Three Chido determinants detected on the B5Rg+ allotype of human C4: their expression in Ch-typed donors and families

Identification of the size and antigenic determinants of the human C4 gene by a polymerase chain-reaction-based amplification method

The human C4 complement components of the C4 locus are encoded by two genes, C4A and C4B, located on chromosome 6p21.3 of the major histocompatibility complex of the human leukocyte antigen class III region. The size difference between the two genes is due to the presence of HERV-K (C4), an endogenous retroviral sequence (6.7 kb long), in intron 9 of the long C4 gene. Whether the C4 is the long (L) or short (S) gene was determined by the Southern blot method, and the antigenic determinants in residues 1,054-1,106 of Rodgers and Chido were generally identified by immunoblot analysis. Herein, we explore a polymerase chain reaction (PCR) amplification method for directly determining the size of C4 loci adjacent to the respective RP1 and RP2 genes and antigenic determinants by DNA sequencing. From the results of this study, we concluded that all of the C4 genes adjacent to the RP1 gene presented the long gene. In addition, 47% of the C4 genes adjacent to the RP2 gene were the short gene and 53% were the long gene. This result was consistent with that of the Southern blot analysis. The PCR method is practical for identifying the C4 genotype and can be used to detect other polymorphisms among variants of C4 genes.

Characterization of a de novo conversion in human complement C4 gene producing a C4B5-like protein

Complement C4 is a highly polymorphic protein essential for the activation of the classical complement pathway. Most of the allelic variation of C4 resides in the C4d region. Four polymorphic amino acid residues specify the isotype and an additional four specify the Rodgers and Chido determinants of the protein. Rare C4 allotypes have been postulated to originate from recombination between highly homologous C4 genes through gene conversions. Here we describe the development of a de novo C4 hybrid protein with allotypic and antigenic diversity resulting from nonhomologous intra or interchromosomal recombination of the maternal chromosomes. A conversion was observed between maternal C4A3a and C4B1b genes producing a functional hybrid gene in one of the children. The codons determining the isotype, Asp(1054), Leu(1101), Ser(1102), Ile(1105) and His(1106), were characteristic of C4B gene, whereas the polymorphic sites in exon and intron 28 were indicative of C4A3a sequence. The protein produced by this hybrid gene was electrophoretically similar to C4B5 allotype. It also possesses reversed antigenicity being Rodgers 1, 2, 3 and Chido-1, -2, -3, 4, -5, and -6. Our case describes the development of a rare bimodular C4B-C4B haplotype containing a functional de novo C4 hybrid gene arisen through gene conversion from C4A to C4B. Overall the data supports the hypothesis of gene conversions as an ongoing process increasing allelic diversity in the C4 locus.

Genetic polymorphism of the fourth component of human complement: population study and proposal for a revised nomenclature based on genomic PCR typing of Rodgers and Chido determinants

The fourth component of human complement (C4) is coded for by two homologous genes, C4A and C4B, located in the class III region of the major histocompatibility complex (MHC). Genetic typing of C4A and B alleles is routinely carried out by high-voltage agarose gel electrophoresis. The electrophoretic C4 polymorphism can be further subdivided by the Rodgers (Rg) and Chido (Ch) blood groups, which are antigenic determinants of the C4A and B alpha-chains, respectively. We have used a recently described direct PCR typing method using sequence-specific primers (PCR-SSP) in combination with electrophoretic C4 typing as well as genomic RFLP analysis to determine the frequency of C4 allotypes, Rg/Ch subtypes and C4A-B haplotypes in a family study of the German population. As the current C4 allele designation does not provide any information about the presence or absence of Rodgers and Chido antigens, we have developed an extension to the existing C4 nomenclature. This revised allele designation combines the existing numerical allotypes defined by electrophoretic mobility with eight subtypes (01-08) based on Rg/Ch PCR genotyping results. Using this approach, most electrophoretic allotypes could be subdivided. Among the C4A allotypes, the most common allele was A*0301 (59.9%), and the most common subtype among all electrophoretic allotypes was 01 (85.1%; = Rg1,2-positive, Ch-negative). For C4B, the most common allele was B*0101 (64.3%), and the most common subtype was 01 (79.6%; = Ch1,2,3,4,5,6-positive, Rg-negative). The subtypes 03, 04, 07 and 08 of the C4A allotypes, and the subtypes 03, 07 and 08 of the C4B allotypes, were not detected in this study. The analysis of duplicated C4 alleles revealed considerable heterogeneity of their subtypes. The results demonstrate that all known C4 allotypes can now be assigned unambiguously, which facilitates the identification of MHC haplotypes relevant for transplantation and disease association studies.

