HLA-Dw antigens in unrelated juvenile, insulin-dependent diabetics

Genetic Modifiers of Thymic Selection and Central Tolerance in Type 1 Diabetes

Type 1 diabetes (T1D) is caused by the T cell-driven autoimmune destruction of insulin-producing cells in the pancreas. T1D served as the prototypical autoimmune disease for genome wide association studies (GWAS) after having already been the subject of many linkage and association studies prior to the development of GWAS technology. Of the many T1D-associated gene variants, a minority appear disease-specific, while most are shared with one or more other autoimmune condition. Shared disease variants suggest defects in fundamental aspects of immune tolerance. The first layer of protective tolerance induction is known as central tolerance and takes place during the thymic selection of T cells. In this article, we will review candidate genes for type 1 diabetes whose function implicates them in central tolerance. We will describe examples of gene variants that modify the function of T cells intrinsically and others that indirectly affect thymic selection. Overall, these insights will show that a significant component of the genetic risk for T1D - and autoimmunity in general - pertains to the earliest stages of tolerance induction, at a time when protective intervention may not be feasible.

Prediabetes in children: natural history, diagnosis, and preventive strategies

The clinical manifestation of type 1 diabetes mellitus is preceded by an asymptomatic prodromal period called prediabetes or preclinical diabetes. It may last from a few months to several years, during which the autoimmune destruction of the insulin-producing beta-cells in the pancreas progresses. The genes on the human leukocyte antigen (HLA) and insulin gene region are major genetic determinants for genetic disease susceptibility, while dietary compounds and viral infections are the most likely environmental factors contributing to the etiopathogenesis. T cells are thought to be the effector cells for the beta-cell destruction, and glutamic acid decarboxylase, insulinoma-associated protein 2 and insulin represent the three major autoantigens. Autoantibodies are early detectable markers of an ongoing disease process and are used to diagnose prediabetes. Among first-degree relatives of patients with type 1 diabetes, the risk for clinical disease can be graded from <5% in those with one or no antibodies to >90% in individuals who carry the HLA-DQB1*02/0302 risk genotype and are positive for multiple autoantibodies. beta-Cell function may also be tested in autoantibody-positive individuals and low first-phase insulin response is highly predictive for rapid progression to the clinical disease. However, dynamic course and individual variation of the disease process complicates the disease prediction, and it is not known whether all individuals with signs of prediabetes will inevitably progress to clinical type 1 diabetes. Until clinically applicable prevention for the condition exists, the screening for the risk markers of type 1 diabetes should actively be undertaken only in the context of research projects. Several major national and international multicenter studies are ongoing to test the potential of various agents (e.g. insulin and nicotinamide) or early elimination of dietary compounds (e.g. cow's milk proteins) to delay or prevent the onset of clinical type 1 diabetes.

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

The aetiology of insulin-dependent diabetes (IDDM) involves genetic predisposition, a major component of which has been mapped in the HLA complex, near to or identical with genes encoding class II molecules. In Caucasian populations IDDM is strongly associated with the serologically defined HLA-DR3 and DR4 antigens, which are widely recognised as markers of susceptibility. The particularly high risk of DR3/DR4 heterozygotes suggests that susceptibility is determined by two genes acting synergistically. The development of recombinant DNA technology has allowed a finer description of the class II region and provided evidence that DQ rather than DR determinants may primarily influence IDDM susceptibility. The search for specific structural changes of the DQA and DQB genes has shown that susceptibility correlates with the absence of aspartic acid at position 57 on the DQ beta chain (DQ beta 57 Asp--) and/or the presence of arginine at position 52 on the DQ alpha chain (DQ alpha 52 Arg+). In Caucasians the formation of a putative DQ susceptibility molecule (DQ alpha 52 Arg+, DQ beta 57 Asp-) accounts best for the disease associations when transcomplementation molecules consisting of DQ alpha and beta chains encoded by different haplotypes are postulated to explain the excess risk of heterozygotes. The HLA-IDDM associations in the Japanese, however, are not explained by this model. These and other unresolved questions indicate that other residues of the DQ alpha and beta chains or other class II molecules (DR beta chains), as well as non-MHC genes, may also contribute to the susceptibility.

