Resistance to thyroid hormone caused by a mutation in thyroid hormone receptor (TR)α1 and TRα2: clinical, biochemical, and genetic analyses of three related patients

Anaemia-based screening for resistance to thyroid hormone alpha in children

BACKGROUND

The hypothyroid phenotype associated with resistance to thyroid hormone alpha (RTH-α) is associated with a diverse clinical picture. On the other hand, thyroid-stimulating hormone (TSH) levels are normal. Free triiodothyronine (fT3) and free thyroxine (fT4) levels can also be normal; however, normo- or macrocytic anaemia is usually present in reported cases. Diagnosis is challenging and there is limited data regarding screening methods.

OBJECTIVE

The study aimed to assess the efficiency of a screening strategy for RTH-α.

SUBJECTS AND METHODS

Out of a total of 6540 children evaluated at the outpatient clinics of paediatric neurology over 2 years and who underwent complete blood count and thyroid function tests, 432 were found to have anaemia. Within this group, we identified 42 children without an underlying specific neurological aetiology who exhibited normo- or macrocytic anaemia, normal TSH levels, fT3 levels in the upper half of the normal range or high, and fT4 levels in the lower half of the normal range or low. We excluded one patient who had already been diagnosed with RTH-α and nine patients could not be reached. Subsequently, clinical evaluation, biochemical assessment, and THRA sequencing analysis were conducted on 32 children. The findings were compared with those of the known RTH-α patients in our unit.

RESULTS

The median age of the patients was 5.7 (5.1-7.4) years, and 22 of them were males (69%). The main reasons for assessment in paediatric neurology clinics were autism spectrum disorder (n = 12, 38%), epilepsy (n = 11, 34%), and delay in developmental stages (n = 8, 25%). Constipation was present in five of the cases (16%), while the closure of the anterior fontanelle and tooth eruption were delayed in two cases (6%) and one case (3%), respectively. The median length/height and weight standard deviation (SD) scores were 0.3 [(-0.8)-(1.1)] and -0.1 [(-0.8)-(0.3)], respectively. The median fT3, fT4, and TSH levels were 4.6 (4.2-5.0) pg/mL, 0.9 (0.8-1.0) ng/dL, and 2.2 (1.8-3.1) uIU/mL, respectively. Thirteen of the patients (41%) had high fT3 levels, while none of them had low fT4 levels. The normo- or macrocytic anaemia rate was 47% (normocytic/macrocytic, n = 8/7) at the time of reassessment. Serum creatine kinase (CK) was elevated in five patients (16%; one had anaemia). None of the subjects had a pathological variant in THRA. Known RTH-α patients had significantly lower median height SD score, higher rates of delayed tooth eruption and closure of the anterior fontanelle, lower haemoglobin levels, and higher mean corpuscular volume (MCV) and CK levels as compared to those found without RTH-α.

CONCLUSIONS

This approach found one known patient with RTH-α but did not reveal any new cases. Notably, normo- or macrocytic anaemia did not persist in nearly half of the screened patients. A screening strategy that takes clinical findings and prominent laboratory features suggestive of RTH-α into account could lower unnecessary genetic analysis of THRA in patients presenting with neurological problems.

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

Thyroid Hormone Regulates the Lipid Content of Muscle Fibers, Thus Affecting Physical Exercise Performance

Skeletal muscle (SkM) lipid composition plays an essential role in physiological muscle maintenance and exercise performance. Thyroid hormones (THs) regulate muscle formation and fuel energy utilization by modulating carbohydrates and lipid and protein metabolism. The best-known effects of THs in SkM include the promotion of mitochondrial biogenesis, the fiber-type switch from oxidative to glycolytic fibers, and enhanced angiogenesis. To assess the role of THs on the lipidic composition of SkM fibers, we performed lipidomic analyses of SkM cells and tissues, glucose tolerance experiments, and exercise performance tests. Our data demonstrated that TH treatment induces remodeling of the lipid profile and changes the proportion of fatty acids in SkM. In brief, THs significantly reduced the ratio of stearic/oleic acid in the muscle similar to what is induced by physical activity. The increased proportion of unsaturated fatty acids was linked to an improvement in insulin sensitivity and endurance exercise. These findings point to THs as critical endocrine factors affecting exercise performance and indicate that homeostatic maintenance of TH signals, by improving cell permeability and receptor stability at the cell membrane, is crucial for muscle physiology.

Clinical and Biochemical Characteristics of Untreated Adult Patients With Resistance to Thyroid Hormone Alpha

Background

Thyroid hormone resistance due to pathogenic variants in thyroid hormone receptor alpha (THRA) is rare and descriptions of patients are sparse. The disorder is probably underdiagnosed as patients may have normal thyroid function tests. Treatment with thyroxine in childhood improves clinical symptoms. However, it is not clear if treatment has beneficial effects if started in adulthood.

Cases

We investigated 4 previously untreated Caucasian adult first-degree-related patients with the THRA c.788C > T, p.(Ala263Val) variant identified by a gene panel for intellectual disability in the index patient. Clinical data and previous investigations were obtained from medical reports.

Results

During childhood and adolescence, short stature, short limbs, metacarpals, and phalanges, and delayed bone age maturation were observed. Delayed motor and language development and decreased intellectual and learning abilities were described. Abdominal adiposity, round face, and increased head circumference were common features. All individuals complained of tiredness, constipation, and low mood. While thyrotropin (TSH) and free thyroxine (FT4) were within the reference range, free triiodothyronine (FT3) was high. FT4/FT3 ratio and reverse T3 were low. Other main features were low hemoglobin and high LDL/HDL ratio.

Conclusion

Investigation of 4 first-degree-related adult patients with untreated resistance to thyroid hormone alpha (RTHα) revealed more pronounced phenotype features and hypothyroid symptoms than previously described in patients treated with levothyroxine from childhood or adolescence. The delay in diagnosis is probably due to normal thyroid function tests. We suggest that THRA analysis should be performed in patients with specific clinical features, as treatment in early childhood may improve outcomes.

Centile reference chart for resting metabolic rate through the life course

OBJECTIVE

Reference centile charts are widely used for the assessment of growth and have progressed from describing height and weight to include body composition variables such as fat and lean mass. Here, we present centile charts for an index of resting energy expenditure (REE) or metabolic rate, adjusted for lean mass versus age, including both children and adults across the life course.

DESIGN, PARTICIPANTS AND INTERVENTION

Measurements of REE by indirect calorimetry and body composition using dual-energy X-ray absorptiometry were made in 411 healthy children and adults (age range 6-64 years) and serially in a patient with resistance to thyroid hormone α (RTHα) between age 15 and 21 years during thyroxine therapy.

SETTING

NIHR Cambridge Clinical Research Facility, UK.

RESULTS

The centile chart indicates substantial variability, with the REE index ranging between 0.41 and 0.59 units at age 6 years, and 0.28 and 0.40 units at age 25 years (2nd and 98th centile, respectively). The 50th centile of the index ranged from 0.49 units (age 6 years) to 0.34 units (age 25 years). Over 6 years, the REE index of the patient with RTHα varied from 0.35 units (25th centile) to 0.28 units (<2nd centile), depending on changes in lean mass and adherence to treatment.

CONCLUSION

We have developed a reference centile chart for an index of resting metabolic rate in childhood and adults, and shown its clinical utility in assessing response to therapy of an endocrine disorder during a patient's transition from childhood to adult.

Resistance to thyroid hormone induced tachycardia in RTHα syndrome

Mutations in thyroid hormone receptor α1 (TRα1) cause Resistance to Thyroid Hormone α (RTHα), a disorder characterized by hypothyroidism in TRα1-expressing tissues including the heart. Surprisingly, we report that treatment of RTHα patients with thyroxine to overcome tissue hormone resistance does not elevate their heart rate. Cardiac telemetry in male, TRα1 mutant, mice indicates that such persistent bradycardia is caused by an intrinsic cardiac defect and not due to altered autonomic control. Transcriptomic analyses show preserved, thyroid hormone (T3)-dependent upregulation of pacemaker channels (Hcn2, Hcn4), but irreversibly reduced expression of several ion channel genes controlling heart rate. Exposure of TRα1 mutant male mice to higher maternal T3 concentrations in utero, restores altered expression and DNA methylation of ion channels, including Ryr2. Our findings indicate that target genes other than Hcn2 and Hcn4 mediate T3-induced tachycardia and suggest that treatment of RTHα patients with thyroxine in high dosage without concomitant tachycardia, is possible.

