Type I human complement C2 deficiency. A 28-base pair gene deletion causes skipping of exon 6 during RNA splicing

Navigating the maze of complement genetics: a guide for clinicians

Genetic deficiencies of nearly all of the 30 complement system proteins have been recognized clinically. In many instances, the molecular basis for the deficiency has been elucidated. As a byproduct of these studies, we now have new insights into the pathophysiologic role of complement studies in several acquired diseases. New targets for drug development are among the practical outcomes of work on complement genetics.

Complement C2 receptor inhibitor trispanning and the beta-chain of C4 share a binding site for complement C2

Complement C2 receptor inhibitor trispanning (CRIT) of the Schistosoma parasite binds human C2 via the C2a segment. The receptor in vivo functions as C2 decoy receptor by directly competing with C4b for binding to C2. As a result, CRIT is able to limit the extent of classical pathway (CP) C3 convertase formation. We report that the CRIT-extracellular domain 1 (ed1) peptide inhibits CP-mediated complement activation with an ICH(50) of approximately 0.1 microM, the C-terminal 11 aa of CRIT-ed1, named H17, even more effectively. The beta-chain region F222-Y232 of C4 shares 55% identity and 73% similarity with H17. Peptides based on this region also inhibit CP in a dose-dependent manner. As further evidence of C2 binding we showed CRIT-ed1 peptides and homologous C4 beta-chain peptides to inhibit complement in C2 hemolytic assays. We have predicted C4 beta-c F222-Y232 as a C2 binding site which we have termed the CRIT-ed1 domain, and the sequence [F/H]EVKX(4/5)P as a consensus C2-binding sequence. Anti-CRIT-ed1 cross-reacts with the C4 beta-chain and F222EVKITPGKPY232 appears to be the key epitope recognized by this Ab. Furthermore, anti-CRIT-ed1 was found to inhibit CP activation in a total hemolytic assay. We believe that Schistosoma CRIT-ed1, as well as C4 beta-chain peptides based on the CRIT-ed1 domain, function as interface peptides. These peptides, based on C2-binding sequences in CRIT, or C4, competitively inhibit the binding of C2 to C4b and thus limit the activation of C. The C4 peptides, unlike CRIT-ed1, did not inhibit the cleavage of C2 by C1s.

Lack of evidence of a specific role for C4A gene deficiency in determining disease susceptibility among C4-deficient patients with systemic lupus erythematosus (SLE)

The aim of the present study was to investigate the prevalence of C4 and C2 deficiencies and to characterize genomic alterations in C4 genes in a large cohort of 125 unselected patients with SLE. We determined the protein concentration and functional activity of C2 and C4, as well as the C4 phenotype. C4 genotyping included Taq 1 restricted fragment lengh polymorphism (RFLP) analysis and polymerase chain reaction using sequence-specific primers (SSP-PCR). Type I C2 deficiency was diagnosed by PCR. Overall, 79.2% of the patients exhibited abnormalities of the C4 genes including deletion, non-expression, gene conversion and duplication. Among C4-deficient patients (n = 66, 52.8% prevalence), 41.0% of the patients exhibited a C4A deficiency and 59.0% a C4B deficiency. Half of the C4 deficiencies were due to a gene deletion. There was a strong association between C4A and C4B gene deletion and the presence of the DRB1*03 allele. Among the silent C4A genes, only two cases were related to a 2-bp insertion in exon 29 of the C4A gene. A gene conversion was demonstrated in eight patients (6.4%). One patient had a homozygous C4A deficiency. Three (2.4%) patients presented with a heterozygous type I C2 deficiency and none with homozygous deficiency. Our results argue against a specific role for C4A gene deficiency in determining disease susceptibility among patients with SLE that are C4-deficient.

