HLA haplotypes in type 1 (insulin-dependent) diabetes mellitus: molecular analysis of the HLA-DQ locus. The DIME Study Group

The emerging global epidemic of type 1 diabetes

Type 1 diabetes mellitus (T1D) can occur at any age, with a peak in incidence around puberty. Classification between T1D and type 2 diabetes becomes more challenging with increasing age of onset of T1D over time develops in genetically predisposed individuals. The main susceptibility is conferred with human leukocyte antigen (HLA) genes. Some of the geographic variation in incidence and familial aggregation is explained by differences in HLA haplotypes. In many populations, the incidence is somewhat higher in males than in females, and a 1.3- to 2.0-fold male excess in incidence after about 15 years of age exists in most populations. The incidence of childhood-onset T1D varies markedly among countries. East Asian and native American populations have low incidences (approximately 0.1-8 per 100 000/year), while the highest rates are found in Finland (>60 per 100 000/year), Sardinia (40 per 100 000/year), and Sweden (47 per 100 000/year). The risk is highest in European-derived populations. About 10 %-20 % of newly diagnosed childhood cases of T1D have an affected first-degree relative. Those with an affected sibling or parent have a cumulative risk of 3 %-7 % up to about 20 years of age, as compared with <1 % in the general population. The cumulative incidence among the monozygotic co-twins of persons with T1D is less than 50 %. Thus, the majority of genetically predisposed people do not develop T1D. Studies assessing temporal trends have shown that the incidence of childhood-onset T1D has increased in all parts of the world. The average relative increase is 3 %-4 % per calendar year. For instance, in Finland, the incidence today is 5 times higher than 60 years ago. At the same time, the age at onset of T1D in children has become younger. It is strongly believed that nongenetic factors are important for the development of T1D and its increase, but the causative evidence is missing. The causes for this increasing trend and current epidemic still remain unknown.

Full likelihood analysis of genetic risk with variable age at onset disease--combining population-based registry data and demographic information

Background

In genetic studies of rare complex diseases it is common to ascertain familial data from population based registries through all incident cases diagnosed during a pre-defined enrollment period. Such an ascertainment procedure is typically taken into account in the statistical analysis of the familial data by constructing either a retrospective or prospective likelihood expression, which conditions on the ascertainment event. Both of these approaches lead to a substantial loss of valuable data.

Methodology and Findings

Here we consider instead the possibilities provided by a Bayesian approach to risk analysis, which also incorporates the ascertainment procedure and reference information concerning the genetic composition of the target population to the considered statistical model. Furthermore, the proposed Bayesian hierarchical survival model does not require the considered genotype or haplotype effects be expressed as functions of corresponding allelic effects. Our modeling strategy is illustrated by a risk analysis of type 1 diabetes mellitus (T1D) in the Finnish population-based on the HLA-A, HLA-B and DRB1 human leucocyte antigen (HLA) information available for both ascertained sibships and a large number of unrelated individuals from the Finnish bone marrow donor registry. The heterozygous genotype DR3/DR4 at the DRB1 locus was associated with the lowest predictive probability of T1D free survival to the age of 15, the estimate being 0.936 (0.926; 0.945 95% credible interval) compared to the average population T1D free survival probability of 0.995.

Significance

The proposed statistical method can be modified to other population-based family data ascertained from a disease registry provided that the ascertainment process is well documented, and that external information concerning the sizes of birth cohorts and a suitable reference sample are available. We confirm the earlier findings from the same data concerning the HLA-DR3/4 related risks for T1D, and also provide here estimated predictive probabilities of disease free survival as a function of age.