The importance of thyroid hormone signaling during early development: Lessons from the zebrafish model

The zebrafish is an optimal experimental model to study thyroid hormone (TH) involvement in vertebrate development. The use of state-of-the-art zebrafish genetic tools available for the study of the effect of gene silencing, cell fate decisions and cell lineage differentiation have contributed to a more insightful comprehension of molecular, cellular, and tissue-specific TH actions. In contrast to intrauterine development, extrauterine embryogenesis observed in zebrafish has facilitated a more detailed study of the development of the hypothalamic-pituitary-thyroid axis. This model has also enabled a more insightful analysis of TH molecular actions upon the organization and function of the brain, the retina, the heart, and the immune system. Consequently, zebrafish has become a trendy model to address paradigms of TH-related functional and biomedical importance. We here compilate the available knowledge regarding zebrafish developmental events for which specific components of TH signaling are essential.

Long-term follow-up results and treatment outcomes of children and adults with resistance to thyroid hormone alpha

PURPOSE

Resistance to thyroid hormone alpha (RTHα) is a rare entity and has no specific treatment. To date, mostly levothyroxine has been used, but there is a lack of knowledge about the long-term outcomes of this treatment. We aimed to evaluate the long-term follow-up results and treatment outcomes of children and their parents diagnosed with RTHα.

METHODS

Four children [the median (minimum-maximum) age at diagnosis, 4.5 (1.4-9.5) years] and three adults [age at diagnosis, 31.7 (28.0-35.3) years] from two families were included in the study, who had RTHα and followed up between 2014 and 2021.

RESULTS

The median duration of treatment was 6.7 (5.9-8.0) years, and the levothyroxine dose at the final visit was 1.4 (1.2-2.2) and 1.9 (1.2-2.4) mcg/kg/day for adults and pediatric patients, respectively. Treatment ameliorated constipation in all patients with this complaint (n = 5). Normal mental functions were achieved and IQ scores improved in most children except one (age at diagnosis, 9.5 years). At the final visit, creatine kinase levels relative to the reference upper limit were significantly lower compared to the pre-treatment ratios [0.9 (0.2-1.3) vs. 1.3 (0.5-1.6), p = 0.028]. Anemia was present in five patients at diagnosis, which resolved in one adult patient but occurred in one child despite treatment (p = 0.999). A minimal pericardial effusion persisted in one pediatric patient.

CONCLUSIONS

We demonstrated that constipation was ameliorated, neuromotor development of some children was improved, and creatine kinase levels were diminished with levothyroxine treatment in patients with RTHα, while some features including anemia did not resolve.

Mediator subunit MED1 differentially modulates mutant thyroid hormone receptor intracellular dynamics in Resistance to Thyroid Hormone syndrome

Thyroid hormone receptor (TR) controls the expression of thyroid hormone (T3)-responsive genes, while undergoing rapid nucleocytoplasmic shuttling. In Resistance to Thyroid Hormone syndrome (RTH), mutant TR fails to activate T3-dependent transcription. Previously, we showed that Mediator subunit 1 (MED1) plays a role in TR nuclear retention. Here, we investigated MED1's effect on RTH mutants using nucleocytoplasmic scoring and fluorescence recovery after photobleaching in transfected cells. MED1 overexpression and knockout did not change the nucleocytoplasmic distribution or intranuclear mobility of C392X and P398R TRα1 at physiological T3 levels. At elevated T3 levels, however, overexpression increased P398R's nuclear retention and MED1 knockout decreased P398R's and A263V's intranuclear mobility, while not impacting C392X. Although A263V TRα1-transfected cells had a high percentage of aggregates, MED1 rescued A263V's impaired intranuclear mobility, suggesting that MED1 ameliorates nonfunctional aggregates. Results correlate with clinical severity, suggesting that altered interaction between MED1 and TRα1 mutants contributes to RTH pathology.

Impact of Technology Driven Mental Health Task-shifting for Accredited Social Health Activists (ASHAs): Results from a Randomised Controlled Trial of Two Methods of Training

Mental health task shifting is a potential way to address the burgeoning treatment gap for mental illness. Easily available and accessible digital technology can be utilised to continuously engage grassroot level health workers (for example, Accredited Social Health Activists (ASHAs). However, the impact of such a strategy is not yet systematically evaluated. In this randomised controlled trial, longitudinal hybrid training of ASHAs [1 day in-person classroom training and seven online sessions (ECHO model), aimed to screen and refer to commonly prevalent mental health issues in communities] was compared with traditional one-day in-person classroom training. ASHAs (n = 75) from six Primary Health Centres in Ramanagara district, Karnataka, India were randomized into study (SG-ASHAs) and control (CG-ASHAs) groups. After excluding drop-outs, 26 ASHAs in each group were included in the final analysis of the scores on their Knowledge, attitude, and practices (KAP) in mental health. Two house-to-house surveys were conducted by both groups to identify and refer possible cases. The number of screen positives (potential persons with mental illnesses) and the KAP scores formed the outcome measures. Online sessions for SG-ASHAs were completed over 18 months, the COVID-19 pandemic being the main disruptor. SG-ASHAs identified significantly higher number of persons with potential alcohol use disorders [n = 873 (83%); p ≤ 0.001] and common mental disorders [n = 96(4%); p = 0.018], while CG-ASHAs identified significantly higher number of those with potential severe mental disorders [n = 61(61.61%); p ≤ 0.001]. As regards KAP, after controlling for baseline scores, the time effect in RMANOVA favoured SG-ASHAs. Mean total KAP score increased from 16.76 to18.57 (p < 0·01) in SG-ASHAs and from 18.65 to 18.84 (p = 0.76) in CG-ASHAs. However, the Time-group interaction effect did not favour either (F = 0.105; p = 0.748). Compared to traditional training, mentoring ASHAs for extended periods is more impactful. Easily accessible digital technology makes the latter feasible. Scaling up such initiatives carry the potential to considerably improve treatment access for those in need.

Genetic disorders of thyroid development, hormone biosynthesis and signalling

Development and differentiation of the thyroid gland is directed by expression of specific transcription factors in the thyroid follicular cell which mediates hormone biosynthesis. Membrane transporters are rate-limiting for cellular entry of thyroid hormones (TH) (T4 and T3) into some tissues, with selenocysteine-containing, deiodinase enzymes (DIO1 and DIO2) converting T4 to the biologically active hormone T3. TH regulate expression of target genes via hormone-inducible nuclear receptors (TRα and TRβ) to exert their physiological effects. Primary congenital hypothyroidism (CH) due to thyroid dysgenesis may be mediated by defects in thyroid transcription factors or impaired thyroid stimulating hormone receptor function. Dyshormonogenic CH is usually due to mutations in genes mediating thyroidal iodide transport, organification or iodotyrosine synthesis and recycling. Disorders of TH signalling encompass conditions due to defects in membrane TH transporters, impaired hormone metabolism due to deficiency of deiodinases and syndromes of Resistance to thyroid hormone due to pathogenic variants in either TRα or TRβ. Here, we review the genetic basis, pathogenesis and clinical features of congenital, dysgenetic or dyshormonogenic hypothyroidism and disorders of TH transport, metabolism and action.

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.

