Genetic heterogeneity between type 1a and type 1b insulin-dependent diabetes mellitus: HLA class II and TAP gene analysis

The convergence of type 1 and type 2 diabetes in childhood: the accelerator hypothesis

It seems likely that type 1 and type 2 diabetes lie at different points of the same spectrum, separated by the misunderstanding that one belongs to childhood and the other to adulthood. The spectrum is that of tempo--the rate at which beta cell function is lost over time. A combination of beta cell up-regulation (insulin demand, largely determined by obesity) and the genetically-determined immune response to it ('autoimmunity') determines tempo, ranging from slow to fast with every variant in between. There is good evidence that people who go on to develop type 1 (fast) diabetes are, like those who develop type 2 (slow diabetes), insulin resistant, and overwhelming evidence that body mass plays a key role. The prevention of type 1 diabetes may lie in weight restriction from an early age.

Changing perspectives in diabetes: their impact on its classification

Type 1 and type 2 diabetes are usually regarded as distinct disorders, but the convergence of their phenotypes over recent years, the relationship of body weight to the risk of type 1 diabetes, the diminishing importance of the type 1 susceptibility genes and the finding of autoantibodies in patients with type 2 diabetes, invite a different interpretation. The possibility that type 1 and type 2 diabetes, rather than being different, are merely poles of a single spectrum, where variation in the tempo of beta cell loss determines age at onset and symptoms at presentation, has important implications. Correct classification is crucial because it directs appropriate treatment and, where available, prevention. This article argues that type 1 diabetes is currently misclassified, provides evidence that insulin resistance drives type 1 diabetes as it does type 2, and proposes how the 'accelerator hypothesis' can be tested in a randomised controlled trial, which could demonstrate, for the first time, the safe and effective prevention of type 1 diabetes.

Is TAP2*0102 allele involved in insulin-dependent diabetes mellitus (type 1) protection?

In this study, we have investigated the frequencies of TAP1 and TAP2 alleles in a group of 226 persons, living in La Reunion Island, consisting of 70 patients with insulin-dependent diabetes mellitus (IDDM) and most of their first degree relatives (i.e., 156 parents and full sibling subjects) and previously HLA DQB1, DQA1, and DRB1 genotyped. The population of this island is constituted by a particular structure of highly crossbreeding people. Interestingly, the new TAP2*0104 allele, previously discovered by our team in Reunion Island, was found to be increased in the IDDM population and the calculated HRR was relatively high (HRR = 3.3). This result seems to be due to a positive linkage disequilibrium between TAP2*0104 allele and the highly diabetogenous DQB1* 0201-DQA1* 0501-DRB1 0301 haplotype (HRR = 9), which suggests that TAP2*0104 cannot be considered as an additional predispositional factor, but more as a genetic susceptibility marker of IDDM. In addition, we show that minor alleles (TAP2D, *0102, *0103, *0104) are associated with a restricted number of HLA DQ-DR haplotypes and each of them exhibits a preferential linkage with one particular haplotype. In contrast with other alleles, and despite a HRR value close to 1, we show that TAP2*0102 allele contributes significantly to a drastic reduction of the diabetogenic effect of DQB1*0201-DQA1*0301.1-DRB*0701 haplotype. Indeed, this haplotype, which is usually preferentially transmitted to affected children, is dominantly transmitted to healthy children when it is associated with TAP2*0102. Therefore, this allele seems to contribute to genetic protection to IDDM.

Polymorphisms of human TAP2 detected by denaturing gradient gel electrophoresis

The human transporter associated with antigen processing (TAP1 and TAP2) genes are located in the human leukocyte antigen (HLA) class II region of the genome and encode proteins that form a heterodimer essential for the transport of endogenous peptides into the endoplasmic reticulum for assembly with HLA class I molecules. Type 1 diabetes is an autoimmune disease that is associated with the HLA region of the genome, with HLA class II genes conferring the greatest statistical risk. The presentation of self-peptides by HLA class I molecules is defective in individuals with this disease, and both TAP1 and TAP2 are potential contributors to this defect. Denaturing gradient gel electrophoresis (DGGE) was applied to screen all 11 exons and the 3' flanking region of TAP2 for polymorphisms in individuals with type 1 diabetes patients and controls. Seventy polymorphisms, including 51 in introns, 4 in the 3' flanking region, and 15 in exons, were identified. Sequencing of polymorphic DNA fragments revealed several new polymorphisms, including a Gln --> Arg substitution at codon 611 and a GT --> GC polymorphism affecting the donor splice site of intron 4, that might be of functional significance. None of the polymorphisms examined differed in frequency between individuals with type 1 diabetes and controls.

