Molecular genetics of the human MHC complement gene cluster

Congenital Adrenal Hyperplasia - A Comprehensive Review of Genetic Studies on 21-Hydroxylase Deficiency from India

Congenital adrenal hyperplasia (CAH) comprises a heterogeneous group of autosomal recessive disorders impairing adrenal steroidogenesis. Most cases are caused by mutations in the CYP21A2 gene resulting in 21-hydroxylase (21-OH) deficiency (21-OHD). The genetics of 21-OH CAH is complexed by a highly homologous pseudogene CYP21A1P imposing several limitations in the molecular analysis. Therefore, genetic testing is still not a part of routine CAH diagnosis and is mainly dependent on 17-hydroxy progesterone (OHP) measurements. There are very few reports of CYP21A2 gene analysis from India and there is no comprehensive review available on genetic testing and the spectrum of CYP21A2 mutations from the country. This review focuses on the molecular aspects of 21-OHD and the genetic studies on CYP21A2 gene reported from India. The results of these studies insist the compelling need for large-scale CYP21A2 genetic testing and newborn screening (NBS) in India. With a high disease prevalence and consanguinity rates, robust and cost-effective genetic testing for 21-OH CAH would enable an accurate diagnosis in routine clinical practice. Whereas establishing affordable genotyping assays even in secondary care or resource-poor settings of the country can identify 90% of the mutations that are pseudogene derived, initiatives on reference laboratories for CAH across the nation with comprehensive genetic testing facilities will be beneficial in those requiring extended analysis of CYP21A2 gene. Further to this, incorporating genetic testing in NBS and carrier screening programmes will enable early diagnosis, better risk assessment and community-based management.

The Path to Conserved Extended Haplotypes: Megabase-Length Haplotypes at High Population Frequency

This minireview describes the history of the conceptual development of conserved extended haplotypes (CEHs): megabase-length haplotypes that exist at high (≥0.5%) population frequency. My career began in internal medicine, shifted to pediatrics, and clinical practice changed to research. My research interest was initially in hematology: on plasma proteins, their metabolism, synthesis, and function. This narrowed to a focus on proteins of the human complement system, their role in immunity and their genetics, beginning with polymorphism and deficiency of C3. My group identified genetic polymorphisms and/or inherited deficiencies of C2, C4, C6, and C8. After defining glycine-rich beta glycoprotein as factor B (Bf) in the properdin system, we found that the genes for Bf (CFB), C2, C4A, and C4B were inherited as a single haplotypic unit which we named the "complotype." Complotypes are located within the major histocompatibility complex (MHC) between HLA-B and HLA-DRB1 and are designated (in arbitrary order) by their CFB, C2, C4A, and C4B types. Pedigree analysis revealed long stretches (several megabases) of apparently fixed DNA within the MHC that we referred to as "extended haplotypes" (later as "CEHs"). About 10 to 12 common CEHs constitute at least 25 - 30% of MHC haplotypes among European Caucasian populations. These CEHs contain virtually all the most common markers of MHC-associated diseases. In the case of type 1 diabetes, we have proposed a purely genetic and epigenetic model (with a small number of Mendelian recessive disease genes) that explains all the puzzling features of the disease, including its rising incidence.

Complement C4, Infections, and Autoimmune Diseases

Complement C4, a key molecule in the complement system that is one of chief constituents of innate immunity for immediate recognition and elimination of invading microbes, plays an essential role for the functions of both classical (CP) and lectin (LP) complement pathways. Complement C4 is the most polymorphic protein in complement system. A plethora of research data demonstrated that individuals with C4 deficiency are prone to microbial infections and autoimmune disorders. In this review, we will discuss the diversity of complement C4 proteins and its genetic structures. In addition, the current development of the regulation of complement C4 activation and its activation derivatives will be reviewed. Moreover, the review will provide the updates on the molecule interactions of complement C4 under the circumstances of bacterial and viral infections, as well as autoimmune diseases. Lastly, more evidence will be presented to support the paradigm that links microbial infections and autoimmune disorders under the condition of the deficiency of complement C4. We provide such an updated overview that would shed light on current research of complement C4. The newly identified targets of molecular interaction will not only lead to novel hypotheses on the study of complement C4 but also assist to propose new strategies for targeting microbial infections, as well as autoimmune disorders.

Revisiting the complement system in systemic lupus erythematosus

Introduction: Systemic lupus erythematosus (SLE) is a multi-system autoimmune disease, characterized by the production of autoantibodies. Numerous mechanisms contribute to the pathogenesis and autoimmunity in SLE. One of the most important mechanisms is the defective function of the early complement components that are involved in clearing the immune-complexes and apoptotic debris. Major evidence supporting this hypothesis is the development of severe lupus in individuals with monogenic defects in any one of the early complement components such as C1q, C1 s, C1 r, C2, or C4.Areas covered: In this review, we discuss hereditary defects in classical complement components and their clinical manifestations, acquired defects of complements in lupus, the role of complements in the pathogenesis of antiphospholipid antibody syndrome and lupus nephritis, and laboratory assessment of complement components and their functions. Articles from the last 20 years were retrieved from PubMed for this purpose.Expert opinion: Complements have a dual role in the pathogenesis of SLE. On one hand, deficiency of complement components predisposes to lupus, while, on the other, excess complement activation plays a role in the organ damage. Understanding the intricacies of the role of complements in SLE can pave way for the development of targeted therapies.

