HLA and complement allotypes in Type 1 (insulin-dependent) diabetes

Genetic variants in the complement system and their potential link in the aetiology of type 1 diabetes

Type 1 diabetes is an autoimmune disease in which one's own immune system destroys insulin-secreting beta cells in the pancreas. This process results in life-long dependence on exogenous insulin for survival. Both genetic and environmental factors play a role in disease initiation, progression, and ultimate clinical diagnosis of type 1 diabetes. This review will provide background on the natural history of type 1 diabetes and the role of genetic factors involved in the complement system, as several recent studies have identified changes in levels of these proteins as the disease evolves from pre-clinical through to clinically apparent disease.

Role of complement in diabetes

Accumulating evidence suggests a role for the complement system in the pathogenesis of diabetes and the vascular complications that characterise this condition. Complement proteins contribute to the development of type 1 diabetes (T1D) by enhancing the underlying organ-specific autoimmune processes. Complement upregulation and activation is also an important feature of insulin resistance and the development of type 2 diabetes (T2D). Moreover, animal and human studies indicate that complement proteins are involved in the pathogenic mechanisms leading to diabetic microvascular and macrovascular complications. The adverse vascular effects of complement appear to be related to enhancement of the inflammatory process and the predisposition to a thrombotic environment, eventually leading to vascular occlusion. Complement proteins have been considered as therapeutic targets to prevent or treat vascular disease but studies have been mainly conducted in animal models, while human work has been both limited and inconclusive so far. Further studies are needed to understand the potential role of complement proteins as therapeutic targets for reversal of the pathological processes leading to T1D and T2D and for the prevention/treatment of diabetic vascular complications.

Complement gene variants in relation to autoantibodies to beta cell specific antigens and type 1 diabetes in the TEDDY Study

A total of 15 SNPs within complement genes and present on the ImmunoChip were analyzed in The Environmental Determinants of Diabetes in the Young (TEDDY) study. A total of 5474 subjects were followed from three months of age until islet autoimmunity (IA: n = 413) and the subsequent onset of type 1 diabetes (n = 115) for a median of 73 months (IQR 54-91). Three SNPs within ITGAM were nominally associated (p < 0.05) with IA: rs1143678 [Hazard ratio; HR 0.80; 95% CI 0.66-0.98; p = 0.032], rs1143683 [HR 0.80; 95% CI 0.65-0.98; p = 0.030] and rs4597342 [HR 1.16; 95% CI 1.01-1.32; p = 0.041]. When type 1 diabetes was the outcome, in DR3/4 subjects, there was nominal significance for two SNPs: rs17615 in CD21 [HR 1.52; 95% CI 1.05-2.20; p = 0.025] and rs4844573 in C4BPA [HR 0.63; 95% CI 0.43-0.92; p = 0.017]. Among DR4/4 subjects, rs2230199 in C3 was significantly associated [HR 3.20; 95% CI 1.75-5.85; p = 0.0002, uncorrected] a significance that withstood Bonferroni correction since it was less than 0.000833 (0.05/60) in the HLA-specific analyses. SNPs within the complement genes may contribute to IA, the first step to type 1 diabetes, with at least one SNP in C3 significantly associated with clinically diagnosed type 1 diabetes.

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.

Pathogenesis of insulin-dependent diabetes mellitus

Insulin-dependent diabetes mellitus is strongly associated with certain HLA types and the presence of islet cell-specific autoantibodies. The pathogenesis is a specific loss of pancreatic beta cells. The dissection of IDDM genes is complicated by the low recurrence rate of the disease among first-degree relatives. HLA-DQ2 and 8 are closest to IDDM with a marked synergistic effect of DQ2/8 heterozygotes. The associations with other HLA genes are often explained by linkage disequilibrium. Genetic factors on other chromosomes which influence the pathogenesis are still to be fully identified but candidates are on chromosomes 11 (insulin gene polymorphisms) and 7 (TCR gene polymorphisms). The autoreactivity against the GAD65 isoform is pronounced both before and at the clinical onset of IDDM. GAD65 autoantibodies show the highest predictive value and may represent an initiating autoantigen. Autoantibodies to numerous other beta cell autoantigens are detected at the clinical onset but may represent a secondary response and antigen spreading during a sustained autoimmune attack on the beta cells. The role of T cells in human IDDM is yet to be defined. GAD65 and other islet autoantibodies have a low positive predictive value for IDDM and further investigations are needed to clarify ways to predict IDDM in the general population.

