Transcriptome, methylome and genomic variations analysis of ectopic thyroid glands

Tongue Base Ectopic Thyroid Tissue-Is It a Rare Encounter?

Failure in the embryological development of the thyroid in adults is rarely seen. We present the case of a 79-year-old female patient who complained of dysphagia and progressive upper respiratory obstruction, which started 12 months prior to her admission. An ENT clinical exam revealed a tongue base, spherical, well-defined tumour covered by normal mucosa. Further assessments established the diagnosis of the tongue base ectopic thyroid tissue. Due to the patient's symptoms, a transhyoid tongue base tumour removal was performed. The selected patient gave consent for participation and inclusion in this paper, in compliance with the 1964 Helsinki declaration.

Stochastic epigenetic mutations as possible explanation for phenotypical discordance among twins with congenital hypothyroidism

PURPOSE

The elevated frequency of discordance for congenital hypothyroidism (CH) phenotype between monozygotic twins suggests the involvement of non-mendelian mechanisms. The aim of the study was to investigate the role of epigenetics in CH pathogenesis.

METHODS

A genome-wide DNA methylation analysis was performed on the peripheral blood of 23 twin pairs (10 monozygotic and 13 dizygotic), 4 concordant and 19 discordant pairs for CH at birth.

RESULTS

Differential methylation analysis did not show significant differences in methylation levels between CH cases and controls, but a different methylation status of several genes may explain the CH discordance of a monozygotic twin couple carrying a monoallelic nonsense mutation of DUOX2. In addition, the median number of hypo-methylated Stochastic Epigenetic Mutations (SEMs) resulted significantly increased in cases compared to controls. The prioritization analysis for CH performed on the genes epimutated exclusively in the cases identified SLC26A4, FOXI1, NKX2-5 and TSHB as the genes with the highest score. The analysis of significantly SEMs-enriched regions led to the identification of two genes (FAM50B and MEG8) that resulted epigenetically dysregulated in cases.

CONCLUSION

Epigenetic modifications may potentially account for CH pathogenesis and explain discordance among monozygotic twins.

Genetics of congenital hypothyroidism: Modern concepts

Congenital hypothyroidism (CH) is the most common neonatal endocrine disorder and one of the most common preventable causes of intellectual disability in the world. CH may be due to developmental or functional thyroid defects (primary or peripheral CH) or be hypothalamic‐pituitary in origin (central CH). In most cases, primary CH is caused by a developmental malformation of the gland (thyroid dysgenesis, TD) or by a defect in thyroid hormones synthesis (dyshormonogenesis, DH). TD represents about 65% of CH and a genetic cause is currently identified in fewer than 5% of patients. The remaining 35% are cases of DH and are explained with certainty at the molecular level in more than 50% of cases. The etiology of CH is mostly unknown and may include contributions from individual and environmental factors. In recent years, the detailed phenotypic description of patients, high‐throughput sequencing technologies, and the use of animal models have made it possible to discover new genes involved in the development or function of the thyroid gland. This paper reviews all the genetic causes of CH. The modes by which CH is transmitted will also be discussed, including a new oligogenic model. CH is no longer simply a dominant disease for cases of CH due to TD and recessive for cases of CH due to DH, but a far more complex disorder.

GWAS of thyroid dysgenesis identifies a risk locus at 2q33.3 linked to regulation of Wnt signaling

Congenital hypothyroidism due to thyroid dysgenesis (TD), presented as thyroid aplasia, hypoplasia or ectopia, is one of the most prevalent rare diseases with an isolated organ malformation. The pathogenesis of TD is largely unknown, although a genetic predisposition has been suggested. We performed a genome-wide association study (GWAS) with 142 Japanese TD cases and 8380 controls, and found a significant locus at 2q33.3 (top SNP, rs9789446: P = 4.4 × 10-12), which was replicated in a German patient cohort (P = 0.0056). A subgroup analysis showed that rs9789446 confers a risk for thyroid aplasia (per allele odds ratio = 3.17) and ectopia (3.12) but not for hypoplasia. Comprehensive epigenomic characterization of the 72-kb disease-associated region revealed that it was enriched for active enhancer signatures in human thyroid. Analysis of chromosome conformation capture data showed long-range chromatin interactions of this region with promoters of two genes, FZD5 and CCNYL, mediating Wnt signaling. Moreover, the finding that rs9789446 is a thyroid-specific expression quantitative trait loci, adding further evidence for a cis-regulatory function of this region in thyroid tissue. Specifically, because the risk rs9789446 allele is associated with increased thyroidal expression of FDZ5 and CCNYL1 and given the recent demonstration of perturbed early thyroid development following over-activation of Wnt signaling in zebrafish embryos, an enhanced Wnt signaling in risk allele carriers provide a biologically plausible TD mechanism. In conclusion, our work found the first risk locus for TD, exemplifying that in rare diseases with relatively low biological complexity, GWAS may provide mechanistic insights even with a small sample size.

