Polymorphism of the second component of human complement (C2). Observation of the rare phenotype (C2 2 (= C2 B) and data on the localization of the C2 locus in the HLA region

Immunofixation for C2 typing: C2 allotypes in Spaniards in relation to HLA, Bf and C4

C2 typing is performed by immunofixation with anti-C2 antiserum instead of by a hemolytic overlay. This method gives sharp band definition, is less cumbersome than the hemolytic overlay, gel files are easily made, and it also enables one to describe putative new nonhemolytic variants. C2 allele frequencies were studied in a sample of the normal Spanish population and were found to be similar to other Caucasoids. HLA-Bw62,-Cw3, and -DR4 were significantly associated with C2 B. Concordantly, the only C2*B extended HLA haplotype found in family material was Bw62-Cw3-Bw6-(DR4)-Bf*S-C2*B-C4A*3 B*2-(GLO*1). C4A*4 B*2 and C4A*4 B*4 are not found within the same haplotype together with C2*B and Bw62 or Bw22 respectively, nor do other C2*B haplotypes occur with common HLA-B alleles. These results may favour the hypothesis that the Bw62-C2*B haplotype is produced by one mutation arising in the Bw62-C2*C haplotype and that subsequent crossovers can explain other C2*B haplotypes (including Bw22-C2*B).

Another family with a silent allele of properdin factor B polymorphism (BF QO)

In five of eight members of a three generation family the existence of a silent allele of the properdin factor B polymorphism (BF QO) was indicated by immunofixation of BF electrophoretic variants and by the hemolytic overlay after isoelectric focusing of BF allotypes. This was further supported by the results of HLA-A, B, C, DR, C2, C4A, C4B, GLO-typing. BF protein was decreased in all heterozygous BF deficient family members. The absolute hemolytic activity, however, was obviously compensated for by an increased relative functional activity of the normal S or F alleles on the other chromosome.

Polymorphism of human C2 detected by immunoblotting

To allotype human complement component C2, thin layer agarose gel isoelectric focusing of human serum and/or EDTA-plasma was performed followed by direct immunofixation or by immunoblotting with a specific anti-human-C2 antibody. Using reference samples for C2 BC phenotypes and local samples from an HLA, C4, and Bf genotyped family, a differentiation of the C2*B and C2*C variants segregating with the respective HLA haplotype was achieved. The C2 BC phenotype is characterized by a double banding pattern similar to that observed in the haemolytic overlay assay usually used for the detection of C2 polymorphism. An homozygous C2*Q0 reference sample determined by functional assays was shown to be biochemically deficient, as demonstrated by immunofixation and immunoblotting. The visual interpretation of C2 phenotypes was definitely easier after immunofixation and immunoblotting than after an haemolytic overlay assay. In addition, the method for C2 allotyping described here has several advantages, in particular it saves time and tolerates repeated thawing and freezing of the samples.

HLA, complement C2, C4, properdin factor B and glyoxalase types in South Indian diabetics

A series of diabetic patients from 3 centres in South India have been tested for HLA A, HLA B, BF, C2, C4A, C4B and GLO types. For insulin-dependent diabetes mellitus (IDDM) patients there was a significant increase in HLA B8, of BF F and decrease of C4 A6. No significant variation in HLA, BF, C2 or GLO frequencies was found in non-insulin-dependent diabetes mellitus (NIDDM) patients, but there was a significant decrease in C4B 1 and an increase in C4B 2. The HLA and BF association in South Indian IDDM patients is very different from that reported previously in North India.

Study of alleles of the second complement component (C2) on Canadian HLA haplotypes

Typing for genetic variation in the second complement component C2 was performed on sera from HLA haplotyped Canadian families. Part of the data has been studied and analysed as a population; in addition there is a further random collection of haplotypes bearing the C2*2 allele. In the population data there were 444 separate haplotypes from unrelated parents or other founders: 4.7% of the haplotypes carried the uncommon allele C2*2; one haplotype carried the rare C2*3. Study of C2 2-1 heterozygotes in the population data revealed 59 haplotypes which carried the common C2*1 allele and one which carried a deficiency allele C2*0. The remaining haplotypes carried either C2*1 or else an undetectable C2*0 allele. In the entire data there were 281 meioses informative for C2. The only recombinant between HLA-B and C2 showed the C2 locus to be on the DR side of the B locus. Strong allelic association between C2 *2 and Bw22 and less strong association between C2 *2 and B15 suggested the possibility of two ancestral C2 mutants. Examination of other markers on these and subsequently collected haplotypes do not conflict with this idea since the B15 haplotypes mostly carry C4A *4, C4B *2 whilst the Bw22 haplotypes mostly carry C4A *4, C4B *4. The alternative idea, that there was one original mutant which crossed over from a B15 to a Bw22 haplotype or vice versa is not excluded, however. Since approximate equilibrium has been reached between Bw22 and the HLA-A locus alleles on these C2 *2 bearing haplotypes, we conclude that this mutation is at least 5000 years old. Other haplotypes carrying C2 *2 are assumed to be ancestral recombinants; if this is true, the C2 locus map position between HLA-B and HLA-DR is confirmed. Study of C2 mutation may provide a model for understanding the genetics of some disease susceptibility genes in the HLA region.