A disease-associated mutation in thyroid hormone receptor α1 causes hearing loss and sensory hair cell patterning defects in mice

Resistance to thyroid hormone due to mutations in THRA, which encodes the thyroid hormone receptor α (TRα1), shows variable clinical presentation. Mutations affecting TRβ1 and TRβ2 cause deafness in mice and have been associated with deafness in humans. To test whether TRα1 also affects hearing function, we used mice heterozygous for a frameshift mutation in Thra that is similar to human THRA mutations (Thra^S1/+ mice) and reduces tissue sensitivity to thyroid hormone. Compared to wild-type littermates, Thra^S1/+ mice showed moderate high-frequency sensorineural hearing loss as juveniles and increased age-related hearing loss. Ultrastructural examination revealed aberrant orientation of ~20% of sensory outer hair cells (OHCs), as well as increased numbers of mitochondria with fragmented morphology and autophagic vacuoles in both OHCs and auditory nerve fibers. Molecular dissection of the OHC lateral wall components revealed that the potassium ion channel Kcnq4 was aberrantly targeted to the cytoplasm of mutant OHCs. In addition, mutant cochleae showed increased oxidative stress, autophagy, and mitophagy associated with greater age-related cochlear cell damage, demonstrating that TRα1 is required for proper development of OHCs and for maintenance of OHC function. These findings suggest that patients with THRA mutations may present underdiagnosed, mild hearing loss and may be more susceptible to age-related hearing loss.

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.

Thyroid Hormone Receptor Isoforms Alpha and Beta Play Convergent Roles in Muscle Physiology and Metabolic Regulation

Skeletal muscle is a key energy-regulating organ, skilled in rapidly boosting the rate of energy production and substrate consumption following increased workload demand. The alteration of skeletal muscle metabolism is directly associated with numerous pathologies and disorders. Thyroid hormones (THs) and their receptors (TRs, namely, TRα and TRβ) exert pleiotropic functions in almost all cells and tissues. Skeletal muscle is a major THs-target tissue and alterations of THs levels have multiple influences on the latter. However, the biological role of THs and TRs in orchestrating metabolic pathways in skeletal muscle has only recently started to be addressed. The purpose of this paper is to investigate the muscle metabolic response to TRs abrogation, by using two different mouse models of global TRα- and TRβKO. In line with the clinical features of resistance to THs syndromes in humans, characterized by THRs gene mutations, both animal models of TRs deficiency exhibit developmental delay and mitochondrial dysfunctions. Moreover, using transcriptomic and metabolomic approaches, we found that the TRs–THs complex regulates the Fatty Acids (FAs)-binding protein GOT2, affecting FAs oxidation and transport in skeletal muscle. In conclusion, these results underline a new metabolic role of THs in governing muscle lipids distribution and metabolism.

Genetics of Thyroid Disorders

Thyroid diseases in children and adolescents include acquired or congenital conditions, including genetic disorders either isolated or part of a syndrome. Briefly, we will review the physiology and pathophysiology of the thyroid gland and its disorders. The aim of this chapter is to describe genetic abnormalities of the thyroid gland.

Thyroid Hormone Resistance: Multicentrical Case Series Study

Resistance to thyroid hormone syndrome (RTHS) is defined as increased thyroxine and triiodothyronine associated with normal or increased thyrotropin. This is usually due to a pathogenic variant of the gene coding for thyroid hormone receptor B (THRB). THRB is a rare genetic disorder characterized by an altered response of target tissue to the thyroid hormone action. Retrospective cross-sectional observational study with diagnosis of RTHS evaluated in secondary and tertiary hospitals for 6 years, from 2014 to 2020, in order to describe variables including age, sex, anthropometric data, clinical and biochemical characteristics of patients, who were divided according to age, in a pediatric group from 0 to 14 years (index cases), and an adult group composed of adult relatives of index cases. A molecular analysis of the THRB gene was performed. The total retrospective cohort included 7 pediatric patients and 15 adults. We found 22 cases with a clear male predominance (14/22). Mean age is 24.8 years old (22 days-70 years). Patients were referred because of symptoms 18.2% (4/22), analysis results 22.7% (5/22), or familial study 59.1% (13/22). About 31.8% (7/22) cases show goiter, 31.8% (7/22) sympathetic symptoms and 13.6% (3/22) abnormalities in behavior. In most cases, 77.3%, (17/22) show familial background of thyroid abnormalities. It is important to remark that 18.2% (4/22) relatives received previous incorrect treatments such as thyroidectomy, because of wrong diagnosis. In conclusion, a better understanding of RTHS, its prompt molecular diagnosis and genetic counseling, could avoid unnecessary tests and inappropriate treatments.

Germ Line Mutations in the Thyroid Hormone Receptor Alpha Gene Predispose to Cutaneous Tags and Melanocytic Nevi

Background: Many physiological effects of thyroid hormone (TH) are mediated by its canonical action via nuclear receptors (TH receptor α and β [TRα and TRβ]) to regulate transcription of target genes. Heterozygous dominant negative mutations in human TRα mediate resistance to thyroid hormone alpha (RTHα), characterized by features of hypothyroidism (e.g., skeletal dysplasia, neurodevelopmental retardation, constipation) in specific tissues, but near-normal circulating TH concentrations. Hitherto, 41 RTHα cases have been recorded worldwide. Methods: RTHα cases (n = 10) attending a single center underwent cutaneous assessment, recording skin lesions. Lesions excised from different RTHα patients were analyzed histologically and profiled for cellular markers of proliferation and oncogenic potential. Proliferative characteristics of dermal fibroblasts and inducible pluripotent stem cell (iPSC)-derived keratinocytes from patients and control subjects were analyzed. Results: Multiple skin tags and nevi were recorded in all cases, mainly in the head and neck area with a predilection for flexures. The affected patients had highly deleterious mutations (p.E403X, p.E403K, p.F397fs406X, p.A382PfsX7) involving TRα1 alone or mild/moderate loss-of-function mutations (p.A263V, p.L274P) common to TRα1 and TRα2 isoforms. In four patients, although lesions excised for cosmetic reasons were benign intradermal melanocytic nevi histologically, they significantly overexpressed markers of cell proliferation (K17, cyclin D1) and type 3 deiodinase. In addition, oncogenic markers typical of basal cell carcinoma (Gli-1, Gli-2, Ptch-1, n = 2 cases) and melanoma (c-kit, MAGE, CDK4, n = 1) were markedly upregulated in skin lesions. Cell cycle progression and proliferation of TRα mutation-containing dermal fibroblasts and iPSC-derived keratinocytes from patients were markedly increased. Conclusions: Our observations highlight frequent occurrence of skin tags and benign melanocytic nevi in RTHα, with cutaneous cells from patients being in a hyperproliferative state. Such excess of skin lesions, including nevi expressing oncogenic markers, indicates that dermatologic surveillance of RTHα patients, monitoring lesions for features that are suspicious for neoplastic change, is warranted.

The Role of Thyroid Hormone in Neuronal Protection

Thyroid hormone is essential for brain development and brain function in the adult. During development, thyroid hormone acts in a spatial and temporal-specific manner to regulate the expression of genes essential for normal neural cell differentiation, migration, and myelination. In the adult brain, thyroid hormone is important for maintaining normal brain function. Thyroid hormone excess, hyperthyroidism, and thyroid hormone deficiency, hypothyroidism, are associated with disordered brain function, including depression, memory loss, impaired cognitive function, irritability, and anxiety. Adequate thyroid hormone levels are required for normal brain function. Thyroid hormone acts through a cascade of signaling components: activation and inactivation by deiodinase enzymes, thyroid hormone membrane transporters, and nuclear thyroid hormone receptors. Additionally, the hypothalamic-pituitary-thyroid axis, with negative feedback of thyroid hormone on thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) secretion, regulates serum thyroid hormone levels in a narrow range. Animal and human studies have shown both systemic and local reduction in thyroid hormone availability in neurologic disease and after brain trauma. Treatment with thyroid hormone and selective thyroid hormone analogs has resulted in a reduction in injury and improved recovery. This article will describe the thyroid hormone signal transduction pathway in the brain and the role of thyroid hormone in the aging brain, neurologic diseases, and the protective role when administered after traumatic brain injury. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

An overview of thyroid function tests in subjects with resistance to thyroid hormone and related disorders

Confirmation of sustained syndrome of inappropriate secretion of thyrotropin (SITSH) is a milestone in diagnosis of β type of resistance to thyroid hormone (RTHβ). The differential diagnoses of RTHβ include TSH-producing pituitary adenoma (TSHoma) and familial dysalbuminemic hyperthyroxinemia (FDH), which also present SITSH. Recently, patients with RTHα caused by a mutation in thyroid hormone receptor α were reported and they did not present SITSH but a decline in the serum T4/T3 ratio. This review was aimed to overview thyroid function tests in RTH and related disorders. First, the characteristics of the thyroid function in RTHβ, TSHoma, and FDH obtained from a Japanese database are summarized. Second, the degrees of SITSH in patients with truncations and frameshifts were compared with those in patients with single amino acid deletions and single amino acid substitutions obtained from the literature. Third, the degrees of SITSH in homozygous patients were compared with those in heterozygous patients with cognate mutations. Finally, the FT3/FT4 ratios in RTHα are summarized. In principle, the TSH values in FDH were within the normal range and apparent FT4 values in FDH were much higher than in RTHβ and TSHoma. The FT3/FT4 values in RTHβ were significantly lower than in TSHoma. The degrees of SITSH in patients with truncations and frameshifts were more severe than those in patients with single amino acid deletions and single amino acid substitutions, and those in homozygous patients were more severe than those in heterozygous patients with cognate mutations. The FT3/FT4 ratios in RTHα were higher than 1.0.

