Resistance to TSH in Patients with Normal TSH Receptors—Where Do We Turn When "Sutton’s Law" Proves False?

Michael A. Levine and Matthew D. Ringel

Division of Endocrinology and Metabolism, Department of Medicine The Johns Hopkins University School of Medicine Baltimore, Maryland 21205; Section of Endocrinology, Department of Medicine, Washington Hospital Center and Medlantic Research Institute, Washington, CD 20010

Address correspondence and requests for reprints to: Michael A. Levine, M.D., Division of Endocrinology and Metabolism, The Johns Hopkins University School of Medicine, Ross Building, Room 863, 720 Rutland Avenue, Baltimore, MD 21205. E-mail: mlevine{at}jhu.edu


    Introduction
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 Introduction
 References
 
Neonatal screening programs for congenital hypothyroidism have been established throughout the world and have shown an incidence of 1:3500 to 1:4500 births. The justification for these programs derives from critical studies that affirm the benefits of early institution of thyroid hormone therapy, and which indicate that the earlier the diagnosis of congenital hypothyroidism is made and therapy started, the better the prognosis for intelligence. A secondary dividend of these screening programs has been the discovery of unusual forms of congenital hypothyroidism. In areas of the world where endemic iodine deficiency does not occur, thyroid dysgenesis is the most common cause of congenital hypothyroidism. Defects that impair synthesis, secretion, or action of thyroid hormone are less common causes of congenital hypothyroidism, but the study of these rare inborn errors of metabolism has often yielded important and unpredicted insights into growth and development of the thyroid gland (1). The identification and characterization of many of these unusual genetic defects has been facilitated by fundamental discoveries over the past decade that have begun to define the molecular physiology of the thyroid gland. Of great interest to many, and of particular relevance to the study presented in this issue of the JCEM, has been the elucidation of the signal transduction cascade that subserves thyrotropin action. Thyrotropin (TSH), the primary regulator of thyroid gland growth and function, exerts its actions by binding to and activating a heptahelical receptor (TSH-Re) that can interact with members of all four families of the signal-transducing G proteins (2, 3). These heterotrimeric proteins, composed of distinct {alpha} chains and tightly bound ß{gamma} dimers, couple receptors for a wide variety of extracellular signals, including hormones, cytokines, neurotransmitters, and pheromones, as well as light, taste, and smell, to intracellular enzymes and ion channels that generate second messengers (4). The G proteins and their associated signal-generating effector molecules have a widespread tissue distribution, albeit with differential expression among different members, and perform generalized signaling functions. The ability of many receptors to couple to several different G proteins, that each regulate different signaling pathways, allows for considerable plasticity in response to a single signal. By contrast, expression of receptor proteins is highly restricted, which confers a high degree of specificity to signal detection by each cell.

In human thyroid, activation of the TSH-Re leads to stimulation of adenylyl cyclase and ß-isoforms of phospholipase C (PLC-ß) via coupling of the receptor to Gs and members of the G{alpha}/11 family, respectively (5, 6). Activation of these signaling pathways leads to the production of second messengers (e.g. cAMP, inositol phosphate, diacyl glycerol, and intracellular calcium) that together promote thyroid cell growth, proliferation, and function. Recognition of the critical role of thyrotropin in initiating and sustaining these signals led to the hypothesis that genetic defects in the TSH-Re might produce significant and tissue-specific phenotypic consequences. These predictions have now been fulfilled in a series of elegant observations (7, 8). First, somatic and germline mutations in the TSH-Re gene that result in constitutive (i.e. ligand independent) activation of the receptor have been found in many toxic thyroid adenomas (9, 10) and in occasional patients with nonimmune hyperthyroidism (11, 12, 13), respectively. Second, TSH-Re gene mutations that inactivate the receptor have been associated with inherited hypothyroidism in both humans (8, 14, 15) and mice (16) with TSH resistance. The thyroid dysfunction produced by these contrasting mutations in the TSH-Re joins a growing list of endocrine and metabolic disorders that have been ascribed to mutations in genes encoding other G-protein coupled heptahelical receptors, including but not limited to activating and inactivating mutations of the calcium sensing receptor and the FSH receptor, activating mutations of the LH receptor and the PTH/PTHrP receptor, and inactivating mutations of the type 2 vasopressin receptor. These findings have suggested that altered receptor function, once considered to be an unusual basis for endocrine dysfunction, may in fact be a common cause of tissue-specific pathology.

With this premise in mind, it was logical for Xie and coworkers to target the TSH-Re gene for analysis in the three families with isolated resistance to TSH reported in this issue of JCEM (17) (see page 3933). It was therefore no doubt surprising, if not disappointing, to the authors that they were unable to detect mutations in the TSH-Re gene in any of these three families. Moreover, using intragenic polymorphic markers to perform linkage analysis, they were able to exclude the TSH-Re gene as a candidate in two of the families. These results should not have been totally unexpected, however. The affected subjects in two of these kindreds demonstrated a dominant mode of transmission of TSH resistance, whereas autosomal recessive mode of inheritance has been found in previously described patients with TSH-Re gene mutations (8). Although the development of TSH resistance, albeit mild, in the affected subjects in these two kindreds could have been explained by a novel dominant negative mutation of the TSH-Re, the phenotype is also consistent with a defect in some other tissue specific component of the signaling cascade.

