The hypothyroidism in an inbred kindred with congenital thyroid hormone and glucocorticoid deficiency is due to a mutation producing a truncated thyrotropin receptor

MicroRNA-489-3p plays a significant role in congenital hypothyroidism through regulating neuronal cell apoptosis via targeting translationally controlled tumor protein 1

Accumulating reports have indicated that congenital hypothyroidism (CH) is an endocrine disorder caused by underdeveloped thyroid gland or thyroid dyshormonogenesis. It has been also reported that certain microRNAs (miRNAs) may exert protective effects against the development of CH. However, whether miR-489-3p regulates CH progression remains unclear. The aim of the present study was to investigate the effects of miR-489-3p on CH and elucidate the underlying mechanisms. Therefore, Sprague Dawley rats were injected with propylthiouracil (50 mg/day) to establish a CH model. Reverse transcription-quantitative PCR (RT-qPCR) assay demonstrated that miR-489-3p was upregulated in the hippocampal tissues of CH rats. Furthermore, the TargetScan software was employed to predict the target gene of miR-489-3p, and a dual luciferase reporter assay revealed that translationally controlled tumor protein 1 (TPT1) was directly targeted by miR-489-3p. Additionally, RT-qPCR and western blot assays suggested that TPT1 was markedly downregulated in the hippocampal tissues of CH rats compared with control rats. In addition, inhibitor control, miR-489-3p inhibitor, control-shRNA or TPT1-shRNA were injected into CH rats. The results of the open-field and forced swimming tests revealed that miR-489-3p inhibitor notably improved the behavior of CH rats. Flow cytometry was applied to explore the effects of miR-489-3p inhibitor on neuronal cell apoptosis, and the findings indicated that miR-489-3p inhibitor attenuated CH-induced neuronal cell apoptosis, whereas these effects were reversed by treatment with miR-489-3p inhibitor and TPT1-shRNA. Finally, the function of miR-489-3p in neuronal cells was investigated in vitro. Neuronal cell viability, apoptosis and the expression of apoptosis-related proteins were determined using MTT assay, flow cytometry and western blot analysis, respectively. The results demonstrated that miR-489-3p inhibitor enhanced cell viability, suppressed apoptosis and upregulated Pim-3, phosphorylated (p)-Bad (Ser112) and Bcl-xL expression. Rescue experiments indicated that these effects were reversed following silencing of TPT1. Taken together, the findings of the present study demonstrated that miR-489-3p inhibitor could relieve CH-induced neurological damage through regulating TPT1 expression.

Congenital Hypothyroidism Patients With Thyroid Hormone Receptor Variants Are Not Rare: A Systematic Review

Background

Primary congenital hypothyroidism (CH) is a common endocrine and metabolic disease. Various genetic factors, including the thyroid hormone receptor (TSHR), play an important role in CH.

Aim

To explore the occurrence of pathogenic TSHR variants in CH.

Methods

We searched published articles in PubMed, Web of Science, and Cochrane Library databases, from the establishment of the database to September 26, 2021. Studies with sequencing partial or full exons of TSHR in CH patients were included. Gene polymorphism was excluded.

Results

A total of 66 articles (44 case-control studies and 22 case reports) were selected from the database. Though case-control studies, we found the incidence of pathogenic TSHR variants were not rare (range from 0% to 30.6%) and varied greatly in different countries and race. The pathogenic genotypes varied in different regions. All the variants were “loss-of-function” mutations, in which the p.(Arg450His) variant was the most common variant. In addition, we analyzed the case reports and found that CH patients with a family genetic background expressed homozygous genotypes. Homozygotes had more obvious symptoms of hypothyroidism and higher risk of comorbidities than heterozygotes.

Conclusion

Pathogenic TSHR variants are not uncommon cause of the CH, especially in the Arabs. The role of TSHR gene detection in the treatment of children with CH needs to be further studied.

The Thyrotropin Receptor Mutation Database Update

The thyrotropin receptor (TSHR) mutation database, consisting of all known TSHR mutations and their clinical characterizations, was established in 1999. The database contents are updated here with the same website (tsh-receptor-mutation-database.org). The new database contains 638 cases of TSHR mutations: 448 cases of gain of function mutations (7 novel mutations and 41 new cases for previously described mutations since its last update in 2012) and 190 cases of loss of function mutations (28 novel mutations and 31 new cases for previously described mutations since its last update in 2012). This database is continuously updated and allows for rapid validation of patient TSHR mutations causing hyper- or hypothyroidism or insensitivity to TSH.

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.

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.

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.

Next-generation sequencing analysis of TSHR in 384 Chinese subclinical congenital hypothyroidism (CH) and CH patients

BACKGROUND

Defects in the human TSHR gene are reported to be one of the causes of CH due to thyroid dysgenesis, the purpose of this study was to examine the TSHR mutation spectrum and prevalence in congenital hypothyroidism (CH) and subclinical congenital hypothyroidism (SCH) patients in the Guangxi Zhuang Autonomous Region of China and to evaluate the genotype-phenotype correlations.

METHODS

Blood samples were collected from 384 patients including 240 CH and 144 SCH patients in Guangxi, China. Genomic DNA was extracted from peripheral blood leukocytes. All exons of the 11 known CH associated genes including TSHR together with their exon-intron boundaries were screened by next-generation sequencing (NGS).

RESULTS

NGS analysis of TSHR revealed nine different variants in ten individuals. Six (4.2%) of 144 patients with SCH were found to harbor monoallelic TSHR variants. Four (1.6%) of 240 patients with CH harbored TSHR variants combined with another monoallelic mutation in either DUOX2 or TG gene. The present study identified five novel variants c.1838A>G (p.Y613C), c.1576G>A (p.A526T), c.2087T>G (p.F696C), c.1631G>A (p.G544E) and c.2051C>A (p.A684D) in TSHR, seven known pathogenic variants c.1349G>A (p.R450H), c.326G>A (p.R109Q), c.2066T>G (p.V689G) and c.2272G>A (p.E758K) in TSHR, IVS3+2T>G in TG, and c.1588A>T (p.K530X) and c.2635G>A (p.E879K) in DUOX2. The previously reported hotspot mutation p.R450H was found in only one SCH patient.

