Structure and organization of the C4 genes

Polysaccharides Extracted From Panax Ginseng C.A. Mey Enhance Complement Component 4 Biosynthesis in Human Hepatocytes

Panax ginseng C.A. Mey (ginseng) is a classic medicinal plant which is well known for enhancing immune capacity. Polysaccharides are one of the main active components of ginseng. We isolated water-soluble ginseng polysaccharides (WGP) and analyzed the physicochemical properties of WGP including molecular weight, monosaccharide composition, and structural characteristics. WGP had minimal effect on the growth of hepatocytes. Interestingly, WGP significantly increased the mRNA and protein levels of complement component 4 (C4), one of the core components of the complement system. Promoter reporter gene assays revealed that WGP significantly enhanced activity of the C4 gene promoter. Deletion analyses determined that the E-box1 and Sp1 regions play key roles in WGP-induced C4 transcription. Taken together, our results suggest that WGP promotes C4 biosynthesis through upregulation of transcription. These results provide new explanation for the intrinsic mechanism by which ginseng boosts human immune capacity.

Complement C4A Regulates Autoreactive B Cells in Murine Lupus

Systemic lupus erythematosus (SLE) is a severe autoimmune disease mediated by pathogenic autoantibodies. While complement protein C4 is associated with SLE, its isoforms (C4A and C4B) are not equal in their impact. Despite being 99% homologous, genetic studies identified C4A as more protective than C4B. By generating gene-edited mouse strains expressing either human C4A or C4B and crossing these with the 564lgi lupus strain, we show that, overall, C4A-like 564Igi mice develop less humoral autoimmunity than C4B-like 564Igi mice. This includes a decrease in the number of GCs, autoreactive B cells, autoantibodies, and memory B cells. The higher efficiency of C4A in inducing self-antigen clearance is associated with the follicular exclusion of autoreactive B cells. These results explain how the C4A isoform is protective in lupus and suggest C4A as a possible replacement therapy in lupus.

Simoni et al. address a long-standing question about how complement C4A and C4B isoforms differ in function in vivo in autoimmunity. They find that C4A leads to an increased protection in humoral autoimmunity relative to C4B. Autoantibody diversity is likewise dependent on the C4 protein isotype.

Increased expression of schizophrenia-associated gene C4 leads to hypoconnectivity of prefrontal cortex and reduced social interaction

Schizophrenia is a severe mental disorder with an unclear pathophysiology. Increased expression of the immune gene C4 has been linked to a greater risk of developing schizophrenia; however, it is not known whether C4 plays a causative role in this brain disorder. Using confocal imaging and whole-cell electrophysiology, we demonstrate that overexpression of C4 in mouse prefrontal cortex neurons leads to perturbations in dendritic spine development and hypoconnectivity, which mirror neuropathologies found in schizophrenia patients. We find evidence that microglia-mediated synaptic engulfment is enhanced with increased expression of C4. We also show that C4-dependent circuit dysfunction in the frontal cortex leads to decreased social interactions in juvenile and adult mice. These results demonstrate that increased expression of the schizophrenia-associated gene C4 causes aberrant circuit wiring in the developing prefrontal cortex and leads to deficits in juvenile and adult social behavior, suggesting that altered C4 expression contributes directly to schizophrenia pathogenesis.

Elevated expression of the gene encoding complement C4 is associated with an enhanced risk of schizophrenia, but the mechanism underlying this link is unclear. This study shows that overexpression of the C4 gene in mice leads to mis-wiring of the prefrontal cortex and deficits in social interactions.

Schizophrenia risk from complex variation of complement component 4

Schizophrenia is a heritable brain illness with unknown pathogenic mechanisms. Schizophrenia's strongest genetic association at a population level involves variation in the major histocompatibility complex (MHC) locus, but the genes and molecular mechanisms accounting for this have been challenging to identify. Here we show that this association arises in part from many structurally diverse alleles of the complement component 4 (C4) genes. We found that these alleles generated widely varying levels of C4A and C4B expression in the brain, with each common C4 allele associating with schizophrenia in proportion to its tendency to generate greater expression of C4A. Human C4 protein localized to neuronal synapses, dendrites, axons, and cell bodies. In mice, C4 mediated synapse elimination during postnatal development. These results implicate excessive complement activity in the development of schizophrenia and may help explain the reduced numbers of synapses in the brains of individuals with schizophrenia.

An integrated genomic analysis of gene-function correlation on schizophrenia susceptibility genes

Schizophrenia is a highly complex inheritable disease characterized by numerous genetic susceptibility elements, each contributing a modest increase in risk for the disease. Although numerous linkage or association studies have identified a large set of schizophrenia-associated loci, many are controversial. In addition, only a small portion of these loci overlaps with the large cumulative pool of genes that have shown changes of expression in schizophrenia. Here, we applied a genomic gene-function approach to identify susceptibility loci that show direct effect on gene expression, leading to functional abnormalities in schizophrenia. We carried out an integrated analysis by cross-examination of the literature-based susceptibility loci with the schizophrenia-associated expression gene list obtained from our previous microarray study (Journal of Human Genetics (2009) 54: 665-75) using bioinformatic tools, followed by confirmation of gene expression changes using qPCR. We found nine genes (CHGB, SLC18A2, SLC25A27, ESD, C4A/C4B, TCP1, CHL1 and CTNNA2) demonstrate gene-function correlation involving: synapse and neurotransmission; energy metabolism and defense mechanisms; and molecular chaperone and cytoskeleton. Our findings further support the roles of these genes in genetic influence and functional consequences on the development of schizophrenia. It is interesting to note that four of the nine genes are located on chromosome 6, suggesting a special chromosomal vulnerability in schizophrenia.

