Genomic organization of human complement component C3

Molecular recognition and binding between human plasminogen Kringle 5 and α-chain of human complement component C3b by frontal chromatography and dynamics simulation

The binding and molecular recognition between α-chain of human complement C3b (α-chain of C3b) and human plasminogen Kringle 5 (Kringle 5) were studied and explored by frontal chromatography and dynamics simulation in the combination of bio-specific technologies. The specific interaction between the α-chain of C3b and Kringle 5 was initially confirmed by ligand blot and ELISA (Kd = 4.243×10-6 L/mol). Furthermore, the binding determination conducted via frontal chromatography showed that the presence of a single binding site between them, with the binding constant of 2.98 × 105 L/mol. Then the molecular recognition by dynamics simulation and molecular docking showed that there were 9 and 13 amino acid residues respective in the Kringle 5 and α-chain of C3b directly implicated in the binding and the main stabilizing forces were electrostatic force (-55.99 ± 11.82 kcal/mol) and Van der Waals forces (-42.70 ± 3.45 kcal/mol). Additionally, a loop structure (65-71) in Kringle 5 underwent a conformational change from a random structure to an α-helix and a loop structure (417-425) in α-chain of C3b was closer to the molecular center, both of them were more conducive to the binding between them. Meanwhile, the involvement of the lysine binding site of Kringle 5 played an important role in the binding process. In addition, the erythrocyte-antibody complement rosette assay substantiated that the presence of Kringle 5 hindered the transportation of α-chain of C3b to antigen-antibody complex in a dose-dependent manner. These findings collectively indicated that the α-chain of C3b is very likely a receptor protein for Kringle 5, which provides a methodology for other similar investigations and valuable insights into expansion of the pharmacological effects and potential application of Kringle 5 in immune-related diseases.

Complement Component C3 Is Highly Expressed in Human Pancreatic Islets and Prevents β Cell Death via ATG16L1 Interaction and Autophagy Regulation

We show here that human pancreatic islets highly express C3, which is both secreted and present in the cytosol. Within isolated human islets, C3 expression correlates with type 2 diabetes (T2D) donor status, HbA1c, and inflammation. Islet C3 expression is also upregulated in several rodent diabetes models. C3 interacts with ATG16L1, which is essential for autophagy. Autophagy relieves cellular stresses faced by β cells during T2D and maintains cellular homeostasis. C3 knockout in clonal β cells impaired autophagy and led to increased apoptosis after exposure of cells to palmitic acid and IAPP. In the absence of C3, autophagosomes do not undergo fusion with lysosomes. Thus, C3 may be upregulated in islets during T2D as a cytoprotective factor against β cell dysfunction caused by impaired autophagy. Therefore, we revealed a previously undescribed intracellular function for C3, connecting the complement system directly to autophagy, with a broad potential importance in other diseases and cell types.

Mutations in CPAMD8 Cause a Unique Form of Autosomal-Recessive Anterior Segment Dysgenesis

Anterior segment dysgeneses (ASDs) comprise a spectrum of developmental disorders affecting the anterior segment of the eye. Here, we describe three unrelated families affected by a previously unclassified form of ASD. Shared ocular manifestations include bilateral iris hypoplasia, ectopia lentis, corectopia, ectropion uveae, and cataracts. Whole-exome sequencing and targeted Sanger sequencing identified mutations in CPAMD8 (C3 and PZP-like alpha-2-macroglobulin domain-containing protein 8) as the cause of recessive ASD in all three families. A homozygous missense mutation in the evolutionarily conserved alpha-2-macroglobulin (A2M) domain of CPAMD8, c.4351T>C (p. Ser1451Pro), was identified in family 1. In family 2, compound heterozygous frameshift, c.2352_2353insC (p.Arg785Glnfs^∗23), and splice-site, c.4549-1G>A, mutations were identified. Two affected siblings in the third family were compound heterozygous for splice-site mutations c.700+1G>T and c.4002+1G>A. CPAMD8 splice-site mutations caused aberrant pre-mRNA splicing in vivo or in vitro. Intriguingly, our phylogenetic analysis revealed rodent lineage-specific CPAMD8 deletion, precluding a developmental expression study in mice. We therefore investigated the spatiotemporal expression of CPAMD8 in the developing human eye. RT-PCR and in situ hybridization revealed CPAMD8 expression in the lens, iris, cornea, and retina early in development, including strong expression in the distal tips of the retinal neuroepithelium that form the iris and ciliary body, thus correlating CPAMD8 expression with the affected tissues. Our study delineates a unique form of recessive ASD and defines a role for CPAMD8, a protein of unknown function, in anterior segment development, implying another pathway for the pathogenicity of ASD.

