Genetic studies provide clues about key disease mechanisms
Genetic predisposition is an important component of the multifactorial pathogenesis of T1DM. Although often diagnosed in individuals with no known family history of T1DM, the disease is about 15 times more common in siblings of a patient than in the general population. Siblings have an average risk of 6%, although individual risk varies significantly in relation to the extent of sharing predisposing genes with the proband, which allele variants are shared, and other factors. The risk to the offspring of affected mothers and fathers is about 2–3% and 6–7%, respectively . Among twins, the observed disease concordance rates are approximately 8–10% in dizygotic twins and, with extended follow-up, more than 60% in monozygotic twins .
Extensive genetic studies have been conducted during the past decades in thousands of families with affected sibling pairs, family trios, patients, and unrelated controls, including genome-wide association studies. The largest coordinated efforts were led by the Type 1 Diabetes Genetic Consortium (www.T1DGC.org), an international collaboration . A single, major susceptibility locus and a multiplicity of other loci (upwards of 50; see http://www.t1dbase.org), individually conferring a much smaller risk, have been identified ; indeed, only four of these genes (HLA Class II, insulin, PTPN22, and IL2RA) are associated with odds ratios greater than 1.5 . Several, but not all, of the genetic loci associated with T1DM have been confirmed in multiple data sets and populations . Many if not most of the susceptibility loci mapped to known genes encode for proteins involved in immune function; a few other genes are expressed in pancreatic islets and putative risk genes remain to be mapped at some recently associated loci . Besides inherited alleles, additional mechanisms may regulate gene expression and function, including epigenetic regulation, typically mediated by changes in DNA methylation ; in addition to contributing to susceptibility, epigenetic regulation may represent a mechanism by which gene–environment interactions can impact disease risk and could help explaining the significant increase in disease incidence reported in many populations . In the next paragraphs we will discuss putative disease mechanisms associated with the best studied risk genes. Figure 29.1 illustrates these mechanisms and compares the effects of predisposing and protective gene variants at the HLA-DQ, insulin, PTPN22, CTLA-4, IL2RA, and IHIF1 loci.
HLA-encoded genetic predisposition points at adaptive autoimmune responses as the dominant effector mechanism leading to β-cell destruction. As noted, the identification of several susceptibility genes involved in key immunologic functions supports a key role for autoimmunity in the disease pathogenesis. Many of these genes control critical functions of the adaptive immune system, in particular, T-cell responses and their regulation. Experimental evidence points at T cells as the main effectors of β-cell destruction. Accordingly, the primary susceptibility locus maps to the human leukocyte antigen (HLA) complex, specifically to HLA class II and class I antigen-presenting molecules; these function as restricting elements for CD4 and CD8 T-cell responses, respectively. HLA-encoded susceptibility confers up to 50–60% of the overall genetic risk from inherited alleles .
Within the HLA complex, HLA class II loci have the strongest association with disease risk; in particular, the HLA-DR3, DQA1*0501-DQB1*0201 (DQ2) and HLA-DR4, DQA1*0301-DQB1*0302 (DQ8) genotype is the strongest susceptibility factor with an odds ratio >40 . This genotype is typically carried by 30–50% of patients, compared to about 2% of the general population; most patients carry at least one of these two high-risk haplotypes. The higher risk conferred by the heterozygous genotype is explained by the formation of a trans-complementing HLA-DQ heterodimer, which can only be formed when the two haplotypes are inherited together, that is capable of presenting islet autoantigens and promote diabetogenic responses . On the other hand, HLA-DR4, DQA1*0301-DQB1*0301 (DQ7) haplotypes are
considered neutral. This observation links diabetes risk with the DQB1*0302 chain. In contrast, HLA-DR2 (DRB1*1501) haplotypes carrying DQA1*0102-DQB1*0602 are negatively associated with T1D Min several populations, with less than 1% of T1DM patients carrying this protective haplotype; reportedly, this haplotype confers strong protection from T1DM development also in first-degree relatives with autoantibodies .
Several HLA-DR4 subtypes are defined by polymorphisms of the DRB1 chain, for example DRB1*0401, DRB1*0404, and so on, and they show variable degrees of association with T1DM risk, even when in linkage with DQA1*0301-DQB1*0302; this suggests that the HLA-DRB1 molecule also plays an important role in modulating immune responsiveness to islet antigens . While there is significant linkage disequilibrium across the HLA complex, selected HLA class I alleles are also associated with T1DM, independently of class II genes, especially HLA-A2, HLA-A24, and HLA-B39 . Many studies suggest that additional loci within the HLA complex may modulate susceptibility. The largest and most systematic evaluation was conducted by the T1DGC, which reported data for more than 3000 single nucleotide polymorphisms, 66 microsatellites, and class I and class II DNA-based allele typing data in 2300 families with affected sibling pairs; these studies identified several associations involving many genes within the HLA complex, suggesting that HLA-encoded susceptibility derives from multilocus effects .
Genetic predisposition impacts central immune tolerance to islet cell antigens. Antigen presentation by HLA molecules is not only a key to the activation of adaptive immune responses, but also to the generation of regulatory T cells and the stimulation of other regulatory mechanisms. Antigen presentation is also required for T-cell selection processes occurring in the thymus during the maturation of the immune system to establish immunologic self-tolerance. Genetic predisposition can impair central (thymic) tolerance to selected autoantigens, and this may well represent the earliest event in T1DM pathogenesis.
Such mechanisms became apparent following the discovery that peripheral or tissue-restricted antigens (TRAs) from multiple organs are “ectopically” produced in the thymus to support the development of immunologic self-tolerance in early life . TRA expression in the thymus is primarily mediated by medullary thymic epithelial cells (mTECs), which transcribe many TRA genes under the control of the AIRE autoimmune regulator transcription factor . Thymic CD11c+ dendritic cells (DCs) also express insulin and other TRAs , but TRA expression levels are lower in DCs compared to mTECs . DCs can also uptake antigens produced by mTECs , a function that is also supported by AIRE .