Clinical and Molecular Characterization of Classical-Like Ehlers-Danlos Syndrome Due to a Novel TNXB Variant

The Ehlers-Danlos syndromes (EDS) constitute a clinically and genetically heterogeneous group of connective tissue disorders. Tenascin X (TNX) deficiency is a rare type of EDS, defined as classical-like EDS (clEDS), since it phenotypically resembles the classical form of EDS, though lacking atrophic scarring. Although most patients display a well-defined phenotype, the diagnosis of TNX-deficiency is often delayed or overlooked. Here, we described an additional patient with clEDS due to a homozygous null-mutation in the TNXB gene. A review of the literature was performed, summarizing the most important and distinctive clinical signs of this disorder. Characterization of the cellular phenotype demonstrated a distinct organization of the extracellular matrix (ECM), whereby clEDS distinguishes itself from most other EDS subtypes by normal deposition of fibronectin in the ECM and a normal organization of the α5β1 integrin.

Does matching for SNPs in the MHC gamma block in 10/10 HLA-matched unrelated donor-recipient pairs undergoing allogeneic stem cell transplant improve outcomes?

BACKGROUND

Matching at the HLA-A, HLA-B, HLA-C, HLA-DRB1, and HLA-DQB1 loci is important in donor selection for patients undergoing unrelated allogeneic hematopoietic stem cell transplantation (ASCT). Additional matching across the MHC gamma region may further improve outcomes.

METHODS

The MHC gamma region was retrospectively genotyped in 66 adult recipients of ASCT and their 10/10 matched unrelated donors. A chart review was performed to determine whether MHC gamma matching impacted survival, relapse, or graft-versus-host disease.

RESULTS

Of 66 donor-recipient pairs, 26(39.4%) were gamma-type matches, 34(51.5%) were mismatches, and 6(9.1%) were "indeterminate." Matching status was not associated with overall survival (p = 0.43), relapse (p = 0.21), acute GVHD (p = 0.43), severe aGVHD (p = 0.31), or chronic GVHD (p = 0.23) in univariate analyses, nor in multivariate analyses (p = 0.28, 0.13, 0.29, 0.16, and 0.67, respectively), with or without adjusting for HLA-DPB1 matching status.

CONCLUSIONS

In our single institution study, gamma-type matching status was not associated with outcomes of adult ASCT recipients.

Genomic and genetic studies of systemic sclerosis: A systematic review

Systemic sclerosis is an autoimmune rheumatic disease characterised by fibrosis, vasculopathy and inflammation. The exact aetiology of SSc remains unknown but evidences show that various genetic factors may be involved. This review aimed to assess HLA alleles/non-HLA polymorphisms, microsatellites and chromosomal abnormalities that have thus far been associated with SSc. PubMed, Embase and Scopus databases were searched up to July 29, 2015 using a combination of search-terms. Articles retrieved were evaluated based on set exclusion and inclusion criteria. A total of 150 publications passed the filters. HLA and non-HLA studies showed that particular alleles in the HLA-DRB1, HLA-DQB1, HLA-DQA1, HLA-DPB1 genes and variants in STAT4, IRF5 and CD247 are frequently associated with SSc. Non-HLA genes analysis was performed using the PANTHER and STRING₁₀ databases. PANTHER classification revealed that inflammation mediated by chemokine and cytokine, interleukin and integrin signalling pathways are among the common extracted pathways associated with SSc. STRING₁₀ analysis showed that NFKB1, CSF3R, STAT4, IFNG, PRL and ILs are the main "hubs" of interaction network of the non-HLA genes associated with SSc. This study gathers data of valid genetic factors associated with SSc and discusses the possible interactions of implicated molecules.

HLA and Hypocomplementemia: The Disadvantage of Carrying the HLA-B35 and the Silent Alleles of the C4 Complement Component

Hypocomplementemia is an extremely complex phenomenon: we devoted our attention to its immunogenetic basis, particularly to the HLA haplotypes involved and to the study of C4 polymorphic genes. With this in mind we analyzed a group of unrelated patients with hypocomplementemia and 15 families suffering from specific C4 deficiency. Firstly, we performed a population analysis in order to identify a statistically significant association: HLA-B35 and C4BQ0 alleles, in the total group of hypocomplementemic individuals, seem to be associated with the primary disease.

Secondly, we defined HLA haplotypes clear-cut segregation in the hypocomplementemic families and we identified the most common HLA haplotypes carrying B35 and C4 null allele associated with this condition. With the aid of correspondence analysis and the Transmission Disequilibrium Test (TDT), we measured the strength of this association. In this work, mainly through family analysis, we envisaged a potentially interesting genomic trait, within HLA, close to B locus, that seems to be involved in hypocomplementemia itself and perhaps in hypocomplementemia-related disorders.

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.