A New Mechanism in THRA Resistance: The First Disease-Associated Variant Leading to an Increased Inhibitory Function of THRA2

The nuclear thyroid hormone receptors (THRs) are key mediators of thyroid hormone function on the cellular level via modulation of gene expression. Two different genes encode THRs (THRA and THRB), and are pleiotropically involved in development, metabolism, and growth. The THRA1 and THRA2 isoforms, which result from alternative splicing of THRA, differ in their C-terminal ligand-binding domain (LBD). Most published disease-associated THRA variants are located in the LBD of THRA1 and impede triiodothyronine (T3) binding. This keeps the nuclear receptor in an inactive state and inhibits target gene expression. Here, we investigated a new dominant THRA variant (chr17:g.38,241,010A > G, GRCh37.13 | c.518A > G, NM_199334 | p.(E173G), NP_955366), which is located between the DNA- and ligand-binding domains and affects both splicing isoforms. Patients presented partially with hypothyroid (intellectual disability, motor developmental delay, brain atrophy, and constipation) and partially with hyperthyroid symptoms (tachycardia and behavioral abnormalities) to varying degrees. Functional characterization of THRA1p.(E173G) by reporter gene assays revealed increased transcriptional activity in contrast to THRA1(WT), unexpectedly revealing the first gain-of-function mutation found in THRA1. The THRA2 isoform does not bind T3 and antagonizes THRA1 action. Introduction of p.(E173G) into THRA2 increased its inhibitory effect on THRA1, which helps to explain the hypothyroid symptoms seen in our patients. We used protein structure models to investigate possible underlying pathomechanisms of this variant with a gain-of-antagonistic function and suggest that the p.(E173G) variant may have an influence on the dimerization domain of the nuclear receptor.

Generation and Characterization of a New Resistance to Thyroid Hormone Mouse Model with Thyroid Hormone Receptor Alpha Gene Mutation

Background: In humans, resistance to thyroid hormone (RTH) caused by mutations in the thyroid hormone receptor alpha (THRA) gene, RTHα, manifests as tissue-specific hypothyroidism and circulating thyroid hormone levels exhibit hypothyroid-like clinical features. Before the identification of patients with RTHα, several Thrα1 knock-in mouse models were generated to clarify the function of TRα1. However, the phenotypes of these mice were not consistent with the clinical presentation of RTHα in humans. For the present study, we generated an RTHα mouse model that carries the Thra1E403X mutation found in human RTHα patients. Here, we report the gross phenotypes of this mouse RTHα model. Methods: Traditional homologous recombination gene targeting techniques were used to introduce a mutation (Thra1E403X) in the mouse Thra gene. The phenotypes of the resulting mice were studied and compared with clinical features observed for RTHα with THRAE403X. Results: Thrα1E403X/E403X homozygous mice exhibited severe neurological phenotypes, such as spasticity and motor ataxia, which were similar to those observed in endemic cretinism. Thrα1E403X/+ heterozygous mice reproduced most clinical manifestations of patient with RTHα, such as a normal survival rate and male fertility, as well as delayed postnatal growth and development, neurological and motor coordination deficits, and anemia. The mice had typical thyroid function with a modest increase in serum triiodothyronine (T3) levels, a low thyroxine (T4)/T3 ratio, and low reverse T3 (rT3) levels. Conclusions: The Thrα1E403X/+ mice faithfully recapitulate the clinical features of human RTHα and thus can provide a useful tool to dissect the role of TRα1 in development and to determine the pathological mechanisms of RTHα.

The Clinical Spectrum of Resistance to Thyroid Hormone Alpha in Children and Adults

Resistance to thyroid hormone alpha occurs due to pathogenic, heterozygous variants in THRA. The entity was first described in 2012 and to date only a small number of patients with varying severity have been reported. In this review, we summarize and interpret the heterogeneous clinical and laboratory features of all published cases, including ours. Many symptoms and findings are similar to those seen in primary hypothyroidism. However, thyroid-stimulating hormone levels are normal. Free triiodothyronine (T3) levels are in the upper half of normal range or frankly high and free thyroxine (T4) levels are low or in the lower half of normal range. Alterations in free T3 and free T4 may not be remarkable, particularly in adults, possibly contributing to underdiagnosis. In such patients, low reverse T3 levels, normo- or macrocytic anemia or, particularly in children, mildly elevated creatine kinase levels would warrant THRA sequencing. Treatment with L-thyroxine results in improvement of some clinical findings.

Thyroid Hormone Resistance due to a Novel De Novo Mutation in Thyroid Hormone Receptor Alpha: First Case Report from the Middle East and North Africa

The physiological actions of thyroid hormone (TH) are mediated through TH alpha and TH beta receptors. Resistance to TH (RTH) is characterized by a lack of peripheral tissues’ response to the active form of TH. TH receptor beta has been extensively studied. Mutations in this receptor were considered the main reason for TH resistance for some time up until the discovery of mutations in TH receptor alpha (TRα) that has attained more focus and interest in recent years. A 13-year-old child with classic hypothyroidism features (coarse facies, growth and developmental delay, skeletal dysplasia, generalized muscular hypertrophy, and severe constipation) associated with near-normal thyroid hormone levels, which did not support the diagnosis of hypothyroidism biochemically. Therefore, progressing with whole-exome sequencing had revealed a de novo heterozygous mutation in a gene encoding TRα that establishes a diagnosis of RTHα. This case report demonstrates a rare form of TH resistance due to mutation of TRα. It also emphasizes that THs act through distinctive receptor subtypes in different target tissues. Moreover, this report aims to raise awareness about this genetic mutation, which is thought to be more common than expected. However, due to its subtle features and insidious presentation, many cases remain undiagnosed; hence, the disorder’s exact incidence is unknown.

Thyroid Hormone Receptor α Mutations Cause Heart Defects in Zebrafish

Background: Mutations of thyroid hormone receptor α1 (TRα1) cause resistance to thyroid hormone (RTHα). Patients exhibit growth retardation, delayed bone development, anemia, and bradycardia. By using mouse models of RTHα, much has been learned about the molecular actions of TRα1 mutants that underlie these abnormalities in adults. Using zebrafish models of RTHα that we have recently created, we aimed to understand how TRα1 mutants affect the heart function during this period. Methods: In contrast to human and mice, the thra gene is duplicated, thraa and thrab, in zebrafish. Using CRISPR/Cas9-mediated targeted mutagenesis, we created C-terminal mutations in each of two duplicated thra genes in zebrafish (thraa 8-bp insertion or thrab 1-bp insertion mutations). We recently showed that these mutant fish faithfully recapitulated growth retardation as found in patients and thra mutant mice. In the present study, we used histological analysis, gene expression profiles, confocal fluorescence, and transmission electron microscopy (TEM) to comprehensively analyze the phenotypic characteristics of mutant fish heart during development. Results: We found both a dilated atrium and an abnormally shaped ventricle in adult mutant fish. The retention of red blood cells in the two abnormal heart chambers, and the decreased circulating blood speed and reduced expression of contractile genes indicated weakened contractility in the heart of mutant fish. These abnormalities were detected in mutant fish as early as 35 days postfertilization (juveniles). Furthermore, the expression of genes associated with the sarcomere assembly was suppressed in the heart of mutant fish, resulting in abnormalities of sarcomere organization as revealed by TEM, suggesting that the abnormal sarcomere organization could underlie the bradycardia exhibited in mutant fish. Conclusions: Using a zebrafish model of RTHα, the present study demonstrated for the first time that TRα1 mutants could act to cause abnormal heart structure, weaken contractility, and disrupt sarcomere organization that affect heart functions. These findings provide new insights into the bradycardia found in RTHα patients.

