Mutations of CpG dinucleotides located in the triiodothyronine (T3)-binding domain of the thyroid hormone receptor (TR) beta gene that appears to be devoid of natural mutations may not be detected because they are unlikely to produce the clinical phenotype of resistance to thyroid hormone

Clinical Consequences of Variable Results in the Measurement of Free Thyroid Hormones: Unusual Presentation of a Family with a Novel Variant in the THRB Gene Causing Resistance to Thyroid Hormone Syndrome

INTRODUCTION

Resistance to thyroid hormone β (RTHβ) is an inherited syndrome caused by dominant negative variants in the THRB gene (NM_000461.5). The clinical picture of RTHβ is variable, and patients harboring the same variant may display different degrees of disease severity.

CASE PRESENTATION

A 30-year-old man presented with thyrotoxicosis and central hyperthyroidism and was found to have a novel variant in the exon 10 of THRB gene (c.C1282G, p.L428V), located within the third hot spot region of the C-terminal of the receptor. Surprisingly, the same variant was found in two other relatives with an apparent normal thyroid function at initial screening. After exclusion of a TSH-secreting adenoma and serum interference in the proband, and the finding that exogenous levothyroxine failed to suppress the TSH in the brother affected by nodular goiter, relatives' thyroid function tests (TFTs) were reassessed with additional analytical method revealing biochemical features consistent with RTHβ in all carriers of the p.L428V variant. Functional studies showed a slightly impaired in vitro transcriptional activity of p.L428V. Interestingly' the expression of the human p.L428V thyroid hormone receptor beta in the zebrafish embryo background generated a phenotype consistent with RTHβ.

CONCLUSION

Variable results of TFTs on some immunoassays can be a cause of RTHβ diagnostic delay, but the genotype-phenotype correlation in this family and functional studies support p.L428V as a novel THRB variant expanding the spectrum of gene variants causing RTHβ. In vivo, rather than in vitro, functional assays may be required to demonstrate the dominant negative action of THRB variants.

Resistance to Thyroid Hormone Beta: A Focused Review

Resistance to thyroid hormone (RTH) is a clinical syndrome defined by impaired sensitivity to thyroid hormone (TH) and its more common form is caused by mutations in the thyroid hormone receptor beta (THRB) gene, termed RTHβ. The characteristic biochemical profile is that of elevated serum TH levels in absence of thyrotropin suppression. Although most individuals are considered clinically euthyroid, there is variability in phenotypic manifestation among individuals harboring different THRB mutations and among tissue types in the same individual due in part to differential expression of the mutant TRβ protein. As a result, management is tailored to the specific symptoms of TH excess or deprivation encountered in the affected individual as currently there is no available therapy to fully correct the TRβ defect. This focused review aims to provide a concise update on RTHβ, discuss less well recognized associations with other thyroid disorders, such as thyroid dysgenesis and autoimmune thyroid disease, and summarize existing evidence and controversies regarding the phenotypic variability of the syndrome. Review of management addresses goiter, attention deficit disorder and "foggy brain". Lastly, this work covers emerging areas of interest, such as the relevance of variants of unknown significance and novel data on the epigenetic effect resulting from intrauterine exposure to high TH levels and its transgenerational inheritance.

A Novel G385E Variant in the Cold Region of the T3-Binding Domain of Thyroid Hormone Receptor Beta Gene and Investigations to Assess Its Clinical Significance

BACKGROUND

Resistance to thyroid hormone beta (RTHβ) is characterized by elevated thyroid hormone and unsuppressed thyroid-stimulating hormone (TSH), caused by thyroid hormone receptor beta gene (THRB) defects. Most mutations producing RTHβ phenotype are located in CG-rich regions of THRB, encoding the T3-binding and hinge domains of the receptor. However, a region encompassing codons 384-425 is virtually devoid of RTHβ-causing mutations, termed "cold region."

CASE

A 49-year-old woman was diagnosed with Hashimoto thyroiditis in her twenties, and levothyroxine (LT4) was initiated. During LT4 treatment she had slightly elevated free thyroxine and TSH levels, suggesting the possibility of RTHβ.

RESULTS

Sequencing of THRB identified a heterozygous missense variant c.1154G>A producing p.G385E in the proband. Since this variant of unknown significance (VUS) has not been reported in RTHβ individuals and considering its location in the "cold region" of THRB, we questioned its relevance. In silico functional prediction algorithms showed conflicting results: PolyPhen-2 predicted this VUS to be probably damaging with a score of 1.000, while SIFT predicted it to be tolerated with a score of 0.07, thus making additional investigations necessary. Genotyping of family members revealed that the proband's mother and sister, without RTHβ phenotype, also harbored the same variant. This indicates that the THRB G385E variant is unlikely to manifest RTHβ phenotype and confirms its "cold" status.

CONCLUSIONS

This study illustrates that assigning causality of a THRB VUS for RTHβ based only on in silico prediction algorithms is not always fully reliable. Additional phenotype-genotype segregation in family members can assist in predicting functional consequences of missense mutations.

