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.

Coexistence of resistance to thyroid hormone and ectopic thyroid: ten-year follow-up

Resistance to thyroid hormone (RTH) coexisting with ectopic thyroid is rare. Here we report a case of RTH with ectopic thyroid. A ten-year-old girl had been misdiagnosed as congenital hypothyroidism and treated with levothyroxine since she was born. Ten-year follow-up showed that the elevated thyrotropin was never suppressed by levothyroxine and no signs indicating hyperthyroidism or hypothyroidism despite elevated FT3 and FT4 levels. Therefore the girl developed no defects in physical and cognitive development. Pituitary adenoma was excluded by magnetic resonance imaging. Ultrasonography did not find the thyroid gland in the normal place, while the thyroid scan found a large lingual thyroid gland. The octreotide inhibition test showed a reduction in thyrotropin by 41.98%. No mutation was detected in the thyroid hormone receptor (THR) β, THRα, thyrotropin receptor (TSHR), and GNAS1 genes. To our knowledge, it is an interesting RTH case coexisting with lingual thyroid.

Update of Thyroid Developmental Genes

Thyroid dysgenesis (TD) is the most common cause of congenital hypothyroidism in iodine-sufficient regions and includes a spectrum of developmental anomalies. The genetic components of TD are complex. Although a sporadic disease, advances in developmental biology have revealed monogenetic forms of TD. Inheritance is not based on a simple Mendelian pattern and additional genetic elements might contribute to the phenotypic spectrum. This article summarizes the key steps of normal thyroid development and provides an update on responsible genes and underlying mechanisms of TD. Up-to-date technologies in genetics and biology will allow us to advance in our knowledge of TD.

JAG1 Loss-Of-Function Variations as a Novel Predisposing Event in the Pathogenesis of Congenital Thyroid Defects

CONTEXT

The pathogenesis of congenital hypothyroidism (CH) is still largely unexplained. We previously reported that perturbations of the Notch pathway and knockdown of the ligand jagged1 cause a hypothyroid phenotype in the zebrafish. Heterozygous JAG1 variants are known to account for Alagille syndrome type 1 (ALGS1), a rare multisystemic developmental disorder characterized by variable expressivity and penetrance.

OBJECTIVE

Verify the involvement of JAG1 variants in the pathogenesis of congenital thyroid defects and the frequency of unexplained hypothyroidism in a series of ALGS1 patients.

DESIGN, SETTINGS, AND PATIENTS

A total of 21 young ALGS1 and 100 CH unrelated patients were recruited in academic and public hospitals. The JAG1 variants were studied in vitro and in the zebrafish.

RESULTS

We report a previously unknown nonautoimmune hypothyroidism in 6/21 ALGS1 patients, 2 of them with thyroid hypoplasia. We found 2 JAG1 variants in the heterozygous state in 4/100 CH cases (3 with thyroid dysgenesis, 2 with cardiac malformations). Five out 7 JAG1 variants are new. Different bioassays demonstrate that the identified variants exhibit a variable loss of function. In zebrafish, the knock-down of jag1a/b expression causes a primary thyroid defect, and rescue experiments of the hypothyroid phenotype with wild-type or variant JAG1 transcripts support a role for JAG1 variations in the pathogenesis of the hypothyroid phenotype seen in CH and ALGS1 patients.

CONCLUSIONS

clinical and experimental data indicate that ALGS1 patients have an increased risk of nonautoimmune hypothyroidism, and that variations in JAG1 gene can contribute to the pathogenesis of variable congenital thyroid defects, including CH.

