Unfolded protein response is involved in the pathology of human congenital hypothyroid goiter and rat non-goitrous congenital hypothyroidism

The p.Cys1281Tyr variant in the hinge module/flap region of thyroglobulin causes intracellular transport disorder and congenital hypothyroidism

Congenital hypothyroidism (CH) due to thyroglobulin (TG) variants causes very low serum TG levels with normal or enlarged thyroid glands, depending on the severity of the defect, and with autosomal recessive inheritance. The purpose of this study was to functionally characterize p.Cys1281Tyr variant in the TG gene in order to increase our knowledge of the molecular mechanisms associated with CH. In order to find evidence that support the hypothesis that the p.Cys1281Tyr variant would affect the TG folding were performed amino acid prediction, 3D modeling and transient expression analysis in HEK293T cells. 18 of the 21″in silico" algorithms predict a deleterious effect of the p.Cys1281Tyr variant. The full-length 3D model p.Cys1281Tyr TG showed disulfide bond cleavage between the cysteines at positions 1249 and 1281 and rearrangement of the TG structure, while transient expression analysis indicated that p.Cys1281Tyr causes retention of the protein inside the cell. Consequently, these results show that this pathogenic variant makes it impossible for TG to fulfill its function in the biosynthesis process of thyroid hormones, causing CH. In conclusion, our results confirm the pathophysiological importance of misfolding of TG as a consequence of p.Cys1281Tyr variant located in the hinge module/flap region of TG.

Iodide Excess Inhibits Thyroid Hormone Synthesis Pathway Involving XBP1-Mediated Regulation

Iodine is an essential micronutrient for producing thyroid hormone (TH); however, iodide excess can lead to adverse thyroidal effects. Unfortunately, the lack of a proper in vitro model system hampered the studies of the effect of iodide excess on thyroid physiology and pathology. Here, we demonstrated that excessive iodide intake downregulated the genes related to TH synthesis in the thyroids of mice. Since sodium iodide has no effect on these genes in cultured cell lines, we developed a three-dimensional (3D) culture system to enable the murine thyrocytes to form organoids in vitro with thyroid follicle-like structures and function and found that the in vivo effect of iodide excess could be mimicked in these thyroid organoids. Our data indicate that iodide excess mainly activated the XBP1-mediated unfolded protein response in both murine thyroid and thyroid organoids, while activation of XBP1 was able to mimic the sodium iodide effect on genes for the synthesis of TH in murine thyroid organoids. Lastly, our results suggest that XBP1 might transcriptionally repress the genes involved in the synthesis of TH. Based on these findings, we propose that iodide excess inhibits the transcription of genes related to TH synthesis through a mechanism involving XBP1-mediated action.

Defective Thyroglobulin: Cell Biology of Disease

The primary functional units of the thyroid gland are follicles of various sizes comprised of a monolayer of epithelial cells (thyrocytes) surrounding an apical extracellular cavity known as the follicle lumen. In the normal thyroid gland, the follicle lumen is filled with secreted protein (referred to as colloid), comprised nearly exclusively of thyroglobulin with a half-life ranging from days to weeks. At the cellular boundary of the follicle lumen, secreted thyroglobulin becomes iodinated, resulting from the coordinated activities of enzymes localized to the thyrocyte apical plasma membrane. Thyroglobulin appearance in evolution is essentially synchronous with the appearance of the follicular architecture of the vertebrate thyroid gland. Thyroglobulin is the most highly expressed thyroid gene and represents the most abundantly expressed thyroid protein. Wildtype thyroglobulin protein is a large and complex glycoprotein that folds in the endoplasmic reticulum, leading to homodimerization and export via the classical secretory pathway to the follicle lumen. However, of the hundreds of human thyroglobulin genetic variants, most exhibit increased susceptibility to misfolding with defective export from the endoplasmic reticulum, triggering hypothyroidism as well as thyroidal endoplasmic reticulum stress. The human disease of hypothyroidism with defective thyroglobulin (either homozygous, or compound heterozygous) can be experimentally modeled in thyrocyte cell culture, or in whole animals, such as mice that are readily amenable to genetic manipulation. From a combination of approaches, it can be demonstrated that in the setting of thyroglobulin misfolding, thyrocytes under chronic continuous ER stress exhibit increased susceptibility to cell death, with interesting cell biological and pathophysiological consequences.