Whole-Exome Sequencing in Congenital Hypothyroidism Due to Thyroid Dysgenesis

Context: Congenital hypothyroidism due to thyroid dysgenesis (CHTD) is a predominantly sporadic and nonsyndromic (NS) condition of unknown etiology. NS-CHTD shows a 40-fold increase in relative risk among first-degree relatives (1 in 100 compared with a birth prevalence of 1 in 4000 in the general population), but a discordance rate between monozygotic (MZ) twins of 92%. This suggests a two-hit mechanism, combining a genetic predisposition (incomplete penetrance of inherited variants) with postzygotic events (accounting for MZ twin discordance). Objective: To evaluate whether whole-exome sequencing (WES) allows to identify new predisposing genes in NS-CHTD. Methods: We performed a case-control study by comparing the whole exome of 36 nonconsanguineous cases of NS-CHTD (33 with lingual thyroid ectopy and 3 with athyreosis, based on technetium pertechnetate scintigraphy at diagnosis) with that of 301 unaffected controls to assess for enrichment in rare protein-altering variants. We performed an unbiased approach using a gene-based burden with a false discovery rate correction. Moreover, we identified all rare pathogenic and likely pathogenic variants, based on in silico prediction tools, in 27 genes previously associated with congenital hypothyroidism (CH) (thyroid dysgenesis [TD] and dyshormonogenesis). Results: After correction for multiple testing, no enrichment in rare protein-altering variants was observed in NS-CHTD. Pathogenic or likely pathogenic variants (21 variants in 12 CH genes) were identified in 42% of cases. Eight percent of cases had variants in more than one gene (oligogenic group); these were not more severely affected than monogenic cases. Moreover, cases with protein-altering variants in dyshormonogenesis-related genes were not more severely affected than those without. Conclusions: No new predisposing genes were identified following an unbiased analysis of WES data in a well-characterized NS-CHTD cohort. Nonetheless, the discovery rate of rare pathogenic or likely pathogenic variants was 42%. Eight percent of the cases harbored multiple variants in genes associated with TD or dyshormonogenesis, but these variants did not explain the variability of hypothyroidism observed in dysgenesis. WES did not identify a genetic cause in NS-CHTD cases, confirming the complex etiology of this disease. Additional studies in larger cohorts and/or novel discovery approaches are required.

[Genetic of congenital hypothyroidism]

Congenital hypothyroidism (CH) is the most frequent neonatal endocrine disorder. CH is due to thyroid development or thyroid function defects (primary) or may be of hypothalamic-pituitary origin (central). Primary CH is caused essentially by abnormal thyroid gland morphogenesis (thyroid dysgenesis, TD) or defective thyroid hormone synthesis (dyshormonogenesis, DH). DH accounts for about 35% of CH and a genetic cause is identified in 50% of patients. However, TD accounts for about 65% of CH, and a genetic cause is identified in less than 5% of patients. The pathogenesis of CH is largely unknown and may include the contribution of individual and environmental factors. During the last years, detailed phenotypic description of patients, next-generation sequence technologies and use of animal models allowed the discovery of novel candidate genes in thyroid development and function. We provide an overview of recent genetic causes of primary and central CH. In addition, mode of inheritance and the oligogenic model of CH are discussed.

