Current loss-of-function mutations in the thyrotropin receptor gene: when to investigate, clinical effects, and treatment

Hypothyroidism: The difficulty in attributing symptoms to their underlying cause

Common symptoms of overt hypothyroidism are non-specific and include fatigue, lethargy, and dry skin. Although the diagnosis is considered to be straightforward, no single symptom can be used to identify patients with overt hypothyroidism, while many patients with subclinical hypothyroidism are asymptomatic. A large population-based study on the spectrum of symptoms in subclinical hypothyroidism showed similar rates of thyroid disease-related symptoms compared with euthyroid subjects, while the TSH concentration had no impact on symptom score. Together, these findings make it challenging to attribute symptoms to their underlying cause. This is also true in the case of unexplained persistent symptoms in levothyroxine-treated patients. Although generally considered a life-long replacement therapy, successful thyroid hormone discontinuation resulting in euthyroidism has been reported in approximately one third of patients. Thus, we overtreat patients with (subclinical) hypothyroidism, highlighting the importance of reliable diagnostic criteria. The diagnostic process, including the implementation of robust TSH and FT4 reference intervals, is especially challenging in specific situations including aging, pregnancy, non-thyroidal illness, and central hypothyroidism. There is a clear need for improved adherence to current guidelines from scientific societies and for willingness to manage symptoms without a clear pathological correlate, especially in the case of mild TSH elevations. This review will highlight recent literature on this topic and offers some practice points.

Variant of TSHR is Not a Frequent Cause of Congenital Hypothyroidism in Chinese Han Patients

PURPOSE

To screen variants of the thyroid stimulating hormone receptor (TSHR) gene among congenital hypothyroidism (CH) patients.

PATIENTS AND METHODS

We conducted a genetic screening of the TSHR gene in a cohort of 125 Chinese CH patients. Variants were detected by customized targeted next-generation sequencing.

RESULTS

A total of 11 TSHR missense heterozygous variants were identified in 14 CH patients. Six variants were in the transmembrane domains, four variants were in the leucine-rich repeats and one variant was located in the hinge region of the TSHR protein. p.F525S was the most prevalent variant with an allele frequency of 0.016, followed by p.R450H with an allele frequency of 0.012. The allele frequency of most variants was higher in our cohort than those of other populations.

CONCLUSION

The prevalence of TSHR variants was 11.2%. Variant p.F525S was the most prevalent variant with an allele frequency of 0.016. The prevalence of TSHR variants was different from other populations.

Subclinical hypothyroidism in children: updates for pediatricians

Subclinical hypothyroidism (SH) is defined as serum thyroid-stimulating hormone (TSH) above the upper limit of the reference range in the presence of normal free T4 concentrations. Depending on the degree of TSH elevation, SH could be defined as mild (TSH, 4.5-10 mIU/L) or severe (TSH>10 mIU/L). While there is a general consensus to treat children with serum TSH levels above 10 mU/L, the management of the mild form is uncertain and should be individualized. In this mini-review, we present a brief review of SH in children based on extensive literature review and long-standing clinical experience. This review provides the prevalence, causes, clinical presentation, consequences, investigation, and up-to-date therapeutic approach of SH in children. Generally, the purpose of the review is to provide pediatricians with an update of this common and continuously evolving condition.

Genetics of primary congenital hypothyroidism-a review

PURPOSE

Congenital primary hypothyroidism (CH) is a state of inadequate thyroid hormone production detected at birth, caused either by absent, underdeveloped or ectopic thyroid gland (dysgenesis), or by defected thyroid hormone biosynthesis (dyshormonogenesis). A genetic component has been identified in many cases of CH. This review summarizes the clinical and biochemical features of the genetic causes of primary CH.

METHODS

A literature review was conducted of gene defects causing congenital hypothyroidism.

RESULTS

Mutations in five genes have predominantly been implicated in thyroid dysgenesis (TSHR, FOXE1, NKX2-1, PAX8, and NKX2-5), the primary cause of CH (85%), and mutations in seven genes in thyroid dyshormonogenesis (SLC5A5, TPO, DUOX2, DUOXA2, SLC6A4, Tg, and DEHAL1). These genes encode for proteins that regulate genes expressed during the differentiation of the thyroid, such as TPO and Tg genes, or genes that regulate iodide organification, thyroglobulin synthesis, iodide transport, and iodotyrosine deiodination. Besides thyroid dysgenesis and dyshormonogenesis, additional causes of congenital hypothyroidism, such as iodothyronine transporter defects and resistance to thyroid hormones, have also been associated with genetic mutations.

