Simon H.S. Pearce
The spectrum of clinical thyroid disease that is strongly influenced by genetic factors runs from congenital hypothyroidism and thyroid agenesis, through transient gestational hyperthyroidism and autoimmune thyroid diseases (AITDs), to benign and malignant thyroid tumors. Indeed, a family history is a particularly frequent finding in patients with all forms of thyroid autoimmunity and for many types of goiter. This chapter reviews the inherited (germ line) basis for thyroid diseases, including those disorders with a strict monogenic (Mendelian) basis and those more common conditions that are inherited as complex multigenic traits. The pathogenesis of thyroid tumors and the role of somatic gene mutations in thyroid disease is covered in detail elsewhere (see section on oncogenes in Chapters 70 and 71).
Monogenic disorders of the thyroid axis frequently present as congenital or childhood hypothyroidism. They are usefully classified according to the level of the defect, i.e., pituitary thyrotrophs, thyroid development or hormonogenesis, hormone transport, and end-organ responsiveness (Table 20.1). Developmental problems of the thyroid account for the majority of infants with congenital hypothyroidism and are broadly referred to as thyroid dysgenesis, which includes thyroid agenesis, hemiagenesis, hypoplasia, and ectopic thyroid (i.e., abnormalities of both morphogenesis and migration) (1,2). Less than 15% of infants with thyroid dysgenesis are found to have one of the currently defined molecular defects (3,4,5,6). Of these gene abnormalities, only infants with a PAX8 gene mutation appear to have a disorder that is manifest as thyroid dysgenesis alone (6). Interestingly, even in families with a characterized PAX8 molecular abnormality, the morphology of the thyroid abnormality is not always consistent between patients (6,7). Defects in thyroid hormonogenesis account for only about 10% of congenital hypothyroidism, and often present with goiter as an additional feature (see Chapter 48). There undoubtedly are other single gene defects, and most likely some complex ones causing or modifying both thyroid dysgenesis and hormonogenesis, that await discovery.
TABLE 20.1. SINGLE GENE DEFECTS THAT CAUSE DISORDERS OF THE HYPOTHALAMIC-PITUITARY-THYROID AXIS
Gene Defect (Inheritance)
Insensitivity to TRH
Hypothyroidism, no response to TRH
Combined pituitary hormone deficiency
Hypothyroidism, serum TSH low to slightly high
Pit1 [POU1F1] (AR, AD)
-with rigid cervical spine
-with septo-optic dysplasia
HESX1 (AR, AD)
Isolated TSH deficiency
Hypothyroidism, ectopic or hypoplastic thyroid
Variable degree of hypothyroidism, pulmonary hypoplasia and choreoathetosis
TTF1 [NKX2–1] (haploinsufficient)
Hypothyroidism, with palatal clefts, spiky hair, choanal atresia [Bamforth-Lazarus syndrome]
Iodide transport defect
Total organification defect
Transient neonatal hypothyroidism
Goiter, variable sensorineural deafness, [Pendred's syndrome]
Variable hypothyroidism, goiter
Complete TSH resistance
Hypothyroidism, thyroid hypoplasia
Partial TSH resistance
Pseudohypoparathyroidism type 1a, parathyroid hormone resistance, osteodystrophy
GNAS1 (AD & paternal imprinting)
Familial gestational hyperthyroidism
Hyperthyroidism in pregnancy
TSH-R (AD) [K183R]
Hereditary toxic thyroid hyperplasia
Congenital hyperthyroidism and goiter
TSH-R activation (AD)
Toxic thyroid adenoma
Autonomous follicular adenoma
TSH-R activation (somatic)
McCune-Albright syndrome, polyostotic fibrous dyplasia, precocious puberty
GNAS1 activation (mosaic)
Thyroid hormone transport
TBG deficiency (complete or partial)
Euthyroid, excess serum total thyroid hormones
TBG duplication (XR)
Increased TTR affinity
Familial dysalbuminemic hyperthyroxinemia
Familial dysalbuminemic hypertriiodothyroninemia
Low serum T4, high serum T3, psychomotor retardation, nystagmus
Resistance to thyroid hormone
High serum T4 and T3, normal to high serum TSH, goiter
Familial medullary carcinoma
MEN2a with pheochromocytoma
MEN2b with ganglioneuroma and pheochromocytoma
Thyroid hamartoma/carcinoma and thyroiditis
Breast adenoma/carcinoma, skin hamartoma [Cowden's syndrome]
Differentiated thyroid carcinoma
Short stature, cataract, skin changes, type 2 diabetes [Werner syndrome]
with polyposis coli [Gardner's syndrome]
Pigmentary adrenal adenoma, cardiac myxoma, [Carney complex]
Hypoparathyroidism, hypoadrenalism, candidiasis [APECED syndrome, APS1]
Immune diabetes, autoimmune enteropathy [IPEX syndrome]
Hypothyroidism, renal failure, corneal deposition
Thyrotoxic periodic paralysis
Muscle paralysis when hypokalemic
aOMIM no. refers to the catalog of “Online Mendelian Inheritance in Man,” as found on www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM .
bAn additional form of dyshormonogenesis, dehalogenase deficiency, has been characterized at a biochemical, but not molecular, level.
