Genetic control of the eighth component of complement

The Path to Conserved Extended Haplotypes: Megabase-Length Haplotypes at High Population Frequency

This minireview describes the history of the conceptual development of conserved extended haplotypes (CEHs): megabase-length haplotypes that exist at high (≥0.5%) population frequency. My career began in internal medicine, shifted to pediatrics, and clinical practice changed to research. My research interest was initially in hematology: on plasma proteins, their metabolism, synthesis, and function. This narrowed to a focus on proteins of the human complement system, their role in immunity and their genetics, beginning with polymorphism and deficiency of C3. My group identified genetic polymorphisms and/or inherited deficiencies of C2, C4, C6, and C8. After defining glycine-rich beta glycoprotein as factor B (Bf) in the properdin system, we found that the genes for Bf (CFB), C2, C4A, and C4B were inherited as a single haplotypic unit which we named the "complotype." Complotypes are located within the major histocompatibility complex (MHC) between HLA-B and HLA-DRB1 and are designated (in arbitrary order) by their CFB, C2, C4A, and C4B types. Pedigree analysis revealed long stretches (several megabases) of apparently fixed DNA within the MHC that we referred to as "extended haplotypes" (later as "CEHs"). About 10 to 12 common CEHs constitute at least 25 - 30% of MHC haplotypes among European Caucasian populations. These CEHs contain virtually all the most common markers of MHC-associated diseases. In the case of type 1 diabetes, we have proposed a purely genetic and epigenetic model (with a small number of Mendelian recessive disease genes) that explains all the puzzling features of the disease, including its rising incidence.

Genetic Basis of Human Complement C8α-γ Deficiency

Deficiency of the α-γ subunit of the eighth component of complement (C8α-γD) is frequently associated with recurrent neisserial infections, especially meningitis caused by Neisseria meningitidis. We here report the molecular basis of C8α-γD in two unrelated Japanese subjects. Screening all 11 exons of the C8α gene and all 7 exons of the C8γ gene and their boundaries by exon-specific PCR/single-strand conformation polymorphism demonstrated aberrant single-stranded DNA fragments in exon 2 of C8α gene in case 1 and in exons 2 and 9 of C8α gene in case 2. Nucleotide sequencing of the amplified DNA fragments in case 1 revealed a homozygous single-point mutation at the second exon-intron boundary, inactivating the universally conserved 5′ splice site consensus sequence of the second intron (IVS2+1G→T). Case 2 was a compound heterozygote for the splice junction mutation, IVS2+1G→T, and a nonsense mutation at Arg394 (R394X). R394X was caused by a C to T transition at nucleotide 1407, the first nucleotide of the codon CGA for Arg394, leading to a stop codon TGA. No mutations were detected in the C8γ gene by our method. Our results indicate that the pathogenesis of C8α-γD might be caused by heterogeneous molecular defects in the C8α gene.

The human complement C8G gene, a member of the lipocalin gene family: polymorphisms and mapping to chromosome 9q34.3

Complement component C8 is a plasma glycoprotein consisting of three nonidentical polypeptide chains (alpha, beta, gamma) which are encoded by three separate genes (C8A, C8B, C8G). The gamma chain whose functional role remains undefined is not related to any other complement protein but is a member of the lipocalins, a family of proteins that bind small hydrophobic ligands. The present report describes the first known polymorphisms for the human C8G gene, namely one polymorphic site in exon 1 (207T/G) and two polymorphic sites in intron 1 (213 + 37G --> A; 213 + 65del3). Specific typing can be performed using simple polymerase chain reaction-based assays. C8G genotyping in eight CEPH reference families demonstrated that C8G is closely linked to a series of marker loci located in the most telomeric region of chromosome 9q. Multipoint analysis placed C8G with 1000:1 support distal to D9S207. C8G is thus located at 9q34.3. Remarkably, this chromosomal region contains at least four other lipocalin genes.