Polymerase chain reaction analysis of the Xba I polymorphism of the human complement C4 genes provides evidence for strong haplotype conservation

The genes coding for the two isotypes of the fourth component of human complement, C4A and C4B, are located between the HLA-B and -DR loci of the MHC. We studied the linkage relationship of the previously described XbaI RFLP to obtain further insight into the evolution of the tandemly arranged C4 genes. Using exon-specific PCR amplification followed by restriction analysis and direct DNA sequencing, the polymorphic site could be located in exon 40 of the C4 gene (cDNA position 5095). The polymorphism does not change an amino acid residue. Using nested PCR amplification with isotype-specific primers to amplify either C4A or C4B alleles the haplotype arrangement of the XbaI sites in both isotypic C4 genes was analyzed independently. It was observed that the XbaI restriction site was either present or absent in both C4 genes of a given haplotype. In a study of 106 Caucasian haplotypes, only two different haplotypes could be identified carrying a C4A gene with and a C4B gene without the XbaI restriction site. Also, the XbaI site could only be detected in long C4 genes possessing the 6.5-kb insertion in intron 9. Our findings provide evidence that the mutation creating the XbaI polymorphism occurred in an ancestral C4 gene already carrying the long intron 9. The duplicating resulting in the presence of two isotypic genes, C4A and C4B, must have taken place subsequently giving rise to haplotypes with or without the XbaI site.

A new PCR-based typing of the Rodgers and Chido antigenic determinants of the fourth component of human complement

The Rodgers (Rg) and Chido (Ch) blood groups are antigenic determinants of the fourth component of human complement C4. They are associated with the two isotypes of C4, C4A and C4B, respectively. They serve as markers to distinguish C4A from C4B as well as for the definition of subtypes of common and rare allotypes. As an alternative to the serological typing method using human alloantisera, a PCR typing procedure with sequence-specific primers (PCR-SSP) was designed. The method was tested on selected DNA samples from individuals with well-defined C4 allotypes. No false-positive or false-negative typing results were obtained and all the determinant combinations could be distinguished. The PCR genotyping allowed the detection of all Rg/Ch sequence determinants of each isotype. Thus, reverse antigenicity could also be established in the presence of other C4 allotypes without a segregation study. To exclude the possibility that PCR-typed determinants originate from a non-expressed C4 null gene, a sequence-specific PCR was established detecting a 2-bp insertion in exon 29 described previously as a cause for C4A non-expression. PCR Rg/Ch genotyping provides a fast and efficient method for routine typing in HLA haplotype and disease association studies.

Effects of C4 null alleles and homoduplications on quantitative expression of C4A and C4B

The availability of MoAbs now allows the accurate quantification of the individual C4 isotypes, C4A and C4B. Using a sensitive two-site immunoradiometric technique to measure serum levels of C4A and C4B, we studied the relationship between genotype and phenotype and physiological factors affecting C4 expression in 129 fully genotyped healthy subjects. Our results confirm that there is extensive phenotypic overlap between genotypic groups and it was not possible to determine the presence of single null alleles from total serum C4. Of the factors which may influence C4 expression, we found that age contributes a very small influence but that gender has no effect and there was no evidence for the presence of feedback of null alleles on the expression of remaining genes. Potential problems in quantifying C4 arising from the complex relationship between isotypic identity and serotypic recognition were highlighted by the finding of reversed antigenic expression of a C4B*5 molecule which was recognized as C4A by the anti-Rg:1 monoclonal used in these studies. We also confirmed that the extended MHC haplotype associated with Felty's syndrome, HLA-B44, C4A*3, C4BQ*O, HLA-DR4, encodes an expressed, duplicated, C4A gene.

