The role of genetic predisposition to type I (insulin-dependent) diabetes mellitus

Natural peptides selected by diabetogenic DQ8 and murine I-A(g7) molecules show common sequence specificity

In this study, a large number of naturally processed peptides was isolated and identified from the human diabetes-susceptible class II MHC molecules HLA-DQ8 (DQA1*0301,DQB1*0302) and from murine I-A species, both of which are expressed in genetically identical APC lines. The peptides presented during the processing of autologous proteins were highly selective in showing sequence specificity, mainly consisting of 1 or more acidic residues at their C terminus. Testing for binding to the MHC molecules revealed that the position 9 (P9) acidic residues of the peptides contributed decisively to binding. For HLA-DQ8, the P1 residue, which was also an acidic amino acid, influenced binding positively. Both HLA-DQ8 and I-A(g7) selected for common peptides that bound in the same register. There was no evidence for selection of peptides having nonspecific or promiscuous binding. Thus, diabetogenic class II MHC molecules are highly selective in terms of the peptides presented by their APCs, and this is governed by the features of their P9 anchor pocket. These results are in striking contrast to those from studies examining synthetic peptide or phage display libraries, in which many peptides were shown to bind.

Potentials and pitfalls in neonatal screening for type 1 diabetes

Childhood-onset diabetes is increasing all over the western world. Only 20-30% of monozygotic twin pairs are concordant for the disease. So far, more than 20 different risk genes have been identified on different chromosomes. The dominating risk genes differ with different age at onset and also in different populations. Thus, the aetiology of the disease is complex, with interactions between different risk genes and environmental risk factors. Several pathogenetic models have been proposed, most of which include autoimmune destruction mechanisms. Screening for genetic markers for diseases with such complex aetiology encounters pitfalls due to the low predictive value of each single marker in the general population. To overcome such problems combinations of markers and decision-tree analysis are necessary. The identification of several immune markers for the disease has provided potential for screening for secondary prevention. When increasing the number of markers to increase the predictive value, sensitivity will be lost. A Swedish population-based study found the best combined positive predictive value in the general population to be 20%, whereas the sensitivity was only 34%. Type 1 diabetes still needs more precise risk markers in addition to a very safe prevention strategy before neonatal screening programmes may be instituted.

The aetiology of type 1 diabetes: an epidemiological perspective

Type 1 diabetes is increasing rapidly in many parts of the Western world, most evidently in Scandinavia. A low concordance rate of insulin-dependent diabetes mellitus among monozygotic twins clearly indicates that genetic risk factors may be necessary, but are not sufficient for the disease to occur. The strongest genetic risk markers are located in the HLA region of chromosome 6, but these DNA specificities differ in different populations. Risk genes are indicated in other chromosomes of the human genome, suggesting a complex interaction between genes and environment as the cause of the disease. The pathogenesis of the disease is proposed to be autoimmune in nature and environmental risk factors may either initiate autoimmunity or accelerate an already ongoing beta-cell destruction. Risk factors disclosed by epidemiological studies that may accelerate the pathogenetic process are: a cold environment, a high growth rate, infections and stressful life events. Risk factors that may initiate the autoimmune process include early exposure to cow's milk proteins, nitrosamines or early foetal events such as blood group incompatibility or foetal viral infections. In conclusion, population-based epidemiological studies have helped to confirm proposed aetiological models that have arisen from experimental research. These epidemiological studies have also introduced important new findings that may reveal the complex aetiology of the disease and advance understanding closer to the ultimate goal of primary prevention.

