Deficiency of the HGF/Met pathway leads to thyroid dysgenesis by impeding late thyroid expansion.

The mechanisms of bifurcation, a key step in thyroid development, are largely unknown. Here we find three zebrafish lines from a forward genetic screening with similar thyroid dysgenesis phenotypes and identify a stop-gain mutation in hgfa and two missense mutations in met by positional cloning from these zebrafish lines. The elongation of the thyroid primordium along the pharyngeal midline was dramatically disrupted in these zebrafish lines carrying a mutation in hgfa or met. Further studies show that MAPK inhibitor U0126 could mimic thyroid dysgenesis in zebrafish, and the phenotypes are rescued by overexpression of constitutively active MEK or Snail, downstream molecules of the HGF/Met pathway, in thyrocytes. Moreover, HGF promotes thyrocyte migration, which is probably mediated by downregulation of E-cadherin expression. The delayed bifurcation of the thyroid primordium is also observed in thyroid-specific Met knockout mice. Together, our findings reveal that HGF/Met is indispensable for the bifurcation of the thyroid primordium during thyroid development mediated by downregulation of E-cadherin in thyrocytes via MAPK-snail pathway.

TSHR Variant Screening and Phenotype Analysis in 367 Chinese Patients With Congenital Hypothyroidism.

Background: Genetic defects in the human thyroid-stimulating hormone (TSH) receptor (TSHR) gene can cause congenital hypothyroidism (CH). However, the biological functions and comprehensive genotype-phenotype relationships for most TSHR variants associated with CH remain unexplored. We aimed to identify TSHR variants in Chinese patients with CH, analyze the functions of the variants, and explore the relationships between TSHR genotypes and clinical phenotypes. Methods: In total, 367 patients with CH were recruited for TSHR variant screening using whole-exome sequencing. The effects of the variants were evaluated by in-silico programs such as SIFT and polyphen2. Furthermore, these variants were transfected into 293T cells to detect their Gs/cyclic AMP and Gq/11 signaling activity. Results: Among the 367 patients with CH, 17 TSHR variants, including three novel variants, were identified in 45 patients, and 18 patients carried biallelic TSHR variants. In vitro experiments showed that 10 variants were associated with Gs/cyclic AMP and Gq/11 signaling pathway impairment to varying degrees. Patients with TSHR biallelic variants had lower serum TSH levels and higher free triiodothyronine and thyroxine levels at diagnosis than those with DUOX2 biallelic variants. Conclusions: We found a high frequency of TSHR variants in Chinese patients with CH (12.3%), and 4.9% of cases were caused by TSHR biallelic variants. Ten variants were identified as loss-of-function variants. The data suggest that the clinical phenotype of CH patients caused by TSHR biallelic variants is relatively mild. Our study expands the TSHR variant spectrum and provides further evidence for the elucidation of the genetic etiology of CH.

Contactin 6, A Novel Causative Gene for Congenital Hypothyroidism, Mediates Thyroid Hormone Biosynthesis Through Notch Signaling.

Background: Congenital hypothyroidism (CH) is the most common neonatal metabolic disorder. In patients with CH in China, thyroid dyshormonogenesis is more common than thyroid dysgenesis; however, the genetic causes of CH due to thyroid dyshormonogenesis remain largely unknown. Therefore, we aimed at identifying novel candidate causative genes for CH. Methods: To identify novel CH candidate genes, a total of 599 patients with CH were enrolled and next-generation sequencing was performed. The functions of the identified variants were confirmed using HEK293T and FTC-133 cell lines in vitro and in a mouse model organism in vivo. Results: Three pathogenic contactin 6 (CNTN6) variants were identified in two patients with CH. Pedigree analysis showed that CH caused by CNTN6 variants was inherited in an autosomal recessive pattern. The CNTN6 gene was highly expressed in the thyroid in humans and mice. Cntn6 knockout mice presented with thyroid dyshormonogenesis and CH due to the decreased expression of crucial genes for thyroid hormone biosynthesis (Slc5a5, Tpo, and Duox2). All three CNTN6 variants resulted in the blocking of the release of the Notch intracellular domain, which could not translocate into the nucleus, impaired NOTCH1 transcriptional activity, and decreased expression of SLC5A5, TPO, and DUOX2. Further, we found that DTX1 was required for CNTN6 to promote thyroid hormone biosynthesis through Notch signaling. Conclusions: This study demonstrated that CNTN6 is a novel causative gene for CH through the mediation of thyroid hormone biosynthesis via Notch signaling, which provides new insights into the genetic background and mechanisms involved in CH and thyroid dyshormonogenesis.

