Thyroid dysgenesis: multigenic or epigenetic ... or both?

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.

Enhanced Canonical Wnt Signaling During Early Zebrafish Development Perturbs the Interaction of Cardiac Mesoderm and Pharyngeal Endoderm and Causes Thyroid Specification Defects

Background: Congenital hypothyroidism due to thyroid dysgenesis is a frequent congenital endocrine disorder for which the molecular mechanisms remain unresolved in the majority of cases. This situation reflects, in part, our still limited knowledge about the mechanisms involved in the early steps of thyroid specification from the endoderm, in particular the extrinsic signaling cues that regulate foregut endoderm patterning. In this study, we used small molecules and genetic zebrafish models to characterize the role of various signaling pathways in thyroid specification. Methods: We treated zebrafish embryos during different developmental periods with small-molecule compounds known to manipulate the activity of Wnt signaling pathway and observed effects in thyroid, endoderm, and cardiovascular development using whole-mount in situ hybridization and transgenic fluorescent reporter models. We used the antisense morpholino (MO) technique to create a zebrafish acardiac model. For thyroid rescue experiments, bone morphogenetic protein (BMP) pathway induction in zebrafish embryos was obtained by manipulation of heat-shock inducible transgenic lines. Results: Combined analyses of thyroid and cardiovascular development revealed that overactivation of Wnt signaling during early development leads to impaired thyroid specification concurrent with severe defects in the cardiac specification. When using a model of MO-induced blockage of cardiomyocyte differentiation, a similar correlation was observed, suggesting that defective signaling between cardiac mesoderm and endodermal thyroid precursors contributes to thyroid specification impairment. Rescue experiments through transient overactivation of BMP signaling could partially restore thyroid specification in models with defective cardiac development. Conclusion: Collectively, our results indicate that BMP signaling is critically required for thyroid cell specification and identify cardiac mesoderm as a likely source of BMP signals.

Familial Aggregation of Endemic Congenital Hypothyroidism Syndrome in Congo (DR): Historical Data

Familial aggregation of endemic congenital hypothyroidism (CH) in an iodine-deficient population from northern Congo (Democratic Republic (DR)) was analysed on data collected four decades ago (1979-1980). During a systematic survey of 62 families, 46 endemic CH subjects (44 myxedematous and 2 neurological) were identified based on clinical evidence within a village cohort of 468 subjects. A distribution analysis showed that two families presented significant excess of cases versus a random background distribution. Both families were characterised by two healthy parents having all of their five offspring affected by some form of endemic CH. Goitre prevalence in endemic CH was lower than that in the general population, while goitre prevalence in the unaffected part of the cohort (parents and siblings) was similar to that of the general population. Some unidentified genetic/epigenetic factor(s) could contribute to the evolution of some iodine-deficient hypothyroid neonates through irreversible and progressive loss of thyroid functional capacity during early childhood (<5 years old). Besides severe iodine deficiency, environmental exposure to thiocyanate overload and selenium deficiency, factors not randomly distributed within families and population, intervened in the full expression of endemic CH. Further exploration in the field will remain open, as iodine deficiency in Congo (DR) was eliminated in the 1990s.

Thyroid isthmus agenesis and its clinical significance in a large-scale multidetector CT-based study

OBJECTIVE

The purpose of this study was to estimate the incidence and clinical significance of thyroid isthmus agenesis based on multi-detector CT imaging in a large-scale study.

METHODS

Data from 1601 patients who underwent neck multi-detector CT at our institution from January 2015 to March 2016 were included in this retrospective study. The morphology of each patient's thyroid gland was evaluated. We classified thyroid isthmus agenesis into two subgroups according to the thickness of the medial margin: clear-cut type (>4 mm) or tapering-edge type (≤4 mm). Associated thyroid pathologies were also evaluated.

RESULTS

Thyroid isthmus agenesis was present in 69 patients (41 males, 28 females), and its incidence was 4.77%. Eleven patients (0.76%) had the clear-cut type and 58 patients (4.01%) had the tapering-edge type. Papillary thyroid carcinoma was diagnosed in 4 patients (5.7% of isthmus agenesis patients). A total of 7 patients underwent thyroid function testing during the course of this study; 1 of these patients presented with borderline hyperthyroidism, and the remaining 6 were in a euthyroid state. The clear-cut type showed a statistically significant narrow gap and a high incidence of pyramidal lobes compared to the tapering-edge type.