Complement deficiencies

The complement proteins play an important role in innate immunity, promoting inflammation and microbial killing. They play a role in the adaptive immune response, as well. Inherited total deficiencies of complement proteins are extremely rare. Table 1 lists more than 40 proteins that comprise the elements of the complement system. Deficiency of the proteins that promote lysis and opsonization is so rare that two papers are able to list all the observed cases to 1991. The exception is mannan-binding lectin, in which deficiency may be commoner. Diseases of regulatory proteins, such as occurs in hereditary angioedema or paroxysmal nocturnal hemoglobinuria, are commoner but still are quite rare. As we learn more about complement proteins and their mechanism of action, we will understand more clearly how these proteins function. Polymorphisms of the proteins exists. Learning how these polymorphisms contribute to the development of disease will be the focus of complement studies in the next decade.

Human genetic diseases of proteolysis

Although in the past protein stability commonly has been considered an inherent property of a given protein, the truth is far more complex. Elaborate enzymatic systems exist in multiple intracellular compartments to hydrolyze proteins. These systems are capable of providing a sensitive mechanism to regulate protein expression, a mechanism that is complementary to the transcriptional and translational control mechanisms that influence protein synthesis. The power of regulated proteolysis has been well-demonstrated in the abrupt degradation of cyclins that underlies eukaryotic cell cycle progression. Coincidental with the recent rapid gains in understanding proteolysis at a biochemical level, several human diseases have been found to result from disordered proteolysis. This article reviews several examples of human disease resulting from mutations of genes encoding serine proteases, cysteine proteases, and their inhibitors. Examples are also presented of human diseases resulting from disorders in the highly intricate ubiquitin-proteasome pathway of protein degradation. It is certain that many more human diseases will be associated in the future with disorders of proteolysis.

Homozygous deletion of the CYP21A-TNXA-RP2-C4B gene region conferring C4B deficiency associated with recurrent respiratory infections

The central class III region of the human major histocompatibility complex contains highly polymorphic genes that are associated with immune disorders and may serve as susceptibility factors for viral infections. Many HLA haplotype specific rearrangements, duplications, conversions and deletions, occur frequently in the C4 gene region. Genetic deficiencies of complement components are associated with recurrent occurrence of bacterial infections. We have studied the complement profile and the class III genes 5'-RP1-C4A-CYP21A-TNXA-RP2-C4B-CYP21B-TNXB -3' in a 4-year-old Caucasian patient. He has suffered from several pneumonias caused by respiratory viruses, eight acute otitis media, prolonged respiratory infections and urinary tract infection. Complement C4 was constantly low, but the other complement components, from C1 to C9, C1INH, factor B and properdin, were within normal limits. Immunological evaluation gave normal lymphocyte numbers and functions with the exception of subnormal T cell response to pokeweed mitogen. Molecular studies of the C4 gene region in the patient revealed homozygous deletion of CYP21A-TNXA-RP2-C4B generating total deficiency of C4B and the flanking 5' region up to C4A, and in the father a missing CYP21A gene. Further investigations are needed to elucidate the relationship between C4B deficiency and susceptibility to infections.

Deficiency of Human Complement Protein C4 Due to Identical Frameshift Mutations in the C4A and C4B Genes

The complement protein C4, encoded by two genes (C4A and C4B) on chromosome 6p, is the most polymorphic among the MHC III gene products. We investigated the molecular basis of C4 deficiency in a Finnish woman with systemic lupus erythematosus. C4-specific mRNA was present at low concentrations in C4-deficient (C4D) patient fibroblasts, but no pro-C4 protein was detected. This defect in C4 expression was specific in that synthesis of two other complement proteins was normal. Analysis of genomic DNA showed that the proposita had both deleted and nonexpressed C4 genes. Each of her nonexpressed genes, a C4A null gene inherited from the mother, a C4A null gene, and a C4B null gene inherited from the father, all contained an identical 2-bp insertion (TC) after nucleotide 5880 in exon 29, providing the first confirmatory proof of the C4B pseudogene. This mutation has been previously found only in C4A null genes. Although the exon 29/30 junction is spliced accurately, this frameshift mutation generates a premature stop at codon 3 in exon 30. These truncated C4A and C4B gene products were confirmed through RT-PCR and sequence analysis. Among the possible genetic mechanisms that produce identical mutations in both genes, the most likely is a mutation in C4A followed by a gene conversion to generate the mutated C4B allele.

