The CTLA-4 gene region of chromosome 2q33 is linked to, and associated with, type 1 diabetes. Belgian Diabetes Registry

What is the evidence of genetic factors in the etiology of Graves' disease? A brief review

Graves' disease (GD) is generally thought of as a multifactorial disorder in which genetic susceptibility interacts with environmental and endogenous factors to cause disease. The importance of genetic factors is suggested by the clustering of GD within families and by a higher concordance rate for disease in monozygotic than in dizygotic twins. This has, however, recently been shown to be less pronounced than previously thought. During the last decade much effort has been put into characterization of the genetic background of GD. Until recently, most studies have examined associations between GD and the human leukocyte antigen (HLA) region, but recent advances in molecular techniques have opened the way for whole genome screening. A number of HLA and non-HLA candidate genes have been proposed, but despite several large investigations within multiplex families no major susceptibility genes have been identified. This brief review discusses relevant articles published from 1940 through 1997 regarding the influence of genetic factors in the etiology of GD. Ongoing studies focus on whole genome screening in multiplex families as well as population-based twin studies. However, the possibility of GD being a heterogeneous disease without a single well-defined genotype and phenotype should be left open.

A second-generation screen of the human genome for susceptibility to insulin-dependent diabetes mellitus

During the past decade, the genetics of type 1 (insulin-dependent) diabetes mellitus (IDDM) has been studied extensively and the disorder has become a paradigm for genetically complex diseases. Previous genome screens and studies focused on candidate genes have provided evidence for genetic linkage between polymorphic DNA markers and 15 putative IDDM susceptibility loci, designated IDDM1-IDDM15. We have carried out a second-generation screen of the genome for linkage and analysed the data by multipoint linkage methods. An initial panel of 212 affected sibpairs (ASPs) was genotyped for 438 markers spanning all autosomes, and an additional 467 ASPs were used for follow-up genotyping. Other than the well-established linkage with the HLA region at chromosome 6p21.3, there was only one region, located on chromosome 1q and not previously reported, where the log likelihood ratio (lod) was greater than 3. Lods between 1.0 and 1.8 were found in six other regions, three of which have been reported in other studies. Another reported region, on chromosome 6q and loosely linked to HLA, also had an elevated lod. Little or no support was found for most reported IDDM loci (lods were less than 1), despite larger sample sizes in the present study.

Genetic susceptibility to systemic lupus erythematosus

Considerable evidence suggests that the development of systemic lupus erythematosus (SLE) has a strong genetic basis. Recent studies have emphasized that this disease, like other autoimmune diseases, is a complex genetic trait with contributions from major histocompatibility complex (MHC) genes and multiple non-MHC genes. Etiologic genes in these disorders determine susceptibility, and no particular gene is necessary or sufficient for disease expression. Studies of murine models of lupus have provided important insight into the immunopathogenesis of IgG autoantibody production and lupus nephritis, and genetic analyses of these mice overcome certain obstacles encountered when studying patients. Genome-wide linkage studies of different crosses have mapped the position of at least 12 non-MHC disease-susceptibility loci in the New Zealand hybrid model of lupus. Although the identity of the actual genes is currently unknown, recent studies have begun to characterize how these genetic contributions may function in the autoimmune process, especially in terms of their role in autoantibody production. Studies of MHC gene contributions in New Zealand mice have shown that heterozygosity for particular haplotypes greatly increases pathogenic autoantibody production and the incidence of severe nephritis. The mechanism for this effect appears to be genetically complex. Studies in human SLE have mostly focused on the association of disease with alleles of immunologically relevant genes, especially in the MHC. Associations with various complement component deficiencies and an allele of a particular Fc gamma receptor gene (FCGR2A) also have been described. In a diversion from previous association studies, a recent directed linkage analysis of sibpairs with SLE was based on mapping studies in murine lupus and may be an important step toward identifying a new disease-susceptibility gene in patients. Since the genes that predispose to autoimmunity are probably related to key events in pathogenesis, their identification in patients and murine models will almost certainly provide important insight into the breakdown of immunological self-tolerance and the cause of autoimmune disease.