Topical L-thyroxine: The Cinderella among hormones waiting to dance on the floor of dermatological therapy?

Topical hormone therapy with natural or synthetic ligands of nuclear hormone receptors such as glucocorticoids, vitamin D analogues and retinoids has a long and highly successful tradition in dermatology. Yet the dermatological potential of thyroid hormone receptor (TR) agonists has been widely ignored, despite abundant clinical, cell and molecular biology, mouse in vivo, and human skin and hair follicle organ culture data documenting a role of TR-mediated signalling in skin physiology and pathology. Here, we review this evidence, with emphasis on wound healing and hair growth, and specifically highlight the therapeutic potential of repurposing topical L-thyroxine (T4) for selected applications in future dermatological therapy. We underscore the known systemic safety and efficacy profile of T4 in clinical medicine, and the well-documented impact of thyroid hormones on, for example, human epidermal and hair follicle physiology, hair follicle epithelial stem cells and pigmentation, keratin expression, mitochondrial energy metabolism and wound healing. On this background, we argue that short-term topical T4 treatment deserves careful further preclinical and clinical exploration for repurposing as a low-cost, effective and widely available dermatotherapeutic, namely in the management of skin ulcers and telogen effluvium, and that its predictable adverse effects are well-manageable.

Cardiac complications of thyroid hormone resistance syndromes

Thyroid hormones exert their action by binding to their thyroid hormone receptors among other mechanisms. They are involved in different cardiac functions, including contractility and rhythm. The mutation of thyroid hormone receptor β is the main cause of thyroid hormone resistance. The cardiac phenotype of mutated patients has been studied in several cohorts of patients with different mutations. Tachycardia, palpitation and cardiac arrhythmia frequently appear; atrial flutter/fibrillation is found in up to 20%. Cardiac systolic and diastolic functions are impaired compared to hyperthyroid or euthyroid subjects, but cases of heart failure have not been reported. No correlation between genotype and cardiac phenotype has been found. Patients with a mutation of thyroid hormone receptor α frequently present bradycardia and systolic and diastolic functions that are similar to those of hypothyroid subjects. Levothyroxine treatment partly improves these parameters.

Role of thyroid hormones in craniofacial development

The development of the craniofacial skeleton relies on complex temporospatial organization of diverse cell types by key signalling molecules. Even minor disruptions to these processes can result in deleterious consequences for the structure and function of the skull. Thyroid hormone deficiency causes delayed craniofacial and tooth development, dysplastic facial features and delayed development of the ossicles in the middle ear. Thyroid hormone excess, by contrast, accelerates development of the skull and, in severe cases, might lead to craniosynostosis with neurological sequelae and facial hypoplasia. The pathogenesis of these important abnormalities remains poorly understood and underinvestigated. The orchestration of craniofacial development and regulation of suture and synchondrosis growth is dependent on several critical signalling pathways. The underlying mechanisms by which these key pathways regulate craniofacial growth and maturation are largely unclear, but studies of single-gene disorders resulting in craniofacial malformations have identified a number of critical signalling molecules and receptors. The craniofacial consequences resulting from gain-of-function and loss-of-function mutations affecting insulin-like growth factor 1, fibroblast growth factor receptor and WNT signalling are similar to the effects of altered thyroid status and mutations affecting thyroid hormone action, suggesting that these critical pathways interact in the regulation of craniofacial development.

Generation of Novel Genetic Models to Dissect Resistance to Thyroid Hormone Receptor α in Zebrafish

Background: Patients with mutations of the thyroid hormone receptor alpha (THRA) gene show resistance to thyroid hormone alpha (RTHα). No amendable mouse models are currently available to elucidate deleterious effects of TRα1 mutants during early development. Zebrafish with transient suppressed expression by morpholino knockdown and ectopic expression of TRα1 mutants in the embryos have been reported. However, zebrafish with germline transmittable mutations have not been reported. The stable expression of thra mutants from embryos to adulthood facilitated the study of molecular actions of TRα1 mutants during development. Methods: In contrast to human and mice, the thra gene is duplicated in zebrafish, thraa, and thrab. Using CRISPR/Cas9-mediated targeted mutagenesis, we created dominant negative mutations in the two duplicated thra genes. We comprehensively analyzed the molecular and phenotypic characteristics of mutant fish during development. Results: Adult and juvenile homozygous thrab 1-bp ins (m/m) mutants exhibited severe growth retardation, but adult homozygous thraa 8-bp ins (m/m) mutants had very mild growth impairment. Expression of the growth hormone (gh1) and insulin-like growth factor 1 was markedly suppressed in homozygous thrab 1-bp ins (m/m) mutants. Decreased messenger RNA and protein levels of triiodothyronine-regulated keratin genes and inhibited keratinocyte proliferation resulted in hypoplasia of the epidermis in adult and juvenile homozygous thrab 1-bp ins (m/m) mutants, but not homozygous thraa 8-bp ins (m/m) mutants. RNA-seq analysis showed that homozygous thrab 1-bp ins (m/m) mutation had global impact on the functions of the adult pituitary. However, no morphological defects nor any changes in the expression of gh1 and keratin genes were observed in the embryos and early larvae. Thus, mutations of either the thraa or thrab gene did not affect initiation of embryogenesis. But the mutation of the thrab gene, but not the thraa gene, is detrimental in postlarval growth and skin development. Conclusions: The thra duplicated genes are essential to control temporal coordination in postlarval growth and development in a tissue-specific manner. We uncovered novel functions of the duplicated thra genes in zebrafish in development. These mutant zebrafish could be used as a model for further analysis of TRα1 mutant actions and for rapid screening of therapeutics for RTHα.

The In Vitro Functional Impairment of Thyroid Hormone Receptor Alpha 1 Isoform Mutants Is Mainly Dictated by Reduced Ligand Sensitivity

Background: Thyroid hormone (TH) acts on TH receptors (TRs) and regulates gene transcription by binding of TRs to TH response elements (TREs) in target gene promoters. The transcriptional activity of TRs is modulated by interactions with TR-coregulatory proteins. Mutations in TRα cause resistance to thyroid hormone alpha (RTHα). In this study, we analyzed if, beyond reduced triiodothyronine (T3) affinity, altered interactions with cofactors or different TREs could account for the differential impaired transcriptional activity of different mutants. Methods: We evaluated four mutants derived from patients (D211G, M256T, A263S, and R384H) and three artificial mutants at equivalent positions in patients with RTHβ (T223A, L287V, and P398H). The in vitro transcriptional activity was evaluated on TRE-luciferase reporters (DR4, IR0, and ER6). The affinity for T3 and interaction with coregulatory proteins (nuclear receptor corepressor 1 [NCoR1] and steroid receptor coactivator 1 [SRC1]) were also determined. Results: We found that the affinity for T3 was significantly reduced for all mutants, except for TRα1-T223A. The reduction in the T3 sensitivity of the transcriptional activity on three TREs, the dissociation of the corepressor NCoR1, and the association of the coactivator SRC1 recruitment for each mutant correlated with the reduced affinity for T3. We did not observe mutation-specific alterations in interactions with cofactors or TREs. Conclusions: In summary, the degree of impaired transcriptional activity of mutants is mainly determined by their reduced affinity for T3.