CONCLUSION

The prevalence of TSHR mutations was 1.6% in CH patients and 4.2% in SCH patients in Guangxi Zhuang Autonomous Region of China. Monoallelic TSHR pathogenic variants were associated with SCH, while TSHR pathogenic variants combined with monoallelic mutations in DUOX2 or TG gene might contribute to CH. Our study expands the TSHR mutation spectrum and provides the best estimation of mutation rate for SCH and CH patients in this Chinese population.

TSH resistance revisited

Genetic defects of hormone receptors are the most common form of end-organ hormone resistance. One example of such defects is TSH resistance, which is caused by biallelic inactivating mutations in the TSH receptor gene (TSHR). TSH, a master regulator of thyroid functions, affects virtually all cellular processes involving thyroid hormone production, including thyroidal iodine uptake, thyroglobulin iodination, reuptake of iodinated thyroglobulin and thyroid cell growth. Resistance to TSH results in defective thyroid hormone production from the neonatal period, namely congenital hypothyroidism. Classically, clinical phenotypes of TSH resistance due to inactivating TSHR mutations were thought to vary depending on the residual mutant receptor activity. Nonfunctional mutations in the two alleles produce severe thyroid hypoplasia with overt hypothyroidism (uncompensated TSH resistance), while hypomorphic mutations in at least one allele produce normal-sized thyroid gland with preserved hormone-producing capacity (compensated TSH resistance). More recently, a new subgroup of TSH resistance (nonclassic TSH resistance) that is characterized by paradoxically high thyroidal iodine uptake has been reported. In this article, the pathophysiology and clinical features of TSH resistance due to inactivating TSHR mutations are reviewed, with particular attention to the nonclassic form.

A nonsense thyrotropin receptor gene mutation (R609X) is associated with congenital hypothyroidism and heart defects

Congenital hypothyroidism (CH), one of the most important preventable causes of mental retardation, is a clinical condition characterized by thyroid hormone deficiency in newborns. CH is most often caused by defects in thyroid development leading to thyroid dysgenesis. The thyroid-stimulating hormone receptor (TSHR) is the main known gene causing thyroid dysgenesis in consanguineous families with CH. In this study, we aim to determine the genetic alteration in a case with congenital hypothyroidism and heart defects coming from a consanguineous family. We utilized genetic linkage analysis and direct sequencing to achieve our aim. Our results revealed that the family showed linkage to the TSHR locus, and we detected a homozygous nonsense mutation (R609X) in the case. Apart from other cases with the same mutation, our case had accompanying cardiac malformations. Although cardiac malformations are not uncommon in sporadic congenital hypothyroidism, here, they are reported for the first time with R609X mutation in a familial case.

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.

Consecutive mutational events in a TSHR allele of Arab families with resistance to thyroid stimulating hormone

BACKGROUND

Our laboratory identified six distinct inactivating TSHR gene mutations in Arab tribes living in Israel. We recently reported three nucleotide substitutions in exon 3 producing p.[L89L;Q90P] and one in exon 9 of the same allele producing p.P264S in Family A. Family B, reported herein, harbors the identical mutation in exon 3 only. We set to determine whether the mutations have common ancestral origin.

METHODS

Coding regions of the TSHR were sequenced and flanking microsatellite markers spanning 5.3 cM were used for haplotyping.

RESULTS

Two siblings of Family B were compound heterozygous for TSHR gene mutations. The paternal allele contained the exon 3 mutation and the maternal allele harbored a mutation in exon 10 (p.L653V). We investigated the possibility of a founder effect with subsequent mutational events for the presence of the same exon 3 mutation in different families. The haplotype of the allele harboring the exon 3 mutation in Family B was identical to that of Family A, also harboring the exon 9 mutation on the same allele, indicating that the latter occurred subsequently. The ancestral wild-type TSHR was present in Family B, suggesting that the mutation in exon 3 was also new in the history of that population.

CONCLUSIONS

It is more likely that two consecutive mutational events occurred on the ancestral wild-type allele instead of a recombination bringing exon 3 and exon 9 mutations together on the same allele. New mutational events contribute to the high prevalence of TSHR mutations in this population in addition to a founder effect and limited gene pool due to inbreeding.

Loss-of-function mutations in the thyrotropin receptor gene as a major determinant of hyperthyrotropinemia in a consanguineous community

CONTEXT

Resistance to TSH (RTSH) is a condition of impaired responsiveness of the thyroid gland to TSH, characterized by elevated serum TSH, low or normal thyroid hormone levels, and hypoplastic or normal-sized thyroid gland.

OBJECTIVES

The aim of the study was to evaluate the clinical course and the genotype-phenotype relationship of RTSH caused by two different TSH receptor (TSHR) gene mutations in a consanguineous population.

PATIENTS AND METHODS

We conducted a clinical and genetic investigation of 46 members of an extended family and 163 individuals living in the same town. In vitro functional studies of the mutant TSHRs were also performed.

RESULTS

Two TSHR gene mutations (P68S and L653V) were identified in 33 subjects occurring as homozygous L653V (five subjects), heterozygous L653V (20 subjects), heterozygous P68S (four subjects), and compound heterozygous L653V/P68S (four subjects). With the exception of one individual with concomitant autoimmune thyroid disease, all homozygotes and compound heterozygotes presented with compensated RTSH (high TSH with free T(4) and T(3) in the normal range). Only nine of 24 heterozygotes had mild hyperthyrotropinemia. The L653V mutation resulted in a higher serum TSH concentration and showed a more severe in vitro abnormality than P68S. Haplotype analysis predicted a founder of the L653V six to seven generations earlier, whereas the P68S is older. Cross-sectional and prospective longitudinal studies indicate that TSH and T(4) concentrations remain stable over time.