CONCLUSION

Thyroid isthmus agenesis is not a rare developmental anomaly of the thyroid gland on multi-detector CT. Based on the metastatic pathophysiology of differentiated thyroid cancer, more extended indications for lobectomy are expected in differentiated thyroid cancer patients with thyroid isthmus agenesis.

Defects of Thyroid Hormone Synthesis and Action

Congenital hypothyroidism (CH) is the most common inborn endocrine disorder and causes significant morbidity. To date, we are only aware of the molecular basis responsible for the defects in a small portion of patients with CH. A better understanding of the pathophysiology of these cases at the genetic and molecular basis provides useful information for proper counseling to patients and their families a well as for the development of better targeted therapies. This article provides a succinct outline of the pathophysiology and genetics of the known causes of thyroid dysgenesis, dyshormonogenesis, and syndrome of impaired sensitivity to thyroid hormone.

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.

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.

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.

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.

Scintigraphic detection of dual ectopic thyroid tissue: experience of a Chinese tertiary hospital

Purpose

To assess scintigraphic pattern, clinical indication and relevance of dual ectopic thyroid tissue (ETT). Literature is reviewed for such cases.

Methods

In this 5-year retrospective study, we reviewed all thyroid scintigraphies in our data base. Patients diagnosed with suspected ETT were identified. Literature is reviewed. Statistics were done by one-way analysis of variance and least significant difference test.

Results

From 11905 thyroid scintigraphies during the 5-year period, we retrieved 121 patients eligible for analysis. The top two indications were assessing a palpable front neck mass to determine whether it was an ETT, and primary hypothyroidism. Patients were divided into 3 groups. Group 1 with single ETT (83 cases); group 2 with dual ETT (6 cases) and group 3 with athyroid (32 cases). Age and thyroid hormones were highest in group 2, and lowest in group 3. Thyrotropin was highest in group 3, and lowest in group 2. Thyroxine was given to hypothyroid patients, while no surgery was performed. There were 42 published cases with dual ETT, most of whom were under 30 years old. 38.10% of them were euthyroid, 33.33% hypothyroid, and 21.43% subclinical hypothyroid. Most frequent ectopic positions included lingual (33.73%), sublingual (27.71%) and subhyoid (22.89%).

Conclusions

In our cohort, incidence of dual ETT was 0.05% if the denominator was total number of thyroid scintigraphies. The incidence was 4.96% if the denominator was the number of patients with suspected ETT. Important clinical indication is a front neck palpable mass suggestive of an ETT. Important clinical relevance of recognizing the dual ETT pattern is to avoid inappropriate surgery. After reviewing all published cases, we find dual ETT is often seen in young patients. Most of such patients are euthyroid or mildly hypothyroid. Thyroid ectopia often resides in lingual, sublingual and subhyoid areas.

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.

A new PAX8 mutation causing congenital hypothyroidism in three generations of a family is associated with abnormalities in the urogenital tract

BACKGROUND

Although thyroid dysgenesis is the most common cause of congenital hypothyroidism (CH), its molecular basis remains largely elusive. Indeed, in only a minority of cases with thyroid dysgenesis (2%-3%) was it possible to identify an underlying genetic defect. The objective of this study was to screen the PAX8 gene and the PAX2 gene in a family with six cases of CH spanning three generations and presenting urogenital malformations. Herein, we report a case series and in vitro characterization of the PAX8 gene mutation.

METHODS

Investigations were conducted at a tertiary care referral center. The index case was diagnosed to have congenital hypothyroidism at 7 months of age when he presented with severe impairment of suckling, constipation, and poor development. Treatment with levothyroxine corrected the symptoms and was associated with catch-up growth. His progeny, including two sons, one daughter, and two granddaughters, were affected by CH, and three of them received the diagnosis at neonatal screening. Ultrasound demonstrated normally located thyroid glands with reduced volumes. Five of the six affected family members, including the index case, had urogenital malformations, including incomplete horseshoe kidney, undescended testicles, hydrocele, and ureterocele. Strabismus was found in three out of six affected patients. No other somatic malformations were found.