Alb, albumin; APC, adenomatous polyposis coli; APECED, autoimmune polyendocrinopathy candidiasis and ectodermal dysplasia; APSI, autoimmune polyendocrinopathy syndrome type 1; CTNS, cystinosin; FOXP3, Forkhead box P3; GNAS1, Gs-alpha subunit gene; KCNE3, Voltage gated potassium channel-3; MCT8, Monocarboxylate transporter-8; NKX2-1, (no full form); PAX8, paired box gene-8; PDS, Pendrin; PRKAR1A, Regulatory c-AMP dependent protein kinase-1α; PTEN, Phosphatase and tensin homolog; RECQL2, RECQ-like2; RET, (no full form); THOX2, thyroid oxidase-2; THRB, Thyroid hormone receptor-β; TTF1, thyroid transcription factor-1; TTF2, thyroid transcription factor-2.
Inherited disorders of thyroid hormone transport comprise abnormalities of both the serum-binding and transmembrane passage of thyroid hormones. Abnormalities of serum transport are due to altered quantity or binding affinity of the serum binding proteins, thyroxine-binding globulin (TBG), transthyretin, and albumin. Thyroxine-binding globulin is the major carrier protein, and abnormalities range from complete TBG deficiency, altered affinity for thyroid hormones, to excess production of TBG (27,28). The TBG gene is located on the X chromosome, and the biochemical manifestations of abnormal TBG production are more marked in males. Alleles encoding TBG isoforms with altered binding affinity are particularly prevalent in certain racial groups (29). Mutations in the albumin gene cause familial dysalbuminemic hyperthyroxinemia (30,31). Interestingly, a kindred with an albumin variant that has a high affinity for triiodothyronine (T3) but not thyroxine (T4) has been reported, demonstrating that the binding of T4 and T3 has different molecular specificities within the albumin molecule (32).
With the widespread use of serum thyrotropin (TSH) and free thyroid hormone assays, these familial states of euthyroid hypothyroxinemia and euthyroid hyperthyroxinemia, which are only apparent on assay of serum total hormone concentrations, rarely cause significant diagnostic confusion. These conditions of altered biochemistry are in marked contrast to the devastating disorder recently described in male children with abnormal transmembrane thyroid hormone transport (33). These children present with hypotonia, severe psychomotor retardation, failure of speech, nystagmus, and a low serum T4 concentration in conjunction with a high serum T3 concentration (33). Serum TSH values are normal at birth and become moderately elevated, but the developmental abnormalities are not reversed by T4 treatment. Mutations in an X-chromosome membrane transporter, MCT8, are responsible for this disorder (33). The disorders of end-organ responsiveness to thyroid hormones are termed resistance to thyroid hormone (RTH) and are caused by a variety of dominantly inherited mutations of the thyroid hormone β receptor (34). RTH syndromes have a variable phenotype, which ranges from asymptomatic goiter or tachycardia to hyperactivity, learning difficulty, developmental delay, and retarded bone growth with short stature. The biochemical hallmarks of these conditions are high serum thyroid hormone and normal or slightly high serum TSH concentrations (see Chapter 81).
Several chromosomal deletion or rearrangement syndromes are associated with thyroid disorders. The most common disorders in which thyroid dysfunction is a frequent accompaniment are listed in Table 20.2. Patients with Down's syndrome, with trisomy of chromosome 21, have a high prevalence of both autoimmune and congenital thyroid disease. Autoimmune hypothyroidism (chronic autoimmune thyroiditis) is the most common thyroid problem, affecting 15% to 20% of adults with Down's syndrome (47,48,49). Graves' disease occurs less frequently, with a prevalence of 1% to 2%, which may not be increased compared with the general population (47,48). Congenital hypothyroidism is found in about 2% of infants with Down's syndrome. Turner's syndrome, which is characterized by complete or partial loss of X-chromosome material in a phenotypic female, is also associated with autoimmune thyroid disease (AITD). About 15% of patients with Turner's syndrome have autoimmune hypothyroidism, with 30% to 40% having high serum antithyroid antibody concentrations (50,51,52). There is a correlation between cytogenetic abnormalities and AITD in patients with Turner's syndrome, with overt hypothyroidism being found in up to 40% of patients with an Xq isochromosome (deleted short arm and duplicated long arm) (52,53,54). The DiGeorge syndrome (cardiac outflow tract defects, thymic hypoplasia, hypoparathyroidism, and facial anomalies) and the overlapping chromosome 22q11 deletion syndromes are associated with Graves' disease in up to 20% of cases (55,56,57). Two rarer chromosomal disorders, 1p terminal deletion and Smith-Magenis syndrome, are each associated with hypothyroidism in about one third of cases (Table 20.2). The pathogenesis of the hypothyroidism is ill defined, although it is likely to be developmental in nature.