Maintaining the thyroid gland in mutant thyroglobulin-induced hypothyroidism requires thyroid cell proliferation that must continue in adulthood

Congenital hypothyroidism with biallelic thyroglobulin (Tg protein, encoded by the TG gene) mutation is an endoplasmic reticulum (ER) storage disease. Many patients (and animal models) grow an enlarged thyroid (goiter), yet some do not. In adulthood, hypothyroid TG^cog/cog mice (bearing a Tg-L2263P mutation) exhibit a large goiter, whereas adult WIC rats bearing the TG^rdw/rdw mutation (Tg-G2298R) exhibit a hypoplastic thyroid. Homozygous TG mutation has been linked to thyroid cell death, and cytotoxicity of the Tg-G2298R protein was previously thought to explain the lack of goiter in WIC-TG^rdw/rdw rats. However, recent studies revealed that TG^cog/cog mice also exhibit widespread ER stress–mediated thyrocyte death, yet under continuous feedback stimulation, thyroid cells proliferate in excess of their demise. Here, to examine the relative proteotoxicity of the Tg-G2298R protein, we have used CRISPR–CRISPR-associated protein 9 technology to generate homozygous TG^rdw/rdw knock-in mice in a strain background identical to that of TG^cog/cog mice. TG^rdw/rdw mice exhibit similar phenotypes of defective Tg protein folding, thyroid histological abnormalities, hypothyroidism, and growth retardation. TG^rdw/rdw mice do not show evidence of greater ER stress response or stress-mediated cell death than TG^cog/cog mice, and both mouse models exhibit sustained thyrocyte proliferation, with comparable goiter growth. In contrast, in WIC-TG^rdw/rdw rats, as a function of aging, the thyrocyte proliferation rate declines precipitously. We conclude that the mutant Tg-G2298R protein is not intrinsically more proteotoxic than Tg-L2263P; rather, aging-dependent difference in maintenance of cell proliferation is the limiting factor, which accounts for the absence of goiter in adult WIC-TG^rdw/rdw rats.

Curating the gnomAD database: Report of novel variants in the thyrogobulin gene using in silico bioinformatics algorithms

Thyroglobulin (TG) is a large glycosylated protein of 2767 amino acids, secreted by the thyrocytes into the follicular lumen. It plays an essential role in the process of thyroid hormone synthesis. TG gene variants lead to permanent congenital hypothyroidism. In the present work, we report a detailed population and bioinformatic prediction analyses of the TG variants indexed in the Genome Aggregation Database (gnomAD). The results showed a clear predominance of nonsense variants in the European (Finnish), European (Non-Finnish) and Ashkenazi Jewish ethnic groups, whereas the splice site variants predominate in South Asian and African/African-American populations. In total, 282 novel TG variants were described (47 missense involving the wild-type cysteine residues, 177 missense located in the ChEL domain and 58 splice site variants) which were not reported in the literature and that would have deleterious effects in prediction programs. In the gnomAD population, the estimated prevalence of heterozygous carriers of the potentially damaging variants was 1:320. In conclusion, we provide an updated and curated reference source for the diagnosis of thyroid disease, mainly to congenital hypothyroidism due to TG deficiency. The identification and characterization of TG variants is undoubtedly a valuable approach to study the TG structure/function relations and an important tool for clinical diagnosis and genetic counseling.