Thyroid Hemiagenesis from Childhood to Adulthood: Review of Literature and Personal Experience

Thyroid hemiagenesis (TH) is a rare congenital abnormality of the thyroid gland, characterised by the absence of one lobe. The true prevalence of this congenital abnormality is not known because the absence of one thyroid lobe usually does not cause clinical symptoms by itself. Between 1970 and 2010, 329 cases of TH have been reported. It is interesting to note that most cases have an agenesis of the left lobe (80% of cases) followed by the isthmus (44-50% of cases). Although the female to male ratio was 1:1.4 in 24,032 unselected 11-to 14-yr-old schoolchildren from South-eastern Sicily, several other reports have documented a higher prevalence in women, which may indicate a possible gender association. Most cases of TH are diagnosed when patients present a lesion in the functioning lobe. The functioning lobe of the thyroid gland can be a site of pathological changes similar to a normally developed gland and may present a spectrum of diseases like multinodular goiter, colloid goiter, follicular adenoma, thyroiditis, hypothyroidism and hyperthyroidism. In three of our patients, TH was associated with Hashimoto thyroiditis (n = 1) and with subclinical hypothyroidism (n = 2). The frequency of thyroid abnormalities in patients with TH varies with age, due to the longer exposure of the hemi-agenetic gland to TSH overstimulation in older patients. This could explain the controversy about the benign character of this anomaly. Other extrathyroidal lesions, such as parathyroid adenoma or hyperplasia, cervical thymic cysts, ectopic sublingual thyroid gland and thyroglossal duct cyst have been reported with TH. Therefore, systematic follow-up of all identified cases is recommended.

A De novo PAX8 mutation in a Chinese child with congenital thyroid dysgenesis

BACKGROUND

Thyroid dysgenesis (TD) is the most frequent cause of congenital hypothyroidism (CH), but its pathogenesis remains unclear. As a thyroid transcription factor, paired box transcription factor 8 (PAX8) is essential for thyroid organogenesis and development.

AIM

To screen PAX8 mutations and characterize the features of these mutations in Chinese TD patients.

MATERIALS AND METHODS

Blood samples were collected from 63 TD patients in Shandong Province, China, and genomic DNA was extracted from peripheral blood leukocytes. Exon 3~4 of PAX8 were analyzed by PCR and direct sequencing.

RESULTS

Direct sequencing of PAX8 revealed a heterozygous missense mutation (c.155G/C, P.Arg52Pro) in one child with agenesis. Genetic screening of the child's family revealed that the clinically unaffected parents do not carry the mutation, suggesting that the identified sequence change is a de novo mutation.

CONCLUSION

We report a heterozygous missense de novo mutation in PAX8 in one out of 63 unrelated Chinese TD patients, showing that the PAX8 mutation rate is very low in TD patients in China. However, de novo mutation and epigenetic mechanisms need to be considered in the future study.

Functional characterization of the novel sequence variant p.S304R in the hinge region of TSHR in a congenital hypothyroidism patients and analogy with other formerly known mutations of this gene portion

CONTEXT

Thyroid dysgenesis may be associated with loss-of-function mutations in the thyrotropin receptor (TSHR) gene.

OBJECTIVES

The aim of this study was to characterize a novel TSHR gene variant found in one patient harboring congenital hypothyroidism (CH) from a cohort of patients with various types of thyroid defects.

MATERIALS AND METHODS

This cross-sectional cohort study involved 118 patients with CH and their family members, including 45 with familial and 73 with sporadic diseases. The thyroid gland was normal in 23 patients, 25 patients had hypoplasia, 25 hemithyroid agenesis, 21 had athyreosis, and 21 had ectopy. Genomic DNA was extracted, and 10 exons of the TSHR gene were amplified and sequenced. Mutations in other candidate genes were investigated. Ortholog alignment was performed, and TSHR functional assays were evaluated.

RESULTS

We identified one previously unknown missense variation in the hinge region (HinR) of the TSHR gene (p.S304R) in one patient with thyroid hypoplasia. This variant is conserved in our ortholog alignment. However, the p.S304R TSHR variant presented a normal glycosylation pattern and signal transduction activity in functional analysis.

CONCLUSION

We report the ocurrence of a novel nonsynonymous substitution in the HinR of the large N-terminal extracellular domain of the TSHR gene in a patient with thyroid hypoplasia. In contrast with four others in whom TSHR mutations of the hinge portion were previously identified, the p.S304R TSHR variation neither affected TSH binding nor cAMP pathway activation. This TSHR gene variant was documented in a CH patient, but the current data do not support its role in the clinical phenotype.

