Congenital hypothyroidism with impaired thyroid response to thyrotropin (TSH) and absent circulating thyroglobulin: evidence for a new inactivating mutation of the TSH receptor gene

Targeting trafficking as a therapeutic avenue for misfolded GPCRs leading to endocrine diseases

G protein-coupled receptors (GPCRs) are plasma membrane proteins associated with an array of functions. Mutations in these receptors lead to a number of genetic diseases, including diseases involving the endocrine system. A particular subset of loss-of-function mutant GPCRs are misfolded receptors unable to traffic to their site of function (i.e. the cell surface plasma membrane). Endocrine disorders in humans caused by GPCR misfolding include, among others, hypo- and hyper-gonadotropic hypogonadism, morbid obesity, familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, X-linked nephrogenic diabetes insipidus, congenital hypothyroidism, and familial glucocorticoid resistance. Several in vitro and in vivo experimental approaches have been employed to restore function of some misfolded GPCRs linked to endocrine disfunction. The most promising approach is by employing pharmacological chaperones or pharmacoperones, which assist abnormally and incompletely folded proteins to refold correctly and adopt a more stable configuration to pass the scrutiny of the cell's quality control system, thereby correcting misrouting. This review covers the most important aspects that regulate folding and traffic of newly synthesized proteins, as well as the experimental approaches targeted to overcome protein misfolding, with special focus on GPCRs involved in endocrine diseases.

Identification of compound heterozygous TSHR mutations (R109Q and R450H) in a patient with nonclassic TSH resistance and functional characterization of the mutant receptors

Genetic defects of the TSH receptor (TSHR) signaling pathway cause a form of congenital hypothyroidism (CH) known as TSH resistance. Consistent with the physiological understanding that thyroidal iodine uptake is up-regulated by TSHR signaling, most patients with TSH resistance have low to normal thyroidal ¹²³I uptake representing the classic TSH resistance. However, paradoxically high ¹²³I uptake was reported in four molecularly-confirmed patients indicating nonclassic TSH resistance. Here, we report the fifth patient with the nonclassic phenotype. He was a 12-yr-old CH patient and treated with levothyroxine. At the age 11 yr, he showed slightly small thyroid gland and elevated thyroidal ¹²³I uptake. Genetic analysis showed that he was compound heterozygous for two known missense mutations (Arg109Gln and Arg450His) in the TSHR gene. Further, the signal transduction of Arg109Gln-TSHR was defective in both Gs- and Gq-coupled pathways, while Arg450His-TSHR showed Gq-dominant defect. ¹²³I uptake was evaluated earlier in 16 patients with TSH resistance, and a correlation between TSH levels and ¹²³I uptake was shown in patients with specific genotypes (Arg450His or Leu653Val). Collectively, we have re-confirmed that the emergence of the nonclassic phenotype requires two factors: mutant TSHR with Gq-dominant coupling defect and relatively high levels of serum TSH.

Resistance to thyrotropin

Resistance to thyrotropin (RTSH) is broadly defined as reduced sensitivity of thyroid follicle cells to stimulation by biologically active TSH due to genetic defects. Affected individuals have elevated serum TSH in the absence of goiter, with the severity ranging from nongoitrous isolated hyperthyrotropinemia to severe congenital hypothyroidism with thyroid hypoplasia. Conceptually, defects leading to RTSH impair both aspects of TSH-mediated action, namely thyroid hormone synthesis and gland growth. These include inactivating mutations in the genes encoding the TSH receptor and the PAX8 transcription factor. A common third cause has been genetically mapped to a locus on chromosome 15, but the underlying pathophysiology has not yet been elucidated. This review provides a succinct overview of currently defined causes of nonsyndromic RTSH, their differential diagnoses (autoimmune; partial iodine organification defects; syndromic forms of RTSH) and implications for the clinical approach to patients with RTSH.

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.

Frequency and effect on serum TSH of phosphodiesterase 8B (PDE8B) gene polymorphisms in patients with sporadic nonautoimmune subclinical hypothyroidism

BACKGROUND

Nonautoimmune subclinical hypothyroidism (NSH) is characterized by elevated serum TSH in presence of normal thyroid hormone levels and absence of anti-thyroid antibodies. As result of a genomic-wide study, a strong association between three polymorphic variants in intron 1 of human PDE8B gene (rs4704397, rs6885099, rs2046045) and serum TSH has been reported in euthyroid subjects.

AIM

The aim of this study was to evaluate frequency and effects on serum TSH of PDE8B gene polymorphisms in patients with sporadic NSH and verify if differences in serum TSH levels are associated to these polymorphic variants.

