The Pro 12 Ala PPAR gamma gene polymorphism: possible modifier of the activity and severity of thyroid-associated orbitopathy (TAO)

A Comprehensive Review of Thyroid Eye Disease Pathogenesis: From Immune Dysregulations to Novel Diagnostic and Therapeutic Approaches

Thyroid eye disease is a complex inflammatory disorder of the orbit that has gained tremendous interest over the past years, and numerous scientific efforts have been deployed to elucidate its pathophysiology for novel drug development. Our manuscript will delve into the molecular dysregulations involved in the pathogenesis of thyroid eye disease that led to its clinical manifestations. Abnormalities within the apoptotic pathway, inflammatory cascade, and autoimmune regulatory systems will be covered. We will further discuss the challenges involved in its diagnosis and management and provide a summary of the current diagnostic tools (i.e., molecular biomarkers, diagnostic scores) from the perspective of clinicians. Finally, our comprehensive literature review will provide a thorough summary of most recent preclinical and clinical studies around the topic of thyroid eye disease, with an emphasis on the manuscripts published within the last five years. We believe our manuscript will bring novelty within the field by bridging the fundamental sciences with the clinical aspect of this disease. This review will be a great tool for clinicians in better understanding the pathogenesis of thyroid eye disease while providing an outlook on future perspectives (i.e., liquid biopsies, artificial intelligence).

The significance of ophthalmological features in diagnosis of thyroid-associated ophthalmopathy

BACKGROUND

Thyroid-associated ophthalmopathy (TAO) is an autoimmune disorder. It has discriminable appearance. This study was conducted to dig the clinical significance of demographic characteristics and ophthalmologic diagram features in TAO diagnosis and stage/severity evaluation.

RESULTS

We included 320 males and 633 females, with an average age of 41.75 ± 13.75. A majority of TAO patients had hyperthyroidism, and most of them were in the inactive stage and at the moderate level. The thyroid function type, stage and severity were closely associated with hypopsia, eyelid congestion, conjunctival congestion, corneal ulcer, ocular motility disorder, best corrected visual acuity, and extraocular muscle thickening. Using these features, we established different logistic regression models to predict thyroid function subtypes, abnormal thyroid function, stage, and severity, in which the AUC of the ROC curve and accuracies were satisfactory.

CONCLUSION

Together, TAO subtype, stage and severity can be diagnosed by auxiliary references including demographic factors, symptoms from complains, and image features. These non-invasive indices can be applied in a timely manner in clinical estimating TAO status.

Thinking inside the box: Current insights into targeting orbital tissue remodeling and inflammation in thyroid eye disease

Thyroid eye disease (TED) is an autoimmune disorder that manifests in the orbit. In TED, the connective tissue behind the eye becomes inflamed and remodels with increased fat accumulation and/or increased muscle and scar tissue. As orbital tissue expands, patients develop edema, exophthalmos, diplopia, and optic neuropathy. In severe cases vision loss may occur secondary to corneal scarring from exposure or optic nerve compression. Currently there is no cure for TED, and treatments are limited. A major breakthrough in TED therapy occurred with the FDA approval of teprotumumab, a monoclonal insulin-like growth factor 1 receptor (IGF1R) blocking antibody. Yet, teprotumumab therapy has limitations, including cost, infusion method of drug delivery, variable response, and relapse. We describe approaches to target orbital fibroblasts and the complex pathophysiology that underlies tissue remodeling and inflammation driving TED. Further advances in the elucidation of the mechanisms of TED may lead to prophylaxis based upon early biomarkers as well as lead to more convenient, less expensive therapies.

Diabetes mellitus and thyroid diseases

Both diabetes mellitus and thyroid diseases are very common in the fields of endocrinology and metabolism. Coexistence of diabetes mellitus and thyroid diseases are frequently experienced. However, higher prevalence of thyroid dysfunction in both type 1 and type 2 diabetes compared with non-diabetics have been reported. Autoimmunity is a key to understand the link between type 1 diabetes and autoimmune thyroid diseases. And, this combination of autoimmune diseases is recognized as autoimmune polyglandular syndrome type 3 variant. On the other hand, the relation between thyroid function and glucose intolerance or type 2 diabetes mellitus is more complicated. Both hyperthyroidism and hypothyroidism could be risk factors for glucose intolerance. In this review, current knowledge about the relationship between diabetes mellitus and thyroid diseases is described, which includes the following topics: effects of thyroid hormone on glucose metabolism, subclinical hypothyroidism and diabetic complications, type 1 diabetes and autoimmune thyroid diseases, and several clinical pitfalls in patients with both diseases.

