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

Gene Therapy for Immunoglobulin E, Complement-Mediated, and Eosinophilic Disorders

Immunoglobulin E, complement, and eosinophils play an important role in host defense, but dysfunction of each of these components can lead to a variety of human disorders. In this review, we summarize how investigators have adapted gene therapy and antisense technology to modulate immunoglobulin E, complement, and/or eosinophil levels to treat these disorders.

The complement system and human autoimmune diseases

Genetic deficiencies of early components of the classical complement activation pathway (especially C1q, r, s, and C4) are the strongest monogenic causal factors for the prototypic autoimmune disease systemic lupus erythematosus (SLE), but their prevalence is extremely rare. In contrast, isotype genetic deficiency of C4A and acquired deficiency of C1q by autoantibodies are frequent among patients with SLE. Here we review the genetic basis of complement deficiencies in autoimmune disease, discuss the complex genetic diversity seen in complement C4 and its association with autoimmune disease, provide guidance as to when clinicians should suspect and test for complement deficiencies, and outline the current understanding of the mechanisms relating complement deficiencies to autoimmunity. We focus primarily on SLE, as the role of complement in SLE is well-established, but will also discuss other informative diseases such as inflammatory arthritis and myositis.

Strong Association of Combined Genetic Deficiencies in the Classical Complement Pathway With Risk of Systemic Lupus Erythematosus and Primary Sjögren's Syndrome

OBJECTIVE

Complete genetic deficiency of the complement component C2 is a strong risk factor for monogenic systemic lupus erythematosus (SLE), but whether heterozygous C2 deficiency adds to the risk of SLE or primary Sjögren's syndrome (SS) has not been studied systematically. This study was undertaken to investigate potential associations of heterozygous C2 deficiency and C4 copy number variation with clinical manifestations in patients with SLE and patients with primary SS.

METHODS

The presence of the common 28-bp C2 deletion rs9332736 and C4 copy number variation was examined in Scandinavian patients who had received a diagnosis of SLE (n = 958) or primary SS (n = 911) and in 2,262 healthy controls through the use of DNA sequencing. The concentration of complement proteins in plasma and classical complement function were analyzed in a subgroup of SLE patients.

RESULTS

Heterozygous C2 deficiency-when present in combination with a low C4A copy number-substantially increased the risk of SLE (odds ratio [OR] 10.2 [95% confidence interval (95% CI) 3.5-37.0]) and the risk of primary SS (OR 13.0 [95% CI 4.5-48.4]) when compared to individuals with 2 C4A copies and normal C2. For patients heterozygous for rs9332736 with 1 C4A copy, the median age at diagnosis was 7 years earlier in patients with SLE and 12 years earlier in patients with primary SS when compared to patients with normal C2. Reduced C2 levels in plasma (P = 2 × 10-9 ) and impaired function of the classical complement pathway (P = 0.03) were detected in SLE patients with heterozygous C2 deficiency. Finally, in a primary SS patient homozygous for C2 deficiency, we observed low levels of anti-Scl-70, which suggests a risk of developing systemic sclerosis or potential overlap between primary SS and other systemic autoimmune diseases.

CONCLUSION

We demonstrate that a genetic pattern involving partial deficiencies of C2 and C4A in the classical complement pathway is a strong risk factor for SLE and for primary SS. Our results emphasize the central role of the complement system in the pathogenesis of both SLE and primary SS.

Innate Immunity: A Balance between Disease and Adaption to Stress

Since first being documented in ancient times, the relation of inflammation with injury and disease has evolved in complexity and causality. Early observations supported a cause (injury) and effect (inflammation) relationship, but the number of pathologies linked to chronic inflammation suggests that inflammation itself acts as a potent promoter of injury and disease. Additionally, results from studies over the last 25 years point to chronic inflammation and innate immune signaling as a critical link between stress (exogenous and endogenous) and adaptation. This brief review looks to highlight the role of the innate immune response in disease pathology, and recent findings indicating the innate immune response to chronic stresses as an influence in driving adaptation.