Analysis of TAP2 polymorphisms in Finnish individuals with type I diabetes

Type I diabetes mellitus is an immune-mediated disease that is known to be associated and linked with genes in the human leukocyte antigen (HLA) region on chromome 6. Functionally, HLA class I antigen presentation may be deranged in type I diabetes. The TAP1 and TAP2 transporters, which mediate the translocation of antigenic peptides into the endoplasmic reticulum and whose genes are located in the HLA class II region, are potential candidates for conferrring predisposition to type I diabetes. Five known coding region variants (codons 379, 565, 651, 665, and 687) as well as three new polymorphisms of TAP2, one silent (codon 604) and two intronic (nucleotide positions 49,270 and 49,471), were typed in a cohort of 146 well-characterized Finnish individuals with type I diabetes and 90 control subjects. Absolute linkage disequilibrium was apparent for the polymorphisms at codons 604, 665, and 687 as well as the two downstream intronic polymorphisms in a 613-bp region of the 3' portion of TAP2; the polymorphism at codon 651, which is also present within this region, was excluded from this linkage. The codon 651 polymorphism defines the allele TAP2F, the frequency of which in HLA-DR4+ diabetic subjects was 5.4 times that in DR4+ controls (27 vs. 5%, p = 0.002, p(c) = 0.01). These data are consistent with the existence of susceptibility haplotypes for type I diabetes in the Finnish population consisting of DRB1*04 (*0401 and *0404), DQ8, and TAP2F.

Association of chronic lymphocytic leukemia with specific alleles of the HLA-DR4:DR53:DQ8 haplotype in German patients

The etiology of chronic lymphocytic leukemia (CLL) remains unknown, though a genetic susceptibility has been suggested. Results of complete DNA typing of HLA alleles in CLL patients are lacking. We compared HLA class I and class II frequencies in 101 German CLL patients and 157 healthy German controls as determined by PCR-SSP/SSO DNA analysis and serologic typing. The most striking difference was the increased frequency of HLA-DRB4*0103 [relative risk (RR) = 2.74, p < 0.0025] among patients. The presence of alleles HLA-DRB1*0401, HLA-DQB1*0302 and HLA-DPB1*0301 as well as of homozygosity for HLA-DQB1 was also associated with a higher risk for CLL, though none of these differences remained significant after correction for multiple comparisons. No association was found for any HLA class I allele. Haplotype analysis revealed a CLL-specific linkage disequilibrium for HLA-DRB1*0401:DRB4*0103 and HLA-DRB4*0103:DQB1*0302. Our results suggest that CLL could be associated with distinct class II alleles of the Caucasian haplotype HLA-DR4:DR53:DQ8, which has also been related to a susceptibility for several auto-immune diseases. The positive, though weak, association of CLL with HLA-DPB1*0301 might represent an independent susceptibility factor since no linkage disequilibrium existed with any of the other CLL-associated alleles. None of the previously reported associations with HLA class I antigens was confirmed. Our results suggest that factors within or close to the human MHC class II region confer susceptibility to CLL.

HLA alleles and IDDM in children in Hungary: a comparison with Finland

It has been postulated that variation in the distribution of human leukocyte antigen (HLA)-encoded susceptibility alleles for insulin-dependent diabetes mellitus (IDDM) is the genetic basis for variation in the incidence of the disease between populations. The aim of this study was to characterize HLA-encoded susceptibility to IDDM in Hungary and to identify whether HLA-DRB1/DQ-encoded susceptibility could account for the five times lower incidence of disease in Hungary compared with Finland. The haplotypes DRB1*03-DQA1*05-DQB1*02 (DRB1*03-DQ2) and DRB1*04-DQA1*0301-DQB1*0302 (DRB1*04-DQ8) were significantly associated with disease in both populations. Three genotypes incorporating either or both of these haplotypes accounted for over 70% of the diabetic population in both races. The combined background frequency and the degree of risk as measured by odds ratios of these HLA-DRB1-DQ genotypes were not significantly different in the two countries. Comparison of the DRB1*0401-DQ8 haplotype between the two races suggested a role for HLA-B alleles in susceptibility. These data indicate that the susceptibility associated with high risk DRB1-DQ genotypes alone is insufficient to account for the fivefold variation in incidence of IDDM between Hungary and Finland. Other genetic and/or environmental influences must be involved.

