Genetic study of thyroid eye disease

Abstract

Thyroid-associated orbitopathy (TO) is an autoimmune-mediated orbital inflammation that affects 25% of patients with Graves’ disease. The close temporal relationship between onset of Graves’ disease and TO suggests they share a common aetiology. While the complex inheritance of Graves’ disease is better characterized, little is known of the genetic susceptibility in TO. Multiple environmental factors such as smoking, male, older age are known risk factors for development of TO, however the extent of gene-environmental interaction remains largely unknown. The molecular mechanisms driving the development of TO is incompletely understood, hence targeted treatment options for TO remained limited.

This thesis is undertaken to test the hypothesis that there is genetic susceptibility that predispose to development of TO. The research project initially examined exogenous risk factors associated with TO in a large Australian cohort with Graves’ disease, in order to identify environmental factors important for subsequent covariates adjustment when analyzing genetic findings. The risk factors association study found smoking, older age and longer duration of Graves’ disease correlated positively with TO, and secondarily there was relative selenium deficiency in TO cases compared to Graves’ disease patients without eye involvement.

A genome-wide association study using deoxyribonucleic acid pooling approach and high-throughout array platform were used to discover gene variants associated with thyroid-associated orbitopathy in a case-control study design. The genetic findings were followed by a second stage individual genotyping targeting fewer markers to validate the genetics variants identified through genome-wide association study in the discovery cohort and independent replication study cohort. MACROD2, a novel gene that encodes an eraser of mono-ADP-ribosylation, possibly has a role in nuclear factor κβ signaling, showed evidence of association with TO in genome-wide association study and also in validation genotyping.

A secondary aim of the thesis is to determine differentially expressed genes a priori in active TO orbital adipose tissue using microarray to explore molecular mechanisms of TO and to correlate gene expression findings with the genetic study. The study found TIMD4, DEFA1, DEFA1B, and DEFA 3 were over-expressed in active TO compared with inactive TO suggesting a pathogenic role of the innate immune response in TO. Active TO was marked by up-regulation of multiple genes involved in cell-mediated, innate and inflammatory responses with concurrent enhancement of orbital adipogenesis. For the first time, epigenetic factors was implicated in the pathogenesis of TO. However MACROD2 were not differentially expressed in active TO when compared with either inactive TO or normal control.

Overall the findings from this thesis further our understanding on the genetic and environmental risk factors involved in thyroid-associated orbitopathy and give new insights into the underlying complex molecular mechanisms. The novel insights into candidate molecules and pathways can be explored to develop alternative treatment strategies for TO.