The eighth component of human complement: molecular basis of C8A (C81) polymorphism

Using an exon-specific polymerase chain reaction (PCR) followed by direct DNA sequence analysis we have analyzed the polymorphism of the alpha-chain of the eighth component of human complement (C8) at the DNA level. We found that two common alleles, C8A*A and C8A*B, are characterized by the substitution of a single amino acid (Gln to Lys), which is caused by a point mutation of a single nucleotide (C to A) in exon 3 at position 187 of the mature C8 alpha cDNA sequence. Based on this mutation, an allele-specific PCR was designed detecting the two alleles of C8A. We applied this method to type the C8A polymorphism using DNA samples from a Chinese Han population. The comparison with the data of protein typing of the same samples proved that the described method is efficient and reliable for the identification of C8A genotypes and may be valuable for further application in population studies and forensic science.

Human complement component C8. Molecular basis of the beta-chain polymorphism

The beta-chain of human complement component C8 exhibits a structural genetic polymorphism: using isoelectric focusing two major allotypes can be identified (C8B B ('basic') and C8B A ('acidic')). In the present report we describe a sequence polymorphism of the C8B gene (codon 63: AGA-->GGA) and demonstrate that the resulting amino acid substitution (Arg-->Gly) consistently differentiates between the two common charge variants of the C8 beta chain; the C8B B allotype is characterized by an Arg and the C8B A allotype by a Gly residue in position 63 of the C8 beta polypeptide chain.

Human complement C81 (C8 A) polymorphism: detection and segregation of new variants

In addition to the earlier detected C81(A) rare variants A1, A2 (now A3) and B1 (now B2), six new rare variants (C81 A2 new, A4, A5, A6, M1 and B1new) are described within the polymorphism of the eighth component of human complement (alpha-gamma chain subunit). Except for A3, all rare C81 A variants are only detected by isoelectric focusing, and not by SDS polyacrylamide gel electrophoresis (PAGE), in the alpha-gamma subunit. In one individual out of approximately 700 individuals studied, a reversed position of the common allele (B vs A) was observed by SDS PAGE and the isofocusing technique. The segregation of A1, A3 and A4 could be followed in putative father/child combinations.

The human complement component C8B gene: structure and phylogenetic relationship

The eighth component of human complement (C8) is a serum protein that consists of three chains (alpha, beta and gamma), encoded by three separate genes, viz., C8A, C8B, and C8G. In serum, the beta-subunit is non-covalently bound to the disulfide-linked alpha-gamma subunit. Using a full-length C8 beta cDNA probe, we isolated several clones from human genomic lambda DNA libraries. Four lambda clones covering the complete cDNA sequence were characterized by TaqI restriction mapping and were "shotgun" subcloned into M13. C8 beta-cDNA-positive clones were partially sequenced to characterize the 12 exons of the gene with sizes from 69 to 347 bp. All intron-exon junctions followed the GT-AG rule. By using polymerase chain reaction (PCR) primers located in the adjacent intron sequences, all 12 exons of the C8B gene could be amplified from genomic DNA. All fragments showed the expected sizes. The sizes of eight introns could be determined by using primer pairs that amplified two exons and the enclosed intron, and by restriction mapping. These analyses and the insert sizes of the genomic lambda clones indicate that the C8B gene has a total size of approximately 40 kb. The polymorphic TaqI site of the C8B gene localized in intron 11 could be demonstrated by direct restriction fragment analysis of a PCR fragment containing exons 11 and 12, and the enclosed intron 11. Homology comparison of the C8B gene with C8A and C9 on the basis of the exon structure confirmed the ancestral relationship known from the protein level.

DNA polymorphism of the human complement C8 beta gene: formal genetics and intragenic localization

The eighth component of human complement consists of three subunits of different molecular mass, which are coded for by three separate genetic loci. Polymorphisms have been described at the protein level for the alpha and beta subunits by means of sodium dodecyl sulfate gel electrophoresis and isoelectric focusing. Using a full-length human C8 beta cDNA probe, we have studied more than 100 individuals by Southern blot analysis to detect DNA polymorphisms. We have found two restriction fragment length polymorphisms (RFLPs) with the enzymes Taq I and Bam HI. The Taq I polymorphism is defined by two alleles, i.e., a single 4.9 kb fragment or two 2.8/2.1 kb fragments. The allele frequencies are 0.68 and 0.32, respectively. The second RFLP with Bam HI is correlated with the Taq I variants: 3 kb Bam HI; 4.9 kb Taq I and 3.3 kb Bam HI; 2.8/2.1 kb Taq I. Both RFLPs could be mapped to the 3' portion of the C8 beta gene. Based on the size of genomic restriction fragments, the C8 beta gene can be estimated to have a size of 32-36 kb. Because of the even frequency distribution, the C8 beta DNA polymorphisms may be useful in gene mapping and disease association studies.