SUBJECTS AND METHODS

A total of 58 Italian selected patients affected by NSH, with elevated serum TSH, normal FT3 and FT4 and without TSHr gene mutations, were subjected to genotyping for specific single nucleotide polymorphism of PDE8B gene.

RESULTS

In all patients, the integrity of TSH receptor gene was attested. The ancestral allele associated with increased serum TSH was present in 42/58 patients (72.4 %) for rs4704397, in 42/58 patients (72.4 %) for rs6885099 and in 44/58 patients (75.9 %) for rs2046045. However, similar values of serum TSH were detected in patients with minor or major allele for each polymorphism.

CONCLUSIONS

A prevalence of the minor allele of PDE8B gene polymorphism associated with elevated serum levels of TSH was demonstrated in patients affected by sporadic NSH; however, significant differences in circulating TSH in patients with minor or major alleles for each polymorphism were not identified demonstrating the lack of association between the polymorphisms and serum TSH levels in these patients.

Frequency of the GPR7 Tyr135Phe allelic variant in lean and obese subjects

BACKGROUND

GPR7, the endogenous coupled receptor for neuropeptide B and neuropeptide W, is expressed in several regions of the central nervous system, which are involved in the regulation of feeding behavior. GPR7 affects the regulation of energy balance through a mechanism independent of leptin and melanocortin pathways.

AIM

Aim of this study was to investigate whether GPR7 gene mutations can be detected in human subjects and, in that event, if they are differently distributed among lean and obese subjects.

SUBJECTS AND METHODS

The coding region of GPR7 were sequenced in 150 obese patients and 100 normal-weight unrelated controls. Functional studies of the allelic variants were performed.

RESULTS

One genetic GPR7 variant was found (Tyr135Phe - rs33977775) in obese subjects (13.3%) and lean control (25%). Functional studies did not reveal significant differences between the wild type and the Tyr135Phe allelic variants in their NPW-mediated capacity to inhibit forskolin-induced cAMP production.

CONCLUSIONS

Screening of GPR7 gene mutations among lean and obese subjects revealed a Tyr135Phe allelic variant that was fairly common in the study population. As indicated by in vitro and in silico studies, this variant is unlikely to cause a functional derangement of the receptor.

The molecular causes of thyroid dysgenesis: a systematic review

BACKGROUND

Congenital hypothyroidism (CH) is a frequent disease occurring with an incidence of about 1/2500 newborns/year. In 80-85% of the cases CH is caused by alterations in thyroid morphogenesis, generally indicated by the term "thyroid dysgenesis" (TD). TD is generally a sporadic disease, but in about 5% of the cases a genetic origin has been demonstrated. In these cases, mutations in genes playing a role during thyroid morphogenesis (NKX2-1, PAX8, FOXE1, NKX2-5, TSHR) have been reported.

AIM

This work reviews the main steps of thyroid morphogenesis and all the genetic alterations associated with TD and published in the literature.

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

Thyroid-stimulating hormone receptor (TSHR) loss-of-function (LOF) mutations lead to a wide spectrum of phenotypes, ranging from severe congenital hypothyroidism (CH) to mild euthyroid hyperthyrotropinemia. The degree of TSH resistance depends on the severity of the impairment of the receptor function caused by the mutation and on the number of mutated alleles In this review data about genotype-phenotype correlation and criteria for clinical work-up will be presented and discussed. Complete TSH resistance due to biallelic LOF TSHR mutations must be suspected in all patients with severe not syndromic CH and severe thyroid hypoplasia diagnosed at birth by neonatal screening. Partial forms of TSH resistance show a more heterogeneous hormonal and clinical pattern . In these cases TSH serum levels are above the upper limit of normal range for the age but with a very variable pattern, free thyroxine (T4) concentrations are within the normal range and thyroid size can be normal or hypoplastic at ultrasound scan. An early substitutive treatment with L-T4 must be mandatory in all patients with severe CH due to complete uncompensated TSH resistance diagnosed at birth by neonatal screening. The usefulness of substitutive treatment appears much more controversial inpatients with subclinical hypothyroidism due to partial TSH resistance in whom the increased TSH concentration should be able to compensate the mild functional impairment of the mutant receptor. Together with standard criteria we recommend also an accurate clinical work-up to select patients who are candidates for a LOF TSHR mutation.