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.

The BF locus and HLA: rare alleles coding for functionally active and inactive factor-B products

The genetic polymorphism of properdin factor B (BF) was studied in different populations. The rarer alleles, BF*F1 and BF*S1, occurred in Caucasians, were less frequent in North American blacks, and were not demonstrated in any of three Oriental populations studied. Two further alleles of BF, termed BF*FM and BF*SM, were found to exist in these populations. The BF*FM allele, which was found only in Caucasians, codes for a functionally inactive factor-B product, whereas the BF*SM allele (found in a single Chinese individual), like other alleles of BF, codes for a functionally active product. HLA haplotype analyses in individuals carrying the rarer alleles of BF revealed not only a strong association between BF*F1 and HLA-B18 and BF*S1 and HLA-Bw50 but an even stronger association between these BF alleles and alleles of the two C4 loci. BF*F1 occurred most frequently on a C4A*3,B*Q0 haplotype, whereas the BF*S1 allele was usually found on a C4A*2,B*1/B*Q0 haplotype. HLA haplotypes carrying the BF*FM and BF*SM alleles all carried the more common C*4A3,B*1 haplotype.

HLA-linked genetic markers in Chinese and other Oriental populations

The polymorphic variants of the HLA-linked genetic markers Bf, C2, C4 and GLO-I were studied in three mongoloid populations. Analysis of linkage dis-equilibrium between these markers and HLA-A, B, C and DR antigens was carried out on test results from 140 unrelated Chinese individuals. The phenotypes BfS and GLO-2 were found at significantly higher frequencies than in Caucasians. BfS was associated with B12 in Japanese but not in Chinese. A single individual with the rare Bf variant S1 was found. No C2 deficient individuals were observed. The C2C (common) allele occurred at a gene frequency of 0.949 and the more basic allele C2B at 0.039. The acidic variant, C2A, was observed at a frequency of 0.011 and appeared to be associated with BfF. Eighty-nine per cent of the Chinese were phenotypically C4FS. In contrast to Bf and C2, each of which is coded for by codominant alleles at a single genetic locus, C4 is coded for by two genes, C4F (Rodgers) and C4S (Chido). The C4F locus allele, C4F1 (extra fast), was strongly associated with HLA-B17, as has been found in other populations, but a new association of the C4S locus deficiency allele, C4so (Ch-), with B17 was also observed. All HLA-B17;C4so haplotypes were BfS position. As has been previously found in Caucasian populations, individuals of the C4F phenotype (i.e. genotypically FsoFso) were all found to be Chido negative. The frequencies of the various HLA-linked genetic markers, however, as much as the frequencies and associations of the HLA antigens themselves, distinguish these populations from other ethnic groups.

Genetic polymorphism of the complement C2 in Japanese

Genetic polymorphism of the second component of human complement (C2) was investigated in 521 unrelated healthy adult Japanese using isoelectric focusing in polyacrylamide gel followed by a specific hemolytic overlay method. Besides the phenotypes reported previously (C, AC and BC), a relatively infrequent double-banded phenotype (tentatively named A'C) was observed. Moreover, a homozygous variant (A) and a heterozygous "double" variant (AB) were observed. The estimated frequencies for the common allele, C2c(= C2(1)), and the variant alleles, C2A, C2B(= C2(2)) and C2A' were 0.939, 0.034, 0.022, and 0.006, respectively. The results of further typing for HLA-A, -B, -C specificities indicated the presence of significant associations of C2A with HLA-B15 and with A26, and of C2B with HLA-Bw61. These findings support our previous observation that in Japanese there are allelic combinations showing linkage disequilibrium between C2 and HLA loci which are different from those in Caucasians, and that the C2 structural locus is more closely linked to HLA-B than to HLA-A. C2 hemolytic activities of each phenotypes were assayed. The mean activity of type AC sera was significantly higher than that of type C or type BC, while there were no differences in the activities among the types C, BC or A'C. Also presented are two pedigrees demonstrating the segregation of C2 with HLA alleles in which a homozygous C2 A or C2 B individual was observed.

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.