CONCLUSIONS

High frequency hyperthyrotropinemia in an Israeli Arab-Muslim consanguineous community is attributed to two inactivating TSHR gene mutations. Concordant genotype-phenotype was demonstrated clinically and by in vitro functional analysis. Retrospective and prospective studies indicate that in the absence of concomitant autoimmune thyroid disease, elevated TSH levels reflect stable compensated RTSH.

TSHR mutations as a cause of congenital hypothyroidism in Japan: a population-based genetic epidemiology study

CONTEXT

The prevalence of congenital hypothyroidism (CH) associated with mutations in the TSH receptor gene (TSHR) has not been established.

OBJECTIVE

We examined the frequency of TSHR mutations among patients with permanent primary CH and in the general population in Japan.

SUBJECTS AND METHODS

We enrolled 102 patients with permanent primary CH [70 with "moderate to severe CH" (TSH, >or=10 mU/liter) and 32 with "mild CH" (TSH, 5-10 mU/liter)], who were identified through newborn screening among 353,000 newborns born in Kanagawa prefecture from October 1979 to June 2006. These subjects were tested for TSHR mutations by PCR-based direct sequencing. We further characterized molecular functions of identified mutant TSHRs in vitro.

RESULTS

We found three patients with moderate to severe CH who had biallelic mutations in TSHR and three patients with mild CH who had monoallelic mutations. Observed mutations included one previously characterized mutation (p.R450H) and three uncharacterized mutations (p.G132R, p.A204V, and p.D403N). In vitro experiments confirmed loss of functions of these four mutants. Among four mutations, p.R450H was particularly frequent: six of nine mutant alleles harbored p.R450H. All six alleles with p.R450H commonly carried a minor single nucleotide polymorphism, suggesting a founder effect. We estimated the prevalence of biallelic TSHR mutations to be 4.3% (three in 70) in Japanese patients with moderate to severe CH, and 1 in 118,000 (three in 353,000) in the general Japanese population.

CONCLUSIONS

In Japan, TSHR mutations are relatively common among patients with CH, and a founder mutation (p.R450H) accounts for about 70% of mutants.

An inactivating mutation within the first extracellular loop of the thyrotropin receptor impedes normal posttranslational maturation of the extracellular domain

The TSH receptor (TSHR), a member of the large family of G protein-coupled receptors, controls both function and growth of thyroid cells; hence, mutations of this receptor result in thyroid dysfunction. Here, we took advantage of the description of a new inactivating TSHR mutation (Q489H) in two brothers with hypothyroidism, to precise maturation, intracellular trafficking, exporting pathways, and activation mechanisms of this receptor. Functional characterization of the Q489H-TSHR in transiently transfected HEK293 cells showed cell surface expression, normal TSH binding affinity, and its inability to generate intracellular cAMP in response to TSH stimulation. Western blot analysis of the whole membrane proteins or proteins expressed at the cell surface showed that Q489H-TSHR expressed in HEK293 transfected cells are restricted to mannose-rich uncleaved receptor. Analysis of the export pathway toward cell surface indicated that both Q489H and wild-type receptors followed the same intracellular route to cell surface throughout endoplasmic reticulum and Golgi apparatus. This study shows that Q489H substitution impedes complete glycosylation of TSHR extracellular domain within the Golgi apparatus and that Q489H-TSHR expressed at the cell surface is unable to undergo intramolecular cleavage as well as to switch toward an active conformation under TSH stimulation. Altogether, our results show that 1) Q489H substitution within the first extracellular loop induces a misfolding of TSHR, blocking it into an inactive conformation and impeding complete glycosylation and intramolecular cleavage, and 2) a misfolded G protein-coupled receptor can bypass endoplasmic reticulum or Golgi apparatus quality control and reach the cell surface as an immature receptor.

Thyrotropin receptor and thyroid transcription factor-1 genes variant in Chinese children with congenital hypothyroidism

The aim of the present study was to investigate the mutation/variant of thyrotropin receptor (TSHR) and thyroid transcription factor-1 (TTF-1) genes in Chinese children with congenital hypothyroidism (CH). Seventy-nine and forty-nine Chinese children with CH were enrolled for molecular analysis of the TSHR gene and TTF-1 gene, respectively. One hundred normal children were evaluated as control. The coding regions of TSHR and TTF-1 genes were amplified by polymerase chain reaction and sequenced. Sequencing of the TSHR gene revealed a compound heterozygous variants (Pro52Thr/Val689Gly) and a heterozygous variant (Gly245Ser) in 2 of 79 patients. In 30 patients and 33 controls the normal cytosine at position 2181 in exon 10 of TSHR gene was replaced by a guanineCresulting in the replacement of Asp (727) by Glu. In 47 patients and 50 controls, the normal thymidine at position 561 in exon 7 of TSHR gene was replaced by a cytosine. This substitution did not change the amino acid in position 187. Sequencing of the TTF-1 gene revealed no mutation or polymorphism in 49 patients and 100 controls. In conclusion, three heterozygous variants (Pro52Thr, Gly245Ser, Val689Gly) of TSHR gene were firstly detected in Chinese children with CH. There were polymorphisms in exon 10 at nucleotide 2181 (C/G) and in exon 7 at nucleotide 561 (T/C) in TSHR gene. No mutation or polymorphism was detected in the coding region of TTF-1 gene. The mutation/variant of TSHR and TTF-1 genes is relatively rare in Chinese children with CH.

Familial glucocorticoid deficiency type 1 due to a novel compound heterozygous MC2R mutation

OBJECTIVE

Description of the clinical, biochemical and genetic features of a Polish patient with familial glucocorticoid deficiency.

METHODS

Detailed clinical investigation, hormonal analysis and sequencing of the coding region of the melanocortin 2 receptor (MC2R) gene in this patient.