CTLA-4 promoter variants in patients with Graves' disease and Hashimoto's thyroiditis

Graves' disease (GD) and Hashimoto's thyroiditis (HT) are T-cell mediated organ-specific autoimmune disorders with a genetic predisposition. The cytotoxic T-lymphocyte antigen 4 (CTLA-4) molecule is the predominant receptor for B7 on activated T cells and represents a negative regulator for T-cell function. Since the CTLA-4-guanine at position 49 of exon 1 is associated with susceptibility to GD as well as to HT and IDDM, we investigated a recently detected cytosine/thymine substitution at position -318 within the CTLA-4 promoter region in patients with GD and HT. 125 patients with GD were significantly more often homozygous for cytosine (86% vs. 73% in controls, P=0.006) and less frequently heterozygous for cytosine and thymine (14% vs. 27%, P=0.008). In 64 patients with HT, the distribution was similar but not significant (81% homozygous for cytosine and 16% heterozygous). When correlating the promoter and the exon 1 polymorphism we found the strongest linkage between thymine (promoter) and adenine (exon 1). In conclusion, a promoter variant of the CTLA-4 gene represents an additional risk marker for GD and HT, but their predisposition is linked to the exon 1 alleles.

Major histocompatibility class II genes polymorphism in insulin dependent diabetes mellitus with or without associated thyroid autoimmunity

Insulin dependent diabetes mellitus (IDDM) is sometimes associated with extrapancreatic organ-specific autoimmune diseases, but whether this phenotype results from a peculiar genetic profile is still unclear. The allelic distribution of the major histocompatibility complex (MHC) class II genes (HLA-DRB1, DQA1, DQB1 and TAP) was analysed in 143 patients with IDDM alone by comparison with 82 IDDM patients with autoimmune thyroid disease (IDDM/AITD). The frequency of the DQB1*0301 IDDM-protective phenotype seemed to be lower in IDDM than in IDDM/AITD patients (16.8% vs 30.5% respectively, p = 0.02). By contrast, the frequency of the DRB1*04-DQB1*0302 IDDM-predisposing phenotype was higher in IDDM than in IDDM/AITD patients (91.3% vs 76.1% of DR4-positive patients respectively, p = 0.007), but these differences were not significant after correcting the p values, except in the case of the DRB1*0405-DQB1*0302 combination (21.3% vs 2.4% of DR4-positive patients, Pc = 0.05). Furthermore, all differences disappeared when patients were matched for age at IDDM-onset. Our data do not long give support for a particular role of MHC class II genes in favouring the occurrence of thyroid autoimmunity in IDDM patients, but rather suggest that some class II alleles or residues might determine the rapidity of progression to IDDM in genetically susceptible individuals. The involvement of non-MHC genes and/or environmental factors remains to be determined.

Cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) regulates the unfolding of autoimmune diabetes

Evidence has been accumulating that shows that insulin-dependent diabetes is subject to immunoregulation. To determine whether cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) is involved, we injected anti-CTLA-4 mAb into a TCR transgenic model of diabetes at different stages of disease. When injected into young mice, months before they would normally become diabetic, anti-CTLA-4 induced diabetes rapidly and essentially universally; this was not the result of a global activation of T lymphocytes, but did reflect a much more aggressive T cell infiltrate in the pancreatic islets. These effects were only observed if anti-CTLA-4 was injected during a narrow time window, before the initiation of insulitis. Thus, engagement of CTLA-4 at the time when potentially diabetogenic T cells are first activated is a pivotal event; if engagement is permitted, invasion of the islets occurs, but remains quite innocuous for months, if not, insulitis is much more aggressive, and diabetes quickly ensues.

CTLA4 codon 17 dimorphism in patients with rheumatoid arthritis

The genetic susceptibility to rheumatoid arthritis is conferred by genes in the human leukocyte antigen (HLA) region on chromosome 6, but additional genes may be involved to determine disease susceptibility. We have studied the distribution of the CTLA4 exon 1 polymorphism (49 A/G) in rheumatoid arthritis. This dimorphism at codon 17 results in an amino acid exchange (Thr/Ala) in the leader peptide of the expressed protein and was analyzed by PCR, SSCP and RFLP in 258 Caucasian rheumatoid arthritis patients and 456 controls. Rheumatoid arthritis patients were characterized by a decreased frequency of homozygotes for the Thr-17 substitution (32% versus 39%) and an overrepresentation of patients heterozygous for the Thr/Ala substitution (54% versus 46%). Gene frequencies for the Ala/Thr substitution differed only marginally from controls. In contrast, analyses of the CTLA4 exon 1 polymorphism with respect to HLA-DRB1*04 revealed significantly more patients with Ala in the homozygous (19% versus 15% controls) or heterozygous state (54% versus 39% controls) and less homozygous for Thr (27% versus 46% controls), with a particular increase of Ala/Ala genotypes among rheumatoid arthritis patients carrying the HLA-DRB1*0401 subtype. Among HLA-DRB1*04 negative rheumatoid arthritis patients, we observed no difference between the allele frequencies of the Ala-17 or Thr-17 substitution.

