A sub-phenotype approach to dissect the genetic control of murine type 1 diabetes

Sammanfattning: The non-obese diabetic (NOD) mouse is a model for human type 1 diabetes (T1D). The disease in the NOD mouse is polygenic and multifactorial and so far at least 20 insulin dependent diabetes (Idd) susceptibility loci have been identified. However, no etiological mutations have been definitely ascribed to the Idd loci. To identify potential etiological mutations, a sub-phenotype approach was undertaken, consisting of the establishment and genetic mapping of immuno-related sub-phenotypes that may contribute to the pathogenesis of T1D in the NOD mouse model. This thesis presents (1) the results of the identification and genetic mapping of four novel NOD immuno-phenotypes to individual Idd loci, and (2) confirmation of these results by the generation and analysis of congenic strains covering those Idd regions.Evidence is provided that gene(s) within the Idd5 region control cyclophosphamide (CY)-induced apoptosis in peripheral lymphocytes and y-irradiation induced apoptosis in NOD thymocytes. Analysis of non-obese resistant (NOR) and NOD-Idd5 congenic mice reveal that CY-induced apoptosis in peripheral lymphocytes and y-irradiation induced apoptosis in thymocytes are controlled by a 20cM and a 6cM region, respectively, both containing the Idd5 region and including the immuno-regulatory Ctla4 gene. Additionally, CTLA4 is shown to be defectively up-regulated in activated NOD peripheral lymphocytes, and CTLA4-deficient mice show similar defects in T cell apoptosis induction. Taken together, these results suggest that a defective up-regulation of CTLA4 mediates apoptosis resistance, contributing to diabetes pathogenesis.Moreover, it is shown that gene(s) within the Idd6 region control low proliferation ofNOD immature thymocytes and resistance to dexamethazone-induced apoptosis in immature DP thymocytes. The decrease of diabetes incidence and the restoration of the apoptosis resistance phenotype in reciprocal Idd6 congenic strains further restrict the chromosomal region controlling the Idd6 locus as well as the locus controlling the apoptosis resistance phenotype. In fact, analysis of NOD-Idd6 congenic mice reveal that Dxm-induced apoptosis in thymocytes is controlled by the distal 3cM region of the Idd6 locus. As the thymic selection process is highly dependent on both proliferation and apoptosis, the hypothesis is raised that the Idd6 locus contributes to the pathogenesis of diabetes by altering thymic selection, resulting in an autoimmune prone peripheral T cell repertoire.