The genomic structure of two ancestral haplotypes carrying C4A duplications

Two major histocompatibility complex (MHC) ancestral haplotypes (AH) HLA A24, Bw52, C2C, BfS, C4A3+2, C4BQO, DRw15, DQw6 (52.1) and HLA A24, Cw7, B7, C2C, BfS, C4A3+3, C4B1, DR1, DQw5 (7.2), which occur with the haplotype frequencies of approximately 10% and 4% respectively in the Japanese population, carry duplicated C4A alleles by C4 allotyping. Southern blot analysis with Taq I indicated that the 52.1 AH has two C4 genes defined by 7.0 kilobase (kb) and 6.0 kb C4 hybridizing fragments but both encode C4A allotypes, being C4A3 and C4A2 respectively. The 7.2 AH carries two C4A3 and one C4B1 alleles and restriction length polymorphism (RFLP) analysis with Taq I showed that 6.0 kb and 7.0 kb fragments are in the proportion of 2:1. By pulsed field gel electrophoresis (PFGE) analysis, the lengths of the Pvul fragments carrying C4 and Cyp21 genes were approximately 390 kb for 52.1 and 440 kb to 7.2. The results indicate that the RFLP markers do not correlate with C4 isotype (A or B) or allotype and that the C4 gene copy number is a function of the number of genomic blocks containing C4 and Cyp21.

C4 Chido 3 and 6 distinguish two diabetogenic haplotypes: HLA-B49, SC01,DR4,DQw8 and B8,SC01,DR3,DQw2

The combination of the HLA complement allotypes BFS, C2C, C4AQ0 (deleted gene) and C4B1, termed SC01 complotype, usually present in the HLA-B8,DR3,DQw2 diabetogenic haplotype, has also been found in a novel "low frequency" HLA-B49,DR4,DQw8 haplotype associated with Spanish insulin-dependent diabetes mellitus (IDDM). Family studies of C4 antigenic determinants Rodgers/Chido and their specific C4d nucleotide sequences confirm that this novel haplotype bearing Chido -3, -6 is not due to a recent recombination from the common HLA-B8,DR3 haplotype bearing Chido 3,6; moreover, Chido analysis at the serological or DNA level is presently the only way to distinguish both SC01 complotypes, since BF, C2, steroid 21-hydroxylase and C4 genes do not reveal other differences by restriction fragment analysis. On the other hand, HLA-B49,SC01,DR4 is the first DR4-bearing IDDM-susceptible haplotype with a deleted C4 gene described so far and the only DR4-bearing haplotype found in the Spanish population. This report further supports the fact that extended haplotypes with deleted (or "not duplicated") genes in the class III region contain IDDM-susceptibility more often than non-deleted (or "duplicated") haplotypes in the Spanish and other Mediterranean populations.

Characterization of two hybrid C4 allotypes (C4A*12 and C4B*3) by electrophoretic, serological and restriction fragment length polymorphism analyses