CTLA4 gene polymorphism confers susceptibility to Graves' disease in Japanese

Susceptibility to Graves' disease (GD) is determined by environmental and genetic factors. The genetic susceptibility to GD is conferred by genes in the human leukocyte antigen (HLA), and several other genes unlinked to HLA are thought to contribute to the development of GD. Three recent papers described the association of GD with the CTLA-4 gene. CTLA-4 is a candidate gene for T-cell mediated autoimmune diseases because it is a negative regulator of T-cell proliferation. As CTLA-4 association with GD may be influenced by the racial composition of the population, it is important to study it in other ethnic groups. We investigated the distribution of CTLA-4 gene polymorphism in 153 Japanese patients with GD (35 males and 118 females) and 200 controls (96 males and 104 females). An A/G transition at position 49 of exon 1 was analyzed by the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. The distribution of genotype frequencies differs between GD and controls (chi2 = 9.46, 2 degrees of freedom, p < 0.01). The presence of at least one G allele (GG or AG) conferred an odds ratio of 2.64 (95% CI = 1.92-3.36). The present study supported the association of the CTLA-4 gene with GD in Japanese and showed that the CTLA4 gene could be one of the non-HLA linked susceptibility genes for GD.

Genetic control of diabetes progression

Autoimmune diabetes in both the human and the nonobese diabetic mouse has elaborate genetics; in the latter case, the disease is influenced by at least 15-20 loci. We anticipated that the genetics would be simpler in the BDC2.5 T cell receptor transgenic mouse model of diabetes, wherein many T cells express a particular diabetogenic specificity. Initiation of insulitis in this model was the same on the two genetic backgrounds analyzed, but the kinetics and penetrance of diabetes were strikingly different, permitting us to focus on genetic influences during a defined window of disease progression. The differences correlated with variations in five genomic intervals, certain ones of which have been previously implicated in susceptibility to autoimmune disease. This reductionist approach indeed simplified the analysis of diabetes susceptibility loci.

Codon 17 polymorphism of the cytotoxic T lymphocyte antigen 4 gene in Hashimoto's thyroiditis and Addison's disease

Endocrine autoimmune disorders share susceptibility and resistance factors of the human leukocyte antigen system on the short arm of chromosome 6, but other gene loci also contribute to predisposition and protection. Because the cytotoxic T lymphocyte antigen 4 (CTLA4) alanine-17 encoded by the CTLA4 gene on chromosome 2q33 confers susceptibility to Graves' disease, as well as to type 1 (insulin-dependent) diabetes mellitus, we investigated this dimorphism in the other endocrine autoimmune disorders: Hashimoto's thyroiditis and Addison's disease. We analyzed the CTLA4 exon 1 polymorphism (49 A/G) in 73 patients with Hashimoto's thyroiditis, 76 with Addison's disease, and 466 healthy controls. This dimorphism corresponds to an aminoacid exchange (Thr/Ala) in the leader peptide of the expressed protein. CTLA4 alleles were defined by PCR, single-strand conformational polymorphism analysis, and restriction fragment length polymorphism analysis using BbvI. Patients with Hashimoto's thyroiditis had significantly more Ala alleles than controls, both as homozygotes (22% vs. 15%) and heterozygotes (53% vs. 46%), and less Thr than controls as homozygotes (25% vs. 39%), P < 0.04. The phenotypic frequency for Ala was significantly higher in patients (75%), compared with controls (61%), P < 0.03. Patients with Addison's disease did not differ significantly from controls, but those carrying the suceptibility marker, human leukocyte antigen DQA1*0501, were significantly more CTLA4 Ala17 positive than controls with the same DQA1 allele (P < 0.05). In conclusion, an alanine at codon 17 of CTLA4 confers genetic susceptibility to Hashimoto's thyroiditis, whereas this applies only to the subgroup of DQA1*0501+ patients with Addison's disease.