The thyroid, the eyes and the gut: a possible connection

INTRODUCTION

Graves' disease (GD) is an autoimmune disorder responsible for 60-90% of thyrotoxicosis, with an incidence of 1 to 2 cases per 1000 population per year in England. Graves' orbitopathy (GO) is the most frequent extrathyroidal manifestation, not provoked directly by abnormal thyroid hormone levels, but by the consequence of the underlying autoimmune process. The aetiology of autoimmune disorders is due to an interplay between susceptibility genes and environmental factors, such as infections and stress. What triggers the autoimmune reaction to a specific site of the body is not yet clearly understood. The lack of knowledge in GD and GO pathogenesis implicates therapies that only limit damage but do not prevent disease onset.

MATERIAL AND METHODS

We performed on PubMed and the Cochrane Library a literature search for the articles published until July 2016 by using the search terms 'graves disease' and 'microbiome', 'orbitopathy' and 'autoimmune pathogenesis'. Reference lists of relevant studies were hand-searched for additional studies.

CONCLUSION

In this scenario, a Marie Sklodowska-Curie funded project INDIGO ( http://www.indigo-iapp.eu/ ) is investigating the role of the gut bacteria in GD and GO pathogenesis. The gut is the first and the widest area of bacteria access, with the highest concentration of T cells in the human body and trained to react to microorganisms. Interestingly, all the environmental factors involved in GD and GO pathogenesis can alter the balance within the microorganisms located in the gut, and influence the immune system, in particular the proportions of regulatory Treg and inflammatory TH17 cells. It is hoped that investigating GD and GO pathogenesis from this novel aspect will identify new targets for prevention and treatment.

Peroxisome Proliferator-Activated Receptor- γ in Thyroid Autoimmunity

Peroxisome proliferator-activated receptor- (PPAR-) γ expression has been shown in thyroid tissue from patients with thyroiditis or Graves' disease and furthermore in the orbital tissue of patients with Graves' ophthalmopathy (GO), such as in extraocular muscle cells. An increasing body of evidence shows the importance of the (C-X-C motif) receptor 3 (CXCR3) and cognate chemokines (C-X-C motif) ligand (CXCL)9, CXCL10, and CXCL11, in the T helper 1 immune response and in inflammatory diseases such as thyroid autoimmune disorders. PPAR-γ agonists show a strong inhibitory effect on the expression and release of CXCR3 chemokines, in vitro, in various kinds of cells, such as thyrocytes, and in orbital fibroblasts, preadipocytes, and myoblasts from patients with GO. Recently, it has been demonstrated that rosiglitazone is involved in a higher risk of heart failure, stroke, and all-cause mortality in old patients. On the contrary, pioglitazone has not shown these effects until now; this favors pioglitazone for a possible use in patients with thyroid autoimmunity. However, further studies are ongoing to explore the use of new PPAR-γ agonists in the treatment of thyroid autoimmune disorders.

PPARg2 Ala¹² variant protects against Graves' orbitopathy and modulates the course of the disease