Skipping of exon 27 in C3 gene compromises TED domain and results in complete human C3 deficiency

Primary deficiency of complement C3 is rare and usually associated with increased susceptibility to bacterial infections. In this work, we investigated the molecular basis of complete C3 deficiency in a Brazilian 9-year old female patient with a family history of consanguinity. Hemolytic assays revealed complete lack of complement-mediated hemolytic activity in the patient's serum. While levels of the complement regulatory proteins Factor I, Factor H and Factor B were normal in the patient's and family members' sera, complement C3 levels were undetectable in the patient's serum and were reduced by at least 50% in the sera of the patient's parents and brother. Additionally, no C3 could be observed in the patient's plasma and cell culture supernatants by Western blot. We also observed that patient's skin fibroblasts stimulated with Escherichia coli LPS were unable to secrete C3, which might be accumulated within the cells before being intracellularly degraded. Sequencing analysis of the patient's C3 cDNA revealed a genetic mutation responsible for the complete skipping of exon 27, resulting in the loss of 99 nucleotides (3450-3549) located in the TED domain. Sequencing of the intronic region between the exons 26 and 27 of the C3 gene (nucleotides 6690313-6690961) showed a nucleotide exchange (T→C) at position 6690626 located in a splicing donor site, resulting in the complete skipping of exon 27 in the C3 mRNA.

Variants in the gene encoding C3 are associated with asthma and related phenotypes among African Caribbean families

Proinflammatory and immunoregulatory products from C3 play a major role in phagocytosis, respiratory burst, and airways inflammation. C3 is critical in adaptive immunity; studies in mice deficient in C3 demonstrate that features of asthma are significantly attenuated in the absence of C3. To test the hypothesis that the C3 gene on chromosome 19p13.3-p13.2 contains variants associated with asthma and related phenotypes, we genotyped 25 single nucleotide polymorphism (SNP) markers distributed at intervals of approximately 1.9 kb within the C3 gene in 852 African Caribbean subjects from 125 nuclear and extended pedigrees. We used the multiallelic test in the family-based association test program to examine sliding windows comprised of 2-6 SNPs. A five-SNP window between markers rs10402876 and rs366510 provided strongest evidence for linkage in the presence of linkage disequilibrium for asthma, high log[total IgE], and high log[IL-13]/[log[IFN-gamma] in terms of global P-values (P = 0.00027, 0.00013, and 0.003, respectively). A three-SNP haplotype GGC for the first three of these markers showed best overall significance for the three phenotypes (P = 0.003, 0.007, 0.018, respectively) considering haplotype-specific tests. Taken together, these results implicate the C3 gene as a priority candidate controlling risk for asthma and allergic disease in this population of African descent.

Mutations in complement C3 predispose to development of atypical hemolytic uremic syndrome

Atypical hemolytic uremic syndrome (aHUS) is a disease of complement dysregulation. In approximately 50% of patients, mutations have been described in the genes encoding the complement regulators factor H, MCP, and factor I or the activator factor B. We report here mutations in the central component of the complement cascade, C3, in association with aHUS. We describe 9 novel C3 mutations in 14 aHUS patients with a persistently low serum C3 level. We have demonstrated that 5 of these mutations are gain-of-function and 2 are inactivating. This establishes C3 as a susceptibility factor for aHUS.

Clinical aspects and molecular basis of primary deficiencies of complement component C3 and its regulatory proteins factor I and factor H

The complement system participates in both innate and acquired immune responses. Deficiencies in any of the protein components of this system are generally uncommon and require specialized services for diagnosis. Consequently, complement deficiencies are clinically underscored and may be more common than is normally estimated. As C3 is the major complement component and participates in all three pathways of activation, it is fundamental to understand all the clinical consequences observed in patients for which this protein is below normal concentration or absent in the serum. C3 deficiencies are generally associated with higher susceptibility to severe infections and in some cases with autoimmune diseases such as systemic lupus erythematosus. Here, we review the main clinical aspects and the molecular basis of primary C3 deficiency as well as the mutations in the regulatory proteins factor I and factor H that result in secondary C3 deficiencies. We also discuss the use of animal models to study these deficiencies.