A Novel Type II Complement C2 Deficiency Allele in an African-American Family

A 9-yr-old African-American male presenting with severe recurrent pyogenic infections was found to have C2 deficiency (C2D). Analysis of his genomic DNA demonstrated that he carried one type I C2D allele associated with the HLA-A25, B18, DR15 haplotype. Screening all 18 exons of the C2 gene by exon-specific PCR/single-strand conformation polymorphism indicated abnormal bands in exons 3, 7, and 6, the latter apparently caused by the 28-bp deletion of the typical type I C2D allele. Nucleotide (nt) sequencing of the PCR-amplified exons 3 and 7 revealed a heterozygous G to A transition at nt 392, causing a C111Y mutation, and a heterozygous G to C transversion at nt 954, causing a E298D mutation and a polymorphic MaeII site. Cys111 is the invariable third half-cystine of the second complement control protein module of C2. Pulse-chase biosynthetic labeling experiments indicated that the C111Y mutant C2 was retained by transfected COS cells and secreted only in minimal amounts. Therefore, this mutation causes a type II C2D. In contrast, the E298D mutation affected neither the secretion of C2 from transfected cells nor its specific hemolytic activity. Analysis of genomic DNA from members of the patient’s family indicated that 1) the proband as well as one of his sisters inherited the type I C2D allele from their father and the novel type II C2D allele from their mother; 2) the polymorphic MaeII site caused by the G954C transversion is associated with the type I C2D allele; and 3) the novel C111Y mutation is associated in this family with the haplotype HLA-A28, B58, DR12.

A Non-Sense Mutation at Arg95 Is Predominant in Complement 9 Deficiency in Japanese

Deficiency of the ninth component of complement (C9D) is one of the most common genetic abnormalities in Japan, with an incidence of one homozygote in 1000. Although C9D individuals are usually healthy, it has been shown that they have an significantly increased risk of developing meningococcal meningitis. In the present study we report the molecular bases for C9D in 10 unrelated Japanese subjects. As a screening step for mutations, exons 2 to 11 of the C9 gene were analyzed using exon-specific PCR/single-strand conformation polymorphism analysis, which demonstrated aberrantly migrating DNA bands in exon 4 in all the C9D subjects. Subsequent direct sequencing of exon 4 of the C9D subjects revealed that eight of the 10 C9D subjects were homozygous for a C to T transition at nucleotide 343, the first nucleotide of the codon CGA for Arg95, leading to a TGA stop codon (R95X). R95X is a novel mutation different from those recently identified in a Swiss family with C9D. Cases 6 and 7 were heterozygous for the R95X mutation. Family study in case 10 confirmed the genetic nature of the defect. In case 6, the second mutation for C9D of the C9 gene was identified to be the substitution of Cys to Tyr at amino acid residue 507 (C507Y), while the genetic defect(s) in the other allele in case 7 remains unknown. Our results indicate that a novel mutation, R95X, is present in most cases of C9D in Japan.

Molecular heterogeneity in deficiency of complement protein C2 type I

Deficiency of the complement protein C2 (C2D), one of the most common genetic deficiencies of the complement system, is associated with rheumatological disorders and increased susceptibility to infection. Two types of C2D have been recognized, each in the context of specific major histocompatibility complex (MHC) haplotypes; type I, a deletion, frameshift and premature stop codon resulting in absence of detectable C2 protein synthesis, and type II, missense mutations resulting in a block in secretion of C2 proteins. Analysis of C2 expression in a child with C2 deficiency, a MHC haplotype different from those associated with type I or II C2D, and recurrent infections revealed additional molecular heterogeneity among C2 deficient patients. No detectable C2 protein was synthesized in the child's fibroblasts under conditions supporting C2 synthesis and secretion in normals and the child's C2 mRNA was reduced to 42% of normal. Nucleotide sequencing of RT-PCR fibroblast mRNA and genomic DNA revealed a type I C2 deficiency (28 base-pair deletion) on one allele and a previously unrecognized two base-pair deletion in exon 2 on the other. Expression of the closely linked factor B gene was markedly decreased (Bf mRNA 25% of normal), though Bf was up-regulated appropriately by interferon-gamma and the flanking sequence containing the Bf promoter was normal in this C2-deficient patient. Moreover, the concentration of Bf protein was normal in the patient's plasma.

