A base mutation of the C-erbA beta thyroid hormone receptor in a kindred with generalized thyroid hormone resistance. Molecular heterogeneity in two other kindreds

Diagnosis of Resistance to Thyroid Hormone due to a Rare Mutation in the Thyroid Hormone Receptor Beta Gene in a Patient Previously Presumed to Have Graves' Disease

Clinicians may confuse an impaired sensitivity to thyroid hormone with hyperthyroidism and offer an inappropriate treatment. We report a diagnosis of resistance to thyroid hormone (RTH) caused by a rare mutation in the thyroid hormone receptor beta gene in a patient previously presumed to have Graves' disease. We have found only one published case of a novel point mutation, c.749T>C (p.Ile250Thr variant) associated with 50% reduction in thyroid hormone receptor binding affinity for triiodothyronine in the I250T mutant; it was found in this patient. A 66-year-old male veteran, with a history of non-ischemic cardiomyopathy and arrhythmias, was referred by a cardiologist with concerns for a possible thyrotropin (TSH) adenoma on account of elevated TSH and free thyroxine (FT4) levels. Pituitary imaging was negative. He was previously treated with radioiodine for presumptive Graves' disease in the civilian sector. Examination revealed a goiter with no nodules. Repeat TSH and FT4 levels were elevated and also free triiodothyronine (FT3) and reverse triiodothyronine. These findings and other test results were consistent with RTH, which was confirmed by genetic testing. Mutation analysis showed the patient to be heterozygous for the p.Ile250Thr variant. He later developed hypothyroidism. Resistance to thyroid hormone can be misdiagnosed as hyperthyroidism with consequent inappropriate treatment. Treatment is not needed in most RTH-beta patients. Thyroid ablation should generally be avoided. Clinicians must be cautious whenever they encounter concurrent elevation of TSH, FT4, and FT3. This RTH-beta patient has a rare I250T mutant of the thyroid hormone receptor beta gene, the second reported case in the literature.

A Historical Reflection on Scientific Advances in Understanding Thyroid Hormone Action

Background: Thyroid hormone (TH) has actions in every tissue of the body and is essential for normal development, as well as having important actions in the adult. The earliest markers of TH action that were identified and monitored clinically, even before TH could be measured in serum, included oxygen consumption, basal metabolic rate, serum cholesterol, and deep tendon reflex time. Cellular, rodent, amphibian, zebrafish, and human models have been used to study TH action. Summary: Early studies of the mechanism of TH action focused on saturable-specific triiodothyronine (T3) nuclear binding and direct actions of T3 that altered protein expression. Additional effects of TH were recognized on mitochondria, stimulation of ion transport, especially the sodium potassium ATPase, augmentation of adrenergic signaling, role as a neurotransmitter, and direct plasma membrane effects. The cloning of the thyroid hormone receptor (THR) genes in 1986 and report of the THR crystal structure in 1995 produced rapid progress in understanding the mechanism of TH nuclear action, as well as the development of modified THR ligands. These findings revealed nuances of TH signaling, including the role of nuclear receptor coactivators and corepressors, repression of positively stimulated genes by the unliganded receptor, THR isoform-specific actions of TRα (THRA) and TRβ (THRB), and THR binding DNA as a heterodimer with retinoid-x-receptor (RXR) for genes positively regulated by TH. The identification of genetic disorders of TH transport and signaling, especially Resistance to Thyroid Hormone (RTH) and monocarboxylate transporter 8 (Mct8) defects, has been highly informative with respect to the mechanism of TH action. Conclusions: The impact of THR isoform, post-translational modifications, receptor cofactors, DNA response element, and selective TH tissue uptake, on TH action, have clinical implications for diagnosing and treating thyroid disease. Additionally, these findings have led to the development of novel TH and TH analogue therapies for metabolic, neurological, and cardiovascular diseases.

Thyroid hormone resistance: Mechanisms and therapeutic development

As an essential primary hormone, thyroid hormone (TH) is indispensable for human growth, development and metabolism. Impairment of TH function in several aspects, including TH synthesis, activation, transportation and receptor-dependent transactivation, can eventually lead to thyroid hormone resistance syndrome (RTH). RTH is a rare syndrome that manifests as a reduced target cell response to TH signaling. The majority of RTH cases are related to thyroid hormone receptor β (TRβ) mutations, and only a few RTH cases are associated with thyroid hormone receptor α (TRα) mutations or other causes. Patients with RTH suffer from goiter, mental retardation, short stature and bradycardia or tachycardia. To date, approximately 170 mutated TRβ variants and more than 20 mutated TRα variants at the amino acid level have been reported in RTH patients. In addition to these mutated proteins, some TR isoforms can also reduce TH function by competing with primary TRs for TRE and RXR binding. Fortunately, different treatments for RTH have been explored with structure-activity relationship (SAR) studies and drug design, and among these treatments. With thyromimetic potency but biochemical properties that differ from those of primary TH (T3 and T4), these TH analogs can bypass specific defective transporters or reactive mutant TRs. However, these compounds must be carefully applied to avoid over activating TRα, which is associated with more severe heart impairment. The structural mechanisms of mutation-induced RTH in the TR ligand-binding domain are summarized in this review. Furthermore, strategies to overcome this resistance for therapeutic development are also discussed.

What is the Role of Thyroid Hormone Receptor Alpha 2 (TRα2) in Human Physiology?

Thyroid hormone receptors are nuclear receptors that function as transcription factors and are regulated by thyroid hormones. To date, a number of variants and isoforms are known. This review focuses on the thyroid hormone receptor α (TRα), in particular TRα2, an isoform that arises from alternative splicing of the THRA mRNA transcript. Unlike the TRα1 isoform, which can bind T3, the TRα2 isoform lacks a ligand-binding domain but still binds to DNA thereby antagonizing the transcriptional activity of TRα1. Although a regulatory role has been proposed, the physiological function of this TRα2 antagonism is still unclear due to limited in vitro and mouse model data. Recently, the first patients with resistance to thyroid hormone due to mutations in THRA, the TRα encoding gene, affecting the antagonistic function of TRα2 were described, suggesting a significant role of this particular isoform in human physiology.

Resistance to thyroid hormone caused by heterozygous mutation of thyroid hormone receptor B gene c.G1378A: Report of one Chinese pedigree and literature review

The same thyroid hormone receptor B gene (THRB) mutation led to thyroid hormone resistance with different clinical manifestations in the kindreds.