Large-scale expression study of human mesial temporal lobe epilepsy: evidence for dysregulation of the neurotransmission and complement systems in the entorhinal cortex

Human mesial temporal lobe epilepsies (MTLE) are the most frequent form of partial epilepsies and display frequent pharmacoresistance. The molecular alterations underlying human MTLE remain poorly understood. A two-step transcriptional analysis consisting in cDNA microarray experiments followed by quantitative RT-PCR validations was performed. Because the entorhinal cortex (EC) plays an important role in the pathophysiology of the MTLE and usually discloses no detectable or little cell loss, resected EC and each corresponding lateral temporal neocortex (LTC) of MTLE patients were used as the source of disease-associated and control RNAs, respectively. Six genes encoding (i) a serotonin receptor (HTR2A) and a neuropeptide Y receptor type 1 (NPY1R), (ii) a protein (FHL2) associating with the KCNE1 (minK) potassium channel subunit and with presenilin-2 and (iii) three immune system-related proteins (C3, HLA-DR-gamma and CD99), were found consistently downregulated or upregulated in the EC of MTLE patients as compared with non-epileptic autopsy controls. Quantitative western blot analyses confirmed decreased expression of NPY1R in all eight MTLE patients tested. Immunohistochemistry experiments revealed the existence of a perivascular infiltration of C3 positive leucocytes and/or detected membrane attack complexes on a subset of neurons, within the EC of nine out of eleven MTLE patients. To summarize, a large-scale microarray expression study on the EC of MTLE patients led to the identification of six candidate genes for human MTLE pathophysiology. Altered expression of NPY1R and C3 was also demonstrated at the protein level. Overall, our data indicate that local dysregulation of the neurotransmission and complement systems in the EC is a frequent event in human MTLE.

A wide range of protein isoforms in serum and plasma uncovered by a quantitative intact protein analysis system

We have implemented an orthogonal 3-D intact protein analysis system (IPAS) to quantitatively profile protein differences between human serum and plasma. Reference specimens consisting of pooled Caucasian-American serum, citrate-anticoagulated plasma, and EDTA-anticoagulated plasma were each depleted of six highly abundant proteins, concentrated, and labeled with a different Cy dye (Cy5, Cy3, or Cy2). A mixture consisting of each of the labeled samples was subjected to three dimensions of separation based on charge, hydrophobicity, and molecular mass. Differences in the abundance of proteins between each of the three samples were determined. More than 5000 bands were found to have greater than two-fold difference in intensity between any pair of labeled specimens by quantitative imaging. As expected, some of the differences in band intensities between serum and plasma were attributable to proteins related to coagulation. Interestingly, many proteins were identified in multiple fractions, each exhibiting different pI, hydrophobicity, or molecular mass. This is likely reflective of the expression of different protein isoforms or specific protein cleavage products, as illustrated by complement component 3 precursor and clusterin. IPAS provides a high resolution, high sensitivity, and quantitative approach for the analysis of serum and plasma proteins, and allows assessment of PTMs as a potential source of biomarkers.

Molecular analysis of a novel hereditary C3 deficiency with systemic lupus erythematosus

A case of inherited homozygous complement C3 deficiency (C3D) in a patient with systemic lupus erythematosus (SLE) and the molecular basis for this deficiency are reported. A 22-year-old Japanese male was diagnosed as having SLE and his medical history revealed recurrent tonsillitis and pneumonia. He was diagnosed as having C3D because of undetectable serum C3 level. His parents were consanguineous. Sequence analysis of C3D cDNA revealed a homozygous deletion of exon 39 (84bp). A single base substitution (AG to GG) in the 3'-splice acceptor site of intron 38 was identified by sequencing the genomic DNA. Expression of C3Delta(ex39) cDNA, the C3cDNA lacking exon 39, in COS-7 cells revealed that C3Delta(ex39) was retained in endoplasmic reticulum-Golgi intermediate compartment because of defective secretion. These data indicate that a novel AG-->GG 3'-splice acceptor site mutation in intron 38 caused aberrant splicing of exon 39, resulting in defective secretion of C3.