Thyroid hormone synthesis continues despite biallelic thyroglobulin mutation with cell death

Complete absence of thyroid hormone is incompatible with life in vertebrates. Thyroxine is synthesized within thyroid follicles upon iodination of thyroglobulin conveyed from the endoplasmic reticulum (ER), via the Golgi complex, to the extracellular follicular lumen. In congenital hypothyroidism from biallelic thyroglobulin mutation, thyroglobulin is misfolded and cannot advance from the ER, eliminating its secretion and triggering ER stress. Nevertheless, untreated patients somehow continue to synthesize sufficient thyroxine to yield measurable serum levels that sustain life. Here, we demonstrate that TG^{W2346R/W2346R} humans, TG^cog/cog mice, and TG^rdw/rdw rats exhibited no detectable ER export of thyroglobulin, accompanied by severe thyroidal ER stress and thyroid cell death. Nevertheless, thyroxine was synthesized, and brief treatment of TG^rdw/rdw rats with antithyroid drug was lethal to the animals. When untreated, remarkably, thyroxine was synthesized on the mutant thyroglobulin protein, delivered via dead thyrocytes that decompose within the follicle lumen, where they were iodinated and cannibalized by surrounding live thyrocytes. As the animals continued to grow goiters, circulating thyroxine increased. However, when TG^rdw/rdw rats age, they cannot sustain goiter growth that provided the dying cells needed for ongoing thyroxine synthesis, resulting in profound hypothyroidism. These results establish a disease mechanism wherein dead thyrocytes support organismal survival.

Structure and genetic variants of thyroglobulin: Pathophysiological implications

Thyroglobulin (TG) plays a main role in the biosynthesis of thyroid hormones (TH), and, thus, it is involved in a wide range of vital functions throughout the life cycle of all vertebrates. Deficiency of TH production due to TG genetic variants causes congenital hypothyroidism (CH), with devastating consequences such as intellectual disability and impaired growth if untreated. To this day, 229 variations in the human TG gene have been identified while the 3D structure of TG has recently appeared. Although TG deficiency is thought to be of autosomal recessive inheritance, the introduction of massive sequencing platforms led to the identification of a variety of monoallelic TG variants (combined with mutations in other thyroid gene products) opening new questions regarding the possibility of oligogenic inheritance of the disease. In this review we discuss remarkable advances in the understanding of the TG architecture and the pathophysiology of CH associated with TG defects, providing new insights for the management of congenital disorders as well as counseling benefits for families with a history of TG abnormalities. Moreover, we summarize relevant aspects of TH synthesis within TG and offer an updated analysis of animal and cellular models of TG deficiency for pathophysiological studies of thyroid dyshormonogenesis while highlighting perspectives for new investigations. All in all, even though there has been sustained progress in understanding the role of TG in thyroid pathophysiology during the past 50 years, functional characterization of TG variants remains an important area of study for future advancement in the field.

Resveratrol Alleviates the Inhibitory Effect of Tunicamycin-Induced Endoplasmic Reticulum Stress on Expression of Genes Involved in Thyroid Hormone Synthesis in FRTL-5 Thyrocytes

Recently, ER stress induced by tunicamycin (TM) was reported to inhibit the expression of key genes involved in thyroid hormone synthesis, such as sodium/iodide symporter (NIS), thyroid peroxidase (TPO) and thyroglobulin (TG), and their regulators such as thyrotropin receptor (TSHR), thyroid transcription factor-1 (TTF-1), thyroid transcription factor-2 (TTF-2) and paired box gene 8 (PAX-8), in FRTL-5 thyrocytes. The present study tested the hypothesis that resveratrol (RSV) alleviates this effect of TM in FRTL-5 cells. While treatment of FRTL-5 cells with TM alone (0.1 µg/mL) for 48 h strongly induced the ER stress-sensitive genes heat shock protein family A member 5 (HSPA5) and DNA damage inducible transcript 3 (DDIT3) and repressed NIS, TPO, TG, TSHR, TTF-1, TTF-2 and PAX-8, combined treatment with TM (0.1 µg/mL) and RSV (10 µM) for 48 h attenuated this effect of TM. In conclusion, RSV alleviates TM-induced ER stress and attenuates the strong impairment of expression of genes involved in thyroid hormone synthesis and their regulators in FRTL-5 thyrocytes exposed to TM-induced ER stress. Thus, RSV may be useful for the treatment of specific thyroid disorders, provided that strategies with improved oral bioavailability of RSV are applied.