Immune responses following meningococcal serogroups A, C, Y and W polysaccharide vaccination in C2-deficient persons: evidence for increased levels of serum bactericidal antibodies

Complement C2 deficiency (C2D) is associated with immunological diseases and increased susceptibility to invasive infections caused by encapsulated bacteria such as Neisseria menigitidis. In this study we evaluate the immunogenicity of vaccination against N. menigitidis in C2D. C2D patients (n=22) and controls (n=52) were given a tetravalent meningococcal polysaccharide vaccine. Serum bactericidal antibody (SBA) titres (serogroups A, C, Y and W) were analysed using a rabbit complement source. Levels of IgG, IgM, and IgA, factor B, and factor H, polymorphisms of MBL and Fc-gamma receptors were determined. The C2D patients responded with an increased SBA titre to all four serogroups (p<0.001). The response rates define as SBA titres ≥8 were found to be between 85.7% and 92.5%. The post-vaccination titres for serogroups C, Y and W were equal to healthy controls. C2D patients with a history of invasive infection had a lower post-vaccination SBA titres both compared to healthy C2D persons (p=0.03) and compared to controls (p<0.0001). We found that the G2M*n/G2M*n genotype were associated with a higher SBA titres after immunization (p=0.03). None of the other investigated immunological factors appear to be important in influencing the vaccine responses. Autoimmune diseases in C2D did not affect the vaccine response. In general, vaccination against meningococci gave rise to antibody responses in the C2D patients that equal healthy controls. The response rate was lower to serogroup A and among C2D patients with history of invasive infections. The presence of G2M*n/G2M*n genotype was associated with higher SBA titres after immunization.

Association of MHC class-III gene polymorphisms with ER-positive breast cancer in Chinese Han population

Polymorphisms of the major histocompatibility complex (MHC) have been linked to many diseases, especially autoimmune disorders. Previous studies have shown that genetic variants in MHC class III are associated with breast cancer. To determine if there is an association between MHC class III and breast cancer risk in the Chinese Han population, we carried out a hospital-based case-control study in Guangdong and Jiangsu Provinces, including 216 histologically confirmed breast cancer patients and 216 healthy controls. Nine SNP markers distributed in the class III-coding region were detected using the Sequenom MassARRAY(®) iPLEX System. Deviation from Hardy-Weinberg equilibrium was observed for seven SNPs. There was no significant association between these seven SNP variants and breast cancer in these Chinese women (unconditional logistic regression analysis). However, chr6_31697494 at BAT2, one of the seven SNPs, was found to be significantly associated with both ER- and PR-positive breast cancer. In addition, both chr6_31911109 at C6orf48 and chr6_31975605 at ZBTB12, another two of the seven SNPs, show relevance with ER-positive breast cancer. In conclusion, this is the first evidence that genetic polymorphisms in the MHC class III region are significantly associated with ER-positive breast cancer in the Han Chinese population.

Complement deficiency states and associated infections

A major function of the immune system is to protect the host from microbial infections. The complement system plays important roles in both the innate and the adaptive immune defense and also acts as a bridge between these arms of immunity. This is obvious from complement deficiencies which in varying degree, depending on which factor is missing, are associated with increased infection susceptibility and also increased risk for other, mainly autoimmune diseases. Genetically determined deficiencies are described for almost all complement proteins but the consequences show a wide variation. Here the genetic defects and molecular abnormalities in complement deficient persons, related clinically relevant infections and the options for prevention and therapy are reviewed. The roles of complement in host defense against common infections are also discussed.

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.

Complement 4 phenotypes and genotypes in Brazilian patients with classical 21-hydroxylase deficiency

The aim of this work was to analyse C4 genotypes, C4 protein levels, phenotypes and genotypes in patients with the classical form of 21-hydroxylase deficiency. Fifty-four patients from 46 families (36 female, 18 male; mean age 10.8 years) with different clinical manifestations (31 salt-wasting; 23 simple-virilizing) were studied. Taq I Southern blotting was used to perform molecular analysis of the C4/CYP21 gene cluster and the genotypes were defined according to gene organization within RCCX modules. Serum C4 isotypes were assayed by enzyme-linked immunosorbent assay. The results revealed 12 different haplotypes of the C4/CYP21 gene cluster. Total functional activity of the classical pathway (CH50) was reduced in individuals carrying different genotypes because of low C4 concentrations (43% of all patients) to complete or partial C4 allotype deficiency. Thirteen of 54 patients presented recurrent infections affecting the respiratory and/or the urinary tracts, none of them with severe infections. Low C4A or C4B correlated well with RCCX mono-modular gene organization, but no association between C4 haplotypes and recurrent infections or autoimmunity was observed. Considering this redundant gene cluster, C4 seems to be a well-protected gene segment along the evolutionary process.

An investigation of the C4 gene arrangement in ethnic Chinese (Taiwanese)

C4 complement components are encoded by two genes, C4A and C4B , located on chromosome 6p21.3 of the major histocompatibility complex class III region. The isotypic residues at position 1101-1106 of the C4A gene contain the Pro-Cys-Pro-Val-Leu-Asp sequence which has a higher affinity for binding amino group-containing antigens, while C4B contains the Leu-Ser-Pro-Val-Ileu-His sequence which has a higher affinity for hydroxyl group-containing antigens. These two genes show different reaction rates which infer solubilization of antibody-antigen aggregates and propagation of the activation pathway to form the membrane attack complex. Using a polymerase chain reaction-based amplification method to identify and differentiate the locations of the C4A and C4B genes adjacent to the respective CYP21A2P and CYP21A2 genes, the isotypic residues at position 1101-1106 for the C4 isotype were categorized into five haplotypes of C4 gene arrangements. Among them, we found that 65% of the gene proportions between C4A and C4B were balanced, while 35% of them were unbalanced in this ethnic Chinese (i.e. Taiwanese) cohort. We consider that the unbalanced arrangements of the C4 locus in the individuals might have influenced the clearance of apoptotic debris and immune complexes which may injure tissue by initiating autoimmune diseases and immunity responses associated with susceptibility to viral and bacterial infections.