Associations between restriction fragment length polymorphisms detected with a probe for human C4 and allotype of C4B5 allele

We studied the fourth component of human complement (C4) allotypes in 58 Japanese individuals. The technique of Southern, with C4 and 21-OH cDNA probes, was used to examine the genomic DNA of 45 individuals typed for C4 by protein electrophoresis. Novel HindIII C4 10- and 5-kb and EcoRI C4 13-Kb restriction fragments were identified in each of nine Japanese individuals. The novel fragments were different from the previously described C4B long (HindIII 31-kb, TaqI 6-kb, BamHI 4.3-kb, and EcoRI 12-kb) and C4B short (HindIII 25-kb, TaqI 5.4-kb, BamHI 3.5-kb, and EcoRI 15-kb) fragments. All novel HindIII- and EcoRI-positive individuals carried C4B5, BfS, and HLA-Bw54. Therefore, the fragments were characteristic for the C4B5 allele. The C4 region was analyzed to determine the restriction sites by single and double digests of uncloned genomic DNA with several restriction endonucleases. It is speculated that an insertion gene lies between the 3' end of the 21-OH and the 5' end of the C4B genes.

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.

Low serum haemolytic function of the fourth complement component (C4) in insulin dependent diabetes

Low serum concentrations of the fourth component of complement (C4) are found in insulin dependent diabetes, and may be important in the aetiology of the disease. To ascertain whether function of C4 is also impaired both its haemolytic activity and its concentration were measured in 34 insulin dependent diabetics, 15 non-insulin dependent diabetics, 20 healthy subjects, and 12 pairs of monozygotic twins discordant for insulin dependent diabetes. C4 function was measured by a radial immune haemolytic assay, and C4 concentration by laser nephelometry. Both measurements were significantly lower in insulin dependent diabetics (C4 function: median 47%, range 4-100%; C4 concentration: 0.22 g/l, 0.10-0.38 g/l) than in non-insulin dependent diabetics (67%, 33-138%, p less than 0.01; 0.27 g/l, 0.16-0.50 g/l, p less than 0.02) and controls (74%, 33-138%, p less than 0.01; 0.27 g/l, 0.18-0.40 g/l, p less than 0.03). C4 function and concentration were lower in both diabetic (48%, 12-100%; 0.17 g/l, 0.08-0.31 g/l) and non-diabetic twins (47%, 12-100%; 0.17 g/l, 0.07-0.36 g/l) than controls (p less than 0.01; p less than 0.01). Thirteen (38%) of the insulin dependent diabetics had a reduction in either C4 function or concentration, but in only five were both features reduced. Values of function and concentration were strongly correlated in both diabetic and non-diabetic twins (r = 0.95, p less than 0.001; r = 0.92, p less than 0.001). These results show defects in C4 function and concentration in insulin dependent diabetes, which--being present in the non-diabetic co-twin of diabetics--may represent a genetic predisposition to the disease.

Study of cis and trans interactions between extended HLA-haplotypes in insulin-dependent diabetes

From the study of HLA, A, B, C, DR, Bf and C4A, C4B alleles in 287 insulin-dependent diabetes mellitus patients and 108 controls, comparisons were made between 424 diabetic and 216 normal extended haplotypes. In the "cis" situation (haplotype), the highest relative risks (RR) for IDDM were borne by multiloci allelic associations, mainly DR/complement alleles, rather than by DR3 or DR4 considered alone. Susceptibility was strongly associated with two extended haplotypes (Aw30, Cw5, B18, C4BQ0, C4A3, BfF1, DR3 and A2, Cw3, B15, C4Bx, C4A3, BfS, DR4) or their smaller segments. Two haplotypes, S31 associated with DR2 or DR5 and F31 associated with DRw6 or DR7 had a protective effect. In the "trans" situation (opposite haplotype) the large excess of DR3/DR4 heterozygotes was not the only distortion observed. An excess of DR1 (57%) and of C4BQ0 (40%) was noted among non DR3, non DR4 haplotypes in diabetics compared to normal individuals (26% and 23%, respectively, P less than 0.01, 0.05). Homozygotes for DR3 or DR4 were not increased, and other homozygotes were decreased compared to controls. The protective antigens HLA DR2, DR5 and DR7 seemed not to be distributed randomly: their putative protective effect was not observed in the case of combination with DR1 or a B18, DR3 haplotype. DR2 was never found homozygous or combined with DR5. These results suggest that susceptibility to IDDM is generated by both cis and trans interactions between genes or gene products of the HLA region.