Genetic analysis of the paired box transcription factor (PAX8) gene in a cohort of Polish patients with primary congenital hypothyroidism and dysgenetic thyroid glands

BACKGROUND

The morphological and biochemical phenotype of PAX8 mutation in patients with congenital hypothyroidism (CH) is variable. The contribution of mutations in PAX8 gene in children with CH and dysgenetic thyroid glands still remains a subject of interest for researchers.

PATIENTS AND METHODS

Some 48 children (37 girls and 11 boys) with CH associated with thyroid ectopy (n=22), agenesis (n=10), hypoplasia (n=6), or thyroid dysgenesis of unknown cause (n=10) were enrolled. The study participants were born in south-eastern Poland in the years 1993-2012 and were selected for neonatal mass screening for CH. DNA was extracted from peripheral blood samples using Master Pure DNA Purification Kit (Epicentre Biotechnologies, Madison, WI, USA). The 12 exons of the PAX8 gene along with their exon-intron boundaries were amplified and sequenced by the Sanger method. Capillary electrophoresis was run on ABI 3500 (Applied Biosystems, Carlsbad, CA, USA).

RESULTS

Novel heterozygous transition in exon 3 (c.68G>A) was detected in a 3-year-old girl with a thyroid hypoplasia. This substitution was not identified in the patient's parents (de novo event). Additionally, a novel genetic variant in 3'UTR region of exon 12 (c.*416C>T) occurred in a 3-year-old boy with ectopic thyroid tissue and concomitant congenital urogenital malformation. This heterozygous variant was also detected in other healthy family members. Thirteen well-described single nucleotide polymorphisms were revealed in the PAX8 gene.

CONCLUSIONS

The study reports on the occurrence of two novel heterozygous substitutions in the PAX8 gene. Estimation of the contribution of the revealed c.68G>A variant to the etiology of CH in a girl with hypoplastic thyroid requires further functional analysis.

DYRK1A BAC transgenic mouse: a new model of thyroid dysgenesis in Down syndrome

The most common thyroid abnormality among Down syndrome (DS) children corresponds to a mildly elevated TSH, with T4 decreased or in the normal range and thyroid hypoplasia, from the neonatal period onward, which aggravate their mental impairment. Transgenic Dyrk1A mice, obtained by bacterial artificial chromosome engineering (mBACTgDyrk1A), have 3 copies of the Dyrk1A gene. The objective is to determine whether this transgenic Dyrk1A (Dyrk1A(+/++)) mouse is an adequate murine model for the study of thyroid dysgenesis in DS. Embryonic thyroid development from embryonic day 13.5 (E13.5) to E17.5 was analyzed in wild-type (WT) and Dyrk1A(+/++) mice by immunofluorescence with anti-Nkx2-1, anti-thyroglobulin, and anti-T4 antibodies, markers of early thyroid development, hormonogenesis, and final differentiation, respectively. The expression of transcription factors Nkx2-1, Pax8, and Foxe1 involved in thyroidogenesis were studied by quantitative RT-PCR at the same embryonic stages. We then compared the adult phenotype at 8 to 12 weeks in Dyrk1A(+/++) and WT mice for T4 and TSH levels, thyroidal weight, and histological analysis. Regarding thyroidal development, at E15.5, Dyrk1A(+/++) thyroid lobes are double the size of WT thyroids (P = .01), but the thyroglobulin stained surface in Dyrk1A(+/++) thyroids is less than a third as large at E17.5 (P = .04) and their differentiated follicular surface half the size (P = .004). We also observed a significant increase in Nkx2-1, Foxe1, and Pax8 RNA levels in E13.5 and E17.5 Dyrk1A(+/++) embryonic thyroids. Dyrk1A(+/++) young adult mice have significantly lower plasma T4 (2.4 ng/mL versus WT, 3.7 ng/mL; P = 0.019) and nonsignificantly higher plasma TSH (114 mUI/L versus WT, 73mUI/L; P = .09). In addition, their thyroids are significantly heavier (P = .04) and exhibit large disorganized regions. Dyrk1A overexpression directly leads to thyroidal embryogenetic, functional and morphological impairment. The young adult thyroid phenotype is probably a result of embryogenetic impairment. The Dyrk1A(+/++) mouse can be considered a suitable study model for thyroid dysgenesis in DS.

