A haemolytically non-active C4 gene product

C4A deficiency in children and adolescents with recurrent respiratory infections

Increased susceptibility to recurrent viral and bacterial respiratory infections in children and young adults is not well understood. To evaluate the role of complement factor C4 in the defense against respiratory infections, we studied complement factor C4 allotypes C4A and C4B and copy numbers of C4A and C4B genes in 84 children and young adults with recurrent acute otitis media, sinusitis, or pneumonia and in 74 healthy controls. The occurrence of C4A gene deficiency was significantly higher in patients compared with controls (26% vs 14%, p = 0.048). Girls predominated in the group of patients with C4A deficiency (73% girls and 27% boys, p = 0.004). The lectin pathway of complement was more often functionally impaired in patients with C4A deficiency than in patients with no C4A deficiency (41% vs 13%, p = 0.033). Classical and alternative pathways were normal in individuals with C4 null alleles. C4A deficiency is 1 of the minor defects of the innate immunity that may predispose children and young adults to recurrent respiratory infections. C4 gene testing should be added to the list of investigations when the cause for recurrent acute otitis media, maxillary sinusitis, or pneumonia in children and young adults is sought.

Dancing with complement C4 and the RP-C4-CYP21-TNX (RCCX) modules of the major histocompatibility complex

The number of the complement component C4 genes varies from 2 to 8 in a diploid genome among different human individuals. Three quarters of the C4 genes in Caucasian populations have the endogenous retrovirus, HERV-K(C4), in the ninth intron. The remainder does not. The C4 serum proteins are highly polymorphic and their concentrations vary from 100 to approximately 1000 microg/ml. There are two distinct classes of C4 protein, C4A and C4B, which have diversified to fulfill (a) the opsonization/immunoclearance purposes and (b) the well-known complement function in the killing of microbes by lysis and neutralization, respectively. Many infectious and autoimmune diseases are associated with complete or partial deficiency of C4A and/or C4B. The adverse effects of high C4 gene dosages, however, are just emerging, as the concepts of human C4 genetics are revised and accurate techniques are applied to distinguish partial deficiencies from differential expression caused by unequal C4A and C4B gene dosages and gene sizes. This review attempts to dissect the sophisticated genetics of complement C4A and C4B. The emphases are on the qualitative and quantitative diversities of C4 genotypes and phenotypes. The many allotypic variants and the processed products of human and mouse C4 proteins are described. The modular variation of C4 genes together with the serine/threonine nuclear kinase gene RP, the steroid 21-hydroxylase CYP21, and extracellular matrix protein TNX (RCCX modules) are investigated for the effects on homogenization of C4 protein polymorphisms, and on the unequal genetic crossovers that knocked out the functions of CYP21 and/or TNX. Furthermore, the influence of the endogenous retrovirus HERV-K(C4) on C4 gene expression and the dispersal of HERV-K(C4) family members in the human genome are discussed.

Genetic, structural and functional diversities of human complement components C4A and C4B and their mouse homologues, Slp and C4

