AT-repeat polymorphisms of the human CTLA-4 gene: Associations with autoimmune diabetes and allele-specific variation of expression

Autoimmune diabetes is a complex disease which involves both environmental and genetic factors which cause a poorly understood breakdown of peripheral tolerance to the beta-cells of the pancreas. This highly specific loss of self-tolerance results in the T cell-mediated destruction of the cells which produce insulin, resulting in type I diabetic disease. CTLA-4 is a candidate gene for tissue-specific autoimmune disease, such as type 1 diabetes, as it is expressed in activated T cells and has been shown to be essential for proper down-regulation and control of T cell responses, and may play an important role in establishing and/or maintaining peripheral tolerance. This gene contains at least two polymorphic regions in many human populations which have been shown to be linked to and associated with both autoimmune diabetes and other tissue-specific autoimmune diseases. Of particular interest is the AT-repeat length polymorphism contained within the 3′ untranslated region (UTR) of the CTLA-4 gene. As part of this dissertation, I show that this polymorphism is associated with an increased risk of autoimmune diabetes in a study of consecutively diagnosed type I diabetes patients from Sweden. To further investigate the possible role of this CTLA-4 polymorphism in human autoimmune disease, I conducted functional T cell studies, created reporter gene constructs, and employed RNA expression assays to examine how this polymorphism associated with diabetes might affect the function and expression of CTLA-4 in human T cells. The novel RT-PCR assay I developed for comparing allelic expression of different CTLA-4 AT-repeat alleles yielded statistically significant results which indicated that the length of the AT-repeats in the 3 ′ UTR of the gene affected CTLA-4 expression at the level of RNA expression. This observation may provide some of the first biochemical data which helps develop a mechanistic model for how polymorphisms in the human CTLA-4 gene contribute to the T-cell immune dysfunction which results in autoimmune diabetes.