Abrogation of the alternative complement pathway by targeted deletion of murine factor B

To investigate the role of complement protein factor B (Bf) and alternative pathway activity in vivo, and to test the hypothesized potential genetic lethal effect of Bf deficiency, the murine Bf gene was interrupted by exchange of exon 3 through exon 7 (including the factor D cleaving site) with the neor gene. Mice heterozygous for the targeted Bf allele were interbred, yielding Bf-deficient offspring after the F1 generation at a frequency suggesting that Bf deficiency alone has no major effect on fertility or fetal development. However, in the context of one or more genes derived from the 129 mouse strain, offspring homozygous for Bf deficiency were generated at less than expected numbers (P = 0.012). Bf-deficient mice showed no gross phenotypic difference from wild-type littermates. Sera from Bf-deficient mice lacked detectable alternative complement pathway activity; purified mouse Bf overcame the deficit. Classical pathway-dependent total hemolytic activity was lower in Bf-deficient than wild-type mice, possibly reflecting loss of the alternative pathway amplification loop. Lymphoid organ structure and IgG1 antibody response to a T-dependent antigen appeared normal in Bf-deficient mice. Sensitivity to lethal endotoxic shock was not significantly altered in Bf-deficient mice. Thus, deficiency of Bf and alternative complement activation pathway led to a less dramatic phenotype than expected. Nevertheless, these mice provide an excellent model for the assessment of the role of Bf and the alternative pathway in host defense and other functions in vivo.

Type II human complement C2 deficiency. Allele-specific amino acid substitutions (Ser189 --> Phe; Gly444 --> Arg) cause impaired C2 secretion

Type II complement protein C2 deficiency is characterized by a selective block in C2 secretion. The Type II C2 null allele (C2Q0) is linked to two major histocompatibility haplotypes (MHC) that differ from the MHC of the more common Type I C2 deficiency. To determine the molecular basis of Type II deficiency the two Type II C2Q0 genes were isolated and transfected separately into L-cells. Subsequent molecular biology, biosynthetic, and immunofluorescence studies demonstrated that C2 secretion is impaired in Type II C2 deficiency because of different missense mutations at highly conserved residues in each of the C2Q0 alleles. One is in exon 5 (nucleotide C566 --> T; Ser189 --> Phe) of the C2Q0 gene linked to the MHC haplotype A11,B35,DRw1,BFS, C4A0B1. The other is in exon 11 (G1930 --> A; Gly444 --> Arg) of the C2Q0 gene linked to the MHC haplotype A2,B5, DRw4,BFS,C4A3B1. Each mutant C2 gene product is retained early in the secretory pathway. These mutants provide models for elucidating the C2 secretory pathway.

Practice parameters for the diagnosis and management of immunodeficiency. The Clinical and Laboratory Immunology Committee of the American Academy of Allergy, Asthma, and Immunology (CLIC-AAAAI)

In this brief review, only the most useful immunologic tests available for defining host defects that lead to susceptibility to infection have been emphasized. It should be pointed out that those evaluations and tests ordered by the physician will rule out the vast majority of the currently recognized defects. Finally, it is important that any patients identified as abnormal by these screening tests be characterized as fully as possible in centers specializing in these diseases before therapy is initiated, since what may appear to be a simple diagnosis on the surface may be an indicator of more complex underlying problems.

Genetic deficiencies of complement

Genetic deficiencies of proteins of the complement system are associated with diverse clinical phenotypes. These clinical manifestations vary as a function of the specific component that is missing. Molecular and cellular biological methods, coupled with more intensive clinical studies, have defined the pathophysiological basis for this set of genetic disorders. Insights into the normal function of complement and its role in immunopathology have been derived from the extensive work in this field during the past few years.