The complement protein C4 is a non-enzymatic component of the C3 and C5 convertases and thus essential for the propagation of the classical complement pathway. The covalent binding of C4 to immunoglobulins and immune complexes (IC) also enhances the solubilization of immune aggregates, and the clearance of IC through complement receptor one (CR1) on erythrocytes. Human C4 is the most polymorphic protein of the complement system. In this review, we summarize the current concepts on the 1-2-3 loci model of C4A and C4B genes in the population, factors affecting the expression levels of C4 transcripts and proteins, and the structural, functional and serological diversities of the C4A and C4B proteins. The diversities and polymorphisms of the mouse homologues Slp and C4 proteins are described and contrasted with their human homologues. The human C4 genes are located in the MHC class III region on chromosome 6. Each human C4 gene consists of 41 exons coding for a 5.4-kb transcript. The long gene is 20.6 kb and the short gene is 14.2 kb. In the Caucasian population 55% of the MHC haplotypes have the 2-locus, C4A-C4B configurations and 45% have an unequal number of C4A and C4B genes. Moreover, three-quarters of C4 genes harbor the 6.4 kb endogenous retrovirus HERV-K(C4) in the intron 9 of the long genes. Duplication of a C4 gene always concurs with its adjacent genes RP, CYP21 and TNX, which together form a genetic unit termed an RCCX module. Monomodular, bimodular and trimodular RCCX structures with 1, 2 and 3 complement C4 genes have frequencies of 17%, 69% and 14%, respectively. Partial deficiencies of C4A and C4B, primarily due to the presence of monomodular haplotypes and homo-expression of C4A proteins from bimodular structures, have a combined frequency of 31.6%. Multiple structural isoforms of each C4A and C4B allotype exist in the circulation because of the imperfect and incomplete proteolytic processing of the precursor protein to form the beta-alpha-gamma structures. Immunofixation experiments of C4A and C4B demonstrate > 41 allotypes in the two classes of proteins. A compilation of polymorphic sites from limited C4 sequences revealed the presence of 24 polymophic residues, mostly clustered C-terminal to the thioester bond within the C4d region of the alpha-chain. The covalent binding affinities of the thioester carbonyl group of C4A and C4B appear to be modulated by four isotypic residues at positions 1101, 1102, 1105 and 1106. Site directed mutagenesis experiments revealed that D1106 is responsible for the effective binding of C4A to form amide bonds with immune aggregates or protein antigens, and H1106 of C4B catalyzes the transacylation of the thioester carbonyl group to form ester bonds with carbohydrate antigens. The expression of C4 is inducible or enhanced by gamma-interferon. The liver is the main organ that synthesizes and secretes C4A and C4B to the circulation but there are many extra-hepatic sites producing moderate quantities of C4 for local defense. The plasma protein levels of C4A and C4B are mainly determined by the corresponding gene dosage. However, C4B proteins encoded by monomodular short genes may have relatively higher concentrations than those from long C4A genes. The 5' regulatory sequence of a C4 gene contains a Spl site, three E-boxes but no TATA box. The sequences beyond--1524 nt may be completely different as the C4 genes at RCCX module I have RPI-specific sequences, while those at Modules II, III and IV have TNXA-specific sequences. The remarkable genetic diversity of human C4A and C4B probably promotes the exchange of genetic information to create and maintain the quantitative and qualitative variations of C4A and C4B proteins in the population, as driven by the selection pressure against a great variety of microbes. An undesirable accompanying byproduct of this phenomenon is the inherent deleterious recombinations among the RCCX constituents leading to autoimmune and genetic disorders.

The purification and properties of some less common allotypes of the fourth component of human complement

Human complement component C4 is coded by two genes situated between HLA-D and HLA-B. Both genes are highly polymorphic; C4-A gene products normally carry the blood group antigen Rodgers and C4-B proteins usually carry the Chido antigen. Using a monoclonal antibody which binds Rodgers-positive and Chido-positive proteins with different affinities, we have purified a number of less common C4 allotypes and compared their properties. All C4-B allotypes tested have similar specific hemolytic activities and binding efficiencies to small molecules. All C4-A proteins tested had similar binding to small molecules and hemolytic activities except for the C4-A6 proteins from two individuals with different extended haplotypes, both of which had identical hemolytic activities and much lower ones than other C4-A allotypes. Two allotypes, C4-A1, Rodgers-negative but Chido-positive, and C4-B5, Chido-negative but probably Rodgers-positive, were found to behave as typical C4-A and C4-B proteins, respectively, apart from the switch in their antigenic properties.

A product of the C4B locus lacking hemolytic activity

An unusual C4B allotype, C4B 4I, was identified in a study of C4 polymorphism in Japanese. This variant was defined as C4B using a murine monoclonal antibody specific for the C4d region of C4B. Hemolytic assay, however, revealed that the C4 variant, C4 BI was hemolytically inactive in contrast to other well-defined C4B locus products.

