Clinical Gems

Clinical utility of T1DM susceptibility genes: One of the goals of genetic studies of complex disease is to identify a profile of susceptibility variants that can be used to predict an individual’s risk of developing a given disease. The long prodrome for T1DM, characterized by progressive loss of beta-cell mass, provides an attractive opportunity for intervention to prevent disease development if “at-risk” individuals can be identified. Currently the best markers of disease risk are a positive family history of T1DM and the presence of autoantibodies to islet cell proteins. Over 90% of T1DM patients have no affected relatives, however, so effective preventive efforts will need to target the general population. Unfortunately screening such huge numbers of individuals for islet autoantibodies is logistically unfeasible, particularly given the need for repeated annual testing for those with a negative result. Genetic markers are therefore needed to stratify risk in the general population,

Other genetic markers—the “missing heritability": Collectively the confirmed T1DM risk loci account for approximately 70% of disease heritability, with around 40–50% being attributed to the HLA genes. These figures are well in excess of the 10–20% of heritability of other complex diseases that can be explained by genetic factors. Experience from GWAS suggests that overall disease risk is likely to be influenced by many genes, most having a weak biologic effect. This may be due to the subtle effects of risk alleles on gene function or the modest contribution of individual gene products to the biologic pathways involved in disease pathogenesis. None of the confirmed T1DM risk variants have complete penetrance and are therefore neither necessary nor sufficient for disease to develop. This makes it difficult to use genetic profiling to predict disease risk as T1DM can develop in the absence of susceptibility variants and does not always occur in subjects with known risk markers.

Linkage studies identify regions of the genome that are shared more frequently than would be expected by chance by relatives affected by a particular disease. Most studies analyze affected sibling pairs and utilize genetic markers that are scattered throughout the genome at moderate density, typically microsatellites. A significant excess of allele sharing identical by-descent (IBD) in affected sibpairs suggests that the region containing the marker locus also contains a disease susceptibility locus. The first linkage scan for T1DM identified 20 chromosomal regions with suggestive evidence of linkage to disease, including the HLA and INS gene regions.

The CTLA4 gene on chromosome 2q33.2 encodes a transmembrane co-receptor expressed on the surface of T cells. This functions as a negative regulator of T-cell activation via interaction with the B7 molecule on antigen-presenting cells (Figure 30.3). The G allele of the +49A/G SNP in exon 1 of CTLA4 has been implicated as a susceptibility marker for T1DM, but was rejected as a causal SNP by a fine-mapping study, which showed that its effect could be explained by more strongly associated variants in a 6.1 kb noncoding region, 3′ of the gene.

Other HLA-encoded susceptibility determinants: The DR and DQ genes cannot completely explain the association between T1DM and the HLA gene region. Teasing out the contribution of other HLA loci, however, is complicated by the high level of linkage disequilibrium within the region, combined with the strong effects of DR and DQ. A number of different approaches have been employed to address this issue, including the analysis of case/control data matched for specific DR-DQ combinations, investigation of the transmission to affected and unaffected offspring of heterozygous markers from parents homozygous for DR-DQ alleles, conditional logistic regression and conditional haplotype analysis.

HLA-DR and -DQ: The highest risk of T1DM is conferred by heterozygosity for the DRB1*0301-DQA1*0501-DQB1*0201 and DRB1*04-DQA1* 0301-DQB1*0302 haplotypes, referred to as the DR3.DQ2/DR4.DQ8 genotype. This allelic combination is carried by 30–40% of individuals with T1DM, but only around 2.5% of the general population [3]. A recent meta-analysis of multiple ethnic groups suggested that this translates into an OR value greater than 16, an unusually large odds ratio for a complex disease [14]. This is consistent with an earlier study which estimated that the risk of developing T1DM was between 1 in 15 and 1 in 25 among those with the DR3.DQ2/DR4.DQ8 genotype, compared with 1 in 300 in the general population [15]. High risk is also conferred by the DR3.DQ2/DR3.DQ2 and DR4.DQ8/DR4.DQ8 homozygous genotypes (OR = 6.32 and OR = 5.68, respectively, from meta-analysis).

Type 1 diabetes (T1DM) is a chronic autoimmune disease in which the beta cells of the islets of Langerhans are selectively destroyed, resulting in insulin deficiency and hyperglycemia. The disease develops in genetically susceptible individuals, most likely as a result of an environmental trigger. T1DM has an uneven geographical distribution; disease prevalence is highest in populations of white European origin and lowest in those of East Asian descent. A marked gradient in disease risk also exists in Europe, with higher prevalence of T1DM in northern countries, particularly Finland, compared with areas around the Mediterranean. This pattern could be attributed to genetic differences between the populations or to the presence/absence of environmental triggers.

The relationship between pathogenic factors and beta-cell destruction remains poorly understood. Longitudinal studies of newborns and children at genetic risk and follow-up of at-risk first-degree relatives in natural history studies link the triggering of autoimmunity with the appearance of autoantibodies to one or more islet autoantigens; individuals with multiple autoantibodies have higher risk of diabetes progression.

Genes and environment: a moving target? Studies from multiple populations have shown shifts of the typical HLA class II gene associations during the last few decades. Indeed, fewer patients carry the high risk heterozygous HLA-DR/DQ genotype, except among younger children [59–63], in whom the disease is becoming more common [4]. In contrast, more patients now carry moderate risk HLA types and genotypes. Such shifts in HLA associations may be explained by stronger environmental pressures that enhance HLA-mediated genetic predisposition and/or broaden the spectrum of diabetogenic gene–environment interactions. Viruses, and especially enteroviruses, rank at the top of the list of environmental factors that have been linked to T1DM.

Insulin itself is a prototypical TRA, its synthesis being virtually restricted to pancreatic beta cells. Thymic insulin production is critical for establishing self-tolerance to beta cells; simply abolishing insulin expression in the thymus leads to the rapid onset of autoimmune diabetes even in mice lacking a diabetogenic genetic background [26]. In humans, thymic insulin expression is modulated by allelic variation and epigenetic effects at the insulin gene locus; this effect is largely mediated by a polymorphic variable nucleotide tandem repeat (VNTR) sequence.