Thyroid-Stimulating Hormone Receptor Mutations

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Chapter 20

Thyroid-Stimulating Hormone Receptor Mutations

Gain-of-Function Mutations

For a hormone receptor, “gain of function” may have three meanings: activation in the absence of ligand (constitutivity), increased sensitivity to its normal agonist, or broadening of its specificity. When the receptor is part of a chemostat, as is the case for the thyroid-stimulating hormone (TSH) receptor, the first situation is expected to lead to tissue “autonomy,” whereas the second would be expected to simply cause adjustment of the agonist concentration to a lower value. In the third case, inappropriate stimulation of the gland is expected to occur because the promiscuous agonist is not expected to be subjected to the normal negative feedback. If a gain-of-function mutation of the first category occurs in a single cell normally expressing the receptor (somatic mutation), it will become symptomatic only if the regulatory cascade controlled by the receptor is mitogenic in this particular cell type. Autonomous activity of the receptor will cause clonal expansion of the mutated cell. If the regulatory cascade also positively controls function, the resulting tumor will progressively take over function of the normal tissue, ultimately resulting in autonomous hyperfunction. If the mutation is present in all cells of an organism (germline mutation), autonomy will be displayed by the whole tissue expressing the receptor. In cases in which the regulatory cascade is mitogenic and activates function, the expected result is hyperplasia associated with hyperfunction.

From what we know of thyroid cell physiology, it is easy to predict the phenotypes associated with gain-of-function mutations of the cyclic adenosine monophosphate (cAMP)-dependent regulatory cascade. Two observations provide pertinent models of this situation. Transgenic mice made to ectopically express the adenosine A2a receptor in their thyroid display severe hyperthyroidism associated with thyroid hyperplasia.1 Because the A2a adenosine receptor is coupled to Gs and displays constitutive activity as a result of continuous stimulation by ambient adenosine, this model closely mimics the situation expected for a gain-of-function germline mutation of the TSH receptor. Patients with the McCune-Albright syndrome are mosaic for mutations in Gs (gsp mutations), which also leads to constitutive stimulation of adenylyl cyclase.2 Hyperfunctioning thyroid adenomas develop in these patients from cells harboring the mutation, which makes them a model for gain-of-function somatic mutations of the TSH receptor. A transgenic model in which gsp mutations are targeted for expression in the mouse thyroid has been constructed. Although with a less dramatic phenotype, this represents a pertinent model for a gain of function in the cAMP regulatory cascade.3

Familial Nonautoimmune Hyperthyroidism or Hereditary Toxic Thyroid Hyperplasia

The major cause of hyperthyroidism in adults is Graves’ disease, in which an autoimmune reaction is mounted against the thyroid gland and autoantibodies are produced that recognize and stimulate the TSH receptor. This origin may explain why the initial description by the group of Leclère of a family showing segregation of thyrotoxicosis as an autosomal dominant trait in the absence of signs of autoimmunity was met with skepticism.4 Reinvestigation of this family together with that of another family from Reims (France) identified two mutations of the TSH receptor gene that segregated in perfect linkage with the disease.5 A series of additional families have been studied since, almost systematically with a different mutation of the TSH receptor gene619 (Fig. 20-1). (For comprehensive lists of activating mutations, see gris.ulb.ac.be/ and www.ssfa-gphr.de/main/ssfa.php.) The functional characteristics of these mutant receptors confirm that they are constitutively stimulated (see below). Hereditary toxic thyroid hyperplasia (HTTH), sometimes called Leclère’s disease, is characterized by the following clinical characteristics: autosomal dominant transmission; hyperthyroidism with a variable age of onset (from infancy to adulthood, even within a given family); hyperplastic goiter of variable size, but with steady growth; and absence of clinical or biological stigmata of autoimmunity. An observation common to cases to date is the need for drastic ablative therapy (surgery or radioiodine) to control the disease once the patient has become hyperthyroid. The autonomous nature of the thyroid tissue from these patients has been elegantly demonstrated by grafting in nude mice.20 In contrast to tissue from patients with Graves’ disease, HTTH cells continue to grow in the absence of stimulation by TSH or thyroid-stimulating antibody.