The molecular genetics of components of complement

Rapid progress has been made in establishing linkages and in chromosome allocation of the genes of some 9 complement components. In the MHC, C2, Factor B, and two C4 or C4 related genes have been placed in some detail in both man and mouse. The gene coding for the cytochrome P-450 21-hydroxylase has been shown to be duplicated and immediately 3' to the two C4 genes, though it appears to be functionally and structurally unrelated to the complement components. Thus six genes have been mapped to this region where particular haplotypes are associated with increased susceptibility to a number of diseases, some of which are autoimmune in character. The complete gene structure of Factor B has been solved in man and rapid progress is being made with the C2 and C4 genes. The structural basis of the polymorphisms of these genes is being established. In C4, the polymorphism is exceptionally complex with varying numbers of loci and probably more than 50 allotypes occurring in man. A structural basis has also been found for the big differences in the biological activity of some of the C4 allotypes in man. Apart from the genes in the MHC, linkage has been found between the genes coding for C4bp, CR1, and Factor H. Remarkably there are sequence homologies between these proteins and C2 and Factor B, probably related to the ability to bind to one or other of the structurally similar proteins C3b and C4b. The complete cDNA sequences of C3 and C4 in mouse and man have given much information on the many posttranslational modifications of these proteins. A partial structure has been obtained for the C3 gene and the homology shown between C3, C4, C5, alpha 2-macroglobulin, and pregnancy zone protein. Although the amount of detailed information in the molecular genetics of complement components is accumulating rapidly, there appears to be a reasonable prospect that linkages and homologies will classify the data into a comprehensible form.

The complement components coded in the major histocompatibility complexes and their biological activities

The complement system has two pathways of activity, both dependent on the sequential conversion of proteolytic zymogens to active proteases leading to a common lytic complex and both with control proteins which inhibit or inactivate different steps in the cascade. Three of the components--C2, factor B and C4--are coded by closely linked genes in the MHC of man and mouse and have been placed relative to each other. The genes are polymorphic, particularly C4, with variable numbers of loci as well as many mutant forms. Some alleles of C4 show strikingly different reactivities in their haemolytic activity and this may be relevant to the association of susceptibility to autoimmune diseases with particular haplotypes in this section of HLA.

Genetic predisposition to acute lymphocytic leukemia in American blacks. A Pediatric Oncology Group study

Recent reports have shown an association between genes lying within the major histocompatibility complex (MHC), particularly HLA and factor B (Bf), and acute lymphocytic leukemia (ALL) in white children. The frequencies of Bf and complement component C4 phenotypes in 90 black American children with ALL were examined to determine if a genetic association existed. The Bf and C4 results for the black children with ALL were compared with frequencies in healthy black Americans from the same geographic region. The BfF allele was carried by 95.6% of the black ALL patients compared with 86.1% of the controls (P = 0.017; relative risk = 3.5). In contrast, only 2.2% of the patients with ALL were homozygous for BfS compared with 9.8% of the controls (P = 0.043; relative risk = 0.2). These findings are similar to those observed in white American children. The C4A6 phenotype was found in 11.9% of the black children with ALL compared with 0.6% of the controls (P = 0.0026; relative risk = 22.7). These findings represent the first reported association of a particular allele whose gene lies within the MHC with ALL in black American children. The results suggest that the occurrence of ALL in black American children may be partially due to a genetic influence.

C4*A6BQO: a 'new' C4 haplotype

A C4 haplotype containing C4*A6 without C4*B1 is described. It occurred in the MHC haplotype HLA-A1, Cw5, Bw44, C2*1, BF*S, C4*A6BQO, DR3, GLO 2. This haplotype was observed in five of eight siblings, including the propositus who was in renal failure. The C4A6 was not haemolytically active and this new haplotype is therefore of particular interest since it is functionally equivalent to the rare C4-deficient haplotype C4*AQOBQO.

Gene and haplotype frequencies of the fourth component of complement (C4) in type 1 diabetics and normal controls

C4 gene and haplotype frequencies were calculated from phenotype data of 380 unrelated Caucasian patients with insulin dependent (type 1) diabetes mellitus and were compared with analogous frequencies of 382 unrelated healthy Caucasian individuals. In diabetics, a significantly increased frequency of the rare allele C4B 3 (p less than 10(-7] and of the silent alleles C4A Q0 (p less than 10(-7] and B Q0 (p less than 0.002) was observed. Accordingly, insulin dependent diabetes is associated with partial C4 deficiency, which may contribute to the pathogenesis of the disease.