CD109 represents a novel branch of the alpha2-macroglobulin/complement gene family

We report here the genomic organization and phylogenic relationships of CD109, a member of the the alpha2-macroglobulin/complement (AMCOM) gene family. CD109 is a GPI-linked glycoprotein expressed on endothelial cells, platelets, activated T-cells, and a wide variety of tumors. We cloned full-length CD109 cDNA from the mammalian U373 cell line by RT-PCR and performed analysis of its corresponding genomic sequence. The CD109 cDNA spans 128 kb of chromosome 6q with its 33 exons constituting approximately 3.3% of the total CD109 genomic sequence. Sequence analysis revealed that CD109 contains specific motifs in its N-terminus, that are highly conserved in all AMCOM members. CD109 also shares motifs with certain other AMCOM members including: (1) a thioester 'GCGEQ" motif, (2) a furin site of four positively charged amino acids, and (3) a double tyrosine near the C-terminus. Based on a phylogenic analysis of human CD109 with other human homologs as well as orthologs from other mammalian species, C. elegans (ZK337.1) and E. coli homologs, we propose CD109 represents a novel and independent branch of the alpha2-macroglobulin/complement gene family (AMCOM) and may be its oldest member.

Molecular characterization of the pig C3 gene and its association with complement activity

The complement system catalyzes direct lysis of micro-organisms and modulates phagocytosis, inflammation, humoral and cellular immune responses. Since the complement protein C3 is the central component within all pathways of complement activation, C3 is a candidate gene for complement activity and also for improved protection against many pathogens. The pig C3 gene was sequenced, screened for polymorphisms, and analyzed for association with hemolytic complement activity of the alternative and classical pathway (AH(50), CH(50)). C3c serum levels and haptoglobin (HP) serum concentrations were measured before and after vaccination against Mycoplasma hyopneumoniae, Aujeszky virus, and porcine reproductive and respiratory syndrome virus in F2 animals of a pig resource population based on crossbreeding of Duroc and Berlin Miniature Pig. The genomic C3 sequence covers 444 bp of promoter region, 41 exons and 40 introns, as well as 881 bp of the 3'-flanking region. The cDNA codes for a 1,661-amino acid precursor C3. Five polymorphic sites were detected in the 5'-UTR, intron 13, exon 15, exon 30, and the 3'-UTR. Within the resource population two haplotypes were found to segregate. Analysis of variance applying a repeated measures model revealed a significant effect of the interaction of C3 genotype and time of measurement relative to immunization on CH(50), AH(50,)and C3c that is likely to be due to variation of C3 expression. In contrast, the time course of the HP acute-phase reaction is not associated with C3 genomic variation. The association of C3 with complement activity indicates the importance of C3 as a candidate gene for natural resistance to micro-organisms, although the causative polymorphism modulating the expression of C3 remains to be delineated.

Transcription factor AP-2 functions as a repressor that contributes to the liver-specific expression of serum amyloid A1 gene

We previously identified transcription factor AP-2 as the nuclear factor that interacts with the tissue-specific repressor element in the rat serum amyloid A1 (SAA1) promoter. In this report, we provide evidence for a second AP-2-binding site and show that both AP-2 sites participate in mediating the transcription repression of SAA1 promoter. This proximal AP-2 site overlaps with the NFkappaB-binding site known to be essential for SAA1 promoter activity. Protein binding competition experiments demonstrated that AP-2 and NFkappaB binding to these overlapping sites were mutually exclusive. Furthermore, the addition of AP-2 easily displaced prebound NFkappaB, whereas NFkappaB could not displace AP-2. These results thus suggest that one mechanism by which AP-2 negatively regulates SAA1 promoter activity may be by antagonizing the function of NFkappaB. Consistent with a repression function, transient expression of AP-2 in HepG2 cells inhibited conditioned medium-induced SAA1 promoter activation. This inhibition was dependent on functional AP-2-binding sites, since mutation of AP-2-binding sites abolished inhibitory effects of AP-2 in HepG2 cells as well as resulted in derepression of the SAA1 promoter in HeLa cells. In addition to SAA1, we found that several other liver gene promoters also contain putative AP-2-binding sites. Some of these sequences could specifically inhibit AP-2.DNA complex formation, and for the human complement C3 promoter, overexpression of AP-2 also could repress its cytokine-mediated activation. Finally, stable expression of AP-2 in hepatoma cells significantly reduced the expression of endogenous SAA, albumin, and alpha-fetoprotein genes. Taken together, our results suggest that AP-2 may function as a transcription repressor to inhibit the expression of not only SAA1 gene but also other liver genes in nonhepatic cells.