A clinician's guide to understanding resistance to thyroid hormone due to receptor mutations in the TRα and TRβ isoforms

There are two genes that express the major thyroid hormone receptor isoforms. Mutations in both these genes have given rise to Resistance to Thyroid Hormone (RTH) syndromes (RTHβ, RTHα) that can have variable phenotypes for mutations of the same receptor isoform as well as between the two receptor isoforms. In general, the relative tissue-specific distribution of TRβ and TRα determine RTH in different tissues for each form of RTH. These differences highlight some of the isoform-specific roles of each TR isoform. The diagnosis of RTH is challenging for the clinician but should be considered whenever a patient presents with unexplained elevated serum free T₄ (fT₄) and unsuppressed TSH levels, as well as decreased serum free T₄/T₃ ratio. Here we provide a guide for the clinician to diagnose and treat both types of RTH.

Implication of thyroid hormone signaling in neural crest cells migration: Evidence from thyroid hormone receptor beta knockdown and NH3 antagonist studies

Thyroid hormones (TH) have been mainly associated with post-embryonic development and adult homeostasis but few studies report direct experimental evidence for TH function at very early phases of embryogenesis. We assessed the outcome of altered TH signaling on early embryogenesis using the amphibian Xenopus as a model system. Precocious exposure to the TH antagonist NH-3 or impaired thyroid receptor beta function led to severe malformations related to neurocristopathies. These include pathologies with a broad spectrum of organ dysplasias arising from defects in embryonic neural crest cell (NCC) development. We identified a specific temporal window of sensitivity that encompasses the emergence of NCCs. Although the initial steps in NCC ontogenesis appeared unaffected, their migration properties were severely compromised both in vivo and in vitro. Our data describe a role for TH signaling in NCCs migration ability and suggest severe consequences of altered TH signaling during early phases of embryonic development.

Characteristics of patients with late manifestation of resistance thyroid hormone syndrome: a single-center experience

Resistance to thyroid hormone (RTH) is a rare genetic disease caused by reduced tissue sensitivity to thyroid hormone. The hallmark of RTH is elevated serum levels of thyroid hormone with unsuppressed thyrotropin (TSH). However, the most common form of RTH results from minor defects in the ligand-binding domain or hinge domain of the TRβ gene, resulting in impaired T3-induced transcriptional activity, often showing mild presentation. Early diagnosis can be challenging. The objective of the current study was to characterize this specific group of RTH patients. This was a retrospective study. Patients diagnosed as RTH with TRβ mutations were enrolled in a single institute between 2004 and 2014. A total of 14 patients were diagnosed as RTH with mutation in THβ gene. The median age at diagnosis was 22.5 (IQR: 13.25-32.75). Goiter was the most common clinical finding. TSH was significantly elevated after TRH injection (median peak was 21.83 μIU/l, IQR: 13.59-31.48), 9.2-fold compared to the basal level. We found 10 mutations in TRβ gene, all located in the last four exons, and including one novel mutation, H271D. In vitro study found that H271D mutation reduced TR affinity to T3. Four patients with intact thyroid were diagnosed after 16 years old, defined as late manifestation. Compared to those diagnosed before 10 years old, patients with late manifestation presented with normal growth and mental development. Interestingly, three of them carried R438H mutation. We identified a novel p.H271D mutation in TRβ associated with RTH. Endocrinologists should be alert that RTH is frequently found in euthyroid patients with mild symptoms and often leads to misleading diagnosis as well as inappropriate treatment.

Incidental identification of a thyroid hormone receptor beta (THRB) gene variant in a family with autoimmune thyroid disease

BACKGROUND

Resistance to thyroid hormone (RTH) is a rare condition usually diagnosed in patients with classic thyroid function tests (TFTs) of elevated thyroid hormone levels with nonsuppressed TSH. The presence of autoimmune thyroid disease (AITD) can confound the clinical diagnosis of RTH. A family was evaluated because several members had elevated TSH and normal or low serum T4 concentrations with AITD. While these individuals were initially reported to have RTH, they were found to have a normal thyroid hormone receptor beta (THRB) gene sequence, and three other asymptomatic family members were found to harbor the variant TRβ G339S.

METHODS

The THRB gene was sequenced in 19 members of a large Mexican/Aztec family. In vitro expression of the mutant TRβ protein was performed, as well as computer modeling of the variant compared to known mutations in the flanking codons.

RESULTS

Investigation of an individual with AITD who was incorrectly diagnosed with RTH led to the fortuitous discovery of a THRB gene variant (G339S) in the proposita's father, paternal aunt, and cousin. This variant was not detected in analysis of 124 unrelated alleles. All individuals harboring G339S had normal TFTs. Normal in vitro expression and function of G339S and molecular modeling predicted that this variant would not have an effect on the hypothalamic-pituitary-thyroid axis as determined by thyroid hormone binding in vitro and thyroid function tests in vivo, despite profound effects seen in mutations in the adjacent codons 338 and 340.

CONCLUSION

We report an individual with normal TFTs and AITD harboring a novel THRB gene variant. In addition to illustrating the importance of accurate diagnosis of thyroid disease so that proper treatment and counseling can be given, TRβ codon 339 is not essential for normal TRβ function.

The syndromes of reduced sensitivity to thyroid hormone

BACKGROUND

Six known steps are required for the circulating thyroid hormone (TH) to exert its action on target tissues. For three of these steps, human mutations and distinct phenotypes have been identified.