Patients with rheumatoid arthritis and gold-induced pneumonitis express two high-risk major histocompatibility complex patterns

Gold salt therapy-induced pneumonitis is a rare complication in patients with rheumatoid arthritis (RA). We studied HLA-A, B, C, D/DR, and complement factor B (Bf) and C4 alleles in 17 patients with RA and gold-induced pneumonitis and found that these patients had strikingly homogeneous major histocompatibility complex (MHC) markers. Eight of them (47 percent) had the alleles HLA-A3 B35 Dwl BfF C4A3,2 (BO), which were shown by family studies of some patients to be inherited as an extended MHC-haplotype with an apparent gene duplication in the C4A locus. The other high-risk phenotype, HLA-B40 with a C4 null allele, was found in eight patients (47 percent). All but three of the 17 patients had at least one of the two high-risk markers, the frequency of these combinations being clearly higher than in the two control groups: patients with RA but with no gold-induced side effects and healthy individuals. Our study shows that use of several MHC markers together results in a strong association between the markers and the disease.

The complement system in type 1 (insulin-dependent) diabetes

The complement proteins C1q, r, s, C2, C4, C3, factor B, C5, C6, and the inhibitors, C1 inhibitors, factors I and H were measured in 35 patients with recently diagnosed Type 1 (insulin-dependent) diabetes, 76 patients with longer-duration disease (30 with complications) and 43 first-degree healthy relatives. We found that C1q, C4 and C3 were reduced significantly in all groups of patients (p less than 0.001 for each protein in recent onset and uncomplicated patients; p less than 0.01, p less than 0.01 and p less than 0.05 respectively, for patients with complications) compared to 60 control subjects and that C4 was also reduced in healthy relatives (p less than 0.001). C4 allotypes were examined in 63 subjects (selected from the patient groups) in order to clarify the role of null alleles in the production of the C4 abnormality. These showed serum C4 to be reduced significantly in 50 patients without null alleles (patient mean 0.24 g/l; control subject mean 0.34 g/l) (p less than 0.0001), although levels were lowest in the 13 patients with one or more null alleles (mean 0.19 g/l). Finally, to examine the metabolic basis for the low concentrations of C4 and C3, the turnover of highly-purified, radiolabelled C4 and C3 was measured in seven recently diagnosed patients; four of these had low levels of C4. The data showed that three out of four of these patients had reduced synthesis of C3 and C4 and normal values for fractional catabolic rate. Two patients showed features of C4 hypercatabolism. We conclude that several early complement proteins are reduced in Type 1 diabetes, irrespective of duration or complications.(ABSTRACT TRUNCATED AT 250 WORDS)

The genetics of diabetic complications

Duration of disease is the major susceptibility factor for microangiopathy. Microangiopathy does not occur without the metabolic abnormality of diabetes and there is much circumstantial evidence to implicate poor diabetic control in its pathogenesis. The rate of development and severity of complications, however, are variable even in patients with apparently similar control and about 25% of diabetics will never develop clinical evidence of microangiopathy. Studies of identical twins suggest a genetic component in the pathogenesis of retinopathy in NIDDM, and less so in IDDM, but increased capillary basement membrane thickness does not occur in the non-diabetic identical co-twins of insulin dependent diabetics. There may also be genetic heterogeneity not only of diabetes, but also of its complications, although for a given type of diabetes the prevalence of microangiopathy is often very similar in different racial groups. Associations between several different HLA molecules (particularly DR4) and microangiopathy in IDDM have been reported but not consistently confirmed. Recently the finding of an increased frequency of the B3 allotype of the fourth component of complement C4B3 in subjects with retinopathy has suggested that there is an HLA linked association. Both complement and the immunoglobulins are concerned with humoral immunity and the report of an association between a phenotype of the IgG heavy chain markers on chromosome 14 and retinopathy is of particular interest. These associations appear to be additive but independent. These reports need confirmation but provide the best evidence we have for an immunogenetic component (HLA and non-HLA linked) of the aetiology of microangiopathy, at least in IDDM. The studies of identical twins, HLA and Gm associations provide good evidence that genetic factors are involved in susceptibility to microangiopathy, at least in some diabetics, although the most relevant genes may not have been identified. Searches for better genetic markers must continue in order to identify those patients at increased risk of developing microangiopathy.