A study of association of the complement C4 mutations with systemic lupus erythematosus in the Malaysian population

The aim of the present study was to investigate the association of C4 gene mutations with systemic lupus erythematosus, in 130 Malaysian SLE patients and 130 healthy controls. Generally, various PCR approaches were used to screen the mutations of the C4 genes, which included 2 bp (+TC) insertions at codon 1213 in exon 29, 1 bp deletions (-C) at codon 811 in exon 20, 1 bp (-C), 2 bp (-GT) deletions at codons 522 and 497 in exon 13 and null alleles. No mutations located at exons 13, 20 and 29 of the C4 gene, were detected amongst the patient and control samples in this study. C4A*Q0 was found in two out of the 130 control samples, while C4B*Q0 was present in two out of the 130 SLE patients. Overall, our results do not demonstrate a significant association to these known C4 mutations identified by previous studies, in the Malaysian scenario.

Defining targets for complement components C4b and C3b on the pathogenic neisseriae

Complement is a key arm of the innate immune defenses against the pathogenic neisseriae. We previously identified lipooligosaccharide on Neisseria meningitidis as an acceptor for complement C4b. Little is known about other neisserial targets for complement proteins C3 and C4, which covalently attach to bacterial surfaces and initiate opsonization and killing. In this study we demonstrate that Neisseria gonorrhoeae porin (Por) 1B selectively binds C4b via amide linkages and C3b via ester linkages. Using strains expressing hybrid Por1A/1B molecules, a region spanned by loops 4 and 5 of Por1B was identified as the preferred binding site for C4b. We also identified the opacity protein (Opa), a major adhesin of pathogenic neisseriae, as a target for C4b and C3b on both N. meningitidis and N. gonorrhoeae. Using N. gonorrhoeae variants that predominantly expressed individual Opa proteins, we found that all Opa proteins tested (A, B, C, D, E, F, and I) bound C4b and C3b via amide and ester linkages, respectively. Amide linkages with Por1B and Opa were confirmed using serum containing only the C4A isoform, which exclusively forms amide linkages with targets. While monomers and heterodimers of C4Ab were detected on bacterial targets, C4Bb appeared to preferentially participate in heterodimer (C5 convertase) formation. Our data provide another explanation for the enhanced serum sensitivity of Por1B-bearing gonococci. The binding of C3b and C4b to Opa provides a rationale for the recovery of predominantly "transparent" (Opa-negative) neisserial isolates from persons with invasive disease, where the bacteria encounter high levels of complement.

Rheumatological manifestations, organ damage and autoimmunity in hereditary C2 deficiency

OBJECTIVE

To analyse rheumatological manifestations, organ damage and autoimmune responses in a large cohort of patients (n = 45) with homozygous C2 deficiency (C2D) and long-term follow-up.

METHODS

Medical records were reviewed and were supplemented with a mailed questionnaire for assessment of cardiovascular disease (CVD) risk factors. Organ damage was evaluated using the Systemic Lupus International Collaborative Clinics/American College of Rheumatology Damage Index (SLICC/ACR DI). Causes for disability pensions were investigated. Autoantibodies were determined with established methods.

RESULTS

Patients with rheumatological diseases had systemic lupus erythematosus (SLE, n = 12), undifferentiated connective tissue disease (n = 5) or vasculitis (n = 3). Judging from annual SLICC/ACR DI, C2D patients with SLE run a similar risk of development of severe disease as other patients with SLE. An increased rate of CVD was observed not explained by Framingham-related risk factors. Disability pensions were mainly related to rheumatological disease. The prevalence of anti-nuclear antibodies in C2D with SLE and of anti-SS-A was 25% while anti-RNP was found in 45%. Only one patient showed antibodies to dsDNA. Formation of anti-cardiolipin antibodies (aCL) appeared to be increased in C2D despite the absence of an anti-phospholipid syndrome. The prevalence of antibodies to the collagen-like region of C1q (C1qCLR) was also remarkably high and was not related to rheumatological manifestations.

CONCLUSIONS

Severity of SLE in C2D is similar to that of SLE in other patients. Conventional risk factors do not explain the occurrence of CVD in C2D. The high prevalence of aCL and anti-C1qCLR indicates mechanisms through which impaired complement function promotes formation of autoantibodies.

Relationship between copy number of genes (C4A, C4B) encoding the fourth component of complement and the clinical course of hereditary angioedema (HAE)

In order to study if in patients with hereditary angioedema (HAE), copy number of the two genes (C4A and C4A) encoded in the central region of main histocompatibility complex (MHC) influences the diagnostically important C4 serum concentration as well as the clinical course of the disease, we determined copy number of the complement C4A and C4B genes in DNA samples of 95 HAE patients and 246 healthy controls. Distribution of both the C4A and C4B copy numbers significantly (p=0.0183 and 0.0318, respectively) differed between the two groups, the most marked difference we observed was the lower frequency of the high (3 or 4) C4A copy numbers in the patients. As it expected, the dosage of both C4A and C4B genes positively correlated to the longitudinally measured serum C4 concentrations. Moreover, we found an unexpected clinical correlation with the dosage of the C4B gene. The course of the disease was milder in the 9/95 patients carrying 3 or 4 copies of C4B gene, compared to the rest of patients, i.e. diagnosis was established at significantly (p=0.0052) older age (36.0 (31.0-39.5)) years versus 20.5 (7.5-31.5 years), bi-yearly attack rate was significantly (p=0.0145) lower (1.0 (0.0-11.0)) versus 11.0 (3.5-21.5), and the over-all activity of the classical pathway and the enzyme-inhibitor activity of the C1-inhibitor (C1-INH) was closer to the normal values. These observations indicate that high copy number of the C4B gene can be a protective factor against disease severity in HAE and therefore its determination is warranted.