RESULTS

Direct sequencing of the PAX8 gene revealed a new heterozygous mutation (c.74C > G) in all affected individuals. This mutation leads to substitution of proline with arginine at codon 25 (P25R). Fluorescence microscopy showed that P25R is normally located in the nucleus. In transient transfection studies, this mutation causes reduced transcriptional activation ability when using a luciferase reporter construct under the control of a thyroglobulin promoter. This diminished transactivation ability is due to loss of DNA binding capability as shown in electrophoresis mobility shift assay. The sequencing analysis of the PAX2 gene was normal.

CONCLUSIONS

We conclude that this novel PAX8 mutation is responsible for a severe form of dominantly inherited CH. The mutation seems to be associated with abnormalities of the urogenital tract.

Genetic, environmental and stochastic factors in monozygotic twin discordance with a focus on epigenetic differences

Genetic-epidemiological studies on monozygotic (MZ) twins have been used for decades to tease out the relative contributions of genes and the environment to a trait. Phenotypic discordance in MZ twins has traditionally been ascribed to non-shared environmental factors acting after birth, however recent data indicate that this explanation is far too simple. In this paper, we review other reasons for discordance, including differences in the in utero environment, genetic mosaicism, and stochastic factors, focusing particularly on epigenetic discordance. Epigenetic differences are gaining increasing recognition. Although it is clear that in specific cases epigenetic alterations provide a causal factor in disease etiology, the overall significance of epigenetics in twin discordance remains unclear. It is also challenging to determine the causality and relative contributions of environmental, genetic, and stochastic factors to epigenetic variability. Epigenomic profiling studies have recently shed more light on the dynamics of temporal methylation change and methylome heritability, yet have not given a definite answer regarding their relevance to disease, because of limitations in establishing causality. Here, we explore the subject of epigenetics as another component in human phenotypic variability and its links to disease focusing particularly on evidence from MZ twin studies.

A high prevalence of dual thyroid ectopy in congenital hypothyroidism: evidence for insufficient signaling gradients during embryonic thyroid migration or for the polyclonal nature of the thyroid gland?

BACKGROUND

Thyroid ectopy results from the failure of the thyroid precursor cells to migrate from the primordial pharynx to the anterior part of the neck. Most ectopic thyroids are revealed by congenital hypothyroidism and present as a single round mass at the base of the tongue, with no other thyroid tissue. However, some cases have dual ectopy, with part of the tissue having partially migrated. We hypothesized that this occurs more frequently than previously reported.

METHODS

To determine the prevalence of dual ectopy, we reviewed the pertechnetate scintigraphies of 81 patients with congenital hypothyroidism from thyroid ectopy diagnosed between 2002 and 2011 at our institution.

RESULTS

We report a series of seven cases (9%) of dual ectopy, representing an incidence ranging from 1:50,000 to 1:70,000.

CONCLUSIONS

Almost one in 10 cases with congenital hypothyroidism due to thyroid ectopy has dual ectopy. This suggests that two populations of cells diverged at an early stage of development, which may arise from insufficient signaling gradients in surrounding tissues during early organogenesis or may indirectly support the polyclonal nature of the thyroid.

Absence of mutations in PAX8, NKX2.5, and TSH receptor genes in patients with thyroid dysgenesis

OBJECTIVES: To precisely classify the various forms of TD, and then to screen for mutations in transcription factor genes active in thyroid development. SUBJECTS AND METHODS: Patients underwent ultrasound, thyroid scan, and serum thyroglobulin measurement to accurately diagnose the form of TD. DNA was extracted from peripheral leukocytes. The PAX8, and NKX2.5 genes were evaluated in all patients, and TSH receptor (TSHR) gene in those with hypoplasia. RESULTS: In 27 nonconsanguineous patients with TD, 13 were diagnosed with ectopia, 11 with hypoplasia, and 3 with athyreosis. No mutations were detected in any of the genes studied. CONCLUSION: Sporadic cases of TD are likely to be caused by epigenetic factors, rather than mutations in thyroid transcription factors or genes involved in thyroid development.