Diabetes and autonomic neuropathy: an immunological association?

Lymphocytic infiltration of autonomic ganglia found at autopsy and a strong clinical association with iritis suggests that diabetic autonomic neuropathy might have an immunological basis. We measured levels of circulating immune complexes, complement (C3, C4), complement breakdown products (C3d), and insulin antibodies in diabetics with autonomic neuropathy and a history of iritis (n = 17), compared to diabetics of similar age and duration with autonomic neuropathy but no history of iritis (n = 20), and with uncomplicated insulin-dependent diabetics (n = 23), together with normal controls (n = 26). We found higher levels of circulating immune complexes in patients with autonomic neuropathy (irrespective of iritis) compared to normal controls, and differences in C3d levels suggesting complement activation. C4 levels were unexpectedly normal in the diabetics with autonomic neuropathy, in contrast to the uncomplicated insulin-dependent diabetic controls. Insulin antibody levels showed no difference between the three groups of diabetics. These findings suggest that immunological mechanisms may be implicated in the aetiology of diabetic autonomic neuropathy and that further studies are indicated.

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.

A DR3-related DX alpha gene polymorphism strongly associates with insulin-dependent diabetes mellitus

HLA-DQ alpha and HLA-DX alpha gene polymorphisms were analyzed by Southern blot techniques in 78 Caucasoid insulin-dependent diabetes mellitus (IDDM) subjects and 55 control subjects. Five restriction fragment length polymorphisms of the HLA-DQ alpha gene correlated with HLA-DR typing. Two allelic DX alpha-related gene fragments, of 2.1 kb (U) and 1.9 kb (L) in size were identified. Genotype frequencies in the IDDM group for UU, UL, and LL were 54%, 38.5%, and 7.5%, respectively, whereas the corresponding frequencies in the control group were 24%, 40%, and 36% (P less than 0.00005 for differences in genotype frequencies). The U allele was associated particularly with IDDM patients who were DR3, with healthy controls who were DR3, as well as with IDDM patients who were not DR3. Thus, if this DX alpha U allele is not the DR3-associated IDDM susceptibility gene, it is the closest marker hitherto studied.

Susceptibility to IDDM is marked by MHC supratypes rather than individual alleles

107 patients with insulin-dependent diabetes mellitus (IDDM) were typed for HLA A, B, C-, and DR antigens, and for complement C4A, C4B, and Bf alleles, and the results were compared with those of a combined reference group of 332 appropriately matched healthy subjects. Supratypes (allelic combinations) were identified from the phenotype of each group, and it was shown that the frequency of several supratypes is increased in patients with IDDM, in particular supratypes (A1 Cw7) B8 C4AQ0 C4B1 BfS DR3 (P = 0.0001), (A30 Cw-) B18 C4A3 C4BQ0 BfF1 DR3 (P = 0.0003), (A2 Cw3) B62 C4AR C4B2.9 BfS DR4 (P = 0.0002), and three other supratypes including DR4. It was also shown that increases in the frequency of individual alleles are secondary to increases in supratype frequency. Moreover, supratypes appeared to interact; the presence of two relevant supratypes being particularly important. The absolute risk of IDDM was approximately 0.5 in subjects who were homozygous for B18 C4A3 C4BQ0 BfF1 DR3. We concluded that genetic susceptibility is best recognized by MHC supratypes rather than isolated alleles, and that supratype combinations make the identification of even greater disease risk possible.

