Roles of GWA genes in controlling pancreatic beta cell function and mass.

Diabetes mellitus affects more than 20 m Europeans and 300 m individuals worldwide. The complications of the disease, including blindness, kidney failure, cardiovascular disease and cancer, drastically reduce the quality of life of sufferers, and consume almost 10 % of health care costs in most westernised nations. The most common form, Type 2 diabetes (T2D), has both genetic and environmental causes, and is particularly prevalent in the overweight. Insulin is the chief "anabolic" hormone responsible for restoring blood sugar levels after a meal and its absence - as well as a failure to act appropriately in T2D - underlies the symptoms of this disease.

Therapeutic approaches towards T2D have relied in the past on enhancing the actions of insulin, and more recently on stimulating insulin secretion. However, none of the existing therapies reverse the progressive loss of functional beta cells and hence the gradual worsening of disease symptoms. New strategies aimed at restoring an adequate number of normally functioning beta-cells are thus urgently sought.

Studies made possible by the human genome project, completed in 2000, and the subsequent mapping of genetic variants (single nucleotide polymorphisms) in man have permitted in the last four years "Genome wide association studies" (GWAS) for T2D. These have identified several genetic variants whose inheritance is associated with an increased risk of diabetes. The identification of these genes, most of which influence insulin production, provides both improved powers of prediction and, just as excitingly, potential new molecular targets for drug treatment.

Our proposed studies will involve two complementary areas aimed at obtaining a fuller understanding of how a subset of T2D genes affect disease risk. Our first aim is to examine in detail the role in the pancreatic beta cell of five genes identified at two novel GWAS "loci" in the human genome. We shall use both a new mouse model engineered to allow genes of interest to be selectively deleted in the pancreatic beta cell, and classical biochemical and mouse genetic approaches to examine the roles of the implicated gene products.

Secondly, we seek to examine the means through which a tumour suppressor termed LKB1, which controls beta cell growth, may be modulated by two relatively well-studied GWA genes for T2D. Inactivation of LKB1 in man leads to a rare form of cancer termed Peutz-Jeghers syndrome. Remarkably, when the LKB1 gene is deleted selectively in the beta cell, mutant mice experience a substantial expansion in beta cell mass and enjoy increased insulin production and improved glucose homeostasis. Importantly, insulin release only occurs when blood glucose levels are high, avoiding the risk of a dangerous "undershoot" in glucose (hypoglycaemia). Here, we shall dissect the signalling pathways lying downstream of LKB1, and explore the potential roles of the GWA genes TCF7L2 and SLC30A8.

Type 2 diabetes (T2D) represents an epidemic of the 21 st century and consumes almost 10 % of the health care budgets in westernised societies. Progressive pancreatic beta cell failure is a sine qua non for the development of the disease. Whilst diabetes risk is markedly increased by factors including obesity, T2D shows a strong element of heritability. Correspondingly, recent genome wide association studies have identified loci affecting disease risk, the majority of which alter insulin production rather than action.

Our laboratories have been in the vanguard of those seeking to dissect the underlying molecular mechanisms. Under Aim 1, we will explore the roles in the beta cell of five genes found at two novel risk loci, selected for study using defined criteria. The first locus, close to a single nucleotide polymorphism (SNP) on chromosome 11, includes two genes: CENTD2, encoding the GTPase activating protein Arap1, and STARD10 encoding a phospholipid transfer protein of presently unknown function. The second locus, on chromosome 9, encodes a cluster of genes implicated in cell cycle control: CDKN2A, CDKN2B and p14Arf, and a non-coding RNA termed ANRIL predicted to regulate the expression of the three former genes. In each case we shall use newly-available human beta cell lines, primary mouse and human islets, and a novel transgenic mouse (INS-Tva) which allows targeted delivery of shRNAs/miRNAs or cDNAs to the beta cell in vivo. In Aim 2 we shall dissect the mechanisms of action of the tumour suppressor LKB1, whose deletion in the beta cell causes a considerable increase in beta cell mass and enhances insulin release. We shall focus on the downstream roles of two well-established GWA genes: TCF7L2, a target of Wnt signalling, and SLC30A8, encoding the secretory granule zinc transporter ZnT8. The Programme will thus provide important insights into the mechanisms controlling beta cell mass and function, and potentially new avenues for therapy.

Impact Summary
The research proposed here is likely to benefit both the general population, in terms of improvements in healthcare, as well as the UK Pharmaceutical industry.

1. The general population of the UK. Type 2 diabetes affects ~3 m UK subjects and ~30 m Europeans (mean prevalence 8.4%; http://www.euphix.org/object_document/o4858n27165.html). These values are predicted to grow further in an "epidemic" driven by increasingly sedentary lifestyles and obesity. The complications of the disease include stroke, retinopathy, neuropathy, renal failure, cardiovascular disease and now cancer. The increased prevalence of this disease was recently predicted to contribute to a lowered overall life expectancy in the UK (http://www.independent.co.uk/life-style/health-and-wellbeing/health-news/diabetes-may-cause-first-fall-in-life-expectancy-for-200-years-966914.html) for the first time in 200 years. Treatment of diabetes is estimated to cost ~£8000 per year per patient, or £ 24 billion in total: diabetic patients are 3.5 times more likely to be admitted for hospital treatment than the rest of the population (http://www.physorg.com/news151077389.html). These direct economic costs together account for 7-13 % of health care costs in most developed societies (IDF Diabetes Atlas, 2003) and are further aggravated by increased absenteeism and decreased individual productivity (ADA: Diabetic Care 31, 596, 2008).

Pancreatic beta-cell loss or dysfunction are cardinal elements of diabetes mellitus, and strategies to rejuvenate or replace these cells, as explored here, are likely to be key to the development of new therapeutic approaches for this disease and its complications.
In particular this work will address roadblocks in diabetes research as identified recently by the European Commission's Support Action "DIAMAP: A Road Map for Diabetes in Europe" (http://www.diamap.eu/; 7th Sep, 2010) including the development of: "novel therapies based on beta cell mass and function" and "a lack of appropriate models that mimic the human condition".

2. The UK Pharmaceutical Industry. The global market for anti-diabetes drugs is estimated to be worth ~$30 billion. Following its joint Workshop with the Association of British Pharmaceutical Industries (ABPI) in March 2011, the MRC concluded that pancreatic beta cell biology should become a Strategic Priority, alongside Stratified Medicine. The present application addresses both of these areas.

New drug targets and leads are desperately needed for the Pharmaceutical industry to produce novel approaches to diabetes treatment. By addressing highly promising new targets, including those identified by GWA screens, and others likely to contribute to beta cell dysfunction, the proposed study will enhance feeds of new Intellectual Property to this sector. As a member of the trans-European diabetes research network "IMIDIA" (http://www.imidia.org/) the PI already interacts with several UK and Europe-based companies (eg Astra Zeneca, Sanofi Aventis, Boehringer-Ingelheim, Novo Nordisk and Novartis), and has established a collaboration with Pfizer (US) to initiate high throughput screens for small molecule regulators of LKB1, which may control beta cell mass. Similarly, with Cenix, an SME based in Dresden, he has begun a collaboration to use genome-wide RNAis screens to identify endogenous regulators of GWA genes including TCF7L2 and ZnT8.

Each of the three research fellows directly involved in the project will enhance their professional skills with training in basic biomedical research and thus develop their skill set for application in both the academic and commercial sectors.