adhesion.

Cloning and recombinant expression of a barred sand bass (Paralabrax nebulifer) cDNA. The encoded protein displays structural homology and immunological crossreactivity to human complement/cofactor related plasma proteins

A new cofactor related cDNA in the bony fish Paralablax nebulifer, (barred sand bass) was isolated from a sand bass liver cDNA library. The clone (c71) is 1040 bp in size and the predicted translation product of 204 amino acids contains a hydrophobic signal peptide, which is followed by a region of three short consensus repeats (SCRs). The three SCRs display high homology to SCRs of the 110 kDa chain of the sand bass plasma cofactor protein, and to a lesser degree to human complement factor H related protein 3 (FHR-3) and to human factor H. Recombinant expression of the c71 cDNA in the baculovirus system shows a product of an apparent molecular mass of 27 kDa, which is secreted and glycosylated. It also contains a His-tag for purification purposes. Removal of the His-tag yields a 24 kDa protein, and deglycosylation further reduces the molecular mass to 21 kDa. This size is in agreement with the calculated molecular mass based on amino acid composition. The sand bass SBCFR-1 protein is immunologically related to the human complement proteins, factor H and factor H-related protein 3. The recombinantly expressed protein reacted with antisera against the human FHR-3 protein and SCRs 19-20 of human factor H. The presence of SCR-containing proteins in sand bass plasma and their structural and immunological homology to human FHR-3 and factor H suggests for a common function between these evolutionary related proteins.

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

The human factor H-related gene 2 (FHR2) encodes a serum protein structurally and immunologically related to complement factor H. We describe the isolation and genomic organization of the human FHR2 gene from a yeast artificial chromosome library. The FHR2 gene is organized in five exons and spans about 7 kilobases (kb) of human genomic DNA. A comparison with the corresponding cDNA sequence (clone DDESK59) shows that the analyzed FHR2 gene has a deleted region within exon 4. A new splice acceptor site created in the truncated exon indicates that the analyzed gene could be translated to a truncated protein. Further, we demonstrate that the genes for FHR2 and beta subunit of coagulation factor XIII are located in the same 165 kb YAC DNA. Thus, the three structurally related genes FXIIIb, FHR2, and factor H are linked on human chromosome 1 in the regulators of complement activation (RCA) gene cluster. The physical linkage of the FHR2 and the factor H genes provides additional evidence for a close relatedness of complement factor H and the factor H-related proteins. The linkage and the almost exclusive organization in short consensus repeat-containing domains indicates a close evolutionary relationship of the FXIIIb, FHR2, and factor H genes.

Polymorphism and deficiency of human factor H-related proteins p39 and p37

We described previously cDNA clones representing a novel factor H-related 1.4 kilobase mRNA. This mRNA species codes for a doublet of serum proteins of M(r) 39,000 and 37,000 (p39/p37). The respective recombinant proteins of the three clones H-69, pFH1.4a, and pFH1.4b differ in the expression of the epitope recognized by the monoclonal antibody (mAb) 3D11. This probably reflects the difference of three amino acid residues of the deduced protein sequence. Here we report evidence for corresponding alterations in the native proteins p39/p37 in human sera. Employing mAb 3D11 and a polyclonal factor H-specific antiserum we detected three different patterns in western blot analyses of human sera which we provisionally termed FH1.4p+m+, FH1.4p+m-, and FH1.4p-m-. In the first pattern, p39/p37 were recognized by both antibodies, while in the second pattern the two proteins reacted only with the polyclonal antiserum. Both antibodies failed to detect p39/p37 in the third pattern. These phenotypes are found in the healthy population with frequencies of 0.556, 0.40, and 0.044, respectively. The frequencies of the alleles FH1.4*p+m+, FH1.4*p+m-, and FH1.4*p-m- were estimated to be 0.33, 0.46, and 0.21, respectively, assuming the gene distribution to be in Hardy-Weinberg equilibrium. Studies of 98 members from 27 families revealed an autosomal Mendelian inheritance. Southern blot data support our assumption of a polymorphism of the factor H-related proteins p39 and p37.

