The molecular genetics of components of complement

Perspective: DNA Copy Number Variations in Cardiovascular Diseases

Human genome contains many variations, often called mutations, which are difficult to detect and have remained a challenge for years. A substantial part of the genome encompasses repeats and when such repeats are in the coding region they may lead to change in the gene expression profile followed by pathological conditions. Structural variants are alterations which change one or more sequence feature in the chromosome such as change in the copy number, rearrangements, and translocations of a sequence and can be balanced or unbalanced. Copy number variants (CNVs) may increase or decrease the copies of a given region and have a pivotal role in the onset of many diseases including cardiovascular disorders. Cardiovascular disorders have a magnitude of well-established risk factors and etiology, but their correlation with CNVs is still being studied. In this article, we have discussed history of CNVs and a summary on the diseases associated with CNVs. To detect such variations, we shed light on the number of techniques introduced so far and their limitations. The lack of studies on cardiovascular diseases to determine the frequency of such variants needs clinical studies with larger cohorts. This review is a compilation of articles suggesting the importance of CNVs in multitude of cardiovascular anomalies. Finally, future perspectives for better understanding of CNVs and cardiovascular disorders have also been discussed.

Human genes involved in copy number variation: mechanisms of origin, functional effects and implications for disease

Copy number variants (CNVs) overlap over 7000 genes, many of which are pivotal in biological pathways. The implications of this are profound, with consequences for evolutionary studies, population genetics, gene function and human phenotype, including elucidation of genetic susceptibility to major common diseases, the heritability of which has thus far defied full explanation. Even though this research is still in its infancy, CNVs have already been associated with a number of monogenic, syndromic and complex diseases: the development of high throughput and high resolution techniques for CNV screening is likely to bring further new insights into the contribution of copy number variation to common diseases. Amongst genes overlapped by CNVs, significant enrichments for certain gene ontology categories have been identified, including those related to immune responses and interactions with the environment. Genes in both of these categories are thought to be important in evolutionary adaptation and to be particular targets of natural selection. Thus, a full appreciation of copy number variation may be important for our understanding of human evolution.

Complement and parasitic trematodes

The complement (C) system acts as a barrier to protect our bodies against invading pathogens. It may react to cytophilic antibodies or directly to foreign molecules presented by the intruder. As well as their cytotoxic activity, C components can attract and attach leucocytes to the surface of the foreign body, and activate them to kill it. Zvi Fishelson describes various strategies used by a parasitic trematode to escape immune damage in the face of potent immune surveillance by C and other effector mechanisms.

Was it there all the time?

The complement system is old, yet it may still have something new to teach us. For many years, research has existed which shows that C3d, in addition to its established role as an adjuvant, could have an immunosuppressive activity. Being true, it suggests that a common mechanism may be used both by organisms and by their pathogens to prevent unwanted immune responses.

Computationally identifying novel NF-kappa B-regulated immune genes in the human genome

Identifying novel NF-kappa B-regulated immune genes in the human genome is important to our understanding of immune mechanisms and immune diseases. We fit logistic regression models to the promoters of 62 known NF-kappa B-regulated immune genes, to find patterns of transcription factor binding in the promoters of genes with known immune function. Using these patterns, we scanned the promoters of additional genes to find matches to the patterns, selected those with NF-kappa B binding sites conserved in the mouse or fly, and then confirmed them as NF-kappa B-regulated immune genes based on expression data. Among 6440 previously identified promoters in the human genome, we found 28 predicted immune gene promoters, 19 of which regulate genes with known function, allowing us to calculate specificity of 93%-100% for the method. We calculated sensitivity of 42% when searching the 62 known immune gene promoters. We found nine novel NF-kappa B-regulated immune genes which are consistent with available SAGE data. Our method of predicting gene function, based on characteristic patterns of transcription factor binding, evolutionary conservation, and expression studies, would be applicable to finding genes with other functions.