The genetic susceptibility to Graves' disease

Graves' disease (GD) develops as a result of a complex interaction between genetic susceptibility genes and likely environmental factors. Most epidemiological data support an important genetic contribution to the development of GD. The concordance rate of GD in monozygotic twins is 30-60% and in dizygotic twins 3-9%, and thyroid autoantibodies have been reported in up to 50% of the siblings of patients with GD. For many years now, HLA studies have consistently shown an increased frequency of HLA-DR3 in Caucasian patients with GD; but with only a risk ratio of 3-5. However, recent advances in human genome mapping techniques have enabled the study of many other candidate genes. Of these additional, non-HLA genes, only CTLA-4 has been consistently found to be associated with GD. Using a linkage based approach which only detects highly significant susceptibility genes we have recently reported preliminary results which demonstrated that a marker located approximately 25 cM from the TSH receptor gene on chromosome 14q31 is linked to GD and in the same vicinity as the IDDM-11 locus. Such results, if confirmed, may signal the presence of a gene family related to endocrine autoimmunity on chromosome 14q31.

Genetics in the prediction of insulin-dependent diabetes mellitus: from theory to practice

Only weak associations have been found for particular environmental factors and development of insulin-dependent diabetes mellitus (IDDM). Very few studies have, however, accounted for genetic susceptibility when cases and controls have been compared. The genetics of IDDM is complex, but the HLA-DQ genes are the most important. There are many different combinations of DQA1 and DQB1 genes conferring disease risk to differing degrees. The strategy of NOBADIA (Norwegian Babies against Diabetes) is to identify at birth the babies in the general population with the highest genetic risk for developing IDDM: those carrying DQA1*0301-DQB1*0302/DQA1*0501-DQB1*0201 (for simplification, hereafter named DQ2/DQ8). Four per cent of Norwegian babies carry this genotype which accounts for 46% of future cases of IDDM. Babies carrying the IDDM high-risk genotype have a lifetime risk of 12% for developing the disease. This is very close to the risk of a first-degree relative with the same genotype. DQ2/DQ8 heterozygotes also acquire the disease earlier than those with a lower genetic risk. Parents of children carrying the DQ2/DQ8 genotype will be informed and offered regular follow-up with blood samples and questionnaires at their public health care centre.

Costimulation and autoimmunity

The past year has seen significant advances in our understanding of the role of the B7-CD28/CTLA-4 pathway in T cell activation and self-tolerance. Recent studies have demonstrated that CTLA-4 is a critical negative regulator of T cell activation and autoreactivity, revealing a previously unsuspected means by which costimulation is involved in the maintenance and breakdown of self-tolerance. Manipulation of this costimulatory pathway in animal models of autoimmunity has shown an important role for this pathway in both the initiation and progression of autoimmune diseases.

Panning for gold: genome-wide scanning for linkage in type 1 diabetes

Genome-wide scans for linkage of chromosome regions to type 1 diabetes in affected sib pair families have revealed that the major susceptibility locus resides within the major histocompatibility complex (MHC) on chromosome 6p21 (lambda S = 2.4). It is recognized that the MHC contains multiple susceptibility loci (referred to collectively as IDDM1), including the class II antigen receptor genes, which control the major pathological feature of the disease: T-lymphocyte-mediated autoimmune destruction of the insulin-producing pancreatic beta cells. However, the MHC genes, and a second locus, the insulin gene minisatellite on chromosome 11p15 (IDDM2; lambda S = 1.25), cannot account for all of the observed clustering of disease in families (lambda S = 15), and the scans suggested the presence of other susceptibility loci scattered throughout the genome. There are four additional loci for which there is currently sufficient evidence from linkage and association studies to justify fine mapping experiments: IDDM4 (FGF3/11q13), IDDM5 (ESR/6q22), IDDM8 (D6S281/6q27) and IDDM12 (CTLA-4/2q33). IDDM4, 5 and 8 were detected by genome scanning, and IDDM12 by a candidate gene strategy. Seven other named loci are not discounted but remain to be replicated widely. Multiple susceptibility loci were expected as genome-wide scans of the mouse model of type 1 diabetes had shown that although the MHC is the major mouse locus, at least 13 genes unlinked to the MHC are involved in the development of disease. Genome-wide scans using 1000 affected sibpair families will be required to be confident that all genes with effects on familial clustering equivalent to the insulin gene locus (lambda S = 1.25) have been detected. The identification of aetiological determinants requires exclusion of hitchhiking polymorphisms in regions of linkage disequilibrium, as demonstrated for the MHC and the insulin gene loci, and functional studies implicating the disease-associated variant in pathogenesis. Ultimately, targeting of specific candidate mutations in mice by homologous recombination and replacement will be necessary to prove the primary role of any candidate mutation.