Genetic screening and functional analysis of TPO variants in Chinese patients with congenital hypothyroidism.

INTRODUCTION: Congenital hypothyroidism (CH), the most common neonatal endocrine disorder world-wide, can be caused by variants in the thyroid peroxidase (TPO) gene. This study aimed to identify TPO variants in Chinese patients with CH, analyze their impact on TPO function, and establish relationships between TPO genotypes and clinical characteristics. METHODS: A total of 328 patients with CH were screened for TPO variants by performing whole exome sequencing. The function of the detected TPO variants was investigated via transfection assays in vitro. The pathogenic effect of five novel variants was further assessed in silico. RESULTS: Among 328 patients with CH, 19 TPO variants, including six novel ones, were identified in 43 patients. Eighteen patients (5.5%) carried biallelic TPO variants. In vitro experiments showed that TPO activity was impaired to varying degrees in 17 variants. Furthermore, we determined that a residual TPO enzyme activity threshold of 15% may serve as a criterion for differentiating CH severity. CONCLUSIONS: According to our study, the prevalence of TPO variants among Chinese patients with CH was 13.1 %. Five novel variants led to impaired TPO function by altering its structure or by affecting its expression or cellular localization, which should result in impaired thyroid hormone synthesis.

Influence of two anti-tumor drugs, pazopanib, and axitinib, on the development and thyroid-axis of zebrafish (Danio rerio) embryos/larvae.

Introduction: In recent years, the potential toxicities of different pharmaceuticals toward the thyroid system have received increasing attention. In this study, we aim to evaluate the toxic effects of pazopanib and axitinib, two anti-tumor drugs with widespread clinical use, on thyroid function in the zebrafish model. Methods: We measured levels of thyroid-related hormones using the commercial Enzyme-Linked Immunosorbent Assay (ELISA) kit. Whole-mount in situ hybridization (WISH) analysis was employed to detect target gene expression changes. Morphology of the thyroid were evaluated by using transgenic Tg (tg: EGFP) fish line under a confocal microscope. The relative mRNA expression of key genes was verified through quantitative real-time polymerase chain reaction (RT‒qPCR). The size and number of the follicles was quantified whereby Hematoxylin-Eosin (H & E) staining under a light microscope. Results: The results revealed that fertilized zebrafish embryos were incubated in pazopanib or axitinib for 96 hours, development and survival were significantly affected, which was accompanied by significant disturbances in thyroid endocrine system (e.g., increased thyroid-stimulating hormone (TSH) content and decreased triiodothyronine (T3) and thyroxine (T4) content, as well as transcription changes of genes associated with the hypothalamus-pituitary-thyroid (HPT) axis. Moreover, based on whole-mount in situ hybridization staining of tg and histopathological examination of zebrafish embryos treated with pazopanib and axitinib, we observed a significantly abnormal development of thyroid follicles in the Tg (tg: EGFP) zebrafish transgenic line. Conclusion: Collectively, these findings indicate that pazopanib and axitinib may have toxic effects on thyroid development and function, at least partially, by influencing the regulation of the HPT axis. Thus, we believe that the potential thyroid toxicities of pazopanib and axitinib in their clinical applications should receive greater attention.

Pathogenic variations in MAML2 and MAMLD1 contribute to congenital hypothyroidism due to dyshormonogenesis by regulating the Notch signalling pathway.

BACKGROUND: In several countries, thyroid dyshormonogenesis is more common than thyroid dysgenesis in patients with congenital hypothyroidism (CH). However, known pathogenic genes are limited to those directly involved in hormone biosynthesis. The aetiology and pathogenesis of thyroid dyshormonogenesis remain unknown in many patients. METHODS: To identify additional candidate pathogenetic genes, we performed next-generation sequencing in 538 patients with CH and then confirmed the functions of the identified genes in vitro using HEK293T and Nthy-ori 3.1 cells, and in vivo using zebrafish and mouse model organisms. RESULTS: We identified one pathogenic MAML2 variant and two pathogenic MAMLD1 variants that downregulated canonical Notch signalling in three patients with CH. Zebrafish and mice treated with N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butylester, a γ-secretase inhibitor exhibited clinical manifestations of hypothyroidism and thyroid dyshormonogenesis. Through organoid culture of primary mouse thyroid cells and transcriptome sequencing, we demonstrated that Notch signalling within thyroid cells directly affects thyroid hormone biosynthesis rather than follicular formation. Additionally, these three variants blocked the expression of genes associated with thyroid hormone biosynthesis, which was restored by HES1 expression. The MAML2 variant exerted a dominant-negative effect on both the canonical pathway and thyroid hormone biosynthesis. MAMLD1 also regulated hormone biosynthesis through the expression of HES3, the target gene of the non-canonical pathway. CONCLUSIONS: This study identified three mastermind-like family gene variants in CH and revealed that both canonical and non-canonical Notch signalling affected thyroid hormone biosynthesis.