cDNA sequence coding for the alpha'-chain of the third complement component in the African lungfish

cDNA clones coding for almost the entire C3 alpha-chain of the African lungfish (Protopterus aethiopicus), a representative of the Sarcopterygii (lobe-finned fishes), were sequenced and characterized. From the sequence it is deduced that the lungfish C3 molecule is probably a disulphide-bonded alpha:beta dimer similar to that of the C3 components of other jawed vertebrates. The deduced sequence contains conserved sites presumably recognized by proteolytic enzymes (e.g. factor I) involved in the activation and inactivation of the component. It also contains the conserved thioester region and the putative site for binding properdin. However, the site for the interaction with complement receptor 2 and factor H are poorly conserved. Either complement receptor 2 and factor H are not present in the lungfish or they bind to different residues at the same or a different site than mammalian complement receptor 2 and factor H. The C3 alpha-chain sequences faithfully reflect the phylogenetic relationships among vertebrate classes and can therefore be used to help to resolve the long-standing controversy concerning the origin of the tetrapods.

Distinction between processing of normal and mutant complement C3 within human skin fibroblasts

Inherited C3 deficiency may result from mutations in the C3 gene affecting transcription or translation (type I deficiency). We described a type II C3 deficiency caused by a mutation yielding an abnormal non-secreted C3. The post-translational processing of mutant and normal C3 was analyzed in fibroblasts grown from skin biopsies. Mutant C3 is located mainly in the endoplasmic reticulum (ER), whereas normal C3 is seen evenly distributed throughout the cytoplasm. Most of the mutant C3 is degraded within the cell, and only a small fraction (around 8%) is secreted after 20 h chase. Processing of C3 at 19 degrees C was reduced in normal fibroblasts but completely blocked in mutant fibroblasts. ATP depletion blocked processing of normal proC3 to C3. In contrast, the mutant proC3 was partly degraded in ATP-depleted cells, yet its complete degradation and secretion were blocked. Intracellular degradation of the mutant C3 was not inhibited by NH4Cl, thus excluding cleavage within lysosomes. These results demonstrate that the type II mutant C3 studied here is retained in the ER probably by a quality contol machinery that identifies abnormal protein folding. Consequently, it is destined to undergo a two-step intracellular degradation; an initial ATP-independent step followed by an ATP-dependent step.

Molecular Analysis of the Third Component of Canine Complement (C3) and Identification of the Mutation Responsible for Hereditary Canine C3 Deficiency

Genetically determined deficiency of the third component of complement (C3) in the dog is characterized by a predisposition to recurrent bacterial infections and to type 1 membranoproliferative glomerulonephritis. The current studies were undertaken to characterize the cDNA for wild-type canine C3 and identify the molecular basis for hereditary canine C3 deficiency. Amplification, cloning, and sequence analysis indicated that canine C3 is highly conserved in comparison with human, mouse, and guinea pig C3. Southern blot analysis failed to show any gross deletions or rearrangements of DNA from C3-deficient animals. Northern blot analysis indicated that the livers of these animals contain markedly reduced quantities of a normal length C3 mRNA. The full-length 5.1-kb canine C3 cDNA was amplified in overlapping PCR fragments. Sequence analysis of these fragments has shown a deletion of a cytosine at position 2136 (codon 712), leading to a frameshift that generates a stop codon 11 amino acids downstream. The deletion has been confirmed in genomic DNA, and its inheritance has been demonstrated by allele-specific oligonucleotide hybridization.

Targeted disruption of the murine gene coding for the third complement component (C3)

Complement is a system of more than 30 proteins found both in plasma and on cell membranes. The complement system has several important functions in the immune response including initiation of inflammation, neutralization and elimination of pathogens, regulation of antibody responses, clearance of immune complexes and disruption of cell membranes. Under certain conditions complement may, however, act as a mediator of deleterious inflammatory reactions and complement activation has been implicated in the pathogenesis of autoimmune disorders, atherosclerosis, neurodegenerative diseases, bioincompatibility reactions and decompression sickness. Using gene targeting, we have generated mice deficient for the third complement component (C3). These mice represent an animal model in which complement activation by any pathway is prevented at an early stage. The C3-deficient mice should be valuable for the study of the roles of the complement system in vivo in a variety of physiological and pathological situations.