[Clinical guideline of «congenital hypothyroidism»]

Congenital hypothyroidism is an important issue of pediatric endocrinology at which timely diagnosis and treatment can prevent the development of severe cases of the disease. The developed clinical guidelines are a working tool for a practicing physician. The target audience is pediatric endocrinologists and pediatricians. They briefly and logically set out the main definition of the disease, epidemiology, classification, methods of diagnosis and treatment, based on the principles of -evidence-based medicine.

Hepatic incidentaloma: An asymptomatic ectopic thyroid tissue

An ectopic thyroid is a form of thyroid dysgenesis in which the entire thyroid gland or parts of it may be located in another part of the body than the usual place. The most frequent location is the base of the tongue. Although most cases are asymptomatic, symptoms related to tumor size and its relationship with surrounding tissues, hormonal dysfunction, and seldom malignancy may also occur. Here, we describe the case of an asymptomatic woman who was thyroidectomized 19 years previously for a toxic goiter and treated with conventional L-thyroxine therapy, until we enacted a progressive reduction of dosage of the replacement therapy. Incidentally, because of occasional abdomen discomfort, she was hospitalized in our Division of Endocrinology as there was ultrasound evidence of a large mass in the liver dislocating and imprinting the choledochal duct in the pre-pancreatic site, the gallbladder, and the cystic duct, which could not be dissociated from the contiguous hepatic parenchyma and was in very close proximity to the second duodenal portion and the head of the pancreas. Imaging techniques, such as TC, MR, TC/PET, and ¹³¹I scintigraphy, confirmed the large lesion with a diameter on the axial plane of about 8 × 5.5 cm and a cranio-caudal extension of about 6 cm. The impossibility of surgical debulking and/or radiometabolic ¹³¹I therapy, in the absence of compression symptoms, led to the multidisciplinary decision of a clinical and instrumental follow-up of this rare lesion.

New genetics in congenital hypothyroidism

INTRODUCTION

Congenital hypothyroidism (CH) is the most frequent neonatal endocrine disorder and one of the most common preventable forms of mental retardation worldwide. CH is due to thyroid development or thyroid function defects (primary) or may be of hypothalamic-pituitary origin (central). Primary CH is caused essentially by abnormal thyroid gland morphogenesis (thyroid dysgenesis, TD) or defective thyroid hormone synthesis (dyshormonogenesis, DH). TD accounts for about 65% of CH, however a genetic cause is identified in less than 5% of patients.

PURPOSE

The pathogenesis of CH is largely unknown and may include the contribution of individual and environmental factors. During the last years, detailed phenotypic description of patients, next-generation sequence technologies and use of animal models allowed the discovery of novel candidate genes in thyroid development, function and pathways.

RESULTS AND CONCLUSION

We provide an overview of recent genetic causes of primary and central CH. In addition, mode of inheritance and the oligogenic model of CH are discussed.

Mucosa-associated lymphoid tissue (MALT) lymphoma developing in ectopic mediastinal thyroid tissue: a case report

BACKGROUND

Normally located in the neck, ectopic mediastinal thyroid tissue consists of very rare ectopic thyroid tissue that does not connect to the thyroid gland. A patient with mucosa-associated lymphoid tissue (MALT) lymphoma that has developed in mediastinal thyroid tissue, to our best knowledge, has not been previously reported.

CASE PRESENTATION

A 67-year-old woman presented with a superior mediastinal mass that was revealed by chest computed tomography (CT), an optional examination she hoped, during a medical checkup. Contrast-enhanced CT scan performed in our hospital for close examination confirmed the presence of a 2 × 1.3 cm poorly enhanced mass anterior to the trachea during the arterial phase. Magnetic resonance imaging depicted low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. I-131 meta-iodobenzylguanidine did not accumulate in the mass. Serum levels of interleukin-2 receptor, catecholamine, and anti-acetylcholine receptor antibody were within the normal range. The mass was resected through a transverse neck incision for the diagnosis and treatment. The histopathological diagnosis of the specimen was ectopic mediastinal thyroid tissue associated with MALT lymphoma and chronic thyroiditis. Immunoglobulin heavy chain class switch recombination was identified. Fine-needle aspiration biopsy of the cervical thyroid showed chronic thyroiditis but not lymphoma. The patient's postoperative thyroid function was normal. To date, the patient's recovery has been uneventful, and she is being monitored without further treatment.

