Tissue-specific RNA processing for the complement C4 gene transcript in the H-2k mouse strain

Analysis of C4 and the C4 binding protein in the MRL/lpr mouse

Systemic lupus erythematosus is a complement-mediated autoimmune disease. While genetic deficiencies of classical pathway components lead to an increased risk of developing systemic lupus erythematosus, end organ damage is associated with complement activation and immune complex deposition. The role of classical pathway regulators in systemic lupus erythematosus is unknown. C4 binding protein (C4bp) is a major negative regulator of the classical pathway. In order to study the role of C4bp deficiency in an established murine model of lupus nephritis, mice with a targeted deletion in the gene encoding C4bp were backcrossed into the MRL/lpr genetic background. Compared with control MRL/lpr mice, C4bp knockout MLR/lpr mice had similar mortality and similar degrees of lymphoproliferation. There were no differences in the extent of proteinuria or renal inflammation. Staining for complement proteins and immunoglobulins in the kidneys of diseased mice revealed no significant strain differences. Moreover, there was no difference in autoantibody production or in levels of circulating immune complexes. In comparison with C57BL/6 mice, MRL/lpr mice had depressed C4 levels as early as 3 weeks of age. The absence of C4bp did not impact serum C4 levels or alter classical pathway hemolytic activity. Given that immune complex renal injury in the MRL/lpr mouse is independent of Fc receptors as well as the major negative regulator of the classical pathway, new mechanisms for immune-complex-mediated renal injury need to be considered.

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.

Genetic, structural and functional diversities of human complement components C4A and C4B and their mouse homologues, Slp and C4

The complement protein C4 is a non-enzymatic component of the C3 and C5 convertases and thus essential for the propagation of the classical complement pathway. The covalent binding of C4 to immunoglobulins and immune complexes (IC) also enhances the solubilization of immune aggregates, and the clearance of IC through complement receptor one (CR1) on erythrocytes. Human C4 is the most polymorphic protein of the complement system. In this review, we summarize the current concepts on the 1-2-3 loci model of C4A and C4B genes in the population, factors affecting the expression levels of C4 transcripts and proteins, and the structural, functional and serological diversities of the C4A and C4B proteins. The diversities and polymorphisms of the mouse homologues Slp and C4 proteins are described and contrasted with their human homologues. The human C4 genes are located in the MHC class III region on chromosome 6. Each human C4 gene consists of 41 exons coding for a 5.4-kb transcript. The long gene is 20.6 kb and the short gene is 14.2 kb. In the Caucasian population 55% of the MHC haplotypes have the 2-locus, C4A-C4B configurations and 45% have an unequal number of C4A and C4B genes. Moreover, three-quarters of C4 genes harbor the 6.4 kb endogenous retrovirus HERV-K(C4) in the intron 9 of the long genes. Duplication of a C4 gene always concurs with its adjacent genes RP, CYP21 and TNX, which together form a genetic unit termed an RCCX module. Monomodular, bimodular and trimodular RCCX structures with 1, 2 and 3 complement C4 genes have frequencies of 17%, 69% and 14%, respectively. Partial deficiencies of C4A and C4B, primarily due to the presence of monomodular haplotypes and homo-expression of C4A proteins from bimodular structures, have a combined frequency of 31.6%. Multiple structural isoforms of each C4A and C4B allotype exist in the circulation because of the imperfect and incomplete proteolytic processing of the precursor protein to form the beta-alpha-gamma structures. Immunofixation experiments of C4A and C4B demonstrate > 41 allotypes in the two classes of proteins. A compilation of polymorphic sites from limited C4 sequences revealed the presence of 24 polymophic residues, mostly clustered C-terminal to the thioester bond within the C4d region of the alpha-chain. The covalent binding affinities of the thioester carbonyl group of C4A and C4B appear to be modulated by four isotypic residues at positions 1101, 1102, 1105 and 1106. Site directed mutagenesis experiments revealed that D1106 is responsible for the effective binding of C4A to form amide bonds with immune aggregates or protein antigens, and H1106 of C4B catalyzes the transacylation of the thioester carbonyl group to form ester bonds with carbohydrate antigens. The expression of C4 is inducible or enhanced by gamma-interferon. The liver is the main organ that synthesizes and secretes C4A and C4B to the circulation but there are many extra-hepatic sites producing moderate quantities of C4 for local defense. The plasma protein levels of C4A and C4B are mainly determined by the corresponding gene dosage. However, C4B proteins encoded by monomodular short genes may have relatively higher concentrations than those from long C4A genes. The 5' regulatory sequence of a C4 gene contains a Spl site, three E-boxes but no TATA box. The sequences beyond--1524 nt may be completely different as the C4 genes at RCCX module I have RPI-specific sequences, while those at Modules II, III and IV have TNXA-specific sequences. The remarkable genetic diversity of human C4A and C4B probably promotes the exchange of genetic information to create and maintain the quantitative and qualitative variations of C4A and C4B proteins in the population, as driven by the selection pressure against a great variety of microbes. An undesirable accompanying byproduct of this phenomenon is the inherent deleterious recombinations among the RCCX constituents leading to autoimmune and genetic disorders.

DNase I hypersensitivity mapping and promoter polymorphism analysis of human C4

Human complement component C4 is encoded by two structurally distinct loci in the major histocompatibility complex (MHC) class III region. The two isotypes, C4A and C4B, differ at only four residues in the C4d fragment, but C4 constitutes the most polymorphic of the complement components. It is not known, however, whether the regions involved in the regulation of C4 expression also display polymorphic variation. By using the technique of DNase I hypersensitivity mapping, we established that the only area of transcriptional activity for C4 in the hepatocyte cell line, HepG2, occurs approximately 500 base pairs upstream of the transcriptional start site. This region was found to be remarkably constant in sequence when analyzed in the context of differing MHC haplotypes including HLA B57, C4A6, C4B1, DR7, which has been correlated with reduced expression of the C4A isotype. Similarly, polymerase chain reaction followed by single-strand conformation polymorphism analysis failed to demonstrate any promoter polymorphisms in 103 individuals comprising 52 systemic lupus erythematosus patients and 51 healthy controls.