Resistance to thyroid hormone is modulated in vivo by the nuclear receptor corepressor (NCOR1).

Mutations in the ligand-binding domain of the thyroid hormone receptor ß (TRß) lead to resistance to thyroid hormone (RTH). These TRß mutants function in a dominant-negative fashion to interfere with the transcription activity of wild-type thyroid hormone receptors (TRs), leading to dysregulation of the pituitary-thyroid axis and resistance in peripheral tissues. The molecular mechanism by which TRß mutants cause RTH has been postulated to be an inability of the mutants to properly release the nuclear corepressors (NCORs), thereby inhibiting thyroid hormone (TH)-mediated transcription activity. To test this hypothesis in vivo, we crossed Thrb(PV) mice (a model of RTH) expressing a human TRß mutant (PV) with mice expressing a mutant Ncor1 allele (Ncor1(ΔID) mice) that cannot recruit a TR or a PV mutant. Remarkably, in the presence of NCOR1ΔID, the abnormally elevated thyroid-stimulating hormone and TH levels found in Thrb(PV) mice were modestly but significantly corrected. Furthermore, thyroid hyperplasia, weight loss, and other hallmarks of RTH were also partially reverted in mice expressing NCOR1ΔID. Taken together, these data suggest that the aberrant recruitment of NCOR1 by RTH TRß mutants leads to clinical RTH in humans. The present study suggests that therapies aimed at the TR-NCOR1 interaction or its downstream actions could be tested as potential targets in treating RTH.

Pathogenic mechanism of mutations in the thyroid hormone receptor ß 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-ß (TRß) 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 TRß mutations was also investigated. MATERIALS AND METHODS: Exons 3-10 of the TRß gene were assessed by PCR-sequencing. Functional characterization of the mutant TRß was determined by the luciferase reporter system. RESULTS: A mutation in exon 9 of the TRß 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 TRß, 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 TRß independent of the ability to activate transcription is a major pathogenic mechanism causing RTH.

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 RTHß. 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 TRß 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 TRß defect. This focused review aims to provide a concise update on RTHß, 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.

Thyroid hormone resistance index and mortality in euthyroid subjects: Di@bet.es study.

OBJECTIVE: It has been proposed that a mild form of acquired resistance to thyroid hormone may occur in the general population. Its clinical significance remains largely unknown. The objective of the study was to explore whether a newly described thyroid hormone resistance index is associated with the risk of mortality in a sample of community-dwelling euthyroid subjects representative of the adult population of Spain. DESIGN: Longitudinal observational study including 3750 individuals, free of thyroid disease, TPO antibodies-negative (<50 IU/mL) and with TSH levels within the euthyroid range (≥0.5 and ≤5.0 mUI/mL) participating in the nationwide study Di@bet.es (2008-2010). METHODS: We used the Thyroid Feedback Quantile-based Index (TFQI) as a marker of resistance to thyroid hormone. The study population was grouped into categories according to their TFQI values at baseline. Fatal events were ascertained from the national death registry (end of follow-up December 2016). RESULTS: A total of 231 deaths were recorded during an average follow-up of 7.3 years. Compared with the category with the highest sensitivity to free thyroxine (TFQI ≤ p5) (reference), the relative risk of mortality in the categories with TFQI > p5 and ≤p25; >p25 and ≤p50; >p50 and ≤p75; >p75 and ≤p95 and >p95 were 1.01, (0.47-2.19), 1.42 (0.68-2.97), 1.54 (0.74-3.22), 1.47 (0.70-3.11) and 2.61 (1.16-5.89), respectively (P for trend 0.003). The association remained significant after multivariate adjustment of the data (P for trend 0.017). CONCLUSIONS: A thyroid hormone resistance index focused on deviations of the average pituitary response to thyroid hormones may be associated with all-cause mortality independently of other conventional risk factors and comorbidities.

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.

[Clinical characteristics of thyroid hormone resistance syndrome in two cases with different subtypes].