Use of a PCR-based amplification analysis as a substitute for the Southern blot method to determine the C4A and C4B genes

The human C4 complement components of the C4 gene are encoded by two genes, C4A and C4B, located on chromosome 6p21.3 of the major histocompatibility complex (MHC) of the human leukocyte antigen (HLA) class III region. Genetic determination of these two genes was by the Southern blot method: the 276- and 191-bp NlaIV fragments represent the C4A gene with the sequence, PCPVLP, at residues 1101-1106; the 467-bp NlaIV fragment represents the C4B gene with the sequence, LSPVIH, at residues 1101-1106. Here, we describe a PCR-based approach for differential amplification of the C4 genes adjacent to the respective CYP21A1P and CYP21A2, followed by NlaIV restriction digestion in a secondary PCR product and direct analysis by electrophoresis on an agarose gel to determine the C4A and C4B genes. From the results of this study, we concluded that 87% and 85% of the C4 genes adjacent to the CYP21A1P and CYP21A2 genes carried the C4A and C4B genes, respectively. The frequencies of the C4A and C4B genes comprising the C4 locus were 51.5 and 49%, respectively in this ethnic Chinese (Taiwanese) cohort. Since no radiolabelling application is involved, the protocol is reliable as a substitute for the Southern blot method for C4A and C4B determination.

Mannan-binding lectin activates C3 and the alternative complement pathway without involvement of C2

Lectin pathway activation of C3 is known to involve target recognition by mannan-binding lectin (MBL) or ficolins and generation of classical pathway C3 convertase via cleavage of C4 and C2 by MBL-associated serine protease 2 (MASP-2). We investigated C3 activation in C2-deficient human sera and in sera with other defined defects of complement to assess other mechanisms through which MBL might recruit complement. The capacity of serum to support C3 deposition was examined by ELISA using microtiter plates coated with O antigen-specific oligosaccharides derived from Salmonella typhimurium, S. thompson, and S. enteritidis corresponding to serogroups B, C, and D (BO, CO, and DO). MBL bound to CO, but not to BO and DO, and efficiently supported C3 deposition in the absence of C2, C4, or MASP-2. The existence of an MBL-dependent C2 bypass mechanism for alternative pathway-mediated C3 activation was clearly demonstrated using CO, solid-phase mannan, and E. coli LPS. MASP-1 might contribute, but was not required for C3 deposition in the model used. Independent of MBL, specific antibodies to CO supported C3 deposition through classical and alternative pathways. MBL-dependent C2 bypass activation could be particularly important in various inherited and acquired complement deficiency states.

Complement deficiency and disease: an update

Complement deficiencies are probably vastly under-diagnosed within clinical medicine. Judging from a Swedish study of C2 deficiency, a deficiency with an estimated prevalence of about 1/20,000 in Western countries, less than 10% of the deficiencies of the classical and alternative pathways and the late complement components are identified in Sweden. C1 inhibitor deficiency and deficiencies of MBL and MASP-2 were not included in the assessment. The introduction of new screening methods should facilitate detection of complement deficiencies in clinical practice. In our study of C2 deficiency (n=40), 57% of the patients had a history of invasive infection with encapsulated bacteria, mainly Streptococcus pneumoniae. This emphasizes the importance of the classical and/or the lectin pathway in defence against severe infection. Rheumatological disease, mainly systemic lupus erythematosus was present in 43% of the patients. In addition, a significant association was found between C2 deficiency and atherosclerosis. Complement-dependent disease mechanisms are discussed together with the potential importance of non-complement genes for disease expression in complement deficiencies. Analysis of larger patient groups is required in order to establish guidelines for investigation and treatment of patients with complement deficiency.

Complete deficiencies of complement C4A and C4B including 2-bp insertion in codon 1213 are genetic risk factors of systemic lupus erythematosus in Thai populations

The complement component C4 is encoded by two genes: C4A and C4B on human chromosome 6p in the major histocompatibility complex (MHC). Most studies have linked the deficiencies in C4 with systemic lupus erythematosus (SLE) in Angio-Irish, North American, Black American, Mexican American, Australian and Japanese populations. Null alleles at either locus (C4AQ0 or C4BQ0) are relatively common in Americans occurring at the C4A and C4B loci in approximately 10% and 16% of normal individuals, respectively. In the present study, we extensively examined the possible association between homozygous C4Q0 and SLE in a large cohort of Thai populations diagnosed as SLE and further attempted to identify the genetic basis of C4Q0. One hundred and eighteen cases of SLE patients and 145 matched controls were genotyped by touchdown PCR. The results confirmed the previous studies that 5.93% (7/118) of C4 null genes: 2.54% (3/118) of C4AQ0 and 3.39% (4/118) of C4BQ0 were found in SLE patients. In contrast to other studies, we found no cases of C4 null genes in normal control (0 from 145 samples). To further investigate the genetic basis of C4 deficiency, all genomic DNAs were also analyzed for 2-bp (TC) insertion at codon 1213 in exon 29 which is a common mutation in many C4A null genes and a novel 1-bp deletion (C) at codon 522 in exon 13 that is common in most C4B null genes. Both mutation results in a flame-shift mutation and premature stop codon using sequence specific primers PCR (SSP-PCR) and direct sequencing. The results showed that there was 2-bp insertion in exon 29 of mutant C4B gene in one SLE patient carrying C4AQ0. There was no 2-bp insertion in exon 29 of both C4A and C4B genes in normal individual and the rest of SLE patients. All patients with C4AQ0 exhibited more than 5 ACR criteria including malar rash, oral ulcers, renal disorder, immunological disorder, anti-nuclear antibody, without hematological disorder. In contrast, all of C4BQ0 SLE patients showed 5 or 6 ACR criteria including hematological disorder, malar rash, oral ulcers, renal disorder, immunological disorder and anti-nuclear antibody. A patient who possesses C4AQ0 and 2-bp insertion in exon 29 of mutant C4B showed 9 ACR criteria but no discoid rash and hematological disorder. In conclusion, both C4AQ0 and C4BQ0 are the strong predisposing factors for SLE in Thais. It was supported by the absence of either C4A or C4B deletion in healthy control. We suggested that the different racial and genetic backgrounds could alter the thresholds for requirement of C4A or C4B protein levels in immune tolerance and regulation.