Complement C4B protein in schizophrenia

Partial and/or complete deficiency of the complement protein C4 is associated with autoimmune and infectious diseases. Infectious or autoimmune processes may have a role in schizophrenia. Previous reports suggest abnormalities in the complement C4B isotype in schizophrenia and other mental disorders. We assessed C4A and C4B isotypes and serum C4B protein concentration in Armenian schizophrenic patients. Although there was no difference in frequency of C4BQ0, C4B serum protein level was significantly decreased in the schizophrenic patients compared with healthy controls.

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.

Deficiencies of human complement component C4A and C4B and heterozygosity in length variants of RP-C4-CYP21-TNX (RCCX) modules in caucasians. The load of RCCX genetic diversity on major histocompatibility complex-associated disease

The complement component C4 genes located in the major histocompatibility complex (MHC) class III region exhibit an unusually complex pattern of variations in gene number, gene size, and nucleotide polymorphism. Duplication or deletion of a C4 gene always concurs with its neighboring genes serine/threonine nuclear protein kinase RP, steroid 21-hydroxylase (CYP21), and tenascin (TNX), which together form a genetic unit termed the RCCX module. A detailed molecular genetic analysis of C4A and C4B and RCCX modular arrangements was correlated with immunochemical studies of C4A and C4B protein polymorphism in 150 normal Caucasians. The results show that bimodular RCCX has a frequency of 69%, whereas monomodular and trimodular RCCX structures account for 17.0 and 14.0%, respectively. Three quarters of C4 genes harbor the endogenous retrovirus HERV-K(C4). Partial deficiencies of C4A and C4B, primarily due to gene deletions and homoexpression of C4A proteins, have a combined frequency of 31.6%. This is probably the most common variation of gene dosage and gene size in human genomes. The seven RCCX physical variants create a great repertoire of haplotypes and diploid combinations, and a heterozygosity frequency of 69.4%. This phenomenon promotes the exchange of genetic information among RCCX constituents that is important in homogenizing the structural and functional diversities of C4A and C4B proteins. However, such length variants may cause unequal, interchromosomal crossovers leading to MHC-associated diseases. An analyses of the RCCX structures in 22 salt-losing, congenital adrenal hyperplasia patients revealed a significant increase in the monomodular structure with a long C4 gene linked to the pseudogene CYP21A, and bimodular structures with two CYP21A, which are likely generated by recombinations between heterozygous RCCX length variants.

Extensive conservation of upstream C4 promoter sequences: a comparison between C4A and C4B

Basal levels of expression of complement C4 vary in individuals but it is unknown whether this is due to gene copy number, protein clearance rates or differences in expression. To investigate whether differences in the promoter region may influence transcription of the C4 genes, we sequenced the promoter region of the C4B1 gene from the HLA-A1, B8, C4AQ0, C4B1, DR3 haplotype and compared this with an equivalent C4A3 region. The promoter regions of the C4 genes were highly conserved, indicating that transcriptional differences are unlikely. An unusual feature of the alignment was the higher level of polymorphism in the RP genes past the breakpoint of deletion/duplication in a region where the highest conservation would be expected.

Complement component C4 gene intron 9 as a phylogenetic marker for primates: long terminal repeats of the endogenous retrovirus ERV-K(C4) are a molecular clock of evolution

The complement component C4 genes of Old World primates exhibit a long/short dichotomous size variation, except that chimpanzee and gorilla only contain short C4 genes. In human it has been shown that the long C4 gene is attributed to the integration of an endogenous retrovirus, HERV-K(C4), into intron 9. This 6.36 kilobase retroviral element is absent in short C4 genes. Here it is shown that the homologous endogenous retrovirus, ERV-K(C4), is present precisely at the same position in the long C4 gene of orangutan and African green monkey. Determination of the short C4 gene intron 9 sequences from human, three apes, two Old World monkeys, and a New World monkey allowed the establishment of consistent phylogenetic trees for primates, which favors a chimpanzee-gorilla clade. The 5' long terminal repeats (LTR) and 3' LTR of ERV-K(C4) in long C4 genes of human, orangutan, and African green monkey have similar sequence divergence values of 9.1%-10.5%. These values are more than five-fold higher than the sequence divergence of the homologous intron 9 sequences between the long and short C4 genes in higher primates. The latter is probably a result of homogenization or concerted evolution. We suggest that the 5' LTR and 3' LTR of an endogenous retrovirus can serve as a reliable reference point or a molecular clock for studies of gene duplication and gene evolution. This is because the 5'/3' LTR sequences were identical at the time of retroviral integration and evolved independently of each other afterwards. Our data provides strong evidence for the short C4 gene being the ancestral form in primates, trans-species evolution, and the "slow-down" phenomenon of the sequence divergence in great apes.

Identification of a novel family of human endogenous retroviruses and characterization of one family member, HERV-K(C4), located in the complement C4 gene cluster

We have identified a novel family of about 10-50 human endogenous retrovirus elements (HERVs) and have characterized one family member (HERV-KC4). This retrovirus element is integrated within intron 9 of and complement C4A genes and also in some C4B genes, and is a principal contribution to interlocus and interallelic length heterogeneity of C4 genes. The HERV-K(C4) sequence has a typical retrovirus structure with elements of gag, pol and env domains, flanked by two long terminal repeats (LTRs) and is similar to type A, B and D retroviruses. Multiple termination codons preclude the existence of long open reading frames, suggesting that the HERV-K(C4) sequence is no longer functional. Zoo blot hybridization reveals that New World monkeys appear to lack sequences similar to HERV-K(C4), suggesting that integration has occurred after the divergence of Old and New World monkeys. Retrotransposition of prototype viruses is presumed to have led to the amplification and integration of the members of the family in different loci, which in humans, appear to be dispersed over several chromosomes. The absence of the HERV-K(C4) element in some C4B genes in both humans and orangutangs indicate that the retrovirus inserted into the C4A gene after the duplication of the cluster. Subsequent spread of the HERV-K(C4) sequence to C4B genes presumably occurred by interlocus sequence exchange mechanisms, such as unequal crossover and gene conversion-like mechanisms.