Relation of gene expression (allotypes) of the fourth component of complement to insulin dependent diabetes and its microangiopathic complications

About a quarter of insulin dependent diabetics have low concentrations of the fourth component of complement (C4), and a low concentration of C4 is associated with diabetic microangiopathy. The variability of the expression of the C4 gene was compared in insulin dependent diabetics with and without microangiopathy and controls. Of the two genes coding for C4, the A gene (C4A) was not expressed--that is, C4A null--in 16 (13%) of the 126 insulin dependent diabetics compared with none of the 93 controls (p less than 0.001), and all these 16 subjects had low concentrations of C4. Lack of expression of the other C4 gene (C4B) was not associated with insulin dependent diabetes, but a rare variant, C4B3, was significantly increased in the diabetics (21/126; 17%) compared with the controls (none) (p less than 0.001). The prevalence of C4B3 was also increased in the diabetics with complications when compared with those without (14/50 (28%) v 7/76 (9%), p less than 0.01). Low plasma C4 concentrations in insulin dependent diabetics are at least partly due to variation in the expression of the C4 gene. The association of the rare C4B3 variant with microangiopathy suggests a genetic component of its aetiology.

Insulin dependence: problems with the classification of 100 consecutive patients

We have evaluated the clinical and immunogenetic features of 100 consecutive patients presenting to an adult diabetic clinic who were judged clinically to need insulin therapy but were not sufficiently ill to be admitted to hospital. Over this same period 15 newly diagnosed patients (aged 13-70 years) were started on insulin as in-patients of whom ten were in ketoacidosis. The 100 out-patients, aged 11-75 years at the time of starting insulin, were followed for at least a year. Fifty-six had islet cell antibodies and/or were heterozygous for HLA DR3 and DR4 (Group A) whereas 44 had neither of these markers (Group B). Islet cell antibodies and/or DR3, DR4 heterozygosity were most common in the 70 patients diagnosed below the age of 40 years but were also found in older patients. Patients in Group A were significantly younger at diagnosis (29 vs. 43 years), had a shorter duration of symptoms (17 vs. 61 weeks), were more likely to have ketonuria, and had a lower random C-peptide level at diagnosis (0.2 vs. 0.31 nmol/l). The two groups could not be distinguished by weight, haemogloblin A1 or blood glucose at diagnosis or by diabetic control or insulin dose after one year. The National Diabetes Data Group (NDDG) definition of insulin dependence stresses the importance of HLA types and islet cell antibodies although we found their prevalence to be low in the 30 patients diagnosed over 40 years who clinically were indistinguishable from the younger patients.(ABSTRACT TRUNCATED AT 250 WORDS)

The pathogenesis of Graves' disease

The abnormally increased thyroid activity that is characteristic of Graves' disease is caused by immunoglobulins which specifically interact with the thyroid cell and stimulate it. Increases and decreases in thyroid activity in Graves' disease can be clearly related to rise and fall of these immunoglobulin-mediated activities. The level of immunoglobulin stimulatory activity can be used for prediction of the likelihood of neonatal Graves' disease and of recurrence of disease after cessation of treatment with antithyroid drugs. Investigation of patients with Graves' disease and their families has led to identification of particular human leukocyte antigens and genetically linked markers on immunoglobulins which both appear to incur increased susceptibility to certain autoimmune diseases. Differences in immune function, when compared with control populations, have been found in patients with these genetically linked markers. Protection against autoimmune disease is maintained by purposeful inhibition of any self-directed activity within each function of the immune system and by the controlling interaction of other immune functions. No single deficiency of immune function can be selected as giving the major risk of autoimmune disease, but rather a sum of relative defects resulting in an increased risk. In some patients with Graves' disease the self-protection mechanisms regain sufficient control of the immune functions to reduce the activity of the autoimmune disease, and the patient may achieve clinical remission. Often, however, there is evidence that abnormal immune activity directed against thyroid tissue has persisted with liability to recurrence of the Graves' disease.