Clathrin heavy and light chain isoforms originated by independent mechanisms of gene duplication during chordate evolution

In humans, there are two isoforms each of clathrin heavy chain (CHC17 and CHC22) and light chain (LCa and LCb) subunits, all encoded by separate genes. CHC17 forms the ubiquitous clathrin-coated vesicles that mediate membrane traffic. CHC22 is implicated in specialized membrane organization in skeletal muscle. CHC17 is bound and regulated by LCa and LCb, whereas CHC22 does not functionally interact with either light chain. The imbalanced interactions between clathrin subunit isoforms suggest a distinct evolutionary history for each isoform pair. Phylogenetic and sequence analysis placed both heavy and light chain gene duplications during chordate evolution, 510-600 million years ago. Genes encoding CHC22 orthologues were found in several vertebrate species, with only a pseudogene present in mice. Multiple paralogons surrounding the CHC genes (CLTC and CLTD) were identified, evidence that genomic or large-scale gene duplication produced the two CHC isoforms. In contrast, clathrin light chain genes (CLTA and CLTB) apparently arose by localized duplication, within 1-11 million years of CHC gene duplication. Analysis of sequence divergence patterns suggested that structural features of the CHCs were maintained after gene duplication, but new interactions with regulatory proteins evolved for the CHC22 isoform. Thus, independent mechanisms of gene duplication expanded clathrin functions, concomitant with development of neuromuscular sophistication in chordates.

Confirmation of the association of the C4B null allelle in autism

The objective of this study was to examine and attempt to confirm our previous findings of an increased frequency of the C4B null allele (C4BQ0) in subjects with autism. Newly identified subjects from Utah and Oregon were studied. Families evaluated included 85 who had a child with autism and 69 control families. Of the subjects with autism studied, 42.4% carried at least one C4BQ0, compared with 14.5% of the control subjects (p = 0.00013), with a relative risk of 4.33. Over half of the C4B null alleles in the subjects with autism involved C4A duplications. A marked increase in the ancestral haplotype 44.1 that lacks a C4B gene and has 2 C4A genes was also observed. The results of this study suggest that the human leukocyte antigen class III C4BQ0 significantly increases the risk for autism.

Hereditary C2 deficiency in Sweden: frequent occurrence of invasive infection, atherosclerosis, and rheumatic disease

Although frequently asymptomatic, homozygous C2 deficiency (C2D) is known to be associated with severe infections and rheumatic disease. We describe the clinical findings in 40 persons with C2D from 33 families identified in Sweden over 25 years. Medical records covering 96% of the accumulated person-years were reviewed, giving a mean observation time of 39 years (range, 1-77 yr). Severe infection was the predominant clinical manifestation in the cohort: 23 patients had a past history of invasive infections, mainly septicemia or meningitis caused by Streptococcus pneumoniae, and 12 patients had repeated infections of this kind. Nineteen patients had at least 1 episode of pneumonia, and recurrent pneumonia was documented in 10 patients. Repeated infections occurred mainly during infancy and childhood. Systemic lupus erythematosus was found in 10 patients. Another 7 patients had undifferentiated connective tissue disease (n = 4) or vasculitis (n = 3). We found no correlation between susceptibility to invasive infection and rheumatologic disease. Cardiovascular disease occurred at a high rate, with a total of 10 acute myocardial infarctions and 5 cerebrovascular episodes in 6 patients. Causes of death among the C2D patients were infection (n = 5), acute myocardial infarction (n = 3), and cancer (n = 1). We suggest that severe infection may be the principal clinical manifestation of C2D. We also provide novel evidence for a possible role of C2D in the development of atherosclerosis consistent with findings in mannan-binding deficiency and experimental C3 deficiency. In addition, we confirm the well-known association between C2D and systemic lupus erythematosus.

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.