HLA and disease

This review describes the greater portion of a large number of new studies on HLA and disease association which has appeared in the literature since 1979. The majority of these are concerned with the association of certain diseases with class II major histocompatibility complex antigens. The possible biologic significance of these associations in terms of their probable etiology appears to be the prevailing theme. Current thinking regarding certain heritable diseases is described. It seems that although much has been done to resolve the genetics of insulin-dependent diabetes mellitus, other diseases such a multiple sclerosis still remain a mystery. Doubtlessly, much will be gained from DNA cloning and sequencing studies proposed for the future. A great deal of new information has been obtained relative to HLA itself. New loci have been postulated in the HLA-D/DR region through the use of powerful immunochemical procedures made possibly by the advent of modern technological advances. The impact of these developments on our understanding of the function of the MHC in man and its possible relationship to disease are discussed.

HLA and susceptibility to type I diabetes. Hypothesis

Positive associations between the antigens D(R)3 and D(R)4 and negative associations of D(R)2 have been reported with insulin dependent or type I diabetes mellitus. It has been suggested that susceptibility factor(s) associated with the D(R)3 and D(R)4 haplotypes and a resistance factor associated with the D(R)2 haplotype may be involved in the pathogenesis of the disease. We propose a hypothesis herein which attempts to unify these findings based on our present understanding suggesting (a) the existence of multiple antigenic determinants associated with any one D(R) haplotype and (b) the sharing of "single" D region encoded determinants between what we now refer to as different D(R) haplotypes. The hypothesis, in its simplest form, focuses on a single D region determinant which can be found associated at different frequencies with the various D haplotypes as potentially explaining the findings.

Genetic studies of insulin-dependent diabetes mellitus: segregation and linkage analyses

The inclusion of HLA data in genetic studies of insulin-dependent diabetes mellitus (IDDM) has not led to conclusive segregation models for IDDM so far. As a new approach, we first applied complex segregation analysis, independently of HLA data, to two combined Danish family materials. Then the best fitting segregation model was entered into linkage analysis of a third material, including family as well as HLA data. The best solution obtained in the segregation analysis was a mixed model, including an intermediate gene, which on the penetrance scale acts as a recessive, together with a polygenic component. The linkage analysis showed an overall recombination fraction of 0.0417 with high coupling frequencies for the HLA-DR3 and HLA-DR4 alleles and the putative disease susceptibility gene. However, when the pedigrees were divided according to whether or not the proband had the heterozygous HLA-phenotype DR3/DR4, a maximum likelihood ratio test for heterogeneity was significant, with estimated recombination fractions of 0.0 and 0.0963 in HLA-DR3/DR4 pedigrees and the remaining pedigrees, respectively. In total, we found convincing evidence that two familial factors contribute to IDDM: a locus within HLA, which very well may be DR; and an unlinked mechanism which is unimportant for HLA-DR3/DR4 but simulates recombination. If confirmed, this conclusion has important implications for further genetic studies of IDDM and complex segregation analyses of family materials sampled according to criteria which include HLA data of the probands are highly needed.

Genetic heterogeneity of diabetes and HLA

Histocompatibility (HLA) antigens and genotypes B, D and DR were studied in large sample of Caucasian insulin dependent diabetic (IDD) probands. The associations between IDD and B8, B15, Dw3, Dw4, and DR3, and DR4 were measured by relative risks (RR) and delta values. Both the homozygotes (B8/8: RR 10, B 15/15: RR 7, DR3/3: RR32, DR4/4: RR34) and the heterozygotes (B8/15: RR 11, DR3/4: RR22) for the high-risk antigens showed highly significant elevation of the relative risks, yet there were no statistically significant differences between the homo- and the heterozygotes. The delta measurements supported the RR results. RR and delta were found significantly decreased for B7, Dw2, and DR2. There were no relationships observed between age at diagnosis or family history and HLA. Although we were unable to demonstrate a statistically significant difference between the RR for the high-risk antigens heterozygote vs. the high-risk antigen homozygotes, our study like many others shows that the RR is higher for the heterozygotes. Thus our data are compatible with genetic heterogeneity of IDD.

Association of B cell alloantigen with juvenile onset diabetes mellitus in the Japanese

Sixty-four Japanese insulin dependent juvenile onset diabetes mellitus (JOD) were studied in relation to HLA-A, B, and DR. Significant deviations were observed. HLA-Bw54 was increased (PF = 49.2%, RR = 6.4) and HLA-B5 was decreased (PF = 7.9%, RR = 0.19). Using radioimmunoassay, two HLA-DR antigens were investigated. Hon 7 antigen, so-called MT3 (WIA4x7), which has linkage disequilibrium between HLA-BW54, is highly associated (PF = 96.9%, RR = 27.8) with JOD found in the Japanese.