The dichotomous size variation of human complement C4 genes is mediated by a novel family of endogenous retroviruses, which also establishes species-specific genomic patterns among Old World primates

The human complement C4 genes in the HLA exhibit an unusual, dichotomous size polymorphism and a four-gene, modular variation involving novel gene RP, complement C4, steroid 21-hydroxylase (CYP21), and tenascin-like Gene X (RCCX). The C4 gene size dichotomy is mediated by an endogenous retrovirus, HERV-K(C4). Nearly identical sequences for this retrotransposon are present precisely at the same location in the long C4 genes from the tandem RCCX Module I and Module II. Specific nucleotide substitutions between the long and short C4 genes have been identified and used for diagnosis. Southern blot analyses revealed that HERV-K(C4) is present at more than 30 locations in the human genome, exhibits variations in the population, and its analogs exist in the genomes of Old World primates with species-specific patterns. Evidence of intrachromosomal recombination between the two long terminal repeats of HERV-K(C4) is found near the huntingtin locus on chromosome 4. It is possible that members of HERV-K(C4) are involved in genetic instabilities including the RCCX modules, and in protecting the host genome from retroviral attack through an antisense strategy.

Organization of C4 and CYP21 loci in gorilla and orangutan

The standard human haplotype contains two C4 and two CYP21 loci arranged in the order C4A ... CYP21P ... C4B ... CYP21 and intercalated between the class I and class II loci of the HLA complex. The C4A gene is 22 kilobases (kb) long; the C4B gene is either 22 kb or 16 kb long. The CYP21P is a pseudogene characterized by an eight base pair (bp) deletion in exon 3 and other defects; the CYP21 is a functional gene. The standard chimpanzee haplotype is arranged in the same way as the standard human haplotype, except that both C4 genes are of the short variety; like the human gene, the chimpanzee CYP21P gene contains the 8 bp deletion. In the present study we demonstrate that a representative gorilla haplotype also consists of two short C4 genes and two CYP21 genes, neither of which, however, has the characteristic 8 bp deletion. On the other hand, the single characterized orangutan haplotype is organized in the following way: C4A ... CYP21 ... C4A ... CYP21 ... C4B ... CYP21. The first two C4 genes are of the long variety, the third gene is short. None of the defects characterizing the human CYP21P gene is present in any of the three orangutan genes. These conclusions are based on the analysis of overlapping clones isolated from cosmid libraries of the indicated species. The observed haplotype organization of the four primate species can be explained by expansion and contraction of the C4-CYP21 region through unequal homologous crossing-over, which preserves the differentiation of the C4 genes into the A and B categories but otherwise homogenizes these genes, as well as the CYP21 genes, within a given species. The 8 bp deletion in the CYP21P gene is postulated to have occurred before the separation of the lineages that led to modern humans and chimpanzees, but after the separation of these two lineages from the lineage that led to modern gorillas. The 6 kb insertion generating the long C4 gene is postulated to have occurred before the separation of the orangutan, gorilla, chimpanzee, and human lineages.

DR3 and nonDR3 associated complement component C4A deficiency in systemic lupus erythematosus

The molecular basis of complement component C4A deficiency in white U.S. and Mexican patients with systemic lupus erythematosus (SLE) was studied. Genomic DNA from SLE patients and non-SLE controls was analyzed for restriction fragments using HindIII and a 5' C4 cDNA probe. C4A gene deletion was recognized by the loss of a 15-kb restriction fragment and the appearance of a 8.5-kb fragment. Thirty-two selected U.S. SLE patients, 7 nonSLE controls, and 11 Mexican SLE patients and 9 relatives were studied. The deletion was recognized in all of the 14 HLA-B8;DR3 SLE patients with a C4A protein deficiency. Two SLE patients with DR3 but without B8 also had this gene deletion. None of the 3 U.S. SLE nonDR3, C4A protein deficient patients nor 20 C4A protein deficient Mexican individuals (11 SLE patients and 9 relatives; none had B8 and/or DR3) showed this deletion. Thus the C4A gene deletion failed to account for the C4A protein deficiency in all the nonDR3 Mexicans and in some U.S. SLE patients. To determine whether gene conversion at the C4A locus would encode a C4B-like protein and be responsible for the C4A protein deficiency (in nonDR3 patients), the C4d region of the gene was amplified by polymerase chain reaction and subjected to Nla IV digestion, and restriction fragment analysis was performed using a C4d region-specific probe. The resulting restriction fragment length polymorphism pattern revealed no changes in the isotype-specific region of the gene as characterized by C4A-specific 276- and 191-bp fragments in Dr3 or nonDR3 individuals. Thus, homoexpression of C4B at both loci was not responsible for C4A deficiency in nonDR3 SLE patients without C4A gene deletion.