The isl2a transcription factor regulates pituitary development in zebrafish.

Background: ISL LIM homeobox 2, also known as insulin gene enhancer protein ISL-2 (ISL2), is a transcription factor gene that participates in a wide range of developmental events. However, the role of ISL2 in the hypothalamus-pituitary-thyroid axis is largely unknown. In the present study, we characterized the expression patterns of ISL2 and revealed its regulative role during embryogenesis using zebrafish. Methods: We used the CRISPR/Cas9 system to successfully establish homozygous ISL2-orthologue (isl2a and isl2b) knockout zebrafish. Moreover, we utilized these knockout zebrafish to analyze the pituitary and thyroid phenotypes in vivo. For further molecular characterization, in situ hybridization and immunofluorescence were performed. Results: The isl2a mutant zebrafish presented with thyroid hypoplasia, reduced whole-body levels of thyroid hormones, increased early mortality, gender imbalance, and morphological retardation during maturity. Additionally, thyrotropes, a pituitary cell type, was notably decreased during development. Importantly, the transcriptional levels of pituitary-thyroid axis hormones-encoding genes, such as tshba, cga, and tg, were significantly decreased in isl2a mutants. Finally, the thyroid dysplasia in isl2a mutant larvae may be attributed to a reduction in proliferation rather than changes in apoptosis. Conclusions: In summary, isl2a regulates the transcriptional levels of marker genes in hypothalamus-pituitary-thyroid axis, and isl2a knockout causing low thyroid hormone levels in zebrafish. Thus, isl2a identified by the present study, is a novel regulator for pituitary cell differentiation in zebrafish, resulting in thyroid gland hypoplasia and phenotypes of hypothyroidism.

Large-scale forward genetic screening of zebrafish affecting thyroid development.

The thyroid follicular cells originate from the foregut endoderm and elucidating which genes and signaling pathways regulate their development is crucial for understanding developmental disorders as well as diseases in adulthood. We exploited unique advantages of the zebrafish model to carry an ENU-based forward mutagenesis screen aiming at identifying genes involved in the development and function of the thyroid follicular cells. ENU is an excellent chemical mutagen due to its high mutation efficiency and an indiscriminate selection of genes. A total of 1606 F2 families from 36 ENU treated founders was raised and embryos from F3 generation were collected at 5dpf to perform the whole embryo in situ hybridization with a cocktail probe of thyroid marker thyroglobulin(tg), pituitary marker thyroid stimulating hormone (tshba) to determine the mutagenic phenotype. Among the 1606 F2 families, 112 F2 mutant families with normal development stages except for thyroid dysfunction were identified and divided into three different groups according to their phenotypic characteristics. Further studies of the mutants are likely to shed more insights into the molecular basis of both the thyroid development and function in the zebrafish and vertebrate.

The influence of sunitinib and sorafenib, two tyrosine kinase inhibitors, on development and thyroid system in zebrafish larvae.

Recently, the potential toxic effects of various pharmaceuticals on the thyroid endocrine system have raised considerable concerns. In this study, we evaluated the adverse effects of sorafenib and sunitinib, two widely used anti-tumor drugs, on the developmental toxicities and thyroid endocrine disruption by using zebrafish (Danio rerio) model. Zebrafish embryos/larvae were exposed to different contentions (0, 10, 50 and 100 nM) of sorafenib and sunitinib for 96 hpf. The results revealed that waterborne exposure to sorafenib and sunitinib exhibited remarkable toxic effects on the survival and development in zebrafish embryos/larvae, which was accompanied by obvious disturbances of thyroid endocrine system (e.g., decreased T3 and T4 content, increased TSH content) and genes' transcription changes within the hypothalamus-pituitary-thyroid (HPT) axis. In addition, we verified a strikingly abnormal thyroid gland organogenesis in zebrafish larvae in response to sorafenib and sunitinib, by assessing the development of thyroid follicles using the WISH staining of tg, the Tg (tg:GFP) zebrafish transgenic line, and histopathological analysis. Taken together, our results indicated sorafenib and sunitinib exposure could induce obvious developmental toxicities and thyroid function disruption in zebrafish embryos/larvae, which might involve a regulatory mechanism, at least in part, by destroying the thyroid follicle structure, and by disturbing the balance of the HPT axis.