Mutational Landscape of Resistance to Thyroid Hormone Beta (RTHβ)

Resistance to thyroid hormone beta (RTHβ) is a syndrome characterized by reduced responsiveness of peripheral tissues to thyroid hormone (TH). In most cases, the disorder is associated with germline pathogenic variants in the thyroid hormone receptor beta (THRB) gene. This paper summarizes the clinical and biochemical presentation of the disease, providing a comprehensive overview on molecular genetic features. Particular care is given in reporting all identified THRB variants with an assessed or unknown clinical significance. Our aim is to offer a useful tool for clinical and genetic specialists in order to ease clinical diagnosis and genetic counseling.

The actions of thyroid hormone signaling in the nucleus

Thyroid hormones are a critical regulator of mammalian physiology. Much of their action is due to effects in the nucleus where T₃ engages thyroid hormone receptor isoforms to mediate its effects. In order to function properly the TR isoforms must be recruited to regulatory sequences within genes that they up-regulate. On these positive regulated target genes the TR can activate or repress depending upon whether the receptor is bound to T₃ or not and the type of co-regulatory proteins present in that cell type. In contrast to T₃ mediated activation, the mechanism by which the TR represses transcription in the presence of T₃ remains unclear. Herein we will review the components of the transcriptional response to T₃ within the nucleus and attempt to highlight the outstanding questions in the field.

Role of Thyroid Hormones in Skeletal Development and Bone Maintenance

The skeleton is an exquisitely sensitive and archetypal T3-target tissue that demonstrates the critical role for thyroid hormones during development, linear growth, and adult bone turnover and maintenance. Thyrotoxicosis is an established cause of secondary osteoporosis, and abnormal thyroid hormone signaling has recently been identified as a novel risk factor for osteoarthritis. Skeletal phenotypes in genetically modified mice have faithfully reproduced genetic disorders in humans, revealing the complex physiological relationship between centrally regulated thyroid status and the peripheral actions of thyroid hormones. Studies in mutant mice also established the paradigm that T3 exerts anabolic actions during growth and catabolic effects on adult bone. Thus, the skeleton represents an ideal physiological system in which to characterize thyroid hormone transport, metabolism, and action during development and adulthood and in response to injury. Future analysis of T3 action in individual skeletal cell lineages will provide new insights into cell-specific molecular mechanisms and may ultimately identify novel therapeutic targets for chronic degenerative diseases such as osteoporosis and osteoarthritis. This review provides a comprehensive analysis of the current state of the art.

Nuclear hormone receptor functions in keratinocyte and melanocyte homeostasis, epidermal carcinogenesis and melanomagenesis

Skin homeostasis is maintained, in part, through regulation of gene expression orchestrated by type II nuclear hormone receptors in a cell and context specific manner. This group of transcriptional regulators is implicated in various cellular processes including epidermal proliferation, differentiation, permeability barrier formation, follicular cycling and inflammatory responses. Endogenous ligands for the receptors regulate actions during skin development and maintenance of tissue homeostasis. Type II nuclear receptor signaling is also important for cellular crosstalk between multiple cell types in the skin. Overall, these nuclear receptors are critical players in keratinocyte and melanocyte biology and present targets for cutaneous disease management.

Long-term 3,5,3'-triiodothyroacetic acid therapy in a child with hyperthyroidism caused by thyroid hormone resistance: pharmacological study and therapeutic recommendations

BACKGROUND

The effectiveness of short-term 3,5,3'-triiodothyroacetic acid (TRIAC) therapy for the treatment of hyperthyroidism caused by thyroid hormone resistance (RTH) has been documented. Here, we report a 3-year course of TRIAC therapy in an RTH boy, with a quantitative evaluation of the therapeutic effects and pharmacological study of TRIAC.

PATIENT FINDINGS

The gene encoding the thyroid hormone receptor beta (THRB) of the patient carries a P453T mutation. During treatment with up to 3.0 mg TRIAC per day, reduction in the thyroid volume, resolution of supraventricular arrhythmia, and decrease in thyroid-stimulating hormone (TSH) and free-thyroxine (FT4) levels were achieved. In addition, attention-deficit hyperactivity disorder (ADHD) symptoms improved, with a concomitant decline in the ADHD Rating Scale score.

SUMMARY

A TRIAC pharmacokinetic study, conducted using triiodothyronine level as a surrogate for TRIAC level, demonstrated that TRIAC disappears from the circulation rapidly and has a shorter duration of TSH secretion inhibitory effect in the RTH patient compared to that in the control subject. Studies of TSH and FT4 levels over a period of 3 years indicated that the TRIAC effect is dose dependent.

CONCLUSIONS

TRIAC was effective and safe in ameliorating the effects of hyperthyroidism and ADHD symptoms in a child with known genetic RTH. Further, it was demonstrated that TRIAC has a short half-life and functions dose dependently.

Pathogenic mechanism of mutations in the thyroid hormone receptor β gene

BACKGROUND

Resistance to thyroid hormone (RTH) is characterized by a variable degree of reduced tissue sensitivity to thyroid hormone (TH). It is usually caused by mutations in the TH receptor-β (TRβ) gene.

AIMS

To characterize clinical and molecular features of a Thai patient with RTH. Functional significance of the identified mutation as well as other uncharacterized TRβ mutations was also investigated.

MATERIALS AND METHODS

Exons 3-10 of the TRβ gene were assessed by PCR-sequencing. Functional characterization of the mutant TRβ was determined by the luciferase reporter system.

RESULTS

A mutation in exon 9 of the TRβ gene resulting in a methionine to threonine substitution at codon 313 was identified. The functional consequence of this mutation and other uncharacterized known mutations (p.I276L, p.I280S, p.L330S, p.G344A, p.M442T) was evaluated by transfection studies. Four out of 6 had a significant impairment of T3-dependent transactivation. When co-transfected with the wild-type TRβ, all exhibited a dominant negative effect.

CONCLUSION

A de novo mutation was identified in the patient with clinical diagnosis of RTH. Our findings provide a strong support that interfering with the T3-mediated transcriptional activation of the wild-type TRβ independent of the ability to activate transcription is a major pathogenic mechanism causing RTH.

Complete activation of thyroid hormone receptor β by T3 is essential for normal cochlear function and morphology in mice

BACKGROUND/AIMS

Thyroid hormones (THs) regulate many developmental processes, including the developmental onset of cochlear differentiation and function. TH action is mediated mostly by triiodothyronine (T3) bound to thyroid hormone nuclear receptors (TRs). At positive regulated genes and in the absence of THs, nuclear co-repressors are bound to TRs and decrease basal transcription rate. Ligand (T(3)) binding results in the dissociation of co-repressors and the recruitment of co-activators to the complex, which results in full transcriptional activation.