Transcriptional regulation of the gene for the second component of human complement: promoter analysis

The 5' flanking region of the human gene for the second component of complement was sequenced and analyzed functionally. RNase protection demonstrated a cluster of four initiation sites in the 5' flanking region utilized in the hepatoma cell line, HepG2. Utilization of all four initiation sites increased in response to gamma-interferon (IFN-gamma). Transient transfection analysis was used to examine cis-acting sequence motifs controlling transcription from the 5'-flanking region. We identified a 228-bp minimal promoter fragment which was able to direct basal and IFN-gamma inducible transcription from authentic initiation sites. Sequence motifs outside of this region may modulate the transcriptional regulation of the second component of complement. Although complement components are not coordinately regulated, we identified four regions of significant homology with the promoters of multiple other complement components. Three of these regions were within the minimal promoter fragment.

Hereditary deficiency of C3 in animals and humans

Inherited deficiency of complement C3 has been described in guinea pigs, dogs and 20 humans. Homozygous deficiency of C3 is associated with recurrent pyogenic infections by encapsulated bacteria, especially H. influenzae, S. pneumoniae and N. meningitidis. In dogs and humans there is also an association with development of glomerulonephritis of the mesangiocapillary type. Some patients also develop transient erythematous rashes in association with pyogenic infections, with histology showing predominantly neutrophil infiltration and small vessel vasculitis. Studies of antibody responses, mainly in experimental animals have shown impaired primary and secondary responses to both thymus-dependent and -independent antigens at low immunizing doses, with a reduced switch from IgM to IgG production. The molecular basis of C3 deficiency has been established in two humans with C3 deficiency. In one it was due to a splice junction mutation and in another, to a partial gene deletion. These mutations are not compatible with the production of functional C3 in any tissue. Such patients with absolute C3 deficiency are a valid model for understanding the physiological role of C3 in vivo.

Homozygous hereditary C3 deficiency due to a partial gene deletion

The molecular mechanism of C3 deficiency in an Afrikaans patient with recurrent pyogenic infections was studied. Restriction enzyme analysis showed a gene deletion of 800 base pairs (bp) mapping to the alpha chain of C3. Amplification of genomic DNA, using the PCR, demonstrated that the deletion included exons 22 and 23 of the C3 gene. Truncated mRNA was shown in an Epstein-Barr virus-transformed B-cell line by PCR amplification of first-strand cDNA. A consequence of this deletion was that the RNA transcribed 3' to the deletion was out of frame, resulting in formation of a stop codon 19 bp downstream from the deletion. The molecular basis of the deletion was compatible with homologous recombination between two Alu sequences located in introns 21 and 23. An unrelated nonconsanguineous relative and two of a sample of 174 Afrikaans-speaking individuals were heterozygous carriers of the same gene deletion. The wide prevalence of this null allele in this population is probably due to the effects of a small founder population. The presence of this deletion in the C3 gene is not compatible with production of any functional C3, supporting the idea that study of such patients offers a valid model for understanding physiological activities of C3 in vivo in humans.

Cis- and trans-acting elements required for constitutive and cytokine-regulated expression of the mouse complement C3 gene

The third component of complement (C3) is an important mediator of inflammation. Murine and human genomic cosmid clones were isolated, characterized and sequenced 5' to the complement C3 gene transcriptional initiation sites to determine cis elements that participate in constitutive and regulated C3 gene expression. The murine and human 5' flanking regions are 51% identical overall, with positions -36 to -1 and -146 to -68 showing 80% identity. Four TATA boxes were identified upstream of the murine transcriptional initiation site, but deletion and transfection analysis using reporter gene constructs in HepG2 cells indicated that only the TATA element at position -30, together with sequences -395 to -111, are essential for constitutive expression of murine C3 in hepatocytes. Deletion analysis also suggested that sequences between -1457 and -800 contain regulatory elements that are involved in suppressing basal expression. Sequences between -90 to -41 confer both enhancer activity and interleukin-1/-6 (IL-1/IL-6)-responsiveness. Mutation analyses showed that both sequences between -88 and -83 and -77 to -72 are essential for enhancer activity and responsiveness to IL-1, but only sequences between -88 and -83 are necessary for IL-6-responsiveness. A gel-retardation assay showed that several nucleoproteins, perhaps of the C/EBP family, from HepG2 cells bound to sequences between -88 to -83. Collectively, these results localize cis-acting elements involved in constitutive and IL-1/IL-6-regulated murine C3 gene expression and provide evidence for specific transacting factors.