Systemic Lupus Erythematosus and Deficiencies of Early Components of the Complement Classical Pathway

The complement system plays an important role in the innate and acquired immune response against pathogens. It consists of more than 30 proteins found in soluble form or attached to cell membranes. Most complement proteins circulate in inactive forms and can be sequentially activated by the classical, alternative, or lectin pathways. Biological functions, such as opsonization, removal of apoptotic cells, adjuvant function, activation of B lymphocytes, degranulation of mast cells and basophils, and solubilization and clearance of immune complex and cell lysis, are dependent on complement activation. Although the activation of the complement system is important to avoid infections, it also can contribute to the inflammatory response triggered by immune complex deposition in tissues in autoimmune diseases. Paradoxically, the deficiency of early complement proteins from the classical pathway (CP) is strongly associated with development of systemic lupus erythematous (SLE) - mainly C1q deficiency (93%) and C4 deficiency (75%). The aim of this review is to focus on the deficiencies of early components of the CP (C1q, C1r, C1s, C4, and C2) proteins in SLE patients.

Early Components of the Complement Classical Activation Pathway in Human Systemic Autoimmune Diseases

The complement system consists of effector proteins, regulators, and receptors that participate in host defense against pathogens. Activation of the complement system, via the classical pathway (CP), has long been recognized in immune complex-mediated tissue injury, most notably systemic lupus erythematosus (SLE). Paradoxically, a complete deficiency of an early component of the CP, as evidenced by homozygous genetic deficiencies reported in human, are strongly associated with the risk of developing SLE or a lupus-like disease. Similarly, isotype deficiency attributable to a gene copy-number (GCN) variation and/or the presence of autoantibodies directed against a CP component or a regulatory protein that result in an acquired deficiency are relatively common in SLE patients. Applying accurate assay methodologies with rigorous data validations, low GCNs of total C4, and heterozygous and homozygous deficiencies of C4A have been shown as medium to large effect size risk factors, while high copy numbers of total C4 or C4A as prevalent protective factors, of European and East-Asian SLE. Here, we summarize the current knowledge related to genetic deficiency and insufficiency, and acquired protein deficiencies for C1q, C1r, C1s, C4A/C4B, and C2 in disease pathogenesis and prognosis of SLE, and, briefly, for other systemic autoimmune diseases. As the complement system is increasingly found to be associated with autoimmune diseases and immune-mediated diseases, it has become an attractive therapeutic target. We highlight the recent developments and offer a balanced perspective concerning future investigations and therapeutic applications with a focus on early components of the CP in human systemic autoimmune diseases.

Classical pathway deficiencies - A short analytical review

Deficiencies in the classical pathway of complement activation have some common features but show also great differences. Deficiencies of each of the components (C1q, C1s, C1r, C4 and C2) imply increased susceptibility to bacterial infections. They are also associated with increased risk to develop systemic lupus erythematosus where deficiency of C1q is strongly associated to the disease while C4 less and C2 much less. Deficiency of C1q affects only activation of the classical pathway while deficiency of C4 and C2 also prevent activation of the lectin pathway. Bypass mechanisms may result in complement activation also in absence of C2 but not in absence of C1q or C4. The genes for C2 and C4 isotypes are closely located within the MHC class III region on chromosome 6p and the genes for the 3 C1q chains are on chromosome 1p. Deficiencies of C1q and of C4 show genetic heterogeneity while deficiency of C2 in the great majority of cases is caused by a specific deletion. The production of C4 and C2 is mainly by the hepatocytes in the liver while C1q is produced by monocytic bone marrow derived cells. This has implications for the possibility to treat the deficiency and hematopoietic stem cell transplantation has been tried in C1q deficiency.

C2 rs547154 polymorphism and polypoidal choroidal vasculopathy susceptibility: a meta-analysis