Mutations in thyroid hormone receptor α1 cause premature neurogenesis and progenitor cell depletion in human cortical development

Mutations in the thyroid hormone receptor α 1 gene (THRA) have recently been identified as a cause of intellectual deficit in humans. Patients present with structural abnormalities including microencephaly, reduced cerebellar volume and decreased axonal density. Here, we show that directed differentiation of THRA mutant patient-derived induced pluripotent stem cells to forebrain neural progenitors is markedly reduced, but mutant progenitor cells can generate deep and upper cortical layer neurons and form functional neuronal networks. Quantitative lineage tracing shows that THRA mutation-containing progenitor cells exit the cell cycle prematurely, resulting in reduced clonal output. Using a micropatterned chip assay, we find that spatial self-organization of mutation-containing progenitor cells in vitro is impaired, consistent with down-regulated expression of cell-cell adhesion genes. These results reveal that thyroid hormone receptor α1 is required for normal neural progenitor cell proliferation in human cerebral cortical development. They also exemplify quantitative approaches for studying neurodevelopmental disorders using patient-derived cells in vitro.

Metabolomic Profiling of Body Fluids in Mouse Models Demonstrates that Nuclear Magnetic Resonance Is a Putative Diagnostic Tool for the Presence of Thyroid Hormone Receptor α1 Mutations

Background: Resistance to thyroid hormone alpha (RTHα) is a rare genetic disease due to mutations in the THRA gene, which encodes thyroid hormone receptor alpha 1 (TRα1). Since its first description in 2012, 46 cases of RTHα have been reported worldwide, corresponding to 26 different mutations of TRα1. RTHα patients share some common symptoms with hypothyroid patients, without significant reduction in thyroid hormone level. The high variability of clinical features and the absence of reliable biochemical markers make the diagnosis of this disease difficult. Some of these mutations have been recently modeled in mice. Methods: In our study, we used four different mouse models heterozygous for frameshift mutations in the Thra gene. Two of them are very close to human mutations, while the two others have not yet been found in patients. We characterized the metabolic phenotypes of urine and plasma samples collected from these four animal models using an untargeted nuclear magnetic resonance (NMR)-based metabolomic approach. Results: Multivariate statistical analysis of the metabolomic profiles shows that biofluids of mice that carry human-like mutations can be discriminated from controls. Metabolic signatures associated with Thra mutations in urine and plasma are stable over time and clearly differ from the metabolic fingerprint of hypothyroidism in the mouse. Conclusion: Our results provide a proof-of-principle that easily accessible NMR-based metabolic fingerprints of biofluids could be used to diagnose RTHα in humans.

RNA-seq and microRNA-seq analysis of Japanese flounder (Paralichthys olivaceus) larvae treated by thyroid hormones

Flatfish pigmentation is a complex process, affected by environmental factors including light, nutrients, and hormones. Of those, the thyroid hormone has been reported to increase the albinism rate of Japanese flounder (Paralichthys olivaceus). However, the underlying mechanism remains unclear. In the present study, triiodothyronine (T3), thyroxine, and thiourea were introduced into P. olivaceus larvae from 16 to 57 days after hatching (DAH). By comparison of albinism rate, T3 treatment and control larvae of 42 DAH were chosen for mRNA and miRNA high-throughput sequencing analyses. A total of 337 miRNAs were identified via miRNA-seq, and 12 miRNAs exhibited significantly differential expression patterns in D42_T3 versus D42_Con (TPM > 10, fold change ≥ 1.5 or ≤ 0.67 and q ≤ 0.05). These differentially expressed miRNAs targeted 3658 putative genes, which further enriched to 10 GO terms (q < 0.05). RNA-seq identified 146 differentially expressed genes (DEGs) in D42_T3 versus D42_Con (|log₂ fold change| > 1 and q < 0.005), including pigmentation-related genes such as the receptor tyrosine-protein kinase erbB-3, pro-opiomelanocortin A, and melanotransferrin, and the growth-related gene somatotropin. These DEGs were significantly enriched to 15 GO terms and 8 KEGG pathways (q < 0.05), which included several sugar metabolic pathways (glycolysis/gluconeogenesis and the pentose phosphate pathway). Integrated analysis revealed that 26 overlapping genes between DEGs and mRNAs were targeted by miRNAs. Furthermore, seven mRNA-miRNA pairs exhibited reversed regulation patterns. This provides important clues to understand the role of thyroid hormones in flatfish pigmentation.

A novel thyroid hormone receptor alpha gene mutation, clinic characteristics, and follow-up findings in a patient with thyroid hormone resistance

Thyroid hormone receptor alpha (THRA) gene mutation is a thyroid hormone resistance syndrome characterized by near-normal thyroid function tests and tissue-specific hypothyroidism. In this case study, we report a novel de novo p.G291S heterozygous mutation in the THRA gene was detected at mutation analysis. A 4-year-old male patient was admitted due to short stature, motor-mental retardation, and constipation. At physical examination, coarse facial appearance, eyelid edema, pallor, and umbilical hernia were observed. Primary thyroid hormone resistance should be considered in patients with phenotypically hypothyroid features. Laboratory analysis found moderate elevation in free triiodothyronine (T3) levels, normochromic normocytic anemia, and elevated creatine kinase levels. In conclusion, THRA gene mutation should be considered in patients with clinical hypothyroid findings and increased/moderately elevated free T3, decreased/ normal free thyroxine, normal thyroid-stimulating hormone levels, and increased muscle enzymes.

Genetic and Pharmacological Targeting of Transcriptional Repression in Resistance to Thyroid Hormone Alpha

Background: Thyroid hormones act in bone and cartilage via thyroid hormone receptor alpha (TRα). In the absence of triiodothyronine (T3), TRα interacts with co-repressors, including nuclear receptor co-repressor-1 (NCoR1), which recruit histone deacetylases (HDACs) and mediate transcriptional repression. Dominant-negative mutations of TRα cause resistance to thyroid hormone alpha (RTHα; OMIM 614450), characterized by excessive repression of T3 target genes leading to delayed skeletal development, growth retardation, and bone dysplasia. Treatment with thyroxine has been of limited benefit, even in mildly affected individuals, and there is a need for new therapeutic strategies. It was hypothesized that (i) the skeletal manifestations of RTHα are mediated by the persistent TRα/NCoR1/HDAC repressor complex containing mutant TRα, and (ii) treatment with the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) would ameliorate these manifestations. Methods: The skeletal phenotypes of (i) Thra1^PV/+ mice, a well characterized model of RTHα; (ii) Ncor1^ΔID/ΔID mice, which express an NCoR1 mutant that fails to interact with TRα; and (iii) Thra1^PV/+Ncor1^ΔID/ΔID double-mutant adult mice were determined. Wild-type, Thra1^PV/+, Ncor1^ΔID/ΔID, and Thra1^PV/+Ncor1^ΔID/ΔID double-mutant mice were also treated with SAHA to determine whether HDAC inhibition results in amelioration of skeletal abnormalities. Results:Thra1^PV/+ mice had a severe skeletal dysplasia, characterized by short stature, abnormal bone morphology, and increased bone mineral content. Despite normal bone length, Ncor1^ΔID/ΔID mice displayed increased cortical bone mass, mineralization, and strength. Thra1^PV/+Ncor1^ΔID/ΔID double-mutant mice displayed only a small improvement of skeletal abnormalities compared to Thra1^PV/+ mice. Treatment with SAHA to inhibit histone deacetylation had no beneficial or detrimental effects on bone structure, mineralization, or strength in wild-type or mutant mice. Conclusions: These studies indicate treatment with SAHA is unlikely to improve the skeletal manifestations of RTHα. Nevertheless, the findings (i) confirm that TRα1 has a critical role in the regulation of skeletal development and adult bone mass, (ii) suggest a physiological role for alternative co-repressors that interact with TR in skeletal cells, and (iii) demonstrate a novel role for NCoR1 in the regulation of adult bone mass and strength.

Thyroid hormone resistance and the value of genetics: Three case reports

RATIONAL

Thyroid hormone resistance (RTH) is a rare disease that is characterised by a lowered sensitivity of the target organs to thyroid hormone. Herein, we present 3 cases of confirmed RTH, with the support of clinical lab results and/or gene sequencing at diagnosis.

PATIENT CONCERNS

The 3 included patients were found to have elevated levels of free T3 (FT3), free T4 (FT4), and non-supressed levels of thyroid stimulating hormone (TSH).

DIAGNOSIS

All patients were tested for thyroid antibodies, somatostatin suppression, vision and hearing at diagnosis. Electrocardiography (ECG), thyroid ultrasonography, and magnet resonance imaging (MRI) of the sellar region were also performed. Furthermore, gene sequencing was used to detect the thyroid hormone receptor beta (THRB) gene mutation.