CONCLUSION

The identification of the underlying genetic defects of CH is important for genetic counseling of families with an affected member, for identifying additional clinical characteristics or the risk for thyroid neoplasia and for diagnostic and management purposes.

Genetics of Gland-in-situ or Hypoplastic Congenital Hypothyroidism in Macedonia

Neonatal screening in Macedonia detects congenital hypothyroidism (CH) with an incidence of 1 in 1,585, and more than 50% of cases exhibit a normally located gland-in-situ (GIS). Monogenic mutations causing dyshormonogenesis may underlie GIS CH; additionally, a small proportion of thyroid hypoplasia has a monogenic cause, such as TSHR and PAX8 defects. The genetic architecture of Macedonian CH cases has not previously been studied. We recruited screening-detected, non-syndromic GIS CH or thyroid hypoplasia cases (n = 40) exhibiting a spectrum of biochemical thyroid dysfunction ranging from severe permanent to mild transient CH and including 11 familial cases. Cases were born at term, with birth weight >3,000 g, and thyroid morphologies included goiter (n = 11), thyroid hypoplasia (n = 6), and apparently normal-sized thyroid. A comprehensive, phenotype-driven, Sanger sequencing approach was used to identify genetic mutations underlying CH, by sequentially screening known dyshormonogenesis-associated genes and TSHR in GIS cases and TSHR and PAX8 in cases with thyroid hypoplasia. Potentially pathogenic variants were identified in 14 cases, of which four were definitively causative; we also detected digenic variants in three cases. Seventeen variants (nine novel) were identified in TPO (n = 4), TG (n = 3), TSHR (n = 4), DUOX2 (n = 4), and PAX8 (n = 2). No mutations were detected in DUOXA2, NIS, IYD, and SLC26A7. The relatively low mutation frequency suggests that factors other than recognized monogenic causes (oligogenic variants, environmental factors, or novel genes) may contribute to GIS CH in this region. Future non-hypothesis-driven, next-generation sequencing studies are required to confirm these findings.

The rs1991517 polymorphism is a genetic risk factor for congenital hypothyroidism

The objective of the current study is to explore the association of thyroid-stimulating hormone receptor (TSHR) rs1991517 polymorphism (c.2337 C > G, p.D727E) with congenital hypothyroidism (CH) through a case-control study followed by a meta-analysis. The case-control study was based on 45 CH subjects and 700 healthy controls. Meta-analysis comprised of seven published studies and our current findings (1044 CH cases and 1649 healthy controls). The allele contrast model showed that the presence of G- allele increased CH risk by 45% (OR: 1.45, 95% CI 1.20-1.76) and 41% (OR: 1.41, 95% CI 1.03-1.93) in fixed effect and random effect models, respectively. The GG- genotype is associated with 2.3-fold (95% CI 1.32-3.99) increased risk for CH in the fixed-effect model. I ² (0.58) and Cochran's Q test (Q: 16.72, p = 0.02) revealed evidence of heterogeneity in the association. No publication bias was observed by Egger's test (p = 0.70). Sensitivity analysis revealed that even after excluding any study this polymorphism is associated with risk for CH. The rs1991517 mutation alters the binding affinity to cAMP (ΔG of 727D vs.727E: - 7.27 vs. - 7.34 kcal/mol). In conclusion, rs1991517 is a genetic risk factor for CH and exerts its impact by altering cAMP-mediated signal transduction.

Mutation spectrum analysis of 29 causative genes in 43 Chinese patients with congenital hypothyroidism

Congenital hypothyroidism (CH) is the most common neonatal endocrine disorder with a genetic origin. The purpose of the present study was to analyze the mutation spectrum of CH patients in China. A targeted next-generation sequencing panel covering all exons of 29 CH-related causative genes was used in 43 Han Chinese patients with CH [11 dysgenesis and 32 glands in situ (GIS)]. The functional impact and pathogenicity of detected variants were analyzed using a comprehensive bioinformatics approach and co-segregation studies. A total of 47 rare non-polymorphic variants in 9 target genes associated with thyroid hormone synthesis (DUOX2, DUOXA2, TPO, TG, SLC26A4 and SLC5A5), thyroid stimulating hormone resistance (TSHR) and central hypothyroidism (PROP1 and TRHR) were identified in 31 patients (31/43, 72%). Of these variants, 8 were novel, including 3 in DUOX2, 2 in TPO, 3 in TSHR and 1 in SLC5A5. Variants were mostly affected by DUOX2, TG, TPO and TSHR. Approximately 44% of the patients (19/43) carried DUOX2 variants. The mutation detection rates in patients with GIS were higher compared with patients with dysgenesis [25/32 (78%) vs. 6/11 (54%)]. Oligogenic mutations were detected in 25.6% of the total cases and 35% of the mutated cases. Genetic basis was ascertained in 13 patients, reaching a diagnosis detection rate of 30%. In conclusion, genetic defects in dyshormonogenesis, mainly in DUOX2, were the main genetic cause of CH in the Chinese population. Oligogenicity is highly involved in CH pathogenesis and may thus be an important factor in common phenotypic variability observed in patients with CH.