Cell death-associated lipid droplet protein CIDE-A is a noncanonical marker of endoplasmic reticulum stress

Secretory protein misfolding has been linked to ER stress and cell death. We expressed a TGrdw transgene encoding TG-G(2298)R, a misfolded mutant thyroglobulin reported to be linked to thyroid cell death. When the TGrdw transgene was expressed at low level in thyrocytes of TGcog/cog mice that experienced severe ER stress, we observed increased thyrocyte cell death and increased expression of CIDE-A (cell death-inducing DFFA-like effector-A, a protein of lipid droplets) in whole thyroid gland. Here we demonstrate that acute ER stress in cultured PCCL3 thyrocytes increases Cidea mRNA levels, maintained at least in part by increased mRNA stability, while being negatively regulated by activating transcription factor 6 - with similar observations that ER stress increases Cidea mRNA levels in other cell types. CIDE-A protein is sensitive to proteasomal degradation yet is stabilized by ER stress, and elevated expression levels accompany increased cell death. Unlike acute ER stress, PCCL3 cells adapted and surviving chronic ER stress maintained a disproportionately lower relative mRNA level of Cidea compared with that of other, classical ER stress markers, as well as a blunted Cidea mRNA response to a new, unrelated acute ER stress challenge. We suggest that CIDE-A is a novel marker linked to a noncanonical ER stress response program, with implications for cell death and survival.

A novel mutation in intron 11 donor splice site, responsible of a rare genotype in thyroglobulin gene by altering the pre-mRNA splincing process. Cell expression and bioinformatic analysis

Thyroglobulin (TG) is a homodimeric glycoprotein synthesized by the thyroid gland. To date, two hundred twenty-seven variations of the TG gene have been identified in humans. Thyroid dyshormonogenesis due to TG gene mutations have an estimated incidence of approximately 1 in 100,000 newborns. The clinical spectrum ranges from euthyroid to mild or severe hypothyroidism. The purpose of the present study was to identify and characterize new variants in the TG gene. We report an Argentine patient with congenital hypothyroidism, enlarged thyroid gland and low levels of serum TG. Sequencing of DNA, expression of chimeric minigenes as well as bioinformatics analysis were performed. DNA sequencing identified the presence of compound heterozygous mutations in the TG gene: the maternal mutation consists of a c.3001+5G > A, whereas the paternal mutation consists of p.Arg296*. Minigen analysis of the variant c.3001+5A performed in HeLa, CV1 and Hek293T cell lines, showed a total lack of transcript expression. So, in order to validate that the loss of expression was caused by such variation, site-directed mutagenesis was performed on the mutated clone, which previously had a pSPL3 vector change, to give rise to a wild-type clone c.3001+5G, endorsing that the mutation c.3001+5G > A is the cause of the total lack of expression. In conclusion, we demonstrate that the c.3001+5G > A mutation causes a rare genotype, altering the splicing of the pre-mRNA. This work contributes to elucidating the molecular bases of TG defects associated with congenital hypothyroidism and expands our knowledge in relation to the pathologic roles of the position 5 in the donor splice site.

Thyroglobulin Interactome Profiling Defines Altered Proteostasis Topology Associated With Thyroid Dyshormonogenesis

Thyroglobulin (Tg) is a secreted iodoglycoprotein serving as the precursor for triiodothyronine and thyroxine hormones. Many characterized Tg gene mutations produce secretion-defective variants resulting in congenital hypothyroidism. Tg processing and secretion is controlled by extensive interactions with chaperone, trafficking, and degradation factors comprising the secretory proteostasis network. While dependencies on individual proteostasis network components are known, the integration of proteostasis pathways mediating Tg protein quality control and the molecular basis of mutant Tg misprocessing remain poorly understood. We employ a multiplexed quantitative affinity purification-mass spectrometry approach to define the Tg proteostasis interactome and changes between WT and several congenital hypothyroidism variants. Mutant Tg processing is associated with common imbalances in proteostasis engagement including increased chaperoning, oxidative folding, and engagement by targeting factors for endoplasmic reticulum-associated degradation. Furthermore, we reveal mutation-specific changes in engagement with N-glycosylation components, suggesting distinct requirements for 1 Tg variant on dual engagement of both oligosaccharyltransferase complex isoforms for degradation. Modulating dysregulated proteostasis components and pathways may serve as a therapeutic strategy to restore Tg secretion and thyroid hormone biosynthesis.