Reevaluation of the importance of polymorphic HLA class II alleles and amino acids in the susceptibility of individuals of different populations to type I diabetes

Several publications have shown that certain alleles at the HLA-DRB1, -DQA1, and -DQB1 loci are associated with insulin-dependent diabetes mellitus (IDDM). Many of these studies have claimed that HLA-DQalpha1Arg52 and DQbeta1Asp57 showed the strongest association with IDDM, but these results could not be confirmed in different populations. We have recently found that DRbeta1Lys71+ provided major susceptibility to IDDM and that DQbeta1Asp57- had an additive effect to DRbeta1Lys71+ [Zamani et al., 1994a: Eur J Hum Genet 2:177-184]. This was confirmed with haplotype analysis in multiplex IDDM families [Zamani et al., 1996a: J Med Genet 33:899-905]. Therefore, we have reanalyzed the data from the literature on the association of the human leucocyte antigen (HLA) DRB1, DQB1, and DQA1 with IDDM in different ethnic groups to determine whether different amino acids in the antigen binding cleft of HLA class II molecules play a preponderant role in the development of IDDM. The results showed that the DRbeta1Lys71+ allele provided the highest relative risk for IDDM in the Belgian, Danish, Greek Taiwanese, and Chinese population while this was not the case in Norwegians, Sardinians, and Algerians. Indeed, in the Sardinian and Algerian population the DRB1*0401 allele encoding Lys71+ is very rare. Nevertheless, the few positive cases were always in the patient group. We also measured the clinical relevance of the testing for DRbeta1Lys71, DQbeta1Asp57, and DQalpha1Arg52 by calculating a prevalence-corrected positive predictive value (PcPPV), a prevalence corrected negative predictive value (PcNPV), the sensitivity and specificity of these tests. The results indicated that the sensitivity of the test for DRbeta1Lys71+ was lower than for DQalpha1Ag52+ and DQbeta1Asp57-, while testing for DRbeta1Lys71+ was more specific than testing for DQbeta1Asp57- and DQalpha1Arg52+ and that the DRbeta1Lys71+ allele had a higher PcPPV than DQalpha1Arg52+ and DQbeta1Asp57- in all studied populations. These results also showed that testing for DRbeta1LyS71+/+ can be useful in IDDM risk assessment particularly in populations with a high prevalence (P) of IDDM such as the Danish (P[IDDM] = 0.65%). PcPPV for DRbeta1Lys71+/+ was 0.2313 in the Danish, indicating a 23.13% risk for an individual who is homozygous for the genotype DRbeta1Lys71+/+ to develop IDDM. Some mechanisms which might explain the role of these HLA class II alleles in susceptibility to IDDM are discussed.

Viruses and other perinatal exposures as initiating events for beta-cell destruction

There is strong evidence that the aetiology of insulin-dependent diabetes mellitus (IDDM) is due to a complex interaction between genes and the environment and that the pathogenesis is autoimmune. In early perinatal life the immune system is induceable and exposures in this period may initiate autoimmunity. Recent findings of time and space clustering of birth dates for later diabetic cases together with the early observation of a very high prevalence of diabetes in cases with rubella embryopathy suggest that foetal virus exposure may be important. Recent findings from Sweden and Finland suggest that enterovirus exposure during foetal life may initiate autoimmunity which may lead to diabetes. Other immune events, such as maternal-foetal blood group incompatibility and pre-eclampsia in the mother have also been associated with IDDM risk. Other more unspecific events in the perinatal period, such as a short gestational age, caesarean section and neonatal respiratory disease, are also indicated to increase the risk. In addition, food components such as nitrosamine components, cow's milk protein and gliadin have been proposed to initiate the slowly progressing autoimmune beta-cell destruction. Most of these epidemiological findings are supported by experimental studies in the nonobese diabetic mice but their exact mechanisms of action are still unclear. It is concluded that new evidence is accumulating indicating that perinatal exposures may be important for the initiation of beta-cell destruction. As such risk factors may be targets for primary prevention strategies further studies are urgently warranted.

