Serum concentration and functional efficiency of factor B alleles

Functional basis of protection against age-related macular degeneration conferred by a common polymorphism in complement factor B

Mutations and polymorphisms in complement genes have been linked with numerous rare and prevalent disorders, implicating dysregulation of complement in pathogenesis. The 3 common alleles of factor B (fB) encode Arg (fB(32R)), Gln (fB(32Q)), or Trp (fB(32W)) at position 32 in the Ba domain. The fB(32Q) allele is protective for age-related macular degeneration, the commonest cause of blindness in developed countries. Factor B variants were purified from plasma of homozygous individuals and were tested in hemolysis assays. The protective variant fB(32Q) had decreased activity compared with fB(32R). Biacore comparison revealed markedly different proenzyme formation; fB(32R) bound C3b with 4-fold higher affinity, and formation of activated convertase was enhanced. Binding and functional differences were confirmed with recombinant fB(32R) and fB(32Q); an intermediate affinity was revealed for fB(32W). To confirm contribution of Ba to binding, affinity of Ba for C3b was determined. Ba-fB(32R) had 3-fold higher affinity compared with Ba-fB(32Q). We demonstrate that the disease-protective effect of fB(32Q) is consequent on decreased potential to form convertase and amplify complement activation. Knowledge of the functional consequences of polymorphisms in complement activators and regulators will aid disease prediction and inform targeting of diagnostics and therapeutics.

Does complement factor B have a role in the pathogenesis of atypical HUS?

Atypical haemolytic uraemic syndrome (aHUS) is a disorder of complement dysregulation. Because complement factor B (fB) carries the catalytic site of the alternative pathway convertase we examined it as both a potential candidate gene and modifier in the pathogenesis of aHUS. No factor B gene (BF) mutations were found in 20 patients with aHUS. There was no statistical difference between controls and aHUS patients in either BF allele or haplotype frequency. In conclusion, in this small series of aHUS patients we found no evidence that fB has a major role in the pathogenesis of aHUS.

Genetic regulation of the impaired immune response to hepatitis-B vaccine associated with low TCR density in end stage renal disease patients: contribution of complement C4 and factor B alleles

We have studied the relationship between T-cell receptor (TCR) density, genetic factors and the specific immune response in 153 end stage renal disease (ESRD) patients on haemodialysis immunised with HBsAg vaccine. One-hundred and nineteen patients raised a protective (> 10 U/ml) antibody response to hepatitis-B vaccination (responder, R), while 34 patients were found to be non-responders (NR). The density of the T-cell receptors was determined by flow cytometry. Proliferation of the T-cells induced by autologous monocytes presenting HBsAg was also measured and expressed as a stimulation index (SI). MHC class I, II and III alleles of the patients were also determined. The densities of TCR/CD3 receptors in NR patients were found to be significantly decreased as compared to the R patients (189 +/- 22 vs. 282 +/- 58 arbitrary units, P = 1.3 x 10(-7). TCR/CD3 receptor densities were found to be strongly associated (Spearman correlation coefficient: 0.84, P < 0.000001) with the SI values. Both parameters were found to be under dual genetic control: (a) very low density of the TCR/CD3 receptors and very low SI were found mainly in NR patients carrying HLA-A1, HLA-B8 and HLA-DR3 alleles; and (b) TCR/CD3 densities and function in R group were found to be significantly lower in carriers than in non-carriers of two MHC class III complement protein alleles: C4A*6, and Bf*F. Non-responsiveness to hepatitis-B vaccination was found to be associated with extremely increased neopterin levels. These findings indicate that both genetic and acquired factors contribute to the hepatitis-B vaccination failure in ESRD patients.