The mutation screening in candidate genes related to thyroid dysgenesis by targeted next-generation sequencing panel in the Chinese congenital hypothyroidism.

OBJECTIVE: Congenital hypothyroidism (CH) is known to be due to thyroid dyshormonogenesis (DH), which is mostly inherited in an autosomal recessive inheritance pattern or thyroid dysgenesis (TD), whose inheritance pattern is controversial and whose molecular etiology remains poorly understood. DESIGN AND METHODS: The variants in 37 candidate genes of CH, including 25 genes related to TD, were screened by targeted exon sequencing in 205 Chinese patients whose CH cannot be explained by biallelic variants in genes related to DH. The inheritance pattern of the genes was analyzed in family trios or quartets. RESULTS: Of the 205 patients, 83 patients carried at least one variant in 19 genes related to TD, and 59 of those 83 patients harbored more than two variants in distinct candidate genes for CH. Biallelic or de novo variants in the genes related to TD in Chinese patients are rare. We also found nine probands carried only one heterozygous variant in the genes related to TD that were inherited from a euthyroid either paternal or maternal parent. These findings did not support the monogenic inheritance pattern of the genes related to TD in CH patients. Notably, in family trio or quartet analysis, of 36 patients carrying more than two variants in distinct genes, 24 patients carried these variants inherited from both their parents, which indicated that the oligogenic inheritance pattern of the genes related to TD should be considered in CH. CONCLUSIONS: Our study expanded the variant spectrum of the genes related to TD in Chinese CH patients. It is rare that CH in Chinese patients could be explained by monogenic germline variants in genes related to TD. The hypothesis of an oligogenic origin of the CH should be considered.

Mutation Screening and Functional Study of SLC26A4 in Chinese Patients with Congenital Hypothyroidism

Objective: Defects in the human solute carrier family 26 member 4 (SLC26A4) gene are reported to be one of the causes of congenital hypothyroidism (CH). We aimed to identify SLC26A4 mutations in Chinese patients with CH and analyze the function of the mutations. Methods: Patients with primary CH were screened for 21 CH candidate genes mutations by targeted next-generation sequencing. All the exons and exon-intron boundaries of SLC26A4 were identified and analyzed. The function of six missense mutation in SLC26A4 were further investigated in vitro. Results: Among 273 patients with CH, seven distinct SLC26A4 heterozygous mutations (p.S49R, p.I363L, p.R409H, p.T485M, p.D661E, p.H723R, c.919-2A>G) were identified in 10 patients (3.66%, 10/273). In vitro experiments showed that mutation p.I363L, p.R409H, p.H723R affect the membrane location and ion transport of SLC26A4, while p.S49R did not. Mutation p.T485M and p.D661E only affected ion transport, but had no effect on the membrane location. Conclusion: The prevalence of SLC26A4 mutations was 3.66% in Chinese patients with CH. Five mutations (p.I363L, p.R409H, p.T485M, p.D661E and p.H723R) impaired the membrane location or ion transport function of SLC26A4, suggesting important roles for Ile363, Arg409, Thr485, Asp661, and His723 residues in SLC26A4 function. As all variants identified were heterozygous, the pathogenesis of these patients cannot be explained, and the pathogenesis of these patients needs further study.

The effect of radioiodine treatment on the characteristics of TRAb in Graves' disease.

BACKGROUND: Graves' disease (GD) is one of the most common autoimmune thyroid diseases (AITDs) in humans, and thyrotropin receptor antibody (TRAb) is a characterized autoantibody in GD. The use of radioactive iodine therapy (RAI) for GD treatment is increasing. OBJECTIVES: We studied the biological properties of TRAb and evaluated the effect of RAI therapy on TRAb in GD patients. METHODS: In total, 225 patients (22 onset GD patients without 131I therapy, 203 GD patients treated with 131I therapy) and 20 healthy individuals as normal controls were included in this study. Clinical assessments were performed, and we examined in vitro the biological properties of TRAb in the 22 onset GD patients and 20 controls as well as 84 GD patients with 131I therapy. RESULTS: Serum TRAb and thyroid peroxidase antibody (TPOAb) levels increased in the initial year of RAI treatment, and both antibodies decreased gradually after one year. After 5 years from radioiodine treatment, TRAb and TPOAb levels decreased in 88% and 65% of GD patients, respectively. The proportion of patients positive for thyroid-stimulatory antibody (TSAb) was significantly higher in the 7-12-month group, and thyroid-blocking antibody (TBAb) levels were elevated after one year in half of the patients who received 131I treatment. CONCLUSIONS: Treatment of GD patients with radioiodine increased TPOAb and TRAb (their main biological properties were TSAbs) within the first year after therapy, and the main biological properties of elevated TRAb were TBAbs after 1 year.