Complement C4 inhibits systemic autoimmunity through a mechanism independent of complement receptors CR1 and CR2

The complement system enhances antibody responses to T-dependent antigens, but paradoxically, deficiencies in C1 and C4 are strongly linked to autoantibody production in humans. In mice, disruption of the C1qa gene also results in spontaneous autoimmunity. Moreover, deficiencies in C4 or complement receptors 1 and 2 (CR1/CR2) lead to reduced selection against autoreactive B cells and impaired humoral responses. These observations suggest that C1 and C4 act through CR1/CR2 to enhance humoral immunity and somehow suppress autoimmunity. Here we report high titers of spontaneous antinuclear antibody (ANA) in C4(-/)- mice. This systemic lupus erythematosus-like autoimmunity is highly penetrant; by 10 mo of age, all C4(-)(/)- females and most males produced ANA. In contrast, titers and frequencies of ANA in Cr2(-)(/)- mice, which are deficient in CR1 and CR2, never rose significantly above those in normal controls. Glomerular deposition of immune complexes (ICs), glomerulonephritis, and splenomegaly were observed in C4(-)(/)- but not Cr2(-)(/)- mice. C4(-)(/)-, but not Cr2(-)(/)-, mice accumulate activated T and B cells. Clearance of circulating ICs is impaired in preautoimmune C4(-)(/)-, but not Cr2(-)(/)-, mice. C4 deficiency causes spontaneous, lupus-like autoimmunity through a mechanism that is independent of CR1/CR2.

Shark complement: an assessment

The classical (CCP) and alternative (ACP) pathways of complement activation have been established for the nurse shark (Ginglymostoma cirratum). The isolation of a cDNA clone encoding a mannan-binding protein-associated serine protease (MASP)-1-like protein from the Japanese dogfish (Triakis scyllia) suggests the presence of a lectin pathway. The CCP consists of six functionally distinct components: C1n, C2n, C3n, C4n, C8n and C9n, and is activated by immune complexes in the presence of Ca++ and Mg++ ions. The ACP is antibody independent, requiring Mg++ ions and a heat-labile 90 kDa factor B-like protein for activity. Proteins considered homologues of C1q, C3 and C4 (C2n) of the mammalian complement system have been isolated from nurse shark serum. Shark C1q is composed of at least two chain types each showing 50% identity to human C1q chains A and B. Partial sequence of the globular domain of one of the chains shows it to be C1q-like rather than like mannan-binding protein. N-terminal amino acid sequences of the alpha and beta chain of shark C3 and C4 molecules show significant identity with corresponding human C3 and C4 chains. A sequence representing shark C4 gamma chain, shows little similarity to human C4 gamma chain. The terminal shark components C8n and C9n are functional analogues of mammalian C8 and C9. Anaphylatoxin activity has been demonstrated in activated shark serum, and porcine C5a desArg induces shark leucocyte chemotaxis. The deduced amino acid sequence of a partial C3 cDNA clone from the nurse shark shows 50%, 30% and 24% homology with the corresponding region of mammalian C3, C4 and alpha 2-macroglobulin. Deduced amino acid sequence data from partial Bf/C2 cDNA clones, two from the nurse shark and one from the Japanese dogfish, suggest that at least one species of elasmobranch has two distinct Bf/C2 genes.

Identification in the HLA class I region of a gene expressed late in keratinocyte differentiation

A gene designated S has been identified in the class I region of the human major histocompatibility complex. The S gene is located 160 kb telomeric of HLA-C. It is expressed at high levels as 2.2-kb and 2.6-kb mRNAs in human skin. No homologous transcripts were detected in other tissues including placenta, liver, spleen, thymus, and brain. In situ hybridization showed that S gene expression was restricted to the differentiating keratinocytes in the granular layer of the epidermis. The predicted amino acid sequence of the S protein was remarkable for its high content of serine, glycine, and proline. There were significant similarities with the amino acid sequences of loricrin, keratin 1, and keratin 10, all major components of the granular-cell layer. The selective expression of the S gene in the granular-cell layer in the epidermis suggests a role in the developmental program of differentiating keratinocytes. Furthermore, in light of the recognized association of psoriasis vulgaris, a disorder of keratinocyte proliferation, with alleles of HLA-C, this gene may contribute primarily to the pathogenesis of this common disorder.