SCOPE OF REVIEW

The clinical, laboratory, genetic and molecular characteristics of these three defects of TH action are the subject of this review. The first defect, recognized 45years ago, produces resistance to TH and carries the acronym, RTH. In the majority of cases it is caused by TH receptor β gene mutations. It has been found in over 3000 individuals belonging to approximately 1000 families. Two relatively novel syndromes presenting reduced sensitivity to TH involve membrane transport and metabolism of TH. One of them, caused by mutations in the TH cell-membrane transporter MCT8, produces severe psychomotor defects. It has been identified in more than 170 males from 90 families. A defect of the intracellular metabolism of TH in 10 individuals from 8 families is caused by mutations in the SECISBP2 gene required for the synthesis of selenoproteins, including TH deiodinases.

MAJOR CONCLUSIONS

Defects at different steps along the pathway leading to TH action at cellular level can manifest as reduced sensitivity to TH.

GENERAL SIGNIFICANCE

Knowledge of the molecular mechanisms involved in TH action allows the recognition of the phenotypes caused by defects of TH action. Once previously known defects have been ruled out, new molecular defects could be sought, thus opening the avenue for novel insights in thyroid physiology. This article is part of a Special Issue entitled Thyroid hormone signaling.

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.

[Clinical and molecular study of five families with resistance to thyroid hormones]

BACKGROUND AND OBJECTIVE

Resistance to thyroid hormone (RTH) is a syndrome mostly caused by mutations in thyroid hormone receptor beta gen (THRB). We present five families with RTH phenotype.

PATIENTS AND METHODS

THRB gene sequencing. In vitro studies to evaluate the mutants response to thyroid hormones and their dominant negative effect. Mechanism of resistance in patients with RTH without THRB mutations quantifying expression of regulator of calcineurin 2 (ZAKI4) and Kruppel-like factor 9 (BTEB) genes in patients fibroblast cultures.

RESULTS

THRB mutations were found in three cases: R243Q, R320C, R429Q. Mutants showed a decreased response to T3. R243Q and R320C had a strong dominant negative effect. One subject without THRB mutation showed changes in ZAKI4 and BTEB expression similar to R320C and the other showed expression levels higher than normal controls.

CONCLUSIONS

Three cases of RTH were caused by THRB heterozygous mutations but in two cases mutations were not found. THRB mutation carriers and one of the patients without mutations share a similar mechanism of resistance and in the other subject RTH is TRβ independent.

Thyroid hormone receptor α and regulation of type 3 deiodinase

Mice deficient in thyroid hormone receptor α (TRα) display hypersensitivity to thyroid hormone (TH), with normal serum TSH but diminished serum T(4). Our aim was to determine whether altered TH metabolism played a role in this hypersensitivity. TRα knockout (KO) mice have lower levels of rT(3), and lower rT(3)/T(4) ratios compared with wild-type (WT) mice. These alterations could be due to increased type 1 deiodinase (D1) or decreased type 3 deiodinase (D3). No differences in D1 mRNA expression and enzymatic activity were found between WT and TRαKO mice. We observed that T(3) treatment increased D3 mRNA in mouse embryonic fibroblasts obtained from WT or TRβKO mice, but not in those from TRαKO mice. T(3) stimulated the promoter activity of 1.5 kb 5'-flanking region of the human (h) DIO3 promoter in GH3 cells after cotransfection with hTRα but not with hTRβ. Moreover, treatment of GH3 cells with T(3) increased D3 mRNA after overexpression of TRα. The region necessary for the T(3)-TRα stimulation of the hD3 promoter (region -1200 to -1369) was identified by transfection studies in Neuro2A cells that stably overexpress either TRα or TRβ. These results indicate that TRα mediates the up-regulation of D3 by TH in vitro. TRαKO mice display impairment in the regulation of D3 by TH in both brain and pituitary and have reduced clearance rate of TH as a consequence of D3 deregulation. We conclude that the absence of TRα results in decreased clearance of TH by D3 and contributes to the TH hypersensitivity.

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.

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.

Mosaicism of a thyroid hormone receptor-beta gene mutation in resistance to thyroid hormone

CONTEXT

Heterozygous mutations in thyroid hormone receptor-beta (TRbeta) gene are the cause of resistance to thyroid hormone (RTH) in more than 85% of families having the syndrome. In 23% of the families, TRbeta gene mutations occur de novo. Of the 141 families with RTH investigated by us, 21 (15%) had no TRbeta gene mutations detectable by sequencing from genomic DNA (gDNA) or cDNA (non-TR RTH).

OBJECTIVE

The objective of the study was to investigate the genotype of a family with RTH and correlate it to the phenotype.

DESIGN

The DNA was isolated from different tissues, and the sequence of the TRbeta gene was determined. Clinical studies involved the administration of incremental doses of T(3).

SETTING

The study was conducted at a referral pediatric endocrinology clinic in Turkey and an academic medical center in the United States.

MAIN OUTCOME AND MEASURES

Measurement included markers of thyroid hormone action and sequencing of TRbeta revealing a R338W mutation. Patients and Family: We studied two siblings with short stature, panic disorder, psychosis, and high free iodothyronine concentrations with nonsuppressed TSH and their father with similar thyroid function tests without growth or psychiatric abnormalities.