Chromosomal assignment of genes encoding the alpha, beta, and gamma subunits of human complement protein C8: identification of a close physical linkage between the alpha and the beta loci

The eighth component of human complement (C8) is a serum protein containing three nonidentical subunits (alpha, beta, gamma) that are arranged as a disulfide-linked alpha-gamma dimer and a noncovalently associated beta chain. In earlier genetic studies, electrophoretic analysis of C8 protein polymorphisms revealed several allelic variants of alpha-gamma and beta. These were governed by separate loci designated C8A and C8B for alpha-gamma and beta, respectively. Genetic linkage analyses indicated that these loci were linked to each other and to chromosome 1 marker loci PGM1 and Rh, but it was unclear at the time if C8A was a single locus coding for a single-chain precursor form of alpha-gamma or if separate loci existed for alpha and gamma. Since evidence now indicates that alpha, beta, and gamma are encoded by separate genes, cDNA probes corresponding to each subunit were used to make direct assignments of the individual loci. Analysis of somatic cell hybrids revealed that only the alpha and beta loci are located on chromosome 1. Parallel analysis of genomic DNA digests using 5' and 3'-specific cDNA probes showed they are physically linked (less than 2.5 kb) and oriented 5' alpha-beta 3'. Further probing of the hybrid panel revealed that gamma is located on chromosome 9q. Thus, the observed genetic linkage of alpha-gamma to beta must be determined solely by alpha. In accordance with these findings, the C8 loci should now be designated C8A, C8B, and C8G for alpha, beta and gamma, respectively.

Deficiency of the eighth component of complement. Evidence for linkage of C8 alpha-gamma pattern with C8 beta deficiency in sera of twelve patients

The C8 alpha-gamma subunit of the eighth component of complement was analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and immunoblotting in sera from 68 normal individuals, 12 C8 beta-deficient patients (from seven unrelated families), and 10 of the parents of the latter. Three different forms of the C8 alpha-gamma subunit were observed: 34/68 normal individuals were found to have a C8 alpha-gamma triple band (termed C8 alpha-gamma 1, C8 alpha-gamma 2, C8 alpha-gamma 3 variants), 23/68 the C8 alpha-gamma 2 and C8 alpha-gamma 3 variants, and 11/68 the C8 alpha-gamma 1 and C8 alpha-gamma 3 variants. In contrast, all C8 beta-deficient patients had detectable C8 alpha-gamma 2 and C8 alpha-gamma 3 variants but lacked the C8 alpha-gamma 1 variant in addition to the C8 beta subunit. Three out of ten parents of the C8 beta-deficient patients were found to have the C8 alpha-gamma triple band, whereas 7/10, like their children, had the C8 alpha-gamma 2 and C8 alpha-gamma 3 variants only. We conclude that there is a linkage between the C8 alpha-gamma pattern and C8 beta deficiency. These data may support earlier findings that in humans the genes encoding for C8 alpha-gamma and C8 beta are closely linked on chromosome 1.

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.

The eighth component of human complement: evidence that it is an oligomeric serum protein assembled from products of three different genes

The eighth component of human complement (C8) consists of three nonidentical subunits arranged asymmetrically as a disulfide-linked alpha-gamma dimer and a noncovalently associated beta chain. Genetic studies of C8 polymorphisms established that alpha-gamma and beta are encoded at different loci. Implicit in this finding was the existence of two different genes and the likelihood that alpha-gamma would be synthesized in single-chain precursor form. However, recent characterization of cDNA clones revealed separate mRNAs for human alpha and beta but no evidence of a single-chain precursor for alpha-gamma. A cDNA clone containing the entire coding region for human gamma has now been characterized, and its sequence supports the existence of a separate gamma mRNA. Included are a consensus translation initiation sequence, an apparent initiation methionine, and a signal peptide. By use of cDNA probes specific for human alpha, beta, or gamma, analysis of poly(A) RNA from normal baboon liver revealed separate mRNAs of 2.5, 2.6, and 1.0 kilobases (kb), respectively. Parallel analysis of poly(A) RNA from rat liver identified mRNAs of 3.4, 2.3, and 0.9 kb. These results argue against the possibility that C8 is assembled from products of two different genes (alpha-gamma and beta) and suggest it is comprised of three different gene products (alpha, beta, and gamma) that undergo both covalent and noncovalent association to yield the mature protein.