Identification of deletions in children with congenital hypothyroidism and thyroid dysgenesis with the use of multiplex ligation-dependent probe amplification

INTRODUCTION

Thyroid dysgenesis (TD) is the most common cause of congenital hypothyroidism (CH). Important genetic factors possibly contributing to TD etiologies include mutations of thyroid transcription factors and TSHR-encoding genes.

OBJECTIVE

Our objective was to determine multiplex ligation-dependent probe amplification (MLPA) utility in detecting the copy number changes in patients with CH and TD.

METHODS

The study included 45 children from southeastern Poland selected via already established neonatal screening for CH. Genomic DNA was extracted from peripheral blood samples and used in MLPA analysis. Genetic variations were analyzed within selected fragments of the PAX8, FOXE1, NKX2-1, thyroid stimulating hormone receptor (TSHR), and TPO genes.

RESULTS

Three heterozygous deletion types in probe hybridization regions were identified for the following genes: PAX8 (exon 7), TSHR (exon 2), and FOXE1 (exon 1). Monoallelic deletions were identified in 5/45 TD subjects.

CONCLUSIONS

MLPA is a useful tool for copy number changes detection and might both improve and expand genetic analysis for CH and TD.

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.

Functional zebrafish studies based on human genotyping point to netrin-1 as a link between aberrant cardiovascular development and thyroid dysgenesis

Congenital hypothyroidism caused by thyroid dysgenesis (CHTD) is a common congenital disorder with a birth prevalence of 1 case in 4000 live births, and up to 8% of individuals with CHTD have co-occurring congenital heart disease. Initially we found nine patients with cardiac and thyroid congenital disorders in our cohort of 158 CHTD patients. To enrich for a rare phenotype likely to be genetically simpler, we selected three patients with a ventricular septal defect for molecular studies. Then, to assess whether rare de novo copy number variants and coding mutations in candidate genes are a source of genetic susceptibility, we used a genome-wide single-nucleotide polymorphism array and Sanger sequencing to analyze blood DNA samples from selected patients with co-occurring CHTD a congenital heart disease. We found rare variants in all three patients, and we selected Netrin-1 as the biologically most plausible contributory factor for functional studies. In zebrafish, ntn1a and ntn1b were not expressed in thyroid tissue, but ntn1a was expressed in pharyngeal arch mesenchyme, and ntn1a-deficient embryos displayed defective aortic arch artery formation and abnormal thyroid morphogenesis. The functional activity of the thyroid in ntn1a-deficient larvae was, however, preserved. Phenotypic analysis of affected zebrafish indicates that abnormal thyroid morphogenesis resulted from a lack of proper guidance exerted by the dysplastic vasculature of ntn1a-deficient embryos. Hence, careful phenotyping of patients combined with molecular and functional studies in zebrafish identify Netrin-1 as a potential shared genetic factor for cardiac and thyroid congenital defects.

Extreme phenotypic variability of thyroid dysgenesis in six new cases of congenital hypothyroidism due to PAX8 gene loss-of-function mutations

CONTEXT

Within the last two decades, heterozygous loss-of-function PAX8 mutations have been reported in patients with a wide degree of thyroid gland dysfunction and growth despite the presence of identical mutations.

OBJECTIVES

To search for PAX8 mutations in a cohort of patients with congenital hypothyroidism (CH) and various types of thyroid gland defects.

DESIGN

A cross-sectional study was conducted in a cohort of patients.

SETTING

The French neonatal screening program was used for recruiting patients.

PATIENTS

A total of 118 patients with CH, including 45 with familial and 73 with sporadic diseases, were included in this study. The thyroid gland was normal in 23 patients had hypoplasia, 25 had hemithyroid agenesis, 21 had athyreosis, and 21 had ectopy.