INTERVENTIONS

Patient treatment was individualised. Patients were given levothyroxine sodium or a low dose of thyroiodin, depending on the individual symptoms.

OUTCOMES

After treatment, thyroid function was stable in 2 of the teenage patients. No evidence of worsening thyrotoxicosis was observed.

LESSONS

Gene sequencing should be considered together with clinical lab results, including somatostatin suppression testing, before approaching a diagnosis of RTH.

Analysis of Physiological Responses to Thyroid Hormones and Their Receptors in Bone

Thyroid hormone has profound effects on skeletal development and adult bone maintenance. Here, we review the current literature concerning thyroid hormone action in bone and cartilage in relation to human disease and animal models. We describe state-of-the-art imaging and biomechanical methods used to determine structural and functional parameters in the skeletal phenotyping of mouse models.

Syndromes of Resistance to Thyroid Hormone Action

Thyroid hormone (TH) action is crucial for the development of several tissues.A number of syndromes are associated with reduced responsiveness to thyroid hormones, expanding the original definition of thyroid hormone resistance, firstly described by Refetoff and collaborators in 1967, which is characterized by elevated circulating levels of T4 and T3 with measurable serum TSH concentrations, as a consequence of mutations of thyroid hormone receptor beta (TRβ), recently named as RTHβ. More recently, another form of insensitivity to TH has been identified due to mutations in the thyroid hormone receptor alpha (TRα), named RTHα. In this chapter we will focus the discussion on the phenotype of RTHβ and RTHα. These diseases share the same pathogenic mechanism caused by dominant negative mutations in TH receptor genes that reduce T3 binding or affect the recruitment of cofactors. As a consequence, thyroid hormone actions are impaired at the tissue level. The phenotypic manifestations of RTHβ and RTHα are to some extent correlated with the degree of disruption and the tissue distribution of the TRs being characterized by variable coexistence of hypothyroid or thyrotoxic manifestations in RTHβ or by a congenital hypothyroid features in RTHα despite normal TSH and borderline low free T4.

TRα Mutations in Human

Resistance to thyroid hormone alpha is an emerging syndrome, with up to now a limited number of published cases. Some features are common to most of the patients, but there is still some work to provide a comprehensive description of the full spectrum of the syndrome. A survey of the strategy to screen for and characterize the mutations in TR α gene is given.

Thyroid Hormone and Skeletal Development

Thyroid hormone (TH) is essential for skeletal development from the late fetal life to the onset of puberty. During this large window of actions, TH has key roles in endochondral and intramembranous ossifications and in the longitudinal bone growth. There is evidence that TH acts directly in skeletal cells but also indirectly, specially via the growth hormone/insulin-like growth factor-1 axis, to control the linear skeletal growth and maturation. The presence of receptors, plasma membrane transporters, and activating and inactivating enzymes of TH in skeletal cells suggests that direct actions of TH in these cells are crucial for skeletal development, which has been confirmed by several in vitro and in vivo studies, including mouse genetic studies, and clinical studies in patients with resistance to thyroid hormone due to dominant-negative mutations in TH receptors. This review examines progress made on understanding the mechanisms by which TH regulates the skeletal development.

Thyroid diseases and bone health

Thyroid hormones are essential for skeletal development and are important regulators of bone maintenance in adults. Childhood hypothyroidism causes delayed skeletal development, retarded linear growth and impaired bone mineral accrual. Epiphyseal dysgenesis is evidenced by classic features of stippled epiphyses on X-ray. In severe cases, post-natal growth arrest results in a complex skeletal dysplasia. Thyroid hormone replacement stimulates catch-up growth and bone maturation, but recovery may be incomplete dependent on the duration and severity of hypothyroidism prior to treatment. A severe phenotype characteristic of hypothyroidism occurs in children with resistance to thyroid hormone due to mutations affecting THRA encoding thyroid hormone receptor α (TRα). Discovery of this rare condition recapitulated animal studies demonstrating that TRα mediates thyroid hormone action in the skeleton. In adults, thyrotoxicosis is well known to cause severe osteoporosis and fracture, but cases are rare because of prompt diagnosis and treatment. Recent data, however, indicate that subclinical hyperthyroidism is associated with low bone mineral density (BMD) and an increased risk of fracture. Population studies have also shown that variation in thyroid status within the reference range in post-menopausal women is associated with altered BMD and fracture risk. Thus, thyroid status at the upper end of the euthyroid reference range is associated with low BMD and increased risk of osteoporotic fragility fracture. Overall, extensive data demonstrate that euthyroid status is required for normal post-natal growth and bone mineral accrual, and is fundamental for maintenance of adult bone structure and strength.

CRISPR/Cas9 Editing of the Mouse Thra Gene Produces Models with Variable Resistance to Thyroid Hormone

BACKGROUND

Resistance to thyroid hormone due to THRA mutations (RTHα) is a recently discovered genetic disease, displaying important variability in its clinical presentation. The mutations alter the function of TRα1, one of the two nuclear receptors for thyroid hormone.

METHODS

The aim of this study was to understand the relationship between specific THRA mutations and phenotype. CRISPR/Cas9 genome editing was used to generate five new mouse models of RTHα, with frameshift or missense mutations.

RESULTS

Like human patients, mutant mice displayed a hypothyroid-like phenotype, with altered development. Phenotype severity varied between the different mouse models, mainly depending on the ability of the mutant receptor to interact with transcription corepressor in the presence of thyroid hormone.

CONCLUSION

The present mutant mice represent highly relevant models for the human genetic disease which will be useful for future investigations.

Increased circulating interleukin-8 in patients with resistance to thyroid hormone receptor α

Innate immune cells have recently been identified as novel thyroid hormone (TH) target cells in which intracellular TH levels appear to play an important functional role. The possible involvement of TH receptor alpha (TRα), which is the predominant TR in these cells, has not been studied to date. Studies in TRα^0/0 mice suggest a role for this receptor in innate immune function. The aim of this study was to determine whether TRα affects the human innate immune response. We assessed circulating interleukin-8 concentrations in a cohort of 8 patients with resistance to TH due to a mutation of TRα (RTHα) and compared these results to healthy controls. In addition, we measured neutrophil and macrophage function in one of these RTHα patients (mutation D211G). Circulating interleukin-8 levels were elevated in 7 out of 8 RTHα patients compared to controls. These patients harbor different mutations, suggesting that this is a general feature of the syndrome of RTHα. Neutrophil spontaneous apoptosis, bacterial killing, NAPDH oxidase activity and chemotaxis were unaltered in cells derived from the RTHαD211G patient. RTHα macrophage phagocytosis and cytokine induction after LPS treatment were similar to results from control cells. The D211G mutation did not result in clinically relevant impairment of neutrophil or pro-inflammatory macrophage function. As elevated circulating IL-8 is also observed in hyperthyroidism, this observation could be due to the high-normal to high levels of circulating T₃ found in patients with RTHα.

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.

Thyroid hormone receptor beta and NCOA4 regulate terminal erythrocyte differentiation

An effect of thyroid hormone (TH) on erythropoiesis has been known for more than a century but the molecular mechanism(s) by which TH affects red cell formation is still elusive. Here we demonstrate an essential role of TH during terminal human erythroid cell differentiation; specific depletion of TH from the culture medium completely blocked terminal erythroid differentiation and enucleation. Treatment with TRβ agonists stimulated premature erythroblast differentiation in vivo and alleviated anemic symptoms in a chronic anemia mouse model by regulating erythroid gene expression. To identify factors that cooperate with TRβ during human erythroid terminal differentiation, we conducted RNA-seq in human reticulocytes and identified nuclear receptor coactivator 4 (NCOA4) as a critical regulator of terminal differentiation. Furthermore, Ncoa4^-/- mice are anemic in perinatal periods and fail to respond to TH by enhanced erythropoiesis. Genome-wide analysis suggests that TH promotes NCOA4 recruitment to chromatin regions that are in proximity to Pol II and are highly associated with transcripts abundant during terminal differentiation. Collectively, our results reveal the molecular mechanism by which TH functions during red blood cell formation, results that are potentially useful to treat certain anemias.

