Improving diagnosis of adult-onset diabetes using islet autoantibodies

Abstract

The diagnosis of diabetes type in adulthood can be difficult due to overlapping phenotypes and lack of clear classification guidelines. Type 1 and type 2 diabetes have very different treatment and care requirements, and incorrect classification can lead to life-threatening consequences. Islet autoantibodies are biomarkers of the autoimmune pathology of type 1 diabetes and can be used in prediction of risk and classification of diabetes type, however current tests have imperfect specificity and sensitivity, and there are a number of remaining questions for optimal use. The overall aim of this thesis was to refine the use of islet autoantibody testing in the diagnosis of adult-onset diabetes by: 1. Exploring optimal test thresholds, and whether these are influenced by age. 2. Determining whether islet antibody level has potential utility in patients with clinically diagnosed type 1 diabetes. 3. Examining whether recently developed assays to specific GAD epitopes, isotype and affinity can improve test utility. In Chapter 1 we provide an introduction to diabetes and the different types, and present some of the difficulties surrounding diabetes classification. Next, we provide a detailed background into islet autoantibodies and review the current evidence for their use in clinical practice and challenges in interpreting their results. In Chapter 2, we looked at whether the use of age-related positivity thresholds are necessary to improve zinc transporter 8 autoantibody (ZnT8A) assay performance using the commercially available RSR ELISA assay. Our first key finding was that the prevalence of detectable ZnT8A differed between those tested under and over 30 years of age in the general population. ZnT8A age-related positivity thresholds improved the specificity of the assay whilst maintaining sensitivity, and that using one positivity threshold can result in misclassification of diabetes type. In Chapter 3, we looked at the utility of islet autoantibody level at diagnosis of type 1 diabetes. Our main finding was a bimodal distribution of levels of glutamate decarboxylase (GADA) and islet antigen-2 autoantibodies (IA-2A) at diagnosis of type 1 diabetes, but not for ZnT8A. Those with high level GADA were older at diagnosis, more likely to be female and to be diagnosed with another autoimmune disease. In contrast, those with high level IA-2A were more likely to be younger at diagnosis and have ZnT8A as additional islet autoimmunity. This was replicated in a second cohort using an alternative method of islet autoantibody assessment. These findings increased our understanding of how islet autoantibody levels at diagnosis are associated with differences in the underlying pathology between age groups at diabetes diagnosis. In Chapter 4 we looked at GADA epitope specificity, affinity and IgG subclass response in those positive for GADA, clinically diagnosed with type 2 diabetes in adult-hood. In full-length (f-)GADA positive adult-onset diabetes, our novel finding was that testing for truncated (t-)GAD(96-585) autoantibodies stratified risk of progression to early insulin therapy (within 5 years) and identified those with a more type 1 diabetes-like phenotype; lower C-peptide, higher type 1 diabetes genetic susceptibility and positivity for IA-2A. In contrast, testing for f-GADA affinity and IgG subclass response did not stratify risk of progression to early insulin requirement. These findings provide evidence to support the testing for t-GADA in adult-onset patients. In Chapter 5, we summarize the key findings, the limitations of this work and its implications. Then we present ideas for future research and how to take these findings further. In summary, here we have provided evidence for the improvement of islet autoantibody testing by taking three different approaches. Firstly, we have shown how using age-related cut-offs improves specificity of a commercially available assay and how these thresholds reduce the risk of misclassification. Secondly, that some differences between child- and adult-onset diabetes are associated with islet autoantibody level and thirdly, that testing for GADA epitope specificity in adult-onset diabetes, can help predict who will require early insulin therapy. Implementation of these findings in clinical testing will improve outcomes for individuals with diabetes.