TABLE 20.2. CHROMOSOMAL DISORDERS WITH THYROID DISEASE AS A COMPONENT
Mental retardation, cardiac anomalies, characteristic facies, others
Cardiac outflow tract anomalies, thymic hypoplasia, hypoparathyroidism
Autoimmune hypothyroidism, Thyroid antibodies
Web neck, ovarian dysgenesis, short stature, renal and aortic root abnormalities
Mental retardation, eye anomalies, self-injury
1p terminal deletion
Mental retardation, microcephaly, large fontanelle, hearing loss, characteristic facies
aOMIM no. refers to the catalog of “Online Mendelian Inheritance in Man,” as found on www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM .
Complex disorders are determined by a combination of genetic and nongenetic factors (including environmental and other influences). Most common complex traits are thought to have a multigenic basis; i.e., in different individuals they are determined by a varying combination of susceptibility alleles in different genes (61). However, even for an apparently simple genetic trait, the phenotype may be dependent to some extent upon environmental influences. For instance, in mendelian forms of thyroid dyshormonogenesis the presence or severity of hypothyroidism and the rate of goiter development may be determined by dietary iodide availability. AITD is the most prevalent of the autoimmune conditions, with at least 2% of women being affected by overt thyroid dysfunction. Similarly, simple euthyroid goiter is a frequent finding in women, even in iodine-replete areas. Both these common disorders have a complex multigenic basis.
Simple (Euthyroid) Goiter
Simple goiter is defined as a diffuse or nodular enlargement of the thyroid that is not the result of neoplasia or inflammation in a subject who is euthyroid (62). It is a heterogeneous disorder that affects up to 15% of women and about 3% of men who live in iodine-sufficient regions (63,64). It is an important problem on a population level due to its prevalence, and also because some subjects with simple goiter may be predisposed to nonmedullary thyroid cancer (65,66,67). However, it is notable that in the majority of patients who have recurrent thyroid nodules after surgery for goiter the new nodules are polyclonal in nature, indicating that the condition is predominantly a hyperplastic rather than neoplastic process (68). Surprisingly, in iodine-deficient regions, goiter has a familial tendency (69), with a higher concordance in monozygotic than dizygotic twins (70). In the absence of iodine deficiency, simple goiter also has a strong genetic basis (71). In a large Danish cohort of female twins, the concordance rate in monozygotic twins was 42%, as compared with 13% in dizygotic twins (72), and data from this study suggested that about 80% of the susceptibility to simple goiter may be determined by genetic factors.
Heterozygous mutations in the thyroglobulin (Tg) molecule have been described in three kindreds with simple goiter, identified from 30 probands (19). In these families, about 50% of patients carrying the mutation had a goiter, with an increasing penetrance after the age of 25 years (19). Thus, at least 10% of cases of simple goiter may be due to mild defects in thyroid hormonogenesis (19,72). However, in contrast to the “dyshormonogenic” presentation as a recessive trait associated with hypothyroidism, these Tg mutations are manifest as euthyroid goiter in later life and are inherited in an autosomal-dominant fashion (19,72,73). In recent years, several large kindreds with dominantly inherited multinodular goiter have been identified, and genome-wide linkage studies have mapped novel susceptibility loci to chromosomes 14q32, Xp22, and 3q26 (designated MNG1 to MNG3) (74,75,76). Of these three loci, only MNG1 on 14q32 has been independently confirmed (77).
Of the three families in whom linkage was initially established, the phenotype of one kindred was distinct in having an onset of goiter in prepubertal children, rapid 123I uptake, and high serum TSH concentrations in some subjects, suggesting that the MNG3 locus may encode another mild form of dyshormonogenesis (76). Reassuringly, these linkages appear distinct from those for familial nonmedullary thyroid cancer (1p21, 2q21, 19p13) (78,79,80) (see section on oncogenes in Chapter 70). Thus, simple goiter is certainly a complex trait in terms of its molecular pathogenesis, with iodine availability as the major environmental component. Whether its inheritance is as a multiple monogenic trait comprising a series of different single gene disorders with a common phenotype, or is a truly complex trait with alleles at several genetic loci contributing to the disorder in each patient, is unknown. It will be interesting to determine whether the gene defects underlying MNG1 to MNG3 are subtle abnormalities of hormonogenesis or whether a proportion of cases will be due to inherited defects in the regulation of cell cycling or growth of thyroid follicular cells.
Autoimmune Thyroid Disease
AITD comprises a series of interrelated conditions including Graves' disease, Hashimoto's (goitrous) thyroiditis, atrophic autoimmune hypothyroidism, postpartum thyroiditis, and thyroid-associated ophthalmopathy. These different manifestations of AITD may occur sequentially, and sometimes synchronously, in the same patient. This clustering of different phenotypes of AITD within a patient suggests that these conditions have a common pathophysiologic basis (see section on pathogenesis of Graves' disease in Chapters 23 and 47). A widely accepted model for the pathogenesis of AITD suggests that each patient has a background inherited predisposition to autoimmunity, with additional environmental and hormonal factors that trigger or contribute to the development of disease. In support of this model, there is good evidence that both cigarette smoking and adverse psychosocial events are associated with the development of Graves' disease (81,82,83). Similarly, the female preponderance of AITD (63), the modulation of animal models of the disease with sex steroids (84), the amelioration of Graves' disease during pregnancy, and the occurrence of postpartum thyroiditis all support the important role of sex steroids in these disorders.