Analysis of the human regulators of complement activation (RCA) gene cluster with yeast artificial chromosomes (YACs)

The human regulators of complement activation gene cluster (RCA cluster) have been partially characterized with yeast artificial chromosomes (YACs). While the data confirm many points previously elucidated, the finer resolution of YAC mapping has allowed the discovery and/or localization of partial gene duplications, the determination of gene orientations, and the measurement of gaps between known genes. Here nine overlapping YACs that encompass a genomic region of 800 kb, encoding four RCA genes and three gene-like elements, are described. The encoded genes and two of the gene-like elements share the same orientation and are ordered (5' to 3') DAF, CR2, CR1, MCP-like, CR1-like, and MCP. A C4bp-like region lies upstream from DAF and is likely to correspond to one recently observed by F. Pardo-Manuel, J. Rey-Campos, A. Hillarp, B. Dahlback, and S. Rodriguez de Cordoba (1990, Proc. Natl. Acad. Sci. USA 87: 4529-4533). MCP-like, a new genetic element, was discovered and found to be homologous to the 5' portion of the MCP gene. Two large gaps of 85 kb (between CR2 and DAF) and 110 kb (between DAF and the C4bp-like element) could carry additional RCA genes. The arrangement of CR1, MCP-like, CR1-like, and MCP, in that order, strongly suggests that this region was generated by a single duplication of neighboring CR1/CR1-like and MCP/MCP-like forerunners. The RCA YACs will now serve as convenient DNA sources for the subcloning and further characterization of this region.

The interaction between complement component C4b-binding protein and the vitamin K-dependent protein S forms a link between blood coagulation and the complement system

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Human and rat chromosomal localization of two genes for 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase by analysis of somatic cell hybrids and in situ hybridization

Two genes encoding 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase were localized in human and rat chromosomes. PFKFB1 (previously PFRX), which encodes the liver and muscle isozymes, was assigned to Xq22-q31 in the rat and to Xq27-q28 in the human by in situ hybridization using probes generated by the polymerase chain reaction. PFKFB2, which encodes the heart isozyme of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, was assigned to chromosome 13 in the rat and to chromosome 1 in the human by hybridization of DNA from somatic cell hybrids. By in situ hybridization, this gene was localized to the regions 13q24-25 in the rat and 1q31 in the human.

Human complement factor H. Tissue specificity in the expression of three different mRNA species

Using cDNA clones H-19 and H-46, we have shown previously that three different mRNA species (4.3 kb, 1.8 kb and 1.4 kb) for complement factor H are expressed constitutively in human liver. Here we report data suggesting that the expression of these different factor-H mRNA species is regulated by tissue-specific control mechanisms. Total RNA and poly(A)-enriched RNA from various human tissues (heart, lung, temporal cortex, kidney, spleen, bone marrow and muscle) various cell lines (HepG2, HepG3, HepG4, Hep3B, H-4, Jurkat, Molt4, H-9, KHos24Os, A-431, U937, Mono Mac 6 and Raji) and from primary cultures of peripheral blood monocytes, fibroblasts and human umbilical vein endothelial cells (HUVEC) were investigated for the expression of factor-H mRNA. In RNA preparations from extrahepatic tissue, factor-H mRNA was only detected in biopsies from the lung. Using 20 micrograms total RNA isolated from all 13 cell lines it was not possible to detect any factor-H mRNA, while mRNA for factor H was expressed in monocytes, HUVEC and fibroblasts. When expressed in extrahepatic tissues, only the 4.3-kb and the 1.8-kb mRNA species were detected, while the 1.4-kb mRNA is expressed abundantly in liver. Interferon-gamma did not induce the expression of factor-H mRNA in any of the cell lines tested. On the other hand, tumour necrosis factor-alpha induced the expression of the 4.3-kb mRNA species in U937 cells. In HUVEC and fibroblasts the relative quantities of the 4.3-kb and the 1.8-kb mRNA species and the regulatory effects of interferon-gamma, interleukin-1, dexamethasone and retinoic acid on their expression showed significant tissue specificity.