RESULTS

Direct sequencing of gDNA obtained from the father's leukocytes, buccal mucosa cells, and prostate tissue showed less amplification of the mutant allele (R338W) than the normal allele as confirmed by PCR/restriction fragment length polymorphism analysis. No sequence abnormalities were detected in gDNA from fibroblasts. Similar results were found in mRNA from the leukocytes and fibroblasts. The sensitivity of various tissues to thyroid hormone was not uniform. The progeny had equal amounts of mutant and wild-type gDNA in leukocytes and skin.

CONCLUSIONS

The father has a mosaicism for the R338W mutation as it was present in some cell lineages, including his germline, because it was transferred to his children but not in fibroblasts. This indicates that the mutation occurred de novo in early embryonic life. Here is the first report of mosaicism in RTH. The possibility of mosaicism should be considered in subjects with RTH without apparent mutations in the TRbeta gene.

Unliganded thyroid hormone receptor beta1 inhibits proliferation of murine fibroblasts by delaying the onset of the G1 cell-cycle signals

Thyroid hormone receptors (TRs) are members of the ligand-inducible transcription factor superfamily. The two major functional TRs (alpha1 and beta1) have different spatial and temporal expression patterns and specific physiological functions for these isoforms are now starting to emerge. By expressing these TR isoforms individually in Swiss 3T3 fibroblasts, we found that TRbeta1 expression, in the absence of hormone, provokes a proliferation arrest in G0/G1, lengthening the cycling time. Upon serum stimulation TRbeta1-expressing cells showed a marked delay in the induction of cyclins D and E, in the phosphorylation of retinoblastoma protein, and in the activation of cyclin-dependent kinase 2, accompanied by increased levels of cyclin-dependent kinase inhibitor p27Kip1. Accordingly, serum-stimulated E2F-1 transcriptional activity was repressed by TRbeta1 in transient transfection experiments. Analysis of the receptor domains required for this effect confirmed that there is no need for a functional ligand-binding domain while the DNA-binding domain is essential. In this work, we demonstrate for the first time that TRbeta1 participates in the molecular mechanisms that control cell proliferation. The unliganded TRbeta1 impairs the normal induction of the G1/S cycle regulators preventing progression into the S phase.

RXR receptor agonist suppression of thyroid function: central effects in the absence of thyroid hormone receptor

High-affinity agonists for the retinoic acid X receptors (RXR) have pleotropic effects when administered to humans. These include induction of hypertriglyceridemia and hypothyroidism. We determined the effect of a novel high-affinity RXR agonist with potent antihyperglycemic effects on thyroid function of female Zucker diabetic rats and nondiabetic littermates and in db/db mice. In both nondiabetic and ZFF rats, AGN194204 causes a 70-80% decrease in thyrotropin (TSH), 3,3',5-triiodothyronine, and thyroxine (T(4)) concentrations. In the db/db mouse, AGN194204 causes a time-dependent decrease in thyroid hormone levels with the fall in TSH that was significant after 1 day of treatment preceding the fall in T(4) levels that was significant at 3 days of treatment. Treatment with AGN194204 caused an initial increase in hepatic 5'-deiodinase mRNA levels which then fell to undetectable levels by 3 days of treatment and continued to be low at 7 days of treatment. After treatment for 5 days with AGN194204, both wild-type and thyroid hormone receptor beta (TR beta(-/-))-deficient mice demonstrated a nearly 50% decrease in serum TSH and T(4) concentrations. The results suggest that a high-affinity RXR agonist with antihyperglycemic activity can cause central hypothyroidism independently of TR beta, the main mediator of hormone-induced TSH suppression.

Impaired interaction of mutant thyroid hormone receptors associated with human hepatocellular carcinoma with transcriptional coregulators

Thyroid hormone (T(3)) exerts its many biological activities through interaction with specific nuclear receptors (TRs) that function as ligand-dependent transcription factors at genes that contain a thyroid hormone response element (TRE). Mutant TRs have been detected in human hepatocellular carcinoma cell lines and tissue, but their contribution to carcinogenesis has remained unclear. The interaction of four such mutant TRs (J7-TRalpha1, J7-TRbeta1, H-TRalpha1, and L-TRalpha1) with transcriptional coregulators has now been investigated. With the exception of J7-TRalpha1, which in the absence of T(3) exhibited transcriptional silencing activity with a TRE-reporter gene construct in transfected cells, the mutant TRs had little effect (compared with that of wild-type receptors) on transcriptional activity of the reporter gene in the absence or presence of T(3), of the transcriptional corepressors SMRT, NCoR or of the transcriptional coactivator SRC. Electrophoretic mobility-shift assays revealed that, in the presence of T(3), the J7-TRss1 mutant did not interact with SRC, whereas J7-TRalpha1 and H-TRalpha1 exhibited reduced abilities to associate with this coactivator and L-TRalpha1 showed an ability to interact with SRC similar to that of wild-type TRalpha1. The dominant negative activity of the mutant TRs in transfected cells appeared inversely related to the ability of the receptors to interact with SRC. Whereas J7-TRss1, H-TRalpha1, and L-TRalpha1 did not interact with SMRT, and NCoR. J7-TRalpha1 bind to corepressors but failed to dissociate from them in the presence of T(3). These aberrant interactions between the mutant TRs and transcriptional coregulators may contribute to the highly variable clinical characteristics of human hepatocellular carcinoma.