Derivation of diabetes-resistant congenic lines from the nonobese diabetic mouse

Autoimmune diabetes is a polygenic disease process in man and rodents. To identify and characterize genes involved in the pathogenesis of diabetes in nonobese diabetic (NOD) mice, we initiated a repetitive backcross of diabetes-resistant C57L/J mice onto the NOD strain. This breeding scheme was based on the premise that selection for the trait of disease resistance among genetically mixed mice could be used to maintain transmission of nonpermissive alleles from the diabetes-resistant strain at critical diabetes susceptibility loci. Each of the three recombinant congenic mouse lines derived by this strategy retains a unique constellation of C57L/J-derived DNA segments. Consistent with the involvement of different genetic loci, the pancreatic histology of disease-resistant mice differs from that in NOD mice in a line-specific manner. Functional studies using these lines demonstrate that pathogenesis of autoimmune diabetes is a multistep process which can be blocked at a minimum of three critical, genetically determined points.

A susceptibility region for myasthenia gravis extending into the HLA-class I sector telomeric to HLA-C

We have analyzed a series of HLA region markers in 207 UK Caucasoids with early-onset myasthenia gravis (EOMG, onset before age 40), where there is a strong female bias. The well known associations with HLA-DR3 and -B8 have now proved to be significantly stronger in the 165 females than in the 42 males. In patients (of either sex) lacking -DR3, there was also a significant increase in HLA-DR2. Although the muscle weakness in EOMG is clearly mediated by autoantibodies, the associations are consistently stronger with HLA-B8 (in class I) than with HLADR3 (in class II), as confirmed here. We therefore typed 87-137 cases for polymorphisms at four loci in the intervening class III region, and also at three in the adjacent stretch of class I. At each locus, one allele tended to co-occur with HLA-B8 and showed strong and highly significant associations in the patients. There appeared to be a region of maximal susceptibility extending from HSP70 (in class III) past HLA-B and HLA-C at least 600 kb telomerically into the class I region, which is now being mapped in detail. Any candidate genes here that act shortly after puberty may allow more precise localization of susceptibility.

Autoimmune diabetes: the role of T cells, MHC molecules and autoantigens

Type 1 diabetes (IDDM) is a T cell mediated autoimmune disease which in part is determined genetically by its association with major histocompatibility complex (MHC) class II alleles. The major role of MHC molecules is the regulation of immune responses through the presentation of peptide epitopes of processed protein antigens to the immune system. Recently it has been demonstrated that MHC molecules associated with autoimmune diseases preferentially present peptides of other endogenous MHC proteins, that often mimic autoantigen-derived peptides. Hence, these MHC-derived peptides might represent potential targets for autoreactive T cells. It has consistently been shown that humoral autoimmunity to insulin predominantly occurs in early childhood. The cellular immune response to insulin is relatively low in the peripheral blood of patients with IDDM. Studies in NOD mice however have shown, that lymphocytes isolated from pancreatic islet infiltrates display a high reactivity to insulin and in particular to an insulin peptide B 9-23. Furthermore we have evidence that cellular autoimmunity to insulin is higher in young pre-diabetic individuals, whereas cellular reactivity to other autoantigens is equally distributed in younger and older subjects. This implicates that insulin, in human childhood IDDM and animal autoimmune diabetes, acts as an important early antigen which may target the autoimmune response to pancreatic beta cells. Moreover, we observed that in the vast majority of newly diagnosed diabetic patients or individuals at risk for IDDM, T cell reactivity to various autoantigens occurs simultaneously. In contrast, cellular reactivity to a single autoantigen is found with equal frequency in (pre)-type 1 diabetic individuals as well as in control subjects. Therefore the autoimmune response in the inductive phase of IDDM may be targeted to pancreatic islets by the cellular and humoral reactivity to one beta-cell specific autoantigen, but spreading to a set of different antigens may be a prerequisite for progression to destructive insulitis and clinical disease. Due to mimic epitopes shared by autoantigen(s), autologous MHC molecules and environmental antigens autoimmunity may spread, intramolecularly and intermolecularly and amplify upon repeated reexposure to mimic epitopes of environmental triggers.

