Gene and haplotype frequencies of the fourth component of complement (C4) in type 1 diabetics and normal controls

Low HERV-K(C4) copy number is associated with type 1 diabetes

Complement component C4 (C4) is a highly variable complement pathway gene situated ∼500 kb from DRB1 and DQB1, the genes most strongly associated with many autoimmune diseases. Variations in C4 copy number (CN), length, and isotype create a highly diverse gene cluster in which insertion of an endogenous retrovirus in the ninth intron of C4, termed HERV-K(C4), is a notable component. We investigated the relationship between C4 variation/CN and type 1 diabetes. We found that individuals with type 1 diabetes have significantly fewer copies of HERV-K(C4) and that this effect is not solely due to linkage with known major histocompatibility complex class II susceptibility alleles. We show that HERV-K(C4) is a novel marker of type 1 diabetes that accounts for the disease association previously attributed to some key HLA-DQB1 alleles, raising the possibility that this retroviral insertion element contributes to functional protection against type 1 diabetes.

Defective prevention of immune precipitation in autoimmune diseases is independent of C4A*Q0

Increased prevalence of C4 null alleles is a common feature of autoimmune diseases. We have shown previously that complement-dependent prevention of immune precipitation (PIP) is defective in patients with systemic lupus erythematosus (SLE), and correlated this defect with C4A*Q0 and low levels of the C4A isotype. To further clarify the role of C4A in the aetiology of SLE, we now extend our studies to other diseases which have been associated with C4A*Q0. The frequency of C4A*Q0 was increased in Icelandic patients with coeliac disease (0.50; P < 0.001), Grave's disease (0.30; P = 0.002) and insulin-dependent diabetes mellitus (0.23; P = 0.04) and in British patients with dermatitis herpetiformis (0.42; P = 0.002) and this was reflected in low levels of C4A. In spite of this, PIP was normal in these patients, and in marked contrast to our previous observations on connective tissue diseases, PIP measurements in these patient groups correlated more strongly with levels of C4B (r = 0.51, P = 0.0000004) than C4A. Patients with increased levels of anti-C1q antibodies had significantly lower PIP than patients without such antibodies (P < 0.01) and a negative association of PIP with anti-C1q antibodies was also reflected in an increased prevalence (P = 0.006) and levels (P = 0.006) of anti-C1q antibodies in patients with subnormal PIP, as well as a negative correlation between PIP and anti-C1q antibodies (r = - 0.25, P = 0.02). These results show that the PIP defect cannot be explained by low levels of C4A alone and suggest that measurements of anti-C1q antibodies may be useful in future studies on the molecular cause of the PIP defect in autoimmune connective tissue disease.

C4 null alleles in a Swedish population

The distribution of C4 null alleles (C4Q0) was studied in a Swedish population comprising 410 male individuals from Stockholm County and Uppsala City. The prevalances of homozygous C4A*Q0 and C4B*Q0 determined by analysing the gene products in serum were 5.0 and 5.6%, respectively, in the Stockholm population and 4.0% each in the Uppsala population. These values are higher than those previously reported from other Caucasian populations. The finding reflects a different genetic composition of the Swedish population and may have relevance to the disease spectrum in this population as compared to other Caucasian populations.

Wegener's granulomatosis and Henoch-Schönlein purpura in a family with hereditary C4 deficiency

Described here is a family in which the father with partial C4 deficiency had Wegener's granulomatosis and a son with complete C4 deficiency presented with Henoch-Schönlein Purpura. The possible role of C4 deficiency in the pathogenesis of both vasculitic disorders is discussed.

A marked drop in the incidence of the null allele of the B gene of the fourth component of complement (C4B*Q0) in elderly subjects: C4B*Q0 as a probable negative selection factor for survival

The incidence of allotypes of the genes of the fourth component (C4) and factor B of the complement system was compared in 252 persons under 45 years of age ("young" group) with 482 people between 61 and 90 years of age ("old" group). One hundred people older than 90 years of age (nonagenarians) were also investigated. A striking difference was found between the "young" and "old" groups in the incidence (16.1% and 5.4%, respectively) of a silent gene of the C4B allele (C4B*Q0). This difference was even more marked among "young" and "old" men (17.6% vs 3.4%). The incidence of the C4B*Q0 allele in women dropped to the level of the men only in the nonagenarian group. The most probable explanation for this finding is that people carrying the C4B*Q0 allele die from as yet unidentified disease(s) in their middle-age. Therefore, male (and to a lesser extent female) carriers of this allele may have a considerably shorter life expectancy than individuals without a silent gene in the C4B locus.