A potential role of activated NF-kappa B in the pathogenesis of euthyroid sick syndrome

Euthyroid sick syndrome, characterized by low serum 3,5, 3'-triiodothyronine (T(3)) with normal L-thyroxine levels, is associated with a wide variety of disorders including sepsis, malignancy, and AIDS. The degree of low T(3) in circulation has been shown to correlate with the severity of the underlying disorders and with the prognosis. Elevated TNF-alpha levels, which accompany severe illness, are associated with decreased activity of type I 5'-deiodinase (5'-DI) in liver, leading us to speculate that high levels of this factor contribute to euthyroid sick syndrome. Here we demonstrate that the activation of NF-kappa B by TNF-alpha interferes with thyroid-hormone action as demonstrated by impairment of T(3)-dependent induction of 5'-DI gene expression in HepG2 cells. Inhibition of NF-kappa B action by a dominant-negative NF-kappa B reversed this effect and allowed T(3) induction of 5'-DI. Furthermore, we show that an inhibitor of NF-kappa B activation, clarithromycin (CAM), can inhibit TNF-alpha-induced activation of NF-kappa B and restore T(3)-dependent induction of 5'-DI mRNA and enzyme activity. These results suggest that NF-kappa B activation by TNF-alpha is involved in the pathogenesis of euthyroid sick syndrome and that CAM could help prevent a decrease in serum T(3) levels and thus ameliorate euthyroid sick syndrome.

A novel point mutation in cluster 3 of the thyroid hormone receptor beta gene (P247L) causing mild resistance to thyroid hormone

Resistance to thyroid hormone (RTH), a syndrome characterized by variable tissue hyposensitivity to thyroid hormone (TH), is linked to mutations in the thyroid hormone receptor (TR) beta gene. We report a new family with a heretofore unreported mutation, P247L. The proposita, a 31-year-old female, presented with goiter and palpitations. RTH was suspected because of elevated serum free thyroxine (FT4) level with a normal thyrotropin (TSH). Sequencing the TRbeta gene revealed a mutation causing replacement of a proline at position 247 with leucine. Seven family members were heterozygous for the mutation, two of whom also had evidence of autoimmune thyroid disease. The mutant TRbeta had a Ka for triiodothyronine (T3) 30% that of the wild-type TRbeta, approximately a threefold reduction in T3-induced transactivation and a low level dominant negative activity when tested with a positively regulated reporter gene. In vivo sensitivity to TH was evaluated in three affected subjects by measurement of the responses to graded doses of levotriiodothyronine (LT3). Peak TSH responses to TRH were reduced and were not completely suppressed at even the highest dose of LT3, (0.9, 0.2, and 0.2, compared to < 0.01 microU/mL in unaffected controls), confirming pituitary resistance to TH in all three subjects. In contrast, peripheral tissues responded variably to LT3: serum cholesterol decreased in all by 15%-25%, serum creatine kinase decreased by 15% in two subjects and increased 35% in another, but serum ferritin and sex hormone-binding globulin increased in only one of the three affected individuals that were tested. Basal metabolic rate and sleeping pulse did not change in three and two individuals, respectively. Hyporesponsiveness to exogenous TH established the clinical diagnosis of RTH in one member of the family with a mutant TRbeta but normal tests of thyroid function at baseline. Three affected subjects had an axis I diagnosis of major depression but had Wechsler Intelligence Scale for Children, III (WISC-III) full-scale IQs (FSIQs) in the normal range. This novel TRbeta mutation is associated with a realtively mild RTH. Results of responses to LT3 underscore the variable phenotype of RTH.

Resistance to thyroid hormone caused by a new mutation (V336M) in the thyroid hormone receptor beta gene

Resistance to thyroid hormone (RTH), usually caused by an inherited defect of the thyroid hormone receptor, (TRbeta), results in a reduced responsiveness of target tissues to thyroid hormone. Until now, more than 600 cases with RTH have been identified. Although usually linked to the TRbeta gene, located on chromosome 3, RTH may also occur in the absence of mutations in the coding region of this gene. We report a 10-month-old boy who had laboratory findings typical of RTH. He was born prematurely on the 34th week of gestation and his thyrotropin (TSH) during neonatal screening was 121 microU/mL, a value very high for RTH or prematurity. Direct sequencing of the TRbeta gene from the patient's genomic DNA revealed a heterozygous substitution of the normal valine with a mutant methionine in codon 336 (V336M) that has not been previously reported. In vitro expression studies showed that this mutant TRbeta has an impaired triiodothyronine (T3)-dependent transactivation that reduces the activity of the wild-type TRbeta (dominant negative effect). While the functional impairment of V336M is not unusual compared to other TRbeta gene mutations, the very high TSH value in this prematurely born infant suggests that fetuses with RTH have altered maturation of the hypothalamo-pituitary-thyroid axis or actually may suffer from hypothyroidism.