Deletion of the RNaseIII enzyme dicer in thyroid follicular cells causes hypothyroidism with signs of neoplastic alterations

Micro-RNAs (miRNAs) are small non-coding RNAs that regulate gene expression, mainly at mRNA post-transcriptional level. Functional maturation of most miRNAs requires processing of the primary transcript by Dicer, an RNaseIII-type enzyme. To date, the importance of miRNA function for normal organogenesis has been demonstrated in several mouse models of tissue-specific Dicer inactivation. However, the role of miRNAs in thyroid development has not yet been addressed. For the present study, we generated mouse models in which Dicer expression has been inactivated at two different stages of thyroid development in thyroid follicular cells. Regardless of the time of Dicer invalidation, the early stages of thyroid organogenesis, preceding folliculogenesis, were unaffected by the loss of small RNAs, with a bilobate gland in place. Nevertheless, Dicer mutant mice were severely hypothyroid and died soon after weaning unless they were substituted with T4. A conspicuous follicular disorganization was observed in Dicer mutant thyroids together with a strong down regulation of Nis expression. With increasing age, the thyroid tissue showed characteristics of neoplastic alterations as suggested by a marked proliferation of follicular cells and an ongoing de-differentiation in the center of the thyroid gland, with a loss of Pax8, FoxE1, Nis and Tpo expression. Together, our data show that loss of miRNA maturation due to Dicer inactivation severely disturbs functional thyroid differentiation. This suggests that miRNAs are mandatory to fine-tune the expression of thyroid specific genes and to maintain thyroid tissue homeostasis.

Gene expression profiling at early organogenesis reveals both common and diverse mechanisms in foregut patterning

The thyroid and lungs originate as neighboring bud shaped outgrowths from the midline of the embryonic foregut. When and how organ specific programs regulate development into structures of distinct shapes, positions and functions is incompletely understood. To characterize, at least in part, the genetic basis of these events, we have employed laser capture microdissection and microarray analysis to define gene expression in the mouse thyroid and lung primordia at E10.5. By comparing the transcriptome of each bud to that of the whole embryo as well as to each other, we broadly describe the genes that are preferentially expressed in each developing organ as well as those with an enriched expression common to both. The results thus obtained provide a valuable resource for further analysis of genes previously unrecognized to participate in thyroid and lung morphogenesis and to discover organ specific as well as common developmental mechanisms. As an initial step in this direction we describe a regulatory pathway involving the anti-apoptotic gene Bcl2 that controls cell survival in early thyroid development.

Minireview: Intrinsic and extrinsic factors in thyroid gland development: an update

In vertebrates the portion of the thyroid gland synthesizing the thyroid hormones develops from a small group of endodermal cells in the foregut. The nature of the signals that lead to the biochemical and morphogenetic events responsible for the organization of these cells into the adult thyroid gland has only recently become evident. In this review we summarize recent developments in the understanding of these processes, derived from evidence collected in several organisms.

Mutations in the NKX2.5 gene and the PAX8 promoter in a girl with thyroid dysgenesis

CONTEXT

Screening of the known candidate genes involved in thyroid organogenesis has revealed mutations in a small subset of patients with congenital hypothyroidism due to thyroid dysgenesis (TD).

OBJECTIVE

We studied a girl with TD who had mutations in two transcription factors involved in thyroid development.

RESULTS

Sequencing analysis of candidate genes involved in thyroid gland development revealed a new paternally inherited heterozygous mutation in the NKX2.5 gene (S265R) and a new maternally inherited heterozygous mutation in the PAX8 promoter region (-456C>T). Both parents and a brother, who was also heterozygous for both mutations, were phenotypically normal. Immunofluorescence microscopy showed a correct nuclear localization of both wild-type (WT) and mutant NKX2.5 proteins. EMSA demonstrated that the mutant NKX2.5 binds to the NKE_2, DIO2, TG, and TPO promoter elements equally well as the WT protein. However, the mutant NKX2.5 protein showed a 30-40% reduced transactivation of the thyroglobulin and the thyroid peroxidase promoters and a dominant-negative effect of the mutant NKX2.5. EMSA studies of the WT and mutant PAX8 promoter sequences incubated with nuclear extracts from PCCL3 cells exhibited a loss of protein binding capacity of the mutant promoter. In addition, the mutant PAX8 promoter showed a significantly reduced transcriptional activation of a luciferase reporter gene in vitro. Thus, this promoter mutation is expected to lead to reduced PAX8 expression.