Inherited deficiencies of complement components in man

Isolated inherited deficiency states of almost every complement protein have been recognized. Almost all are autosomal recessive traits. Deficiency of the early-acting components C1, C4 and C2 is associated with increased risk of immune complex disease, particularly systemic lupus erythematosus. Patients with deficiency of C3, factor I or factor H have increased susceptibility to infection by pyogenic bacteria, whereas those with deficiencies of properdin, C5, C6, C7 or C8 are prone to systemic neisserial infection. Inherited deficiency of C1 inhibitor is transmitted as an autosomal dominant trait, is genetically heterogeneous, and is associated with attacks of angioedema and consumption of C4 and C2. There is evidence that a plasmin-modified fragment of C2 is responsible for the angioedema in this disorder. Administration of androgens tends to correct the biochemical abnormalities of hereditary angioedema and to prevent attacks.

The C8A and C8B loci are closely linked on chromosome 1

Close linkage was demonstrated between the loci governing the polymorphisms of complement component C8 alpha-gamma (C8A) and beta (C8B). Both C8 loci were linked to the chromosome 1 marker loci PGM1 and Rh. The distance between the two C8 loci and PGM1 appeared identical in males and females. A female/male ratio of 1.6 was observed between the two C8 loci and Rh. No evidence for linkage between the C8 loci and Fy was found. Preliminary results of this study were presented at the Eighth International Workshop on Human Gene Mapping, Helsinki, August 1985 (Rogde et al. 1985b).

Human C81 (alpha-gamma) polymorphism: detection in the alpha-gamma subunit on SDS-PAGE, formal genetics and linkage relationship

The molecular basis of human C81 (alpha-gamma) polymorphism could be elucidated by immunoprecipitation of human C81 allotypes and separation of the alpha-gamma and beta subunits on sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) under nonreducing conditions. If the C8 molecules were completely reduced, C81 polymorphism was no longer detectable on SDS-PAGE. It is concluded that C81 variation depends on charge rather than molecular weight differences. Four C81 allotypes, the common A and B and two rare allotypes provisionally named A2 and B1, could be distinguished. The rare allotype A1 as detected by isoelectric focusing with subsequent C8 (alpha-gamma)-dependent functional overlay could no longer be visualized on SDS-PAGE. This allotype may therefore be elicited only in the intact C8 molecule. The beta-chain polymorphism named C82, probably also reflecting charge variation of the C8 molecule, could not be detected yet on SDS-PAGE. The distributions of C81 phenotypes and their respective allele frequencies were in good agreement with previously reported data. In the study of 30 families with 100 offspring, no deviation from the rule of at least four codominant alleles at one genetic locus was found. Linkage between C81 gene(s) and PGM1a encoded on chromosome 1 could be confirmed. The following estimates were obtained: (formula; see text) with S theta being the standard error of the maximum likelihood estimate theta. The new technique for allotyping human C81 at the subunit may provide a new tool for the differentiation of qualitative and quantitative variation of the eighth component of human complement.

Genetic polymorphism of human complement component C81 in the Japanese population

Genetic polymorphism of human C81 has been investigated using polyacrylamide gel isoelectric focusing (PAGIEF) in the presence of 3.1 M urea followed by electroblotting with enzyme immunoassay. In 448 individuals phenotypes of C81 were classified into three common and four rare patterns, and these were considered to be controlled by two common alleles, C81 A and C81 B, and three rare alleles which were tentatively designated C81 A1J and C81 A2J for acidic variants and C81 B1J for the basic variant. The alleles of C81 A2J and C81 B1J are new rare alleles, but C81 A1J might correspond to C81 A1 in the former studies. Family data were in accordance with the hereditary rules. The gene frequencies were estimated as C81 A is 0.6228, C81 B is 0.3672, C81 A1J is 0.0078, C81 A2J is 0.0011, and C81 B1J is 0.0011, respectively. The gene frequencies of the two common alleles agreed approximately with other ethnic groups. PAGIEF of neuraminidase-treated plasma samples followed by electroblotting with enzyme immunoassay is applicable to the study of heterogeneity of C81.