C4 phenotypic variation suggests an unusual class III gene organization

The class III gene markers, C4A, C4B, C2 and BF were determined in 26 HLA genotyped families. In 6 of these families the inheritance of the C4A electrophoretic phenotype, C4A 3, A 2 could not be explained assuming the genetic model that the C4A component is controlled by only one locus (C4A) and suggests that both C4A 3 and C4A 2 are encoded by the same chromosome. This apparently duplicated C4A locus haplotype is common in Northern Europeans occurring at a frequency of at least 6%. In 4 families the C4A 3, A 2 'variant' occurred together with HLA-Bw35 and in 2 together with HLA-Bw55. Furthermore, it was striking that in each family the C4A 3, A 2 'variant' was found exclusively with the complotype FC3/20 which encodes no phenotypically expressed C4B locus product.

The genetic susceptibility to insulin-dependent diabetes mellitus: combined segregation and linkage analysis

We report a combined segregation and linkage analysis of a Danish sample of 216 insulin-dependent diabetes mellitus (IDDM) nuclear families: of these 216, twenty multiplex families were haplotyped regarding HLA-DR and -B markers. The analysis was conducted using the computer program COMBIN, which includes a modifier to absorb family resemblance that is additional to the effect of the major locus that is assumed linked to a marker locus, eg, within the HLA region. The initial analysis could clearly reject a dominant major locus but could not discriminate between other models with or without modifier. However, after adding supplementary information on population associations between HLA and IDDM together with the identity-by-descent (IBD) distribution to the analysis, a final model was identified. This invokes an additive major locus, linked to HLA with recombination not significantly different from 0, a disease gene frequency of 0.217, plus a dominant modifier. From this model it can be predicted that about 0.15% of the general population is at 100% risk of IDDM, about 5% is at intermediate risk (approximately 10%), while the remaining population has a risk of 0. The model predicts recurrence risks compatible with empirically estimated values. Particularly strong, positive haplotype associations were found for the DR3,B8, DR3,B18, and DR4,B15 haplotypes, but detailed analyses showed that neither these particular haplotypes nor the DR3 and DR4 haplotypes in general could entirely explain the HLA-associated susceptibility. The DR2 haplotypes showed a strong negative association. The results are discussed in the light of available data on the epidemiology of IDDM in order to provide a framework for further epidemiological studies.

Possible association between HLA-DR5 and superficial spreading melanoma (SSM)

Previous analyses of possible associations between MHC determinants and cutaneous malignant melanoma (MM) have been inconclusive. We have investigated 98 patients with special emphasis on histologically determined subtypes of MM, and 5 multiple-case families. In addition to HLA-ABC and DR typing, complement allotypes of C2, C4A, C4B, BF were determined. Among the unrelated patients HLA-DR5 and, secondarily, B49 were observed to be associated with superficial spreading (SSM) but not with nodular (NM) or lentigo maligna melanoma (LMM). In families with MM, no definite segregation of the disease according to HLA and complement haplotypes was discernible. Moreover, no coincidence of haplotypes occurred in patients of the different families. HLA region recombinations including B/DR and/or DR/GLO were found in all the families investigated.

Low plasma C4 concentrations: association with microangiopathy in insulin dependent diabetes

Plasma C4 concentrations were measured in insulin dependent diabetics with and without microangiopathy and in controls. The diabetics had significantly lower C4 values than controls (p less than 0.001), and patients with insulin dependent diabetes and microangiopathy had lower values than those without this complication (p less than 0.001). There was a 7.1-fold increase in the prevalence of complications in the diabetics with low C4 values. Of 41 diabetics whose rate of albumin excretion was measured, 13 had increased rates and 11 of these had low C4 concentrations. Low plasma C4 concentration in insulin dependent diabetes is strongly associated with microvascular disease and may identify diabetics with a particular propensity to develop this complication.

C4*A6BQO: a 'new' C4 haplotype

A C4 haplotype containing C4*A6 without C4*B1 is described. It occurred in the MHC haplotype HLA-A1, Cw5, Bw44, C2*1, BF*S, C4*A6BQO, DR3, GLO 2. This haplotype was observed in five of eight siblings, including the propositus who was in renal failure. The C4A6 was not haemolytically active and this new haplotype is therefore of particular interest since it is functionally equivalent to the rare C4-deficient haplotype C4*AQOBQO.