Objective: To analyze the clinical characteristics of children with two types of thyroid hormone resistance (RTH) syndrome, and to detect the variants of thyroid hormone receptor alpha(TRα) and TRß gene in children. Method: Two children with RTH were reported in regard to clinical manifestation, laboratory examination and genetic variants. Some related reports in literature were reviewed. Result: Case 1 was a girl, 10 years old, with thyroid enlargement for several days and without thyrotoxicosis. Laboratory work-up revealed that free thyroxine (FT(4)) was 65.77 pmol/L (reference 12-22) , free triiodothyronine (FT(3)) was 15.36 pmol/L (reference 3.1-6.8) and thyroid stimulating hormone (TSH) level was normal. There was a likely pathogenic missense variant detected in TRß gene and this patient was diagnosed with RTHß. Case 2 was a boy, 3 years old, with classic features of hypothyroidism(growth retardation, developmental retardation, skeletal dysplasia) but had only borderline-abnormal thyroid hormone levels. Targeted sequencing showed a de novo heterozygous nonsense variant in TRα gene which is a pathogenic variant and this patient been diagnosed with RTHα. Conclusion: Thyroid enlargement is a common clinical manifestation of RTHß, with laboratory work-up reveals elevated FT(4) and FT(3) levels but TSH level is normal. The clinical manifestations of RTHα are similar to those of hypothyroidism, but the thyroid hormone levels are almost normal. The gene sequence and the pathogenicity analysis for TRα and TRß will help to make a definitive diagnosis.

Pituitary resistance to thyroid hormone syndrome is associated with T3 receptor mutants that selectively impair beta2 isoform function.

Resistance to thyroid hormone (RTH) syndrome is an inherited inability to respond appropriately to T3 hormone. In generalized RTH, the T3 response of both the pituitary and periphery is disrupted. In pituitary (or central) RTH, the ability of the pituitary to sense (and down-regulate) elevated T3 is selectively impaired, whereas the periphery remains relatively T3 responsive, resulting in peripheral thyrotoxicity. Both forms of disease are linked to mutations in thyroid hormone receptor (TR)-beta. TRbeta is expressed by alternate mRNA splicing as two isoforms: TRbeta2, found primarily in the pituitary/hypothalamus, and TRbeta1, expressed broadly in many tissues. We report here that the wild-type TRbeta2 isoform displays an enhanced T3 response relative to the TRbeta1 isoform. Mutations associated with generalized RTH (P453S, G345S) impair both TRbeta2 and TRbeta1 function proportionally, whereas mutations associated with pituitary-specific RTH (R338L, R338W, R429Q) disproportionately disrupt TRbeta2 function. We propose that in the normal organism, and in generalized RTH, TRbeta2 in the pituitary can sense rising T3 levels in advance of TRbeta1 in the periphery, preventing thyrotoxicity. In contrast, the TRbeta mutations associated with pituitary RTH disproportionately disrupt the pituitary's ability to sense and suppress elevated T3 levels in advance of the periphery, producing symptoms of thyrotoxicity.

Targeted disruption of the type 2 selenodeiodinase gene (DIO2) results in a phenotype of pituitary resistance to T4.

The type 2 deiodinase (D2), a selenoenzyme that catalyzes the conversion of T4 to T3 via 5'-deiodination, is expressed in the pituitary, brain, brown adipose tissue (BAT), and the reproductive tract. To examine the physiological role of this enzyme, a mouse strain lacking D2 activity was developed using homologous recombination. The targeting vector contained the Neo gene in place of a 2.6-kb segment of the Dio2 gene. This segment comprises 72% of the coding region and includes the TGA codon that codes for the selenocysteine located at the active site of the enzyme. Mice homologous for the targeted deletion [D2 knockout (D2KO)] had no gross phenotypic abnormalities, and development and reproductive function appeared normal, except for mild growth retardation (9%) in males. No D2 activity was observed in any tissue in D2KO mice under basal conditions, or under those that normally induce this enzyme such as cold-exposure (BAT) or hypothyroidism (brain, BAT, and pituitary gland). Furthermore, no D2 activity was present in cultured astrocytes, nor could it be induced by treatment of the cells with forskolin. Although D2 mRNA transcripts were detected in BAT RNA obtained from cold-exposed wild-type (WT) mice, none was detected in BAT RNA from comparably-treated D2KO mice. Levels of D1 in the liver, thyroid, and pituitary were the same in WT and D2KO animals, whereas D3 activity in D2KO cerebrum was twice that in WT cerebrum. Serum T3 levels were comparable in adult WT and D2KO mice. However, serum T4 and TSH levels were both elevated significantly (40% and 100%, respectively) in the D2KO mice, suggesting that the pituitary gland of the D2KO mouse is resistant to the feedback effect of plasma T4. This view was substantiated by the finding that serum TSH levels in hypothyroid WT mice were suppressed by administration of either T4 or T3, but only T3 was effective in the D2KO mouse. The data also suggest that the clearance of T4 from plasma was reduced in the D2KO mouse. In summary, targeted inactivation of the selenodeiodinase Dio2 gene results in the complete loss of D2 activity in all tissues examined. The increased serum levels of T4 and TSH observed in D2KO animals demonstrate that the D2 is of critical importance in the feedback regulation of TSH secretion.