Mapping of the SLA complex of miniature swine: mapping of the SLA gene complex by pulsed field gel electrophoresis

The overall order of the regions of the swine major histocompatibility complex (MHC), the SLA complex, was determined by pulsed field gel electrophoresis (PFGE). It was found that the order of the regions is class II-class III-class I. A class I probe hybridized to a 420 kb Mlu I and a 420 kb Not I fragment as did a class III probe for C2. None of the class II probes hybridized to these fragments. Thus, linkage of class I to class III was shown. The class III C2, Bf, and C4 genes were found to residue in a 190 kb Not I fragment. Linkage of class III and class II genes was shown when both the class III C4 and the class II DR probes hybridized to the same 195 kb Sac II and 340 kb Not I fragments. The class I probe did not hybridize to these fragments. The order of the regions, class II-class III-class I, is similar to that of human MHC genes and may have been conserved in evolution so that coordinated expression of MHC genes could be achieved.

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.

Counterregulatory effects of interferon-gamma and endotoxin on expression of the human C4 genes

Susceptibility to autoimmune disease is associated with null alleles at one of the two genetic loci encoding complement protein C4. These two genetic loci, C4A and C4B, are highly homologous in primary structure but encode proteins with different functional activities. Expression of C4A and C4B genes is regulated by IFN-gamma in human hepatoma cells and in murine fibroblasts transformed with the respective genes. In these cell lines, IFN-gamma has a significantly greater and longer-lasting effect on expression of C4A than that of C4B. In this study we examined synthesis and regulation of C4A and C4B in peripheral blood monocytes from normal, C4A-null, and C4B-null individuals. Synthesis of C4 in human peripheral blood monocytes decreases during time in culture. IFN-gamma mediates a concentration- and time-dependent increase in steady-state levels of C4 mRNA and a corresponding increase in synthesis of C4 in normal human monocytes. LPS decreases monocyte C4 expression and completely abrogates the effect of IFN-gamma on the expression of this gene. In contrast, LPS and IFN-gamma have a synergistic effect in upregulating expression of another class III MHC gene product, complement protein factor B. The effect of LPS on constitutive and IFN-gamma-regulated C4 synthesis is probably not mediated via release of endogenous monokines IL-1 beta, TNF-alpha, or IL-6. Synthesis of C4, and regulation of its synthesis by IFN-gamma and LPS, are similar in normal, C4A-, and C4B-null individuals. These results demonstrate the synthesis of C4 at extrahepatic sites and tissue-specific regulation of C4 gene expression.

Marked shortage of C4B DNA polymorphism among insulin-dependent diabetic patients

TaqI, BamHI and HinddIII polymorphisms of the C4 genes were studied with a 500-bp C4 cDNA probe (pAT-A153) specific for the 5' end of the gene. The restriction patterns obtained were correlated with the C4A and C4B genotypes in 35 patients suffering from insulin-dependent diabetes mellitus (IDDM), and results were compared to those from 40 healthy individuals. The controls, all Caucasian, were genotyped for HLA-A, B, C, DR, Bf, C2 and C4, together with 10 diabetics and their families; haplotypes for the other patients had been deduced using DNA and protein polymorphism, and taking into consideration linkage disequilibrium for neighbouring loci. No significant difference between genotypes at the C4A locus was seen in either population. The C4A gene deletion, associated with a C4B "short" gene (66.7%), was found mainly in the haplotype B8,Cw7,DR3,BfS,C2C, C4AQOB1, and the C4B gene deletion in the haplotype B18,Cw5,DR3,BfF1, C2C,C4A3BQO. When diabetic patients were compared with normal individuals, we observed, at the C4B locus, a decrease in the C4B "long" gene (22% vs. 49% respectively, p less than 0.001). A compensatory increase was observed in patients vs. controls for the frequency of C4BQO, both in the deleted and intact form (26% vs. 10% respectively, p less than 0.03).

The "Sardinian" HLA-A30,B18,DR3,DQw2 haplotype constantly lacks the 21-OHA and C4B genes. Is it an ancestral haplotype without duplication?

The C4 and 21-OH loci of the class III HLA have been studied by specific DNA probes and the restriction enzyme Taq 1 in 24 unrelated Sardinian individuals selected from completely HLA-typed families. All 24 individuals had the HLA extended haplotype A30,Cw5,B18, BfF1,DR3,DRw52,DQw2, named "Sardinian" in the present paper because of its frequency of 15% in the Sardinian population. Eighteen of these were homozygous for the entire haplotype, and six were heterozygous at the A locus and blank (or homozygous) at all the other loci. In all completely homozygous cells and in four heterozygous cells at the A locus, the restriction fragments of the 21-OHA (3.2 kb) and C4B (5.8 kb or 5.4 kb) genes were absent, and the fragments of the C4A (7.0 kb) and 21-OHB (3.7 kb) genes were present. It is suggested that the "Sardinian" haplotype is an ancestral haplotype without duplication of the C4 and 21-OH genes, practically always identical in its structure, also in unrelated individuals. The diversity of this haplotype in the class III region (about 30 kb less) may be at least partially responsible for its misalignment with most haplotypes, which have duplicated C4 and 21-OH genes, and therefore also for its decreased probability to recombine. This can help explain its high stability and frequency in the Sardinian population. The same conclusion can be suggested for the Caucasian extended haplotype A1,B8,DR3 that always seems to lack the C4A and 21-OHA genes.

Direct observation of the gene organization of the complement C4 and 21-hydroxylase loci by pulsed field gel electrophoresis

Pulsed field gel electrophoresis and enzymes that cut genomic DNA infrequently have been used to define large RFLPs at the human C4 loci. With the enzymes BssH II or Sac II, and C4 or 21-hydroxylase DNA probes, it has been possible to observe directly the number of C4 genes present on a haplotype, and also whether the C4 genes are long (6-7-kb intron present) or short (6-7-kb intron absent). Haplotypes that have either two long C4 genes or one long and one short C4 gene generate BssH II fragments of approximately 115 or approximately 105 kb, respectively. Haplotypes that have either a single long or a single short C4 gene generate BssH II fragments of approximately 80 or approximately 70 kb, respectively. This technique has been used to analyze the DNA isolated from PBMC and allows the complete definition of the C4 gene organization of an individual without the need for family studies.