Molecular analysis of the beta-chain of human C4b-binding protein

C4b-binding protein (C4BP) is a multimeric glycoprotein in plasma with important regulatory functions in the complement system. It occurs in two forms, as free protein and in a non-covalent bimolecular complex with the vitamin K-dependent protein S. Protein S is an important anticoagulant and enhances the rate of inactivation by activated protein C of blood coagulation factors, Va and VIIIa. Protein S bound to C4BP is inactive as an anticoagulant, indicating C4BP to have a regulatory function in the blood coagulation process. Approximately 50% of C4BP in plasma circulates in complex with protein S, but little has been known about as to how these proteins interact. This report describes the structure of C4BP and its relation to protein S binding. A novel C4BP subunit, designated the beta-chain, which in all likelihood contains the protein S binding site, has been identified, isolated and characterized. The major form of C4BP is composed of seven alpha-chains and one beta-chain, and the subunits are covalently linked by their carboxy-terminal regions giving the molecule a spider-like quaternary structure. A subpopulation of C4BP, which does not bind protein S, was found to lack the beta-chain. This provides support for the concept that the single protein S binding site is located on the beta-chain. The beta-chain is structurally related to the alpha-chain of C4BP, and both subunits belong to the superfamily of C3b/C4b-binding proteins. The genes coding for the alpha- and beta-chains of C4BP were found to be closely linked within a cluster of genes, coding for structurally related proteins, on the long arm of human chromosome 1.

Interferon-induced transcriptional and post-transcriptional modulation of factor H and C4 binding-protein synthesis in human monocytes

Cultured human monocytes synthesize factor H (H) and C4 binding protein (C4-bp), as assessed by measuring their presence in culture fluids, and demonstrating the presence of their corresponding mRNAs in total monocyte RNA by Northern blot analysis and by nuclear run-on experiments. H mRNA (4.3 kb and 1.8 kb) could only be detected when cycloheximide (2.5 micrograms/ml) was present in monocyte culture fluid. Recombinant interferon-gamma (IFN-gamma) and lymphoblastoid interferon-alpha (IFN-alpha) produce dose-related increases in the synthesis of H and C4-bp and in the abundance of C4-bp mRNA (2.5 kb). Increased abundance of H mRNA was also seen when cycloheximide (2.5 micrograms/ml) was present in the cultures. Both cytokines increased the transcription rates of the H and C4-bp genes. These changes in transcription were rapid, with significant increases being observed within 30 min of exposure. Following the removal of the cytokines from the cultures the transcription rates of both genes returned to control levels within 4 h. The effects of combining IFN-alpha and IFN-gamma on H and C4-bp transcription rates, mRNA abundances and protein secretion rates were not quantitatively additive. The half-life of H mRNA in monocytes was 15 min, whereas that of C4-bp mRNA was 2 h 45 min. Neither half-life was altered by IFN-gamma.

Genetic polymorphism of complement factor H in cattle

Bovine factor H was found to be polymorphic by the combined techniques of SDS-polyacrylamide electrophoresis of bovine plasma and immunoblotting. Three phenotypes (S, SF, F) were identified in a sample population of 149 cattle. Variant S and F differed by an apparent molecular weight of 5000 daltons. Family studies demonstrated Mendelian segregation of variants S and F. The data indicate that these genetic variants of bovine factor H are encoded by two codominant alleles at a single autosomal locus.

Factor H

While the mouse and human H proteins are structurally and functionally similar, they differ in their genetics. Whereas there is no evidence in humans for more than one gene; in mice the H locus is complex. Based on cDNA sequence and hybridization analysis of genomic cosmid clones, there are at least three distinct genes, all highly related to one another. The consensus repeating unit that comprises this molecule has obviously been duplicated numerous times, since it is present in many other molecules. Thus, it is not surprising to discover that there are several genes related to H in the mouse. A similar case has been described for two other members of this family. In humans, CR1 cDNA hybridizes to two distinct genomic clusters in the CR1 locus (Wong et al. 1989), and in mice, mCRY hybridizes to two regions in the genome, one on chromosome 1 and another on chromosome 8 (Aegerter-Shaw et al. 1987). It will be of interest to see if any other members of this family display as complex a genetic locus as murine H.