ROR alpha augments thyroid hormone receptor-mediated transcriptional activation

This study is designed to clarify the role of an orphan nuclear hormone receptor, ROR alpha, on thyroid hormone (TH) receptor (TR)-mediated transcription on a TH-response element (TRE). A transient transfection study using various TREs [i.e., F2 (chick lysozyme TRE), DR4 (direct repeat), and palindrome TRE] and TR and ROR alpha1 was performed. When ROR alpha1 and TR were cotransfected into CV1 cells, ROR alpha1 enhanced the transactivation by liganded-TR on all TREs tested without an effect on basal repression by unliganded TR. By electrophoretic mobility shift assay, on the other hand, although ROR alpha bound to all TREs tested as a monomer, no (or weak) TR and ROR alpha1 heterodimer formation was observed on various TREs except when a putative ROR-response element was present. The transactivation by ROR alpha1 on a ROR-response element, which does not contain a TRE, was not enhanced by TR. The effect of ROR alpha1 on the TREs is unique, because, whereas other nuclear hormone receptors (such as vitamin D receptor) may competitively bind to TRE to exert dominant negative function, ROR alpha1 augmented TR action. These results indicate that ROR alpha1 may modify the effect of liganded TR on TH-responsive genes. Because TR and ROR alpha are coexpressed in cerebellar Purkinje cells, and perinatal hypothyroid animals and ROR alpha-disrupted animals show similar abnormalities of this cell type, cross-talk between these two receptors may play a critical role in Purkinje cell differentiation.

A role for helix 3 of the TRbeta ligand-binding domain in coactivator recruitment identified by characterization of a third cluster of mutations in resistance to thyroid hormone

Resistance to thyroid hormone (RTH) has hitherto been associated with thyroid hormone beta receptor (TRbeta) mutations which cluster in two regions (alphaalpha 310-353 and alphaalpha 429-461) of the hormone-binding domain and closely approximate the ligand-binding cavity. Here, we describe a third cluster of RTH mutations extending from alphaalpha 234-282 which constitute a third boundary of the ligand pocket. One mutant, T277A, exhibits impaired transactivation which is disproportionate to its mildly reduced ligand affinity (Ka). T3-dependent recruitment of coactivators (SRC-1, ACTR) by mutant receptor-RXR heterodimers was reduced in comparison with wild-type. Cotransfection of SRC-1 restored transactivation by T277A. In the TRbeta crystal structure this helix 3 residue is surface-exposed and is in close proximity to residues L454 and E457 in helix 12 which are known to be critical for coactivator interaction, suggesting that they all constitute part of a receptor-coactivator interface. The transcriptional function of other mutants (A234T, R243W/Q, A268D, Delta276I, A279V, R282S) in this cluster correlated with their reduced Ka and they inhibited wild-type TRbeta action in a dominant negative manner. DNA binding, heterodimerization and corepressor recruitment were preserved in all mutants, signifying the importance of these attributes for dominant negative activity and correlating with the absence of natural mutations in regions bordering the third cluster which mediate these functions.

Selective pituitary resistance to thyroid hormone produced by expression of a mutant thyroid hormone receptor beta gene in the pituitary gland of transgenic mice

Resistance to thyroid hormone (RTH) has been subdivided into generalized resistance (GRTH) and pituitary resistance (PRTH) based on the clinical impression of absence or presence of thyrotoxicosis. However, due to lack of objective clinical and genetic criteria, the existence of PRTH as a distinct entity became controversial. To determine what the phenotype would be if RTH was confined to the pituitary, a transgenic mouse was developed in which expression of the mutant thyroid hormone receptor (TR) beta (G345R) was targeted to the pituitary thyrotrophs by placing it downstream of the mouse thyrotropin beta promoter. This construct exhibited an antagonistic effect on the thyroid hormone-dependent transactivation, mediated through the wild-type TRbeta1, only when cotransfected with the thyrotroph embryonic factor in a heterologous cell line. As expected the transgene was transcribed predominantly in the pituitary gland but not in liver. These mice showed a significant, though modest, increase in serum T4 concentration. A decrease in the serum cholesterol was observed in keeping with the selective tissue hyposensitivity to thyroid hormone.

Congenital hypothyroidism due to mutations in the sodium/iodide symporter. Identification of a nonsense mutation producing a downstream cryptic 3' splice site

A 12-yr-old hypothyroid girl was diagnosed at birth as athyreotic because her thyroid gland could not be visualized by isotope scanning. Goiter development due to incomplete thyrotropin suppression, a thyroidal radioiodide uptake of < 1%, and a low saliva to plasma ratio of 2.5 suggested iodide (I-) transport defect. mRNA isolated from her thyroid gland and injected into Xenopus oocytes failed to increase I- transport. Sequencing of the entire Na+/I- symporter (NIS) cDNA revealed a C to G transversion of nucleotide (nt) 1146 in exon 6, resulting in a Gln 267 (CAG) to Glu (GAG) substitution. This missense mutation produces an NIS with undetectable I- transport activity when expressed in COS-7 cells. Although only this missense mutation was identified in thyroid and lymphocyte cDNA, genotyping revealed that the proposita and her unaffected brother and father were heterozygous for this mutation. However, amplification of cDNA with a primer specific for the wild-type nt 1146 yielded a sequence lacking 67 nt. Genomic DNA showed a C to G transversion of nt 1940, producing a stop codon as well as a new downstream cryptic 3' splice acceptor site in exon 13, responsible for the 67 nt deletion, frameshift, and premature stop predicting an NIS lacking 129 carboxy-terminal amino acids. This mutation was inherited from the mother and present in the unaffected sister. Thus, although the proposita is a compound heterozygote, because of the very low expression (< 2.5%) of one mutant allele, she is functionally hemizygous for an NIS without detectable bioactivity.