CONCLUSION

We treated the patient with MALT lymphoma that developed in ectopic mediastinal thyroid tissue. This novel case illustrates a new differential diagnosis associated with ectopic mediastinal thyroid tissue.

Molecular defects in thyroid dysgenesis

Congenital hypothyroidism (CH) is a neonatal endocrine disorder that might occur as itself or be associated to congenital extra-thyroidal defects. About 85% of affected subjects experience thyroid dysgenesis (TD), characterized by defect in thyroid gland development. In vivo experiments on null mice paved the way for the identification of genes involved thyroid morphogenesis and development, whose mutation has been strongly associated to TD. Most of them are thyroid-specific transcription factors expressed during early thyroid development. Despite the arduous effort in unraveling the genetics of TD in animal models, up to now these data have been discontinuously confirmed in humans and only 5% of TD have associated with known null mice-related mutations (mainly PAX8 and TSHR). Notwithstanding, the advance in genetic testing represented by the next-generation sequencing (NGS) approach is steadily increasing the list of genes whose highly penetrant mutation predisposes to TD. In this review we intend to outline the molecular bases of TD, summarizing the current knowledge on thyroid development in both mice and humans and delineating the genetic features of its monogenetic forms. We will also highlight current strategies to enhance the insight into the non-Mendelian mechanisms of abnormal thyroid development.

DIAGNOSIS OF ENDOCRINE DISEASE: Congenital hypothyroidism: update and perspectives

Congenital hypothyroidism (CH) may be primary, due to a defect affecting the thyroid gland itself, or central, due to impaired thyroid-stimulating hormone (TSH)-mediated stimulation of the thyroid gland as a result of hypothalamic or pituitary pathology. Primary CH is the most common neonatal endocrine disorder, traditionally subdivided into thyroid dysgenesis (TD), referring to a spectrum of thyroid developmental abnormalities, and dyshormonogenesis, where a defective molecular pathway for thyroid hormonogenesis results in failure of hormone production by a structurally intact gland. Delayed treatment of neonatal hypothyroidism may result in profound neurodevelopmental delay; therefore, CH is screened for in developed countries to facilitate prompt diagnosis. Central congenital hypothyroidism (CCH) is a rarer entity which may occur in isolation, or (more frequently) in association with additional pituitary hormone deficits. CCH is most commonly defined biochemically by failure of appropriate TSH elevation despite subnormal thyroid hormone levels and will therefore evade diagnosis in primary, TSH-based CH-screening programmes. This review will discuss recent genetic aetiological advances in CH and summarize epidemiological data and clinical diagnostic challenges, focussing on primary CH and isolated CCH.

High-resolution melting analysis (HRM) for mutational screening of Dnajc17 gene in patients affected by thyroid dysgenesis

BACKGROUND

Congenital hypothyroidism is a frequent disease occurring with an incidence of about 1/1500 newborns/year. In about 75% of the cases, CH is caused by alterations in thyroid morphogenesis, defined "thyroid dysgenesis" (TD). TD is generally a sporadic disease but in about 5% of the cases a genetic origin has been demonstrated. Previous studies indicate that Dnajc17 as a candidate modifier gene for hypothyroidism, since it is expressed in the thyroid bud, interacts with NKX2.1 and PAX8 and it has been associated to the hypothyroid phenotype in mice carrying a single Nkx2.1 and Pax8 genes (double heterozygous knock-out).

PURPOSE

The work evaluates the possible involvement of DNAJC17 in the pathogenesis of TD.

METHODS

High-resolution DNA melting analysis (HRM) and direct sequencing have been used to screen for mutations in the DNAJC17 coding sequence in 89 patients with TD.

RESULTS

Two mutations have been identified in the coding sequence of DNAJC17 gene, one in exon 5 (c.350A>C; rs79709714) and one in exon 9 (c.610G>C; rs117485355). The last one is a rare variant, while the rs79709714 is a polymorphism. Both are present in databases and the frequency of the alleles is not different between TD patients and controls.

CONCLUSIONS

DNAJC17 mutations are not frequently present in patients with TD.