CONCLUSIONS

We identified new heterozygous mutations in both NKX2.5 and PAX8 genes of a girl with TD. Both defects might contribute to the phenotype.

Space-time clustering of elevated thyroid stimulating hormone levels

Previous studies of congenital hypothyroidism (CHT) have reported an increasing incidence which may suggest that environmental factors play an aetiological role. If so, then cases may exhibit space-time clustering, where cases occur at similar times and close proximities to other cases. In this study we investigated whether space-time clustering of elevated thyroid stimulating hormone (TSH) in newborns exists. All infants born in the Northern Region of England are screened by measuring levels of circulating TSH using a blood spot assay. Data on 207 cases of elevated TSH values, as a proxy for CHT, in newborns born from 1994 to 2006 inclusive were available and analysed using rigorous space-time clustering statistical methods. Analysis showed statistically significant evidence of space-time clustering. The strength of clustering was most marked for cases born within 0.1-0.7 year (1-8 months) of one another. This is the first study to find significant space-time clustering of cases of elevated TSH levels in newborns, a surrogate for space-time clustering of CHT. Whilst the reasons for the clustering are unclear, it would appear from this analysis that transient environmental exposures are likely to be involved, although environmental determinants of genetic mutations and epigenetic factors cannot be ruled out. Further research is required to a) validate these results in other populations and b) to assess in more detail the potential environmental determinants of increased CHT risk.

Variation by ethnicity in the prevalence of congenital hypothyroidism due to thyroid dysgenesis

BACKGROUND

The scant data on ethnic differences in the prevalence of congenital hypothyroidism (CH) have generally not taken etiology of CH into account. Our hypothesis is that the prevalence of CH due to thyroid dysgenesis (TD) varies by ethnicity.

METHODS

This case-control study included all patients with CH due to TD (a condition of unknown origin) or to dyshormonogenesis (DH, of known autosomal recessive transmission) between 1987 and 2009. Etiology was established by (99m)Tc scintigraphy. The parents self-assessed their ethnicity, which we grouped in Caucasian, Hispanic, black, Asian, and Maghreb/Middle East. We compared ethnicity between the 190 patients with TD (147 ectopies, 40 athyreoses, and 3 orthotopic hypoplasias) and the 44 patients with DH. Ethnicity was also compared to the reference population of the city of Montreal. Prevalence odds ratios (POR) were calculated and compared by the bilateral Fisher's exact test.

RESULTS

The ethnic composition of the DH group was similar to that of the reference population. In blacks, TD prevalence of 1 in 190 (0.5%) was low compared to that of DH (4 in 44; 9.1%; POR 0.06; 95% confidence interval: 0.001-0.56; p = 0.005). In contrast, Caucasians showed an increased TD prevalence of 166 in 190 (87.3%) compared to that of DH (30 in 44; 68.2%; POR 3.21; 95% confidence interval: 1.37-7.34; p = 0.0052). No statistically significant differences were observed between other ethnic groups.

CONCLUSION

TD is less prevalent in blacks and more prevalent in Caucasians. Blacks being more genetically diverse, this is an argument for an oligogenic inheritance of susceptibility to TD.

Characterization of a novel loss-of-function mutation of PAX8 associated with congenital hypothyroidism

BACKGROUND

Congenital hypothyroidism (CH) is a common endocrine disease that occurs in about 1:3000 newborns. In 80-85% of the cases, CH is presumably secondary to thyroid dysgenesis (TD), a defect in the organogenesis of the gland leading to an ectopic (30-45%), absent (agenesis, 35-40%) or hypoplastic (5%) thyroid gland. The pathogenesis of TD is still largely unknown. Most cases of TD are sporadic, although familial occurrences have occasionally been described. Recently, mutations in the PAX8 transcription factor have been identified in patients with TD.

OBJECTIVE

Our aim was to identify and functionally characterize novel PAX8 mutations with autosomal dominant transmission responsible for TD.