Genetic polymorphism of complement component C8

Extensive genetic polymorphism of complement component C8 was demonstrated by isoelectric focusing of serum or plasma samples followed by immunoblotting procedures. Using these methods, we could detect both alpha-gamma (C81) and beta (C82) chain polymorphisms in the same gel. Two-dimensional (2D) electrophoresis of C8 immunoprecipitates was used to obtain further information of the C8 patterns. Evidence was obtained that the C81 polymorphism resides in the structural gene of the C8 alpha chain. Both C8 systems show autosomal, chiefly codominant inheritance, and the distribution of phenotypes agrees with the Hardy-Weinberg equilibrium. Our findings suggest at least five different alleles in the C81 system; the gene frequencies of the two most common ones, C81*A and C81*B being 0.59 and 0.39, respectively. In C82 we found evidence for at least three codominant alleles, the gene frequencies for the two most common ones, C82*B and C82*A being 0.94 and 0.05, respectively. In addition, family studies disclosed the existence of a null allele, C82*Q0.

Inherited C8 beta subunit deficiency in a patient with recurrent meningococcal infections: in vivo functional kinetic analysis of C8

A 16 year old with recurrent meningococcal infections is reported. Absence of haemolytic activity in both the classical and alternative pathways resulted from an absence of functional C8. Addition of functional C8 restored hemolytic activity. Antigenically deficient C8 was present in the serum and isoelectric focusing of serum confirmed the absence of the C8 beta chain. Following the infusion of fresh frozen plasma, we followed the decay in C8 functional activity as well as total haemolytic activity. C8 activity peaked at about 3 h with a half-life survival estimated to be 28 h. The kinetics of total haemolytic activity showed a slower decay with an exponential decline over 72 h and a half-life of 55 h. Fresh frozen plasma may be of value in the treatment of patients with C8 deficiency and acute Neisserial infections.

Complotype genetic loci segregate more frequently with HLA-DR than with HLA-B

The loci for BF, C2, C4A, and C4B are very closely linked to each other so that alleles of these plasma protein markers occur in populations in linkage disequilibrium and are inherited as single genetic units called complotypes. These complotypes are coded by a DNA region of the short arm of chromosome 6 embracing approximately 100 kilobases, which serve as a marker of the major histocompatibility complex. We have studied the complotypes of nine families with known HLA-B/DR crossovers. In seven families, the complotypes were inherited with HLA-DR, including in one family with a double recombination. The haplotype HLA-A28, Cw1, B27, FC3, 20, DR4 of JTr resulted from two recombinations between HLA-A2, Cw1, B27, SC42, DR7 and HLA-A28, Cwx or Cw1, B37, FC3, 20, DR4. In the remaining two families (Ro and Lo) the complotypes were inherited with HLA-B. The haplotype A2, Cw5, Bw44, SC30, DR3 of StLo resulted from paternal recombination between the haplotypes A2, Cw5, Bw44, SC30, DR4 and A24, B8, SC01, DR3, and the haplotype A24, Cw4, Bw35, SC31, DR3 of NaRo resulted from maternal recombination between A24, Cw4, Bw35, SC31, DR4 and A26, Bw41, FC31, DR3. Our data suggest that the complotype region maps closer to HLA-D than to HLA-B.

Phenotypic genetics of complement components

Both charge and size-dependent electrophoretic techniques have been used to investigate genetic polymorphisms of complement proteins. Of the seventeen complement proteins, ten have been shown to have genetic variants and only one (C9) has been extensively investigated without revealing variants. These investigations give information on the numbers of cistrons and their linkage relations. They demonstrate or confirm the linkage of C2, Factor B and C4 to the MHC. In the cases of both human and mouse C4, it has been shown that the loci are (usually) duplicated. C4 in humans is extremely polymorphic and exhibits a number of strong allelically associated haplotypes. Some of these have only one expressed C4 gene and are associated with disease susceptibility. C8 has at least two cistrons coding for associating subunits. C6 and C7 are linked in several species and sometimes C7 is duplicated. This gene pair is discussed in relation to natural selection and gene conversion.