Informative pedigree analysis of two rare C4 allotypes is reported. One proband was C4A deficient as a consequence of having one haplotype with a deleted C4A gene, and the second haplotype with two C4B genes--one encoding the common C4B*1 and one encoding a unique hybrid gene product C4B*3. C4B*3 had approximately normal C4B hemolytic activity, a single alpha-chain of MR 94,000 by SDS-PAGE but was positive for Rg:1,2 by hemagglutination inhibition (HAI) and for Rg:1 by Western blotting. The hybrid nature was confirmed by RFLP analysis with a Rg:1-associated fragment by Eco0109 digestion but no C4A-associated fragments by N1aIV digestion were identified. A gene conversion at Locus I which included just the C4 isotype region could explain the structure of C4B*3. The second pedigree had a Rodgers negative C4A*12 allotype. This C4A gene, which segregated with a single 7.0 kb TaqI fragment, encoded a C4A alpha-chain, which was negative for Rg:1 epitope. The affected haplotype lacked the Rg:1-associated fragment by Eco0109 digestion yet had the C4A specific N1aIV digestion fragment. These studies successfully employed RFLP analyses to confirm serologic and electrophoretic observations.

An epitope on C4 beta light (L) chains detected by human anti-Rg; its relationship with beta chain polymorphism and MHC associations

Two out of ten Rg-specific antisera tested contain a third antibody specific for the beta chain of C4. Analysis of the beta chains of 66 unrelated individuals by sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed that the epitope detected is located exclusively on the light (L) beta chain. A strong, but incomplete, association between the beta chain epitope and the expression of the Rg:2 determinant on the alpha chain of the same protein was also observed. While H (heavy) and L beta chains were not associated with a particular C4 isotype, previously unrecorded associations of beta chain polymorphism with the DR locus have been established.

An estimate on the frequency of duplicated haplotypes and silent alleles of human C4 protein polymorphism. I. Investigations in healthy Caucasoid families

The frequency of duplicated and non-expressed C4 alleles was determined by segregation analysis in 31 German and five French families with altogether 274 individuals by submitting the complete data from C4 protein phenotyping, including C4 beta chains, and the other classical MHC markers to the family analysis programme (FAP). From 120 unrelated German haplotypes the following frequencies were derived for silent alleles: C4A*Q0 0.2000, C4B*Q0 0.2083, and for the total of homo- and heteroduplicated C4A resp. C4B alleles: C4"DA"* 0.1333, C4"DB"* 0.1000. The true occurrence of the duplicated C4A*2, "DB*21" haplotype, first observed in French families, was found to be 0.0250 in the German sample. While the frequency of duplicated C4 haplotypes confirms earlier estimates, the increase in the frequency of silent alleles corresponds to those assumed from investigations at the DNA level. The results demonstrate classical protein typing with inclusion of C4 beta chain types to be an indispensable and powerful tool for haplotype recognition; they support the hypothesis that deletion at one C4 locus is accompanied by duplication at the other in a majority of haplotypes.

High-titer, low-avidity (HTLA) antibodies and antigens: a review

Antibodies that react to HTLA characteristics cause difficulties in serologic testing because of the weak reactions they produce in the indirect antiglobulin test. Those specificities that are more frequently encountered (anti-Yka, -McCa, -Kna, -Ch) are directed toward antigens of high incidence in both the white and black populations. They have not been shown to cause significant destruction of transfused antigen-positive red cells. The antibodies create problems in serologic tests because the reactions they produce interfere with the identification of reactions due to other, clinically significant antibodies.