Complement gene expression in human brain: comparison between normal and Alzheimer disease cases

Many investigators have reported the presence of complement proteins in Alzheimer disease (AD) brains, but their origin is uncertain. We report the expression of complement genes C1q, C3 and C4 in RNA extracted from temporal cortex samples of post-mortem human brain. The transcripts for C3 and C4 were detected by Northern hybridization analysis, and the mRNAs for C1q, C3 and C4 were detected by polymerase chain reaction (PCR) amplification of brain derived complementary DNA (cDNA). The relative abundances of mRNAs for C3 and C4 were compared between samples of temporal cortex from neurologically normal and Alzheimer disease (AD) cases, using a semiquantitative PCR assay. There was a 3.01 fold mean increase in expression of C3 and a 3.27-fold mean increase in expression of C4 in AD samples compared to control cases. These results indicate that localized synthesis of some of the major complement components can occur in human brain. The factors causing activation of complement in AD still remain undetermined.

Identification and characterization of two nonessential regions of the rabbitpox virus genome involved in virulence

We have developed a means to identify genes associated with particular aspects of virulence. By beginning with an avirulent deletion mutant of rabbitpox virus and systematically reintroducing overlapping segments of the deleted region, we have identified two regions of the viral genome associated with increased virulence in mice. Evaluation of illness has been aided by the exploitation of weight loss as an indicator of pathogenesis. One of the regions identified by this method contains several open reading frames and includes two previously described genes. A third, as yet undescribed, gene within this region potentially encodes a product related to the C5 protein of human complement. The second region of DNA associated with increased virulence is the HindIII M fragment, which contains only one complete open reading frame. Analysis of this previously unreported gene shows coding potential for a polypeptide of 254 amino acids (approximately 25 kDa) which is related to the C4 component of human complement. The elucidation of two new viral genes related to complement components, taken together with the recent report of the biological activity of a poxvirus-encoded complement-binding protein, suggests the importance of interactions of the virus with the complement system during a normal infection.

Increased frequency of HLA-DR3 and complotype SC01 in Mexican mestizo patients with amoebic abscess of the liver

Our preliminary study (31 patients) of HLA frequencies and amoebic abscess of the liver (AAL) in Mexican mestizos was extended to include 110 patients with this condition. The previously found increase in HLA-B16 was not confirmed, but the frequency of HLA-DR3 was again found significantly increased in patients with AAL when compared to the normal, ethnically matched control population, both in its isolated (35.5% vs 12.7%) and in the HLA-A2, DR3 haplotypic version (20.9% vs 4.5%). Moreover, seven of the 17 HLA specificities that were found to be individually different at P (yet not at PC) level in patients with AAL when compared to the control population, were actually HLA-DR3 containing haplotypes. HLA-DR3 may thus encode a risk factor(s) for AAL, at least in the Mexican mestizo population. Furthermore a significant increase in the complotype SC01 and its haplotypic version SC01, DR4 were identified in 45 non selected patients with AAL when compared to normal controls (31.1% vs 6.7% and 17.8% vs 0% respectively). Even though a relationship between allelic forms of complement components and their function has not been fully established, this complotype could represent a risk factor as well, since complement appears to play a role in host defence against amoebic invasion. Finally, no extended haplotype preference was found in these AAL patients.

Associations between restriction fragment length polymorphisms detected with a probe for human C4 and allotype of C4B5 allele

We studied the fourth component of human complement (C4) allotypes in 58 Japanese individuals. The technique of Southern, with C4 and 21-OH cDNA probes, was used to examine the genomic DNA of 45 individuals typed for C4 by protein electrophoresis. Novel HindIII C4 10- and 5-kb and EcoRI C4 13-Kb restriction fragments were identified in each of nine Japanese individuals. The novel fragments were different from the previously described C4B long (HindIII 31-kb, TaqI 6-kb, BamHI 4.3-kb, and EcoRI 12-kb) and C4B short (HindIII 25-kb, TaqI 5.4-kb, BamHI 3.5-kb, and EcoRI 15-kb) fragments. All novel HindIII- and EcoRI-positive individuals carried C4B5, BfS, and HLA-Bw54. Therefore, the fragments were characteristic for the C4B5 allele. The C4 region was analyzed to determine the restriction sites by single and double digests of uncloned genomic DNA with several restriction endonucleases. It is speculated that an insertion gene lies between the 3' end of the 21-OH and the 5' end of the C4B genes.