The prevalence of hereditary toxic thyroid hyperplasia is difficult to estimate at the present time. It is likely that many cases have been (and still are) mistaken for Graves’ disease. This may be explained by the high frequency of thyroid autoantibodies (antithyroglobulin, antithyroperoxidase) in the general population. It is expected that wider knowledge of the existence of the disease will lead to better diagnosis. This is not a purely academic problem, in that presymptomatic diagnosis in children of affected families may prevent the developmental or psychological complications associated with infantile or juvenile hyperthyroidism. A country-wide screening for the condition has been performed in Denmark. It was found in 1 out of 121 patients with juvenile thyrotoxicosis (0.8%; 95% confidence interval [CI]: 0.02% to 4.6%), which corresponds to 1 in 17 patients with presumed nonautoimmune juvenile thyrotoxicosis (6%; 95% CI: 0.15% to 28.69%).21

Sporadic Toxic Thyroid Hyperplasia

Cases of toxic thyroid hyperplasia have been described in children of unaffected parents.7,2229 Conspicuously, congenital hyperthyroidism was present in most of the patients and required aggressive treatment. Mutations of one TSH receptor allele were identified in the children but were absent in the parents. Because paternity was confirmed by minisatellite or microsatellite testing, these cases qualify as true neomutations. When the amino acid substitutions implicated in hereditary and sporadic cases are compared, for the majority, they do not overlap (see Fig. 20-1). Although most of the sporadic cases harbor mutations that are also found in toxic adenomas, most of the hereditary cases have “private” mutations. Analysis of the functional characteristics of the individual mutant receptors in COS cells and the clinical course of individual patients suggests an explanation for this observation: “sporadic” mutations seem to have a stronger activating effect than “hereditary” mutations do. From their severe phenotypes, it is likely that newborns with neomutations would not have survived if not treated efficiently. On the contrary, from inspection of the available pedigrees, it seems that the milder phenotype of patients with hereditary mutations has only a limited effect on reproductive fitness. The fact that hereditary mutations are rarely observed in toxic adenomas is compatible with the suggestion that they would cause extremely slow tissue growth and, accordingly, would rarely cause thyrotoxicosis, if somatic. There is, however, no a priori reason for neomutations to cause stronger gain of function than hereditary mutations. Accordingly, an activating mutation of the TSH receptor gene has been found in a 6-month-old child with subclinical hyperthyroidism who presents with weight loss as the initial symptom.30

Somatic Mutations: Autonomous Toxic Adenomas

Soon after mutations of Gsα had been found in adenomas of the pituitary somatotrophs,31 similar mutations (also called gsp mutations) were found in some toxic thyroid adenomas and follicular carcinomas.3235 The mutated residues (Arg201, Glu227) are homologous to those found mutated in the ras proto-oncogenes, that is, the mutations decrease the endogenous guanine triphosphatase activity of the G protein, thereby resulting in a constitutively active molecule.

Toxic adenoma was found to be a fruitful source of somatic mutations activating the TSH receptor, probably because the phenotype is very conspicuous and easy to diagnose. Whereas mutations are distributed all along the serpentine portion of the receptor36-42 and even in the extracellular aminoterminal domain,43 there is clearly a hotspot at the cytoplasmic side of the sixth transmembrane segment (see Fig. 20-1). (For a complete list of TSH receptor gene mutations with their functional characteristics, see gris.ulb.ac.be/ and www.ssfa-gphr.de/main/ssfa.php.) The clustering reflects the pivotal role of this portion of the molecule in activation mechanisms.4449

Despite some dispute about the prevalence of TSH receptor mutations in toxic adenomas, which may be due to different origins among patients50 or different sensitivities of methods, the current consensus is that activating mutations of the TSH receptor are the major cause of solitary toxic adenomas and account for about 60% to 80% of cases.36,42,5153 Contrary to initial suggestions, the same percentage of TSH receptor mutation is observed in Japan, an iodine-sufficient country with low prevalence of toxic adenomas.54 In some patients with multinodular goiter and two zones of autonomy at scintigraphy, a different mutation of the TSH receptor was identified in each nodule.5558 This finding indicates that the pathophysiologic mechanism responsible for solitary toxic adenomas can be at work on a background of multinodular goiter. In agreement with this notion, activating mutations of the TSH receptor have been identified in hyperfunctioning areas of multinodular goiter.57,5961 The independent occurrence of two activating mutations in a patient may seem highly improbable at first. However, the multiplicity of the possible amino acid targets for activating mutations within the TSH receptor makes this event less unlikely. It is also possible that a mutagenic environment is created in glands exposed to chronic stimulation by TSH in which H2O2 is produced.62,63 Finally, involvement of TSH receptor mutations in thyroid cancer has been implicated in a limited proportion of follicular thyroid carcinomas.61,6469

Structure-Function Relationships of The TSH Receptor, as Deduced from Activating Mutations