RESULTS

We report on a 3-month-old boy with familial glucocorticoid deficiency who presented at the age of 3 months with skin hyperpigmentation, muscle weakness, mild jaundice and constipation. Hormonal analyses revealed high ACTH and TSH serum concentrations, low serum cortisol concentration along with normal blood electrolytes. On hydrocortisone supplementation, the disease symptoms disappeared and the child recovered completely. His physical and mental development progresses normally. Genetic analysis disclosed a novel compound heterozygous MC2R mutation p.Leu46fs and p.Val49Met.

CONCLUSION

The heterozygous p.Leu46fs mutation adds to the small number of MC2R nonsense mutations and is the first frameshift mutation within the first transmembrane domain of the receptor. According to molecular modeling the Val49Met mutation results in a structural change of the first transmembrane domain and in a potential novel interaction of the transmembrane domains I and VII.

Recombinant thyrotropin in the diagnosis of congenital hypothyroidism

CONTEXT

A modern approach to congenital hypothyroidism requires a definitive diagnosis of the underlying mechanisms; this can be achieved within the first weeks of life. When uncertainty persists, treatment is commenced, and the definitive diagnosis of congenital hypothyroidism is deferred to the age of 3 yr.

OBJECTIVES

The interruption of thyroid replacement treatment is perceived as risky by parents and physicians. The aim of this pilot study was to test the possibility of a definitive diagnosis during thyroid replacement treatment, using stimulation of thyroid tissue by recombinant human (rh)TSH.

SUBJECTS

Eight patients, three boys and five girls, age 5-15 yr (mean, 9.5+/-3.7 yr), with congenital hypothyroidism that had been diagnosed by the neonatal screening program, and having their diagnosis verified between the ages of 3-4 yr, were reevaluated while on thyroid replacement therapy.

INTERVENTIONS

Patients received im 0.6 mg/m2 rhTSH on two consecutive days.

RESULTS

rhTSH pharmacokinetics, maximal concentration, t1/2, and area under the curve in children were different as compared with adults. In the patients with intact TSH receptors, free T4 levels decreased after the first and the second injection of rhTSH (P=0.0137 and P=0.0149, respectively). All eight children showed identical scintigraphy after rhTSH administration as compared with thyroid replacement withdrawal.

CONCLUSIONS

The use of rhTSH is effective for definitive diagnosis of congenital hypothyroidism during thyroid replacement treatment, and no safety issues were encountered.

A novel mutation in the thyrotropin (thyroid-stimulating hormone) receptor gene in a case of congenital hypothyroidism

Congenital hypothyroidism (CH) occurs approximately with a frequency of 1 in 3000-4000 births, being a disease caused by defects in thyroid hormone synthesis associated either with goiter presence or with agenesis or ectopy of the thyroid gland. A study of some familial cases has allowed identification of mutations in several known genes, including that encode the thyroid-stimulating hormone receptor (TSHR). We report a familial case of CH that transmitted as a recessive trait and caused by a novel homozygous nonsense mutation in TSHR with an initial diagnosis of thyroid agenesis hypoplasia. Genomic DNA was obtained from two siblings and their parents; TSHR was amplified using pairs of overlapping exonic primers; and polymerase chain reaction products were automatically sequenced. The propositus was homozygous (genotype: M/M) for a novel C to G transversion (1431C>G), producing a nonsense mutation, Y444X, in the first intracellular loop of TSHR, rendering a truncated receptor. Thus, the observed unresponsiveness to TSHR may be due to absent insertion of the truncated receptor into the cell membrane (if it gets translated at all) or the truncation may lead to nonsense-mediated mRNA degradation (its unresponsive to TSH). Both parents were heterozygous (wWt/M) and unrelated, as known from family history. The other daughter was homozygous for both wild-type alleles (wWt/wWt).

Syndromes of hormone resistance in the hypothalamic-pituitary-thyroid axis

Forty years have elapsed since the first description of a syndrome of resistance in the hypothalamic-pituitary-thyroid axis, i.e., resistance to thyroid hormone action. In the last two decades many other types of resistance have been discovered, including resistance to the action of thyrotropin-releasing hormone (TRH), of thyroid-stimulating hormone (TSH), and of thyroid hormones (THs); the latter can be due not only to thyroid hormone receptor defects but also to alteration in genes encoding TH-specific transporters or components involved in metabolic pathways of THs. Moreover, alteration in genes encoding for second messengers may cause forms of resistance other than those due to receptor mutations, the most important one being that of an inactivating mutation in the G-protein alpha-subunit leading to TSH resistance in the setting of pseudohypoparathyroidism type 1a. Recognition of these rare thyroid disorders is of great importance not only for informed genetic counselling but also for avoiding diagnostic mistakes that may lead to incorrect and potentially dangerous treatments.

Inactivating mutations of G protein-coupled receptors and diseases: Structure-function insights and therapeutic implications

Since the discovery of the first rhodopsin mutation that causes retinitis pigmentosa in 1990, significant progresses have been made in elucidating the pathophysiology of diseases caused by inactivating mutations of G protein-coupled receptors (GPCRs). This review aims to compile the compelling evidence accumulated during the past 15 years demonstrating the etiologies of more than a dozen diseases caused by inactivating GPCR mutations. A generalized classification scheme, based on the life cycle of GPCRs, is proposed. Insights gained through detailed studies of these naturally occurring mutations into the structure-function relationship of these receptors are reviewed. Therapeutic approaches directed against the different classes of mutants are being developed. Since intracellular retention emerges as the most common defect, recent progresses aimed at correcting this defect through membrane permeable pharmacological chaperones are highlighted.

Identification and functional analysis of novel inactivating thyrotropin receptor mutations in patients with thyrotropin resistance

OBJECTIVE

We identified and analyzed novel thyrotropin (TSH) receptor mutations in three Japanese families with resistance to TSH.

DESIGN

The TSH receptor gene was sequenced and the mutations were determined. The mutant TSH receptors were transfected into COS-7 cells, and their functions were analyzed.