Previous studies have indicated the association between C2 rs547154 polymorphism and polypoidal choroidal vasculopathy (PCV) risk, while the results are controversial and inconsistent. Herein, we perform a meta-analysis to gain a precise estimation of the association using 5 eligible studies involving 4076 subjects, of which 1220 were PCV cases, 1073 were age-related macular degeneration (AMD) cases and 1783 were controls. Allelic frequencies of C2 rs547154 polymorphism between PCV and AMD were also compared. Both crude and adjusted odds ratios (OR) with their 95% confidence interval (CI) were included to assess the strength of the association. The pooled OR in random-effect model for allele T versus G was 0.64 (95% CI, 0.52–0.80; p < 0.0001), for genotype TG versus GG was 0.65 (95% CI, 0.52–0.83; p, 0.0004), and for genotype TT + TG versus GG was 0.64 (95% CI, 0.51–0.80; p, 0.0002). No difference in allelic frequency was observed between PCV and AMD (OR, 0.86; 95% CI, 0.64–1.16; p, 0.32). Sensitivity analysis proved the robustness of our data. No significant ethnic divergence was suggested by subgroup analysis, and no publication bias was detected via Egger's test. In conclusion, our data indicate that C2 rs547154 polymorphism plays a protective role in the development of PCV.

Molecular characterization of the complement C1q, C2 and C4 genes in Brazilian patients with juvenile systemic lupus erythematosus

OBJECTIVE

To perform a molecular characterization of the C1q, C2 and C4 genes in patients with juvenile systemic lupus erythematosus.

METHODS

Patient 1 (P1) had undetectable C1q, patient 2 (P2) and patient 3 (P3) had decreased C2 and patient 4 (P4) had decreased C4 levels. All exons and non-coding regions of the C1q and C2 genes were sequenced. Mononuclear cells were cultured and stimulated with interferon gamma to evaluate C1q, C2 and C4 mRNA expression by quantitative real-time polymerase chain reaction.

RESULTS

C1q sequencing revealed heterozygous silent mutations in the A (c.276 A>G Gly) and C (c.126 C>T Pro) chains, as well as a homozygous single-base change in the 3' non-coding region of the B chain (c*78 A>G). C1qA mRNA expression without interferon was decreased compared with that of healthy controls (p<0.05) and was decreased after stimulation compared with that of non-treated cells. C1qB mRNA expression was decreased compared with that of controls and did not change with stimulation. C1qC mRNA expression was increased compared with that of controls and was even higher after stimulation. P2 and P3 had Type I C2 deficiency (heterozygous 28 bp deletion at exon 6). The C2 mRNA expression in P3 was 23 times lower compared with that of controls and did not change after stimulation. The C4B mRNA expression of P4 was decreased compared with that of controls and increased after stimulation.

CONCLUSIONS

Silent mutations and single-base changes in the 3' non-coding regions may modify mRNA transcription and C1q production. Type I C2 deficiency should be evaluated in JSLE patients with decreased C2 serum levels. Further studies are needed to clarify the role of decreased C4B mRNA expression in JSLE pathogenesis.

Complement and antibody primary immunodeficiency in juvenile systemic lupus erythematosus patients

OBJECTIVE

To evaluate the frequency of primary immunodeficiencies (PID) in juvenile systemic lupus erythematosus (JSLE) patients.

METHODS

Some 72 JSLE patients were analyzed for levels of immunoglobulin classes and IgG subclasses and early components of the classical complement pathway. Determination of C4 gene copy number (GCN) and detection of type I C2 deficiency (D) were also performed.

RESULTS

PID was identified in 16 patients (22%): C2D in three, C4D in three, C1qD in two, IgG2D (<20 mg/dl) in four, IgAD (<7 mg/dl) in three, and IgMD (<35 mg/dl) in three; one of these patients presented IgA, C2 and C4D. Two patients had low C4 GCN and two had type I C2D. Demographic data, family history of autoimmune disease and PID, JSLE clinical findings, occurrence of infections, disease activity and therapies were similar in patients with and without PID (p > 0.05). Remarkably, the median of Systemic Lupus International Collaborating Clinics/ACR-damage index (SLICC/ACR-DI) was significantly higher in JSLE patients with PID compared with patients without these abnormalities (p = 0.0033), likewise the high frequency of SLICC/ACR-DI > 1 (p = 0.023).

CONCLUSIONS

A high frequency of PID was observed in JSLE patients, suggesting that these defects may contribute to lupus development. Our findings indicate that these two groups of PID should be investigated in severe pediatric lupus.