Relationship between complement components C4A and C4B diversities and two TNFA promoter polymorphisms in two healthy Caucasian populations

The RP-C4-CYP21-TNX (RCCX) modules and the tumor necrosis factor (TNF) gene cluster are probably the most polymorphic genomic regions in the human central major histocompatibility complex (MHC). Using definitive methods for genotypic and phenotypic analyses of complement components C4A and C4B, determination of the RCCX length variants, and SSP-PCR/RFLP analyses of TNFA promoter polymorphisms at positions -308 and -238, we studied the complex relationships between the C4 and TNFA polymorphisms in two normal Caucasian populations. The patterns of the RCCX modular structures and the allelic frequency of -308A TNFA (TNF2) were similar between the Budapest (n = 125) and the Ohio (n = 80) Caucasians. However, the frequency of the -238A allele was significantly higher in the Ohio (11.3%) than in the Budapest (1.6%) study population (p < 0.0001). Marked features were found in the RCCX length variants in the TNF2 carriers and noncarriers. Strong associations were found between the C4AQ0 B1 haplotype from the monomodular short (mono-S) RCCX structure and the TNF2 allele, and between the C4A6 B1 haplotype from the bimodular long-short (LS) structure of the RCCX and the TNFA -238A allele. However, 36%-46% of the TNF2 carriers did not associate with a mono-S in both study cohorts, and 57.1% of the TNFA -238A carriers in Ohio did not associate with C4A6, which has a defective complement C5 convertase activity. The carriers of TNF2 allele had significantly lower C4A serum concentration (0.17 +/- 0.08 g/l) than noncarriers (0.23 +/- 0.09 g/l) (p < 0.001). The lowest C4A serum levels were found in TNF2 carriers with mono-S structures (0.14 +/- 0.06 g/l). In essence, our results demonstrated the heterogeneities of the TNFA promoter polymorphisms, and the linkage disequilibrium of TNFA -308A and -238A alleles with complement C4A deficiency and impaired C4A protein function, respectively.

Rapid detection of the ERV-K(C4) retroviral insertion reveals further structural polymorphism of the complement C4 genes in old world primates

The fourth component of complement (C4) is coded for by two tandem-duplicated genes located in the class III region of the MHC of humans as well as a number of primates. A C4 gene size polymorphism giving rise to two gene variants of 16 and 22.3 kb length can be attributed to a complete endogenous retroviral insertion of 6.3 kb termed ERV-K(C4) in intron 9 of the long C4 genes. We developed a simple PCR-based screening assay to detect the presence of this insertion, and tested a number of unrelated animals from old world primate species. The presence of the ERV insertion in the orangutan, rhesus macaque and green monkey as well as its absence in gorillas and chimpanzees could be confirmed. In addition, the insertion was also detected in the baboon and the cynomolgus macaque whereas it was not found in a single gibbon. Among rhesus and cynomolgus macaques one individual was identified in each species only carrying short C4 genes demonstrating further structural heterogeneity in these species. Based on these findings we propose that the primigenial retroviral integration occurred prior to the radiation of old world primate species, and that both the long and the short forms of the C4 gene have existed side by side since then.

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.

The human and mouse MHC class III region: a parade of 21 genes at the centromeric segment

The human major histocompatibility complex (MHC) class III region contains 57-60 structural genes spanning 654-759 kb of genomic DNA. Analysis of the sequence identities of the human and mouse genomic regions between NOTCH4 and complement C2 yields important information on the locations of the coding and regulatory sequences. It also provides insights into the relationship between protein function and level of sequence conservation, and on the clustering of genes with related functions.

Deficiencies of human complement component C4A and C4B and heterozygosity in length variants of RP-C4-CYP21-TNX (RCCX) modules in caucasians. The load of RCCX genetic diversity on major histocompatibility complex-associated disease

The complement component C4 genes located in the major histocompatibility complex (MHC) class III region exhibit an unusually complex pattern of variations in gene number, gene size, and nucleotide polymorphism. Duplication or deletion of a C4 gene always concurs with its neighboring genes serine/threonine nuclear protein kinase RP, steroid 21-hydroxylase (CYP21), and tenascin (TNX), which together form a genetic unit termed the RCCX module. A detailed molecular genetic analysis of C4A and C4B and RCCX modular arrangements was correlated with immunochemical studies of C4A and C4B protein polymorphism in 150 normal Caucasians. The results show that bimodular RCCX has a frequency of 69%, whereas monomodular and trimodular RCCX structures account for 17.0 and 14.0%, respectively. Three quarters of C4 genes harbor the endogenous retrovirus HERV-K(C4). Partial deficiencies of C4A and C4B, primarily due to gene deletions and homoexpression of C4A proteins, have a combined frequency of 31.6%. This is probably the most common variation of gene dosage and gene size in human genomes. The seven RCCX physical variants create a great repertoire of haplotypes and diploid combinations, and a heterozygosity frequency of 69.4%. This phenomenon promotes the exchange of genetic information among RCCX constituents that is important in homogenizing the structural and functional diversities of C4A and C4B proteins. However, such length variants may cause unequal, interchromosomal crossovers leading to MHC-associated diseases. An analyses of the RCCX structures in 22 salt-losing, congenital adrenal hyperplasia patients revealed a significant increase in the monomodular structure with a long C4 gene linked to the pseudogene CYP21A, and bimodular structures with two CYP21A, which are likely generated by recombinations between heterozygous RCCX length variants.