PATIENTS

The patients were compound-heterozygotes for the R450H mutation and novel mutations in the TSH receptor gene. The first patient was a compound-heterozygote for R450H and V473I. The second sibling possessed R450H and R519C. The third sibling had R450H and R519G.

RESULTS

The R450H mutant exhibited moderately impaired receptor functions and a moderately decreased cell surface expression in agreement with previous results. The V473I mutant exhibited an almost normal TSH binding, a slightly decreased cyclic adenosine monophosphate (cAMP) response, a moderately decreased inositolphosphate (IP) response, and an almost normal cell surface expression. TSH binding and TSH stimulation of cAMP and IPs were markedly decreased in the R519C and R519G mutants. Cell surface expression was decreased in the R519C mutant and negligible in the R519G mutant. All of these mutants showed normal intracellular synthesis of TSH receptors.

CONCLUSIONS

These novel inactivating mutations contribute to understanding of the structure-function relationship of the TSH receptor. To date, all of the patients with TSH resistance resulting from TSH receptor mutations identified in Japan possessed the R450H mutation at least in one allele. These observations suggest that the R450H mutation is a commonly observed TSH receptor mutation in patients with TSH resistance in Japan.

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.

Intracellular entrapment of wild-type TSH receptor by oligomerization with mutants linked to dominant TSH resistance

TSH resistance is one of the causes of congenital hypothyroidism with thyroid gland in situ. We recently identified families with dominant transmission of partial TSH resistance due to heterozygous inactivating mutations in TSH receptor (TSHR) gene. Although we documented a poor routing of TSHR mutants to the cell membrane, the mechanism responsible for dominant inheritance of partial TSH resistance remained unexplained. We therefore co-transfected Cos-7 cells with wild-type TSHR and mutant receptors found in these patients. A variable impairment of cAMP response to bTSH stimulation was observed, suggesting that inactive TSHR mutants can exert a dominant negative effect on wild-type TSHR. We then generated chimeric constructs of wild-type or inactive TSHR mutants fused to different reporters. By fluorescence microscopy and immunoblotting, we documented an intracellular entrapment, mainly in the endoplasmic reticulum, and reduced maturation of wild-type TSHR in the presence of inactive TSHR mutants. Finally, fluorescence resonance energy transfer and co-immunoprecipitation experiments were performed to study the molecular interactions between wild-type and mutant TSHRs. The results are in agreement with the presence of oligomers formed by wild-type and mutant receptors in the endoplasmic reticulum. Such physical interaction represents the molecular basis for the dominant negative effect of inactive TSHR mutants. These findings provide an explanation for the dominant transmission of partial TSH resistance. This is the first report linking dominant negative mutations of a G protein-coupled receptor to an abnormal endocrine phenotype in heterozygous patients.

Thyrotropin receptor gene mutations and TSH resistance: variable expressivity in the heterozygotes

OBJECTIVE

TSH resistance ranges from overt nonautoimmune hypothyroidism to subclinical hypothyroidism, defined as mild hyperthyrotrophinaemia but a euthyroid state clinically. To date, 23 inactivating mutations of the TSH receptor (TSHR) gene have been proven responsible for the clinical condition, but an absence of mutations in the TSHR gene has been reported for several cases of TSH resistance as well. In this paper, we aimed to investigate the actual role of the TSHR gene in the development of both subclinical and congenital hypothyroidism.

PATIENTS

14 hypothyroid newborns, 116 subclinical hypothyroid subjects and 120 healthy controls.

MEASUREMENTS

Through denaturing high performance liquid chromatography (DHPLC), we screened for mutations the TSHR gene (the proximal promoter, the exons and their flanking regions), and evaluated the association between serum TSH and functionally characterized alleles identified.

RESULTS

In the hypothyroid patients, one patient was heterozygous for a new missense variation, E34K, whereas two others patients were either homozygous or heterozygous for the P162A substitution. In the subclinical hypothyroid subjects, we detected only heterozygous substitutions: they are mostly new (123-124insTGCA, P27T, R46P, 555-561delTATTCTT, D403N, W488R, M527T), while six correspond to already published mutations (P162A, R109Q, L252P and three C41S). We only focused on those mutations that had been functionally characterized in vitro, and in whom serum TSH was available from family members.

CONCLUSIONS

A single grossly mutated allele (such as C41S or 555-561del) invariably leads to a condition of subclinical hypothyroidism, whereas in case of heterozygous carriers of mutations partially affecting the receptor function (such as P162A or L252P), a remarkable variable expressivity was detected among individuals belonging to different generations.

Molecular mechanisms of thyroid dysgenesis

Thyroid dysgenesis (TD) is the most prevalent form of congenital hypothyroidism. Ttf-1, Ttf-2, Pax8 and the Tshr are expressed at early stages of thyroid development and are implicated in thyroid ontogeny. Mutations in these genes have been found in some cases of TD. The prevalence of familial forms of TD is significantly higher than expected if the disease was only sporadic, allowing to postulate a genetic basis of the disease. Linkage analysis and mutational screening of the four above-mentioned genes in familial forms of TD showed their exclusion as contributors to the disease in some families, implicating genetic heterogeneity and involving other genetic mechanisms. Strategies to uncover new genes involved in TD are therefore needed. We underscore differences in the temporal expression patterns during the human thyroid development with those in animal models. Further, the extrathyroid expression of these genes during human development enables to define the gene-specific malformations that may be present in patients bearing mutations. The data gathered on molecular thyroid development enable precise genetic counselling of affected families. By increasing our knowledge of thyroid development, we hope to uncover new perspectives of genetic screening and eventually of early in utero treatment.