RESULTS

We found four different PAX8 mutations (p.R31C, p.R31H, p.R108X, and p.I47T) in ten patients (six patients with CH and four family members), two with sporadic and eight with familial diseases. Imaging studies performed in the index cases showed ectopic thyroid gland (n=2), hypoplasia (n=2), eutopic lobar asymmetry (n=1), and eutopic gland compatible with dyshormonogenesis (n=1). The previously reported p.R31C and the novel p.I47T PAX8 mutations are devoid of activity.

CONCLUSION

Four different PAX8 mutations were detected in six index patients with CH (ten total subjects). The p.R31C, p.R31H, and p.R108X mutations have been reported. The novel p.I47T PAX8 mutation presented loss of function leading to CH. Thyroid ectopy was observed in two cases of PAX8 (p.R31H) mutation, a finding that has not been reported previously. We observed a high inter-individual and intra-familial variability of the phenotype in PAX8 mutations, underlining that population genetic studies for CH should include patients with various clinical presentations.

An essential splice site mutation (c.317+1G>A) in the TSHR gene leads to severe thyroid dysgenesis

Congenital hypothyroidism (CH) is the most common neonatal endocrine disorder and 2% of cases have familial origin. Our aim in this study was to determine the genetic alterations in two siblings with CH coming from a consanguineous family. Because CH is often inherited in autosomal recessive manner in consanguineous/multicase-families, we first performed genetic linkage studies to all known causative CH loci followed by conventional sequencing of the linked gene. The family showed potential linkage to the TSHR locus, and we detected an essential splice site mutation (c.317+1G>A) in both siblings. RT-PCR analysis confirmed the functionality of the mutation. The mutation was homozygous in the cases whereas heterozygous in carrier parents and an unaffected sibling. Here we conclude that thyroid agenesis in both siblings in this study originates from c.317+1G>A splice site mutation in the TSHR gene, and this study underlines the importance of detailed molecular genetic studies in the definitive diagnosis and classification of CH.

Identification of a novel pax8 gene sequence variant in four members of the same family: from congenital hypothyroidism with thyroid hypoplasia to mild subclinical hypothyroidism

BACKGROUND

Congenital hypothyroidism is often secondary to thyroid dysgenesis, including thyroid agenesis, hypoplasia, ectopic thyroid tissue or cysts. Loss of function mutations in TSHR, PAX8, NKX2.1, NKX2.5 and FOXE1 genes are responsible for some forms of inherited congenital hypothyroidism, with or without hypoplastic thyroid. The aim of this study was to analyse the PAX8 gene sequence in several members of the same family in order to understand whether the variable phenotypic expression, ranging from congenital hypothyroidism with thyroid hypoplasia to mild subclinical hypothyroidism, could be associated to the genetic variant in the PAX8 gene, detected in the proband.

METHODS

We screened a hypothyroid child with thyroid hypoplasia for mutations in PAX8, TSHR, NKX2.1, NKX2.5 and FOXE1 genes. We studied the inheritance of the new variant R133W detected in the PAX8 gene in the proband's family, and we looked for the same substitution in 115 Caucasian European subjects and in 26 hypothyroid children. Functional studies were performed to assess the in vitro effect of the newly identified PAX8 gene variant.

RESULTS

A new heterozygous nucleotide substitution was detected in the PAX8 DNA-binding motif (c.397C/T, R133W) in the proband, affected by congenital hypothyroidism with thyroid hypoplasia, in his older sister, displaying a subclinical hypothyroidism associated with thyroid hypoplasia and thyroid nodules, in his father, affected by hypothyroidism with thyroid hypoplasia and thyroid nodules, and his first cousin as well, who revealed only a subclinical hypothyroidism. Functional studies of R133W-PAX8 in the HEK293 cells showed activation of the TG promoter comparable to the wild-type PAX8.

CONCLUSIONS

In vitro data do not prove that R133W-PAX8 is directly involved in the development of the thyroid phenotypes reported for family members carrying the substitution. However, it is reasonable to conceive that, in the cases of transcriptions factors, such as Pax8, which establish several interactions in different protein complexes, genetic variants could have an impact in vivo.