Most of the activating mutations of the TSH receptor have been studied by transient expression in COS or HEK293T cells. There is no guarantee that the mutants will function in an identical way in these artificial systems as they do in thyrocytes. Arguments have been obtained for such cell-type specific effects.70 In thyrocytes, a better relation has been observed between adenylyl cyclase stimulation and differentiation than with growth.70 However, the built-in amplification associated with transfection of constructs in COS or HEK 293T cells makes it possible to detect even slight increases in constitutive activity of TSH receptor mutants. An important observation has been that the wild-type receptor itself displays significant constitutive activity.38,71 This characteristic is not unique to the TSH receptor, but it is interesting to note that it is not shared by its close relatives, the luteinizing hormone/chorionic gonadotropin (LH/CG) receptor and the follitropin receptor (FSH). The effect of activating mutations accordingly must be interpreted in terms of “increase in constitutive activity.” Most receptor mutants found in toxic adenomas and/or toxic thyroid hyperplasia share common characteristics: (1) they increase the constitutive activity of the receptor toward stimulation of adenylyl cyclase; (2) with a few notable exceptions (see Fig. 20-1),72 they do not display constitutive activity toward the inositol phosphate/diacylglycerol pathway; (3) their expression at the cell surface is decreased (from slightly to severely); (4) most but not all of them keep responding to TSH for stimulation of cAMP and inositol phosphate generation, with a tendency to do so at decreased median effective concentrations; and (5) they bind 125I-labeled bovine TSH with an apparent affinity higher than that of the wild-type receptor.

No simple relationship exists between the position of the mutations or the nature of the amino acid substitution and their functional characteristics. Mutations found in transmembrane segments 1, 2, 3, 6, and 7 and the third cytoplasmic loop all have similar phenotypes; they involve amino acids belonging to all classes (charged, polar, hydrophobic), with substitutions not necessarily involving a shift to another class. Mutations involving Ile486 and Ile568 in the first and second extracellular loops, respectively, and Pro639 in transmembrane segment 6 are exceptional in that, in addition to stimulating adenylyl cyclase, they cause constitutive activation of the inositol phosphate pathway.

No direct relationship has been found between the level of cAMP achieved by different mutants in transfected COS cells and their level of expression at the cell membrane,73 which means that individual mutants have widely different “specific constitutive activity” (measured as the stimulation of cAMP accumulation/receptor number at the cell surface). Although this specific activity may tell us something about the mechanisms of receptor activation, it is not a measure of the actual phenotypic effect of the mutation in vivo. Indeed, one of the relatively mild mutations, observed up until now only in a family with HTTH (Cys672Tyr), is among the strongest according to this criterion. It would be logical to expect the best correlation to be found between the phenotype and the actual level of cAMP achieved, irrespective of the level of receptor expression. However, differences between the effects of the mutants in transfected COS or HEK293 cells and thyrocytes in vivo may render these correlations a difficult exercise.70

According to a current model of G protein–coupled receptor (GPCR) activation, the receptor would exist under at least two interconverting conformations: R (silent conformations) and R* (the active forms).44 The unliganded receptor would shuttle between both forms, the equilibrium being in favor of R. Binding of the ligand to the slit between the transmembrane segments (for biogenic amines) and/or the residues of the N-terminal segment or extracellular loops (for neuropeptides) is believed to stabilize the R* conformations. The resulting R-to-R* transition is supposed to involve a conformational change that modifies the relative position of transmembrane helices, which in turn would translate into conformational changes in the cytoplasmic domains interacting with trimeric G proteins. Seminal studies with the adrenergic receptor α1b have shown that a variety of amino acid substitutions in the C-terminal portion of the third intracellular loop lead to their constitutive activation.74 The observation that all amino acid substitutions at Ala293 were effective in activating the receptor led to the concept that the silent form of GPCRs would be submitted to a structural constraint requiring the wild-type primary structure of the third intracellular loop. This constraint could be released by a wide spectrum of amino acid substitutions in this segment.44,75

The observation that amino acid substitutions in a large number of residues scattered over the serpentine portion of the TSH receptor cause an increase in its constitutive activity is fully compatible with the above model and provided arguments for its extension. The fact that mutations in residues distributed over most of the serpentine portion of the receptor are equally effective in activating it (which does not seem to be a general characteristic in all GPCRs) suggests that the unliganded TSH receptor might be less constrained than others. The readily measurable constitutive activity of the wild-type receptor is compatible with this contention. Being already “noisy,” the TSH receptor would be more prone to further destabilization by a variety of mutations.

The precise effects of individual mutations in structural terms are beginning to emerge from analogies with the limited list of GPCRs whose tridimensional structure has been solved.76