Thyroid hemiagenesis is a rare variant of thyroid dysgenesis with a familial component but without Pax8 mutations in a cohort of 22 cases

Thyroid hemiagenesis is a rare form of thyroid dysgenesis of which some familial cases have been reported, including one associated with a heterozygous mutation in the Pax8 gene. However, the physiopathology remains not well known. The objectives of this study were 1) to describe the clinical features, 2) to look for familial clustering, and 3) to search for Pax8 mutations in a relatively large cohort of affected patients. A family history of thyroid dysgenesis was found in nine patients (40%), whose affected relatives had ectopic thyroid (n = 4), athyreosis (n = 1), thyroid hemiagenesis (n = 2), or thyroglossal duct cysts (n = 2). Screening for Pax8 mutations identified abnormal migration profiles by SSCP analysis in 3 patients, but direct sequencing did not show coding region mutations in any of the 22 patients. In conclusion, this study provides the first evidence that thyroid hemiagenesis can occur as a familial disorder associated with any form of thyroid dysgenesis. This finding supports both a common underlying mechanism to the various abnormalities in thyroid development and a role for genetic factors; however, our results from Pax8 analysis suggest that this gene may not be a key factor.

Congenital primary hypothyroidism in a turkish family caused by a homozygous nonsense mutation (R609X) in the thyrotropin receptor gene

Congenital primary hypothyroidism can be caused by reduced or absent responsiveness of the thyroid gland to thyrotropin (TSH), which normally binds to the thyrotropin receptor (TSHR) and exerts its effects mainly by stimulating intracellular cyclic adenosine monophosphate (cAMP). So far 22 causative loss-of-function mutations in the TSHR gene have been reported as occurring in families from different ethnic and geographical origins. We here report a family of Turkish origin in which all four children were diagnosed as having congenital primary hypothyroidism. A homozygous nonsense mutation in codon 609 (R609X) of the TSHR gene was identified in all affected individuals as the underlying molecular defect. The mutation leads to a truncated TSHR, which is likely to be biologically inactive. This mutation has been identified before to cause resistance to TSH in a large Bedouin kindred where two affected hypothyroid individuals were also resistant to adrenocorticotrophic hormone (ATCH). Interestingly none of the affected individuals in our family had symptoms consistent with hypocortisolism, which excludes a direct impact of this TSHR mutation on the hypothalamicpituitary-adrenal axis.

Thyroid development and its disorders: genetics and molecular mechanisms

Thyroid gland organogenesis results in an organ the shape, size, and position of which are largely conserved among adult individuals of the same species, thus suggesting that genetic factors must be involved in controlling these parameters. In humans, the organogenesis of the thyroid gland is often disturbed, leading to a variety of conditions, such as agenesis, ectopy, and hypoplasia, which are collectively called thyroid dysgenesis (TD). The molecular mechanisms leading to TD are largely unknown. Studies in murine models and in a few patients with dysgenesis revealed that mutations in regulatory genes expressed in the developing thyroid are responsible for this condition, thus showing that TD can be a genetic and inheritable disease. These studies open the way to a novel working hypothesis on the molecular and genetic basis of this frequent human condition and render the thyroid an important model in the understanding of molecular mechanisms regulating the size, shape, and position of organs.

Analysis of the PAX8 gene in congenital hypothyroidism caused by different forms of thyroid dysgenesis in a father and daughter

Thyroid dysgenesis occurs sporadically with only rare familial presentation. We report a father and daughter with congenital hypothyroidism caused by different forms of thyroid dysgenesis. The father had a severely hypoplastic thyroid gland in a normal location, whereas the daughter had an ectopic thyroid gland in a sublingual position. Her brother had a hypoplastic thyroid but was euthyroid. The involvement of the candidate gene, PAX8, as the cause of thyroid dysgenesis in this family was partially excluded by linkage analysis, and the possibility of a de novo mutation excluded by sequencing.

Premature birth and low birth weight associated with nonautoimmune hyperthyroidism due to an activating thyrotropin receptor gene mutation

OBJECTIVE

Nonautoimmune hyperthyroidism (NAH), a rare autosomal dominantly inherited condition characterized by nonremitting thyrotoxicosis and the absence of features of autoimmune thyrotoxicosis, can result from activating germline mutations in the thyrotropin receptor (TSHR) gene. We report clinical and genetic features of a new family with NAH, and highlight that premature delivery and low birth weight are important characteristics of this condition.

PATIENTS AND METHODS

Thyrotoxicosis was diagnosed in two children at the ages 20 months and 4 years and in their father at the age of 9 years. Both children were born prematurely by Caesarian section at 33 and 30 weeks following early rupture of the membranes. Their birth weights were 1750 g (27th centile) and 790 g (< 3rd centile), respectively. Mutation analysis of the TSHR gene was performed in both children and their parents by direct DNA sequencing.

RESULTS

A heterozygous germline mutation of the TSHR gene resulting in the substitution of serine (AGC) by asparagine (AAC) at codon 505 (S505N) was found, which co-segregated with thyrotoxicosis in the family. A review of all previously reported cases of NAH due to an activating TSHR germline mutation showed that the mean duration of gestation in these patients was significantly lower than in patients with inactivating TSHR mutations causing congenital hypothyroidism (35.8 weeks vs. 39.4 weeks, P = 0.003). In addition, the mean birth weight in patients with activating TSHR mutations was lower than in patients with inactivating TSHR mutations (2338 g vs. 3470 g, P = 0.004).

CONCLUSIONS

Premature delivery and low birth weight are consistent features of NAH due to activating TSHR germline mutations. This suggests a possible role for the fetal thyroid axis in the regulation of the timing of delivery and possibly fetal growth.

Congenital hypothyroidism and apparent athyreosis with compound heterozygosity or compensated hypothyroidism with probable hemizygosity for inactivating mutations of the TSH receptor

OBJECTIVE

We wished to ascertain whether mutations in the TSH receptor (TSHR) gene were present in two siblings with congenital hypothyroidism with no parental consanguinity.

DESIGN

The pituitary-thyroid axis and thyroid gland morphology were investigated in both affected siblings and their parents. The TSHR gene was analysed in each subject.