The molecular genetics of components of the complement system

Rapid progress has been made recently on the elucidation of the structural components of the complement system by the application of recombinant DNA techniques. The derived amino acid sequences of most of the complement proteins are now available through cDNA cloning, and significant progress has been made in the discovery of the genetic organization of the corresponding genes. The linkage of some of the complement component genes has been established through the study of phenotypic genetics. Of particular interest has been the mapping of two clusters of genes which encode proteins involved in the activation of C3. C2, C4 and factor B, three of the structural components of the classical and alternative pathway C3 convertases, are encoded by genes which map to the MHC on human chromosome 6. The linkage of the genes with each other in a 100 kb segment of DNA has been established through the isolation of overlapping cosmid clones of genomic DNA, and PFGE has defined the molecular map position of these genes within the class III region of the MHC. The regulatory proteins factor H, C4BP, CR1 and DAF, which are involved in the control of C3 convertase activity, are encoded by closely-linked genes (termed the regulators of complement activation or RCA linkage group) that have been mapped to human chromosome 1. PFGE has defined the linkage of the CR1, C4BP and DAF genes, together with the CR2 gene in an 800 kb segment of DNA, and it is clear that this technique will eventually be applied to the molecular mapping of other complement genes in relation to their flanking loci. Polymorphism is a feature of many of the complement proteins, especially those encoded by genes in the MHC class III region. Of these, C4 is by far the most polymorphic, and differences in gene size and gene number, in addition to the functional and antigenic differences in the gene products, have been recognized. Null alleles at either of the C4 loci are rather common and may be important susceptibility factors in some HLA-associated diseases, particularly SLE. The molecular basis of complement deficiency states has begun to be elucidated. In many cases, the deficiency is not caused by a major gene deletion or rearrangement, and techniques which detect single point mutations in DNA (Cotton et al, 1988) will have to be applied to fully characterize the nature of the defect.

The antigenic determinants, Rg/Ch/WH, expressed by Japanese C4 allotypes

The expression of antigenic determinants, Rg/Ch/WH, on Japanese C4 allotypes has been studied. Although the Japanese C4 allotype frequencies are known to differ from Europeans, the antigenic expression of their C4 allotypes correlates with associations described previously. All 89 random donors and 17 selected donors were Rg:1,2 so neither Rg:1,-2 nor Rg:1,-2 was found. The frequency of Ch:1,-2,3 was elevated while that of Ch:1,2,3 was reduced, which was seen as a direct result of the higher frequency of B2 and B5 allotypes. None of the Japanese were Ch:1,2,-3, but this can be accounted for by the absence of the A*6,B*1 haplotype. The WH determinant, which has been associated completely with Rg:1,-2 in Caucasians, was found at a higher frequency, 32%, in association with an A*3,2,B*QO haplotype expressing Rg:1,2, which has not been described previously. Detailed investigation showed that the A3 allotype was Rg:1,2 whereas the A2 allotype only expressed Rg1 (Rg:1,-2 WH+).

Antigenic determinants expressed by human C4 allotypes; a study of 325 families provides evidence for the structural antigenic model

The antigenic determinants of human C4 have been defined by human IgG antisera, Rodgers (Rg) and Chido (Ch), in hemagglutination-inhibition assays (HAI). Eight (2 Rg and 6 Ch) are of high frequency, greater than 90%, and 1, WH, is of low frequency, 15%. The phenotypic combinations are complex; generally, C4A expresses Rg, and C4B has Ch, but reverse antigenicities have been established both by HAI and by sequence data of selected C4 allotypes. A study of 325 families provides data on the antigenic expression of each C4 allotype and demonstrates strong associations. A structural model for the antigenic determinants of C4 proteins has been proposed and is completely supported by the family material. Of the 16 possible antigenic combinations for C4 proteins, only 3 are undetected. A new Ch combination has been recorded in two French families. The reported sequence variation within the C4d region can account for the antigenic determinants but leaves the location of electrophoretic variation in C4 still unclear.

An immunoblotting technique for direct visualization of Chido and Rodgers reactivity on C4 variants after electrophoresis

An immunoblotting technique allows direct visualization of Chido and Rodgers antigenic determinants on intact C4 proteins. C4 molecules separated by electrophoresis are selectively transferred to nitrocellulose membranes saturated with goat antiserum to human C4. The membranes are then incubated in alloanti-Chido or anti-Rodgers followed by enzyme-conjugated goat antihuman IgG. Molecules with Chido or Rodgers reactivity are visualized by incubation with an indicator substrate for the bound enzyme.