DUOX2/DUOXA2 Mutations Frequently Cause Congenital Hypothyroidism that Evades Detection on Newborn Screening in the United Kingdom

Background: The etiology, course, and most appropriate management of borderline congenital hypothyroidism (CH) are poorly defined, such that the optimal threshold for diagnosis with bloodspot screening thyrotropin (bsTSH) measurement remains controversial. Dual oxidase 2 (DUOX2) mutations may initially cause borderline elevation of bsTSH, which later evolves into significant hypothyroidism on venous blood measurement. It was hypothesized that mutations in both DUOX2 and its accessory protein DUOXA2 may occur frequently, even in patients with borderline bsTSH elevation, such that higher diagnostic thresholds in bsTSH screening may fail to detect such cases, with consequent risk of undiagnosed neonatal hypothyroidism of sufficient magnitude to require thyroxine therapy. This study aimed to investigate the frequency and characteristics of DUOX2 and DUOXA2 mutations in a borderline CH cohort. Methods: A cross-sectional study of patients with borderline CH was undertaken at Great Ormond Street Hospital, a tertiary British pediatric center. DUOX2 was sequenced in 52 patients with a bsTSH of 6-19.9 mIU/L, venous TSH of >25 mIU/L, and eutopic thyroid gland in situ. DUOXA2 was sequenced in DUOX2 mutation-negative cases, and novel DUOXA2 mutations were functionally characterized. Results: A total of 26 (50%) patients harbored likely pathogenic mutations in DUOX2 (n = 20; 38%) or DUOXA2 (n = 6; 12%), including novel gene variants (DUOX2, n = 3; DUOXA2, n = 7). Two recurrent DUOX2 mutations (p.Q570L, p.F966Sfs*29) occurred frequently in population databases (MAF ≥0.01). Despite bsTSH being <10 mIU/L in 46% of DUOX2 and DUOXA2 mutation-positive cases, venous free thyroxine levels in these patients were in the moderate CH range (M = 9.3 pmol/L, range <3.9-15.8 pmol/L), Conclusions: Targeted DUOX2 and DUOXA2 sequencing in a borderline CH cohort has a high diagnostic yield. These findings might argue for a lowering of bsTSH thresholds, but follow-up studies are required to assess whether cases with borderline bsTSH harboring DUOX2/DUOXA2 mutations will benefit from an early diagnosis and subsequent levothyroxine treatment.

Mild Hypothyroidism in Childhood: Who, When, and How Should Be Treated?

Mild hypothyroidism, also known as subclinical hypothyroidism (SH), is biochemically defined as serum TSH levels above the upper limit of the reference range, in the presence of normal serum concentrations of total T4 and free T4 (FT4). In the neonatal period, mild hypothyroidism can be defined by the presence of a TSH value between 6 and 20 mIU/L and normal FT4 levels. After the neonatal period, SH can be defined mild if TSH ranges between 4.5 and 10 mIU/L. The management of mild hypothyroidism in childhood is challenging. The major concern is to establish whether this condition should always be considered an expression of mild thyroid dysfunction. Indeed, the effects of untreated mild hypothyroidism are still not completely defined. In the neonatal period, concern exists about neurocognitive outcome; in children, although there is no clear evidence of alterations in growth or neurocognitive development, subtle cardiovascular abnormalities have been documented. Therefore, there is still uncertainty about the need of treatment across all ages, and the management should be based on the age of the child, the etiology, and the degree of TSH elevation, as well as on other patient factors. This review updates current evidences on diagnosis and management of mild hypothyroidism in childhood.

Medical relevance of protein-truncating variants across 337,205 individuals in the UK Biobank study

Protein-truncating variants can have profound effects on gene function and are critical for clinical genome interpretation and generating therapeutic hypotheses, but their relevance to medical phenotypes has not been systematically assessed. Here, we characterize the effect of 18,228 protein-truncating variants across 135 phenotypes from the UK Biobank and find 27 associations between medical phenotypes and protein-truncating variants in genes outside the major histocompatibility complex. We perform phenome-wide analyses and directly measure the effect in homozygous carriers, commonly referred to as “human knockouts,” across medical phenotypes for genes implicated as being protective against disease or associated with at least one phenotype in our study. We find several genes with strong pleiotropic or non-additive effects. Our results illustrate the importance of protein-truncating variants in a variety of diseases.