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.

Heterogeneity in the gene locus for steroid 21-hydroxylase deficiency

DNA was analysed from 33 patients with congenital adrenal hyperplasia due to steroid 21-hydroxylase deficiency. In each case Southern blots were prepared from a number of restriction enzyme digests and hybridised with probes for both the 21-hydroxylase and the adjacent fourth component of complement (C4). Evidence for deletion of the active 21-hydroxylase gene (CYP21B) was found in 13 cases and in 10 of these the deletion included the adjacent C4B gene, leading to a hybrid CYP21A/CYP21B gene. Deletion of CYP21B alone was found in one patient, the remaining two cases appearing to have the active gene replaced by the inactive pseudogene. Duplications of the CYP21A-C4B region and deletion of the pseudogene are also described. In a further 12 cases no gross abnormality could be found.

Italian extended HLA haplotypes in congenital adrenal hyperplasia

In order to complete the data on human 21-Hydroxylase deficiency, we present a study on HLA markers in 35 Italian families (14 from Northern, eight from Central and 13 from Southern Italy) with one affected child. Three children from the issue of first cousin marriages were homozygous for the whole HLA haplotype. Extended haplotypes shared by unrelated patients were not found, and a total absence of the HLA Bw47 allele among the haplotypes carrying the disease as well as normal haplotypes was observed. The absence of A1 Cw7 B8 BfS C4AQ0 C4B1 DR3 extended haplotype was instead confirmed. Allele frequencies in the different clinical forms were analyzed: BfSO7 allele frequency was significantly increased on haplotypes of the salt-wasting form (p less than 0.01). We noticed two duplications (C4B1-2) of C4B genes, on haplotypes involved in the disease. Allele distribution in the regions studied showed that Bw22 (w55), Cw3 and DR2 were characteristic of Northern patients, while B15 was found in patients from Central Italy.

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.

A high frequency of inherited deficiency of complement component C4 in Darwin Aborigines

A high frequency of serum complement component C4A deficiency may explain the higher prevalence and greater severity of systemic lupus erythematosus reported in Australian Aborigines. Inherited deficiencies of serum complement components C4A, C4B, and C2 were examined in two Australian Aboriginal populations from Darwin and Alice Springs and compared with the prevalence of complement deficiencies in white Australian blood donors. The frequency of C4A deficiency alleles was 29% in Darwin Aborigines compared with 12% in Alice Springs and 17% in Canberra blood donors. Partial C4B deficiency was also higher in Darwin Aborigines than in the other populations. Inherited deficiency of serum complement component C2 was not observed.

Association of HLA-Bw65 with two major complotypes

The association between the HLA-B14 subtypes Bw64 and Bw65 and complement allotypes (C2, Bf and C4) was investigated in both population and family studies. Bf, C4A and C4B allotyping was performed on 37 Bw64 and 35 Bw65 positive unrelated Welsh/English subjects. Sixteen HLA-Bw65 bearing haplotypes were characterized for HLA-ABC, DR and DQ antigens and complement allotypes, including C2. The findings of the population study suggested that the complement haplotype associated with Bw64 is BfS, C4A2, C4B2. The population and family studies revealed two major complement haplotypes associated with HLA-Bw65: (i) C2C, BfF, C4A3, C4A1 - often associated with HLA-A3, Cw8 and DRw13, and (ii) C2C, BfS, C4A2, C4B2 - often associated with HLA-Aw33, Cw8 and DR1 or with A28, Cw8 and DRw13. The HLA-Bw65 bearing haplotypes of three families carried a C4B2B1 duplication of the C4B locus. In these families three C4B gene products were identified in the Bw65 positive members using an anti-C4B monoclonal antibody. It is suggested that most, if not all, HLA-Bw65 bearing haplotypes may possess a C4B locus duplication.

Definitive RFLPs to distinguish between the human complement C4A/C4B isotypes and the major Rodgers/Chido determinants: application to the study of C4 null alleles

Definitive restriction fragment length polymorphisms (RFLPs) representing the exact locations responsible for isotypicity between the human complement components C4A and C4B, and their generally associated major Rodgers (Rg1) and Chido (Ch1) antigenic determinants, have been designed. By means of C4d-specific genomic probe for Southern blot analysis, a C4A gene can be defined by the presence of the 276 bp and 191 bp N1a IV fragments, while a C4B gene can be defined by a single 467 bp N1aIV fragment. In addition, an Rg1-expressing C4 gene can be represented by a 565 bp EcoO 109 fragment, and a Ch1-expressing C4 gene by a 458 bp EcoO 109 fragment, under the same conditions. All these polymorphic restriction fragments can be unambiguously and conveniently detected. In combination with the Taq I polymorphic patterns specific for the C4 loci and for the neighboring 21-hydroxylase genes, the nature and structure of the tandem C4,21-hydroxylase gene complex can be elucidated. In this study, it is inferred that the null allele of the HLA haplotype B44 DR6 C4A3 C4BQO is not a C4B allele, but probably encodes another C4A 3 allotype at the second C4 locus.