METHODS

We measured cochlear function in two knock-in mouse models: TRβ(E457A/E457A), with the TRβ co-activator binding surface (AF-2) disrupted to prevent co-activator binding; and TRβ(Δ337T/Δ337T), which is unable to bind T(3). Cochlear morphology and function were analyzed in 10-week-old normal and mutated mice. Cochlear function was determined by measuring auditory brainstem responses, cochlear tuning and compound action potential (CAP) thresholds.

RESULTS

All TRβ(Δ337T/Δ337T) and 85% of the TRβ(E457A/E457A) mice presented elevated CAP thresholds (P < 0.05 or less). Five percent of the TRβ(E457A/E457A) mice presented normal CAP thresholds with broadened cochlear tuning. TRβ(E457A/E457A) and TRβ(Δ337T/Δ337T) presented developmental defects that led to a decreased width (P < 0.01) and an increased thickness (P<0.01) of the tectorial membrane. In addition, TRβ(Δ337T/Δ337T) animals showed an increased tectorial membrane area (P<0.01).

CONCLUSION

Both mutations were deleterious to tectorial membrane development and led to important alterations in cochlear morphology and loss of cochlear function.

Thyromimetics: a journey from bench to bed-side

Selective thyromimetics are synthetic analogs of thyroid hormones with tissue-specific thyroid hormone actions. Tissue selectivity is partly mediated by selectivity for the thyroid hormone receptor-β isoform, but is also enhanced by tissue-selective uptake. Several preclinical animal models and recent human clinical trials have provided sound evidence that thyromimetics can serve as pharmacological tools to improve serum lipids without affecting heart rate. Thyromimetics consistently and efficiently lowered low-density lipoprotein cholesterol and lipoprotein (a) plasma levels without positive chronotropic effects. Most importantly, thyromimetics had a synergistic action when used in addition to 3-hydroxy-3-methylglutaryl CoA reductase inhibitors. Animal data have further suggested that thyromimetics might be useful in the treatment of obesity, hepatic steatosis and atherosclerosis. However, only long-term phase III clinical trials will tell if the observed lipid lowering effects of thyromimetics will improve cardiovascular outcome in humans, too. At the moment, the treatment of dyslipidemia seems to be the major indication for the therapeutic use of thyromimetics, which are now rapidly moving from bench to bed-side.

Approach to the patient with resistance to thyroid hormone and pregnancy

Resistance to thyroid hormone (RTH), a syndrome of reduced end-organ responsiveness to thyroid hormone (TH), is mostly caused by mutations in the TH receptor (TR) beta gene. Diagnosis is based on persistent elevations of serum free T(4) and often T(3) levels in the absence of TSH suppression, and confirmation in most cases is by way of genetic testing. The mainstay in the management of RTH patients who are asymptomatic is to recognize the correct diagnosis and avoid antithyroid treatment. Deciding whether to manage these patients with TH replacement is made even more challenging when an affected individual is pregnant. How one approaches such a patient with pregnancy and RTH would depend on the genotype of the fetus. This requires obtaining prenatal information on the genotype of the fetus and a thorough history of the outcome of previous pregnancies as well as a history of the course and outcome of other family members with RTH. If the TRbeta mutation is known in the mother, the fetus can be rapidly genotyped from DNA from amniocentesis for the same mutation, and then management decisions could be made regarding thyroid or antithyroid hormone treatment.

Multiple genomic aberrations in a patient with mental retardation and hypogonadism: 45,X/46,X,psu dic(Y) karyotype, thyroid hormone receptor beta (THRB) mutation and heterozygosity for Wilson disease

We report on multiple genomic aberrations in a patient with mental retardation. In addition, he had hypogonadism, elevated thyroid hormone levels, hearing loss, delayed speech development and mild dysmorphic features. First, we identified a mosaic karyotype, 45,X/46,X,psu dic(Y). The pseudo-dicentric Y chromosome has three short arm segments. Second, we found a germline mutation (Pro453Thr) of the thyroid hormone receptor beta (THRB) which is associated with resistance to thyroid hormone. Third, he was found to be a carrier of a heterozygous ATP7B mutation (c.2575 + 5G > C), the Wilson disease gene. Even though an array-CGH (with a density of approximately 1 Mb) did not reveal any further genomic gains or losses, we cannot exclude that all contributing factors have been identified. However, this case report shows that with increasing technological possibilities we can find more than one cause for developmental problems in a single patient. The identification of multiple causes in a single patient may complicate explaining the disorder and genetic counseling.

A thyroid hormone receptor mutation that dissociates thyroid hormone regulation of gene expression in vivo

Resistance to thyroid hormone (RTH) is most often due to point mutations in the beta-isoform of the thyroid hormone (TH) receptor (TR-beta). The majority of mutations involve the ligand-binding domain, where they block TH binding and receptor function on both stimulatory and inhibitory TH response elements. In contrast, a few mutations in the ligand-binding domain are reported to maintain TH binding and yet cause RTH in certain tissues. We introduced one such naturally occurring human RTH mutation (R429Q) into the germline of mice at the TR-beta locus. R429Q knock-in (KI) mice demonstrated elevated serum TH and inappropriately normal thyroid-stimulating hormone (TSH) levels, consistent with hypothalamic-pituitary RTH. In contrast, 3 hepatic genes positively regulated by TH (Dio1, Gpd1, and Thrsp) were increased in R429Q KI animals. Mice were then rendered hypothyroid, followed by graded T(3) replacement. Hypothyroid R429Q KI mice displayed elevated TSH subunit mRNA levels, and T(3) treatment failed to normally suppress these levels. T(3) treatment, however, stimulated pituitary Gh levels to a greater degree in R429Q KI than in control mice. Gsta, a hepatic gene negatively regulated by TH, was not suppressed in R429Q KI mice after T(3) treatment, but hepatic Dio1 and Thrsp mRNA levels increased in response to TH. Cardiac myosin heavy chain isoform gene expression also showed a specific defect in TH inhibition. In summary, the R429Q mutation is associated with selective impairment of TH-mediated gene repression, suggesting that the affected domain, necessary for TR homodimerization and corepressor binding, has a critical role in negative gene regulation by TH.

Thyroid hormone receptor beta gene mutation (P453A) in a family producing resistance to thyroid hormone

BACKGROUND

Resistance to thyroid hormone (RTH) is a dominantly inherited syndrome characterized by decreased responsiveness of target tissues to thyroid hormone. Two members of a Turkish family, a mother and son, had thyroid function tests suggestive of resistance to thyroid hormone (RTH).

METHODS

The clinical presentation was, however, different. The mother (proposita) had palpitation, weakness, tiredness, nervousness, dry mouth and was misdiagnosed as having multinodular toxic goiter which was treated with antithyroid drugs and partial thyroidectomy. Her younger son had attention deficit hyperactivity disorder and primary encopresis, but normal intellectual quotient. Both had elevated serum iodothyronine levels with nonsuppressed thyrotropin.