Genetic organization of complement receptor-related genes in the mouse

Using an interspecific cross, gene linkage relationships among members of the murine complement receptor-related genes, C4bp, Cfh, Mcry, and Mcr2, were analyzed by segregation of RFLP in 200 mice. The human homologues of these genes are tightly linked, composing the RCA locus, which maps to human chromosome (Chr.)1q32, within a large linkage group conserved between human Chr.1q21-32 and mouse Chr.1. RFLP associated with C4bp and Cfh map within this conserved linkage group; Cfh is located 9 cM telomeric to C4bp, which is consistent with linkage data for their human homologues. Mcry and Mcr2, while tightly linked, are located outside the conserved group, 40 cM telomeric to C4bp. These data suggest that a translocation or inversion occurred within the RCA family during the evolution of the mouse, defining a breakpoint of this large conserved linkage group.

The molecular genetics of components of the complement system

Rapid progress has been made recently on the elucidation of the structural components of the complement system by the application of recombinant DNA techniques. The derived amino acid sequences of most of the complement proteins are now available through cDNA cloning, and significant progress has been made in the discovery of the genetic organization of the corresponding genes. The linkage of some of the complement component genes has been established through the study of phenotypic genetics. Of particular interest has been the mapping of two clusters of genes which encode proteins involved in the activation of C3. C2, C4 and factor B, three of the structural components of the classical and alternative pathway C3 convertases, are encoded by genes which map to the MHC on human chromosome 6. The linkage of the genes with each other in a 100 kb segment of DNA has been established through the isolation of overlapping cosmid clones of genomic DNA, and PFGE has defined the molecular map position of these genes within the class III region of the MHC. The regulatory proteins factor H, C4BP, CR1 and DAF, which are involved in the control of C3 convertase activity, are encoded by closely-linked genes (termed the regulators of complement activation or RCA linkage group) that have been mapped to human chromosome 1. PFGE has defined the linkage of the CR1, C4BP and DAF genes, together with the CR2 gene in an 800 kb segment of DNA, and it is clear that this technique will eventually be applied to the molecular mapping of other complement genes in relation to their flanking loci. Polymorphism is a feature of many of the complement proteins, especially those encoded by genes in the MHC class III region. Of these, C4 is by far the most polymorphic, and differences in gene size and gene number, in addition to the functional and antigenic differences in the gene products, have been recognized. Null alleles at either of the C4 loci are rather common and may be important susceptibility factors in some HLA-associated diseases, particularly SLE. The molecular basis of complement deficiency states has begun to be elucidated. In many cases, the deficiency is not caused by a major gene deletion or rearrangement, and techniques which detect single point mutations in DNA (Cotton et al, 1988) will have to be applied to fully characterize the nature of the defect.

Organization of the genes encoding complement receptors type 1 and 2, decay-accelerating factor, and C4-binding protein in the RCA locus on human chromosome 1

The organization and physical linkage of four members of a major complement locus, the RCA locus, have been determined using the technique of pulsed field gradient gel electrophoresis in conjunction with Southern blotting. The genes encoding CR1, CR2, DAF, and C4bp were aligned in that order within a region of 750 kb. In addition, the 5' to 3' orientation of the CR1 gene (5' proximal to CR2) was determined using 5'- and 3'-specific DNA probes. The proximity of these genes may be related to structural and functional homologies of the protein products. Overall, a restriction map including 1,500 kb of DNA was prepared, and this map will be important for positioning of additional coding sequences within this region on the long arm of chromosome 1.

Sequence polymorphism of human complement factor H

Factor H is a major regulatory protein of the complement system. The complete cDNA coding sequence has been derived from overlapping clones, and a polymorphism at base 1277 has been characterized. In four clones there is a T at nucleotide 1277 and in two others there is a C. This T/C change represents a tyrosine/histidine polymorphism at position 384 in the derived amino acid sequence. Protein sequence studies on peptides generated by trypsin digestion of factor H, purified from pooled plasma from 12 donors, confirmed the presence of both tyrosine and histidine at this position. Tyrosine and histidine were observed in a ratio of 2:1, respectively, and therefore this polymorphism is likely to represent a sequence difference between the two most abundant charge variants, FH1 and FH2, of factor H.