MEASUREMENTS

Basal thyroid function together with circulating thyroglobulin levels were measured in each subject. In addition, a TRH stimulation test was undertaken in each parent. All family members underwent thyroid ultrasonography. The TSHR gene was amplified from genomic DNA using the polymerase chain reaction and receptor mutations were identified by sequence analysis.

RESULTS

Two siblings were diagnosed with severe congenital hypothyroidism (total T4 19-21 nmol/l, TSH 160-230 mU/l on neonatal screening). Although radioiodine scanning showed no tracer uptake and ultrasound imaging in both individuals failed to demonstrate thyroid tissue, suggesting complete athyreosis, circulating thyroglobulin levels were measurable. The thyroid status of the parents was discordant: in the father, baseline thyroid function (FT4 13 pmol/l, TSH 4 mU/l), the peak TSH level after TRH stimulation (30 mU/l) were normal and he exhibited an appropriate rise in circulating thyroid hormones in response to the elevated TSH; in contrast, in the mother, baseline thyroid function was abnormal (FT4 10 pmol/l, TSH 15 mU/l), the TSH response to TRH was exaggerated (110 mU/l), with no subsequent rise in circulating thyroid hormones. An eutopic, slightly hypoplastic thyroid gland was visualized on ultrasonography in the mother and her thyroid antibody status was negative. Both children were compound heterozygotes for missense (alanine to threonine at codon 553, A553T) and premature stop (tryptophan at codon 546, W546X) mutations in the fourth transmembrane domain of the TSH receptor. The mother and father were heterozygous for W546X and A553T mutations, respectively. Each mutation is known to abolish the function or cellular surface expression of the TSH receptor.

CONCLUSIONS

Inactivating mutations in the TSH receptor can be associated with severe TSH resistance presenting as congenital hypothyroidism with apparent athyreosis. Our observations also suggest that heterozygosity for an inactivating TSHR mutation may be associated with compensated hypothyroidism and thyroid hypoplasia.

Aspectos genéticos do hipotireoidismo congênito

Hipotireoidismo congênito (HC) afeta cerca de 1:3000 a 1:4000 recém-nascidos (RN). Numerosos genes são essenciais, tanto para o desenvolvimento normal do eixo hipotálamo-hipófise-tireóide quanto para a produção hormonal, e estão associados ao HC. Cerca de 85% do hipotireoidismo primário é denominado disgenesia tireoidiana e evidências sugerem que mutações nos fatores de transcrição (TTF2, TTF1 e PAX-8) e no gene do receptor de TSH podem ser responsáveis pela doença. Os defeitos hereditários da síntese hormonal podem ser devidos a mutações nos genes NIS (natrium-iodide symporter), pendrina, tireoglobulina (TG), peroxidase (TPO). Mais recentemente, mutações no gene THOX-2 têm sido descritas para defeitos na organificacão. O hipotireoidismo central afeta cerca de 1:20.000 RN e tem sido associado com mutações nos fatores transcricionais hipofisários (POUIF1, PROP1, LHX3, HESX1). A síndrome de resistência periférica ao hormônio tireoidiano é uma doença rara que cursa com hipotireoidismo em alguns tecidos e, freqüentemente, está associada a mutações autossômicas dominantes no receptor beta (TRß).

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.

Development of the thyroid gland: lessons from congenitally hypothyroid mice and men

Congenital hypothyroidism is the most common congenital endocrine disorder (one newborn in 3000) and represents the most common cause of preventable mental retardation. In 10-20% of cases, it is due to autosomal recessive functional disorders leading to goiter formation (thyroid dyshormonogenesis). In the remainder, it is due to thyroid dysgenesis, which comprises usually isolated defects in: (1) migration of the median thyroid anlage, leading to a round cluster of ectopic cells (usually in a sublingual position) with no other thyroid tissue present; (2) differentiation or survival of the thyroid follicular cells leading to athyreosis; and (3) growth of a thyroid with the normal bilobed shape and in the normal cervical position (orthotopic hypoplasia). Mouse knock-outs have demonstrated that thyroid transcription factor-1 (TTF-1) and PAX8 are required for the survival and proliferation of thyroid follicular cell precursors, TTF-2 for their downward migration and the thyrotropin receptor (TSHR) for post-natal thyroid growth. In humans, thyroid dysgenesis is generally a sporadic malformation but an affected relative is found in 2% of cases, a figure 15-fold higher than by chance alone. Pedigree analysis is most compatible with dominant inheritance with variable penetrance. However, mutations in TTF-1, TTF-2, PAX8 and TSHR are found in <10% of patients with congenital hypothyroidism and these predominantly have orthotopic thyroid hypoplasia, often associated with other malformations. This low yield and the discordance of >90% of monozygotic twin pairs suggests that isolated thyroid ectopy or athyreosis most often results from early somatic mutations, epigenetic modifications or stochastic developmental events.

11 beta-Hydroxysteroid dehydrogenase activity in hypothalamic obesity

After extensive suprasellar operations for hypothalamic tumor removal, some patients develop Cushing-like morbid obesity while they receive replacement doses of glucocorticoids. In this study, we examined the hypothesis that target tissue conversion of inactive 11-ketosteroids to active 11 beta-OH glucocorticoids might explain the obesity of some patients with hypothalamic lesions. Toward this aim, we studied 10 patients with hypothalamic obesity and secondary adrenal insufficiency and 6 control Addisonian patients while they were on glucocorticoid replacement therapy. Pituitary hormone deficiencies were replaced when medically indicated. Twenty-four-hour urine was collected after a single oral dose of 12 mg/m(2) hydrocortisone acetate. The ratios of free and conjugated cortisol (F) to cortisone (E) and their metabolites, [tetrahydrocortisol (THF)+5 alpha THF]/tetrahyrdocortisone (THE), dihydrocortisols/dihydrocortisones, cortols/cortolones, and (F+E)/(THF+THE+5 alpha THF), were considered to represent 11 beta-hydroxysteroid dehydrogenase (HSD) activity. The 11-OH/11-oxo ratios were significantly higher in the urine of patients with hypothalamic obesity. The 11-OH/11-oxo ratios, however, did not correlate with the degree of obesity, yet a significant correlation was found between conjugated F/E and the ratio of visceral fat to sc fat measured by computerized tomography at the umbilical level. The consequence of increased 11 beta-HSD1 activity and the shift of the interconversion toward cortisol may contribute to the effects of the latter in adipose tissue. We propose that deficiency of hypothalamic messengers after surgical injury induces a paracrine/autocrine effect of enhanced glucocorticoid activity due to up-regulated 11 beta-HSD1 activity.