RESULTS

A mutation in one allele of the thyroid hormone receptor beta gene (P453A) was identified, providing a genetic confirmation for the diagnosis of RTH.

CONCLUSION

Mutational analysis of the TRss gene allows definitive diagnosis of RTH, potentially avoiding the need for protracted and expensive pituitary function testing.

Clinical features and genetic analysis of four Brazilian kindreds with resistance to thyroid hormone

OBJECTIVE

Resistance to thyroid hormone (RTH) is a dominantly inherited syndrome of reduced tissue responsiveness to thyroid hormone usually due to mutations located in the ligand-binding domain and adjacent hinge region of the thyroid hormone receptor beta (TRbeta). In the present report we describe the clinical and laboratory characteristics and the genetic analysis of patients with this rare disorder from a Brazilian population.

PATIENTS

Four unrelated Brazilian families with diagnosis of RTH were studied. Age at diagnosis varied from 14 months to 29 years.

RESULTS

All affected individuals were clinically euthyroid, except for one patient who presented immediately after birth with hyperthyroidism. All individuals had tachycardia and goitre, elevated concentrations of free thyroid hormones and reduced sensitivity to thyroid hormone. Direct sequencing analysis of the TRbeta gene revealed four previously reported mutations: c.949G-->A, c.1313G-->A, c.1357C-->A and c.1358dupC in families A, B, C and D, respectively.

CONCLUSION

The present report shows that the frequent mutations described in the thyroid hormone receptor worldwide are also present in the Brazilian population, which is characterized by a variable ethnic background.

Syndromes of reduced sensitivity to thyroid hormone: genetic defects in hormone receptors, cell transporters and deiodination

At least six major steps are required for secreted thyroid hormone (TH) to exert its action on target tissues. Mutations interfering with three of these steps have been so far identified. The first recognized defect, which causes resistance to TH, involves the TH receptor beta gene and has been given the acronym RTH. Occurring in approximately 1 per 40,000 newborns, more than 1000 affected subjects, from 339 families, have been identified. The gene defect remains unknown in 15% of subjects with RTH. Two novel syndromes causing reduced sensitivity to TH were recently identified. One, producing severe psychomotor defects in > 100 males from 26 families, is caused by mutations in the cell-membrane transporter of TH, MCT8; the second, affecting the intracellular metabolism of TH in four individuals from two families, is caused by mutations in the SECISBP2 gene, which is required for the synthesis of selenoproteins, including TH deiodinases.

The clinical relevance of steroid hormone receptor corepressors

Steroid hormone receptors are ligand-dependent transcription factors that control a variety of essential physiologic and developmental processes in humans. The functional activity of a steroid receptor is regulated not only by hormones but also by an array of regulatory proteins such as coactivators, corepressors, and chromatin modifiers. Contrary to an earlier notion that corepressors and coactivators exist in separate complexes, these molecules, which have apparently opposite functions, are increasingly being found in the same complex, which allows for efficient transcriptional control mechanisms. These control mechanisms are in turn regulated by an array of post-translational modifications under the influence of upstream and local signaling networks. Because the outcome of steroidal hormone receptor transcriptional complexes is measured in terms of the expression of target genes, any dysregulation of coregulator complexes perturbs normal homeostasis and could contribute to the development and maintenance of malignant phenotypes. Increasing evidence implicating steroid hormone receptors and their coregulators in various pathophysiologic conditions has elicited interest in their structure and biology. Further advances in this field of study should open up a unique window for novel targeted therapies for diseases such as cancer. Here we briefly review the clinical relevance of corepressors, with a particular focus on their role in the development of cancerous phenotypes.

DNA-based diagnosis of thyroid hormone resistance syndrome: A novel THRB mutation associated with mild resistance to thyroid hormone

BACKGROUND

Thyroid hormones govern a wide range of metabolic processes in the body via thyroid hormone receptors (TR). We report a patient with mild resistance to thyroid hormone who was initially misdiagnosed and treated as having thyrotoxicosis.

METHODS

We used direct DNA sequencing of the THRB gene.

RESULTS

We identified a novel missense mutation, I276L, located in exon 8 of the gene. The mutation is located in cluster 3 of the ligand-binding domain, a protein domain associated with resistance to thyroid hormone.

CONCLUSION

DNA-based diagnosis of thyroid hormone resistance syndrome is simple, reliable, and economical compared to traditional biochemical tests. Once the mutation is identified, targeted screening for the whole family can be performed and the unnecessary use of anti-thyroid drugs or thyroidectomy can be avoided.

Resistance to thyroid hormone associated with autoimmune thyroid disease in a Turkish family

The syndrome of resistance to thyroid hormone (RTH) is characterized by impaired tissue responses to thyroid hormone. Hashimoto's thyroiditis is the most common thyroid autoimmune disease. We present a Turkish family with both RTH and Hashimoto's thyroiditis. RTH was detected through the presence of point mutation in thyroid hormone receptor (TR), and Hashimoto's thyroiditis was diagnosed due to the presence of thyroid autoantibodies. The proposita, her affected mother as well as her unaffected sister have thyroid autoantibodies consistent with Hashimoto's thyroiditis, and a heterozygous point mutation in exon 10 encoding the ligand (3,3',5-L-T3)-binding domain of the TRbeta gene was detected in both the proposita and the mother. The mutation is a replacement of cytosine for guanine in codon 453 (CCT->GCT) producing a missense mutation substituting a normal proline with an alanine (P453A), which reduces the affinity for T3 to 17% of that of the normal TRbeta. Both also have modest elevation of serum TSH levels. In severe RTH, marked elevation of thyroid hormone concentrations in the absence of suppressed TSH supports the laboratory diagnosis of RTH. However, when RTH is mild and associated with thyroiditis, even a modest thyroid gland insufficiency can obliterate the serum T4 and T3 elevations, typical of RTH. This will manifest as elevated serum TSH. Demonstration of TRbeta gene mutation is then necessary to establish the diagnosis. In addition, under these circumstances, treatment with thyroid hormone should be considered.

Resistance to thyroid hormone: report of 2 kindreds with 35 patients

OBJECTIVE

To report two kindreds with resistance to thyroid hormone (RTH), particularly the clinical and laboratory evolution after thyroidectomy and with antithyroid drug treatment.

METHODS

Clinical data and thyroid function were determined in 82 members from three different generations of both families. Unaffected relatives were used as a control group. Response of thyroid-stimulating hormone (TSH) to the hypothalamic thyrotropin-releasing hormone (TRH) was evaluated in five patients and compared with five nonaffected relatives.