Stimulation of histamine receptors of human monocytoid and hepatoma-derived cell lines and mouse hepatocytes modulates the production of the complement components C3, C4, factor B, and C2

The influence of histamine (and the related agonists and antagonists) alone or in the presence of recombinant human interleukin 1 alpha (IL-1 alpha) and gamma interferon (IFN-gamma) was studied on the production of complement components C3, C2, factor B, and C4 in vitro with human monocytoid cell line U937, hepatoma-derived cell line HepG2, and mouse hepatocytes. Both U937 and HepG2 cells responded to histamine through H1 and H2 histamine receptors. The effect of histamine on the biosynthesis and gene expression of complement proteins was predominantly enhancing via the H1 histamine receptors and inhibitory through the H2 receptors. The actual predominance of the histamine receptor involved (and the outcome of the ligand interaction) seemed to be greatly affected by the simultaneous activation of the cells by IL-1 or IFN-gamma.

Purification and characterisation of ovine C4: evidence for two molecular forms in ovine plasma

A relatively rapid procedure is described for the isolation of the fourth component of complement (C4) from ovine plasma. The method, which recovers approximately 30% C4, is based upon DEAE Sephacel anion exchange chromatography of PEG precipitated plasminogen depleted plasma followed by cation exchange chromatography on CM Sepharose and finally gel filtration. SDS-PAGE of purified ovine C4 under reducing conditions revealed a complex pattern of bands which was interpreted on the basis of a three polypeptide chain structure for each of two distinct species, or isotypes, of C4 molecule herein termed C4A and C4B. Each isotype differs in the mol. wt of the alpha chain--108 and 95 K respectively. Nucleophilic substitution of immunoprecipitated ovine C4 with radiolabelled methylamine revealed that both C4 species contained a reactive thiol ester site and that each could be cleaved into an activated form (presumably C4b) characterised by a truncated alpha' chain some 8 K lower in mol. wt. A comparison of the isotype composition of purified C4 with that of immunoprecipitated C4 from the same animal indicated that the purification procedure favoured isolation of the C4B isotype. The mol. wts of both the alpha and beta chains were lowered following digestion of ovine C4 with neuraminidase.

Extent of the mouse t complex and its inversions shown by in situ hybridization

Probes for loci situated near one end of the proximal (Tcp-1) and distal (Qa-2, 3) inversions of the mouse t complex have been hybridized to chromosomes of mice with and without t complexes and with morphologically distinguishable chromosome 17s. Both the probe for Tcp-1 and that for Qa-2, 3 hybridized to clearly different positions on t and non-t chromosomes, thus making visible the extent of the two inversions. The proximal inversion extends from roughly the junction of bands A1 and A2 to band A3, and the distal inversion from band A3 to band C. Thus, the whole t complex extends from the band A1-A2 junction to band C, and is therefore somewhat larger than previously thought, and occupies about 1.2% of the genome. A probe for complement component 3 (C3-1), genetically known to be several cM distal to the t complex, was found by in situ hybridization to lie in band E1. The proximal part of chromosome 17 is one of the best known parts of the mouse genome, at both the genetic and molecular levels. It may soon be possible to correlate the length of the t complex in terms of chromosomal distance with its physical length in megabases.