Regional variation in C4 phenotype in patients with IgA nephropathy

The relationship between complete deficiency for either isotype of the fourth component of complement, C4A or C4B, and glomerulonephritis was initially examined in white patients from Kentucky with either IgA nephropathy or Schönlein-Henoch purpura. Subsequently, C4B deficiency was found to be associated with IgA nephropathy for pediatric patients followed in Cincinnati, Ohio. We later reported that at least 60% of the original patients from Kentucky were related to at least one other patient with the disease. This finding raised the possibility that the C4 phenotype frequencies for these patients may have been biased by the fact that they were based on a sample of related patients. In our study, C4 phenotyping was performed for 52 related and 63 unrelated patients from Kentucky, 81 unrelated patients from the Mid-South region of the United States, and 39 unrelated patients from the Puglia region of southeastern Italy. In addition, data from patients with IgA nephropathy from Spain were available for comparative studies. Neither C4A deficiency nor C4B deficiency was significantly increased for groups of unrelated patients from the Mid-South, Italy, Spain, or Kentucky in comparison with regional control subjects. In fact, C4A and C4B deficiencies did not occur in any of the Italian patients. With the exception of C4A.6, frequencies for the most common C4A and C4B alleles did not differ among the unrelated patient and control groups from Kentucky and the Mid-South. In addition, no significant differences in C4A and C4B allelic frequencies were observed in comparisons of pediatric patients (diagnostic biopsy before age 18 years) and adult patients with IgA nephropathy in either U.S. population. Italian control subjects had a significantly lower frequency for C4A null alleles in comparison with control subjects from both Kentucky and the Mid-South; a significantly higher frequency of C4B null alleles was found among Kentucky control subjects than in Mid-South, Italian, and Spanish control samples. The importance of recognizing the ethnic background of study subjects and of eliciting a good family history to minimize unsuspected sampling of related patients should be considered in future disease association studies.

C4B deficiency in two siblings with IgA nephropathy

The development of IgA nephropathy in first degree relatives is a well-described, yet relatively uncommon, occurrence. The association of a C4 isotype deficiency or partial deficiency of another complement protein has been previously documented for patients with IgA nephropathy. The present report describes a family in which two siblings and their father had biopsy-confirmed IgA nephropathy; both siblings were deficient for the C4B isotype. In addition, one of the biopsied siblings and a third sibling with microscopic hematuria but no renal biopsy apparently had a partial deficiency of the complement regulatory protein, I. The findings in this family may be compatible with the hypothesis that a C4 isotype deficiency and/or a partial deficiency of an individual complement protein allows clinical expression of IgA nephropathy in an individual with a genetic susceptibility for the disease.

Extended HLA haplotypes in families with insulin-dependent diabetes mellitus in northern Finland

Haplotypes including HLA, Bf and C4 loci were analyzed in a material comprising 55 families with diabetic children. One hundred and ten haplotypes found in IDDM patients were compared with 101 haplotypes present only in healthy family members. Two complotypes, BfSC4A3B3 and SC4A0B1, were significantly more common (P less than 0.05) in the diabetic haplotypes, and these were in most cases found in haplotypic combinations with HLA-B15,Dw4,DR4 and HLA-B8,Dw3,DR3 genes, respectively. The B8/DR3 haplotype was better conserved, as 72% included the BfSC4A0B1 complotype as compared with only 35% of the B15/DR4 haplotypes with "high risk" C4A3B3 complement alleles (p less than 0.05). DR3 was found in 26% of the diabetic haplotypes and DR4 in 43%. DR4 associated with the Dw4 in 69% of cases and with Dw14 in 26% of the diabetic haplotypes. Our results confirm that the two phenotypes found earlier to be associated with IDDM in Northern Finland, e.g. "B15,BfS,C4A3B3,Dw4,DR4" and "B8,Bfs,C4A0B1,Dw3,DR3" are inherited as haplotypes.

Allelic variants for complement factors C3, C4, and B in acute necrotizing ulcerative gingivitis

Impaired immune defense mechanisms and genetic factors appear to play a role in susceptibility to acute necrotizing ulcerative gingivitis (ANUG). Therefore, possible etiological mechanisms might involve genes at the Major Histocompatibility Complex, which include the complement factor loci. We have tested for a possible association between certain complement factor alleles and ANUG using a case-control study design. Specific alleles at complement factors C3 and C4, and properdin factor B (Bf) loci were determined indirectly by high voltage agarose gel electrophoresis in 58 subjects with a history of ANUG and in 58 age-sex-matched healthy controls. The highest relative risk of ANUG, as obtained by conditional logistic regression, for alleles at the C3 locus was 1.9 (90% confidence limits 0.8 to 4.8; p = 0.229) for C3*F-positive individuals. The highest relative risk for alleles at the C4 locus was 2.6 (0.5 to 14.9; p = 0.358) for C4A*3-positive individuals. There was no evidence for an association between Bf allotype and risk of ANUG, with a relative risk of 1.2 for Bf*F- and relative risk of 1.0 for B*S-positive individuals. None of our estimates was statistically significant. We conclude, therefore, that it is unlikely that there is any association between complement factor gene haplotype and susceptibility to ANUG.

HLA antigens and complotypes in insulin-dependent diabetes mellitus

One hundred and thirty-six Finnish patients with insulin-dependent (type I) diabetes mellitus were investigated for the HLA-A, B, D and DR antigens as well as the Bf and C4 allotypes. The statistically significant increase in the frequencies of HLA-A9, B8, B15, Dw3, Dw4, DR3, DR4, C4A0 and C4B3 was observed when compared with the healthy controls. About 79% of the patients had HLA-DR4, and 53% had HLA-DR3 antigens. A rare C4 allele C4B3 was found in 21% of the patients, whereas only in 2% among the controls (relative risk 16.35). The etiological fraction (EF) values indicated that HLA D/DR alleles were the best markers for IDDM, the observed EF for HLA-DR4 in diabetes was as high as 0.70. Examination of HLA, Bf and C4 phenotypes suggested that at least two supratypes "B15 BfS C4A3B3 D(R)4" and "B8 BfS C4A0B1 D(R)3" were markers for the susceptibility to type I diabetes, one third of our patients had either of these supratypes. The protective role of DR2 and Dw2 antigens was also confirmed: no HLA-Dw2 positive patients and only one with HLA-DR2 was found.