Organizations and gene duplications of the human and mouse MHC complement gene clusters

The MHC complement gene cluster (MCGC) in most people contains thirteen structural genes, pseudogenes and gene segments. Novel genes RD, SKI2W, DOM3Z and RP1 are organized as two head-to-head gene pairs between complement gene Bf and the first locus of C4. Southern blot analysis shows that single-copy genes for DOM3Z are detectable in primates and other mammals. Sequence analyses revealed that the exon- intron structures of human and mouse DOM3Z genes are identical. Both human and mouse DOM3Z transcripts exhibit splice variants at the 5' regions, although the open reading frames remain identical. Cloning and characterization of the mouse RP1 cDNA revealed a reading frame for 254 amino acids with a bipartite nuclear localization signal close to the amino terminus. The mouse RP1 gene consists of 7 exons and spans 12.9 kb. Located in intron 4 of mouse RP1 is an endogenous retrovirus that probably confers the androgen-responsive expression of the Slp protein in certain male mice. The availability of the complete human and mouse MCGC genomic and cDNA sequences allows further deliberate analyses of gene duplications and evolution. The intergenic region between mouse SLP and C4 genes is more than six times larger than the corresponding region in humans. It contains the functional gene steroid CYP21A, long stretches of repetitive DNA elements, and three partially duplicated gene segments TNXA, SKI2W2 and RP2. The modular duplications of human and mouse RP-C4-CYP21-TNX (RCCX) are sharply different as SKI2W2 is absent in the human MCGC, and TNXA and RP2 are smaller in size but higher in sequence conservation in humans.

Vaccination responses to capsular polysaccharides of Neisseria meningitidis and Haemophilus influenzae type b in two C2-deficient sisters: alternative pathway-mediated bacterial killing and evidence for a novel type of blocking IgG

Meningitis caused by Neisseria meningitidis serogroup W-135 was diagnosed in a 14-year-old girl with a history of neonatal septicemia and meningitis caused by group B streptococci type III. C2 deficiency type I was found in the patient and her healthy sister. Both sisters were vaccinated with tetravalent meningococcal vaccine and a conjugate Haemophilus influenzae type b vaccine. Three main points emerged from the analysis. First, vaccination resulted in serum bactericidal responses demonstrating anticapsular antibody-mediated recruitment of the alternative pathway. Second, addition of C2 to prevaccination sera produced bactericidal activity in the absence of anticapsular antibodies, which suggested that the bactericidal action of antibodies to subcapsular antigens detected in the sera might strictly depend on the classical pathway. A third point concerned a previously unrecognized type of blocking activity. Thus, postvaccination sera of the healthy sister contained IgG that inhibited killing of serogroup W-135 in C2-deficient serum, and the deposition of C3 on enzyme-linked immunosorbent assay plates coated with purified W-135 polysaccharide. Our findings suggested blocking to be serogroup-specific and dependent on early classical pathway components. Retained opsonic activity probably supported post-vaccination immunity despite blocking of the bactericidal activity. The demonstration of functional vaccination responses with recruitment of alternative pathway-mediated defense should encourage further trial of capsular vaccines in classical pathway deficiency states.

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.

Modular variations of the human major histocompatibility complex class III genes for serine/threonine kinase RP, complement component C4, steroid 21-hydroxylase CYP21, and tenascin TNX (the RCCX module). A mechanism for gene deletions and disease associations

The frequent variations of human complement component C4 gene size and gene numbers, plus the extensive polymorphism of the proteins, render C4 an excellent marker for major histocompatibility complex disease associations. As shown by definitive RFLPs, the tandemly arranged genes RP, C4, CYP21, and TNX are duplicated together as a discrete genetic unit termed the RCCX module. Duplications of the RCCX modules occurred by the addition of genomic fragments containing a long (L) or a short (S) C4 gene, a CYP21A or a CYP21B gene, and the gene fragments TNXA and RP2. Four major RCCX structures with bimodular L-L, bimodular L-S, monomodular L, and monomodular S are present in the Caucasian population. These modules are readily detectable by TaqI RFLPs. The RCCX modular variations appear to be a root cause for the acquisition of deleterious mutations from pseudogenes or gene segments in the RCCX to their corresponding functional genes. In a patient with congenital adrenal hyperplasia, we discovered a TNXB-TNXA recombinant with the deletion of RP2-C4B-CYP21B. Elucidation of the DNA sequence for the recombination breakpoint region and sequence analyses yielded definitive proof for an unequal crossover between TNXA from a bimodular chromosome and TNXB from a monomodular chromosome.

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.

Four ubiquitously expressed genes, RD (D6S45)-SKI2W (SKIV2L)-DOM3Z-RP1 (D6S60E), are present between complement component genes factor B and C4 in the class III region of the HLA

The association of the HLA class III region with many diseases motivates the investigation of unidentified genes in the 30-kb segment between complement component genes Bf and C4. RD, which codes for a putative RNA binding protein, is 205 bp downstream of Bf. SKI2W (HGMW-approved symbol SKIV2L), a DEVH-box gene probably involved in RNA turnover, is 171 bp downstream of RD (HGMW-approved symbol D6S45). RP1 (HGMW-approved symbol D6S60E) is located 611 bp upstream of C4. The DNA sequence between human RD and RP1 was determined and the exon-intron structure of SKI2W elucidated. SKI2W consists of 28 exons. The putative RNA helicase domain of Ski2w is encoded by 9 exons. Further analysis of the 2.5-kb intergenic sequence between SKI2W and RP1 led to the discovery of DOM3Z. The full-length cDNA sequence of DOM3Z encodes 396 amino acids with a leucine zipper motif. Dom3z-related proteins are present in simple and complex eukaryotes. In Caenorhabditis elegans, Dom3z-related protein could be involved in the development of germ cells. Human RD-SKI2W and DOM3Z-RP1 are arranged as two head-to-head oriented gene pairs with unmethylated CpG sequences at the common 5' regulatory region of each gene pair. The ubiquitous expression pattern suggests that these four genes are probably housekeeping genes.