Gene dosage and suceptibility to insulin-dependent diabetes

1. All members of 33 families in which two or more sibs have insulin-dependent diabetes mellitus (IDDM) were HLA-typed. The results strongly support the hypothesis that, closely linked to the HLA region, there is a locus (S) for susceptibility to IDDM. We use Sd for alleles at this locus which confer susceptibility to disease, and Sa for all other alleles. 2. Published prevalence (recurrence) rates for relatives of an IDDM proband, which are essential for the analysis, are reviewed critically, and a new estimate of prevalence at age 30 in the general population is calculated from published data. 3. The analysis allows for greater risk (penetrance) of the genotype SdSd (two doses of Sd) than of SaSd (one dose of Sd) and for recombination between the HLA region and S. With either our own data on 26 affected sib pairs or a much larger sample assembled by pooling with published data, the maximum-likelihood estimate of the two-dose penetrance is 71%, of the one dose penetrance 6.5%. The hypothesis of differential penetrance according to gene dosage provides a significantly better explanation than either simple dominant or simple recessive inheritance. 4. Estimates of the recombination fraction between HLA and S are very sensitive to the proportion of affected sib pairs sharing neither HLA haplotype, which varies considerably among the samples pooled. With the pooled data the recombination fraction is estimated as 3%, but in view of the strong associations with particular HLA alleles seen in population data, this should probably be considered an upper limit.

Histocompatibility antigen frequencies in diabetic retinopathy

We studied the histocompatibility antigens A and B in 300 insulin-dependent diabetics: 200 had proliferative retinopathy and 100 did not. The two groups were matched for known duration of diabetes and other clinical features. In both groups the frequencies of HLA-B8, HLA-B18, and HLA-B8/HLA-B15 were significantly higher, and those of HLA-B7 and HLA-B12 were significantly lower than in healthy controls. The patients with proliferative retinopathy were significantly less often positive for HLA-B7 (X2 = 10.0; Pc < .03) than patients with nonproliferative retinopathy. When both groups were stratified for age at diagnosis, there were additional differences. HLA-B15 was significantly more frequent in the proliferative retinopathy group with age at diagnosis between 15 and 40 years (nonproliferative retinopathy = 16.4%; proliferative retinopathy = 39.4%; X2 = 7.89, Pc < .03; relative risk = 3.32) and HLA-B7 significantly less frequent (nonproliferative retinopathy = 23.6%; proliferative retinopathy = 5.6%; X2 = 8.0, Pc < .03; relative risk = 0.19). These differences in histocompatibility frequencies between patients with and without proliferative retinopathy indicate a genetic contribution to diabetic retinopathy.

Linkage analysis between the major histocompatibility system and insulin-dependent diabetes in families with patients in two consecutive generations

We have histocompatibility (HLA) genotyped 28 families with insulin-dependent diabetics in two or more consecutive generations, usually parent and child. This strategy of ascertainment was used to maximize the likelihood of obtaining a homogeneous type of disease within a family, and an autosomal dominant mode of inheritance. 76 diabetics and 169 nondiabetics were studied in these families. The frequencies of the antigens Dw3 and Dw4, and the genotype Dw3/Dw4 among the diabetics are 59, 68, and 30%, respectively, as compared with 15, 12, and 2% in normal controls, and 43, 41, and 10% in the nondiabetic relatives of the diabetics. Dw2 is present in only one diabetic (4%), as compared with 18% in normal controls and 17% in nondiabetic relatives.HLA haplotype concordance was analyzed for sib pairs in relation to the haplotype shared by the affected parent/child pair, and for the diabetic sib pairs within each sibship. The results failed to reveal deviations in the expected HLA haplotype assortment. Assuming an autosomal dominant mode and several penetrance levels, linkage analysis between the HLA and diabetes was performed. The total lod score is 0.37 for a recombination fraction of 0.29 at 50% penetrance. Although the linkage and concordance analysis results are inconclusive, they seem to be different from those reported by us for families with normal parents and two or more diabetic sibs. Because ascertainment biases may have influenced these results in an unquantifiable manner, it is not certain whether the two types of families are genetically different. However, the marked difference in the lod scores for the 50% penetrant autosomal recessive model between the two types of families is compatible with a genetic dissimilarity between them. The high frequency of the Dw3 and Dw4 antigens, the Dw3/Dw4 genotype, and the decreased frequency of Dw2, however, indicate the existence of two or more important diabetic genetic factors associated with the D region of the HLA in these families.