Structure and organization of the C4 genes

This 200 000 Mr serum protein is coded for by at least two separate loci, C4A and C4B, which map in the HLA Class III region on chromosome 6 in man. Both loci are highly polymorphic with more than 30 alleles, including null alleles assigned to the two loci. The complete nucleotide sequence of a full length C4A cDNA clone and a substantial part of a C4b cDNA clone has shown class differences which can be used to synthesize nucleotide probes specific for C4A and C4B. Three C4 loci of approximately 16 kilobases each spaced by 10 kilobases have been identified in DNA from one individual and aligned 30 kilobases from the factor B gene by overlapping cloned genomic fragments from a cosmid library. Characterization of these genes by restriction mapping, nucleotide sequence analysis and hybridization with C4A and C4B specific synthetic oligonucleotides show that these genes are very similar.

Extended major histocompatibility complex haplotypes in type I diabetes mellitus

We have studied major histocompatibility complex markers in Caucasian patients with type I diabetes mellitus and their families. The frequencies of extended haplotypes that were composed of specific HLA-B, HLA-DR, BF, C2, C4A, and C4B allelic combinations, which occurred more commonly than expected, were compared on random diabetic and normal chromosomes in the study families. We demonstrated that all of the previously recognized increases in HLA-B8, B18, B15, DR3, and perhaps DR4 could be ascribed to the increase among diabetic haplotypes of a few extended haplotypes: [HLA B8, DR3, SC01, GLO2]; [HLA-B18, DR3, F1C30]; [HLA-B15, DR4, SC33]; and [HLA-BW38, DR4, SC21]. In fact, HLA-DR3 on nonextended haplotypes was "protective", with a relative risk considerably less than 1.0. There was a paucity or absence among diabetic patients of several extended haplotypes of normal chromosomes, notably [HLA-B7, DR2, SC31] and [HLA-BW44, DR4, SC30]. The extended haplotype [HLA-BW38, DR4, SC21] is found only in Ashkenazi Jewish patients, which suggests that extended haplotypes mark specific mutations that arise in defined ethnic groups. The data show that no known MHC allele, including HLA-DR3 and possibly HLA-DR4, is per se a marker for or itself a susceptibility gene for type I diabetes. Rather, extended haplotypes, with relatively fixed alleles, are either carriers or noncarriers of susceptibility genes for this disease. Thus, the increased frequency (association) or the decreased frequency (protection) of individual MHC alleles is largely explainable by these extended haplotypes.

Genetic polymorphism of the fourth component of complement and type 1 (insulin-dependent) diabetes

The complement proteins, Bf, C2, C4A and C4B, are closely linked to HLA. In 74 propositi and their families, and 97 controls genotyped for HLA-A, -B, -C, DR, -Bf, a high incidence of the C4BQ0 variant was detected in the patient group (33% versus 12%, p less than 0.00001); C4BQ0 was more frequent in propositi than in non-affected siblings (40 out of 74 versus 36 out of 92, p less than 0.05). When comparing the distribution of the phenotype C4BQ0 in Type 1 diabetic patients and normal control subjects, the difference was significant in patients bearing DR3 or DR4 (56% and 25%, respectively, p less than 0.003). The main linkage disequilibria were observed among the 74 propositi: B18, BfF1, C4, A3, BQ0, DR3; B12, BfS, C4, A3, BQ0, DR4. The existence of a silent allele at the C4 B locus is known to be associated with a defective immune response.

Thyrogastric autoimmunity and MHC associated alleles at the C4 locus in patients with type 1 (insulin-dependent) diabetes

HLA antigens, complement allotypes, insulin antibodies and thyrogastric autoantibodies were determined in 69 patients with Type 1 (insulin-dependent) diabetes defined by a tendency to ketosis, non-obesity and insulin requirement within 2 years of diagnosis. Analysis of HLA and C4 allotypes suggested that Type 1 diabetes was associated with only certain DR3- and DR4-containing supratypes. Low antibody response to insulin was associated with all HLA-DR3, being present in 89% of those with DR3 compared with 48% of those without. Thyrogastric autoantibodies were associated with a null allele at the C4A locus, usually with HLA-B8-C4AQO-C4B1-BfS-DR3. These results indicate that, unlike Type 1 diabetes, low insulin antibody response was associated with all HLA-DR3. Thyrogastric autoantibodies, on the other hand, were associated with a null allele at the C4A locus. It is probable that while interaction between certain HLA-DR3 and DR4-containing supratypes is important in conferring susceptibility to Type 1 diabetes, other manifestations of autoimmunity are associated with supratypes containing C4AQ0, and in particular the diabetogenic supratype HLA-B8-C4AQ0-C4B1-BfS-DR3.