A physical map of the human regulator of complement activation gene cluster linking the complement genes CR1, CR2, DAF, and C4BP

We report the organization of the human genes encoding the complement components C4-binding protein (C4BP), C3b/C4b receptor (CR1), decay accelerating factor (DAF), and C3dg receptor (CR2) within the regulator of complement activation (RCA) gene cluster. Using pulsed field gel electrophoresis analysis these genes have been physically linked and aligned as CR1-CR2-DAF-C4BP in an 800-kb DNA segment. The very tight linkage between the CR1 and the C4BP loci, contrasted with the relative long DNA distance between these genes, suggests the existence of mechanisms interfering with recombination within the RCA gene cluster.

The complete amino acid sequence of human complement factor H

The complete amino acid sequence of the human complement system regulatory protein, factor H, has been derived from sequencing three overlapping cDNA clones. The sequence consists of 1213 amino acids arranged in 20 homologous units, each about 60 amino acids long, and an 18-residue leader sequence. The 60-amino-acid-long repetitive units are homologous with those found in a large number of other complement and non-complement proteins. Two basic C-terminal residues, deduced from the cDNA sequence, are absent from factor H isolated from outdated plasma. A tyrosine/histidine polymorphism was observed within the seventh homologous repeat unit of factor H. This is likely to represent a difference between the two major allelic variants of factor H. The nature of the cDNA clones indicates that there is likely to be an alternative splicing mechanism, resulting in the formation of at least two species of factor H mRNA.

Human complement factor H: expression of an additional truncated gene product of 43 kDa in human liver

The human complement factor H is an important factor in the control of the alternative pathway and also induces the stimulation of B cells and macrophages in vitro. Using a human factor H cDNA clone as probe, two factor H-specific transcripts of 4.4 and 1.8 kb were detected in four human livers. Both mRNAs were found independently of the expression of an acute phase marker SAA, in these livers, indicating that their presence is not linked to an acute phase state. The shorter transcript was cloned in two cDNA plasmids H-19 and H-20, lacking only seven amino acids of the N-terminus of the mature factor H protein. The deduced protein sequence showed that this protein is identical to the N-terminal portion of the large classical factor H of 150 kDa mol. mass. Parallel to the finding that the N-terminal sequence of factor H is expressed by two distinct mRNA species, evidence is presented that the C-terminal sequence is also contained on two different transcripts, the common 4.4-kb mRNA and an additional 1.0-kb mRNA. A novel, short form of human complement factor H of 43 kDa was detected in human sera which represents the translation product of the 1.8-kb factor H-specific mRNA detected in human liver. Five distinct epitopes detected with 6 monoclonal antibodies are present on both the 150-kDa factor H protein and the truncated 43-kDa molecule. We conclude that additional H-specific mRNAs are found in human liver and that at least one of them is translated yielding a truncated form of factor H.

Decay-accelerating factor. Genetic polymorphism and linkage to the RCA (regulator of complement activation) gene cluster in humans

We have investigated the genetic relationships between the human decay-accelerating factor (DAF) and a group of complement components including the C3b/C4b receptor (CR1), C4-binding protein (C4bp), and factor H (H), to which DAF is structurally and functionally related. CR1, C4bp, and H were previously demonstrated to be encoded by a cluster of closely linked genes, which we have designated regulator of complement activation (RCA). Southern blot analysis of genomic DNA using a DAF cDNA probe unraveled the existence of restriction fragment length polymorphism (RFLP) for both Bam HI and Hind III restriction endonucleases. Segregation analysis of these polymorphic fragments in families informative for the segregation of alleles at the CR1, C4BP, and H loci (RCA-haplotypes), demonstrated that, in humans, the gene encoding DAF is located within the RCA gene cluster. No recombinants between DAF and C4BP/CR1 were encountered in 32 informative meioses. In addition, in two individuals showing recombination between the CR1/C4BP and H loci, DAF segregated with the CR1/C4BP segment. Thus, the DAF gene maps closer to the CR1/C4BP loci than to the H gene, from which it can be separated by genetic recombination.