Orbital fibroblast differentiation to adipocytes is a peroxisome proliferator-activated receptor g (PPARg)-dependent process essential for pathogenic tissue remodeling in Graves' orbitopathy (GO). PPARg2 Pro¹²Ala polymorphism modulates expression and/or function of the molecule encoded by this gene and is a promising locus of GO. Here, we analyzed associations of PPARg2 Pro¹²Ala with clinical manifestation of GO in 742 Polish Caucasians including 276 Graves' disease (GD) patients. In our study, the Ala¹² allele and Ala¹² variant (Ala¹²Ala and/or Pro¹²Ala genotype) decreased the risk of GO (p = 0.000012 and p = 0.00013). Moreover, Ala¹²Ala genotype was observed only in patients without GO (p = 0.002). GD patients with Ala¹² variant had less active and less severe eye symptoms. Female carriers of the Ala¹² allele rarely developed GO, but the marker was not related to symptoms of GO. The opposite finding was recorded in males, in whom the studied polymorphism was related to activity, but not to the development, of GO. In Ala¹² variant carriers without familial history of thyroid disease, risk of GO was lower than in persons with a familial background. The Ala¹² allele seemed to protect smokers from GO, but in nonsmokers, such a relation was not obvious. A multivariate analysis indicated the Pro¹²Ala marker as an independent risk factor of eye symptoms (p = 0.0001) and lack of Ala increases the risk of GO 3.24-fold. In conclusion, the gain-of-function Ala¹² variant protects against GO and modulates the course of the disease.

Differential involvement of orbital fat and extraocular muscles in graves' ophthalmopathy

Graves' ophthalmopathy (GO) is characterized by swelling of orbital fat and extraocular muscles, but little attention has been given to differential involvement of fat and muscles. Advancements in imaging allow rather accurate measurements of orbital bony cavity volume (OV), fat volume (FV) and muscle volume (MV), and are the topics of this review. Ratios of FV/OV and MV/OV neutralize gender differences. In adult Caucasian controls, mean values ± SD of FV/OV are 0.56 ± 0.11 and of MV/OV are 0.15 ± 0.02. FV increases substantially and MV decreases slightly with advancing age, requiring age-specific reference ranges. In 95 consecutive untreated Caucasian GO patients, both FV and MV were within normal limits in 25%, increased FV but normal MV was present in 5%, normal FV but increased MV was detected in 61%, and both increased FV and MV was evident in 9%. Increased FV was associated with more proptosis and longer GO duration. Increased MV was associated with older age, more severe GO (more proptosis and diplopia, worse eye muscle ductions), higher TBII and current smoking. At the cellular and molecular level differential involvement of fat and muscles might be related to differences between fibroblast phenotypes and cytokine profiles in each compartment, to different orbital T cell subsets during the course of the disease and to peroxisome proliferator activator receptor-γ polymorphisms and modulation of 11β-hydroxysteroid dehydrogenase-1. Enlarged muscles are apparently a rather early phenomenon in GO, whereas increases in fat mass occur relatively late. Why a minor subset of GO patients presents with an increase of only fat remains poorly understood.

Risk factors for development or deterioration of Graves' ophthalmopathy

BACKGROUND

Graves' ophthalmopathy (GO) significantly impairs the quality of life of affected individuals and the most severe cases can be sight threatening. Given the limited therapeutic options, a strong emphasis should be placed on disease prevention to diminish the significant morbidity associated with this disease.

SUMMARY

GO is most prevalent in women and most severe in men. Although some genetic differences between GO patients and Graves' disease patients without ophthalmopathy have been identified, none of the polymorphisms identified to date impart a high enough risk of GO to justify genetic testing to guide therapy or preventive strategies. Poorly defined mechanical factors that appear also to play a role in GO susceptibility will likely be better elucidated with advances in imaging techniques. Tobacco smoking has been consistently linked to development or deterioration of GO. Smokers who receive radioactive iodine have the highest incidence of unfavorable GO outcome, which is proportional to the number of cigarettes smoked per day. Several studies have reported an association between radioactive iodine treatment for Graves' disease and worsening or development of GO. Observational studies suggest that the same appears to be true for thyroid dysfunction, including both hyper- and hypothyroidism. While thyrotropin receptor antibody levels appear to be useful in predicting the course of disease and response to therapy, it is not known whether they are predictive of GO development. The puzzling scenarios of euthyroid or clinically unilateral GO, the large number of nonsmoking GO patients, and the occasional development of GO years after thyroid dysfunction has been treated all underline the multifactorial etiology of this disorder in which no single factor determines the clinical outcome.

CONCLUSIONS

GO appears to have a complex genetic basis with multiple susceptibility alleles that act in combination with nongenetic factors to contribute to disease expression.