Rare variant of complement C4 is seen in high frequency in patients with primary glomerulonephritis

59 unselected patients with primary glomerulonephritis were phenotyped for alleles of the MHC-linked complement genes, C4A, C4B, and BF. A rare variant of the C4B locus, C4B*2.9, was found in 25% of these patients compared with only 2% of the normal population--a relative risk of 22.1 for glomerulonephritis in individuals with this variant. Subdivision of patients by histological classification of glomerulonephritis revealed a significant association of C4B*2.9 with the membranoproliferative form. There were no significant associations between primary glomerulonephritis or its subtypes and the other HLA markers tested.

A solution to the genetic and environmental puzzles of insulin-dependent diabetes mellitus

Studies of the segregation of heterozygous immunoglobulin allotypes in families with several cases of insulin-dependent diabetes mellitus (IDDM) show that germline heavy-chain V (variable region) genes are not major genetic determinants for IDDM, but data for IDDM and Graves' disease together suggest involvement of kappa light-chain V genes. Absence of IDDM at birth, the semi-random age of onset, and the 50% discordance of identical twins suggest that somatic mutation of germline V genes is involved in the development of the pathogenetic anti-beta-cell clones. The effect of histocompatibility and other alloantigens on the prevalence of IDDM is readily accounted for by the effect of the "holes" they induce, by natural tolerance, in the immune response repertoire; these alterations apparently affect the chance of emergence of anti-beta-cell clones by the somatic mutations and network of interclonal deletions that constantly change the fringes of the repertoire. Histocompatibility antigens can also influence repertoire development by changing the specificity of conjoint presentation of foreign antigens by macrophages. Antigenic stimulation by particular environmental microorganisms is probably essential to the repertoire development necessary for the occurrence of IDDM. Additionally, beta-cell damage by local infection may play a part by facilitating autoantigen presentation to the immune system.

MHC-linked class III genes. Analysis of C4 gene frequencies, complotypes and associations with distinct HLA haplotypes in German Caucasians

The class III complement components, C4, C2 and factor B (BF), are encoded in the human major histocompatibility complex (MHC). The two genes determining C4 (C4A and C4B) display considerable polymorphism and, thus, are important markers for HLA. In combination with alleles of C2 and BF they can be grouped into unique complotypes. We have analyzed the C4 alleles in a panel of 204 unrelated German Caucasians and studied their segregation with HLA haplotypes in 24 normal families. Inclusion of the class III markers with the class I and II alleles provides a more refined picture of the genetic structure of the MHC in these families. When charted according to the HLA-B locus specificities the MHCs can be clustered into groups showing distinctly homogenous or heterogenous complotypes. The identification of such groups is valuable for the selection of genetic material to analyze the molecular genetics of the human MHC.

Correlation between a DNA restriction fragment length polymorphism and C4A6 protein

The fourth component of complement (C4) in man, is coded for by two separate but closely linked loci (C4A and C4B) within the major histocompatibility region (MHC), on the short arm of chromosome 6. Like class I and II loci of this region, the C4 genes are highly polymorphic with more than 30 alleles, including null alleles, assigned to the two loci. This extensive polymorphism, based mainly on electrophoretic mobility, provides a useful marker for studies of disease susceptibility. Several disorders, including systemic lupus erythematosus and type I diabetes, show associations with C4 phenotypes. We have used the technique of Southern with a C4 specific probe to examine the genomic DNA of individuals typed for C4 by protein electrophoresis. We have identified 10.7 and 3.8 kilobase (kb) BglII restriction fragments in each of 9 unrelated individuals with a C4A6 allele, and in none of 22 unrelated individuals in whom this allele was not expressed. This clear correlation of restriction fragment length polymorphism with C4 phenotype provides a precise basis for analysis of C4 polymorphism. It is likely to be of value in clinical investigations of autoimmune disease.