Complement

The complement system mediates a wide range of important biological functions. The use of modern techniques in protein chemistry and molecular biology has greatly facilitated our understanding of the interactions between the fluid phase and cell-bound components of the system. Structural and genetic analysis has shown that while many of these components are polymorphic, there are major similarities between many of the proteins serving enzymatic and regulatory roles in both the alternative and classical pathways. The regulation of complement activation and Class III genes, on chromosomes 1 and 6 respectively, encode nine of the major proteins in the system. The genetic basis of C4 and C3 polymorphisms is now well established, and further study may reveal functional differences between polymorphic variants of other components. The study of individuals with either genetic or acquired deficiencies of complement proteins and receptors has provided insight into the function of these components, leukocyte adherence deficiency (LAD) providing the best example. An appreciation of the genetics, structure and functions of the regulatory proteins decay-accelerating factor (DAF) and homologous restriction factor has enhanced our understanding of the pathogenesis of paroxysmal nocturnal haemoglobinuria. The full importance of CD59 glycoprotein, the newest member of the complement family, remains to be determined.

A 58-base-pair region of the human C3 gene confers synergistic inducibility by interleukin-1 and interleukin-6

We have cloned the promoter for the human third component of complement (C3) gene and have identified sequences involved in its regulation during the acute-phase response. A construct linking 199 bp of the C3 promoter to the firefly luciferase gene was found to be very responsive to interleukin-1 (IL-1) and modestly responsive to interleukin-6 (IL-6) by transfection analysis in the human hepatoma line Hep3B2. Simultaneous treatment with the two cytokines showed a strong synergy between the actions of the two molecules. A 58-bp fragment (-127 to -70 bp) was shown by 5' and 3' deletional mutagenesis to contain cis-acting elements that mediated both the IL-1 response and the IL-1-plus-IL-6 synergistic response of this promoter. When coupled to a heterologous promoter, this fragment enabled the synergistic induction by IL-1 plus IL-6. Sequences homologous to the palindrome ACATTGCACAATCT, which mediates the induction of the IL-6 gene by IL-1 (S. Akira, H. Isshiki, T. Sugita, O. Tanabe, S. Kinoshita, Y. Nishio, T. Nakajima, T. Hirano, and T. Kishimoto, EMBO J. 9:1897-1906, 1990), and the core sequence of the IL-6-responsive element of the rat alpha 2-macroglobulin gene (CTGGGA; M. Hattori, L. J. Abraham, W. Northemann, and G. H. Fey, Proc. Natl. Acad. Sci. USA 87:2364-2368, 1990) are contained within this fragment in immediate juxtaposition and partially overlapping. Site-directed mutagenesis within this homology region drastically reduced the inducibility of the C3 promoter by either cytokine. DNase I footprinting analysis defined a binding site for the transcription factor CCAAT/enhancer-binding protein (C/EBP), which included the IL-1-responsive element-like sequence. No differences were seen between the footprints generated by using extracts from unstimulated and IL-1-stimulated Hep3B2 cells. However, gel retardation analyses revealed two IL-1-specific bands. The data suggest that the induction by IL-1 is mediated by a factor belonging to the family of C/EBP-related proteins.

A 58-base-pair region of the human C3 gene confers synergistic inducibility by interleukin-1 and interleukin-6

