Functional evaluation of the type 1 diabetes (T1D) susceptibility candidate genes.

Progress has been made in investigating the genetic factors involved in type 1 diabetes (T1D) development for the past few years. While Linkage disequilibrium (LD) mapping has been useful for both the confirmation and fine-mapping of susceptibility intervals, as well as identification of etiological mutations, identification of specific disease genes has been a challenge and limited to known candidate genes. The overall risk for T1D from the HLA DR and DQ molecules (IDDM1) is determined by combinations of polymorphic alleles. Functional studies indicate that the susceptible and protective HLA-DR and -DQ bind and present non-overlapping peptides. Although consistent linkage evidence was reported for the susceptibility intervals IDDM2, IDDM5 and IDDM12, evidence for most other intervals varies in different data sets. The variable number of tandem repeats at the 5' end of the insulin gene (IDDM2) regulates insulin expression in the thymus. Studies on IDDM5 have led to the discovery of a novel polymorphism 163 A-->G (M55V) in SUMO4 gene, which was found to be associated with T1D patients with Asian origin. Functionally SUMO4 conjugates to IkBalpha and negatively regulates NFkB transcriptional pathway. The M55V substitution reduces the sumoylation activity of the V55 variant, which resulted in higher NFkB dependent transcriptional activity. The polymorphisms of the cytotoxic T lymphocyte antigen 4 gene (CTLA4, IDDM12) encoding a regulatory molecule in the immune system associate with T1D and autoimmune thyroid diseases (ATD). The 3' untranslated region of this gene determines the level of soluble CTLA-4. Genetic mapping of variants conferring a small disease risk can identify pathways in complex disorders, as evidenced by quantitative alterations of candidate genes contributing to autoimmune tissue destruction. Moreover, the identification of two transcription factors that, when mutated, are responsible for severe autoimmune disease is leading to a better understanding of T cell tolerance. Both AIRE and Foxp3, identified initially via their association with genetically manipulated mice, are involved in tolerance induction in humans. Although mutations in these genes may cause rare but serious diseases, it is likely that other transcription factors will contribute to the genetic load that predisposes certain individuals to disease.