Genetic Epidemiology of Autoimmune Thyroid Disease
Twin studies show an increased concordance of Graves' disease and autoimmune hypothyroidism in monozygotic twins as compared to dizygotic twins (reviewed in reference 85). In a series of careful investigations of 8966 Danish twins, concordance for Graves' disease in monozygotic twins was 35% as compared with 3% in dizygotic twins (86). Similarly, there was 55% concordance of autoimmune hypothyroidism (combining both atrophic and goitrous forms of the disease) in monzygotic twins, but no concordance among dizygotic twins (87). A statistical model based on these data suggested that 79% of the predisposition to Graves' disease is due to genetic factors, with only 21% due to nongenetic (environmental and hormonal) factors (86), confirming the dominant role of heredity in the pathogenesis of the disease.
In addition to twin studies, a measure of the heritability of a disorder can be gained from the ratio of the risk to a relative of an affected proband compared with the background population prevalence (a value termed λs for siblings and λo for parents/offspring) (61). A study from Hungary found 5.3% of 435 probands with Graves' disease had siblings with the disease (21 sisters, 2 brothers), compared with a background population frequency of Graves' disease of 0.6% (88). This allows estimation of the λs for Graves' disease as 8.1 in this population. In a similar study carried out in England, the λs for Graves' disease was 9.9 (89). To put these values into context, the λs for type 1 diabetes mellitus is 15, and that for rheumatoid arthritis is 8 (90). An important message for patients with Graves' disease is that their female siblings and children have a 5% to 8% chance of also being affected by the disease, and an approximately similar risk of developing autoimmune hypothyroidism.
Heritability of Thyroid Antibodies
Soon after the first description of thyroid antibodies (91), their occurrence in 22 of 39 siblings (56) of probands with AITD was noted, and a dominant pattern of inheritance was suggested (92). The finding of thyroid antibodies in 30% to 50% of first-degree female relatives has been confirmed using sensitive assays for serum anti-thyroglobulin (Tg) and antithyroid peroxidase (TPO) antibodies (93). The prevalence of these antibodies in male relatives is less (~10% to 30); this pattern has been attributed to dominant inheritance with reduced penetrance in males (93). Naturally occurring TPO antibodies are not uniform in specificity, and tend to be directed against certain defined “immunodominant” epitopes of TPO. Interestingly, these patterns of TPO antibody response (epitopic fingerprints) can be inherited in a dominant fashion (94), and can persist for many years. It is striking that even in families in which several members have antibodies to TPO or Tg, clinically overt AITD is not the rule (95). This demonstrates that, although the generation of a B-cell immune response to thyroid antigens is one component of thyroid autoimmunity, it is not in itself sufficient to cause disease, and that other tissue-specific responses or immune system factors are also necessary.
Association of Autoimmune Thyroid Disease with Other Autoimmune Disorders
The occurrence of Hashimoto's thyroiditis in patients with type 1 diabetes mellitus, and in their family members, is well recognized. In large groups of families with type 1 diabetes in the United Kingdom and the United States, at least one case of AITD was reported in relatives of 22% and 40% of patients with type 1 diabetes, respectively (96,97). A higher-than-expected prevalence of AITD has been found in patients with other autoimmune disorders and in their families (98). In two studies of families with two or more rheumatoid arthritis cases, about 50% of the families had a patient with rheumatoid arthritis or a relative had AITD (99,100). Similarly, AITD is often found in patients with autoimmune Addison's disease, being present in 40% to 50% of patients with sporadic Addison's disease [i.e., those who do not have the autoimmune polyendocrinopathy, candidiasis, or ectodermal dysplasia (APECED) syndrome] (101,102). Other weaker associations exist between AITD and other autoimmune endocrinopathies (e.g., autoimmune premature ovarian failure) and nonendocrine autoimmune disorders (e.g., pernicious anemia, celiac disease, myasthenia gravis, multiple sclerosis) (103). This clustering of different autoimmune conditions suggests that several different autoimmune disorders are likely to have a disease susceptibility allele (or alleles) in common.
Monogenic Disorders with Autoimmune Thyroid Disease as a Component
AITD is a component of three monogenic syndromes. The most common of these rare disorders is the APECED (type 1 polyendocrinopathy) syndrome (Table 20.1). The cardinal features of this condition are autoimmune hypoparathyroidism, Addison's disease, and chronic mucocutaneous candidiasis, which begin in childhood or early adolescence (see reference 42 for a comprehensive review). Other autoimmune disorders such as type 1 diabetes, pernicious anemia, and hypogonadism occur in 15% to 30% of patients with the APECED syndrome, but autoimmune hypothyroidism is comparatively uncommon, affecting only about 5% of patients. The extreme rarity of Graves' disease in APECED is notable. APECED is an autosomal recessive syndrome caused by mutations in the autoimmune regulator (AIRE) gene on chromosome 21 (43). AIRE is a nuclear transcription factor that is expressed predominantly in dendritic antigen presenting cells in the thymus and peripheral lymphoid tissues. AIRE gene mutations lead to defective negative selection of potentially autoreactive thymocytes and ineffective peripheral antigen presentation for some antigens (e.g., Candida). One large family has been described with dominantly inherited severe and persistent candidiasis and hypothyroidism with goiter, a condition that is distinct from APECED. Linkage studies found that the disorder mapped to chromosome 2p, but the causative gene has not been identified (104). A third, rarer disorder is the immune dysregulation, polyendocrinopathy, and enteropathy (X-linked) syndrome (IPEX). This is a devastating disorder of male infants with failure to thrive due to autoimmune enteropathy (44). Autoimmune hypothyroidism and type 1 diabetes develop in the first year of life in about 50% and 90% of affected males, respectively. IPEX is caused by mutations in the FOXP3 gene on chromosome Xp11. FOXP3 is expressed in T lymphocytes and is critical to the development of regulatory T cells, without which inappropriate activation and proliferation of CD4 lymphocytes occurs (44). These monogenic syndromes point towards two cell types, namely the dendritic antigen-presenting cell and the CD4 lymphocyte, whose normal functioning is critical to the maintenance of immune tolerance to thyroid tissues.
