Immunogenetic studies in autoimmune endocrine diseases

Abstract

Type 1 diabetes mellitus (T1DM) is an autoimmune disease characterised by Tlymphocyte infiltration of islets of Langerhans with consequent destruction of beta cells. A permissive genetic background is required for the development of the islet autoimmune process. The strongest genetic association identified is that with HLA class II genes located on the short arm of chromosome 6. It is well known that both HLA DRB1*04-DQA1*0301DQB1*0302 (DR4-DQ8) and DRB1*03DQA1*0501-DQB1*0201 (DR3-DQ2) are positively, and DRB1*15-DQA1*0102-DQB1*0602 is negatively, associated with T1DM. However, only a minority of die subjects carrying the high-risk haplotypes/genotypes develops the disease, which suggests that additional genes play a crucial role in conferring either protection or susceptibility to T1DM. The heterogeneity of the clinical and immunological features of T1 DM in relation to age at clinical onset is a complicating factor.

In our project we identified, by first, LADA patients among subjects with clinical diagnosis of type 2 diabetes. The presence of GAD65Ab, but not that of IA2/ICA512Ab, identified a large subgroup of autoimmune diabetic subjects, initially classified as having type 2 diabetes, with an increased risk for thyroid and adrenal autoimmunity and increased frequency of the haplotype HLA-DR3/DQ2. With the aim of increasing the diagnostic specificity of GAD65Ab, we studied the selection of GAD65 antibody epitopes. Autoantibodies directed to the COOH-terminal end of GAD65 (GAD65CAb) were more frequent in insulin treated subjects, while the exclusive presence of autoantibodies to the middle region of the autoantigen (GAD65-MAb) was more frequent in subjects treated with hypoglycaemic agents and/or diet The diagnostic specificity for insulin requirement increased from 96.9% of the typical GAD65Ab assay to 99.4% of GAD65-CAb.

Recently, a distinct family of MHC genes has been identified within the class III region and has been denominated MHC class I chain-related genes (MIC). Sequence analysis of the MICA gene revealed a trinucleotide repeat (GCT) microsatellite polymorphism within the transmembrane region and 5 alleles were identified and named A4, A5, A6, A9 and A5.1.

In our study, we demonstrated the age-dependent association of MICA gene polymorphism with autoimmune diabetes. More specifically, MICA allele 5 was associated with childhood T1DM, even in the absence of HLA DR3/DQ2-DR4/DQ8. On the other hand, both MICA allele 5.1 and HLA DR3/DQ2 and/or DR4/DQ8 were required to significantly increase the risk for adult onset of T1DM and LADA.

The availability of distinct genetic markers for either rapid or slowly progressive T1DM may prove important for the understanding of the molecular mechanisms of genetic predisposition for this autoimmune diseases, as well as for the organization of clinical trials of primary and secondary prevention.

MICA gene polymorphism marks the genetic risk also for autoimmune Addison's disease.

According to our data, the combination of MICA allele 5.1 and HLA-DR3/DQ2 is now to be seen as the most important genetic marker for autoimmune Addison's so far identified.