Deletion of C4A genes in patients with systemic lupus erythematosus

To define the relationship between inheritance of major histocompatibility complex (MHC) alleles and susceptibility to the development of systemic lupus erythematosus (SLE), we examined the MHC class I, II, and III phenotypes of white SLE patients and characterized the structures of their class III MHC genes, using Southern blotting. Nine of 88 SLE patients (10.2%) were C4A null. As detected by Southern blot analysis, the C4A gene was deleted from both chromosomes in 8 of the 9 C4A-null patients. Deletions affecting only 1 chromosome (heterozygous) were detected in the remaining C4A-null patient and in 34.5% of SLE patients who were not C4A deficient (compared with 12.5% of controls; P less than 0.05). These results indicate that deletion of the C4A gene is a common genetic marker for SLE. Deletions of C4A were observed most commonly as part of the HLA-B8;DR3 extended haplotype, although deletions were also detected in different HLA haplotypes. Because of the critical role of C4A in the processing of immune complexes, deficiency of C4A may, itself, confer susceptibility to the development of SLE.

Covalent association of C3b with C4b within C5 convertase of the classical complement pathway

The C5 convertase of the classical complement pathway is a complex enzyme consisting of three complement fragments, C4b, C2a, and C3b. Previous studies have elucidated functional roles of each subunit (4, 6, 7), but little is known about how the subunits associate with each other. In this investigation, we studied the nature of the classical C5 convertase that was assembled on sheep erythrocytes. We found that one of the nascent C3b molecules that had been generated by the C3 convertase directly bound covalently to C4b. C3b bound to the alpha' chain of C4b through an ester bond, which could be cleaved by treatment with hydroxylamine. The ester bond was rather unstable, with a half-life of 7.9 h at pH 7.4 and 37 degrees C. Formation of the C4b-C3b dimer is quite efficient; e.g., 54% of the cell-bound C3b was associated with C4b when 25,000 molecules of C4b and 12,000 molecules of C3b were present per cell. Kinetic analysis also showed the efficient formation of the C4b-C3b dimer; the rate of dimer formation was similar to or even faster than that of cell-bound monomeric C3b molecules. These results indicate that C4b is a highly reactive acceptor molecule for nascent C3b. High-affinity C5-binding sites with an association constant of 2.1 X 10(8) L/M were demonstrated on C4b-C3b dimer-bearing sheep erythrocytes, EAC43 cells. The number of high-affinity C5-binding sites coincided with the number of C4b-C3b dimers, but not with the total number of cell-bound C3b molecules. Anti-C4 antibodies caused 80% inhibition of the binding of C5 to EAC43 cells. These results suggest that only C4b-associated C3b serves as a high-affinity C5 binding site. EAC14 cells had a small amount of high-affinity C5 binding sites with an association constant of 8.1 X 10(7) L/M, 100 molecules of bound C4b being necessary for 1 binding site. In accordance with the hypothesis that C4b-associated C4b might also serve as a high-affinity C5-binding site, a small amount of C4b-C4b dimer was detected on EAC14 cells by SDS-PAGE analysis. Taken together, these observations indicate that the high-affinity binding of C5 is probably divalent, in that C5 recognizes both protomers in the dimers. The high-affinity binding may allow selective binding of C5 to the convertase in spite of surrounding monomeric C3b molecules.

The pore-forming protein (perforin) of cytolytic T lymphocytes is immunologically related to the components of membrane attack complex of complement through cysteine-rich domains

Structural, functional and immunological similarities between the ninth component of complement (C9) and the lymphocyte pore-forming protein (PFP, perforin) have recently been described (8-10). PFP is shown here to be immunologically related to all other components of the membrane attack complex (MAC) of human complement, namely, C5b-6, C7, C8, and C9. Polyclonal antibodies raised against purified human C5b-6, C7, C8, or C9 react with other components of the MAC and with mouse lymphocyte PFP. The antigenic epitopes shared by human complement proteins and mouse lymphocyte PFP are limited to cysteine-rich domains. Only complement proteins that have been reduced and alkylated elicit the production of crossreactive antibodies when used as immunogens. The nonreduced forms of complement components or lymphocyte PFP neither react with these antibodies nor give rise to crossreactive antibodies. The homologous domains of complement proteins and lymphocyte PFP may play related functions in their attachment to lipid membranes and assembly of membrane lesions.