We have cloned the promoter for the human third component of complement (C3) gene and have identified sequences involved in its regulation during the acute-phase response. A construct linking 199 bp of the C3 promoter to the firefly luciferase gene was found to be very responsive to interleukin-1 (IL-1) and modestly responsive to interleukin-6 (IL-6) by transfection analysis in the human hepatoma line Hep3B2. Simultaneous treatment with the two cytokines showed a strong synergy between the actions of the two molecules. A 58-bp fragment (-127 to -70 bp) was shown by 5' and 3' deletional mutagenesis to contain cis-acting elements that mediated both the IL-1 response and the IL-1-plus-IL-6 synergistic response of this promoter. When coupled to a heterologous promoter, this fragment enabled the synergistic induction by IL-1 plus IL-6. Sequences homologous to the palindrome ACATTGCACAATCT, which mediates the induction of the IL-6 gene by IL-1 (S. Akira, H. Isshiki, T. Sugita, O. Tanabe, S. Kinoshita, Y. Nishio, T. Nakajima, T. Hirano, and T. Kishimoto, EMBO J. 9:1897-1906, 1990), and the core sequence of the IL-6-responsive element of the rat alpha 2-macroglobulin gene (CTGGGA; M. Hattori, L. J. Abraham, W. Northemann, and G. H. Fey, Proc. Natl. Acad. Sci. USA 87:2364-2368, 1990) are contained within this fragment in immediate juxtaposition and partially overlapping. Site-directed mutagenesis within this homology region drastically reduced the inducibility of the C3 promoter by either cytokine. DNase I footprinting analysis defined a binding site for the transcription factor CCAAT/enhancer-binding protein (C/EBP), which included the IL-1-responsive element-like sequence. No differences were seen between the footprints generated by using extracts from unstimulated and IL-1-stimulated Hep3B2 cells. However, gel retardation analyses revealed two IL-1-specific bands. The data suggest that the induction by IL-1 is mediated by a factor belonging to the family of C/EBP-related proteins.

Molecular basis of polymorphisms of human complement component C3

C3 exhibits two common allotypic variants that may be separated by gel electrophoresis and are called C3 fast (C3 F) and C3 slow (C3 S). C3 F, the less common variant, occurs at appreciable frequencies only in Caucasoid populations (gene frequency = 0.20). An increased prevalence of the C3 F allele has been reported in patients with partial lipodystrophy, IgA nephropathy, and Indian childhood hepatic cirrhosis. Studies of the genomic organization of the human C3 gene led to the identification of a single change (C to G) between C3 S and C3 F at nucleotide 364 in exon 3. This leads, at the translation level, to the substitution of an arginine residue (positively charged) in C3 S for a glycine residue (neutral) in C3 F. This substitution results in a polymorphic restriction site for the enzyme HhaI. The resulting restriction fragment length polymorphism (RFLP) was investigated using genomic DNA, amplified using the polymerase chain reaction; there was absolute concordance between the genomic polymorphism and the distribution of C3 S and C3 F in 50 normal subjects. The molecular basis of a second structural polymorphism, defined by the monoclonal antibody HAV 4-1, was also characterized. The polymorphic determinant was identified at codon 314 in the exon 9 of the beta chain where a leucine residue (HAV 4-1+) is substituted for a proline residue (HAV 4-1-). Identification of the amino acid sequences of these polymorphic variants will facilitate characterization of possible functional differences between different allotypes of C3. Three RFLPs (BamHI, EcoRI, and SstI) were located to introns in the C3 gene. There was no allelic association between these three RFLPs, or between the RFLPs and the C3 F/S polymorphic site. Genetic equilibration of these polymorphisms has occurred within a gene of 41 kb.

Molecular basis of hereditary C3 deficiency

Hereditary deficiency of complement component C3 in a 10-yr-old boy was studied. C3 could not be detected by RIA of serum from the patient. Segregation of C3 S and C3 F allotypes within the family confirmed the presence of a null gene for C3, for which the patient was homozygous. 30 exons have been characterized, spanning the entire beta chain of C3 and the alpha chain as far as the C3d region. Sequence analysis of the exons derived from the C3 null gene showed no abnormalities in the coding sequences. A GT-AT mutation at the 5' donor splice site of the intervening sequence 18 was found in the C3 null gene. Exons 17-21 were amplified by the polymerase chain reaction (PCR) from first-strand cDNA synthesized from mRNA obtained from peripheral blood monocytes stimulated with LPS. This revealed a 61-bp deletion in exon 18, resulting from splicing of a cryptic 5' donor splice site in exon 18 with the normal 3' splice site in exon 19. This deletion leads to a disturbance of the reading frame of the mRNA with a stop codon 17 bp downstream from the abnormal splice in exon 18. His parents had both the normal and abnormal C3 mRNA and were shown to be heterozygous for this mutation by sequence analysis of genomic DNA amplified by PCR. Similar splice mutants have previously been reported in the beta-globin, phenylalanine hydroxylase, and porphobilinogen deaminase genes. This mutation is sufficient to cause the deficiency of C3 in the patient.