Human complement C4 locus is duplicated on some chromosomes

HLA and disease

Many human diseases are associated with HLA class I, class II and class III antigens. It appears that the class III antigen disease associations can be explained by a direct defect operating at the level of either the class III gene or its gene product. The mechanism underlying class I and class II antigen disease associations is at present unknown. In this review we have considered thirty diseases which have been ranked according to their relative risk as defined by the frequency of a given HLA antigen in patient and control populations. The chronic inflammatory disorder, ankylosing spondylitis and its association with HLA B27 has been used as a model to study the HLA linked diseases. We have suggested that the disease may be caused by the Gram-negative microorganism Klebsiella which has antigenic similarity to HLA B27. It is proposed that some antibodies made against Klebsiella bind to HLA B27, thereby acting as autoantibodies leading to the pathological sequelae of chronic inflammatory arthritis. This is the crosstolerance hypothesis or molecular mimicry model and it has been compared to the receptor model. It is further suggested that the crosstolerance hypothesis can be utilised as a general theory to explain the association of other diseases with the class I and class II antigens, and offer a possible explanation for the polymorphism of HLA.

Use of DNA amplification (PCR) and direct DNA sequencing in the characterization of C4 alleles

A procedure for detailed characterization of individual C4 alleles has been developed. DNA containing the two polymorphic clusters of C4 was amplified in the polymerase chain reaction (PCR). Direct DNA sequencing of amplified DNA was then performed by a modification of previously described techniques. The results were confirmed by M13 sequencing. Single C4A3 and C4B1 allele sequences were in accordance with previous reports. An individual typed C4A3B1 revealed double bands in the autoradiogram in the positions corresponding to the polymorphic nucleotides. We did not find the reported thymine in position 3641 specific for the C4A4 allele in an individual typed C4A4B2.

Restriction fragment length polymorphisms of the complement component C4 loci on chromosome 6: studies with emphasis on the determination of gene number

Restriction fragment length polymorphisms of the C4 region of human chromosome 6 have been studied in family material where the haplotypes are defined with regard to other genetic markers in this region. Employing one near full-length C4 probe and the combination of BglII and XbaI enzymes, five different C4 genes were characterized. Studies of the segregation of DNA patterns in families made possible the reliable determination of DNA C4 haplotype pattern including gene number. In the total material of 76 haplotypes, 13 different types with regard to number and/or DNA type of C4 gene(s) were encountered. Twelve of the haplotypes had one C4 gene only, 58 had two genes, while 6 had three C4 genes. This fits fairly well with the hypothesis that the one- and three-gene haplotypes have originated through unequal crossing-over between chromosomes carrying duplicated C4 genes.

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.

Structure and organization of the C4 genes

This 200 000 Mr serum protein is coded for by at least two separate loci, C4A and C4B, which map in the HLA Class III region on chromosome 6 in man. Both loci are highly polymorphic with more than 30 alleles, including null alleles assigned to the two loci. The complete nucleotide sequence of a full length C4A cDNA clone and a substantial part of a C4b cDNA clone has shown class differences which can be used to synthesize nucleotide probes specific for C4A and C4B. Three C4 loci of approximately 16 kilobases each spaced by 10 kilobases have been identified in DNA from one individual and aligned 30 kilobases from the factor B gene by overlapping cloned genomic fragments from a cosmid library. Characterization of these genes by restriction mapping, nucleotide sequence analysis and hybridization with C4A and C4B specific synthetic oligonucleotides show that these genes are very similar.

The structural basis of the multiple forms of human complement component C4

cDNA clones of human complement components C4A and C4B alleles were prepared from mRNA obtained from the liver of a donor heterozygous at both loci. cDNA from one C4A allele was sequenced to give the derived complete amino acid sequence of 1722 amino acid residues of the C4 single chain precursor molecule and the estimated sequences of the three peptide chains of secreted C4. Comparison with partial sequences of a second C4A allele and a C4B allele has led to the tentative identification of some class differences in nucleotide sequences between C4A and C4B and of allelic differences between C4A alleles in this highly polymorphic system.

Partial deficiency of the fourth component of human complement (C4) and autoantibody directed against C4 in a patient with SLE

The finding of dramatically depressed levels of C4 in a 17-year old patient with severe systemic lupus erythematosus (SLE) prompted a genetic study of her family. This study revealed the existence of a partial C4 deficiency; we found the presence of a C4A3,C4BQo haplotype which was transmitted by the mother to each of her children. This patient possessed, in her serum, an autoantibody with anti-C4 specificity. The immunochemical characterization of this autoantibody revealed that it was IgM and belonged to the immunoconglutinin family. We have studied the effects of this autoantibody on the formation and dissociation kinetics of classical C3-convertase.