Defects in protein folding in congenital hypothyroidism

Primary congenital hypothyroidism (CH) is the most common endocrine disease in children and one of the most common preventable causes of both cognitive and motor deficits. CH is a heterogeneous group of thyroid disorders in which inadequate production of thyroid hormone occurs due to defects in proteins involved in the gland organogenesis (dysembryogenesis) or in multiple steps of thyroid hormone biosynthesis (dyshormonogenesis). Dysembryogenesis is associated with genes responsible for the development or growth of thyroid cells: such as NKX2-1, FOXE1, PAX8, NKX2-5, TSHR, TBX1, CDCA8, HOXD3 and HOXB3 resulting in agenesis, hypoplasia or ectopia of thyroid gland. Nevertheless, the etiology of the dysembryogenesis remains unknown for most cases. In contrast, the majority of patients with dyshormonogenesis has been linked to mutations in the SLC5A5, SLC26A4, SLC26A7, TPO, DUOX1, DUOX2, DUOXA1, DUOXA2, IYD or TG genes, which usually originate goiter. About 800 genetic mutations have been reported to cause CH in patients so far, including missense, nonsense, in-frame deletion and splice-site variations. Many of these mutations are implicated in specific domains, cysteine residues or glycosylation sites, affecting the maturation of nascent proteins that go through the secretory pathway. Consequently, misfolded proteins are permanently entrapped in the endoplasmic reticulum (ER) and are translocated to the cytosol for proteasomal degradation by the ER-associated degradation (ERAD) machinery. Despite of all these remarkable advances in the field of the CH pathogenesis, several points on the development of this disease remain to be elucidated. The continuous study of thyroid gene mutations with the application of new technologies will be useful for the understanding of the intrinsic mechanisms related to CH. In this review we summarize the present status of knowledge on the disorders in the protein folding caused by thyroid genes mutations.

The role of thyroglobulin in thyroid hormonogenesis

In humans, the thyroid hormones T₃ and T₄ are synthesized in the thyroid gland in a process that crucially involves the iodoglycoprotein thyroglobulin. The overall structure of thyroglobulin is conserved in all vertebrates. Upon thyroglobulin delivery from thyrocytes to the follicular lumen of the thyroid gland via the secretory pathway, multiple tyrosine residues can become iodinated to form mono-iodotyrosine (MIT) and/or di-iodotyrosine (DIT); however, selective tyrosine residues lead to preferential formation of T₄ and T₃ at distinct sites. T₄ formation involves oxidative coupling between two DIT side chains, and de novo T₃ formation involves coupling between an MIT donor and a DIT acceptor. Thyroid hormone synthesis is stimulated by TSH activating its receptor (TSHR), which upregulates the activity of many thyroid gene products involved in hormonogenesis. Additionally, TSH regulates post-translational changes in thyroglobulin that selectively enhance its capacity for T₃ formation - this process is important in iodide deficiency and in Graves disease. 167 different mutations, many of which are newly discovered, are now known to exist in TG (encoding human thyroglobulin) that can lead to defective thyroid hormone synthesis, resulting in congenital hypothyroidism.