Recombinant human complement component C2 produced in a human cell line restores the classical complement pathway activity in-vitro: an alternative treatment for C2 deficiency diseases

Background

Complement C2 deficiency is the most common genetically determined complete complement deficiency and is associated with a number of diseases. Most prominent are the associations with recurrent serious infections in young children and the development of systemic lupus erythematosus (SLE) in adults. The links with these diseases reflect the important role complement C2 plays in both innate immunity and immune tolerance. Infusions with normal fresh frozen plasma for the treatment of associated disease have demonstrated therapeutic effects but so far protein replacement therapy has not been evaluated.

Results

Human complement C2 was cloned and expressed in a mammalian cell line. The purity of recombinant human C2 (rhC2) was greater than 95% and it was characterized for stability and activity. It was sensitive to C1s cleavage and restored classical complement pathway activity in C2-deficient serum both in a complement activation ELISA and a hemolytic assay. Furthermore, rhC2 could increase C3 fragment deposition on the human pathogen Streptococcus pneumoniae in C2-deficient serum to levels equal to those with normal serum.

Conclusions

Taken together these data suggest that recombinant human C2 can restore classical complement pathway activity and may serve as a potential therapeutic for recurring bacterial infections or SLE in C2-deficient patients.

Clinical significance of complement deficiencies

The complement system is composed of more than 30 serum and membrane-bound proteins, all of which are needed for normal function of complement in innate and adaptive immunity. Historically, deficiencies within the complement system have been suspected when young children have had recurrent and difficult-to-control infections. As our understanding of the complement system has increased, many other diseases have been attributed to deficiencies within the complement system. Generally, complement deficiencies within the classical pathway lead to increased susceptibility to encapsulated bacterial infections as well as a syndrome resembling systemic lupus erythematosus. Complement deficiencies within the mannose-binding lectin pathway generally lead to increased bacterial infections, and deficiencies within the alternative pathway usually lead to an increased frequency of Neisseria infections. However, factor H deficiency can lead to membranoproliferative glomerulonephritis and hemolytic uremic syndrome. Finally, deficiencies within the terminal complement pathway lead to an increased incidence of Neisseria infections. Two other notable complement-associated deficiencies are complement receptor 3 and 4 deficiency, which result from a deficiency of CD18, a disease known as leukocyte adhesion deficiency type 1, and CD59 deficiency, which causes paroxysmal nocturnal hemoglobinuria. Most inherited deficiencies of the complement system are autosomal recessive, but properidin deficiency is X-linked recessive, deficiency of C1 inhibitor is autosomal dominant, and mannose-binding lectin and factor I deficiencies are autosomal co-dominant. The diversity of clinical manifestations of complement deficiencies reflects the complexity of the complement system.

The genetic and molecular bases of monogenic disorders affecting proteolytic systems

Complete and limited proteolysis represents key events that regulate many biological processes. At least 5% of the human genome codes for components of proteolytic processes if proteases, inhibitors, and cofactors are taken into account. Accordingly, disruption of proteolysis is involved in numerous pathological conditions. In particular, molecular genetic studies have identified a growing number of monogenic disorders caused by mutations in protease coding genes, highlighting the importance of this class of enzymes in development, organogenesis, immunity, and brain function. This review provides insights into the current knowledge about the molecular genetic causes of these disorders. It should be noted that most are due to loss of function mutations, indicating absolute requirement of proteolytic activities for normal cellular functions. Recent progress in understanding the function of the implicated proteins and the disease pathogenesis is detailed. In addition to providing important clues to the diagnosis, treatment, and pathophysiology of disease, functional characterisation of mutations in proteolytic systems emphasises the pleiotropic functions of proteases in the body homeostasis.

Frequency in Spanish population of familial complement factor 2 type I deficits and associated HLA haplotypes

We present two familial cases of complement factor 2 (C2) type I deficiency. Probands had experienced severe pyogenic bacteria infections in childhood and had undetectable levels of C2 and very low level of CH50. Both children were homozygous for the deletion of 28 bp in exon 6 of the C2 gene. Human leukocyte antigen (HLA) typing in family 1 had the commonly reported associations, but family 2 demonstrated a new association of the mutated C2 gene to HLA-A*3101, -Cw*0602, -B*1801, and -DRB1*0901. In addition, for the first time, the frequency of the 28-bp deletion of C2 and its HLA haplotypic associations have been analyzed in a sample of the Spanish population containing 790 haplotypes and 105 phenotypes. Cw*0602 is frequently found in Spanish haplotypes linked to the C2 mutated gene instead of the commonly reported -Cw*1203. The presence of heterozygous or homozygous individuals for the C2 deletion with low levels of IgD in both families supports the existence of a putative dominant susceptibility gene for IgD deficiency in haplotype HLA-B18, -S042, and -DR2. The frequency of the C2 28-bp deletion in heterozygosis is 1.4% (gene frequency 0.007) in Spanish healthy controls, similar to that reported in other white populations.