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.

Inherited disorders of complement

Isolated complement component deficiencies are uncommon. Deficiencies of all eleven components and two inhibitors of the classical pathway have been described. Complete absence of the components of the alternative pathway has not been described. The consequences of a single defect in complement are often predictable from an understanding of the biologic activities associated with activation of the complement system. Deficiency of C1 esterase inhibitor gives rise to the disease, hereditary angioedema; deficiency of the early components of the classical pathway are associated with lupus erythematosus; C3 and C3 inactivator deficiencies with pyogenic infections; C5 dysfunction with Leiner's disease; deficiencies of the terminal components with recurrent Neisseria bacteremia; and C9 deficiency with normal health. The complement system and its associated biologic activities are reviewed. The present knowledge of the inherited complement deficiencies and associated diseases, with particular emphasis on the dermatologic manifestations, genetics, and diagnosis, is summarized.

Genetic deficiency of C4 presenting with recurrent infections and a SLE-like disease. Genetic and immunologic studies

A young girl presenting with recurrent pulmonary infections and atypical lupus erythematosus was totally deficient in C4. In one sister, also deficient in C4, the same symptoms developed. Results of family studies were consistent with an autosomal recessive mode of transmission and with linkage of the genes determining C4 deficiency to those of the major histocompatibility complex. The patient's serum and red cells were Chido- and Rodgers-negative. Humoral and cellular immunity were normal, except for a low lymphocyte response in mixed lymphocyte culture. The cellular function of the patient's polymorphonuclear leukocytes was normal, for both phagocytosis and bactericidal activity using Candida albicans. However, in the presence of C4-deficient serum, opsonin generation and bactericidal indexes were diminished. These defects were completely reversible upon addition of purified C4.

Isolation of tryptic fragments of human C4 expressing Chido and Rodgers antigens

It has been shown previously that methylamine is incorporated into the alpha-chain of human C4, resulting in a loss of haemolytic function and the appearance of a free thiol group in the molecule. In the present study it was demonstrated that a fragment resembling C4d is liberated from C4 by trypsin. The fragment--Try-C4d--contains both the methylamine binding site and the free thiol group. When separated on DEAE-Sepharose, four types of Try-C4d, differing in charge and size, could be defined. The size difference was found to parallel the presence of Chido and Rodgers blood group antigens. Fragments of Mr 30,000 carried the Rodgers antigen and the Chido antigen was expressed on fragments of Mr 28,000.

Genetics of complement C4. Two homoduplication haplotypes C4S C4S and C4F C4F in a family

A family in which two homoduplicated C4 haplotypes (or supergenes) segregate is described. One haplotype C4F*3 C4F*2.2 is composed of two C4F alleles and the other C4S*5.1 C4S*1 of two C4S alleles. The C4F duplication haplotype is a partial inhibitor of the Rodgers antigen, and judged from our family and population material, it seems to be rather frequent and associated with HLAB*35, Bf*F, and HLAD/DR*1. The C4S duplication haplotype is Rg(a-) and is not identified in individuals without another S, Ch(A+) variant.

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.

Family studies of complement C4 and HLA in man

At least 12 different C4 gene products with a three band pattern have been identified after electrophoresis of sera pretreated with neuraminidase. Segregation analysis showed at least 12 different C4 haplotypes (or supergenes), of which five represent a single gene product and seven are duplications each composed of an F and an S gene. The data analyzed with respect to linkage showed one recombinant between the C4 and HLAB loci in 154 meioses giving a map distance of C4 HLAB of 0.6 cM. Another recombinant between the C4 and the HLAD loci was found in 101 meioses giving a map distance of C4 HLAD of 1.0 cM. Linkage disequilibrium was found between at least eight C4 haplotypes and certain alleles at the HLAB as well as the HLAD loci. Examinations of 15 families selected through a proband with HLAA 25, HLAB 18 and C2Q0 showed that in almost all cases a slight variant of the C4 supergene F3S2 followed the haplotype HLAA25 HLAB18 C2Q0. No associations were found between the two duplications of C4F3 C4S2 and C4F3 C4S1 and the loci. These findings may indicate that these C4 haplotypes were the original ones preceding the other C4 haplotypes.

Complement loci of the HLA complex. Studies on families with intra-HLA crossovers and haplotype associations

We report genetic studies of families with defined crossovers in the HLA complex. Haplotypic associations between the different alleles have been analyzed. The object of the studies has been to determine the precise location of complement loci on the HLA complex on chromosome 6. Based on direct observation of recombinations and on indirect evidence from haplotypes, we postulate that the C4 and Bf loci are located between the HLA-B and -D regions and probably closer to the former. Available information also points towards the same localization for the C2 locus.