Lethal deletion of the complement component C4 and steroid 21-hydroxylase genes in the mouse H-2 class III region, caused by meiotic recombination

A recombinant H-2 haplotype, designated aw18, was produced that underwent meiotic recombination in the E alpha (I-E alpha chain)--Slp (sex-limited protein) interval of the H-2 class III region between B10.A (H-2a) and wild-derived B10.MOL-SGR (H-2wm7) strains. It appeared that the H-2aw18 haplotype has a single, recessive, lethal mutation, since homozygotes for H-2aw18 were not detected in progeny generated from the intercross of mice that were heterozygous for this H-2 haplotype. Nine newly established recombinant H-2 haplotypes, which arose from the heterozygous mice that resulted from a cross between common inbred H-2 haplotypes and the aw18 haplotype, allowed us to map the lethal gene to the class III region of the H-2 complex. Southern blot analysis indicated that the aw18 haplotype has a deletion of the C4 gene and a deletion of one of the steroid 21-hydroxylase genes. This result was confirmed by an immunodiffusion test that demonstrated the absence of production of the C4 protein in mice of haplotype H-2aw18. All data that were obtained supported the hypothesis that the meiotic, presumably unequal, recombination between homologous chromosomes of the H-2a and H-2wm7 haplotypes caused the deletion of the C4 and the 21-hydroxylase genes.

Heterogeneity of human C4 gene size. A large intron (6.5 kb) is present in all C4A genes and some C4B genes

In this article we present a study showing that the human C4 genes differ in length because of the presence or absence of a 6.5 kb intron near the 5' end of the gene. DNA from individuals of known HLA, factor B, and C4 haplotypes was analyzed for restriction fragment length polymorphism (RFLP) by Southern blot analysis with C4-specific cDNA probes. The RFLP patterns obtained showed that the C4 genes are either 22.5 kb or 16 kb in length. They are referred to as long and short C4 genes, respectively. A population study was carried out to examine the distribution of the gene size according to C4 allotypes and haplotypes. Long C4 genes included all C4A genes studied and also some C4B allotypes, e.g., B1 on most C4 A3B1 haplotypes. Similarly, C4B null genes were found to be of the long form. Other C4B allotypes tested were found to be coded for by short C4 genes, including B2, B1 in C4 A6B1 and C4 AQOB1 (with a single C4B gene haplotype).

Phenotyping of human complement component C4, a class-III HLA antigen

The plasma complement protein C4 is encoded at two highly polymorphic loci, A and B, within the class-III region of the major histocompatibility complex. At least 34 different polymorphic variants of human C4 have been identified, including non-expressed or 'null' alleles. The main method of identification of C4 polymorphic allotypes is separation on the basis of charge by agarose-gel electrophoresis of plasma. On staining by immunofixation with anti-C4 antibodies, each C4 type gives three major bands, but, since individuals can have up to five allotypes, the overlapping banding pattern is difficult to interpret. We show that digestion of plasma samples with carboxypeptidase B, which removes C-terminal basic amino acids, before electrophoresis, produces a single, sharp, distinct band for each allotype and allows identification of the biochemical basis of the multiple banding pattern previously observed in C4 phenotype determination.

Steroid 21-hydroxylase deficiency and the major histocompatibility complex

Steroid 21-hydroxylase (21-OHase) deficiency is an HLA-linked recessive disorder of cortisol biosynthesis that can occur in several forms which differ in severity. Because they are in genetic linkage disequilibrium with different HLA antigens, the inheritance of these forms is consistent with the existence of several alleles at a single locus. When severe 21-OHase deficiency occurs in association with the HLA haplotype A3;Bw47;DR7, there is a simultaneous null allele at one of the C4 loci. This was hypothesized to result from a single deletion or rearrangement affecting the 21-OHase and C4 loci and perhaps the HLA-B gene as well. To test this hypothesis and identify the 21-OHase gene, a cDNA clone was isolated that encoded the cytochrome P450 specific for steroid 21-hydroxylation in the bovine adrenal gland. This clone hybridized to two genes in normal human DNA, but to only one gene in DNA from an individual homozygous for A3;Bw47;DR7. All individuals heterozygous for A3;Bw47;DR7 carry a heterozygous deletion of a cytochrome P450 gene. Cosmid clones have been used to locate the 21-OHase genes both in man and mouse. In both species, there are two 21-OHase genes each located immediately 3' of one of the two C4 genes, and oriented in the same direction as the C4 genes. In man, the gene located 3' of the C4B gene is deleted in 21-OHase deficiency on the Bw47 haplotype, but the gene 3' of the C4A gene is deleted in hormonally normal individuals on the A1;B8;C4AQ0, C4B1;DR3 haplotype. Thus the 21-OHase B gene is normally active in man, but the 21-OHase A gene is not.

Rearrangement of 21-hydroxylase genes in disease-associated MHC supratypes

Human cDNA probes for 21-hydroxylase (21-OH) and for complement component C4 are used on restriction digests of the members of several families with interesting supratypes. The presence of two Taq I fragments of 3.7 kb and 3.2 kb in size with a 21-OH probe is confirmed in most individuals who show no evidence of C4 deletions or 21-OH deficiency. Most individuals also show a doublet of weakly hybridizing bands at approximately 2.5 kb, the smaller of which is part of the 21 A gene. The arrangement of the 21-OH genes on disease-associated supratypes was examined, and it is shown that copies of the same supratype from unrelated individuals are usually identical. Evidence is provided for deletions of 21A on the B8, C4AQ0, C4B1, BfS, DR3 and B18, C4A3, C4BQ0, BfF1, DR3 supratypes and a duplication of 21A on the B14, C4A2, C4B1/B2, BfS supratype. Gene rearrangements may be relevant to diseases such as juvenile onset diabetes mellitus.

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.