RESULTS

Thirty-five (22 male and 13 female) patients had high serum levels of thyroxine and triiodothyronine in conjunction with nonsuppressed TSH. Goiter was present in 66% of affected family members; prevalence was 10% in childhood and 88% in adulthood. The prevalence of goiter was 18% in nonaffected relatives. No patient showed evidence of thyroid hypofunction or hyperfunction, and the TSH response to TRH was similar to that of nonaffected relatives. The incidence of RTH in offspring was 60%, consistent with autosomal dominant transmission. Four patients had undergone thyroidectomy, and two required a second operation because of recurrence of goiter. Patients who had undergone thyroidectomy had normal or near-normal levels of thyroid hormone and very high values of TSH despite thyroid hormone therapy. Five patients were treated with antithyroid drugs for 2 to 10 years, but their thyroid function remained similar to that of nontreated patients.

CONCLUSION

RTH is a relatively rare and benign condition that can be misdiagnosed as hyperthyroidism. The disease is responsible for a high prevalence of goiter in affected families. Thyroidectomy and treatment with antithyroid drugs are not indicated and may have lifelong implications.

Overexpression of estrogen receptor-α in the endometrial carcinoma cell line Ishikawa: inhibition of growth and angiogenic factors

BACKGROUND

A high level of estrogen receptor-alpha (ER-alpha) is believed to be favorable in the prognosis and treatment of endometrial, ovarian, and breast cancer. High levels of ER-alpha have been shown to inhibit the growth and invasive, metastatic potential of breast cancer cell lines. To bring about these inhibitory effects, ER-alpha probably acts through other cellular factors involved in the regulation of cell growth.

OBJECTIVE

To investigate the role of high levels ER-alpha in growth inhibition of endometrial cancer cells.

METHODS

A human ER-alpha cDNA was stably overexpressed in an endometrial cancer cell line, namely, Ishikawa. ER-alpha-overexpressing, parent, and control Ishikawa cells were grown in vitro and their growth rates were compared by cell count. ER-alpha-overexpressing and parent Ishikawa cells were also grown in vitro as tumors in a chicken chorioallantoic membrane (CAM) model, and tumor growth and angiogenesis was measured. Finally, levels of angiogenesis-modulating factors, nitric oxide synthase (NOS), and vascular endothelial growth factor (VEGF) were examined in relation to ER overexpression.

RESULTS

The growth of Ishikawa cells was found inhibited in culture as well as in the CAM model. Angiogenesis of CAM tumors was also found inhibited in ER-overexpressing cells. Angiogenic factor VEGF was inhibited whereas the activity of NOS was found elevated following ER overexpression.

CONCLUSION

Our work on the Ishikawa cell line indicates that high levels of ER-alpha in endometrial cancer may inhibit cancer growth by modulating angiogenic factors, thereby limiting the blood supply to the growing tumor. Our results support the earlier data from other groups that have shown a positive correlation between high ER content and better prognosis of endometrial cancers.

Molecular basis of resistance to thyroid hormone

Resistance to thyroid hormone (RTH) is a syndrome in which patients have raised serum thyroid hormone (TH) levels and raised or inappropriately normal thyrotropin (TSH) levels. In general, patients exhibit TH resistance in the pituitary and peripheral tissues. Novel techniques and genetically engineered mouse model systems have increased our understanding of thyroid hormone receptor (TR) action, and shed new light on the underlying molecular mechanisms for RTH. In particular, we are learning how mutant TRs from RTH patients can block wild-type TR function, with consequent effects in various tissues and cells. This dominant-negative activity has important implications for other hormone-resistant conditions and in hormone-sensitive tumors. This article examines the molecular basis of RTH.

Nuclear receptor superfamily: Principles of signaling

Nuclear receptors (NRs) comprise a family of 49 members that share a common structural organization and act as ligand-inducible transcription factors with major (patho)physiological impact. For some NRs (“orphan receptors”), cognate ligands have not yet been identified or may not exist. The principles of DNA recognition and ligand binding are well understood from both biochemical and crystal structure analyses. The 3D structures of several DNA-binding domains (DBDs),in complexes with a variety of cognate response elements, and multiple ligand-binding domains (LBDs), in the absence (apoLBD)and presence (holoLBD) of agonist, have been established and reveal canonical structural organization. Agonist binding induces a structural transition in the LBD whose most striking feature is the relocation of helix H12, which is required for establishing a coactivator complex, through interaction with members of the p160 family (SRC1, TIF2, AIB1) and/or the TRAP/DRIP complex. The p160-dependent coactivator complex is a multiprotein complex that comprises histone acetyltransferases (HATs), such as CBP,methyltransferases, such as CARM1, and other enzymes (SUMO ligase,etc.). The agonist-dependent recruitment of the HAT complex results in chromatin modification in the environment of the target gene promoters, which is requisite to, or may in some cases be sufficient for, transcription activation. In the absence of ligands, or in the presence of some antagonists, certain NRs are bound to distinct multiprotein complexes through the interaction with corepressors, such as NCoR and SMRT. Corepressor complexes comprise histone deacetylases (HDACs) that have the capacity to condense chromatin over target gene promoters. Ligands have been designed that selectively modulate the interaction between NRs and their coregulators. Both HATs and HDACs can also modify the acetylation status of nonhistone proteins, but the significance in the context of NR signaling is unclear. NRs communicate with other intracellular signaling pathways on a mutual basis, and their functionality may be altered, positively or negatively, by post-translational modification. The majority of NRs act as retinoid X receptor (RXR) heterodimers in which RXR cannot a priori respond autonomously to its cognate ligand to activate target gene transcription. This RXR subordination allows signaling pathway identity for the RXR partner. The corresponding mechanism is understood and reveals cell and NR selectivity, indicating that RXR can, under certain conditions, act autonomously. NRs are regulators of cell life and death,and NR malfunction can be at the basis of both disease and therapy, as is impressively documented in the case of acute promyelocytic leukemia. Recently, several pathways have been uncovered that link NR action with cell proliferation and apoptosis.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

In vitro effect of Triac on resistance to thyroid hormone receptor mutants: potential basis for therapy

Resistance to thyroid hormone (RTH) is a syndrome caused by a mutation in the carboxyl-terminal domain of the thyroid hormone receptor beta (TRbeta) gene. 3,5,3'-triiodothyroacetic acid (Triac) has been used on an empirical basis to treat RTH but its efficacy is still controversial. In previous studies, we demonstrated that Triac has TR isoform- and TRE-specific effects. In this report, we used five natural RTH mutations of the ligand-binding domain in both TRbeta1 and TRbeta2 isoforms for the evaluation of the effect of T3 and Triac on regulation of transcription and binding affinity. We show that Triac has superior activity on negatively and positively regulated promoters and higher binding affinity than T3 for a majority of TRbeta1 and TRbeta2 mutants. However, the difference of transcriptional activity and binding affinity between both ligands is less for RTH mutants than for wild type receptors. These results suggest that Triac could be a potential treatment for RTH patients.