Anemia in Patients With Resistance to Thyroid Hormone α: A Role for Thyroid Hormone Receptor α in Human Erythropoiesis

CONTEXT

Patients with resistance to thyroid hormone (TH) α (RTHα) are characterized by growth retardation, macrocephaly, constipation, and abnormal thyroid function tests. In addition, almost all RTHα patients have mild anemia, the pathogenesis of which is unknown. Animal studies suggest an important role for TH and TH receptor (TR)α in erythropoiesis.

OBJECTIVE

To investigate whether a defect in TRα affects the maturation of red blood cells in RTHα patients.

DESIGN, SETTING, AND PATIENTS

Cultures of primary human erythroid progenitor cells (HEPs), from peripheral blood of RTHα patients (n = 11) harboring different inactivating mutations in TRα (P398R, F397fs406X, C392X, R384H, A382fs388X, A263V, A263S), were compared with healthy controls (n = 11). During differentiation, erythroid cells become smaller, accumulate hemoglobin, and express different cell surface markers. We assessed cell number and cell size, and used cell staining and fluorescence-activated cell sorter analysis to monitor maturation at different time points.

RESULTS

After ∼14 days of ex vivo expansion, both control and patient-derived progenitors differentiated spontaneously. However, RTHα-derived cells differentiated more slowly. During spontaneous differentiation, RTHα-derived HEPs were larger, more positive for c-Kit (a proliferation marker), and less positive for glycophorin A (a differentiation marker). The degree of abnormal spontaneous maturation of RTHα-derived progenitors did not correlate with severity of underlying TRα defect. Both control and RTHα-derived progenitors responded similarly when differentiation was induced. T3 exposure accelerated differentiation of both control- and RTHα patient-derived HEPs.

CONCLUSIONS

Inactivating mutations in human TRα affect the balance between proliferation and differentiation of progenitor cells during erythropoiesis, which may contribute to the mild anemia seen in most RTHα patients.

Surgical treatment and outcome of TSH-producing pituitary adenomas

BACKGROUND

Thyrotropin-producing adenomas (TSHomas) account for approximately 1-2% of all pituitary tumors. Recently, there has been debate on primary treatment as some studies suggest a high rate of hypopituitarism after transsphenoidal surgery and therefore suggest primary use of somatostatin analogs (SSA) instead. We would like to challenge this assumption by presenting our single-center experience with transsphenoidal surgery for TSHoma.

METHODS

Fifteen patients treated consecutively between May 2010 and December 2016 were analyzed for long-term postoperative remission and pituitary function.

RESULTS

Data on the development of TSH, fT3 and fT4 were available for 12 of 15 patients (mean follow-up was 18 months, 8 patients >12 months), showing mean TSH levels of 1.289 mU/l (0.02-2.04, SD 0.66), mean fT3 levels of 3.76 pmol/l (0.5-6.16, SD 1.8) and mean fT4 levels of 16.5 pmol/l (11.7-21.9, SD 3.66). Six of those patients were substituted with a mean of 85.4 μg L-thyroxine after a median follow-up of 20.7 months. The other six patients did not receive L-thyroxine at a median follow-up of 15.5 months. One patient with a known tumor remnant on MRI stayed euthyreotic with cabergoline at the timepoint of follow-up 22 months after the operation. Control of the corticotrop axis was also available in 12 of 15 with no patient showing a corticotroph insufficiency in the long term.

CONCLUSIONS

We argue that transsphenoidal surgery for TSHoma should be considered as the treatment of choice as remission following surgery is highly probable and postoperative hypopituitarism is very unlikely if patients are referred to centers with high pituitary surgery case loads.

Contrasting Phenotypes in Resistance to Thyroid Hormone Alpha Correlate with Divergent Properties of Thyroid Hormone Receptor α1 Mutant Proteins

BACKGROUND

Resistance to thyroid hormone alpha (RTHα), a disorder characterized by tissue-selective hypothyroidism and near-normal thyroid function tests due to thyroid receptor alpha gene mutations, is rare but probably under-recognized. This study sought to correlate the clinical characteristics and response to thyroxine (T4) therapy in two adolescent RTHα patients with the properties of the THRA mutation, affecting both TRα1 and TRα2 proteins, they harbored.

METHODS

Clinical, auxological, biochemical, and physiological parameters were assessed in each patient at baseline and after T4 therapy.

RESULTS

Heterozygous THRA mutations occurring de novo were identified in a 17-year-old male (patient P1; c.788C>T, p.A263V mutation) investigated for mild pubertal delay and in a 15-year-old male (patient P2; c.821T>C, p.L274P mutation) with short stature (0.4th centile), skeletal dysplasia, dysmorphic facies, and global developmental delay. Both individuals exhibited macrocephaly, delayed dentition, and constipation, together with a subnormal T4/triiodothyronine (T3) ratio, low reverse T3 levels, and mild anemia. When studied in vitro, A263V mutant TRα1 was transcriptionally impaired and inhibited the function of its wild-type counterpart at low (0.01-10 nM) T3 levels, with higher T3 concentrations (100 nM-1 μM) reversing dysfunction and such dominant negative inhibition. In contrast, L274P mutant TRα1 was transcriptionally inert, exerting significant dominant negative activity, only overcome with 10 μM of T3. Mirroring this, normal expression of KLF9, a TH-responsive target gene, was achieved in A263V mutation-containing peripheral blood mononuclear cells following 1 μM of T3 exposure, but with markedly reduced expression levels in L274P mutation-containing peripheral blood mononuclear cells, even with 10 μM of T3. Following T4 therapy, growth, body composition, dyspraxia, and constipation improved in P1, whereas growth retardation and constipation in P2 were unchanged. Neither A263V nor L274P mutations exhibited gain or loss of function in the TRα2 background, and no additional phenotype attributable to this was discerned.

CONCLUSIONS

This study correlates a milder clinical phenotype and favorable response to T4 therapy in a RTHα patient (P1) with heterozygosity for mutant TRα1 exhibiting partial, T3-reversible, loss of function. In contrast, a more severe clinical phenotype refractory to hormone therapy was evident in another case (P2) associated with severe, virtually irreversible, dysfunction of mutant TRα1.

Thyroid Hormone Receptor Beta in the Ventromedial Hypothalamus Is Essential for the Physiological Regulation of Food Intake and Body Weight

The obesity epidemic is a significant global health issue. Improved understanding of the mechanisms that regulate appetite and body weight will provide the rationale for the design of anti-obesity therapies. Thyroid hormones play a key role in metabolic homeostasis through their interaction with thyroid hormone receptors (TRs), which function as ligand-inducible transcription factors. The TR-beta isoform (TRβ) is expressed in the ventromedial hypothalamus (VMH), a brain area important for control of energy homeostasis. Here, we report that selective knockdown of TRβ in the VMH of adult mice results in severe obesity due to hyperphagia and reduced energy expenditure. The observed increase in body weight is of a similar magnitude to murine models of the most extreme forms of monogenic obesity. These data identify TRβ in the VMH as a major physiological regulator of food intake and energy homeostasis.

Genetic disorders of nuclear receptors

Following the first isolation of nuclear receptor (NR) genes, genetic disorders caused by NR gene mutations were initially discovered by a candidate gene approach based on their known roles in endocrine pathways and physiologic processes. Subsequently, the identification of disorders has been informed by phenotypes associated with gene disruption in animal models or by genetic linkage studies. More recently, whole exome sequencing has associated pathogenic genetic variants with unexpected, often multisystem, human phenotypes. To date, defects in 20 of 48 human NR genes have been associated with human disorders, with different mutations mediating phenotypes of varying severity or several distinct conditions being associated with different changes in the same gene. Studies of individuals with deleterious genetic variants can elucidate novel roles of human NRs, validating them as targets for drug development or providing new insights into structure-function relationships. Importantly, human genetic discoveries enable definitive disease diagnosis and can provide opportunities to therapeutically manage affected individuals. Here we review germline changes in human NR genes associated with "monogenic" conditions, including a discussion of the structural basis of mutations that cause distinctive changes in NR function and the molecular mechanisms mediating pathogenesis.