Genetic and functional studies of novel type 2 diabetes susceptibility genes

Diabetes is a complex metabolic disease, with both genetic and environmental causes, in which the body?s ability to maintain normal blood glucose levels is compromised. In recent years, diabetes has developed into a global health problem of epidemic proportions, consuming as much as 10 % of the health care budgets of many westernised countries. Increases in blood glucose concentrations have very serious health implications, and can lead to blindness, heart disease, kidney malfunction and nerve damage. There are two main clinical subtypes of diabetes: type 1 diabetes is characterised by the destruction of the insulin-secreting pancreatic cells that regulate blood glucose levels; and type 2 diabetes (T2D) is characterised by insulin resistance and dysfunctional insulin secretion. Of the two subtypes, T2D is by far the predominant form of diabetes, accounting for up to 90% of the total diabetes prevalence.

Our research is focussed on the identification of the genes that predispose an individual to develop T2D and on elucidating the molecular mechanisms by which the products of these T2D ?susceptibility genes? influence an individual?s risk of T2D. We have recently completed the first ever genetic scan of all 46 human chromosomes for T2D susceptibility genes (the results of this research will be published in a top journal, Nature, later in 2007). Our results confirmed a previously known susceptibility gene on chromosome 10 (called TCF7L2) and identified novel T2D susceptibility genes, including a zinc transporter gene called ZnT-8. Zinc is very important for regulating the pancreatic secretion of the hormone insulin that acts to control blood glucose levels.

The proposed project aims to follow up on our genetic scan by: a) carrying out an exhaustive genetic analysis of these novel genetic loci in the European population and b) elucidating the molecular mechanisms by which the key genes, TCF7L2 and ZnT-8, contribute to T2D risk.

Type 2 diabetes is a major and growing health problem with both genetic and environmental causes. Together with our collaborators at McGill University, Montreal, Canada, we have performed, in a French population, the first published genome wide association (GWA) scan for T2D genes (Sladek et al., Nature, 2007, in press). Our results replicated the established strong association with the TCF7L2 SNP rs7903146 (p=1.51 x 10-34) and confirmed associations for eight SNPs in four novel T2D susceptibility loci.

This project aims to carry out exhaustive fine mapping of these 4 loci (and additional loci ones likely to appear from ongoing WGA studies) in the European population and to elucidate the molecular mechanisms by which the transcription factor TCF7L2 and the Zinc transporter ZnT-8 contribute to T2D risk.

We have three specific aims:

Aim 1: We will undertake fine mapping of the novel susceptibility loci in order to identify the putative aetiological variants that are responsible for the association signals. We shall genotype the entire catalogue of common SNPs at all four loci in our extensive French case-control samples. T2D-associated SNPs will be subjected to large scale replication and prospective cohort studies.

Aim 2: TCF7L2 is a transcription factor targeted by the Wnt signalling pathway, important for pancreatic development. We seek to determine the following: what is the impact of TCF7L2 silencing on glucose- and other nutrient-stimulated insulin secretion, beta-cell proliferation/survival, and the expression of key beta-cell genes? What is the impact of TCF7L2 over-expression on beta-cell secretory function, proliferation/apoptosis, and beta-cell gene expression? How does the TCF7L2 rs7903146 polymorphism affect mRNA stability, splicing and transcriptional activity? What is the phenotype of knock-in mice bearing the at-risk rs7903146 allele?

Aim 3. Apart from rs7903146 in TCF7L2, the strongest WGA signal was provided by the coding SNP rs13266634, a R325W variant in SLC30A8 (ZnT-8), a beta-cell restricted vesicular zinc transporter. We will firstly examine the importance of ZnT-8 for normal beta cell glucose metabolism, basal and stimulated insulin secretion as well as beta-cell growth and survival. We will then determine the effect of the R325W variant on ZnT-8 activity and function, in terms of protein stability, sub-cellular localisation, and zinc uptake into insulin vesicles.

We believe that this integrated genetic and functional approach to the study of novel T2D susceptibility loci should bring new insights to the molecular determinants of this disease, opening new avenues for predicting and treating diabetes and its complications.