Population and formal genetics of the human C81(alpha-gamma) polymorphism

One hundred and ninety-six unrelated healthy individuals and 30 families with 75 offspring have been studied for the C81(alpha-gamma) polymorphism. The following allele frequencies were calculated: C81*A = 0.5536; C81*B = 0.4286; C81*A1 = 0.0178. Observed and expected phenotype frequencies were in a good agreement according to the Hardy Weinberg law. No exceptions from the mode of inheritance were found. In family W the segregation of the rare allele C81*A1 could be followed. Comparing the results of this study with previous data from Boston and Oslo, a combined technology including C8-dependent lysis and C8 structural variation is suggested for future investigations.

Genetic polymorphism of the seventh component of complement in a Japanese population

Genetic polymorphism of C7 in a Japanese population has been described, using polyacrylamide gel isoelectric focusing electrophoresis followed by an electrophoretic blotting technique. Phenotypes of C7 were classified into six common patterns, and observed phenotypes were produced by autosomal codominant at a single locus with three alleles. Three common alleles, designated C7*B, C7*M and C7*A, were found, and gene frequencies calculated from 494 individuals showed C7*B = 0.858, C7*M = 0.096 and C7*A = 0.046, respectively. It is noteworthy that both C7*M and C7*A have polymorphic frequencies in the Japanese population. The distribution of phenotypes fitted the Hardy-Weinberg equilibrium. Results indicate that the electrophoretic blotting technique, which has high specificity and sensitivity, is applicable in the study of heterogeneity of protein antigens.

Genetic polymorphism of complement C6 and haplotype analysis between C6 and C7 in a Japanese population

Genetic polymorphism of C6 in the Japanese population has been described using polyacrylamide gel isoelectric focusing electrophoresis followed by the electrophoretic blotting technique, and haplotype analysis between C6 and C7 has also been investigated. In 565 plasma samples five different common patterns and three rare variant patterns were observed, and these were controlled by autosomal codominance at a single locus with three common and one rare alleles. These alleles were designated C6*B, C6*A, C6*B2, and C6*M, and gene frequencies were estimated to be 0.50265, 0.43186, 0.06018, and 0.00531 for C6*B, C6*A, C6*B2, and C6*M, respectively. It is noteworthy that C6*B2 has a polymorphic frequency in the Japanese population. The distribution of phenotypes fitted the Hardy-Weinberg equilibrium. Two combinations between C6 and C7 alleles, namely C6B-C7B and C6M-C7B, were shown to be in significant positive linkage disequilibrium. The presence of allelic combinations showing linkage disequilibrium suggests the close proximity between the C6 and C7 loci.

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.

Inherited complement component deficiencies in membranoproliferative glomerulonephritis

Anecdotal reports of complement component deficiencies in patients with immune complex disease led to a systematic study of the levels of seven complement components in serum specimens from 178 patients with glomerulonephritis and 163 normal subjects. Deficiencies were found with significantly higher frequency (22.7%) among 44 patients with membranoproliferative glomerulonephritis (MPGN) types I and III, than among the normal subjects (6.7%, P less than 0.002) or among 134 patients with other glomerulonephritides (5.2%, P less than 0.001). The component deficiencies in MPGN were partial in nine patients and subtotal in one. They could not be ascribed to acquired hypocomplementemia or to a nephrotic syndrome. They were present over long periods, were found in family members, and involved C2, C3, factor B, C6, C7, and C8. Six were presumably the result of null structural genes, two were associated with a structurally abnormal component, and two were of unknown cause. The results give evidence that partial deficiency of one or more complement components is a factor predisposing to MPGN.

Genetic polymorphism in C8 beta-chains. Evidence for two unlinked genetic loci for the eighth component of human complement (C8)

Genetic polymorphism in the beta-subunit of the eighth component of human complement, C8, was defined by isoelectric focusing of serum in polyacrylamide gel in the presence of urea and development of specific patterns of hemolysis in an overlay gel containing antibody-sensitized erythrocytes and C8 beta-chain-deficient serum. Bands of hemolysis induced by serum from unrelated Caucasians suggested autosomal codominant inheritance of three structural alleles at a single locus, C82: C82 degrees A (acidic), C82 degrees B (basic), and C82 degrees A1 (very acidic) with frequencies of 0.952, 0.044, and 0.004, as well as the probable null allele C82 degrees Q0. The distribution of phenotypes agreed with the Hardy-Weinberg equilibrium. The previously described genetic polymorphism in human C8 defined with the use of "complete" C8 (C8 alpha-gamma-chain)-deficient serum was distinct from and independent of the inherited structural variation at C82. Therefore, the locus for C8 alpha-gamma-chains has been redesignated C81, and has the alleles C81 degrees A, C81 degrees A1, and C81 Q0. Linkage studies failed to show close linkage between the two loci for C8, C81, and C82, and between C82 and the major histocompatibility complex or C6.