A deletion including exon 2 of the TSHR gene is associated with thyroid dysgenesis and severe congenital hypothyroidism

Congenital hypothyroidism (CH) is the most common neonatal endocrine disorder and 2% of cases have a familial origin. Our aim in this study was to determine the genetic alterations in two siblings with CH coming from a consanguineous family. As CH is often inherited in an autosomal recessive manner in consanguineous/multi case-families, we first performed genetic linkage studies to all known causative CH loci followed by conventional sequencing of the linked gene. The family showed potential linkage to the TSHR locus and our attempts to amplify and sequence exon 2 of the TSHR gene continuously failed. Subsequent RT-PCR analysis using mRNA and corresponding cDNA showed a large deletion including the exon 2 of the gene. The deletion was homozygous in affected cases whilst heterozygous in carrier parents. Here we conclude that CH in both siblings of this study originates from a large deletion including the exon 2 of the TSHR gene. This study demonstrates that full sequence analysis in a candidate CH gene might not always be enough to detect genetic alterations, and additional analyses such as RT-PCR and MLPA might be necessary to describe putative genetic causes of the disease in some cases. It also underlines the importance of detailed molecular genetic studies in the definitive diagnosis and classification of CH.

A new mutation in the promoter region of the PAX8 gene causes true congenital hypothyroidism with thyroid hypoplasia in a girl with Down's syndrome

BACKGROUND

Thyroid dysfunction is common in newborn infants with Down's syndrome (DS), but defects causing classic thyroid dysgenesis (TD) with permanent congenital hypothyroidism (CH) have not been described.

OBJECTIVE

We studied a girl with DS and CH who had a mutation in the promoter sequence of the PAX8 gene.

RESULTS

A female infant was found to have trisomy 21 and CH, with a venous thyrotropin (TSH) of >150 mU/L and a free thyroxine (fT4) of 15.1 pmol/L (day 12). Thyroid peroxidase antibodies and thyroglobulin antibodies were elevated. Scintigraphy showed normal uptake, but ultrasound identified a small gland with heterogenous echotexture and cystic changes. Sequence analysis of the PAX8 gene revealed a new heterozygous maternally inherited mutation (-3C>T) close to the transcription initiation site. Electromobility shift assay studies of the wild type and the mutant PAX8 sequence incubated with nuclear extracts from PCCL3 cells exhibited that the sequence at position -3 is not involved in specific protein binding. However, the mutant PAX8 promoter showed a significantly reduced transcriptional activation of a luciferase reporter gene in vitro tested in HEK, PCCL3, as well as in HeLa cells, indicating that this mutation is very likely to lead to reduced PAX8 expression.

CONCLUSIONS

The persistent CH in this patient with DS is likely to be attributable to the diminished PAX8 expression due to a new heterozygous mutation in the PAX8 promoter sequence. Our case shows that true CH may occur in DS, as in the general population. Furthermore, it is possible that the trisomy 21 itself may have resulted in a more severe phenotypic expression of the PAX8 mutation in the child than the mother.

A novel FOXE1 mutation (R73S) in Bamforth-Lazarus syndrome causing increased thyroidal gene expression

BACKGROUND

Homozygous loss-of-function mutations in the FOXE1 gene have been reported in several patients with partial or complete Bamforth-Lazarus syndrome: congenital hypothyroidism (CH) with thyroid dysgenesis (usually athyreosis), cleft palate, spiky hair, with or without choanal atresia, and bifid epiglottis. Here, our objective was to evaluate potential functional consequences of a FOXE1 mutation in a patient with a similar clinical phenotype.

METHODS

FOXE1 was sequenced in eight patients with thyroid dysgenesis and cleft palate. Transient transfection was performed in HEK293 cells using the thyroglobulin (TG) and thyroid peroxidase (TPO) promoters in luciferase reporter plasmids to assess the functional impact of the FOXE1 mutations. Primary human thyrocytes transfected with wild type and mutant FOXE1 served to assess the impact of the mutation on endogenous TG and TPO expression.