Autoimmune Thyroid Diseases as Complex Genetic Traits
In contrast to the unusual monogenic forms of autoimmunity mentioned above, most cases of AITD, along with other common autoimmune disorders, are now thought to have a complex genetic basis; i.e., the genetic predisposition to the disease is determined by a series of interacting susceptibility alleles of several different genes (90). These various genetic loci may also have differing influences on the predisposition to AITD in different populations (locus heterogeneity), which makes the identification of disease susceptibility genes a more difficult task. There are two standard approaches for identifying disease genes for either monogenic or complex traits (61). First, candidate gene studies involve examining polymorphic markers within a particular gene, which has been selected because it is thought that disruption of its function may result in the phenotype. There has been some success in using this approach to identify susceptibility genes for AITD. A second approach is linkage scanning, in which widely spaced anonymous genetic markers (usually microsatellite repeat polymorphisms between genes) are used to detect chromosomal segments with evidence for linkage in affected families. Linkage analysis is more robust than association analysis, because it may detect genetic effects many millions of nucleotides away and does not rely on the same allele being linked to disease in each kindred. However, linkage studies may lack sensitivity to detect loci with small effects and are poor at localizing a disease allele. Several genetic tests now combine elements of familial linkage and association analysis (linkage in the presence of association), the most widely used analysis being the transmission disequilibrium test (105).
Luckily for investigators interested in AITD, linkage studies of type 1 diabetes have been prominent in the field of complex trait mapping. It is now clear from genome-wide linkage studies encompassing more than 750 families with type 1 diabetes that there are seven loci where there is firm or suggestive evidence of genetic linkage (106,107). Of these loci, one is the major histocompatibility complex (MHC) on chromosome 6p21, another is the insulin gene region on chromosome 11p15, and a third is the cytotoxic T lymphocyte antigen-4 (CTLA-4) gene on chromosome 2q33 (see later in the chapter). The other four linked loci, and a further 12 putative type 1 diabetes loci, have all been defined on the basis of linkage to anonymous markers, and there is no defined allelic association. These studies of type 1 diabetes have advanced the field by demonstrating that a common autoimmune disorder has a complex (multigenic) genetic basis (90). Indeed, similar findings have subsequently been made for rheumatoid arthritis, multiple sclerosis, and systemic lupus erythematosus, albeit with smaller numbers of families (108,109). There is likely to be a similar number of genetic loci implicated in the pathogenesis of AITD, and because of the familial clustering of the disease with type 1 diabetes, some susceptibility alleles are likely to be shared by both disorders.
Genetic studies of AITD have proceeded in recent years with a series of linkage and association studies, and by fine-mapping studies. Although some progress has been made, there is still no detailed molecular view of the pathogenesis of AITD. The limiting factors for these genetic approaches have been small sample sizes, disease loci with small effects, and the expense and relatively slow speed of genotyping large numbers of people for multiple markers.
The Major Histocompatibility Complex
The MHC, which contains the human leukocyte antigen (HLA) genes, is located on chromosome 6p21. It is subdivided into three regions: the class I region, which encodes the HLA antigens A, B, and C; the class II region, which encodes HLA antigens DR, DQ, and DP, each with one or more α and β chains; and the class III region, encoding several immunoregulatory molecules including complement components, heat shock protein 70 (HSP70), and tumour necrosis factors (TNF) (Fig. 20.1). The class II region also contains the peptide transporters associated with antigen processing (TAP) and large multifunctional protease (LMP) genes. Tight linkage disequilibrium (i.e., conserved haplotypes) exists between the alleles of the MHC region. Major histocompatibility complex class II molecules play a critical part in the initiation of adaptive immune responses. Peptide antigens can only be recognized by T-cell antigen receptors when they are attached to the binding groove of an MHC molecule on the surface of an antigen-presenting cell.