Non-autoimmune subclinical hypothyroidism due to a mutation in TSH receptor: report on two brothers

Subclinical hypothyroidism (SH) is a condition characterized by a mild persistent thyroid failure. The main cause is represented by autoimmune thyroiditis, but mutations in genes encoding proteins involved in TSH pathway are thought to be responsible for SH, particularly in cases arising in familial settings. Patients with the syndrome of TSH unresponsiveness may have compensated or overt hypothyroidism with a wide spectrum of clinical and morphological alterations depending on the degree of impairment of TSH-receptor (TSH-R) function. We describe the case of two brothers with non autoimmune SH carrying the same heterozygous mutation in the extracellular domain of TSH-R and presenting with different clinical, biochemical and morphological features. The first one had only a slight persistent elevation of TSH, a normal thyroid ultrasound and did never require l- thyroxine (L-T4) replacement treatment. The second one had a neonatal persistent moderate TSH levels increase associated with a thyroid gland hypoplasia and was treated with L-T4 since the first months of life.

These two cases support the recent association of TSH-R mutations inheritance as an autosomal dominant pattern with variable expressivity and suggest that the decision to start replacement therapy in patients with persistent SH due to TSH resistance should be individualized.

TSH receptor function is required for normal thyroid differentiation in zebrafish

TSH is the primary physiological regulator of thyroid gland function. The effects of TSH on thyroid cells are mediated via activation of its membrane receptor [TSH receptor (TSHR)]. In this study, we examined functional thyroid differentiation in zebrafish and characterized the role of TSHR signaling during thyroid organogenesis. Cloning of a cDNA encoding zebrafish Tshr showed conservation of primary structure and functional properties between zebrafish and mammalian TSHR. In situ hybridization confirmed that the thyroid is the major site of tshr expression during zebrafish development. In addition, we identified tpo, iyd, duox, and duoxa as novel thyroid differentiation markers in zebrafish. Temporal analyses of differentiation marker expression demonstrated the induction of an early thyroid differentiation program along with thyroid budding, followed by a delayed onset of duox and duoxa expression coincident with thyroid hormone synthesis. Furthermore, comparative analyses in mouse and zebrafish revealed for the first time a thyroid-enriched expression of cell death regulators of the B-cell lymphoma 2 family during early thyroid morphogenesis. Knockdown of tshr function by morpholino microinjection into embryos did not affect early thyroid morphogenesis but caused defects in later functional differentiation. The thyroid phenotype observed in tshr morphants at later stages comprised a reduction in number and size of functional follicles, down-regulation of differentiation markers, as well as reduced thyroid transcription factor expression. A comparison of our results with phenotypes observed in mouse models of defective TSHR and cAMP signaling highlights the value of zebrafish as a model to enhance the understanding of functional differentiation in the vertebrate thyroid.

An intracellular loop (IL2) residue confers different basal constitutive activities to the human lutropin receptor and human thyrotropin receptor through structural communication between IL2 and helix 6, via helix 3

The human lutropin receptor (hLHR) and human TSH receptor (hTSHR) are G protein-coupled receptors that play key roles in reproductive and thyroid physiology, respectively. We show using a quantitative assessment of cAMP production as a function of cell surface receptor expression that the hTSHR possesses greater basal constitutive activity than the hLHR. Further studies were undertaken to test the hypothesis that different potential Gs-coupling motifs identified in IL2 of the hTSHR and hLHR contribute to their different basal constitutive activities. Although mutating the receptors to interchange their potential Gs-coupling motifs reversed their relative activities, we show this to be due to the swapping of one IL2 residue (Q476 in the hLHR; R531 in the hTSHR). Molecular dynamics simulations show that the effect of the hLHR(Q476R) mutation, switching the structural features of the hLHR toward those of the hTSHR, is greater than the switching effect of the hTSHR(R531Q) mutant toward the hLHR. The structural model of the hLHR(Q476R) mutant can be considered as a hybrid of wild-type (wt) hTSHR and constitutively active mutant hLHR forms. In this hLHR(Q476R) mutant, IL2 adopts a structure similar to IL2 of the wt hTSHR, but it shares with the hLHR constitutively active mutant the solvent exposure and the reciprocal arrangement of helices 3, 5, and 6, including the weakening of the wt native R3.50-D6.30 interaction. Our results suggest a H3-mediated structural connection between IL2 and the cytosolic extension of H6. Thus, IL2 contributes significantly to the inactive and active state ensembles of these G protein-coupled receptors.