Mutation in the thyroid hormone receptor beta gene (A317T) in a Thai subject with resistance to thyroid hormone

Analysis of the thyroid hormone receptor beta (TRbeta) gene of a Thai female with the syndrome of resistance to thyroid hormone (RTH) revealed a missense mutation at codon 317, changing the guanine in nucleotide 1234 to an adenine that results in the replacement of the normal alanine (GCT) with a threonine (ACT). The proposita was heterozygous, and this mutation was not present in her parents and her sister, compatible with a neomutation. This is the first report of TRbeta gene mutation causing RTH in an individual of Thai origin.

Autosomal dominant postaxial polydactyly, nail dystrophy, and dental abnormalities map to chromosome 4p16, in the region containing the Ellis-van Creveld syndrome locus

We have studied a four-generation family with features of Weyers acrofacial dysostosis, in which the proband has a more severe phenotype, resembling Ellis-van Creveld syndrome. Weyers acrofacial dysostosis is an autosomal dominant condition with dental anomalies, nail dystrophy, postaxial polydactyly, and mild short stature. Ellis-van Creveld syndrome is a similar condition, with autosomal recessive inheritance and the additional features of disproportionate dwarfism, thoracic dysplasia, and congenital heart disease. Linkage and haplotype analysis determined that the disease locus in this pedigree resides on chromosome 4p16, distal to the genetic marker D4S3007 and within a 17-cM region flanking the genetic locus D4S2366. This region includes the Ellis-van Creveld syndrome locus, which previously was reported to map within a 3-cM region between genetic markers D4S2957 and D4S827. Either the genes for the condition in our family and for Ellis-van Creveld syndrome are near one another or these two conditions are allelic with mutations in the same gene. These data also raise the possibility that Weyers acrofacial dysostosis is the heterozygous expression of a mutation that, in homozygous form, causes the autosomal recessive disorder Ellis-van Creveld syndrome.

Hypothyroidism in a Brazilian kindred due to iodide trapping defect caused by a homozygous mutation in the sodium/iodide symporter gene

A 36 year old man with a large goiter was suspected of having iodide (I-) transport defect based on low thyroidal and salivary gland radioiodide uptake. Thyroid surgery was performed, because thyroid cytology could not exclude a malignancy. Sequencing of the entire Na+/I- symporter (NIS) cDNA derived from thyroidal mRNA revealed a homozygous substitution of the normal cytosine in nucleotide (nt) 1163 with an adenine, resulting in a stop (TGA) at codon 272. This nonsense mutation produces a truncated NIS with undetectable I- transport activity when expressed into COS-7 cells. Genotyping confirmed that the propositus was homozygous for the mutation whereas his unaffected mother, son, and paternal aunt were heterozygous. This nt substitution was not detected in any of 50 normal individuals, ruling out a polymorphism. While the homozygous mutant NIS-272X causes congenital hypothyroidism, expression of one normal allele in the heterozygote (C272X) is sufficient to maintain active thyroidal I- uptake and function.

A splicing variant of Steroid Receptor Coactivator-1 (SRC-1E): the major isoform of SRC-1 to mediate thyroid hormone action

Steroid Receptor Coactivator-1 (SRC-1) interacts with nuclear receptors only when they are bound to the ligands and enhance the transactivation. We identified splicing variants encoding three isoforms, SRC-1, SRC-1(-Q), and SRC-1E, generated by alternative usage of an exon(s) and splicing acceptor sites. RT-PCR analysis showed that SRC-1E was more abundantly expressed than SRC-1 or SRC-1(-Q) at the mRNA level in all the cell lines tested. SRC-1E lacks 56 amino acids of SRC-1 and has unique 14 amino acids at the carboxyl terminus, while SRC-1(-Q) differs from SRC-1 by deletion of only one glutamine residue. Since the C-terminal domain of SRC-1 has been shown to be involved in the interaction with nuclear receptors, the enhancement of transactivation by these three isoforms was tested. SRC-1E enhanced thyroid hormone dependent transactivation of reporter gene expression more profoundly than SRC-1 or SRC-1(-Q). Taken together, it was suggested that SRC-1E is the major isoform of SRC-1 to mediate thyroid hormone action.

Modification of thyroid hormone and 9-cis retinoic acid signaling by overexpression of their cognate receptors using adenoviral vector

Tissue responsiveness to a hormone is dependent on the amounts of its receptor expressed under physiological conditions. In the present report, we compared the magnitude of ligand-dependent transactivation mediated by two nuclear hormone receptors, thyroid hormone receptor beta (TR) and retinoid X receptor alpha (RXR), when overexpressed in a variety of cell lines. TR, RXR and reporter (luciferase) genes under the control of artificial hormone response elements were introduced into the cells using recombinant adenovirus (Ad) vectors, to ensure highly efficient gene delivery. Although the amounts of TR expressed were similar in the cell lines infected with Ad-TR, T3 dependent induction of reporter gene expression was significantly greater in HepG2 than in Cos7, GH3, or JEG3 cells, indicating that factors other than TR are limiting the responsiveness to T3. The enhanced response to 9-cis retinoic acid in cells overexpressing RXR was much greater in JEG3 than in HepG2 which had the highest responsiveness to T3 under TR overexpression. These results indicate that the factors affecting T3 responsiveness are not identical to those affecting the 9-cis retinoic acid responsiveness. On the other hand, overexpression of RXR in addition to TR resulted in a decrease in T3-responsiveness in all the cell lines tested, suggesting that some cofactors are common to TR and RXR.