DESIGN

The PAX8 gene was sequenced in a mother and child both suffering from congenital hypothyroidism (CH) because of thyroid hypoplasia. Subsequently, expression vectors encoding the mutated PAX8 were generated, and the effects of the mutation on both the DNA-binding capability and the transcriptional activity were evaluated.

RESULTS

PAX8 gene sequencing revealed a heterozygous mutation that consists of the substitution of a histidine residue with a glutamine at position 55 of the PAX8 protein (H55Q). When tested in cotransfection experiments with a thyroglobulin promoter reporter construct, the mutant protein turned out to be still able to bind DNA in Electrophoretic Mobility Shift Assay assays but transcriptionally inactive.

CONCLUSIONS

Our findings confirm the important role of PAX8 in normal thyroid development and support the evidence that in humans haploinsufficiency of PAX8 is associated with TD.

A locus on mouse chromosome 2 is involved in susceptibility to congenital hypothyroidism and contains an essential gene expressed in thyroid

We report here the mapping of a chromosomal region responsible for strain-specific development of congenital hypothyroidism in mice heterozygous for null mutations in genes encoding Nkx2-1/Titf1 and Pax8. The two strains showing a differential predisposition to congenital hypothyroidism contain several single-nucleotide polymorphisms in this locus, one of which leads to a nonsynonymous amino acid change in a highly conserved region of Dnajc17, a member of the type III heat-shock protein-40 (Hsp40) family. We demonstrate that Dnajc17 is highly expressed in the thyroid bud and had an essential function in development, suggesting an important role of this protein in organogenesis and/or function of the thyroid gland.

Increasing Incidence, but Lack of Seasonality, of Elevated TSH Levels, on Newborn Screening, in the North of England

Previous studies of congenital hypothyroidism have suggested an increasing incidence and seasonal variation in incidence, which may suggest nongenetic factors involved in aetiology. This study describes the incidence of elevated thyroid stimulating hormone (TSH) values in newborns, a surrogate for congenital hypothyroidism, measured as part of the screening programme for congenital hypothyroidism, over an eleven-year period (1994-2005), and assesses whether seasonal variation exists. All infants born in the Northern Region of England are screened by measuring levels of circulating TSH using a blood spot assay. Data on all 213 cases born from 1994 to 2005 inclusive were available. Annual incidence increased significantly from 37 per 100,000 in 1994 to a peak of 92.8 per 100,000 in 2003. There was no evidence of seasonal variation in incidence. The reasons for the increasing incidence are unclear, but do not appear to involve increasing exposure to seasonally varying factors or changes in measurements methods.

[New aspects of genetics and molecular mechanisms on thyroid morphogenesis for the understanding of thyroid dysgenesia]

The elucidation of the molecular mechanisms underlying the very early steps of thyroid organogenesis and the etiology of most cases of thyroid dysgenesis are poorly understood. Many genes have been identified as important contributors to survival, proliferation and migration of thyroid cells precursors, acting as an integrated and complex regulatory network. Moreover, by generation of mouse mutants, the studies have provided better knowledge of the role of these genes in the thyroid morphogenesis. In addition, it is likely that a subset of patients has thyroid dysgenesis as a result of mutations in regulatory genes expressed during embryogenesis. This review summarizes molecular aspects of thyroid development, describes the animal models and phenotypes known to date and provides information about novel insights into the ontogeny and pathogenesis of human thyroid dysgenesis.

Clinical and molecular analysis of thyroid hypoplasia: a population-based approach in southern Brazil

BACKGROUND

Congenital hypothyroidism (CH) is mainly due to developmental abnormalities leading to thyroid dysgenesis (TD). TD encompasses very distinct morphologic subtypes of disease. This study examined and compared the phenotype in TD variants and searched for genetic alterations in sporadic thyroid hypoplasia (TH), the most misdiagnosed form of CH. This was a longitudinal study over a 14-year period (1990-2004).

METHODS

A continuous series of 353 children with TD was identified using thyroid function tests [thyroxine (T4) and TSH], scintigraphy, and ultrasound as diagnostic tools. Individual phenotypes were analyzed in 253 children with TD. Mutations in the most likely candidate genes were studied in 35 cases of TH.

