Null alleles of human complement C4. Evidence for pseudogenes at the C4A locus and for gene conversion at the C4B locus.

Abstract

C4 is the only component of the human complement system that is coded for by two genes, C4A and C4B, showing 99% homology. The genes for the two C4 isotypes are located with the genes for the second component (C2), factor B (BF), and steroid 21-hydroxylase (21-OHA and 21-OHB) between HLA-B and -DR in the MHC on chromosome six (1-4). The C4 and 21-OH genes are tandemly arranged and have probably arisen by duplication (Fig . 1 a). Based on the direction of transcription, C4A is usually expressed at C4 locus I, whereas C4B is usually expressed at locus II (5).C4A and C4B are highly polymorphic with more than 35 alleles including null alleles (C4QO) at both loci (6). The polymorphism can be defined by electrophoretic mobility ofthe intact protein or its subunits (7-9), by serology of Rodgers (Rg) and Chido (Ch) determinants (10), and by functional studies of complement activation and binding characteristics (11,12). A sequence of four amino acids in the C4d region (Chido 4 determinant on C4B molecules) is responsible for the major structural and functional differences of the C4 isotypes (Fig. 1 c; 9, 11-13) . The antigenic determinants Rodgers and Chido are generally expressed on the C4A and C4B isotypes, respectively, but rare reversed associations have been described (14). Gene conversion has been discussed as a possible mechanism for the generation of aberrant allotypes (15,16).Null alleles of C4A or C4B (AQO or BQO) occur at frequencies of 0.1-0 .3 in the normal population (17), and are assessed by the absence of gene products. The structural analysis of the C4 genes at the DNA level has revealed that only a proportion of C4 null alleles result from gene deletions affecting an entire C4 gene and one adjacent 21-OH gene, and consequently other null alleles were due to nonexpressed genes (pseudogenes) (18,19). Alternatively, it has been suggested that null alleles represent the expression of two identical, and therefore undistinguishable, allotypes on a haplotype, and this results from gene conversion of C4A to C4B and the reverse (5, 20).

C3 and C4 gene expression and interferon-gamma-mediated regulation in human glomerular mesangial cells

“…The C4. Nla IV digest yielded five clear fragments, of 367, 279, 203, 164 and 123 bp [32], These data prove that lhe sequences amplified are those of C3 and C4 (data not shown).…”

Association of Fcγ receptor IIIB, but not of Fcγ receptor IIA and IIIA, polymorphisms with systemic lupus erythematosus in Japanese

“…Primers C4-L3 (5′-TGCGGATCCAGCAGTTTCGGAAG-3′) and C4-L4 (5′-ATAGGATCCTAAGGTCCCCTGGGCCT-3′) were included in all reactions and were used to amplify an internal positive control PCR product, 926 bp fragment from C4 gene. 41 PCR conditions were as follows: 96°C for 10 min, and then 30 cycles of: 96°C for 1 min, 52°C for 30 sec, 72°C for 1 min, and finally 72°C for 5 min. The amplified fragments were analyzed on a 10% polyacrylamide gel with SYBR Gold staining.…”

The Molecular Basis of Complete Complement C4A and C4B Deficiencies in a Systemic Lupus Erythematosus Patient with HomozygousC4AandC4BMutant Genes

“…Oligonucleotides of human C4 and RCCX constituents were designed based on published DNA sequences (51)(52)(53) 1,3,4,5,6,8,9,10,11,12,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,31,32,33,34,35,37,38,39,40, and 41. The primers were designed based on the genomic DNA sequence of C4A3a (51).…”

“…The presence of a single C4A or C4B gene in one or both MHC haplotypes and/or the isoexpression of C4A proteins only (or C4B proteins only) from bimodular and trimodular RCCX structures constitute complete or partial deficiencies of either C4A or C4B. Close to 30% of the normal population have relatively lower levels of either C4A or C4B proteins caused by the presence of only one C4A gene or only one C4B gene in a diploid genome (5,19,20). Also, in HLA A2 Cw3 B60 DR6 haplotypes with bimodular LS structures for RCCX, the long C4A gene does not code for a protein product whereas the short C4B gene codes for C4B1 or C4B2 protein (21).…”