Resistance to thyroid hormone in a family caused by a new point mutation L330S in the thyroid receptor (TR) beta gene

Resistance to thyroid hormone (RTH) is an inherited defect manifesting as variable tissue hyporesponsiveness to thyroid hormone, usually caused by mutations in the thyroid hormone receptor beta (TR beta) gene. Up to now 78 mutations in this gene have been identified, mostly clustered in two regions located in exon 9 and 10. We describe a new point mutation replacing the normal thymidine-1274 with a cytosine that results in the substitution of the normal leucine-330 with a serine (L330S) in the receptor protein. This mutation was identified in an 11-year-old boy who presented with symptoms and signs suggestive of both hyperthyroidism and hypothyroidism. Interestingly a mutation in the same codon (L330F) has been previously described in a patient who presented with stigmata suggestive of thyrotoxicosis.

A natural transactivation mutation in the thyroid hormone beta receptor: impaired interaction with putative transcriptional mediators

The syndrome of resistance to thyroid hormone is characterized by elevated serum free thyroid hormones, failure to suppress pituitary thyrotropin secretion, and variable peripheral refractoriness to hormone action. Here we describe a novel leucine to valine mutation in codon 454 (L454V) of the thyroid hormone beta receptor (TR beta) in this disorder, resulting in a mutant receptor with unusual functional properties. Although the mutant protein binds ligand comparably to wild-type receptor and forms homo- and heterodimers on direct repeat, everted repeat, or palindromic thyroid response elements, its ability to activate transcription via these elements is markedly impaired. The hydrophobic leucine residue lies within an amphipathic alpha-helix at the carboxyl terminus of TR beta and the position of the homologous residue in the crystal structure of TR alpha indicates that its side chain is solvent-exposed and might interact with other proteins. We find that two putative transcriptional mediators (RIP140 and SRC-1) exhibit hormone-dependent association with wild-type TR. In comparison, the interaction of this natural mutant (L454V) and artificial mutants (L454A, E457A) with RIP140 and SRC-1 is markedly reduced. Furthermore, coexpression of SRC-1 is able to restore the transcriptional activity of the L454V mutant receptor, indicating that the interaction of this residue with accessory proteins is critical for transcriptional activation. Finally, the occurrence of the L454V mutation in resistance to thyroid hormone, together with impaired negative regulation of the thyroid-stimulating hormone alpha promoter by this mutant, suggests that the amphipathic alpha-helix also mediates hormone-dependent transcriptional inhibition, perhaps via interaction with these or other accessory factors.

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.

New point mutation (R243W) in the hormone binding domain of the c-erbA beta 1 gene in a family with generalized resistance to thyroid hormone

Two years after the first mutation on exon 7 in the carboxy-terminal part of the hinge domain (D) was reported (Behr and Loos 1992), we have identified the second mutation on exon 7 in patients with GRTH. Interestingly, our mutation it is not located in the two previously described "hot spot regions", but instead very close to the hinge domain (D) of the receptor protein that is essential for the function of the hormone binding domain (E) (Lin et al., 1991). Confirming the observation that the majority of single base substitutions causing human genetic diseases or DNA polymorphisms follow the hot spot mutation rule of CG to TG and CG to CA transition (Barker et al., 1984), an additional CpG dinucleotide transition has been identified.

Dominant negative and DNA-binding properties of mutant thyroid hormone receptors that are defective in homodimerization but not heterodimerization

Thyroid hormone receptors (TRs) bind to thyroid hormone response elements (TREs) as monomers, homodimers, and heterodimers. Mutations that cause resistance to thyroid hormone (RTH) have proven useful for identifying important functional domains in the receptor. Previous studies have shown that RTH mutants must retain the ability to form heterodimers with RXR to exert dominant negative inhibition of wild-type receptor function. In this report, we examined in detail the dimerization properties, function, and dominant negative activity of RTH mutations at R316H and R338W--two mutations that have a propensity to cause the pituitary form of RTH. These mutants show selective loss of homodimerization, with preservation of heterodimerization with RXR alpha. The selective loss of homodimerization was independent of the orientation of the half sites in the TRE. The R316H mutant was transcriptionally inactive in transient expression assays, consistent with its markedly reduced T3 binding. In contrast, R338W was activated at nanomolar concentrations of T3, precluding quantitative analyses of its dominant negative properties. In cotransfection assays with wild-type TR beta, the R316H mutant functioned in a dominant negative manner to block positively (TRE-pal; DR4) and negatively (TSH alpha) regulated reporter genes, although its inhibitory potential was reduced compared with other RTH mutants. Introduction of the R316H mutation into a receptor containing a potent RTH mutant (G345R) reduced its dominant negative activity to the level of the R316H mutant alone. These results suggest that mutations that alter homodimerization have reduced dominant negative activity for some target genes, a feature that may account, in part, for phenotypic variability in RTH.