The importance of adrenocortical glucocorticoids for adrenomedullary and physiological response to stress: a study in isolated glucocorticoid deficiency

Glucocorticoids are required for the normal functioning of chromaffin cells and their capacity to produce epinephrine. This was modeled in a unique clinical syndrome of isolated glucocorticoid deficiency due to unresponsiveness to ACTH. The working hypotheses were that in patients with isolated glucocorticoid deficiency, adrenomedullary epinephrine would be suppressed despite replacement therapy; that norepinephrine might show a compensatory response; and that the physiological response to stress would reflect these changes. Toward these hypotheses, patients with ACTH unresponsiveness on glucocorticoid replacement were subjected to three levels of acute stress: assumption of upright posture, cold pressor, and exercise. Their catecholamine and physiological response were monitored. Patients with isolated glucocorticoid deficiency of this study had severe adrenomedullary dysfunction, characterized by a minimal resting production of epinephrine (6 +/- 2 pg/ml compared with 64 +/- 22 pg/ml of the controls) and a minimal response to stress. A slight compensatory increase of norepinephrine was found in response to cold pressor test (754 +/- 200 pg/ml compared with 431 +/- 73 pg/ml of the control). The physiological response is characterized by low systolic blood pressure and high pulse rate in rest and mild stress and in a pressor response to exercise (diastolic 87 +/- 5 mm Hg, compared with 73 +/- 2 mm Hg of the control). It is concluded that intra-adrenal glucocorticoids are essential for epinephrine secretion, that norepinephrine may be compensatory, and that these result in a distinct physiological response. The implications of the pressor response to exercise, the declining pulse pressure, and the increased pulse response insinuate a lower physical fitness in patients with adrenal insufficiency.

A familial case of congenital hypothyroidism caused by a homozygous mutation of the thyrotropin receptor gene

Most of the time congenital hypothyroidism appears as a sporadic disease. In addition to the rare defects in hormonosynthesis associated with goiters, the causes of congenital hypothyroidism include agenesis and ectopy of the thyroid gland. The study of some familial cases has allowed the identification of a few genes responsible for congenital hypothyroidism. We report here a familial case of congenital hypothyroidism, transmitted as a recessive trait, and caused by a homozygous mutation in the thyrotropin receptor (TSH-R). The initial diagnosis of thyroid agenesis, based on the absence of tracer uptake on scintiscan, was incorrect, because ultrasound examination identified severely hypoplastic thyroid tissue in the cervical region.

Novel inactivating missense mutations in the thyrotropin receptor gene in Japanese children with resistance to thyrotropin

We describe Japanese siblings with resistance to thyrotropin (TSH) who are compound heterozygotes for two novel mutations in the TSH receptor gene. The affected siblings had increased serum TSH, normal serum thyroid hormones, and normal positioned but slightly hypoplastic thyroid glands. The mutated paternal allele has the substitution of His (CAC) in place of Arg (CGC) at codon 450 (R450H) of the TSH receptor. The mutated maternal allele has the substitution of Ser (AGT) in place of Gly (GGT) at codon 498 (G498S) of the TSH receptor. COS-7 cells transfected with the R450H mutant exhibited a slightly decreased TSH binding and a slightly decreased cyclic adenosine monophosphate (cAMP) response to TSH, whereas cells transfected with the G498S mutant exhibited a markedly decreased TSH binding and a markedly decreased cAMP response to TSH. Flow immunocytofluorometry analysis demonstrated that the G498S mutant resulted in extremely low expression at the cell surface as compared with the wild type receptor and the R450H mutant, in spite of a normal intracellular synthesis. The present cases are the first Japanese patients with TSH resistance in whom mutations in the TSH receptor gene have been identified. These novel mutations may contribute to understanding of the struc-ture-function relationship of the TSH receptor.

Interpretation of thyroid function tests

The introduction of sensitive thyrotropin assays and free thyroid hormone measurements has simplified the interpretation of thyroid function tests. However, important pitfalls and difficult cases still exist. In this review, thyroid function test results are grouped into six different patterns. We propose that if assays for thyrotropin, free T3, and free T4 are all done, knowledge of these patterns coupled with clinical details and simple additional tests allow a diagnosis to be made in almost all cases.

Disorders of thyrotropin synthesis, secretion, and function

Advances related to thyrotropin during 1999 included better understanding of the genetic basis of pituitary development and genetic advances in identifying clinical entities and their mechanisms and enabling new therapies. Initial clinical use of recombinant thyrotropin in evaluation of thyroid cancer recurrence was described. The importance of glycosylation pattern was clarified including the role of thyrotropin-releasing hormone in synthesis of thyrotropin molecules with mature glycosylation, and the impact of abnormal glycosylation in loss-of-function and gain-of-function mutations of the thyrotropin receptor. Causes of excessive thyrotropin secretion were evaluated, including pituitary thyrotropin-secreting adenomas. The fairly common causes of central hypothyroidism including ischemic injury, cranial irradiation, psychiatric conditions, or medical illness were assessed. The action of thyrotropin at the thyroid cell was assessed as a growth factor and as an influence on tyrosine sulfate content of thyroglobulin. Such basic and clinical science advances are rapidly affecting clinical care.