RESULTS

The overall birth prevalence of permanent CH was 1:4795. Ectopy represented 37% of all cases of permanent primary CH, dyshormonogenesis 28%, agenesis 24%, hypoplasia 10%, and hemiagenesis 1%. The lowest screening T4 level and the highest TSH level were in the agenetic group, followed by TH. The TH group had an improvement in the thyroid function showing less-severe phenotype with aging. In the molecular analysis, one patient was identified with a mutation in the PAX8 gene (155G>C; R52P); four patients had a heterozygous G>C substitution in position -569; two patients showed a (234C>A; P52T) or (2181C>G; D727E) polymorphic variants of the TSH-R gene; and one patient presented a novel heterozygous nonsynonymous substitution, 293G>A; S98N, in the NKX2.5 gene.

CONCLUSIONS

The prevalence of CH was within the previously reported range of 1:3000-4000. Ectopy was the most common etiology. Clinical analysis revealed distinct hormonal patterns in TH subgroup when compared with other variants of TD, with genetic abnormalities identified only in few cases in the TSH-R, PAX8, and NKX2.5 genes.

Identification and characterization of four PAX8 rare sequence variants (p.T225M, p.L233L, p.G336S and p.A439A) in patients with congenital hypothyroidism and dysgenetic thyroid glands

CONTEXT

Thyroid dysgenesis may be associated with mutations in the paired box transcription factor 8 (PAX8) gene and is characterized by congenital hypothyroidism transmitted in an autosomal dominant mode.

OBJECTIVES

The aim of this study was to identify new mutations in the PAX8 gene. Sixty congenital hypothyroidism-affected individuals with dysgenetic (agenesis, ectopia and hypoplasia) and eutopic thyroid glands were studied.

METHODS

The 12 exons of the PAX8 gene along with their exon-intron boundaries were amplified from genomic DNA and a mutational screening was performed by single-strand conformational polymorphism (SSCP) followed by direct sequencing of samples with abnormal migration patterns. The PAX8 mutations were functionally characterized by transient transfection experiments.

RESULTS

Molecular analysis of the PAX8 gene indicated that four affected individuals had four sequence differences: three novel variations [c.699C>T (p.L233L), c.1006G>A (p.G336S) and c.1317A>G (p.A439A)] and one recently reported [c.674C>T (p.T225M)], whereas the 56 remaining patients showed only wild-type alleles of PAX8. p.T225M, p.L233L and p.G336S variants were not detected in 530 chromosomes from 265 subjects randomly selected from the general population, whereas the p.A439A variant was identified in only one of the 530 chromosomes analysed. Functional analysis of the nonsynonymous substitutions showed that the p.T225M and p.G336S proteins had not lost their ability to bind a specific DNA sequence and to activate the transcription of the thyroglobulin (TG) promoter in synergy with thyroid transcription factor 1 (TTF1).

CONCLUSIONS

We report the occurrence of two nonsynonymous substitutions, one recently reported (p.T225M) and one novel (p.G336S), and two novel synonymous substitutions (p.L233L and p.A439A) in the PAX8 gene. p.T225M and p.G336S are rare sequence variants or may act by inhibiting an unknown particular function. Our study also confirms the very low prevalence of PAX8 mutations in thyroid dysgenesis.

Random variability in congenital hypothyroidism from thyroid dysgenesis over 16 years in Québec

CONTEXT

Research on the etiology of congenital hypothyroidism from thyroid dysgenesis (CHTD) (comprising mostly ectopy and agenesis) over the past decade has focused on genetic mechanisms. However, the possibility that environmental factors might be involved has been raised by studies showing a seasonal variability of the incidence of CHTD.

OBJECTIVES

The objective of this study was to assess the variability in incidence of CHTD in the province of Québec, Canada.

DESIGN, SETTING, PATIENTS, AND MAIN OUTCOME MEASURE

The Québec provincial newborn screening database was analyzed from January 1990 to December 2005. Only cases of permanent congenital hypothyroidism with thyroid ectopy or agenesis on scintigraphy were analyzed.

RESULTS

During the study period, 1,303,341 children were screened, and 424 cases of permanent congenital hypothyroidism were diagnosed, giving an overall incidence of 1:3074. Of these, 306 had CHTD (overall incidence 1:4259) from either ectopy (n = 231) or agenesis (n = 75). Over the 16 yr of the study, this incidence remained stable (P = 0.57), and no significant variability in monthly incidence was found (P = 0.87).