Human complement factor B: cDNA cloning, nucleotide sequencing, phenotypic conversion by site-directed mutagenesis and expression

A full-length cDNA clone, BHL4-1, encoding factor B was isolated from a human liver cDNA library and sequenced in its entirety. It consists of 2388 bp which include a 5'-untranslated region of 40 bp, a single open reading frame, 2292 bp in length, and a 3'-untranslated region of 56 bp followed by a poly-A tail. The deduced amino acid sequence comprises 25 residues of a putative leader peptide and 739 residues of the mature polypeptide chain of the F allele of factor B. We constructed an S allele-like Q7R mutant of BHL4-1 by site-directed mutagenesis. Both the wild-type and mutant factor B cDNA were expressed transiently in a eukaryotic system. The specific hemolytic activities of the two recombinant factor B alleles and of native B were not significantly different from each other.

Allelic differences in hemolytic activity and protein concentration of BF molecules are found in association with particular HLA haplotypes

After separating the *F and *S alleles by electrophoresis the allele-specific hemolytic activity was detected by agarose overlay method using the programmable densitometer for scanning. The hemolytic activity of BF allotypes was analyzed from 81 individuals. In thirteen FS heterozygous serum samples BF F had lower hemolysis than BF S. Four FF homozygous samples also exhibited lower hemolysis than a homozygous control sample. The low hemolytic activity of F in FS heterozygotes was not due to decreased protein concentrations relative to S. On the contrary, BF F was associated with higher protein concentration than BF S. The relative quantitation of the allele specific BF protein was done by crossed immunoelectrophoresis. BF F with low hemolytic activity but with high protein concentration associated strongly with HLA B35 phenotype and the family material confirmed the association with the haplotypes A3, Cw4, B35, DR1, BFFB, C4A3BQO (or A2BQO, A3,2BQO). The results suggest that particular MHC haplotypes contain a factor B allele with encoding for poor hemolytic activity or that MHC haplotype specific regulatory elements affect pre- or post-translational activity levels.

Two BF F subtypes, but no BF S, BF F1 or BF S07 subdivision, are found by isoelectric focusing

When studied by immunoelectrophoresis the factor B (BF)*F allele is a monomorphic protein, but by using isoelectric focusing (IEF), it turns out to be polymorphic. Two BF*F allelic subdivisions, BF*FA and BF*FB, were detected in samples from unrelated Spanish donors; *FA and *FB are in Hardy-Weinberg equilibrium, segregate as autosomal codominant alleles in families, and BF*FA (but not BF*FB) is found to be in linkage disequilibrium with B44 and DR7 and is within the A29, B44, Bw4, Cx, CFA31, DR7, DRw53, DQ2 haplotype. Furthermore, BF FA individuals have a higher BF serum concentration than the BF FB individuals. The subdivision of BF S observed with IEF was found to be due to nonspecific BF S cleavage by serum proteases in inadequately collected or aged samples. Thus, the subdivision of BF S is spurious and was not found in our sera. BF F1 and BF S07 could not be further subdivided by IEF in our subjects. The BF F1 banding pattern was characterised by the presence of a cathodal band which corresponds to the Bb activation fragment. Finally, IEF combined with immunoblotting and monoclonal Ba and Bb antibodies may be used for accurately distinguishing BF phenotypes and doubtful bands.

Factor B (BF) subtyping by isoelectric focusing: methods, nomenclatures, genetics and forensic application

Usually factor B (BF) typing is performed by means of the traditional agarose gel electrophoresis. Using isoelectric focusing, the system can be extended by two common subtypes of BF F. The existence of BF F subtypes has in the meantime been confirmed by various authors and in different populations. Their inheritance has been proven by family- and mother/child analyses and molecular-genetic studies (correlation with restriction fragment length polymorphism). Different typing methods as well as different nomenclatures seem to indicate that the subtypes FA and FB (according to Geserick et al.) are identical with the Fb and Fa subtypes (according to Teng and Tan). At present, some confusion still exists for the less frequent variants and subtypes which possibly could be identified by direct comparison of the patterns. The BF system is a valuable marker in paternity testing. Its chance for exclusion of paternity in Caucasian populations has been calculated to be about 14% for agarose gel electrophoresis and increases to about 16% for BF F subtyping by isoelectric focusing. Preliminary results indicate that BF may also be used for typing of bloodstains (up to 2 weeks old).

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.