A pituitary tumor in a patient with thyroid hormone resistance: a diagnostic dilemma

Resistance to thyroid hormone (RTH) is due to mutations in the beta-isoform of the thyroid hormone receptor (TR-beta). RTH patients display inappropriate secretion of thyrotropin-releasing hormone (TRH) from the hypothalamus and thyrotropin (TSH) from the anterior pituitary, despite elevated levels of thyroid hormone thyroxine (T4) and triiodothyronine (T3). Thyrotropin-secreting tumors are presumed to represent clonal expansion of abnormal cells. Because the diagnosis of TSH-secreting tumors tends to be delayed and curative surgical resection remains under 50%, early diagnosis is paramount. Current diagnostic strategies suggest that RTH patients are distinguishable from patients with TSH-secreting pituitary tumors by the use of standard laboratory tests and imaging. Here, we present a woman in whom the standard evaluation for inappropriate TSH secretion was insufficient to distinguish these entities. The patient had a low-normal TRH stimulation test and an unmeasurable alpha-glycoprotein subunit level; however, a pituitary magnetic resonance imaging (MRI) revealed an adenoma. More testing using a T3 suppression test supported a RTH diagnosis and a R438H mutation was found in the TR-beta gene. To our knowledge, this represents the first report of an apparently incidental pituitary adenoma in the setting of documented resistance to thyroid hormone. As such, it raises the question of whether RTH predisposes to pituitary hyperplasia and adenoma development.

Syndromes associated with Homo sapiens pol II regulatory genes

The molecular basis of human characteristics is an intriguing but an unresolved problem. Human characteristics cover a broad spectrum, from the obvious to the abstract. Obvious characteristics may include morphological features such as height, shape, and facial form. Abstract characteristics may be hidden in processes that are controlled by hormones and the human brain. In this review we examine exaggerated characteristics presented as syndromes. Specifically, we focus on human genes that encode transcription factors to examine morphological, immunological, and hormonal anomalies that result from deletion, insertion, or mutation of genes that regulate transcription by RNA polymerase II (the Pol II genes). A close analysis of abnormal phenotypes can give clues into how sequence variations in regulatory genes and changes in transcriptional control may give rise to characteristics defined as complex traits.

The thyroid hormone receptor-beta gene mutation R383H is associated with isolated central resistance to thyroid hormone

Resistance to thyroid hormone (RTH) action is due to mutations in the beta-isoform of the thyroid hormone receptor (TR-beta). RTH patients display inappropriate central secretion of TRH from the hypothalamus and of TSH from the anterior pituitary despite elevated levels of thyroid hormone (T4 and T3). RTH mutations cluster in three hot spots in the C-terminal portion of the TR-beta. Most individuals with TR-beta mutations have generalized resistance to thyroid hormone, where most tissues in the body are hyporesponsive to thyroid hormone. The affected individuals are clinically euthyroid or even hypothyroid depending on the severity of the mutation. Whether TR-beta mutations cause a selective form of RTH that only leads to central thyroid hormone resistance is debated. Here, we describe an individual with striking peripheral sensitivity to graded T3 administration. The subject was enrolled in a protocol in which she received three escalating T3 doses over a 13-day period. Indexes of central and peripheral thyroid hormone action were measured at baseline and at each T3 dose. Although the patient's resting pulse rose only 11% in response to T3, her serum ferritin, alanine aminotransferase, aspartate transaminase, and lactate dehydrogenase rose 320%, 117%, 121%, and 30%, respectively. In addition, her serum cholesterol, creatinine phosphokinase, and deep tendon reflex relaxation time fell (25%, 36%, and 36%, respectively). Centrally, the patient was sufficiently resistant to T3 that her serum TSH was not suppressed with 200 microg T3, orally, daily for 4 days. The patient's C-terminal TR exons were sequenced revealing the mutation R383H in a region not otherwise known to harbor TR-beta mutations. Our clinical evaluation presented here represents the most thorough documentation to date of the central thyroid hormone resistance phenotype in an individual with an identified TR-beta mutation.

Stable over-expression of estrogen receptor-alpha in ECV304 cells inhibits proliferation and levels of secreted endothelin-1 and vascular endothelial growth factor

Studies with mammalian vascular cells have suggested growth inhibitory effects of estrogen on the vascular wall. To investigate the involvement of estrogen receptor-alpha (ER) in the control of endothelial cell proliferation, we have stably transfected human estrogen receptor-alpha cDNA into the endothelial cell line ECV304. The clone ECV-ER, thus obtained, over-expresses estrogen receptor to a level approximately 10-fold higher than the parent cell line. Effects of this over-expression were studied on the cell growth rate, and on the levels of secreted endothelin-1 and vascular endothelial growth factor (VEGF). Similar to the previously reported data in other cell types, we found the transfection of ER in ECV304 cells to be inhibitory to their growth. Our ER-over-expressing clone of ECV304 also showed an inhibition of secreted endothelin-1 and VEGF levels. Moreover, the growth inhibition of this ER-over-expressing clone was reversed by the addition of endothelin-1 or VEGF to the medium. In view of the growth-stimulatory effect of endothelin-1 and VEGF on vascular cells, our results indicate that estrogen receptor-alpha may bring about its growth inhibition partly by suppressing endothelin-1 and/or VEGF production in ECV304 cells.

Altered thyroid hormone binding to plasma lipoproteins in the syndrome of resistance to thyroid hormones

Lipoproteins, especially HDL, are carriers of a small fraction of the thyroid hormones in plasma and participate in the intracellular transport of T4. In previous work we showed that a brief period of hypothyroidism alters the hormone distribution among the lipoproteins, causing a decrease in VLDL and LDL binding despite a relative increase in VLDL and LDL cholesterol, an increase in HDL binding, and a reversal of T4 and T3 binding to the smallest HDL size subgroup. The present study of three patients with thyroid hormone resistance and largely compensated hypothyroidism showed thyroid hormone distribution that differed markedly from both normal and hypothyroid subjects. The most striking difference was a much lower binding of both T4 and T3 to HDL and a much higher binding to LDL. If confirmed in a larger group of patients, this might serve as a marker for thyroid hormone resistance.