Protein-truncating variants (PTVs) are predicted to significantly affect a gene’s function and, thus, human traits. Here, DeBoever et al. systematically analyze PTVs in more than 300,000 individuals across 135 phenotypes and identify 27 associations between PTVs and medical conditions.

Generation and characterization of a hypothyroidism rat model with truncated thyroid stimulating hormone receptor

Thyroid stimulating hormone receptor (TSHR), a G-protein-coupled receptor, is important for thyroid development and growth. In several cases, frameshift and/or nonsense mutations in TSHR were found in the patients with congenital hypothyroidism (CH), however they have not been functionally studied in an animal model. In the present work, we generated a unique Tshr Df/Df rat model that recapitulates the phenotypes in TSHR Y444X patient by CRISPR/Cas genome editing technology. In this rat model, TSHR is truncated at the second transmembrane domain, leading to CH phenotypes as what was observed in the patients, including dwarf, thyroid aplasia, infertility, TSH resistant as well as low serum thyroid hormone levels. The phenotypes can be reversed, at least partially, by levothyroxine (L-T4) treatment after weaning. The thyroid development is severely impaired in the Tshr Df/Df rats due to the suppression of the thyroid specific genes, i.e., thyroperoxidase (Tpo), thyroglobulin (Tg) and sodium iodide symporter (Nis), at both mRNA and protein levels. In conclusion, the Tshr Df/Df rat serves as a brand new genetic model to study CH in human, and will greatly help to shed light into the development of terminal organs that are sensitive to thyroid hormones.

Identification and functional characterization of a novel thyrotropin receptor mutation (V87L) in a Chinese woman with subclinical hypothyroidism

The thyrotropin receptor (TSHR) gene has been defined as a highly mutable gene. Mutations in the TSHR gene result in either gain or loss of the receptor function. Subclinical hypothyroidism (SH) is a clinical condition defined as an elevated serum TSH level associated with normal free thyroxine and free triiodothyronine. Chronic autoimmune thyroiditis is the most frequent cause of subclinical hypothyroidism in adults. In rare cases, a loss-of-function mutation of TSHR is the cause of SH. In the present study, a novel TSHR mutation (V87L; confirmed to be a loss-of-function mutation) was identified in a 59-year-old Chinese woman, as the potential cause of the patient's subclinical hypothyroidism. The case may provide valuable insight into the etiology of SH.

Subclinical hypothyroidism in childhood - current knowledge and open issues

Subclinical hypothyroidism is defined as serum levels of TSH above the upper limit of the reference range, in the presence of normal concentrations of total T₄ or free T₄. This biochemical profile might be an indication of mild hypothyroidism, with a potential increased risk of metabolic abnormalities and cardiovascular disease recorded among adults. Whether subclinical hypothyroidism results in adverse health outcomes among children is a matter of debate and so management of this condition remains challenging. Mild forms of untreated subclinical hypothyroidism do not seem to be associated with impairments in growth, bone health or neurocognitive outcome. However, ongoing scientific investigations have highlighted the presence of subtle proatherogenic abnormalities among children with modest elevations in their TSH levels. Although current findings are insufficient to recommend levothyroxine treatment for all children with mild asymptomatic forms of subclinical hypothyroidism, they highlight the potential need for assessment of cardiovascular risk among children with this condition. Increased understanding of the early metabolic risk factors associated with subclinical hypothyroidism in childhood will help to improve the management of affected individuals.