Resistance to thyroid hormone due to defective thyroid receptor alpha.

Thyroid hormones act via nuclear receptors (TRα1, TRß1, TRß2) with differing tissue distribution; the role of α2 protein, derived from the same gene locus as TRα1, is unclear. Resistance to thyroid hormone alpha (RTHα) is characterised by tissue-specific hypothyroidism associated with near-normal thyroid function tests. Clinical features include dysmorphic facies, skeletal dysplasia (macrocephaly, epiphyseal dysgenesis), growth retardation, constipation, dyspraxia and intellectual deficit. Biochemical abnormalities include low/low-normal T4 and high/high-normal T3 concentrations, a subnormal T4/T3 ratio, variably reduced reverse T3, raised muscle creatine kinase and mild anaemia. The disorder is mediated by heterozygous, loss-of-function, mutations involving either TRα1 alone or both TRα1 and α2, with no discernible phenotype attributable to defective α2. Whole exome sequencing and diagnostic biomarkers may enable greater ascertainment of RTHα, which is important as thyroxine therapy reverses some metabolic abnormalities and improves growth, constipation, dyspraxia and wellbeing. The genetic and phenotypic heterogeneity of RTHα and its optimal management remain to be elucidated.

TRα receptor mutations extend the spectrum of syndromes of reduced sensitivity to thyroid hormone.

Since 2012, eight different abnormalities have been described in the THRA gene (encoding the TRα1 thyroid hormone receptor) of 14 patients from 9 families. These mutations induce a clinical phenotype (resistance to thyroid hormone type α) associating symptoms of untreated mild congenital hypothyroidism and a near-normal range of free and total thyroid hormones and TSH (the T4/T3 ratio is nevertheless usually low). The phenotype can diversely include short stature (due to growth retardation), dysmorphic syndrome (face and limb extremities), psychoneuromotor disorders, constipation and bradycardia. The identified genetic abnormalities are located within the ligand-binding domain and result in defective T3 binding, an abnormally strong interaction with corepressors and a dominant negative activity against still functional receptors. The identification of patients with consistent phenotypes and the underlying mutations are warranted to better delineate the spectrum of the syndromes of reduced sensitivity to thyroid hormone.

Magnetic resonance imaging of cerebral anomalies in subjects with resistance to thyroid hormone.

OBJECTIVE: Resistance to thyroid hormone (RTH) is an autosomal dominant disease caused by mutations in the human thyroid receptor beta gene on chromosome 3. Individuals with RTH have an increased incidence of attention deficit hyperactivity disorder (ADHD). The purpose of this study was to search for developmental brain malformations associated with RTH. METHOD: Forty-three subjects (20 affected males [AM], 23 affected females [AF]) with resistance to thyroid hormone and 32 unaffected first degree relatives (18 unaffected males [UM], 14 unaffected females [UF]) underwent MRI brain scans with a volumetric acquisition that provided 90 contiguous 2 mm thick sagittal images. Films of six contiguous images beginning at a standard sagittal position lateral to the insula were analyzed by an investigator who was blind with respect to subject characteristics. The presence of extra or missing gyri in the parietal bank of the Sylvian fissure (multimodal association cortex) and multiple Heschl's transverse gyri (primary auditory cortex) were noted. RESULTS: There was a significantly increased frequency of anomalous Sylvian fissures in the left hemisphere in males with RTH (AM: 70%; AF: 30%; UM: 28% UF: 28%). Also, there was an increased frequency of anomalous Sylvian fissures on the left combined with multiple Heschl's gyri in either hemisphere in males with RTH (AM: 50%; AF: 9%; UM: 6%; UF: 0%). However, RTH subjects with anomalies did not have an increased frequency of ADHD as compared with RTH subjects with no anomalies. CONCLUSIONS: Abnormal thyroid hormone action in the male fetus early during brain development may be associated with grossly observable cerebral anomalies of the left hemisphere. The effects of mutations in the thyroid receptor beta gene provide a model system for studying the complex interaction of genetic and nongenetic factors on brain and behavioral development.

Stanniocalcin 1 induction by thyroid hormone depends on thyroid hormone receptor ß and phosphatidylinositol 3-kinase activation.