Evaluation of TAP1 polymorphisms with insulin dependent diabetes mellitus in Finnish diabetic patients. The Childhood Diabetes in Finland (DiMe) Study Group

Insulin dependent diabetes mellitus (IDDM) is an autoimmune disease with a strong association between disease and the HLA class II region. Because abnormal antigen processing, in part characterized by altered class I processing, has been identified in patients with IDDM, the TAP (transporter associated with antigen processing) genes located in the HLA class II region make attractive candidate genes for IDDM. Five coding region variants of TAP1 were typed in a cohort of well characterized Finnish patients with diabetes (n = 119) and compared to racially marched control subjects (n = 92). We found that although no single TAP1 polymorphism was associated with IDDM, a genotypic combination of Ile/Val at codon 333 with Asp/Asp at codon 637 was found more frequently in subjects with IDDM (9.4%) compared to controls (1.2%; p = 0.025). This could not be accounted for by an association with any particular haplotype defined by class I or class II serology.

An association between type 1 diabetes and the interleukin-1 receptor type 1 gene. The DiMe Study Group. Childhood Diabetes in Finland

The polygenic susceptibility to type 1 diabetes is well established and recent studies have demonstrated linkage of a further locus on chromosome 2q to disease. We have studied a polymorphism of the interleukin-1 receptor type 1 gene (IL1R1) on chromosome 2q in type 1 diabetic and control subjects from Finland, the United Kingdom, South India: three populations in which the risk of disease varies from very high to very low. In the medium-risk U.K. population we find a very strong association of IL1R1 with type 1 diabetes (p = 0.0002) but we find no overall association in either the high-risk Finnish or low-risk South-Indian populations. However, we do find heterogeneity of risk at IL1R1 amongst Finnish diabetic subjects according to the possession of HLA-DR associated susceptibility (p = 0.0001); there is an association with IL1R1 in only those Finnish diabetic subjects who do not possess high-risk HLA-DR4 or DR3 haplotypes (p = 0.006), as recently demonstrated for the insulin gene region in this population. We find no such heterogeneity of risk in either the U.K. or South-Indian populations. This study further demonstrates the genetic heterogeneity of disease susceptibility between and within populations and also supports the hypothesis of an interaction of the IL1R1 locus with genes within the HLA and insulin gene regions in the susceptibility to type 1 diabetes.

In Finland insulin gene region encoded susceptibility to IDDM exerts maximum effect when there is low HLA-DR associated risk. DiMe (Childhood Diabetes in Finland) Study Group

An association between insulin-dependent diabetes mellitus (IDDM) and polymorphisms of the insulin gene on chromosome 11p15 (INS) is a consistent finding in Europid populations. While one study suggested that the INS association is restricted to HLA-DR4-positive individuals, studies in other Europid populations have shown the disease-associated INS genotype to confer susceptibility independently of HLA-DR. We have investigated the role of INS in susceptibility to IDDM in Finland, which has the highest incidence of diabetes mellitus in the world, at two polymorphic restriction sites, 5' and 3' to the insulin gene. From the DiMe (Childhood Diabetes in Finland) Study we studied 154 diabetic children without regard to HLA-DR type; 108 DR4 positive/non-DR3 diabetic children; 39 DR3 positive/non-DR4 diabetic children; 30 DR4/DR3 positive diabetic children; 31 non-DR4/non-DR3 diabetic children; 96 matched DiMe control subjects and 86 other healthy, non-diabetic Finnish control subjects. We found an overall association between IDDM and INS in the high-risk Finnish population only with the 5' polymorphism and identified an INS haplotype negatively associated with IDDM in Finland. However, among diabetic subjects with a reduced HLA-associated susceptibility (non-DR4/non-DR3) both 3' and 5' INS loci showed an association with IDDM (p values 0.02 and 0.0002, respectively). Thus, in the Finnish population insulin gene-encoded susceptibility to IDDM exerts a maximum effect in those with reduced HLA-associated risk.