Diagnosis and management of a mediastinal ectopic thyroid laying on the right bronchus: case report and review of literature

Background

The mediastinal ectopic thyroid is very rare, accounting for less than 1% of all cases of ectopic thyroid tissue. The differential diagnoses with other diseases such as lymphomas, thymic tumors and dermoid cysts is mandatory, in fact each one, needs different management and treatment.

Case presentation

Here, we discuss a rare case of mediastinal ectopic thyroid presenting with a paratracheal mass laying on the right bronchus without symptoms. A 63-year-old male presented with an abnormal well-defined mass along the right paratracheal side, detected by chest x-ray. The CT scan confirmed the presence of a 6 × 8 cm heterogeneously enhanced mass, located behind the superior vena cava and left brachiocephalic artery, reaching azygos vein and right bronchus, without a mass effect. Taking into account the clinical importance of a mediastinal mass, we removed it surgically, through a double surgical approach consisting in a classical transverse cervicotomy for the left thyroid lobe, followed then by a longitudinal sternal splitting to remove the mediastinal mass and complete the thyroidectomy.

Conclusions

In case of mediastinal masses, the surgical excision is recommended, presenting the double advantage to clarify the diagnosis and to treat the pathology. As demonstrated in this case, a mediastinal ectopic thyroid should be taken into account in the differential diagnosis, considering its clinical importance.

Histopathological Characterization and Whole Exome Sequencing of Ectopic Thyroid: Fetal Architecture in a Functional Ectopic Gland from Adult Patient

Ectopic thyroid results from a migration defect of the developing gland during embryogenesis causing congenital hypothyroidism. But it has also been detected in asymptomatic individuals. This study aimed to investigate the histopathological, functional, and genetic features of human ectopic thyroids. Six samples were histologically examined, and the expression of the specific thyroid proteins was assessed by immunohistochemistry. Two samples were submitted to whole exome sequencing. An oropharynx sample showed immature fetal architecture tissue with clusters or cords of oval thyrocytes and small follicles; one sample exhibited a normal thyroid pattern while four showed colloid goiter. All ectopic thyroids expressed the specific thyroid genes and T4 at similar locations to those observed in normal thyroid. No somatic mutations associated with ectopic thyroid were found. This is the first immature thyroid fetal tissue observed in an ectopic thyroid due to the arrest of structural differentiation early in the colloid stage of development that proved able to synthesize thyroid hormone but not to respond to TSH. Despite the ability of all ectopic thyroids to synthetize specific thyroid proteins and T4, at some point in life, it may be insufficient to support body growth leading to hypothyroidism, as observed in some of the patients.

Disorders of thyroid morphogenesis

Developmental anomalies of the thyroid gland, defined as thyroid dysgenesis, underlie the majority of cases of congenital hypothyroidism. Thyroid dysgenesis is predominantly a sporadic disorder although a reported familial enrichment, variation of incidence by ethnicity and the monogenic defects associated mainly with athyreosis or orthotopic thyroid hypoplasia, suggest a genetic contribution. Of note, the most common developmental anomaly, thyroid ectopy, remains unexplained. Ectopy may result from multiple genetic or epigenetic variants in the germline and/or at the somatic level. This review provides a brief overview of the monogenic defects in candidate genes that have been identified so far and of the syndromes which are known to be associated with thyroid dysgenesis.

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.

Demonstration of Autosomal Monoallelic Expression in Thyroid Tissue Assessed by Whole-Exome and Bulk RNA Sequencing

BACKGROUND

Congenital hypothyroidism due to thyroid dysgenesis (CHTD) is a disorder with a prevalence of 1/4000 live births, the cause of which remains unknown. The most common diagnostic category is thyroid ectopy, which occurs in up to 80% of CHTD cases. CHTD is predominantly not inherited and has a high discordance rate (>92%) between monozygotic (MZ) twins. The sporadic nature of CHTD might be explained by somatic events such as autosomal monoallelic expression (AME), given that genes expressed in a monoallelic way are more vulnerable to otherwise benign monoallelict genetic or epigenetic mutations.

OBJECTIVE

The aim of this study was to search for complete (90%) AME in normal and dysgenetic thyroid tissues.

