The molecular genetics of insulin secretion and signalling
Keywords: genetics
Type 2 diabetes
monogenic diabetes

Type 2 diabetes (T2D) and fasting plasma glucose (fpg) levels have distinct genetic components which are as yet only modestly understood. Understanding the genetics of this complex disorder and its related traits is likely to be of significant benefit to the field. Not only will it shed light on critical genes, pathways and mechanisms of regulation, but it may also contribute to the development of pharmaceutical anti-hyperglycaemic agents via the identification of key therapeutic targets. Therefore the aim of this thesis was to utilise a broad, multidisciplinary approach to study the genetics of insulin secretion and signalling.
Traditionally genes which harbour rare variants causing monogenic beta-cell dysfunction have also been found to harbour common variants associated with T2D and fpg. As genome-wide association studies (GWAS) identify an increasing number of common variants and genes, they also increase the number of genes which act as monogenic candidates. I screened G6PC2, a novel fpg associated gene, in patients with monogenic forms of beta-cell dysfunction and demonstrated that rare variants in this gene are unlikely to be a common cause of monogenic beta-cell dysfunction.
Although GWAS have been of considerable benefit to our understanding of complex disease genetics, they are not without their own limitations, primarily concerning signal refinement. To try to overcome this barrier for T2D and fpg signals I established a pipeline for fluorescence activated cell sorting of human islets to obtain pure beta-cells. In these cells, I performed transcript profiling of genes falling within T2D and fpg associated loci, demonstrating how this approach, alongside physiological analysis, can be of benefit for GWAS researchers and provide a starting point for fine mapping. Access to human beta-cells also enabled me to follow up one novel fpg association signal, SLC2A2. Through analysis within this metabolically relevant tissue I was able to establish that the mechanism for increased fpg levels is unlikely to be mediated via a beta-cell pathway.
Although GWAS have highlighted a number of key genes associated with beta-cell dysfunction; they have been far less successful at identifying genes associated with insulin resistance, another key component of T2D pathogenesis. Additional approaches, including the study of rodent models, may be required to study this aspect of T2D. PTEN is known to negatively regulate the insulin signalling pathway and adipose tissue specific Pten-/- animals were shown to be markedly insulin sensitive. To assess the role of PTEN in human insulin sensitivity I performed mRNA expression profiling of PTEN in human adipose tissue biopsies from subjects with T2D and matched controls, demonstrating that PTEN is significantly reduced in the subcutaneous adipose tissue of the former. This response is likely to be a compensatory mechanism to counteract muscular insulin resistance although further investigation needs to be performed to determine the mechanism of compensatory downregulation.
These data provide insights into a number of aspects of T2D genetics, and demonstrate how a multidisciplinary approach is of benefit to T2D genetic research.