Human complement protein C2. Alternative splicing generates templates for secreted and intracellular C2 proteins

Alternative splicing of the primary transcript for human complement protein C2 generates templates for translation of a secreted (C2 long) protein and an intracellular (C2 short) form in liver, bronchoalveolar macrophages, and fibroblasts. The approximate ratio of C2 long to C2 short mRNA is 2:1. The C2 short mRNA does not contain the 396-base pair encompassed by exons 2 and 3 of the full-length C2 long and thus lacks codons for the 5 carboxyl-terminal residues of the signal peptide. Synthesis of C2 in cells transfected with full-length RNA corresponding to each of the transcripts show that C2 long is secreted within a half-time of approximately 1 h and that C2 short is not secreted. Cell-free biosynthesis in the presence of microsomes demonstrate that this intracellular C2 protein (70 kDa) is apparently capable of traversing the membrane of the endoplasmic reticulum. Though the function of the intracellular C2 protein is unknown, it is abundant in all cell types that express the C2 gene.

Recurrent infections, pericarditis and renal disease in a patient with total C2 deficiency and decreased NK cell function consistent with acute rheumatic fever and systemic lupus erythematosus

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Prevalence of the type I complement C2 deficiency gene in Swedish systemic lupus erythematosus patients

The prevalence of type I complement C2 deficiency in Swedish systemic lupus erythematosus (SLE) patients was investigated by DNA analysis. The characteristic 28 base pair deletion was determined by polymerase chain reaction analysis followed by gel electrophoresis. Five of the 86 patients (5.8%) retrieved from a defined population of 160,000 individuals were heterozygous for the C2Q0 gene compared with one heterozygote of 100 local blood donors (1%), the difference in prevalence not being significant. Among 26 other SLE patients, two patients who are siblings were C2Q0 homozygous. No distinctive clinical features among the patients with C2Q0 genes were obvious, although none had renal involvement.

Molecular basis for human immunodeficiencies

Major emphasis is currently being placed on unraveling the molecular basis of various forms of primary human immunodeficiencies. It is clear from recent studies that not only can different mutations give rise to different phenotypes but the same mutation may result in quite diverse clinical pictures. A correct diagnosis at the DNA level therefore becomes increasingly important in view of the possibility of gene therapy.

Kniest and Stickler dysplasia phenotypes caused by collagen type II gene (COL2A1) defect

Kniest and Stickler dysplasia are two chondrodysplasias characterized by specific phenotypes. No basic defect has been found in patients with Kniest dysplasia, whereas Stickler dysplasia is one of four chondrodysplasias for which mutations of type II procollagen gene (COL2A1) have been identified. We studied a 2-year-old girl presenting with manifestations of Kniest dysplasia and her mother showing a Stickler phenotype. Analysing COL2A1 in both patients, we detected the same 28 basepair deletion spanning the 3'-exon/intron boundary of exon 12 in mother and daughter. We were able to prove a somatic mosaic status for this mutation in the mother which accounts for her milder Stickler-like phenotype.

Genetic deficiencies of the complement system and association with disease--early components

Genetic deficiency of one of the early components of the classical pathway of complement (C1q, C1r, C1s, C4 and C2) is often associated with clinical symptoms and immunochemical abnormalities common in idiopathic autoimmune diseases, such as lupus erythematosus, but also with an increased incidence of various, local and generalized infections. These observations are consistent with the current view of the complement system's role in handling immune complexes and combating microbial invasion. However, the absence of absolute correlations in these experiments of nature suggests that genetic defects of the classical pathway act only epistatically to other host factors and the primary etiologies of the associated diseases. In contrast, the strong association of properdin and factor D deficiency with serious infections caused by encapsulated Gram-negative bacteria suggests a more immediate involvement of the alternative pathway in a specific segment of immunity and its pathology. This concept is also supported by the primordial role of the alternative pathway in the evolution of the complement system and the apparent lethality of factor B deficiency. The gene structures of most of these early components have now been elucidated providing the basis for detailed analyses of the defective alleles, the determination of carrier status, and prenatal diagnosis.