RESULTS

We identified and characterized the function of a new homozygous FOXE1 missense mutation (p.R73S) in a boy with a typical phenotype (athyreosis, cleft palate, and partial choanal atresia). This new mutation located within the forkhead domain was inherited from the heterozygous healthy consanguineous parents. In vitro functional studies in HEK293 cells showed that this mutant gene enhanced the activity of the TG and TPO gene promoters (1.5-fold and 1.7-fold respectively vs. wild type FOXE1; p<0.05), unlike the five mutations previously reported in Bamforth-Lazarus syndrome. The gain-of-function effect of the FOXE1-p.R73S mutant gene was confirmed by an increase in endogenous TG production in primary human thyrocytes.

CONCLUSION

We identified a new homozygous FOXE1 mutation responsible for enhanced expression of the TG and TPO genes in a boy whose phenotype is similar to that reported previously in patients with loss-of-function FOXE1 mutations. This finding further delineates the role for FOXE1 in both thyroid and palate development, and shows that enhanced gene activity should be considered among the mechanisms underlying Bamforth-Lazarus syndrome.

The absence of mutations in homeobox candidate genes HOXA3, HOXB3, HOXD3 and PITX2 in familial and sporadic thyroid hemiagenesis

BACKGROUND

The molecular mechanisms leading to the formation of the two thyroid symmetrical lobes, which are impaired in thyroid hemiagenesis (TH), are little known.

OBJECTIVE

The aim of this work was to search for mutations in thyroid developmental candidate genes HOXA3, HOXB3, HOXD3 and PITX2.

METHODS

Total DNA from peripheral blood was extracted and then the entire coding region of all these genes was amplified by polymerase chain reaction and direct sequencing.

RESULTS

Herein we describe familial cases of TH in two generations (proband and his father), in addition to other two sporadic cases. We have found polymorphisms in the HOXB3 (rs2229304), HOXD3 (rs34729309, rs1051929, c.543-199G>T and c.543-34G>A; and a new synonymous variant, NP_008829.3:p.314;C>G) and PITX2 (c.45+76C>T) genes, but no deleterious mutations.

CONCLUSION

These results suggest the existence of other left-right thyroid asymmetry candidate genes in humans such as classical Mendelian mutation-causing disease, as well as other etiopathogenic mechanisms such as epigenetic modifications, especially for sporadic hemiagenesis.

Genetics of normal and abnormal thyroid development in humans

The most frequent cause of congenital hypothyroidism is thyroid dysgenesis. Thyroid dysgenesis summarizes a spectrum of developmental abnormalities of the embryonic thyroid ranging from complete absence of the thyroid gland (athyreosis), to a normally located but too small thyroid (hypoplasia), or an abnormally located thyroid gland (ectopy). Although considered a sporadic disease, distinct genetic forms of isolated or syndromic thyroid dysgenesis have been described in recent years. However, genetics of thyroid dysgenesis (TD) are mostly not following simple Mendelian patterns, and beside monogenic, multigenic and epigenetic mechanisms need to be considered. The review will highlight the molecular mechanisms of thyroid organogenesis, clinical and genetic features of the different monogenetic forms of thyroid dysgenesis, the aspects relevant for diagnosis and counseling of affected families and current research strategies to get more insight into the non-Medelian mechanisms of normal and abnormal thyroid development.

Clinical genetics of congenital hypothyroidism

Congenital hypothyroidism (CH) is a state of insufficient thyroid hormone supply to the organism, starting in utero. Two forms of permanent primary or thyroidal CH are known. Thyroid dysgenesis (TD) describes a spectrum of defects of thyroid organogenesis. Five monogenetic forms due to mutations in TSHR, PAX8, NKX2-1, FOXE1 and NKX2-5 have been identified so far. Thyroid dyshormonogenesis comprises defects at every step of thyroid hormone synthesis. Mutations in 7 genes are well described causing iodine transport defect (SLC5A5), iodine organification defect (TPO, DUOX2, DUOXA2, SLC26A4), thyroglobulin (TG) synthesis or transport defect or iodotyrosine deiodinase (IYD/DEHAL1) deficiency. The new consensus guidelines for CH recommend genetic counseling for each family with an affected child. Mode of inheritance, recurrence rate and possible associated malformations in the context of syndromic forms should be outlined. Molecular genetic studies should be preceded by a detailed phenotypic description of the patient's thyroid disease and a detailed family history. This review summarizes clinical, biochemical and radiological phenotypes and molecular aspects of the known genetic forms of TD and thyroid dyshormonogenesis relevant for genetic counseling and molecular studies.