Immunoglobulin deficiencies and susceptibility to infection among homozygotes and heterozygotes for C2 deficiency

About 25% of C2-deficient homozygotes have increased susceptibility to severe bacterial infections. C2-deficient homozygotes had significantly lower serum levels of IgG2, IgG4, IgD, and Factor B, significantly higher levels of IgA and IgG3 and levels of IgG1 and IgM similar to controls. Type 1 (28 bp deletion in C2 exon 6 on the [HLA-B18, S042, DR2] haplotype or its fragments) and type II (non-type I) C2-deficient patients with increased susceptibility to bacterial infection had significantly lower mean levels of IgG4 (p < 0.04) and IgA (p < 0.01) than those without infections (who had a higher than normal mean IgA level) but similar mean levels of other immunoglobulins and Factor B. Of 13 C2-deficient homozygotes with infections, 85% had IgG4 deficiency, compared with 64% of 25 without infections. IgD deficiency was equally extraordinarily common among infection-prone (50%) and noninfection-prone (70%) homozygous type I C2-deficient patients. IgD deficiency was also common (35%) among 31 type I C2-deficient heterozygotes (with normal or type II haplotypes), but was not found in 5 type II C2-deficient heterozygotes or 1 homozygote. Thus, C2 deficiency itself is associated with many abnormalities in serum immunoglobulin levels, some of which, such as in IgG4 and IgA, may contribute to increased susceptibility to infection. In contrast, IgD deficiency appears not to contribute to increased infections and appears to be a dominant trait determined by a gene or genes on the extended major histocompatibility complex (MHC) haplotype [HLA-B 18, S042, DR2] (but probably not on type II C2-deficient haplotypes) similar to those previously identified on [HLA-B8, SC01, DR3] and [HLA-B18, F1C30, DR3].

Early manifestation and recognition of C2 complement deficiency in the form of pyogenic infection in infancy

OBJECTIVE

Although frequently asymptomatic, C2 complement component deficiency may lead to severe pyogenic infections or lupus-like illness. In the present report, we describe infectious manifestations in infancy and childhood in our C2-deficient patients.

METHOD

A retrospective study of clinical manifestation in three patients was carried out. C2 deficiency was proved both by undetectable serum C2 level and typical homozygous 28 bp deletion of the C2 gene.

RESULTS

All patients were hospitalized at least once by the age of 12 months, each had one episode of meningitis in infancy, one also had arthritis with septicaemia. Infections of the respiratory tract were the causes of other hospitalizations. Two patients also suffered from frequent mild respiratory tract infections; in both patients, decreased immunoglobulin IgA and immunoglobulin IgG2 or immunoglobulin IgG3 levels were recorded.

CONCLUSION

Our observations point to an early manifestation of C2 deficiency within the first year of life, with meningitis as the most severe complication. The severity of immunodeficiency may be influenced by concomitant deficiencies of immunoglobulin isotypes.

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.