CONTEXT: Thyroid hormone (TH) mediated changes in gene expression were thought to be primarily initiated by the nuclear TH receptor (TR) binding to a thyroid hormone response element in the promoter of target genes. A recently described extranuclear mechanism of TH action consists of the association of TH-liganded TRß with phosphatidylinositol 3-kinase (PI3K) in the cytosol and subsequent activation of the PI3K pathway. OBJECTIVE: The aim of this study was to examine the effect of TH, TRß and PI3K on stanniocalcin 1 (STC1) expression in human cells. DESIGN: We treated human skin fibroblasts with triiodothyronine (T3) in the absence or presence of the PI3K inhibitor LY294002, a dominant negative PI3K subunit, Δp85α, and the protein synthesis inhibitor cycloheximide (CHX). The role of the TRß was studied in cells from patients with resistance to thyroid hormone (RTH). STC-1 mRNA expression was measured by real-time PCR. RESULTS: We found an induction of STC1 by T3 in normal cells, but less in cells from subjects with RTH (2.7 ± 0.2 vs. 1.6 ± 0.04, P < 0.01). The effect of T3 was completely abrogated by blocking PI3K with LY294002 (3.9 ± 0.5 vs. 0.85 ± 0.5; P < 0.05) and greatly reduced after transfection of a dominant negative PI3K subunit, demonstrating dependency on the PI3K pathway. CONCLUSION: These results establish STC1 as a TH target gene in humans. Furthermore, we show that STC1 induction by TH depends on both TRß and PI3K activation.

A novel mutation of thyroid hormone receptor beta (I431V) impairs corepressor release, and induces thyroid hormone resistance syndrome.

Resistance to thyroid hormone (RTH) is a rare disorder characterized by variable tissue hyporesponsiveness to thyroid hormone, usually caused by mutations in the thyroid hormone receptor beta (TRbeta). We describe a large Brazilian family harboring a novel mutation affecting TRbeta gene and inducing RTH. A 14-year-old girl was found to have elevated free T4 and free T3 plasma concentrations in coexistence with unsuppressed TSH and a questionable goiter. The diagnosis of RTH was verified by identification of a novel mutation (I431V) in the TRbeta gene. Sixteen asymptomatic relatives of the proposita are also affected by the mutation. Functional studies showed that I431V mutation exerts dominant-negative effect on wild type TRbeta, mainly by impairment of ligand-dependent release of corepressor SMRT. The presence of this mutation reduces potency, but does not affect efficacy of thyroid hormone action, in accordance with the clinical picture of eumetabolism of the affected individuals.

A novel mutation (M310L) in the thyroid hormone receptor beta causing resistance to thyroid hormone in a Brazilian kindred and a neonate.

Resistance to thyroid hormone (RTH) is an inherited syndrome of reduced tissue responsiveness to thyroid hormone (T3) caused by mutations in the thyroid hormone receptor beta (TRbeta). The index patient of the family reported here, a 17-year-old woman, came to medical attention because of a diffuse goiter, short stature, and learning disabilities. Biochemical tests revealed an elevated free T4 of 5.2 ng/dl (0.8-2.1), a T3 of 270 ng/dl (80-220), and a nonsuppressed TSH of 1.79 mU/l (0.4-4). Administration of exogenous T4 or T3 did not result in the usual TSH suppression, prompting the clinical diagnosis of RTH. Her father and one of her brothers also had clinical and biochemical findings consistent with RTH. Direct sequence analysis of the TRbeta gene revealed a heterozygous transition 928A>G in exon 9 resulting in substitution of methionine 310 by leucine (M310L). This novel receptor mutant has a reduced affinity for T3 ( approximately 10% of normal) and dominant negative properties that are similar in comparison to other RTH mutations. The index patient had a normal pregnancy and delivery. At birth, the female neonate had no goiter, a significantly elevated T4, and increased TSH. The diagnosis of RTH was confirmed by sequencing the TRbeta gene. She was underweight at birth and her length was between the 5th and 10th percentile. At 26 months, her height remained at the 10th percentile but her bone age was 18 months, suggesting mild hypothyroidism at the level of the bone. In contrast, increased heart rate and restlessness are consistent with hyperthyroidism in other tissues, such as the heart and possibly the brain.

Genetic analyses and evaluation of peripheral parameters of thyroid hormone action for the differential diagnosis of RTH. A novel heterozygous missense mutation (M334T) discovered.