Identification of PENDRIN (SLC26A4) mutations in patients with congenital hypothyroidism and "apparent" thyroid dysgenesis

CONTEXT

Congenital hypothyroidism, the most frequent endocrine congenital disease, can occur either based on a thyroid hormone biosynthesis defect or can predominantly be due to thyroid dysgenesis. However, a genetic cause could so far only be identified in less than 10% of patients with a thyroid dysgenesis.

OBJECTIVES

Exome sequencing was used for the first time to find additional genetic defects in thyroid dysgenesis.

PATIENTS AND METHODS

In a consanguineous family with thyroid dysgenesis, exome sequencing was applied, and findings were further validated by Sanger sequencing in a cohort of 94 patients with thyroid dysgenesis.

RESULTS

By exome sequencing we identified a homozygous missense mutation (p.Leu597Ser) in the SLC26A4 gene of a patient with hypoplastic thyroid tissue, who was otherwise healthy. In the cohort of patients with thyroid dysgenesis, we observed a second case with a homozygous missense mutation (p.Gln413Arg) in the SLC26A4 gene, who was additionally affected by severe hearing problems. Both mutations were previously described as loss-of-function mutations in patients with Pendred syndrome and nonsyndromic enlarged vestibular aqueduct.

CONCLUSION

We unexpectedly identified SLC26A4 mutations that were hitherto diagnosed in thyroid dyshormonogenesis patients, now for the first time in patients with structural thyroid defects. This result resembles the historic description of thyroid atrophy in patients with the so-called myxedematous form of cretinism after severe iodine deficiency. Most likely the thyroid defect of the two homozygous SLC26A4 gene mutation carriers represents a kind of secondary thyroid atrophy, rather than a primary defect of thyroid development in the sense of thyroid agenesis. Our study extends the variable clinical spectrum of patients with SLC26A4 mutations and points out the necessity to analyze the SLC26A4 gene in patients with apparent thyroid dysgenesis in addition to the known candidate genes TSHR, PAX8, NKX2.1, NKX2.5, and FOXE1.

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.

Two cases of thyroid dysgenesis caused by different novel PAX8 mutations in the DNA-binding region: in vitro studies reveal different pathogenic mechanisms

BACKGROUND

Mutations in PAX8, a transcription factor gene, cause thyroid dysgenesis (TD). The extreme variability of the thyroid phenotype makes it difficult to identify individuals harboring PAX8 gene mutations. Here we describe two patients with TD and report two novel PAX8 gene mutations (S54R and R133Q). We performed in vitro studies to functionally characterize these mutations.

METHODS

Using PAX8 expression vectors, we investigated whether the PAX8 mutants localized correctly to the nucleus. To analyze the DNA-binding properties of S54R and R133Q, electrophoretic mobility shift assays were performed. Furthermore, we measured whether the mutant PAX8 proteins were able to activate the thyroglobulin (TG)- and the thyroperoxidase (TPO)-promoters.

RESULTS

S54R had an impaired binding to DNA and a negligible activity on the TG- and the TPO-promoters. The DNA-binding property of R133Q, which is located in the highly conserved terminal portion of the PAX8 DNA-binding domain, was normal. Interestingly, it also exhibited dramatically impaired activation of the TG- and TPO-promoters. However, R133Q has no dominant negative effect on the WT protein in vitro. Thus, the underlying molecular mechanism by which the function of R133Q is impaired remains to be elucidated.

CONCLUSIONS

We identified and functionally characterized two novel mutations of the PAX8 gene that lead to TD by distinct mechanisms. A structural defect of the mutant R133Q leading to a reduced capability for induced fit upon DNA interaction might explain the disparity between its apparently normal binding to DNA, but lack of promoter activation.