Rare thyroid non-neoplastic diseases

Rare diseases are usually defined as entities affecting less than 1 person per 2,000. About 7,000 different rare entities are distinguished and, among them, rare diseases of the thyroid gland. Although not frequent, they can be found in the everyday practice of endocrinologists and should be considered in differential diagnosis. Rare non-neoplastic thyroid diseases will be discussed. Congenital hypothyroidism's frequency is relatively high and its early treatment is of vital importance for neonatal psychomotor development; CH is caused primarily by thyroid dysgenesis (85%) or dyshormonogenesis (10-15%), although secondary defects - hypothalamic and pituitary - can also be found; up to 40% of cases diagnosed on neonatal screening are transient. Inherited abnormalities of thyroid hormone binding proteins (TBG, TBP and albumin) include alterations in their concentration or affinity for iodothyronines, this leads to laboratory test abnormalities, although usually with normal free hormones and clinical euthyroidism. Thyroid hormone resistance is most commonly found in THRB gene mutations and more rarely in THRA mutations; in some cases both genes are unchanged (non-TR RTH). Recently the term 'reduced sensitivity to thyroid hormones' was introduced, which encompass not only iodothyronine receptor defects but also their defective transmembrane transport or metabolism. Rare causes of hyperthyroidism are: activating mutations in TSHR or GNAS genes, pituitary adenomas, differentiated thyroid cancer or gestational trophoblastic disease; congenital hyperthyroidism cases are also seen, although less frequently than CH. Like other organs and tissues, the thyroid can be affected by different inflammatory and infectious processes, including tuberculosis and sarcoidosis. In most of the rare thyroid diseases genetic factors play a key role, many of them can be classified as monogenic disorders. Although there are still some limitations, progress has been made in our understanding of rare thyroid diseases etiopathogenesis, and, thanks to these studies, also in our understanding of how normal thyroid gland functions.

Long-term outcome of loss-of-function mutations in thyrotropin receptor gene

BACKGROUND

Loss-of-function mutations in the thyrotropin receptor (TSHR) gene lead to resistance to TSH (RTSH) presenting with either congenital hypothyroidism (CH) or subclinical hypothyroidism (SCH). Despite several reports of patients with TSHR mutations, data on the long-term outcome of this condition are limited, and no consensus exists on the need for hormone replacement therapy. The aim of the present study was to assess the long-term outcome in children and adolescents with RTSH due to TSHR mutations.

METHODS

The TSHR gene was sequenced in 94 subjects (aged 3 days-21 years) with either nonautoimmune SCH or CH with RTSH.

RESULTS

Twenty-seven subjects (29%) carried mutations in TSHR. Fifteen infants were identified by neonatal screening, and the other 79 patients were detected in the process of testing for various other conditions or because of family occurrence of thyroid test abnormalities. Six different mutations were identified: c.484C>G (p.P162A), c.202C>T (p.P68S), c.790C>T (p.P264S), c.269A>C (p.Q90P), c.1957C>G (p.L653V), and c.1347C>T (p.R450C). Twelve subjects were homozygous, three were compound heterozygous, and 12 were heterozygous. Mean serum TSH levels at diagnosis and at last visit were significantly higher in patients with TSHR mutations than in those without mutations (29.04 vs. 14.15, p=0.002; 31.73 vs. 6.19, p<0.0001, respectively). Homozygous patients had a more severe phenotype (TSH 53.6 vs. 9.24, p<0.0001). Mean serum free thyroxine (fT4) levels at the last visit were significantly lower than at the first visit in the homozygous individuals (p=0.05) for a follow-up period of as long as 11 years. Heterozygous subjects had only mild hyperthyrotropinemia with stable TSH levels. However, homozygous subjects showed a trend toward increased TSH and decreased fT4 with time.

CONCLUSION

SCH in heterozygotes with TSHR mutations is a stable compensated condition with an appropriately adjusted set point for pituitary-thyroid feedback that does not require replacement therapy. However, homozygous subjects, with incompletely compensated SCH, show reduced fT4 levels over time and may require levothyroxine treatment. Replacement therapy should be considered on an individual basis, and long-term follow up is recommended.

Novel insights on thyroid-stimulating hormone receptor signal transduction

The TSH receptor (TSHR) is a member of the glycoprotein hormone receptors, a subfamily of family A G protein-coupled receptors. The TSHR is of great importance for the growth and function of the thyroid gland. The TSHR and its endogenous ligand TSH are pivotal proteins with respect to a variety of physiological functions and malfunctions. The molecular events of TSHR regulation can be summarized as a process of signal transduction, including signal reception, conversion, and amplification. The steps during signal transduction from the extra- to the intracellular sites of the cell are not yet comprehensively understood. However, essential new insights have been achieved in recent years on the interrelated mechanisms at the extracellular region, the transmembrane domain, and intracellular components. This review contains a critical summary of available knowledge of the molecular mechanisms of signal transduction at the TSHR, for example, the key amino acids involved in hormone binding or in the structural conformational changes that lead to G protein activation or signaling regulation. Aspects of TSHR oligomerization, signaling promiscuity, signaling selectivity, phenotypes of genetic variations, and potential extrathyroidal receptor activity are also considered, because these are relevant to an understanding of the overall function of the TSHR, including physiological, pathophysiological, and pharmacological perspectives. Directions for future research are discussed.