HLA-linked complement polymorphisms (C2, BF) in psoriasis

Starting from the known association between psoriasis and several HLA antigens and from the fact that the HLA chromosomal region contains the structural genes for at least three complement components, the authors have looked for an association between psoriasis and allotypes of C2 and BF. C2 and BF polymorphism were examined in 230 psoriatic patients. Two rare complement genes were found to be significantly increased when compared with controls: the frequency of the C2*2 gene was 0.061 among patients and 0.035 among controls (P less than 0.05); for BF*SO7, the frequencies were 0.0304 in patients and 0.0092 in controls (P less than 0.0005). The BF*F gene frequency, however, was significantly decreased among patients: 0.1196 vs. 0.1743 (P less than 0.01). The relative risks were 1.79 for the C2 2, 3.44 for the BF SO7, and 0.6 for the BF F gene product. From previous studies, it is known that these three complement alleles (C2*2, BF*SO7, BF*F) are in linkage disequilibrium with HLA alleles that have also been found increased or decreased, respectively, in psoriasis.

Pigeon breeder's lung: association with HLA-DR 3

52 symptomatic (SPB) and 64 asymptomatic (APB) pigeon breeders were investigated for the HLA-A, -B, -C, -DR; C2, C3, C4 and Bf systems, and C3, C4 and factor B serum concentrations. HLA-DR3 and Bf S frequencies were significantly higher in the SPB than the APB group. Mean factor B concentrations were lower in the SPB than in the APB group. A positive two-way association between HLA-DR3 and the disease was found. It is concluded that a gene or genes responsible for type III and IV allergic reactions leading to the disease is associated with HLA-DR3. The increase of Bf S allotype and low mean factor B concentrations, however, can be explained by the strong linkage disequilibrium between BfS and HLA-DR3.

Isoelectrofocusing in the Study of the Bf System: Existence of Two Common Subtypes of the BfFAllele

The inherited polymorphism of the properdin factor B (Bf system) was studied by isoelectrofocusing in polyacrylamide gel (PAGIF, pH range 5.0-8.0), followed by immunofixation. Using the described technique, two different subtypes of F bands are visible (designated as F' and F"). Studies of 13 families and 48 mother-child pairs are not in contradiction to the assumption of a codominant inheritance by two alleles (BfF' and BfF") at a single locus.

The HLAB*18,BF*F1 in haplotype in type 1 diabetes

We have investigated the properdin factor B (BF) locus as a possible marker for Type 1 diabetes. The frequencies of the four BF alleles were determined among 70 Type 1 diabetics with onset before age 18 years. The frequency of BF F1 among the diabetics was increased compared with 206 controls (15.7 vs 3.4%). The relative risk (RR) associated with BF F1 was 5.30. The suggestion that BF is most strongly associated with onset of diabetes under 10 years of age was not supported. HBLA 18 was increased (RR = 4.91) and B7 was decreased in frequency (RR = 0.41) among the 62 diabetics tested compared with 238 controls. An association was observed between HLAB 18 and BF F1 among the diabetics. A study of 12 families in which BF*F1 was segregating suggested that BF*F1 and HLAB*18 are components of a haplotype associated with Type 1 diabetes in this population and that both haplotypes of an affected parent may contribute to the development of Type 1 diabetes.

On the significance of C2, C4, and factor B polymorphisms in disease

In this review article, recent evidence is presented that some diseases like insulin-dependent diabetes mellitus, multiple sclerosis, and idiopathic membranous nephropathy, which are primarily associated with HLA-D,DR, are also related to the rare C2, C4, and Factor B alleles. Circumstantial evidence is available that at least some of these rare variants may be functionally deficient. Based on the concept of functionally interacting gene clusters, mutant complement genes may lead to impaired effector mechanisms in virus neutralization or lysis of virus-infected cells. Other mechanisms such as alteration of vascular permeability may be involved in the development of proliferative retinopathy and familial hypertension. In lepromatous lepra, an impaired cell-mediated lysis of M. leprae may be related to the hemolytically inactive C4F1 allelic product.