Human C4 polymorphism: pedigree analysis of qualitative, quantitative, and functional parameters as a basis for phenotype interpretations

Ten families with 82 members were investigated for C4A- and B polymorphism in a blind trial. Phenotyping was done on neuraminidase treated sera by immunofixation and simultaneously by hemolytic overlay electrophoresis. In addition Rg, Ch, BF, C2, HLA-A, B, C, DR, and GLO were determined. After decoding the samples the reliability of blind typing was found to be 84.4% according to segregation patterns. Inconsistencies occurred mostly when A4, A2, or A92 were present. The detection of silent A*Q0 and B*Q0 alleles was more critical than that of "difficult" allotypes. The quantitation of the C4A/B ratio by densitometry of stained gels or by conventional immunochemical measurements of serum C4 level could not substantially improve the identification of A*Q0 or B*Q0. C4 dependent activity in radial diffusion hemolysis showed satisfactory correspondence with the number of expressed C4B alleles. At least three haplotypes with two C4A genes (duplicated A genes) were observed as ascertained from offspring analysis in accordance with the MHC segregation pattern. Individuals with the duplicated C4A gene (C4A*3, A*2, in the absence of any other expressed A allele or together with C4A*92) showed only partial inhibition of Rodgers antisera. Partial inhibition of Chido antisera was seen in individuals with C4B 2 (in the absence of other B allotypes). The findings support the hypothesis of at least two structural C4 loci. They also demonstrate the inconsistency of quantitative data in the recognition of silent alleles.

The complement component C4 in black Americans with type 1 (insulin-dependent) diabetes mellitus

The complement component C4 variants C4A 4, C4B 4 and C4B Q0 were found to be significantly increased in 64 black patients with Type 1 (insulin-dependent) diabetes compared with 169 black control subjects, yielding relative risks of 3.3, 2.9 and 3.4, respectively. The increased frequencies of C4B 4 and C4B Q0 in black Type 1 diabetic patients are similar to those found in Caucasoid Type 1 diabetic patients. The data suggest that Type 1 diabetes in black Americans may be partially due to admixture of genes from whites.

Genetics of human C4 polymorphism: detection and segregation of rare and duplicated haplotypes

Applying a combined technology for the detection of allotypic variation of the fourth component of human complement (C4), including immunofixation with anti-C4 and C4-dependent lysis after agarose electrophoresis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis of C4 to separate the C4A and B alpha-chains, and the determination of Rodgers (Rg) and Chido (Ch) determinants of C4 in serum and at the blotted C4 alpha-chains, we detected rare human C4 allotypes and studied the genetic linkage. Partial inhibitors (p.i.) of anti-Rg and anti-Ch sera were found; the C4A51 allotype characterized as Rg p.i. and the C4A1 and C4B51 allotypes as Ch p.i. were genetically inherited. The C4A1 allotype has a unique Rg- Ch+ C4A alpha-chain. Duplicated C4A loci, A*3, A*2, and A*5, A*2 were both associated with a C4BQO and the HLA haplotype A3-Cw4-Bw35-DR1. These additions to the already known extensive C4 polymorphism may help to sort out their significance for the biological functions of human C4.

The complement components of the major histocompatibility locus

Polymorphism of complement components, recognized by differences in either their antigenic specificity or their electrophoretic mobility, together with studies of inherited deficiencies, has enabled many of their structural genes to be mapped. In humans, three genes (for C2, C4, and factor B) have been placed between HLA-D and HLA-B on chromosome 6 and in mice, C4 between H2-I and H2-D, chromosome 17. Structural studies show that these components have exceptional features. C2 and factor B which contain the proteolytic active site of the C3 and C5 convertases are of the classical and alternative pathway respectively and are similar in structure and function. Both are novel types of serine proteases. C4 (as C3) contains an intrachain thioester bond essential for hemolytic activity. Molecular genetic investigations are determining the relative positions of these genes, and their precise structure, and should clarify their relation to the inherited diseases which are associated with defects in this section of the human genome.