METHODS

Aggregated analysis of whole-exome and bulk RNA sequencing was performed on two ectopic thyroids, four normal thyroids, and the human thyroid cell line Nthy-ori.

RESULTS

A median of 5062 (range 2081-5270) genes per sample showed sufficient numbers of heterozygous single nucleotide polymorphisms to be informative. The median monoallelic expression represented 22 (range 16-32) of the informative genes for each thyroid sample. Examples of genes displaying AME are FCGBP, ZNF331, USP10, BCLAF1, and some HLA genes; these genes are involved in epithelial-mesenchymal transition, cell migration, cancer, and immunity.

CONCLUSIONS

AME may account for the high discordance rate observed between MZ twins and for the sporadic nature of CHTD. These findings also have implications for other pathologies, including cancers and autoimmune disorders of the thyroid.

Conserved Telomere Length in Human Ectopic Thyroids: An Argument Against Premature Differentiation Causing Arrested Migration

BACKGROUND

In humans, the cause of arrested migration of the median thyroid anlage resulting in an ectopic sublingual gland is unknown. These ectopic glands have a normal follicular architecture but their thyrotropin-induced growth is insufficient, leading to congenital hypothyroidism in the vast majority of affected subjects. We hypothesized that arrested migration is due to premature differentiation [reflected by decreased telomere length (TL)], as observed in neural tube defects in mice.

METHODS

Absolute TL and telomerase reverse transcriptase (hTERT) expression was measured in four ectopic and six orthotopic thyroids. TL was measured by quantitative polymerase chain reaction of genomic DNA, whereas hTERT expression was measured by quantitative polymerase chain reaction of total RNA.

RESULTS

The mean±standard deviation TL (in kilobases per diploid genome) was 140.45±40.07 in ectopic and 97.50±30.48 in orthotopic thyroids (p=0.12). Expression of hTERT was quiescent in both ectopic and orthotopic thyroids.

CONCLUSIONS

Compared with orthotopic thyroids, TL shortening is not observed in ectopic thyroid tissues and, consequently, no compensatory hTERT expression was measured. This makes premature differentiation an unlikely cause of arrested migration and it suggests, indirectly, that ectopic thyroids are not at higher risk of cancer than orthotopic thyroids.

Sublingual thyroid ectopy: similarities and differences with Kallmann syndrome

Permanent primary congenital hypothyroidism (CH), the commonest cause of preventable intellectual disability, is due to defects in the embryonic development of the thyroid in the vast majority of cases. These defects are collectively called thyroid dysgenesis. The thyroid may be absent (athyreosis) but, more commonly, a sublingual thyroid ectopy without lateral lobes, is the only thyroid tissue present. Such an ectopy presumably results from an arrest in the downward migration of the median anlage. Thyroid ectopy almost always occurs in a sporadic fashion. However, first-degree relatives are affected more often than chance alone would predict. On the other hand, almost all reported monozygotic twin pairs are discordant for thyroid ectopy. Current research is aimed at reconciling these contradictory epidemiological data. We propose a two-hit mechanism associating a germline predisposing factor with another genetic or epigenetic alteration within the ectopic thyroid tissue itself or, as in some forms of Kallmann syndrome, in the structures surrounding the thyroid during embryogenesis. Thyroid ectopy, a model for sporadic congenital malformations in humans, is also associated with congenital heart disease, and molecular mechanisms common to thyroid and heart development are being unraveled.

Thyroid transcription factors in development, differentiation and disease

Identification of the thyroid transcription factors (TTFs), NKX2-1, FOXE1, PAX8 and HHEX, has considerably advanced our understanding of thyroid development, congenital thyroid disorders and thyroid cancer. The TTFs are fundamental to proper formation of the thyroid gland and for maintaining the functional differentiated state of the adult thyroid; however, they are not individually required for precursor cell commitment to a thyroid fate. Although knowledge of the mechanisms involved in thyroid development has increased, the full complement of genes involved in thyroid gland specification and the signals that trigger expression of the genes that encode the TTFs remain unknown. The mechanisms involved in thyroid organogenesis and differentiation have provided clues to identifying the genes that are involved in human congenital thyroid disorders and thyroid cancer. Mutations in the genes that encode the TTFs, as well as polymorphisms and epigenetic modifications, have been associated with thyroid pathologies. Here, we summarize the roles of the TTFs in thyroid development and the mechanisms by which they regulate expression of the genes involved in thyroid differentiation. We also address the implications of mutations in TTFs in thyroid diseases and in diseases not related to the thyroid gland.

