**3. Pathophysiology of Graves' Disease**

In GD, four standard thyroid antigens: thyroglobulin, thyroid peroxidase, sodium-iodide symporter and the thyrotropin receptor are recognized to direct B and T lymphocyte-mediated autoimmunity. However, the primary auto antigen of GD is the thyrotropin receptor itself and is responsible for the manifestation of hyperthyroidism. In this disease, the antibody and cell-mediated thyroid antigenspecific immune responses are properly defined. The development of hyperthyroidism in healthy subjects by transferring thyrotropin receptor antibodies in serum from patients with GD and the passive transfer of thyrotropin receptor antibodies to the foetus in pregnant women are the direct proof of an autoimmune disorder that is mediated by means of autoantibodies. By circulating autoantibodies against the thyrotropin receptor, the thyroid gland is under continuous stimulation, and because of the increased production of thyroid hormones pituitary thyrotropin secretion is suppressed [16]. In the immunoglobulin G1 subclass, the stimulating activity of thyrotropin receptor antibodies is found mostly. The release of thyroid hormone and thyroglobulin that is mediated via 3,'5′-cyclic adenosine monophosphate (cyclic AMP) are caused by these thyroid-stimulating antibodies, and they also stimulate iodine uptake, protein synthesis, and thyroid gland growth. In the etiology of hyperthyroidism in GD the anti-thyroglobulin, anti-sodium-iodide symporter, and anti-thyroid peroxidase antibodies seem to have a very little role. However, against the thyroid, these are markers of autoimmune disease. In persons with autoimmune thyroid disease, intrathyroidal lymphocytic infiltration is the initial histologic abnormality which has a direct correlation with thyroid antibodies' titer [17, 18]. In addition to autoantigens, the cells of thyroid produce specific immune mediators such as cytokines and Fas which are involved in various immune process including complement legislation and T cell adhesion. Those individuals who are suffering from Graves' Disease have lesser percentage of CD4 lymphocytes in thyroid as compared to their peripheral blood. In addition, the CD4 reduction in these patients may also be related to the elevated Fas expression in intrathyroidal CD4 T lymphocytes. *CD40, CTLA-4, thyroglobulin, TSH receptor,* and *PTPN22* are several autoimmune thyroid disease susceptibility genes that have been identified. Either to GD or *Hashimoto thyroiditis*, some of these susceptibility genes are unique, while others confer susceptibility to both conditions. With environmental factors or activities to precipitate the onset of GD the genetic predisposition to thyroid autoimmunity might also interact [17–19]. The RNASET2-FGFR1OP-CCR6 region at 6q27 and an intergenic region at 4p14 are two new susceptibility loci that had been found [20]. Moreover, thyroid-stimulating hormone receptor and major histocompatibility complex class II versions have strong associations with thyroid stimulating hormone receptor autoantibodies (TRAb)-positive GD [21]. Compared with healthy controls, GD patients have higher rate of peripheral blood mononuclear cell conversion into CD34<sup>+</sup> fibrocytes. The production of inflammatory cytokines like Inter-leukin 6 (IL-6) and TNF-alpha by these cells after piling up in orbital tissues also contribute to the pathophysiology of thyroid eye disease (opthalmopathy) [22]. In a whole genome association study of more than 1500 individuals suffering from Graves' Disease and equal controls, six susceptible loci which are (*CTLA4; cytotoxic T-lymphocyte-associated protein 4, MHC; major histocompatibility complex, FCRl3; Fc receptor-like protein 3, TSHR; thyroid stimulating hormone receptor, RNASET2-FGFR1OP-CCR6 region at 6q27, and an intergenic region at 4p14*) have been discovered to be associated with GD. **Figure 2** describes the pathophysiology of Graves' Disease [23]*.*

## **4. Genetics of Graves' Disease**

GD is a complex autoimmune disorder which affects the functioning of the thyroid gland, which is the butterfly shaped gland in the lower neck. Specific antibodies targetting the thyrotropin receptor are found in about 95% of patients with GD. GD is thought to result from a combination of environmental and genetic factors most of which are unknown. A number of genes predispose to the GD which include *HLA region*, *protein tyrosine phosphatase-22 (PTPN22)*, *cluster of differentiation 40(CD40)*, *the cytototoxic T lymphocyte- Associated factor4 (CTLA4 or CD152), thyrotropin receptor (TSHR), thyroglobulin (Tg), FCRL3 (FC receptor-like-3), Secretoglobulin 3A2 (SCGB3A2) gene* encoding secretory uteroglobin- related protein 1 (UGRP) and many others. The role of these genes in the pathophysiology of GD is discussed below:

#### **4.1 HLA region**

Human leukocyte antigen (HLA) region (6p21) within the human genome codes for 252 expressed loci including numerous key immune response genes is the most gene dense region [24]. This region contains the highest degree of polymorphism within the genome and is divided into different classes which includes the extended class I, classical class I, classical class III, classical class II and extended class II [24] (**Figure 3**). The densest linkage disequilibrium (LD) is also shown by this gene, extending up to 540 kb [25], which compares with distances of between 1 and 173 kb seen in the rest of the genome [26]. When trying to tease out the exact site of etiological variants, the degree of LD within the region is challenging. Most studies highlights the importance on the role of HLA class II encoded HLA-DR and –DQ molecules, which present exogenous antigens for recognition by CD4+ T helper (Th) cells. Including GD, strong associations of *HLA* with almost all autoimmune disorders have been detected. Many studies regarding association of *HLA* alleles with GD have been done. Among different ethnic populations, association of *HLA* alleles with GD varies like *HLA-B\*08, DR3* and *DQA1\*05:01* are associated with a

*Graves' Disease: Pathophysiology, Genetics and Management DOI: http://dx.doi.org/10.5772/intechopen.98238*

**Figure 3.**

*HLA region on chromosome 6p21. a) nucleotide length of genes of HLA, b) regions of HLA genes.*

high risk of GD, and *HLA-DRB1\*07:01* is a protective allele against GD in Caucasian populations [27]. *HLA* alleles have been shown to predispose certain groups of people to the disease and vary regionally. British Caucasians showed role of HLA class II alleles DRB1-0304, DQB1-02, and DQA1-0501 [28]. The *HLA* complex shows strong linkage disequilibrium. In Caucasians populations, It is found that there is strong linkage disequilibrium between the genes that codes DR and DQ molecules therefore the existence of a particular DRB1 variant to a larger degree determines DQA1 and DQB1 alleles. *DRB1\*03:01-DQA1\*05:01-DQB1\*02:01 (DR17, DQ2)* and *DRB1\*04:01- DQA1\*03:01-DQB1\*03:02 (DR4, DQ8)* are HLA haplotype combinations in GD sibling pairs. The maximum risk related to this disease is associated with *DR17, DQ2 while as the HLA-DRB1\*07 (DR7)* is protective for Graves' Disease. The recessive inheritance of MHC related susceptibility is favored by the distribution of *HLA-B8* genotypes and it is in close accord with Hardy–Weinberg equilibrium proportions. The possibility that an individual will be affected with GD depends on sex, *HLA* genotype, and family history. 14.9% of DR3-positive women with an affected first-degree relative are liable to be affected [29]. *HLA Class I* is also linked with GD, the disease may be mainly associated with alleles of *HLA class I*, in particular *HLA-C\*07* whereas *C\*03* and *C\*16* provides protection [30]. *HLA-DPB1\*05:01* was the main gene predisposing to GD. Other alleles included *B\*46:01, DRB1\* 15:02* and *16:02* whereas *DRB1\*12:02* and *DQB1\*03:02* provide protection. The association of *DQA1\*05:01* with GD was not supported by Linkage Disequilibrium patterns observed in Asians [31]. Peptides derived from TSHR are the cause of association with HLA and development of immune response. Other cause is owing to thymic selection affecting positive and negative selection of T cell clones with regulatory or effector functions. Moreover, impact on NK cell repertoire through interactions with killer immunoglobulin-like receptors (KIR) and/or serving directly as a source of auto antigens after misfolding and presentation by HLA class II molecules [30]. Development of Graves' Disease is related to HLA-DR3. The extracellular domain (ECD) of human TSH receptor (hTSH-R) is a crucial antigen in Graves' Disease. hTSH-R peptide 37 (amino acids 78-94) is an important immunogenic peptide [32]. HLA is the cause of many diseases, the disease occurred at an earliest age in HLA-DR3 positive patients and important link between exophthalmos and either exophthalmos and/or soft tissue modifications were found with DR3. HLA-DR3 positive patients were found to be more resilient to radioiodine therapy than patients negative for these antigens [33]. It has been hypothesized that arginine at position 74 of the HLA- DRB1 chain has role in GD pathogenesis. But the most common residues at position 74 of *DRB1\*15:01* and *DRB1\*16:02* reported in our association study are both alanine and it is considered to be neutral for GD risk [34]. On the other hand, HLA region is linked to GD susceptibility in both

Caucasian and Chinese Han populations [35]. The associated alleles vary from those in Causacians. *HLA-DPB1\*05:01* is the major gene of GD in our population, *B\*46:01, DQB1\*03:02, DRB1\*15:01* and *DRB1\*16:02* were closely linked with GD [31]. As per the other meta-analysis study, the *HLA-B\*46* allele is a risk factor for GD in Asian populations. The distribution of *HLA-B\*46* and *HLA-B\*08* vary between European and Asian populations. The allelic frequency of *HLA-B\*08* is around 12%, while the allelic frequency is 0.3 to 0.5% in most Asian populations. By contrast, the allelic frequency of *HLA-B\*46* is 3.9 to 8.6% in Asian populations and almost zero in Europe populations [35]. **Figure 4** depicts the classical HLA Class I and II pathways.