No association between anti-bovine serum albumin antibodies and islet cell reactive antibodies in newly diagnosed type 1 diabetic patients

Serological findings have suggested that antibodies (Ab) to bovine serum albumin (BSA-Ab) are associated with type 1 diabetes mellitus. The aim of our study was to evaluate a competitive fluid-phase radioimmunoassay for detecting BSA-Ab using different incubation times and to study a possible association of these BSA-antibodies with autoantibodies (AAb) frequently detected in type 1 diabetic patients. For the overnight incubation time, there was an enormous overlap in the [125I]BSA binding by serum samples between 52 newly diagnosed type 1 diabetic patients (mean [125I]BSA binding 23.6 +/- 17.4%) and 54 healthy blood donors (mean [125I]BSA binding 10.2 +/- 15.7%). By an incubation time of only 3 min the BSA-antibody prevalence was found to be 15.4% (8/52) for type 1 diabetic patients and 3.7% (2/54) for control subjects. However, there was no association between BSA-Ab and type 1 diabetes-associated antibodies as cytoplasmic islet cell antibodies (ICA), or glutamate decarboxylase autoantibodies. Our results confirm that (i) BSA-Ab occur more frequently in newly diagnosed type 1 diabetic patients compared with a healthy control group and (ii) that the BSA-Ab detected by the fluid-phase radioimmunoassay with an incubation time of 3 min are more disease-associated than the [125I]BSA binding after an overnight incubation. The competitive BSA-Ab fluid-phase radioimmunoassay described is a simple and rapid method to detect antibodies specifically reactive with BSA. It is suggested that the humoral immune reactivity to BSA in type 1 diabetic patients probably reflects an unspecific defect of the immune system and gives no additionally diagnostic value about the type 1 diabetes.

Insulin autoantibodies and high titre islet cell antibodies are preferentially associated with the HLA DQA1*0301-DQB1*0302 haplotype at clinical type 1 (insulin-dependent) diabetes mellitus before age 10 years, but not at onset between age 10 and 40 years. The Belgian Diabetes Registry

Demographic and biological data were collected from all Caucasian Type 1 diabetic patients (n = 279) who were recruited at clinical onset by the Belgian Diabetes Registry over 34 months. The male/female ratio was significantly higher for onset between age 20 and 40 years (2.4) than before age 20 years (1.0); no age-or sex-differences were noticed in serum fructosamine concentration. Total and high concentrations of insulin autoantibodies and islet cell antibodies were preferentially associated with the HLA DQA1*0301-DQB1*0302 susceptibility haplotype. The occurrence of both types of antibodies was also correlated, irrespective of haplotype. At onset before age 10 years, the high risk genotype DQA1*0301-DQB1*0302/DQA1*0501-DQB1*0201 was more prevalent than all other DQA1-DQB1 genotypes taken together, leading to a higher prevalence of the DQA1*0301-DQB1*0302 haplotype in this age group (75%) than in the 10-39 years age group (54%). Under age 10 years, the presence of DQA1*0301-DQB1*0302 was strongly associated with insulin autoantibodies (90%) and islet cell autoantibodies (92% with 85% of high titre), whereas patients without this haplotype were less frequently positive for insulin autoantibodies (31%) or islet cell autoantibodies (38% high titre). In the group with onset at age 10-39 years, the DQA1*0301-DQB1*0302 haplotype presented a lower association with insulin autoantibodies (approximately 40%) and islet cell autoantibodies (50 to 65% high titre), prevalences which no longer differed from those in subjects lacking this haplotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Etiological aspects of insulin-dependent diabetes mellitus: an epidemiological perspective

The mechanism of beta-cell destruction leading to insulin dependent diabetes is probably a cell mediated auto-immune process occurring in genetically susceptible individuals. Since 50-70% of monozygotic twins will not get the disease non-genetic risk factors must play an important role in the etiology of the disease. During the past decade population based epidemiological studies have identified several risk determinants for insulin dependent diabetes. Based on these studies a multifactorial hypothesis of causation is proposed. Some risk determinants (maternal child blood group incompatibility, fetal viral infections, early exposure to cow's milk proteins, a high exposure level of nitrosamines) may independently initiate the autoimmune process by causing the initial damage of the beta-cell, leading to antigen release. Other risk determinants may promote an already ongoing autoimmune destructive process through induction of lymphokine release or by causing an increased work load on the beta-cell. Risk factors that may increase the peripheral need for insulin (infectious diseases, cold environment, a high growth rate and stressful life events) may act as promoters of the beta-cell destruction but also disclose the beta-cell impairment and make the disease clinically overt. Possibilities of different risk profiles in different age groups and of synergism between different risk factors are also discussed.