CONCLUSIONS

The incidence of CHTD did not vary over the observation period, and its monthly variation was random. Therefore, environmental factors do not appear to play a significant role in the etiology of CHTD.

Genetic analysis of the PAX8 gene in children with congenital hypothyroidism and dysgenetic or eutopic thyroid glands: identification of a novel sequence variant

OBJECTIVE

To analyse the coding region of PAX8 in individuals with congenital (CH) or post neonatal hypothyroidism due to dysgenetic (TD) or eutopic thyroid glands.

DESIGN AND PATIENTS

Forty-three children with CH and TD (13 agenesis, 23 ectopia, and seven hypoplasia), one subject with post neonatal onset of hypothyroidism and thyroid ectopia, 15 children with CH and eutopic thyroid glands and six euthyroid adults with thyroid hemiagenesis were enrolled as cases, along with 120 healthy individuals as controls.

MEASUREMENTS

Exons 2-8 of the PAX8 were directly sequenced. HeLa and HEK293 cells were transfected with PAX8 wild-type (PAX8-WT), mutant PAX8, p300, thyroid transcription factor 1 (TTF-1) and thyroglobulin promoter pGL3 (TG prom-pGL3). Synergism of TTF-1 with PAX8-WT vs. mutant and activity of PAX8-WT vs. mutant in accompaniment with p300 on TG prom-pGL3 were also assessed. The luminescence produced by PAX8-WT and mutant PAX8 was measured.

RESULTS

Among patients and controls only a 15-year-old girl with thyroid ectopia showed a heterozygous transition of cytosine to thymine at position 674 in exon 6, which changed a conserved threonine at position 225 to methionine (PAX8-T225M). Her father and sister harboured PAX8-T225M without abnormal thyroid phenotypes. PAX8-T225M and PAX8-WT similarly increased luciferase activity and had a similar synergistic effect with TTF-1. At 500 ng p300, however, PAX8-T225M could not significantly increase TG promoter activity when compared to PAX8-T225M alone, while PAX8-WT +500 ng p300 induction was significantly higher than PAX8-WT alone (P < 0.001). Cotransfection of TTF-1 together with PAX8-T225M resulted in rescuing of the lack of synergism with p300.

CONCLUSIONS

PAX8 mutations in congenital hypothyroidism due to dysgenetic or orthotopic thyroid glands are rare. PAX8-T225M is probably a rare variant.

Missense mutation in the transcription factor NKX2-5: a novel molecular event in the pathogenesis of thyroid dysgenesis

CONTEXT

Congenital hypothyroidism (CH) is a common endocrine disorder with an incidence of 1:3000-4000 at birth. In 80-85% of cases, CH is caused by defects in thyroid organogenesis, resulting in absent, ectopically located, and/or severely reduced gland [thyroid dysgenesis (TD)]. Mutations in genes controlling thyroid development have demonstrated that in a few cases, TD is a Mendelian trait. However, accumulating evidence supports the view that the genetics of TD are complex, possibly with a polygenic/multifactorial basis. A higher prevalence of congenital heart disease has been documented in children with CH than in the general population. Such an association suggests a possible pathogenic role of genes involved in both heart and thyroid development. NKX2-5 encodes a homeodomain-containing transcription factor with a major role in heart development, and mutations affecting this gene have been reported in individuals with congenital heart disease.

OBJECTIVE

In the present work we investigated the possible involvement of NKX2-5 mutations in TD.

RESULTS

Our results indicate that Nkx2-5(-/-) embryos exhibit thyroid bud hypoplasia, providing evidence that NKX2-5 plays a role in thyroid organogenesis and that NKX2-5 mutations contribute to TD. NKX2-5 mutational screening in 241 patients with TD allowed the identification of three heterozygous missense changes (R25C, A119S, and R161P) in four patients with TD. Functional characterization of the three mutations demonstrated reduced DNA binding and/or transactivation properties, with a dominant-negative effect on wild-type NKX2-5.

CONCLUSION

Our results suggest a previously unknown role of NKX2-5 in the pathogenesis of TD.