Genetic regulation of a structural polymorphism of human C3b receptor

Two forms of the human C3b receptor (C3bR), which have relative molecular weights (Mr) of 250,000 and 260,000 and are designated F and S, respectively, have been identified in specific immunoprecipitates from erythrocytes and leukocytes by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Both forms of the receptor were visualized on gels by autoradiography of 125I-labeled antigen and by silver nitrate staining. Individual donors expressed one of three possible patterns of C3bR, either the F or S form alone or both, and these patterns represented stable phenotypic characteristics of their erythrocytes and polymorphonuclear and mononuclear leukocytes. Removal of N-linked oligosaccharides by endoglycosidase-F treatment decreased the Mr of both forms but did not abolish the difference in their electrophoretic mobilities. That both forms of the receptor were functional was indicated by the capacity of all antigenic C3bR sites on erythrocytes from individuals having any of the three phenotypes to bind dimeric C3b with affinities ranging from 3 to 5 X 10(7) M-1. Analyses of the occurrence of the F and S forms of C3bR in 76 individuals from 15 families revealed that this polymorphism was regulated by two alleles transmitted in an autosomal codominant manner. Of 111 normal unrelated individuals, 64.9% were homozygous for the F form (FF), 1.8% were homozygous for the S form (SS), and 33.3% were heterozygotes (FS). This distribution did not differ from that calculated by the Hardy-Weinberg equilibrium based on two codominant alleles that regulate the expression of the F and S forms and that have frequencies of 81.5 and 18.5%, respectively. The locus regulating structural polymorphism of C3bR is designated C3BRM (M for mobility or Mr), and is distinct from the recently described locus regulating the quantitative expression of C3bR on erythrocytes.

Inherited deficiency of C8 in a patient with recurrent meningococcal infections: further evidence for a dysfunctional C8 molecule and nonlinkage to the HLA system

An adult male with recurrent meningococcal infections is reported whose serum lacked functional C8 activity but possessed antigenic C8. The addition of 1500 U of purified C8/ml of serum restored hemolytic activity to normal. Four to five times more C8 was required to restore bactericidal activity than to restore hemolytic activity. Bactericidal activity could also be restored by mixing the patient's serum with a second C8-deficient serum that lacked detectable antigenic or functional C8. The patient's serum contained bactericidal antibody for groups A, B, C, and Y meningococci and specific antibody to group Y capsular polysaccharide. There was two to three times more bactericidal antibody activity in the serum than in a pool of normal sera for the infecting strain. Family studies disclosed a sibling who was HLA identical to the patient but whose serum contained normal amounts of total hemolytic and C8 functional activity.

The location of C2, C4, and BF relative to HLA-B and HLA-D

The loci for HLA-A,B,C,D, and DR are known to be closely linked to the structural loci for the complement components C2, BF, and the duplicated loci for C4, C4A and C4B. Conflicting evidence has been presented for the order of these genes. However, new techniques have made possible identification of markers in the HLA-D and C4 region for nearly all identified haplotypes. In our population we have confirmed five HLA-B-D crossovers and in each case informative allotypes of C2, BF, or C4A and C4B segregated with HLA-D or DR suggesting that the loci for these proteins lie close to HLA-D and DR. These findings may be of importance for resolving problems encountered in the assignment of HLA-D alleles.

Genetic analysis of C4 deficiency

The inherited structural polymorphism in the fourth component of complement was studied in the family of a child with homozygous deficiency of this protein. It was shown that a number of family members, including the child's parents, carried a C4 haplotype, C4A*QO C4B*QO, that produced no detectable protein at either the Chido (C4B) or Rodgers (C4A) locus. The family contained individuals with one, two, three, or four expressed C4 genes, and the mean serum C4 levels in such individuals roughly reflected the number of structural genes.