FIGURE 20.1. Linkage disequilibrium map of the human major histocompatility complex (MHC) region. A gene and single nucleotide polymorphism (SNP) map showing seven regions (*) with highly conserved haplotype blocks, the linkage disequilibrium measure (D′), and recombination rates across the MHC region. The terminal end of the 6p arm is to the left in the upper chain, and the centromere is to the right of the lower chain. The numerous genes in the MHC region can be seen, with some areas of particularly strong linkage disequilibrium. (From Walsh EC, Mather KA, Schaffner SF, et al. An integrated haplotype map of the human major histocompatibility complex. Am J Hum Genet 2003;73:580, with permission.)
The first recognition of an association between Graves' disease and alleles of MHC class I molecules was the finding of a higher frequency of the HLA-B8 allele in patients with Graves' disease (47), as compared with normal subjects (21) (110). Several studies in white populations confirmed this association (111,112). However, a stronger association of Graves' disease was found with the MHC class II allele, HLA-DR3, which is in strong linkage disequilibrium with HLA-B8 (113). Many case-control studies in white populations have since found Graves' disease to be associated with HLA-DR3, with relative risks between 2.5 and 5 (reviewed in references 85 and 114). More recently, this association of MHC with Graves' disease was confirmed in a study using the transmission disequilibrium test, which showed preferential transmission of the HLA DRB1*0304-DQB1*02-DQA1*0501 (DR3-DQ2) haplotype (115). Although there is no doubt about the association of Graves' disease with the HLA-DR3 haplotype in whites, the primary disease susceptibility allele in the region remains unknown. An association of Graves' disease, particularly in males, with the HLA-DQA1*0501 allele, which was stronger than, and independent of, the HLA-DR3 status, has also been reported (116). This independent association of HLA-DQA1*0501 with Graves' disease has been supported by some studies (115,117,118), but not by others (119,120,121). In other populations, Graves' disease has been found to be associated with several different HLA alleles. For example, it has been found to be associated with HLA-B35, -B46, -A2, and -DPB1*0501 in Japanese patients (122,123,124); HLA-A10, -B8, and -DQw2 in Indians (125); HLA-DR1 and -DR3 in South African blacks (126); the DRB3*020/DQA1*0501 haplotype in black Americans (127); and HLA-B46, -DR9, -DRB1*303, and -DQB1*0303 in Hong Kong Chinese (128). Furthermore, case-control studies have also shown an association of Graves' disease with alleles of several different non-HLA genes within the MHC complex, including the HSP70, TNF, TAP, and LMP genes (121,129,130,131,132). Thus, it is likely that other, non-HLA genes within the MHC locus contribute to susceptibility to AITD. Due to the strong linkage disequilibrium within this region (Fig. 20.1), it is difficult to determine the independent effect of a particular allele in disease susceptibility, and much larger association studies are needed to resolve these issues.
The results of HLA association studies in autoimmune thyroiditis have been less consistent. In whites, autoimmune hypothyroidism has been associated with HLA alleles that include HLA-B8, -DR3, -DR4, -DR5, -DQA1* 0201/*0301, and -DQB1*03 (121,133,134,135,136,137). Small sample sizes and phenotypic heterogeneity (for example, goitrous vs. atrophic autoimmune hypothyroidism) make it difficult to draw a firm conclusion from these studies, but they suggest a modest association of autoimmune hypothyroidism with HLA-DR3, -DR4, and -DR5. Postpartum thyroiditis has also been found to be associated with HLA-DR4 (135,138) and -DR5 (139,140), suggesting a close relationship between these two disorders.
Although case-control studies in whites have consistently demonstrated an association of Graves' disease with HLA alleles, the results of familial linkage studies of the HLA locus in the disease have been discrepant. Among early studies, several found excess sharing of HLA alleles in sibs affected with the disease, suggesting linkage (88,141), but linkage was not confirmed in other studies (reviewed in reference 89). However, all the studies were small, and there was phenotypic heterogeneity in the kindreds in some studies (e.g., inclusion of members with both Graves' disease and autoimmune hypothyroidism) (89). Based on a transmission disequilibrium test approach in a larger cohort of Graves' disease families, Graves' disease was linked to HLA (115). Linkage analysis in a cohort of sib-pairs with Graves' disease in the United Kingdom also found modest evidence to support linkage of the disease to the MHC region (142). However, three recent genome-wide scans in families with AITD have failed to detect linkage at chromosome 6p21 markers (143,144,145). Therefore, the contribution of genes in the MHC region to the genetic susceptibility to Graves' disease is comparatively small, perhaps accounting for 10% to 20% of the inherited susceptibility in certain populations (142). This is in marked contrast to type 1 diabetes, in which MHC is the dominant locus, accounting for 30% to 40% of the total genetic susceptibility (106,107). In addition, most of the susceptibility to type 1 diabetes within the MHC locus is thought to be encoded by HLA-DQB and -DRB alleles, with lesser contributions from other adjacent genes (146,147). For AITD, it is likely that other, non-HLA genes within 6p21 determine a more substantial part of the inherited predisposition encoded by this region of the genome.