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.

Association between the electrophoretically-determined C4M haplotype product and partial inhibition of anti-Cha

C4-coated Ch(a+) red blood cells (RBC) were used as indicator cells in a serum inhibition reaction of anti-Ch(a), for the determination of the Ch group of serum. This serological study, combined with electrophoretic studies of C4 in a family material, showed that the C4M haplotype product was associated with partial inhibition of anti-Ch(a).

Different HLA antigen associations for the functionally active and inactive products of the complement C4F1 allele

The genetic polymorphism of the fourth component of human complement, C4, was studied in 945 unrelated Caucasian individuals. A third allele of the C4F (Rodgers) locus, termed C4F1 was demonstrated. This allele is characterized using immunofixation electrophoresis, by the presence of an additional fast-moving anodal band of C4 which distinguishes it clearly from the common C4F variant. The allelic frequencies fit the Hardy-Weinberg equilibrium assuming three alleles at the C4F locus: C4F, C4Fo, and C4F1. The functional activity of the C4F variants was investigated using a specific hemolytic overlay technique for C4. It was found that in almost all individuals (75 out of 78), the C4F1 allele codes for a functionally inactive C4 product only when it occurs on an HLA-B17 positive haplotype but that the same allele codes for a functionally active fast variant of C4 when it occurs on an HLS-B37 positive haplotype (18 out of 18). Very strong genetic linkage disequilibrium was observed for the C4F1 allele with HLA-B17 and B37. The active and inactive C4F1 variant also has marked nonrandom gametic association to different alleles of the Bf locus and to HLA-C locus determinants. No further variants of the C4S (Chido) locus have been identified so far. Rodgers (Rg) typing by the plasma inhibition test of anti-Rg antiserum has shown that plasma from individuals homozygous for the C4F1 allele is only able to partially inhibit anti-Rg whereas all C4F positive individuals totally inhibited the reaction.

Inherited structural polymorphism of the fourth component of human complement

Human fourth component of complement (C4) was found to be highly polymorphic by agarose gel electrophoresis of neuraminidase-treated plasma. The system allows clear-cut separation of the products of the two C4 genetic loci, C4A (acidic or Rodgers) and C4B (basic or Chido). There are at least six structural variants and a deletion allele at the C4A locus and two structural variants and a deletion allele at the C4B locus. Close linkage with no crossovers was found between the two C4 loci, allowing the definition of C4AB haplotypes, and between C4 haplotypes and the C2 and BF loci of the human histocompatibility complex. Nine C4 haplotypes, each with a frequency of 0.005 or more in Caucasians, were found. These studies provide direct evidence for two distinct but closely linked genetic loci for human C4 in the major histocompatibility complex on the short arm of chromosome 6.

Genetic basis of acquired C4 deficiency

A study of the family of a patient who had an SLE-like syndrome and an extremely low serum C4 revealed an inheritance of C4 types and HLA region markers which indicated that the patient had 60--70% of "normal" C4 level prior to the onset of disease. Thus the extremely low C4 level during her disease may result from a combination of genetically determined low normal C4 and increased consumption/hyposynthesis secondary to her SLE.

Chido, Rodgers and C4. In vivo and in vitro coating of red blood cells, grouping and antibody detection

C4 sucrose/low ionic strength (LIS)-coated red blood cells (RBC) are excellent for the detection of the previously 'nebulous' antibodies, anti-Chido and anti-Rodgers, as well as for serum/plasma typing of these antigens by an inhibition technique. By enzyme treatment of such cells, it is confirmed that the Ch and Rg antigens reside on the C4d part of the C4 molecule. Freshly taken RBC from normal individuals were examined with a sensitive Auto-Analyzer technique with anti-Chido, anti-C4 and anti-C3 sera. All normal RBC were shown to have C4d and C3d components on their surface. The technique was also very sensitive for the detection of the Ch antigen, which was detected on the RBC of all Chido-positive individuals, and which did not show great variation in strength by this method. The mode of in vivo C4 fixation on normal RBC seems to be different from the fixation in LIS or by RBC antibody-mediated activation.

The C4 system: formal and population genetics

The present study shows that the C4 system as investigated by high voltage agarose gel electrophoresis is highly polymorphic. In a series of unrelated Norwegian adults, where C4 types have been ascertained through segregation in families, six different haplotypes have been found to occur with a frequency exceeding 1%. The genotype frequencies in the population fit expected Hardy-Weinberg distribution. In family material comprising 89 matings with 327 children the distribution of offspring is as expected according to autosomal codominant inheritance of haplotypes.