A new marker in the HLA class I region is associated with the age at onset of IDDM

The (MHC) class II association with insulin-dependent diabetes mellitus (IDDM) is well documented. However, it is likely that genes within the MHC class III and the class I region also play a role in determining susceptibility to IDDM. In this study we have used a novel molecular probe to investigate the class I P3A and P3B loci of 179 patients with IDDM and 142 normal control subjects. A highly significant increase in the frequency of the class I P3 4.0;1.5 kilobase (kb) and 4.0;1.8;1.5 kb genotypes was found in patients compared to the control subjects (chi 2 46.8, 6 df, p < 0.0001). The association with the P3B 1.5 kb allele was strongly associated with the age at onset of diabetes, being present in 96.2% of subjects who developed diabetes between the age of 10-20 years compared to 55.0 and 74.6% who developed diabetes before 10 years or after 20 years, respectively (chi 2 31.4, p < 0.0001). There was no evidence for linkage disequilibrium between the DQA1 and DQB1 loci and P3B suggesting that this is an independent association. In conclusion, these results suggest that genes in both the MHC class I and II regions confer susceptibility to IDDM and are related to the age at onset of the disease.

Effect of genetic risk load defined by HLA-DQB1 polymorphism on clinical characteristics of IDDM in children

Clinical and autoimmune characteristics of 150 diabetic children of mean age 7.8 years (SD 4.1 years) were recorded at clinical manifestation and during the first 2 years of IDDM in order to investigate whether subjects with high risk HLA-DQB1 genotypes differ from those without these risk markers. When comparing subjects with the DQB1*0302/0201, DQB1*0302/x, DQB1*0201/x, or other DQB1 genotypes (x = no protective allele), no differences were found in the age of the subjects at diagnosis, the duration of hyperglycaemic symptoms, or the length of clinical remission. The frequency of islet cell antibodies (ICA) and quantitative serum levels of these antibodies were of the same magnitude in all four groups. During the initial 2 years of IDDM serum C-peptide concentrations were observed to be inversely related to the degree of genetic risk (P < 0.001 in two-way analysis of variance for repeated measures), the lowest C-peptide levels being observed in the group of DQB1*0302/0201 heterozygotes (P < 0.001 vs. DQB1*0201/x; P < 0.01 vs. DQB1*0302/x; P = 0.05 vs. others). On the other hand, the subjects with the DQB1*0201 genotype had the highest serum C-peptide concentrations, the levels being even higher than those of the patients carrying neutral or protective DQB1 genotypes (P < 0.01). These subjects also had lower daily insulin doses and blood glycated haemoglobin A1 (HbA1) levels over the initial 2 years of the disease when compared with the DQB1*0302/0201 heterozygotes (P < 0.05 and P < 0.01, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Variation in HLA-associated risks of childhood insulin-dependent diabetes in the Finnish population: I. Allele effects at A, B, and DR loci. DiMe Study Group. Childhood Diabetes in Finland

Variation in the risk of insulin-dependent diabetes mellitus (IDDM) across alleles at HLA-A, B, and DR loci was investigated in a population-based study of 801 families of children with newly diagnosed IDDM in Finland nationwide. Parallel analyses assessed the relative frequencies of alleles in IDDM children compared with age-matched sibling controls and with the four possible genotypes which could have been inherited from the parents. The joint effects of DR3 and DR4 alleles were investigated under dominant, recessive, and additive models of gene expression. The additive model gave the best fit, though the relative risk for DR4 homozygotes was smaller than predicted. To investigate other alleles, we fitted the standard multiplicative model for alleles at each locus. After controlling for the correlation among alleles, significantly elevated risks were found for B13, DR3, DR4, and DR14. Subjects with these alleles have more than twice the risk of IDDM as those without. Alleles A24 and B62 incurred relative risks between one and two. DR2 and DR5 were significantly negatively associated with IDDM, incurring less than half the risk. These findings support an independent role of class I antigens in the etiology of IDDM.