Cytotoxic T Lymphocyte Antigen-4
CTLA-4 is an immunoregulatory molecule that is expressed on the surface of activated T lymphocytes. Several lines of evidence support its role as a key negative regulator of T-cell activation. CTLA-4 knockout mice develop a rampant lymphoproliferative disorder resulting in splenomegaly, lymphadenopathy, and death from autoimmunity before 3 to 4 weeks of age (148). The administration of CTLA-4 blocking antibodies also precipitates or exacerbates autoimmunity in several murine models (149). Conversely, a soluble fusion protein of CTLA-4 and the immunoglobulin G1 Fc region (CTLA-4-Ig) ameliorates different experimental autoimmune disorders, including diabetes (150). Furthermore, the engagement of CTLA-4 with its ligands arrests the progression of activation-induced T-cell cycling (151), possibly by inducing apoptosis of activated T cells (152). This regulatory role of CTLA-4 in T-cell activation makes the CTLA-4 gene an attractive candidate locus for autoimmune disorders.
In 1995, Yanagawa and coworkers reported a significant association of Graves' disease with an allele of a microsatellite polymorphism (designated CTLA-4[AT]n) within the 3′-untranslated region of the CTLA-4 gene (153). Subsequently, the G allele of a single-nucleotide polymorphism that encoded a threonine to alanine change within the signal peptide of CTLA-4 (CTLA-4A/G) was also found to be associated with Graves' disease (154). The association of these two CTLA-4 polymorphisms with Graves' disease has been confirmed by subsequent studies in many different populations, with odds ratios between 1.4 and 3.2 (reviewed in reference 89). Of note, one study in Tunisians found the association with Graves' disease was with the A allele at CTLA-4A/G (155), suggesting that despite the change in the coding sequence encoded by this polymorphism, it was probably not the disease allele. The G allele at the CTLA-4A/G polymorphism was also found to be associated with autoimmune hypothyroidism in several different populations, although with less consistency than with Graves' disease (reviewed in reference 156). This is most likely to be due to the small size of some of the studies, and perhaps the phenotypic heterogeneity in ascertainment of autoimmune hypothyroidism.
Despite the clear finding of association of CTLA-4 alleles with Graves' disease and autoimmune hypothyroidism in many studies from different populations, initial family studies failed to detect linkage at CTLA-4 (143,157). However, detailed analysis of the CTLA-4 locus in a cohort of sib pairs in the United Kingdom revealed strong evidence of linkage to CTLA-4 (peak nonparametric linkage score 3.4), conferring up to one third of the total genetic susceptibility to Graves' disease in this population (142). Significant linkage of CTLA-4 to thyroid autoantibody status was also subsequently reported (158), but genome-wide scans in Chinese families with Graves' disease and Japanese families with AITD did not show linkage at CTLA-4 (144,145).
In order to identify the underlying susceptibility polymorphism within CTLA-4, a detailed fine-mapping study of the entire CTLA-4 locus using markers for 108 single nucleotide polymorphisms was done (Fig. 20.2) (159). Using a regression model, the marker most highly associated with the disease (designated CT60) was located in the 3′ untranslated region of the gene, and the susceptibility allele had an odds ratio of about 1.5 for both Graves' disease and autoimmune hypothyroidism, but a lower contribution to type 1 diabetes (odds ratio ~1.2). The CT60 polymorphism appears to regulate a splice variant of an upstream exon, which determines the expression of membrane-bound versus soluble forms of the mature CTLA-4 molecule. It is hypothesized that under-expression of the mRNA for the soluble CTLA-4 isoform (encoded by the CT60 allele) leads to an imbalance in positive versus “negative” costimulatory signaling to activated T lymphocytes (Fig. 20.3). The CTLA-4 CT60 susceptibility allele has a high population prevalence (60% carrier rate in whites in the United Kingdom), and is likely to be a susceptibility allele not just for AITD and type 1 diabetes, but also for other autoimmune disorders. The high frequency of this low penetrance “disease” allele/haplotype in the normal population is the probable explanation as to why several linkage studies failed to identify the locus (160). A key question is whether this allele can be used to predict the outcome of treatment of Graves' disease, or the development of Graves' ophthalmopathy (161,162). Finally, the effect of nongenetic factors (e.g., smoking, pregnancy, sex hormones) on the expression of the soluble form of CTLA-4 are needed to understand better the interaction of the genetic predisposition and “environmental” factors.
FIGURE 20.2. Fine-mapping of markers of single nucleotide polymorphisms markers at CTLA-4 and adjacent loci. A plot of -log P values in favor of association against the physical map of the markers in the adjacent transcripts CD28, CTLA-4, and ICOS on chromosome 2q33, each of which has a role in lymphocyte costimulation. The most evidence for association is found at the CT60 polymorphism within the 3′ untranslated region of CTLA-4. Alleles of this polymorphism were found to correlate with relative amounts of membrane-bound and soluble CTLA-4 production. (From Ueda H, Howson JM, Esposito L, et al. Association of the T-cell regulatory gene CTLA4 with susceptibility to autoimmune disease. Nature 2003;423:506, with permission.)