Novel Compound Heterozygous Pathogenic Mutations of SLC5A5 in a Chinese Patient With Congenital Hypothyroidism.

Background and Objectives: Defects in the human sodium/iodide symporter (SLC5A5) gene have been reported to be one of the causes of congenital hypothyroidism (CH). We aimed to identify SLC5A5 mutations in Chinese patients with CH and to evaluate the function of the mutation. Methods: Two hundred and seventy-three patients with primary CH were screened for mutations in SLC5A5 using next-generation sequencing. We investigated the expression and cellular localization of the novel compound heterozygous mutation in SLC5A5. The functional activity of the mutants was further examined in vitro. Results: In 273 patients with CH, two previously undescribed pathogenic mutations p.Gly51AlafsTer45 (G51fs) and p.Gly421Arg (G421R) in a compound heterozygous state in SLC5A5 were identified in a pediatric patient. G51fs was located in the first intercellular loop connecting transmembrane segment I and II, whereas G421R was in the transmembrane segment (TMS) XI. G51fs and G421R resulted in a truncated NIS and reduced protein expression, respectively. In vitro experiments further showed that the normal function of iodine transport of sodium-iodide symporter (NIS) mutants was markedly impaired. Conclusion: The undescribed compound heterozygous mutation of SLC5A5 was discovered in a Chinese CH patient. The mutation led to significantly reduced NIS expression and impaired iodide transport function accompanied by the impaired location of the NIS on the plasma membrane. Our study thus provides further insights into the roles of SLC5A5 in CH pathogenesis.

Molecular and clinical genetics of the transcription factor GLIS3 in Chinese congenital hypothyroidism.

The transcription factor GLIS3 is an important factor in hormone biosynthesis and thyroid development, and mutations in GLIS3 are relatively rare. Deletions of more than one of the 11 exons of GLIS3 occur in most patients with various extrathyroidal abnormalities and congenital hypothyroidism (CH), and only 18 missense variants of GLIS3 related to thyroid disease have been reported. The aim of this study was to report the family history and molecular basis of patients with CH who carry GLIS3 variants. Three hundred and fifty-three non-consanguineous infants with CH were recruited and subjected to targeted exome sequencing of CH-related genes. The transcriptional activity and cellular localization of the variants in GLIS3 were investigated in vitro. We identified 20 heterozygous GLIS3 exonic missense variants, including eight novel sites, in 19 patients with CH. One patient carried compound heterozygous GLIS3 variants (p.His34Arg and p.Pro835Leu). None of the variants affected the nuclear localization. However, three variants (p.His34Arg, p.Pro835Leu, and p.Ser893Phe) located in the N-terminal and C-terminal regions of the GLIS3 protein downregulated the transcriptional activation of several genes required for thyroid hormone (TH) biosynthesis. This study of patients with CH extends the current knowledge surrounding the spectrum of GLIS3 variants and the mechanisms by which they cause TH biosynthesis defects.

The TPO mutation screening and genotype-phenotype analysis in 230 Chinese patients with congenital hypothyroidism.

Inborn defects in thyroid hormone biosynthesis contribute to nearly half of congenital hypothyroidism (CH) cases in China. The thyroid peroxidase (TPO) mutation is one of the most frequent mutations that results in thyroid dyshormonogenesis. In this study, 35 non-synonymous mutations in 15 TPO sites, including 6 novel mutations, were identified in 230 Chinese patients with CH. The enzyme activity of the mutations in TPO was investigated in vitro, and patients with less than 15% residual enzyme activity showed severe CH, such as markedly increased thyroid-stimulating hormone (TSH) at diagnosis (>100 µIU/mL) and pronounced goiter, and required a higher dose of L-thyroxine to maintain the euthyroid. However, CH patients with greater than 16% TPO activity showed mild CH, a typical childhood socially without L-thyroxine treatment before 3 years of age, and the appearance of a macroscopic goiter at childhood. The findings indicated that the residual enzymatic activity of TPO was correlated with clinical phenotypes of CH patients with TPO biallelic mutations.