Phenotypic genetics of complement components

Complement

The complement system mediates a wide range of important biological functions. The use of modern techniques in protein chemistry and molecular biology has greatly facilitated our understanding of the interactions between the fluid phase and cell-bound components of the system. Structural and genetic analysis has shown that while many of these components are polymorphic, there are major similarities between many of the proteins serving enzymatic and regulatory roles in both the alternative and classical pathways. The regulation of complement activation and Class III genes, on chromosomes 1 and 6 respectively, encode nine of the major proteins in the system. The genetic basis of C4 and C3 polymorphisms is now well established, and further study may reveal functional differences between polymorphic variants of other components. The study of individuals with either genetic or acquired deficiencies of complement proteins and receptors has provided insight into the function of these components, leukocyte adherence deficiency (LAD) providing the best example. An appreciation of the genetics, structure and functions of the regulatory proteins decay-accelerating factor (DAF) and homologous restriction factor has enhanced our understanding of the pathogenesis of paroxysmal nocturnal haemoglobinuria. The full importance of CD59 glycoprotein, the newest member of the complement family, remains to be determined.

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 C8A and C8B loci are closely linked on chromosome 1

Close linkage was demonstrated between the loci governing the polymorphisms of complement component C8 alpha-gamma (C8A) and beta (C8B). Both C8 loci were linked to the chromosome 1 marker loci PGM1 and Rh. The distance between the two C8 loci and PGM1 appeared identical in males and females. A female/male ratio of 1.6 was observed between the two C8 loci and Rh. No evidence for linkage between the C8 loci and Fy was found. Preliminary results of this study were presented at the Eighth International Workshop on Human Gene Mapping, Helsinki, August 1985 (Rogde et al. 1985b).

The molecular genetics of components of complement

Rapid progress has been made in establishing linkages and in chromosome allocation of the genes of some 9 complement components. In the MHC, C2, Factor B, and two C4 or C4 related genes have been placed in some detail in both man and mouse. The gene coding for the cytochrome P-450 21-hydroxylase has been shown to be duplicated and immediately 3' to the two C4 genes, though it appears to be functionally and structurally unrelated to the complement components. Thus six genes have been mapped to this region where particular haplotypes are associated with increased susceptibility to a number of diseases, some of which are autoimmune in character. The complete gene structure of Factor B has been solved in man and rapid progress is being made with the C2 and C4 genes. The structural basis of the polymorphisms of these genes is being established. In C4, the polymorphism is exceptionally complex with varying numbers of loci and probably more than 50 allotypes occurring in man. A structural basis has also been found for the big differences in the biological activity of some of the C4 allotypes in man. Apart from the genes in the MHC, linkage has been found between the genes coding for C4bp, CR1, and Factor H. Remarkably there are sequence homologies between these proteins and C2 and Factor B, probably related to the ability to bind to one or other of the structurally similar proteins C3b and C4b. The complete cDNA sequences of C3 and C4 in mouse and man have given much information on the many posttranslational modifications of these proteins. A partial structure has been obtained for the C3 gene and the homology shown between C3, C4, C5, alpha 2-macroglobulin, and pregnancy zone protein. Although the amount of detailed information in the molecular genetics of complement components is accumulating rapidly, there appears to be a reasonable prospect that linkages and homologies will classify the data into a comprehensible form.