FIGURE 20.3. Schematic drawing of an antigen-presenting cell interacting with a T lymphocyte. The dendritic antigen-presenting cell presents a cleaved peptide antigen bound to the groove of the MHC class II molecule. This complex is recognized by a T lymphocyte with an antigen receptor (CD3 complex) with high affinity for the peptide-MHC complex. For the lymphocyte to become activated, a second signal must be delivered. This occurs when CD28 molecules on the T cell bind to B7 (CD80 and CD86) molecules on the antigen-presenting cell. This binding results in T-cell proliferation and activation. In contrast, T cells become quiescent or die when B7 molecules bind to CTLA-4 molecules. Either membrane-bound or soluble CTLA-4 may inhibit B7 binding to CD28, thereby stopping the costimulatory activation of the lymphocyte via the B7-CD28 interaction. Subjects with the susceptibility allele for autoimmune thyroid disease at CTLA-4 produce relatively small amounts of soluble CTLA-4 messenger RNA (159); if reflected at the protein level, this could allow a greater B7 binding to CD28, and therefore greater activation of T cells.
Other Candidate Autoimmune Thyroid Disease Genes and Results of Linkage Scanning
Case-control studies of many other candidate immune system and thyroid-specific genes for AITD have not led to substantial advances in our understanding of pathogenesis. Immunoregulatory genes, including the immunoglobulin heavy-chain (Gm), T-cell receptor β-chain, interleukin-1 receptor antagonist, interleukin-4, interferon-γ, and Fas ligand genes have been studied, but no allele of any of these genes has been consistently associated with AITD (reviewed in references 89 and 163). Similarly, studies of the TSH receptor, the thyroid receptor-β and TPO genes have not yielded significant associations (89). Studies of the vitamin D receptor and the vitamin D–binding protein gene have suggested a possible role in Graves' disease (164,165) but await replication.
In recent years, four different cohorts of patients have been examined for genetic linkage to a large number of anonymous chromosomal markers by US, UK, Japanese, and Chinese investigators (142,143,144,145,157,158,166,167,168). The results of these studies need to be viewed in the context that with 102 families (US study), 82 sib-pairs (UK study), 123 sib-pairs (Japanese study), and 54 families (Chinese study), no individual study has been large enough to detect susceptibility loci with modest effects (169). Thus, only highly significant linkages, or those that are replicated in a second cohort of patients, are likely to stand the test of time. Despite this caution, several promising chromosomal loci have emerged (Table 20.3) (170). In particular, loci for AITD on chromosomes 5q31–q33, 8q23, 18q21, and 20q13 have each been replicated to some degree by a second study (143,144,145,166,167,168,171), suggesting that novel disease susceptibility alleles are likely to be identified from at least some of these loci. Of particular interest, the chromosome 8 linkage appears to be strongest in patients with autoimmune hypothyroidism, and it overlaps the Tg locus, and in one study preliminary evidence for association within the Tg gene was found (172,173).
TABLE 20.3. LINKAGE OF AUTOIMMUNE THYROID DISEASE IN DIFFERENT POPULATIONSa
U.S. Study (Reference Nos. 143,157,158,168)
Japanese Study (Reference No. 144)
Chinese Study (Reference No. 145)
UK Study (Newcastle) (Reference No. 142,161,166,167)
UK Study (Birmingham) (Reference No. 115,131)
AITD1, Autoimmune thyroid disease-susceptibility to 1; CTLA-4, cytotoxic T lymphocyte antigen-4; GD1, Graves' disease-susceptibility to 1; GD2, Graves' disease-susceptibility to 2; GD3, Graves' disease-susceptibility to 3; HT1, Hashimoto's thyroiditis-susceptibility to 1; HT2, Hashimoto's thyroiditis-susceptibility to 2; MHC, major histocompatibility complex; ND, not done; TDT, transmission disequilibrium test.
aSignificance levels for linkage are designated according to the criteria of Lander and Kruglyak:—, no linkage; +, nominal linkage (LOD score >1.0); ++, suggestive linkage (LOD score >1.9); +++, significant linkage (LOD score >3.3). Equivalent nonparametric linkage scores are >1.2, >2.2, and >3.6 for nominal, suggestive and significant linkage, respectively (169).
bLoci found linked in more than one dataset are shown in bold.
cLinkage with thyroid autoantibody production (158).
dEvidence for linkage found only in the Graves' disease families with apparent dominant mode of transmission (a tentative finding).
Summary of Autoimmune Thyroid Disease Genetics
AITD can be considered a complex trait with a predominant genetic component. Several gene loci determine susceptibility to the disease, with a major contribution from CTLA-4. Currently, there is moderate evidence that other loci are involved to a lesser degree, including one or more genes in the MHC region. As work progresses, it is likely that other disease genes and immune system pathways will be identified that predispose to the disease. The evaluation of gene-environment interaction at a molecular level is likely to cast further light on disease pathogenesis.
In the last few years there have been unprecedented advances in knowledge about the sequence and organization of the human and murine genomes, such that there are now comprehensive gene maps of each chromosome, and tissue-specific expression profiles at both the mRNA and protein level are rapidly being developed. In time, these advances will more fully inform our knowledge of the physiology of normal thyroid function and its disturbance in many more disease states. Impressive progress in understanding both monogenic and complex thyroid disorders at a genomic level has been made in the last decade. The next decade holds the promise of allowing the integration of genetic and epidemiologic approaches to understanding disease, through a determination of the underlying molecular pathology.
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