Molecular genetics of the human MHC complement gene cluster

The human major histocompatibility complex (MHC) complement gene cluster (MCGC) is a highly variable region that is characterized by polymorphisms, variations in gene size and gene number, and associations with diseases. Deficiencies in complement C2 are either due to abolition of C2 protein synthesis by mini-deletions that caused frameshift mutations, or blocked secretion of the C2 protein by single amino acid substitutions. One, two or three C4 genes may be present in a human MCGC haplotype and these genes may code for C4A, C4B, or both. Deficiencies of C4A or C4B proteins are attributed to the expression of identical C4 isotypes or allotypes from the C4 loci, the absence or deletion of a C4 gene, 2-bp insertion at exon 29 or 1-bp deletion at exon 20 that caused frameshift mutations. The C4 genes are either 21 or 14.6 kb in size due to the presence of endogenous retrovirus HERV-K(C4) in the intron 9 of long C4 genes. A deletion or duplication of a C4 gene is always accompanied by its neighboring genes, RP at the 5' region, and CYP21 and TNX at the 3' region. These four genes form a genetic unit termed the RCCX module. In an RCCX bimodular structure, the pseudogene CYP21A, and partially duplicated gene segments TNXA and RP2 are present between the two C4 loci. The RCCX modular variations in gene number and gene size contributed to unequal crossovers and exchanges of polymorphic sequences/mutations, resulting in the homogenization of C4 polymorphisms and acquisitions of deleterious mutations in RP1, C4A, C4B, CYP21B and TNXB genes. RD, SKI2W, DOM3Z and RP1 are the four novel genes found between Bf and C4. RD and Ski2w proteins may be related to RNA splicing, RNA turnover and regulation of translation. The functions of Dom3z and RP1 are being investigated. The complete genomic DNA sequence between C2 and TNX is now available. This should facilitate a complete documentation of polymorphisms, mutations and disease associations for the MCGC.

Liver injury in alpha 1-antitrypsin deficiency

Alpha 1-antitrypsin deficiency is the most common genetic cause of liver disease in children. It is also associated with chronic liver disease, hepatocellular carcinoma, and pulmonary emphysema in adults. Liver injury is caused by hepatotoxic effects of retention of the mutant alpha 1-antitrypsin molecule within the endoplasmic reticulum of liver cells, and emphysema is caused by uninhibited proteolytic damage to elastic tissue in the lung parenchyma. Recent studies of the biochemistry and cell biology of the mutant alpha 1-antitrypsin molecule have led to advances in understanding susceptibility to liver injury and in developing new strategies for prevention of both liver and lung disease.

New structural motifs on the chymotrypsin fold and their potential roles in complement factor B

Factor B and C2 are two central enzymes for complement activation. They are multidomain serine proteases and require cofactor binding for full expression of proteolytic activities. We present a 2.1 A crystal structure of the serine protease domain of factor B. It shows a number of structural motifs novel to the chymotrypsin fold, which by sequence homology are probably present in C2 as well. These motifs distribute characteristically on the protein surface. Six loops surround the active site, four of which shape substrate-binding pockets. Three loops next to the oxyanion hole, which typically mediate zymogen activation, are much shorter or absent. Three insertions including the linker to the preceding domain bulge from the side opposite to the active site. The catalytic triad and non-specific substrate-binding site display active conformations, but the oxyanion hole displays a zymogen-like conformation. The bottom of the S1 pocket has a negative charge at residue 226 instead of the typical 189 position. These unique structural features may play different roles in domain-domain interaction, cofactor binding and substrate binding.

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.

Disruption of disulfide bonds is responsible for impaired secretion in human complement factor H deficiency

Factor H, a secretory glycoprotein composed of 20 short consensus repeat modules, is an inhibitor of the complement system. Previous studies of inherited factor H deficiency revealed single amino acid substitutions at conserved cysteine residues, on one allele arginine for cysteine 518 (C518R) and on the other tyrosine for cysteine 941 (C941Y) (Ault, B. H., Schmidt, B. Z., Fowler, N. L., Kashtan, C. E., Ahmed, A. E., Vogt, B. A., and Colten, H. R. (1997) J. Biol. Chem. 272, 25168-25175). To ascertain if the phenotype, impaired secretion of factor H, is due to the C518R substitution or the C941Y substitution and to ascertain the mechanism by which secretion is impaired, we studied COS-1 and HepG2 cells transfected with wild type and several mutant factor H molecules. The results showed markedly impaired secretion of both C518R and C941Y factor H as well as that of factor H molecules bearing alanine or arginine substitutions at the Cys518-Cys546 disulfide bond (C518A, C546A, C546R, C518A-C546A). In each case, mutant factor H was retained in the endoplasmic reticulum and degraded relatively slowly as compared with most other mutant secretory and membrane proteins that are retained in the endoplasmic reticulum. These data indicate that impaired secretion of the naturally occurring C518R and C941Y mutant factor H proteins is due to disruption of framework-specific disulfide bonds in factor H short consensus repeat modules.

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.

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.