These results confirm and extend a similar negative report by Ahlbo et al. (18) published elsewhere earlier this year. Together, these two negative studies lend further support to the notion that most genetic disorders display genetic heterogeneity, that is, similar phenotypes (i.e. phenocopies) can arise from defects in different genes. Despite the fact that both of these studies (17, 18) report essentially "negative" results, they nevertheless represent highly significant contributions. First, they remind us that receptor mutations may not account for all forms of hormone resistance, and second, they oblige us to consider how defects in other components of the signaling cascade might impair hormone action. It was no doubt this latter consideration that led Xie et al. (17) to study both an upstream signal (the ß subunit of thyrotropin) as well as a downstream target (the {alpha} subunit of Gs) of the TSH-Re. These analyses also failed to disclose gene mutations or biological abnormalities that could account for TSH resistance in these unusual patients.

These results invite comparison to another syndrome of hormone resistance, pseudohypoparathyroidism (PHP; reviewed in ref. 19). PTH resistance is the hallmark of PHP, a syndrome described by Fuller Albright in 1942 as the first example of a human disorder in which endocrine deficiency resulted not from a lack of hormone but from failure of target tissues to respond appropriately to the hormone (20). The subsequent observation that intravenous administration of PTH to normal subjects stimulates a marked increase in the urinary excretion of nephrogenous cAMP led to the development of PTH infusion protocols that have enabled distinction between the several variants of PHP. Thus, patients with PHP type 1 fail to show either a phosphaturic or a nephrogenous cAMP response after administration of exogenous PTH (21). By contrast, patients with the far more uncommon type 2 variant of PHP fail to show a phosphaturic response to PTH but manifest a normal nephrogenous cAMP response (22). These findings implicate defects in the G protein-coupled PTH receptor-adenylyl cyclase signaling cascade as the basis for PTH resistance in patients PHP type 1 and suggest that more distal defects must account for apparent hormone resistance in patients with PHP type 2. This hypothesis has been confirmed in patients with PHP type 1a, in whom hormone resistance is accompanied by an unusual constellation of developmental defects that are collectively termed Albright’s hereditary osteodystrophy. This autosomal dominant disorder is characterized by an approximately 50% deficiency of Gs activity, which arises as a consequence of mutations of the Gs{alpha} gene that reduce function or expression of the Gs{alpha} protein (19). Patients with PHP type 1a have decreased responsiveness not only to PTH, but to other hormones as well, including TSH, whose receptors are coupled by Gs to activation of adenylyl cyclase. Although generalized deficiency of Gs{alpha} likely accounts for multiple hormone resistance, the relationship of the Gs{alpha} defect to Albright’s hereditary osteodystrophy remains uncertain. Patients with a second autosomal dominant form of PTH resistance, PHP type 1b, lack features of Albright’s hereditary osteodystrophy, have normal levels of Gs{alpha} protein, and manifest target tissue resistance only to PTH. These features are certainly consistent with a defect in PTH receptor action, but nucleotide sequence analysis of the gene (23, 24) and cDNA (25) encoding the type 1 PTH-receptor (also termed the PTH/PTHrP-receptor), as well as linkage analysis using intragenic polymorphisms (26), have excluded this candidate gene as the basis for PHP type 1b. Thus, the primary genetic defect(s) in this disorder remains as great a mystery as the cause of TSH resistance in the three kindreds described in this issue of JCEM (17).

When the notorious criminal Willie Sutton (1901–1980) was once asked why he robbed banks, Sutton is reputed to have replied, "Because that’s where the money is." This apocryphal parable has long inspired investigators to seek straightforward explanations for their observations and to examine first the most obvious candidates. So what are the molecular defects that cause TSH resistance in these three kindreds and PTH resistance in patients with PHP type 1b, now that "Sutton’s law" has apparently led us to where the money "ain’t?" With normal genes encoding the TSH-Re and PTH-Re, what are the appropriate next steps in defining the molecular basis for these disorders? What other genes can be proposed as likely, or even potential, candidates, to thereby justify the arduous (if not tedious) efforts necessary to accomplish a comprehensive evaluation? The answers to these questions are many, and we invite the readers to consider the possibilities. Certainly other G proteins, receptor kinases, isoforms of adenylyl cyclase or phosphodiesterase, as well as protein kinases and their tissue specific substrates are all attractive possibilities. We will not review here all the possible gene defects that may impair TSH action in the patients described by Xie et al. (17), but we will suggest that speculation is made all the more difficult by our lack of knowledge concerning the ability of their thyroid glands to generate cAMP in response to TSH.

An alternative to the candidate gene approach is to perform whole genome linkage analysis to identify genes that are tightly linked to the disorder. This method is highly ambitious; one must analyze a large number of chromosomal markers that are distributed throughout the genome using DNA samples from affected and nonaffected members of large, multigenerational families. To be successful, one needs a sufficiently large number of subjects to ensure that a convincing statistical odds of linkage (LOD) score can be achieved. This approach assumes that the same gene, albeit not the same mutation, is defective in all the families. For TSH resistance, the clinical variability may create significant problems, as inclusion of even one misidentified family can invalidate or obscure linkage.

We eagerly look forward to studies that advance our understanding of the molecular pathogenesis of hormone resistance disorders. The negative studies of TSH resistance and PHP type 1b provide timely reminders that looking "where the money is" might not always be straightforward: while the money is likely to be in the bank, it may not always be kept within the vault.

Received October 14, 1997.

Accepted October 14, 1997.


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