A gene in the HLA class I region contributes to susceptibility to IDDM in the Finnish population. Childhood Diabetes in Finland (DiMe) Study Group

In Finland the haplotype A2, Cw1, B56, DR4, DQ8 is the third most common haplotype in insulin-dependent diabetic (IDDM) patients and has the highest haplotype-specific absolute risk for IDDM. Cw1, B56, DR4, DQ8 haplotypes containing HLA-A alleles other than A2 are infrequent in the population and are not associated with IDDM. Comparison of the A2 and non-A2 haplotypes at the DNA level showed that they were identical at HLA-B, -DR, and -DQ loci. Evidence that class I alleles confer susceptibility to IDDM was obtained from the two HLA-C, -B, -DR and -DQ haplotypes most frequently found in IDDM patients in Finland. A24, A3 and A2 on the Cw3, B62, DR4, DQ8 haplotype, and A28, A2 and A1 on the Cw7, B8, DR3, DQ2 were all found to be associated with IDDM. In Finland these seven haplotypes, including A2, Cw1, B56, DR4, DQ8, account for 33% of diabetic haplotypes and 10.3% of non-diabetic haplotypes (p < 0.00001). The contribution of the class I region to IDDM susceptibility was also apparent in those IDDM patients lacking the disease-predisposing class II alleles. Significantly more non-DR3/non-DR4 IDDM patients (47 of 55) possessed two of the IDDM-associated HLA-A alleles compared to non-DR3/non-DR4 control subjects (40 of 58; p = 0.038). Moreover, IDDM patients confirmed by oligotyping as unable to form a 'diabetes-susceptibility' DQ heterodimer, tended to possess two diabetes-associated HLA-A alleles (12 of 13) compared to control subjects (12 of 20; p = 0.056).

Multiple sclerosis: multiple etiologies, multiple genes?

Multiple sclerosis is a chronic inflammatory disease characterized by multifocal damage of the central nervous system myelin. Both humoral and cell-mediated immune abnormalities have been observed in patients with multiple sclerosis, but their relation to the demyelination process is not understood. The etiology of the disease is still unknown; however, evidence exists for an interplay between environmental and genetic factors. Several genes are involved in determining the disease susceptibility, at least one of them encoded within human leukocyte antigen gene complex. Other genomic regions coding for components of the immune system or myelin have also been suggested. Clinical, immunological and genetic data suggest that multiple sclerosis may turn out to be a heterogeneous disease. Therefore, molecular genetic dissection of this complex disease should provide important clues to its pathogenesis as well as unravel metabolic pathways for potential therapeutic or preventive strategies. This review will give an overview of recent progress and future challenges in identifying susceptibility genes for multiple sclerosis.

Genetic susceptibility to non-insulin dependent diabetes mellitus and glucose intolerance are located in HLA region

OBJECTIVES

To test the hypothesis that the genetic susceptibility to non-insulin dependent diabetes mellitus is the same as that to insulin dependent disease and to see whether glucose intolerance is associated with specific HLA haplotypes.

DESIGN

Population based study of men in 1989 first tested for glucose tolerance in 1984. HLA haplotypes, including HLA-A, C, B, DR, and DQ, were defined serologically. HLA haplotype data from a population based Finnish study of childhood diabetes were used for predicting non-insulin dependent diabetes and impaired glucose tolerance.

SETTING

Two communities in Finland.

SUBJECTS

Representative cohort of Finnish men aged 70-89, comprising 98 men with non-insulin dependent diabetes mellitus and a randomly selected group of 74 men, who served as controls, who were tested for glucose tolerance twice within five years.

MAIN OUTCOME MEASURES

Non-insulin dependent diabetes, impaired glucose tolerance, blood glucose concentration.

RESULTS

Diabetes associated HLA haplotypes were present in 94% (85/90) of diabetic subjects, 79% (27/34) of subjects with impaired glucose tolerance, and only 13% (3/23) of non-diabetic subjects. In this group of elderly men sensitivity of the diabetes associated HLA haplotypes for non-insulin dependent diabetes and impaired glucose tolerance was 90%, specificity 87%, and predictive power 97%. Mean fasting blood glucose concentration was only just significantly higher in men with diabetes associated haplotypes than in men with no such haplotypes, but there was a substantial difference in blood glucose values two hours after glucose loading (10.4 and 6.4 mmol/l in men with diabetes associated HLA haplotypes and men with no such haplotypes, respectively (p < 0.0001)).