Clinical significance of heterozygous carriers associated with compensated hypothyroidism in R450H, a common inactivating mutation of the thyrotropin receptor gene in Japanese

Loss-of-function mutations in the thyrotropin receptor (TSHR) gene were described as a syndrome characterized by thyroid hyposensivity to biologically active TSH, ranging from euthyroid to severe hypothyroidism. In Japanese, a common mutation in the TSHR gene is R450H, which demonstrated moderately impaired receptor function. We studied six subjects of Japanese origin whose major abnormality was persistent hyperthyrotropinemia by genetic sequence analysis of the TSHR gene. Three subjects were homozygous for the R450H mutation, whereas the three remaining subjects were single heterozygous. Homozygous subjects displayed mild hypothyroidism confirmed by moderately elevated basal TSH levels and excessive TSH response to TRH administration. Heterozygous subjects also demonstrated fully or partially compensated hypothyroidism, but less severe than that of homozygous subjects. More frequent involvement of the R450H mutation in the TSHR gene in Japanese was identified. In addition, a good correlation between phenotype and genotype was demonstrated in respect to biochemical analysis and drug dosage. Our observations showed clinical significance of heterozygosity associated with compensated hypothyroidism in spite of only mildly impaired receptor function.

Genetics of congenital hypothyroidism

Congenital hypothyroidism is the most common neonatal metabolic disorder and results in severe neurodevelopmental impairment and infertility if untreated. Congenital hypothyroidism is usually sporadic but up to 2% of thyroid dysgenesis is familial, and congenital hypothyroidism caused by organification defects is often recessively inherited. The candidate genes associated with this genetically heterogeneous disorder form two main groups: those causing thyroid gland dysgenesis and those causing dyshormonogenesis. Genes associated with thyroid gland dysgenesis include the TSH receptor in non-syndromic congenital hypothyroidism, and Gsalpha and the thyroid transcription factors (TTF-1, TTF-2, and Pax-8), associated with different complex syndromes that include congenital hypothyroidism. Among those causing dyshormonogenesis, the thyroid peroxidase and thyroglobulin genes were initially described, and more recently PDS (Pendred syndrome), NIS (sodium iodide symporter), and THOX2 (thyroid oxidase 2) gene defects. There is also early evidence for a third group of congenital hypothyroid conditions associated with iodothyronine transporter defects associated with severe neurological sequelae. This review focuses on the genetic aspects of primary congenital hypothyroidism.

TSH receptor and Gs(alpha) genetic analysis in children with Down's syndrome and subclinical hypothyroidism

The prevalence of thyroid diseases in children with Down's syndrome (DS) is about 3%. The most frequently observed condition is autoimmune subclinical hypothyroidism (SH). Autoimmune SH must be distinguished from defects in the biological activity of the TSH molecule or from the rare inherited condition of thyroid resistance to TSH. To investigate this last aspect we studied 12 patients with DS that had moderately elevated TSH with normal free thyroid hormones without signs of autoimmunity. For the genetic analysis the genomic DNA was extracted from peripheral lymphocytes. All the exons of the TSH receptor (TSHr) and Gs(alpha) genes were sequenced. The genetic analysis of the TSHr gene revealed the presence of four polymorphic variants. In two patients there was an allelic variant in the exon 1 (Pro52Thr--in one patient in the heterozygous state and in the other as a homozygous substitution). In one patient there was an allelic variant in the exon 1 (Asp36His) in the heterozygous state. In 11 patients there was a silent polymorphism in the exon 7 at nucleotide 561. All patients were homozygous for a silent polymorphism in the exon 9 at nucleotide 855. No inactivating mutations of TSHr or Gs(alpha) genes were identified in the 12 patients. In conclusion, our results seem to exclude the role of TSHr or Gs(alpha) gene mutations in the pathogenesis of the non-autoimmune SH observed in some children with DS.

Resistance to thyrotropin

Resistance to TSH is a syndrome of reduced sensitivity to a biologically active TSH molecule. Subjects have elevated TSH levels but no goiter. However, thyroid hormone concentration may vary from normal to very high, depending on the severity of the resistance. Individuals with very high TSH, low T4 and hypoplastic thyroid glands can be mistakenly diagnosed as having primary hypothyroidism due to a defective development of the thyroid gland. Those with normal or slightly decreased T4 can be misdiagnosed as having central hypothyroidism especially if their serum TSH concentration is only slightly elevated. Mutations in the TSH receptor (TSHr) gene have been reported in 16 families with homozygous or compound heterozygous inheritance. The mutant TSHrs show reduced or no function due to either altered ligand binding or defect in membrane targeting. Some individuals, heretozygous for a TSHr gene mutation can present mild resistance to TSH manifesting as euthyroidism with slight hyperthyrotropinemia. A larger proportion of families express the phenotype of resistance to TSH in the absence of a TSHr defect. In many the inheritance is dominant and the genetic cause has not been yet determined.