Resistance to thyroid hormone (RTH) is a rare disease characterized by goiter and elevated free thyroid hormone (TH) levels in the presence of detectable concentrations of TSH. Most RTH patients harbor mutations in the ligand binding domain (LBD) of thyroid hormone receptor beta (TRbeta) gene, without a clear correlation between genotype and phenotype. Clinical, biochemical and genetic analyses were performed in several members of one family, because the index case presented with elevated free TH, measurable TSH and no hyperthyroid manifestations, but with a pituitary lesion at MRI. High free TH levels and TSH concentrations in the normal range were found also in 4 relatives. The presence of euthyroidism in all patients together with peripheral parameters of TH action in the normal range led to the diagnosis of generalized RTH (GRTH). In the five affected members, the genetic analysis revealed a novel heterozygous missense mutation at codon 334 (M334T). A different mutation at codon 334 was previously described in association with selective pituitary resistance to thyroid hormone (PRTH). Therefore, we confirm that substitutions at Methionine 334 are critical for the structural integrity of TRbeta LBD. The association of different phenotypes with substitutions affecting the same codon is another contribution confirming that RTH phenotype does not generally depend upon the site of the mutation in the LBD of TRbeta1.

Thyroid hormone resistance and enlargement of the sella turcica during pregnancy.

CASE REPORT: A 26-year-old pregnant woman was admitted to our institution running her 30 weeks of gestation. The patient had a past history of total thyroidectomy cause of a thyroid papillary carcinoma and presented with increased supraphysiological TSH levels under 250 microg T4, while slightly hyperthyroid, from the clinical point of view. Partial resistance to thyroid replacement therapy or TSH-secreting tumour was evoked. Pituitary MRI revealed a pituitary enlargement without excluding a pituitary adenoma. To avoid further stress on pituitary a caesarean section was performed at 38 weeks of gestation. MRI 7 months later was normal, while the patient remained under high doses of T4 replacement therapy and TSH was found at the upper limits of normalcy, while T3, T4 and FTI were above normalcy. CONCLUSION: We conclude that, in the absence of thyroid gland, high TSH levels due to thyroid hormone resistance could be erroneously attributed to a pituitary TSH secreting tumour, when associated with a pregnancy-related pituitary enlargement.

A novel mutation of thyroid hormone receptor beta (I431V) impairs corepressor release, and induces thyroid hormone resistance syndrome / Uma nova mutação no TR beta (I431V) prejudica a liberação de correpressores e induz síndrome de resistência ao hormônio tireoideano

Resistance to thyroid hormone (RTH) is a rare disorder characterized by variable tissue hyporesponsiveness to thyroid hormone, usually caused by mutations in the thyroid hormone receptor beta (TRβ). We describe a large Brazilian family harboring a novel mutation affecting TRβ gene and inducing RTH. A 14-year-old girl was found to have elevated free T4 and free T3 plasma concentrations in coexistence with unsuppressed TSH and a questionable goiter. The diagnosis of RTH was verified by identification of a novel mutation (I431V) in the TRβ gene. Sixteen asymptomatic relatives of the proposita are also affected by the mutation. Functional studies showed that I431V mutation exerts dominant-negative effect on wild type TRβ, mainly by impairment of ligand-dependent release of corepressor SMRT. The presence of this mutation reduces potency, but does not affect efficacy of thyroid hormone action, in accordance with the clinical picture of eumetabolism of the affected individuals.

A resistência ao hormônio tireoideano (RHT) é uma doença rara, causada por variável hiporresponsividade dos tecidos aos hormônios tireoideanos, usualmente causada por mutações no receptor beta do hormônio tireoideano (TRβ). Descrevemos uma grande família brasileira portadora de uma nova mutação afetando o gene do TRβ, induzindo RHT. Uma paciente de 14 anos de idade apresentou concentrações plasmáticas elevadas de T4 e T3 livres, associadas a TSH não-suprimido e bócio questionável. O diagnóstico de RHT foi estabelecido pela identificação da mutação I431V no gene do TRβ. Dezesseis parentes assintomáticos da probanda também são afetados pela mutação. Estudos funcionais mostram que a mutação I431V exerce efeito dominante negativo sobre o TR selvagem, basicamente, por prejudicar a liberação do correpressor SMRT ligante-dependente. A presença desta mutação reduz a potência, mas não afeta a eficácia da ação do hormônio tireoideano, o que está de acordo com a apresentação clínica de eumetabolismo dos indivíduos afetados.