Thyroid C-Cell Biology and Oncogenic Transformation

The thyroid parafollicular cell, or commonly named "C-cell," functions in serum calcium homeostasis. Elevations in serum calcium trigger release of calcitonin from the C-cell, which in turn functions to inhibit absorption of calcium by the intestine, resorption of bone by the osteoclast, and reabsorption of calcium by renal tubular cells. Oncogenic transformation of the thyroid C-cell is thought to progress through a hyperplastic process prior to malignancy with increasing levels of serum calcitonin serving as a biomarker for tumor burden. The discovery that multiple endocrine neoplasia type 2 is caused by activating mutations of the RET gene serves to highlight the RET-RAS-MAPK signaling pathway in both initiation and progression of medullary thyroid carcinoma (MTC). Thyroid C-cells are known to express RET at high levels relative to most cell types; therefore, aberrant activation of this receptor is targeted primarily to the C-cell, providing one possible cause of tissue-specific oncogenesis. The role of RET signaling in normal C-cell function is unknown though calcitonin gene transcription appears to be sensitive to RET activation. Beyond RET, the modeling of oncogenesis in animals and screening of human tumors for candidate gene mutations have uncovered mutation of RAS family members and inactivation of Rb1 regulatory pathway as potential mediators of C-cell transformation. A growing understanding of how RET interacts with these pathways, both in normal C-cell function and during oncogenic transformation, will help in the development of novel molecular-targeted therapies.

Somatic mutations are not observed by exome sequencing of lymphocyte DNA from monozygotic twins discordant for congenital hypothyroidism due to thyroid dysgenesis

BACKGROUND/AIMS

Congenital primary hypothyroidism (CH) is a rare pediatric disorder estimated to occur in about 1:2,500 live births. Approximately half of these cases entail ectopic thyroid tissue, which is believed to result from a migration defect during embryogenesis. Approximately 3% of CH cases are explained by mutation(s) in known genes, most of which are transcription factors implicated in the embryology of the thyroid gland. Surprisingly, monozygotic (MZ) twins are usually discordant for CH due to thyroid dysgenesis, suggesting that most cases are not caused by transmitted genetic variation. One possible explanation is somatic mutation in genes involved in thyroid migration occurring after zygotic twinning. Such mutations should be observed only in the affected twin.

METHODS

To test the hypothesis of somatic mutation, we performed whole exome sequencing of DNA from three pairs of MZ twins discordant for CH with ectopic glands.

RESULTS

We found no somatic mutations exclusive to any of the three affected twins or in any of the unaffected twins.

CONCLUSION

Either somatic mutations are not significant for the etiology of CH or else such mutations lie outside regions of the genome accessible by exome sequencing technology.

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.

Role for tissue-dependent methylation differences in the expression of FOXE1 in nontumoral thyroid glands

BACKGROUND

Discordance of monozygotic twins for thyroid dysgenesis suggests that epigenetic mechanisms may underlie defects in thyroid gland development. This prompted us to evaluate whether differentially methylated regions (DMRs) can be found between human thyroids (either eutopic or ectopic) and matched leukocytes.

METHODS

To compare the genome-wide methylation profile of thyroids and leukocytes, immunoprecipitated methylated DNA was interrogated on human promoter plus CpG island tiling arrays. In addition, the methylation profile of the human FOXE1, PAX8, and NKX2.1 promoter was examined using bisulfite sequencing. Finally, the functional impact of CpG methylation of the promoter on FOXE1 expression was assessed with luciferase assays.