Distribution of TAP gene polymorphisms and extended MHC haplotypes in Mexican Mestizos and in Seri Indians from northwest Mexico

The study of the genetic structure is very useful for investigating the biological significance of polymorphism and may provide clues to understand population origins. We present TAP1/TAP2 gene analysis in the Seri indians from Sonora, and in Mestizos from the highlands of Mexico. Thirty-two Seri and 89 Mestizos were studied. TAP genes were typed using the ARMS-PCR technique. The most frequent alleles in Seri were: TAP1*0101/02, (68.8%); TAP1*02011/02012, (31.2%); TAP2*0201, (38.7%) and TAP2*0101, (29.0%). TAP1*0301, TAP1*0401, TAP2*0102 TAP2*0103 and TAP2H were absent in them. For Mestizos, the prevalent alleles were: TAP1*0101/02 (75.8%); TAP1*02011/12 (20.3%); TAP2*0101 (45.4%) and TAP2*0201 (29.3%). These results are similar to those found in Kaingang and Caucasians from Brazil, four Mediterranean, other Caucasians, two Oriental and one African group. In Seri, the extended prevalent haplotypes are typically Amerindian, such as TAP1*0101/2-TAP2*0201-QBP3.21-DQB1*0302-QAP*3.1-DQA1*03011-DRB1*0407-B*3501-A*0201 (HF = 16.6%). Thirty-two extended haplotypes were found in Seri, although TAP contributed scarcely to diversity. Mestizos show Amerindian and Caucasian combinations. No difference was detected in the distribution of amino acids in the individual variable sites, between both groups. These findings are the basis for further anthropological studies and to explore the contribution of TAP genes to disease expression in Mexicans.

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.

TAPI polymorphisms in several human ethnic groups: characteristics, evolution, and genotyping strategies

Genetic variations in the locus encoding the transporter associated with antigen processing, subunit 1 (TAP1), were systematically studied using samples from Caucasians, Africans, Brazilians, and compared with data from chimpanzees. PCR-amplified genomic sequences corresponding to the 11 exons were analyzed by single-strand conformation polymorphism (SSCP) and sequencing. Six nonsynonymous and 2 synonymous single nucleotide polymorphisms (SNPs) were found to be common in one ethnic group or another, and they involved codons 254 (Gly-GGC/Gly-GGT) in exon 3, 333 (Ile-ATC/Val-GTC) in exon 4, 370 (Ala-GCT/Val-GTT) in exon 5, 458 (Val-GTG/Leu-TTG) in exon 6, 518 (Val-GTC/Ile-ATC) in exon 7, 637 (Asp-GAC/Gly-GGC), 648 (Arg-CGA/Gln-CAA) and 661 (Pro-CCG/Pro-CCA) in exon 10. At each SNP site the sequence listed first was predominant in all ethnic groups. Several SNPs segregated on the same chromosome regardless of populations and species. Together, the SNPs produced 5 major human TAP1 alleles, 4 of which matched the officially recognized alleles *0101, *02011, *0301, and *0401; the 5th allele differed from each of those by at least 4 SNPs. Overall, TAP1*0101 was the predominant allele in all ethnic groups, with frequencies ranging from 0.667 in Zambians to 0.808 in US Caucasians. The TAP1*0401 frequency showed the greatest difference between Africans (0.221-0.254) and Caucasians (0.033), with Brazilians (0.058) fitting in the middle. Consistent with earlier work based on Caucasians and gorillas, *0101 appeared to be the newest human TAP1 allele, suggesting a dramatic spread of *0101 into all human populations examined. Characterization of TAP1 polymorphisms allowed the design of a PCR-based genotyping scheme that targeted 7 SNP sites and required 2 separate genotyping techniques.

T cell autoreactivity to proinsulin epitopes in diabetic patients and healthy subjects

We investigated the immune response to proinsulin, a potential autoantigen in IDDM secreted exclusively by pancreatic beta-cells. A total of 2,142 short-term cell lines were generated from 19 individuals; seven IDDM patients at the disease onset and 12 control subjects. No increase in the frequency of proinsulin reactive cells was observed in the IDDM group. To define proinsulin epitopes, proliferative responses of proinsulin-specific lines were examined against 10 overlapping 15 amino acid peptides encompassing the human proinsulin sequence. The predominant immune response was directed against the proinsulin p35-50 peptide located in the (C) connecting peptide between the alpha- and beta-chain of insulin. Recognition of the proinsulin p35-50 peptide could be shown by generating specific T cell clones against the peptide. However, unlike responses to other tissue-specific autoantigens there were only low proliferative responses to proinsulin as measured by 3H-thymidine incorporation. This low reactivity may be partially explained by the location of the p35-50 peptide in the C-peptide which is released into the circulation and therefore, may induce a clonal anergy of T reactive cells. However, the significantly higher 3H-thymidine incorporation after CD3-CD28 triggering showed that peptide specific T cells were capable of a significant response with a stronger TCR signal.

A novel PCR-based methodology to determine TAP allele frequencies in population studies

The TAP genes have been extensively studied as candidate susceptibility genes for autoimmune and infectious diseases. TAP1 and TAP2 have two and three polymorphic sites respectively which are used for the allele assignment following WHO nomenclature. The usual techniques employed to determine the TAP alleles cannot unequivocally ascertain the alleles present in subjects which are heterozygous at more than one polymorphic position within the genes. This results in an inability to determine absolute TAP allele frequencies in population studies. The aim of this study was to devise a PCR-based method to unambiguously assign TAP alleles to all subjects. The novel method was tested in Oriental Type I diabetic and control subjects. The technique is a valuable tool for allele assignment in heterozygous subjects solely using PCR.

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.