Restriction fragment analysis of duplication of the fourth component of complement (C4A)

The two genes encoding the fourth component of complement (C4A and C4B) reside between HLA-B and HLA-DR on human chromosome 6. Two kilobases downstream from each C4 gene lies a 21-hydroxylase gene (CA21HA and CA21HB, respectively). Utilizing the method of Southern blotting and a 5'-end 2.4-kb BamHI/KpnI fragment of the C4 cDNA, we have analyzed TaqI-digested DNA from four pedigrees with one or more extended haplotypes containing a C4A duplication, as demonstrated by protein electrophoresis and segregation analysis. Two C4A protein duplications (C4A*2,A*3,C4B*QO and C4A*3,A*5,C4B*QO) segregated with two large TaqI DNA restriction fragments (7.0 and 6.0). In pedigree Fi, one individual homozygous for HLA-A3,B35,C4,DR1,DQ1,BFF,C2C,-C4A2,3,C4BQO had TaqI 7.0- and 6.0-kb restriction fragments with equal hybridization intensities as measured by two-dimensional densitometry (7.0/6.0 kb = 0.83, SD = 0.12, N = 7). A hybridization probe for the 21-hydroxylase gene also demonstrated equal gene dosage (CA21HA/CA21HB = 1.01). DNA from another individual (Ma I-2) with a different C4A gene duplication (C4A*3,A*5,C4B*QO) also had equal densitometry measurements (7.0/6.0 kb = 1.07). We conclude that two extended haplotypes from unrelated pedigrees have two C4 genes and both C4 genes encode separate C4A alleles. These findings are compatible with a gene conversion event of C4B to C4A.

The study of a French family with two duplicated C4A haplotypes

The finding of two duplicated C4A haplotypes in a normal French family led to a detailed study of their C4 polymorphism. The father had an extremely rare A*6A*11, B*QO haplotype inherited by all of his children and the mother had the more common A*3A*2, B*QO haplotype. Two HLA identical daughters only have four C4A alleles. The father's A11 allotype expresses Ch:1 (Chido) rather than Rg:1 (Rodgers) and represents a new Ch phenotype Ch:1,-2,-3,-4,-5,-6. In order to clarify the genetic background in this unusual family, DNA studies of restriction fragment length polymorphisms (RFLPs) were undertaken. The father's rare haplotype, which expresses two C4A allotypes, results from a long and a short C4 gene normally associated with the A*6, B*1 that also exhibits the Bg/II RFLP. As it travels in an extended MHC haplotype HLA A2, B57(17), C2*C, BF*S, DR7 that is most frequently associated with A*6, B*1, we postulate that the short C4B has been converted in the alpha chain region to a C4A gene which produces a C4A protein. This report of a short C4A gene is the first example in the complex polymorphism of C4.

Definitive RFLPs to distinguish between the human complement C4A/C4B isotypes and the major Rodgers/Chido determinants: application to the study of C4 null alleles

Definitive restriction fragment length polymorphisms (RFLPs) representing the exact locations responsible for isotypicity between the human complement components C4A and C4B, and their generally associated major Rodgers (Rg1) and Chido (Ch1) antigenic determinants, have been designed. By means of C4d-specific genomic probe for Southern blot analysis, a C4A gene can be defined by the presence of the 276 bp and 191 bp N1a IV fragments, while a C4B gene can be defined by a single 467 bp N1aIV fragment. In addition, an Rg1-expressing C4 gene can be represented by a 565 bp EcoO 109 fragment, and a Ch1-expressing C4 gene by a 458 bp EcoO 109 fragment, under the same conditions. All these polymorphic restriction fragments can be unambiguously and conveniently detected. In combination with the Taq I polymorphic patterns specific for the C4 loci and for the neighboring 21-hydroxylase genes, the nature and structure of the tandem C4,21-hydroxylase gene complex can be elucidated. In this study, it is inferred that the null allele of the HLA haplotype B44 DR6 C4A3 C4BQO is not a C4B allele, but probably encodes another C4A 3 allotype at the second C4 locus.