Defective release of corepressor by hinge mutants of the thyroid hormone receptor found in patients with resistance to thyroid hormone

On positive thyroid hormone response elements (pTREs), thyroid hormone receptor (TR) binding to DNA in the absence of ligand (thyroid hormone, T3) decreases transcription (silencing). Silencing is due to a family of recently described nuclear corepressor proteins (NCoR and SMRT) which bind to the CoR box in the hinge region of TR. Ligand-dependent activation of TR is associated with displacement of corepressors and recruitment of coactivating proteins. Resistance to thyroid hormone (RTH) is due to mutations in the beta isoform of the thyroid hormone receptor (TR-beta). To date, three RTH mutations reportedly with near-normal T3 binding (A234T, R243Q, and R243W) have been described in or near the CoR box. To determine the mechanism of RTH caused by these mutants, the interaction of wild type (wt) and mutant TRs with the corepressor, NCoR, and the coactivator, SRC-1, was tested in gel-shift assays. As expected, NCoR bound wt TR in the absence of T3 and dissociated from TR with increasing T3 concentration. SRC-1 failed to bind wt TR in the absence of T3, but bound to TR with increasing avidity as T3 concentrations rose. At no T3 concentration did both NCoR and SRC-1 bind to wt TR, indicating that their binding to TR was mutually exclusive. Hinge mutants bound NCoR normally in the absence of T3; however, dissociation of NCoR and recruitment of SRC-1 was markedly impaired except at very high T3 concentrations. Importantly, hinge mutant TRs when complexed to DNA bound T3 poorly despite their near-normal T3 binding in solution. These binding studies correlated with functional assays showing defective transactivation of pTREs by hinge mutants except at high T3 concentrations. Thus, we describe a novel mechanism of RTH whereby TR hinge mutants selectively affect T3 binding when complexed to DNA, and prevent NCoR dissociation from TR. Our data also suggest that solution T3 binding by RTH mutants may not accurately reflect physiologically relevant T3 binding by TR when bound to DNA.

The Syndrome of Resistance to Thyroid Hormone, Misdiagnosed and Treated as Thyrotoxicosis

OBJECTIVE

To report two cases of resistance to thyroid hormone and to promote increased awareness of this syndrome, which is frequently misdiagnosed and incorrectly treated.

METHODS

We describe a young woman and her father, both of whom were diagnosed at a younger age as having thyrotoxicosis and were treated with thyroidectomy and radioactive iodine. Both patients later proved to have resistance to thyroid hormone and required supraphysiologic doses of levothyroxine to normalize the thyroid-stimulating hormone (TSH) while remaining euthyroid.

RESULTS

Laboratory evaluation revealed increased serum total thyroxine and triiodothyronine levels as well as normal to increased TSH levels. The free alpha sub-unit/TSH ratio was normal, and magnetic resonance imaging of the pituitary gland showed no tumor. Metabolic studies in the daughter, with use of graded doses of triiodothyronine, supported the diagnosis.

CONCLUSION

Both patients shown to have resistance to thyroid hormone were misdiagnosed and inappropriately treated in the past. The resultant hypothyroidism has been difficult to treat, particularly in the father who has coronary artery disease.

Progesterone receptor variants found in breast cells repress transcription by wild-type receptors

Progesterone, through its nuclear receptors (PR), regulates the development and growth of breast cancers. PR also serve as markers of hormone dependence and prognosis in patients with this disease, and functional PR are required to mediate the antiproliferative effects of progestin therapies. We find that normal and malignant breast cells and tissues can express anomalous forms of PR transcripts. We have isolated four variant PR mRNAs that contain precise deletions of exons encoding sections of the DNA- and hormone-binding domains. The transcripts lack exon 2 (PRdelta2), exon 4 (PRdelta4), exon 6 (PRdelta6), or exons 5 and 6 (PRdelta5,6). On immunoblots, PRdelta4, delta6. and delta5, 6 cloned into the background of the PR A-isoform comigrate with similar proteins present in breast tumor extracts; delta6 and delta5, 6 are dominant-negative transcriptional inhibitors of wild-type A- and B-receptors. We propose that expression of variant PR can compromise the accuracy of receptor measurements as markers of hormone-dependent cancers, and can modify the responses of tumors to progestin therapies.

Nuclear corepressors enhance the dominant negative activity of mutant receptors that cause resistance to thyroid hormone

The syndrome of resistance to thyroid hormone (RTH) is caused by multiple distinct mutations in the ligand-binding domain of the thyroid hormone receptor-beta (TRbeta). Although the mutant receptors are transcriptionally inactive, they inhibit normal receptor function in a dominant negative manner to cause hormone resistance. Recently, a group of transcriptional cofactors, referred to as corepressors (CoRs), was shown to induce ligand-independent silencing of genes that contain positive T3 response elements. CoRs also play a role in the ligand-independent basal activation of genes that are negatively regulated in response to T3. We hypothesized that CoR might play a role in the dominant negative inhibition by TRbeta mutants that cause RTH. In gel mobility shift assays, RTH mutants retained interactions with CoRs even in the presence of T3, whereas the ligand dissociated CoR from wild-type TRbeta. Using Gal4-TR chimeric receptors and a VP16-CoR fusion protein in an interaction assay, a strong positive correlation was found between mutant receptor interactions with CoR and transcriptional silencing activity. A mutation (P214R) that impairs CoR interactions with TR was introduced into the RTH mutants to assess the role of CoR in dominant negative activity. In transient transfection assays, introduction of the P214R CoR mutation decreased RTH mutant silencing of positively regulated genes and basal activation of negatively regulated genes. The dominant negative activity of several different RTH mutants, studied by cotransfection with wild-type receptor, was greatly diminished by the CoR mutation, and this effect was seen with both positively and negatively regulated genes. These results suggest that CoR interactions play a critical role in the dominant negative effect of RTH mutants and support the idea that these proteins are involved in the regulation of genes that are positively as well as negatively regulated by T3.

Resistance to thyroid hormone: implications for neurodevelopmental research on the effects of thyroid hormone disruptors

Thyroid hormones are essential for normal behavioral, intellectual, and neurological development. Congenital hypothyroidism, if not treated, can result in irreversible mental retardation, whereas thyroid diseases with more moderate impairment of thyroid function, such as resistance to thyroid hormone, cause less severe intellectual and behavioral abnormalities, including attention deficit hyperactivity disorder. There is increasing evidence that exposure to certain synthetic compounds, including dioxins and polychlorinated biphenyls (PCBs), during the perinatal period can also impair learning, memory, and attentional processes in offspring. Animal and human studies suggest that exposure to these environmental toxicants impair normal thyroid function. Although the precise mechanisms of action of the adverse effects these toxicants have on neurodevelopment have not yet been elucidated, it is possible that they are partially or predominantly mediated by alterations in hormone binding to the thyroid hormone receptor. The convergence of studies that examine the neurodevelopmental consequences of moderate impairment of thyroid function, such as is found in resistance to thyroid hormone, with those studies that demonstrate the adverse behavioral and cognitive effects of perinatal exposure to dioxins and PCBs serves to generate new hypotheses to test in a research setting. Such studies may provide new insights into the basic pathogenesis of developmental neurotoxicity following exposure to thyroid-disrupting synthetic compounds.