Correlation between a DNA restriction fragment length polymorphism and C4A6 protein

The fourth component of complement (C4) in man, is coded for by two separate but closely linked loci (C4A and C4B) within the major histocompatibility region (MHC), on the short arm of chromosome 6. Like class I and II loci of this region, the C4 genes are highly polymorphic with more than 30 alleles, including null alleles, assigned to the two loci. This extensive polymorphism, based mainly on electrophoretic mobility, provides a useful marker for studies of disease susceptibility. Several disorders, including systemic lupus erythematosus and type I diabetes, show associations with C4 phenotypes. We have used the technique of Southern with a C4 specific probe to examine the genomic DNA of individuals typed for C4 by protein electrophoresis. We have identified 10.7 and 3.8 kilobase (kb) BglII restriction fragments in each of 9 unrelated individuals with a C4A6 allele, and in none of 22 unrelated individuals in whom this allele was not expressed. This clear correlation of restriction fragment length polymorphism with C4 phenotype provides a precise basis for analysis of C4 polymorphism. It is likely to be of value in clinical investigations of autoimmune disease.

HLA-linked complement polymorphisms (C2, BF) in psoriasis

Starting from the known association between psoriasis and several HLA antigens and from the fact that the HLA chromosomal region contains the structural genes for at least three complement components, the authors have looked for an association between psoriasis and allotypes of C2 and BF. C2 and BF polymorphism were examined in 230 psoriatic patients. Two rare complement genes were found to be significantly increased when compared with controls: the frequency of the C2*2 gene was 0.061 among patients and 0.035 among controls (P less than 0.05); for BF*SO7, the frequencies were 0.0304 in patients and 0.0092 in controls (P less than 0.0005). The BF*F gene frequency, however, was significantly decreased among patients: 0.1196 vs. 0.1743 (P less than 0.01). The relative risks were 1.79 for the C2 2, 3.44 for the BF SO7, and 0.6 for the BF F gene product. From previous studies, it is known that these three complement alleles (C2*2, BF*SO7, BF*F) are in linkage disequilibrium with HLA alleles that have also been found increased or decreased, respectively, in psoriasis.

Phenotypes of the fourth complement component (C4) in black Americans from the southeastern United States

C4 is composed of two tightly linked genes (C4A and C4B) lying within the major histocompatibility complex of chromosome 6 that can be demonstrated by agarose gel electrophoresis. Seven alleles and five alleles at the C4A and C4B loci, respectively, were detected in 169 black individuals from the southeastern United States. Furthermore, the phenotypic frequencies of C4A6, C4A5, C4A4, C4B4, C4B3 and C4BQ0 were significantly different between black and white Americans.

Electrophoretic polymorphism of human C4 is due to charge differences in the alpha chain, presumably in the C4d fragment

Various common C4 gene products were isolated from serum by immunoprecipitation. After reduction the C4 alpha-, beta-, and gamma-polypeptide chains were studied by two-dimensional electrophoresis. Isoelectrofocusing was performed in the first dimension and sodium dodecyl sulphate polyacrylamide gradient gel electrophoresis in the second. The charge differences behind the electrophoretic C4 polymorphism were shown to reside in the 95,000-u (atomic mass units) alpha-chain. Charge variation closely mirroring the alpha-chain differences were also found in a 49,000-u fragment of the alpha-chain, most probably C4d. The basic beta-chain could not be studied in detail, but no differences were observed with regard to molecular weight or charge of the gamma-chains of the different C4 gene products.

On the significance of C2, C4, and factor B polymorphisms in disease

In this review article, recent evidence is presented that some diseases like insulin-dependent diabetes mellitus, multiple sclerosis, and idiopathic membranous nephropathy, which are primarily associated with HLA-D,DR, are also related to the rare C2, C4, and Factor B alleles. Circumstantial evidence is available that at least some of these rare variants may be functionally deficient. Based on the concept of functionally interacting gene clusters, mutant complement genes may lead to impaired effector mechanisms in virus neutralization or lysis of virus-infected cells. Other mechanisms such as alteration of vascular permeability may be involved in the development of proliferative retinopathy and familial hypertension. In lepromatous lepra, an impaired cell-mediated lysis of M. leprae may be related to the hemolytically inactive C4F1 allelic product.