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

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.

Molecular analysis of complement component C4 gene copy number

Classical, alternative, or lectin pathways may activate the complement system cascade. The classical pathway includes the C4 protein and functions in the prevention of immune complex precipitation and in clearance of immune complexes.Two isotypes of C4-C4A and C4B-are coded by genes located at two loci within the major histocompatibility complex (MHC) on chromosome 6. While these isotypes share over 99% amino acid sequence homology, five nucleotide differences located in exon 26 are responsible for major structural and functional differences between the C4 isotypes.C4A and C4B are highly polymorphic with over 40 alleles, gene duplications, and "null alleles". C4 genes may be short (14.6 kb) or long (21 kb), due to the absence or presence of an endogenous retroviral sequence-HERV-K(C4)-in intron 9, respectively. The C4 gene copy number (GCN) can vary from 1-3 per haplotype or 2-6 per diploid genome. The variation in GCN leads to a range of C4 plasma protein concentrations among healthy subjects. In subjects with equal numbers of C4 genes, subjects with short genes have C4 plasma levels relatively higher than subjects with long genes.Variation of the C4 GCN, the gene size (long or short) and the C4 isotypes (C4A and C4B) may also lead to susceptibility to autoimmune disease. Therefore, in subjects with autoimmune disease, a low serum C4 level may be due to ongoing disease activity associated with complement activation and consumption or it may be due to genetic factors. Distinguishing between these will have clinical implications.Exact determination of GCN can be difficult, at least in part due to the high degree of homology between C4A and C4B and a variety of techniques has been described. This chapter describes a quantitative TaqMan real-time PCR (qPCR) copy number assay, based on our laboratory experience using this assay.

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.

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.

Characterization of non-expressed C4 genes in a case of complete C4 deficiency: identification of a novel point mutation leading to a premature stop codon

The genetic basis of complete C4 deficiency in a patient with SLE was investigated. Previous studies have demonstrated that this patient has two different major histocompatibility complex (MHC) haplotypes that each contain a major deletion and a non-expressed C4 gene. In the present study, non-expression of the C4 genes was explained by the finding of two distinct C4 gene mutations. A previously described two base pair insertion in exon 29 of the C4 gene was detected in the paternal MHC haplotype [HLA-A2, B40, SC00, DR6]. The maternal haplotype [HLA-A30, B18, F1C00, DR3] carried a C4 gene with a one base pair deletion in exon 20 generating a premature stop codon. This mutation was neither found in 10 individuals with known non-expressed C4 genes nor in 9 individuals homozygous for the complotype F1C30. The isotype and allotype specific regions of the patient's C4 genes were sequenced, and both contained C4A3a sequence. In conclusion, two different MHC haplotypes resembling the extended haplotypes [HLA-A2, B40, SC02, DR6] and [HLA-A30, B18, F1C30, DR3] both contained a non-expressed C4A gene that was due to either of two distinct mutations, demonstrating the heterogeneous genetic background of C4 deficiency.