CONCLUSIONS

These findings support the hypothesis that specific HLA haplotypes exhibit a common genetic determinant for insulin dependent and non-insulin dependent diabetes. Furthermore, HLA is a major genetic determinant of glucose intolerance in elderly Finnish men. The belief that the HLA predisposition to diabetes is specific for insulin dependent diabetes mellitus is largely incorrect.

The immunogenetics of insulin-dependent diabetes

Insulin-dependent (type 1) diabetes mellitus (IDDM) is a multifactorial disease with a strong genetic component. The majority of the genetic component can be explained by associations between IDDM and genes in the major histocompatibility complex (MHC). The best single marker for IDDM is based on amino acid polymorphism of the HLA-DQ gene. Current evidence, however, indicates that the MHC susceptibility to IDDM is determined by a combination of HLA class I, II and III genes contained on HLA haplotypes. A non-MHC genetic component to IDDM also exists. To date, the most consistent association is between IDDM and markers of the insulin gene locus.

DQA1 and DQB1 heterodimers in insulin-dependent diabetes mellitus: a genetic-epidemiological study in Finland. DiMe Study Group

Susceptibility to insulin-dependent diabetes mellitus (IDDM) correlates with the absence of aspartic acid in position 57 of the DQB1 and/or the presence of arginine in position 52 of the DQA1. It has been postulated that transcomplementation between the DQ alpha and beta chains of the two haplotypes could create new molecules conferring susceptibility to IDDM. Finland has the highest incidence of IDDM in the world (35/100,000). In a nationwide study of IDDM in childhood (DiMe study) HLA genotyping using conventional serology was carried out according to genetic-epidemiological principles. We simulated DQA1 and DQB1 alleles in 707 consecutively diagnosed IDDM probands and 98 non-diabetic children based on serology, restriction fragment length polymorphism results and sequence data assuming no recombination between DQ and DR. In 34% of Finnish children with IDDM all four combinations (two in cis and two in trans) could lead to SS heterodimers. Two-thirds of these combinations were explained by DR3,DR4 heterozygotes. In 50% of IDDM children half and in 11% a quarter of the combinations could lead to heterodimers. In 38 IDDM patients (5%) the formation of hybrid molecules was not possible. In 59% of the controls SS heterodimers were possible and should therefore have an underlying genetic susceptible for IDDM assuming the theory of transcomplementation is correct. These findings, together with the fact that the lowest frequency of DR3,DR4 heterozygosity (21%) was seen in Finland, show that heterozygosity for DQ and DR cannot explain the differences seen in IDDM incidence.

Genetic epidemiology of insulin-dependent diabetes mellitus: international comparisons using molecular genetics

International comparative molecular studies of insulin-dependent diabetes mellitus can be viewed from an immunogenetical and an epidemiological perspective. Each approach independently contributes to our knowledge concerning the etiology and prevention of the disease. However, the combination of these perspectives, representing an interface between molecular biology and epidemiology, will achieve more in terms of scientific advancement and public health than either perspective alone. An excellent example of the development of molecular epidemiology has been the evolution of international comparative research through the WHO Multinational Project for Childhood Diabetes. Case-control molecular studies are currently being conducted to test the hypothesis that population variation in the frequency of HLA class II susceptibility alleles is the primary determinant of the global patterns of insulin-dependent diabetes mellitus incidence. Preliminary data suggest that this hypothesis is correct. Future international molecular epidemiological studies, encompassing host-environment interactions, will lead to a better understanding of the relationships among specific risk factors within populations and across the world. Molecular epidemiology represents a new paradigm for studying the etiology and epidemiology of infectious and non-communicable diseases, and no doubt represents a major challenge for the 1990s.