Clinical and hormonal outcome after two years of triiodothyroacetic acid treatment in a child with thyroid hormone resistance

We report here a new family with thyroid hormone resistance (RTH), with phenotypic variability among subjects. Particular emphasis is given to the clinical and hormonal outcome after 2 years of triiodothyroacetic acid (TRIAC) treatment in an affected child with peripheral thyrotoxic features (pituitary RTH [PRTH]). The genetic defect was a substitution in position 1642 (C to A) within the exon 10 of thyroid hormone receptor beta1 (TRbeta1) gene, resulting in the codon change P453T. The mutant receptor had a significantly reduced triiodothyronine (T3) binding affinity. Within this family, the child and the mother suffered from hyperthyroidism and were clinically classified as PRTH, while the maternal grandmother was clinically euthyroid, indicating a generalized form of the disease (GRTH). Rapid normalization of heart rate was initially obtained by the association of the cardioselective beta-blocker atenolol with TRIAC. Nevertheless, long-term TRIAC therapy, through its lowering action of serum thyrotropin (TSH) and thyroid hormone levels, maintained a normal heart rate after atenolol discontinuation and normalized the neurological disturbances and the clinical signs in the child, without any apparent side effect. In fact, growth velocity remained unchanged and no alteration of several parameters of thyroid hormone action at the tissue level was observed, whereas soluble interleukin-2 receptor levels improved significantly, confirming the safety and efficacy of long-term TRIAC therapy for PRTH also during childhood. We thus recommend testing the efficacy of TRIAC therapy in all RTH patients presenting with clinical features of hyperthyroidism.

A mutant thyroid hormone receptor beta 1 identified in a patient with resistance to thyroid hormone inhibits the activities of not only the wild-type TRs, but also other nuclear receptors

Although mutations of human thyroid hormone receptor beta (hTR beta) have been associated with resistance to thyroid hormone (RTH), the molecular basis by which the mutant TRs cause the various clinical symptoms is unknown. We show here that a mutant TR beta [corrected] identified in a patient with RTH inhibited the transcriptional activities of, not only the wild-type TR beta, but also other nuclear receptors including retinoid X receptor alpha (RXR alpha), vitamin D3 receptor (VDR) and retinoic acid receptor (RAR alpha). We provide evidence that these inhibitions by the mutant TR beta [corrected] occur by different mechanisms. Namely, the mutant TR beta interferes with VDR and RAR alpha by competition for binding to the corresponding response elements, but the pathway through RXR alpha is mainly inhibited by squelching of RXR alpha in solution. These findings suggest that in patients with RTH, not only the T3 responsive genes but also other responsive genes are inhibited by the mutant TRs, which might explain the variety of clinical symptoms in RTH.

Thyroid hormone resistance syndrome manifests as an aberrant interaction between mutant T3 receptors and transcriptional corepressors

Nuclear hormone receptors are hormone-regulated transcription factors that play critical roles in chordate development and homeostasis. Aberrant nuclear hormone receptors have been implicated as causal agents in a number of endocrine and neoplastic diseases. The syndrome of Resistance to Thyroid Hormone (RTH) is a human genetic disease characterized by an impaired physiological response to thyroid hormone. RTH is associated with diverse mutations in the thyroid hormone receptor beta-gene. The resulting mutant receptors function as dominant negatives, interfering with the actions of normal thyroid hormone receptors coexpressed in the same cells. We report here that RTH receptors interact aberrantly with a newly recognized family of transcriptional corepressors variously denoted as nuclear receptor corepressor (N-CoR), retinoid X receptor interacting protein-13 (RIP-13), silencing mediator for retinoid and thyroid hormone receptors (SMRT), and thyroid hormone receptor-associating cofactor (TRAC). All RTH receptors tested exhibit an impaired ability to dissociate from corepressors in the presence of thyroid hormone. Two of the RTH mutations uncouple corepressor dissociation from hormone binding; two additional RTH mutants exhibit an unusually strong interaction with corepressor under all hormone conditions tested. Finally, artificial mutants that abolish corepressor binding abrogate the dominant negative activity of RTH mutants. We suggest that an altered corepressor interaction is likely to play a critical role in the dominant negative potency of RTH mutants and may contribute to the variable phenotype in this disorder.

Mutations in the c-erbA beta 1 gene: do they underlie euthyroid fibromyalgia?

Fibromyalgia, a chronic condition of widespread pain, stiffness, and fatigue, has proven unresponsive to drugs, the use of which is based on the 'serotonin-deficiency hypothesis'. An alternative hypothesis-failed transcription regulation by thyroid hormone-can explain the serotonin deficiency and other objective findings and symptoms of euthyroid fibromyalgia. Virtually every feature of fibromyalgia corresponds to signs or symptoms associated with failed transcription regulation by thyroid hormone. In hypothyroid fibromyalgia, failed transcription regulation would result from thyroid-hormone deficiency. In euthyroid fibromyalgia, failed transcription regulation may result from low-affinity thyroid hormone receptors coded by a mutated c-erbA beta 1 gene, yielding partial peripheral resistance to thyroid hormone. The hypothesis of this paper is that, in euthyroid fibromyalgia, a mutant c-erbA beta 1 gene (or alternately, the c-erbA alpha 1 gene) results in low-affinity thyroid-hormone receptors that prevent normal thyroid hormone regulation of transcription. As in hypothyroidism, this would cause a shift toward alpha-adrenergic dominance and increases in both cyclic adenosine 3'-5'-phosphate phosphodiesterase and inhibitory Gi proteins. The result would be tissue-specific hypothyroid-like symptoms despite normal circulating thyroid-hormone levels.

Effect of thyroid hormone on growth. Lessons from the syndrome of resistance to thyroid hormone

Thyroid hormone deprivation results in deleterious effects on bone growth. The delayed bone development is mediated by a direct effect of thyroid hormone on bone and an indirect effect of the hormone on GH release and IGF-1 action. Both TR alpha and TR beta are expressed in bone cells. To examine the role of TR beta on bone, we have reviewed the growth abnormalities in the human syndrome of RTH caused by mutations in the TR beta gene. The mutant TR beta reduces the tissue responsiveness to thyroid hormone, producing in some tissues variable degrees of thyroid hormone deprivation. With regard to bone, relative thyroid hormone deficiency caused by the mutant TR beta produces short stature and delayed bone growth but does not attenuate growth to the extent that absolute thyroid hormone deficiency does. These observations indicate that an intact TR beta is required for normal bone development and growth.