Molecular biology of the human and mouse MHC class III genes: phylogenetic conservation, genetics and regulation of expression

The generation of complementary and genomic DNA clones for the human and mouse MHC class III genes has advanced the study of the organization, structure, genetics and expression of these loci. These clones have been useful in defining new polymorphic markers in each species and therefore permit a more complete genetic analysis of the complement cluster and the MHC as a whole. The coding sequences of the factor B and C4 genes are extensively conserved both within and between species, in contrast to the coding sequences of other MHC products. In human and mouse, the organization of the class III genes is similar with respect to order and position between the class II and class I regions of the MHC. However, these inter-species similarities in the organization and products of the class III genes does not extend to their regulation. In addition to complement gene expression being regulated differently between tissue sites within a species, expression is apparently regulated differently in analogous tissues between species. The considerable progress which has been made in the molecular analysis of C2, factor B and C4 using DNA clones forms the basis for the future study of the biology of the class III genes and the role of complement in inflammatory processes and in the immune system.

The structure and genetics of the C2 and factor B genes

This review summarises our current knowledge of the genetic organisation, structure and polymorphism of the loci for the complement proteins, C2 and Factor B--class III gene products of the major histocompatibility complex. cDNA probes specific for C2 and Factor B have been used to screen cosmid libraries of human genomic DNA, and this has allowed isolation and characterisation of the corresponding genes. Southern blot analysis of the cosmid clones and of uncloned genomic DNA has shown that there are single C2 and Factor B loci that are less than 500 bp apart. Molecular mapping has revealed that the C2 gene spans approximately 18 kb of DNA. This is in marked contrast to the Factor B gene which is 6 kb in length. The entire gene structure of the Factor B gene has been determined and the interesting features of this gene which have emerged from an examination of the intron-exon boundaries are discussed. C2 and Factor B are polymorphic and structural variants have been detected by differences in charge. The degree of polymorphism at the C2 and Factor B loci has been examined by Southern blot analysis of restriction digests of genomic DNA. Three DNA polymorphisms have been identified in the C2 gene. These polymorphisms subdivide the common allelic variant of C2 (C2C) and reveal that there is much greater variability at the C2 locus than that detected by protein typing. It is suggested that these DNA polymorphisms may serve as useful markers in the genetic analysis of diseases that are related to the major histocompatibility complex.

Expression of complement proteins C2 and factor B in transfected L cells

Factor B and C2 are structurally and functionally similar complement proteins encoded by genes that are closely linked within the class III region of the major histocompatibility complex (MHC). In this study, restriction endonuclease digestion of cosmid DNAs isolated from an H-2d murine genomic library indicated that the chromosomal organization of these two genes was similar in mouse to that in man. To further characterize their expression, cosmid DNAs encoding human and murine factor B and C2 were introduced into mouse L cells by DNA-mediated gene transfer. Factor B expression was demonstrated in cells transfected with either the human or the murine gene, but not in cells transfected with a control plasmid. Synthesis and secretion of factor B by L cells transfected with the human and murine cosmids was similar to that of human and murine cells in primary culture. An interspecies variation between human and murine factor B was identified and reproduced with extraordinary fidelity by the mouse fibroblast. In contrast, C2 RNA and protein were expressed by L cells alone and by L cells transfected with a control plasmid, as well as by L cells transfected with cosmids encoding human and murine complement genes. Expression of the transferred human C2 gene was demonstrated by the presence of a new distinct C2 RNA transcript and secretion of biologically active human C2. These results demonstrate the similarity of organization of the murine and human class III MHC regions. Expression of the two closely linked gene products, C2 and factor B, after DNA-mediated gene transfer provides a system for further analysis of regulation in both normal and deficient states.

Adrenal 21-hydroxylase cytochrome P-450 genes within the MHC class III region

Genes encoding several serum complement components and the gene(s) for steroid 21-hydroxylase (21-OH) have been located in the class III region of the major histocompatibility complex (MHC). All these genes are highly polymorphic in man, and these polymorphisms have been used to draw conclusions about the structure and function of these genes. For example, electrophoretic polymorphisms of the fourth component of complement (C4) have been shown to be controlled by two closely linked genes, which also control expression of the red cell antigens Rodgers and Chido. Steroid 21-OH deficiency (D) can occur in several forms which differ in severity, and because of genetic linkage disequilibrium with different HLA antigens the inheritance of these forms is consistent with the existence of several alleles at a single locus. When severe 21-OH D occurs in association with the HLA haplotype A3;Bw47;DR7, there is a simultaneous null allele at one of the C4 loci. This was hypothesized to result from a single deletion or rearrangement affecting the 21-OH and C4 loci and perhaps the HLA-B gene as well. To test this hypothesis and identify the 21-OH gene, a cDNA clone was isolated which encoded the cytochrome P450 specific for steroid 21-hydroxylation in the bovine adrenal gland. This clone hybridized to two genes in normal human DNA, but to only one gene in DNA from an individual homozygous for A3;Bw47;DR7. All individuals heterozygous for A3;Bw47;DR7 carry a heterozygous deletion of a gene. These experiments showed that at least one structural gene for the cytochrome P450 specific for 21-hydroxylation is located in the MHC, probably very near the C4 genes, and a mutation in this gene results in 21-OH D. Cosmid clones have been used to locate the 21-OH genes both in man and mouse. In both species, there are two 21-OH genes, each located immediately 3' of one of the two C4 genes, and oriented in the same direction as the C4 genes. In man, the gene located 3' of the C4B gene is deleted in 21-OH D on the Bw47 haplotype, but the gene 3' of the C4A gene is deleted in hormonally normal individuals on the A1;B8;C4AQO;C4B1;DR3 haplotype. Thus the 21-OH B gene is normally active in man, but the 21-OH A gene is not.