RESULTS

Genome-wide methylation profiling and bisulfite sequencing of CpG islands of PAX8 and NKX2.1 promoters revealed no DMR between thyroid and leukocytes. However, bisulfite sequencing revealed that the methylation level of two consecutive CpG dinucleotides (CpG14 and CpG15, which were not covered by the genome-wide array) in one CpG island of the FOXE1 promoter (-1600 to -1140 from the transcription start site) is significantly higher in leukocytes than in eutopic or ectopic thyroid tissues, suggesting that methylation of this region may decrease FOXE1 gene expression. Indeed, luciferase activities were decreased when FOXE1 promoter constructs were methylated in vitro. Moreover, derepression of luciferase activity was observed when the methylation of CpG14 and CpG15 was prevented by mutations.

CONCLUSION

We report a tissue-dependent DMR in the FOXE1 promoter. This DMR contains two consecutive CpG dinucleotides, which are epigenetic modifiers of FOXE1 expression in nontumoral tissues.

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.

Mechanisms of thyroid development and dysgenesis: an analysis based on developmental stages and concurrent embryonic anatomy

Thyroid dysgenesis is the most common cause of congenital hypothyroidism that affects 1 in 3000 newborns. Although a number of pathogenetic mutations in thyroid developmental genes have been identified, the molecular mechanism of disease is unknown in most cases. This chapter summarizes the current knowledge of normal thyroid development and puts the different developmental stages in perspective, from the time of foregut endoderm patterning to the final shaping of pharyngeal anatomy, for understanding how specific malformations may arise. At the cellular level, we will also discuss fate determination of follicular and C-cell progenitors and their subsequent embryonic growth, migration, and differentiation as the different thyroid primordia evolve and merge to establish the final size and shape of the gland.

Ectopic lingual thyroid

Thyroid ectopy is characterized by the presence of thyroid tissue outside its normal position resulting from a defect of the thyroid diverticulum migration from the base of the tongue until its final pre-tracheal position. One case is presented in a 12-year-old girl patient who consults for a failure to thrive estimated at less than three standard deviations (SD). Bone age was estimated at 8 years late compared to chronological age. The hormonal assessment showed hypothyroidism with negative thyroid antibodies. Cervical ultrasound was revealed thyroid parenchyma pre-dominantly left in place while sweeping the area under chin showed a nodular formation of the base of the tongue. Thyroid scan with technetium 99 m showed a selective uptake of radiotracer in sublingual position. Cervical computed tomography revealed a posterior median sublingual mass spontaneously hyperdense and enhancing sharply after injection of contrast. Treatment with thyroxine allowed obtaining euthyroidism. This case asks us to be careful before aetiological diagnosis of hypothyroidism in children, because although this is rare, the presence of a thyroid parenchyma up to the cervical ultrasound does not eliminate the presence of ectopic tissue.

Evidence for calcitonin-producing cells in human lingual thyroids

CONTEXT

The thyroid contains two types of cells, the thyroid follicular cells and the calcitonin-producing cells. The site of origin of the thyroid follicular cells is the median thyroid anlage, an endothelial diverticulum in the midline of the ventral pharynx between the first and the second pharyngeal pouches. The ultimobranchial bodies (UBB), a pair of transient embryonic structures evaginated from the fourth pharyngeal pouch and located symmetrically on the sides of the developing neck, are the source of calcitonin-producing cells. In human embryos, the thyroid bud starts its migration at embryonic day 24 and reaches its final location in front of the trachea at embryonic day 45-50. The UBB fuse with the primitive thyroid when thyroid migration is completed. Lingual thyroids result from the failure of the thyroid precursor cells to migrate from the primordial pharynx to the anterior part of the neck. Therefore, calcitonin-producing cells are not expected to be present in lingual thyroids.

OBJECTIVE

Our objective was to determine whether calcitonin-producing C cells are present in ectopic lingual thyroids.

DESIGN, SETTING, PATIENTS, AND MAIN OUTCOME MEASURE

We performed calcitonin immunolabeling and transcript detection on four flash-frozen ectopic lingual thyroids. Additional calcitonin immunolabeling was performed on two other paraffin-embedded ectopic lingual thyroids.

RESULTS

We report evidence of calcitonin-producing cells in six independent cases of ectopic lingual thyroids.

CONCLUSION

The UBB are not the only source of calcitonin-producing cells in humans. Interactions between calcitonin-producing and thyroid follicular cells occur earlier than previously accepted.

TSH receptor function is required for normal thyroid differentiation in zebrafish

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