Molecular organization and in vitro expression of murine class III genes

These experiments demonstrate that at least two types of gene duplications have occurred during the evolution of the S region. The first type, which produced the C2 and factor B genes, involved a short segment of the chromosome encompassing a single gene. The related products have subsequently diverged yielding sequences which do not cross-hybridize. Further duplication of these genes has not been observed. The second type of duplication consisted of a much longer primordial sequence, spanning approximately 55 kb of genomic DNA and including at least two genes, C4/Slp and 21-hydroxylase. The duplicated sequences are separated by a segment of single copy sequence of as yet undefined length. These duplicated sequences have been relatively conserved. There is evidence that further duplication of this region is possible (as seen in the H-2w7 strain) although the exact nature of the increase in gene number has not been fully characterized. Detailed analysis of cosmid clones which span these two duplications has permitted the assignment of a new pair of loci to the S region, encoding 21-hydroxylase A and B. The advantage conferred by linkage of the gene encoding this adrenal steroid biosynthesis enzyme to the genes encoding complement components C2, factor B, and C4 is unclear, as is the advantage of the association of all of the class III genes with the remainder of the MHC. The availability of cloned sequences containing all of the class III genes permits further study of the factors which govern the tissue specificity of their expression and which confer androgen responsiveness on certain of the Slp alleles.

Two genes encoding steroid 21-hydroxylase are located near the genes encoding the fourth component of complement in man

Two genes encoding steroid 21-hydroxylase [21-OHase; steroid 21-monooxygenase; steroid, hydrogen-donor: oxygen oxidoreductase (21-hydroxylating); EC 1.14.99.10], a cytochrome P-450 enzyme, have been located within the HLA major histocompatibility complex. Congenital adrenal hyperplasia due to 21-OHase deficiency is a common inherited disorder of cortisol biosynthesis which is in genetic linkage disequilibrium with certain extended HLA haplotypes. These haplotypes include characteristic serum complement allotypes. A series of cosmid clones was isolated from a human genomic library by using a probe encoding part of the fourth component of complement, C4. These clones also hybridized with a probe encoding most of human 21-OHase. Restriction mapping and hybridization analysis showed that there are two 21-OHase genes, each located near the 3' end of one of the two C4 genes. Hybridization with probes specific for the 5' and 3' ends of the 21-OHase gene showed that the 21-OHase and C4 genes all have the same orientation. The 21-OHase genes 3' to C4A and C4B carry T aq I fragments of 3.2 and 3.7 kilobases (kb), respectively. Both of these fragments are found in genomic DNA of most individuals. In DNA from an individual with the severe, "salt-wasting" form of 21-OHase deficiency who was homozygous for HLA-A3;Bw47;C4A*1;C4B*Q0(null); DR7, the 3.7-kb Taq I fragment is absent, whereas hormonally normal individuals homozygous for HLA-A1;B8;C4A*Q0;C4B*1;DR3 do not carry the 3.2-kb Taq I fragment. These data suggest that the 21-OHase "B" gene (3.7-kb Taq I fragment) is functional, but the 21-OHase "A" gene (3.2-kb Taq I fragment) is not.

The polymorphism of the complement genes in HLA

Genes coding for the complement proteins C2, C4A, C4B and factor B lie between HLA-D and HLA-B in HLA, the major histocompatibility complex in man. All the complement components are polymorphic, particularly C4, which has many alleles at each locus. The genetic complexity of C4 extends to the number of loci each of which may be deleted or duplicated on the chromosome. The different forms of C4 showed markedly different reactivities with small molecules and on haemolytic activity in the complement system. Surprisingly, amino acid sequences of the several allelic forms of C4 appear to be very similar, with less than 1% of residue positions being changed between alleles of C4A and C4B. These results may be relevant to the increased susceptibility to autoimmune disease which is associated with particular haplotypes of the HLA complex.

Polymorphism of human complement component C4

An assessment has been made of the polymorphism of human complement component C4 by comparing derived amino acid sequences of cDNA and genomic DNA with limited amino acid sequences. In all, one complete and six partial sequences have been obtained from material from three individuals and include two C4A and two C4B alleles. Differences were found between the 4 alleles from 2 loci in only 15 of the 1722 amino acid residues, and 12 lie within one section of 230 residues, which in 1 allele also contains a 3-residue deletion. In three variable positions, an allelic difference in one C4 type was common to the other types. Three nucleotide differences were found in four introns. In spite of marked differences in their chemical reactivity, the many allelic forms appear to differ in less than 1% of their amino acid residue positions. This unusual pattern of polymorphism may be due to recent duplication of the C4 gene, or may have arisen by selection as a result of the biological role of C4, which interacts in the complement sequence with nine other proteins necessitating conservation of much of the surface structure.

Mapping of steroid 21-hydroxylase genes adjacent to complement component C4 genes in HLA, the major histocompatibility complex in man

The genes for four components (C) of complement in the human major histocompatibility complex (HLA) have been aligned previously in a series of overlapping cosmid cloned inserts. Those inserts, which contained the two C4 genes C4A and C4B, hybridized with human adrenal mRNA, indicating that they contain a gene expressed in the adrenal. The mRNA fraction of 2.4 kilobases (kb) hybridizes with genomic DNA of 4.5 kb, which is duplicated and lies about 1.5 kb 3' of both the C4A and the C4B complement genes. Sequencing of a 430-base section and comparison with the published cDNA sequence of bovine cytochrome P-450 21-hydroxylase, peptide sequences of porcine 21-hydroxylase, and a cDNA sequence of a rat liver cytochrome P-450 identified the gene as coding for human steroid 21-hydroxylase [steroid,hydrogen-donor:oxygen oxidoreductase (21-hydroxylating), EC 1.14.99.10]. Mapping of the gene was helped by use of a synthetic oligonucleotide based on the bovine cDNA sequence.