Perigestational exposure to low doses of PBDE-47 induces excessive ER stress, defective autophagy and the resultant apoptosis contributing to maternal thyroid toxicity

Brominated flame retardant 2,2',4,4'‑tetrabromodiphenyl ether (PBDE-47) is known to induce developmental neurotoxicity by disturbing thyroid hormones (THs). Evidence shows that maternal THs are crucial for brain development and growth of fetuses and infants. However, little is known about the effects of PBDE-47 on maternal thyroid status and its mode of action. Here, using female Sprague-Dawley rats orally exposed to low doses of PBDE-47 (0.1, 1.0, 10 mg/kg/day) from pre-pregnancy until weaning of offspring to mimic human exposure, we show that perigestational exposure to PBDE-47 elevated serum triiodothyronine and thyroxine levels in mother rats. This is accompanied by disrupted thyroid follicle structure including expanded follicles, hyperplastic epithelial cells and shed cell remnants filled in the exhausted follicular lumen. Mechanistically, PBDE-47 enhanced apoptosis in thyroid tissue, as demonstrated by Caspase-3 activation, PARP cleavage and DNA fragmentation. Further study identified that PBDE-47 upregulated the levels of GRP78, ATF4, active Caspase-12 and CHOP, suggesting endoplasmic reticulum (ER) stress and unfolded protein response activation. Moreover, PBDE-47 reduced the levels of LC3-II, an autophagy marker protein essential for the autophagosomes formation, while increased the autophagy substrate p62 accumulation, indicating autophagy defect. Importantly, the colocalization of apoptotic cells with CHOP, a key mediator of ER stress-induced apoptosis, or p62, uncovered the contribution of excessive ER stress and defective autophagy to apoptosis. Collectively, our results suggest that excessive ER stress, defective autophagy and the resultant apoptosis are implicated in maternal thyroid injury following perigestational PBDE-47 exposure, which offers insight into a better understanding of PBDE-47-induced maternal thyroid toxicity.

Molecular analysis of thyroglobulin mutations found in patients with goiter and hypothyroidism

Thyroid dyshormonogenesis due to thyroglobulin (TG) gene mutations have an estimated incidence of approximately 1 in 100,000 newborns. The clinical spectrum ranges from euthyroid to mild or severe hypothyroidism. Up to now, one hundred seventeen deleterious mutations in the TG gene have been identified and characterized. The purpose of the present study was to identify and characterize new mutations in the TG gene. We report eight patients from seven unrelated families with goiter, hypothyroidism and low levels of serum TG. All patients underwent clinical, biochemical and image evaluation. Sequencing of DNA, genotyping, as well as bioinformatics analysis were performed. Molecular analyses revealed three novel inactivating TG mutations: c.5560G>T [p.E1835*], c.7084G>C [p.A2343P] and c.7093T>C [p.W2346R], and four previously reported mutations: c.378C>A [p.Y107*], c.886C>T [p.R277*], c.1351C>T [p.R432*] and c.7007G>A [p.R2317Q]. Two patients carried homozygous mutations (p.R277*/p.R277*, p.W2346R/p.W2346R), four were compound heterozygous mutations (p.Y107*/p.R277* (two unrelated patients), p.R432*/p.A2343P, p.Y107*/p.R2317Q) and two siblings from another family had a single p.E1835* mutated allele. Additionally, we include the analysis of 48 patients from 31 unrelated families with TG mutations identified in our present and previous studies. Our observation shows that mutations in both TG alleles were found in 27 families (9 as homozygote and 18 as heterozygote compound), whereas in the remaining four families only one mutated allele was detected. The majority of the detected mutations occur in exons 4, 7, 38 and 40. 28 different mutations were identified, 33 of the 96 TG alleles encoded the change p.R277*. In conclusion, our results confirm the genetic heterogeneity of TG defects and the pathophysiological importance of the predicted TG misfolding and therefore thyroid hormone formation as a consequence of truncated TG proteins and/or missense mutations located within its ACHE-like domain.

A novel mutation in the TG gene (G2322S) causing congenital hypothyroidism in a Sudanese family: a case report

Background

Congenital hypothyroidism (CH) has an incidence of approximately 1:3000, but only 15% have mutations in the thyroid hormone synthesis pathways. Genetic analysis allows for the precise diagnosis.

Case presentation

A 3-week old girl presented with a large goiter, serum TSH > 100 mIU/L (reference range: 0.7–5.9 mIU/L); free T₄ < 3.2 pmol/L (reference range: 8.7–16 pmol/L); thyroglobulin (TG) 101 μg/L. Thyroid Tc-99 m scan showed increased radiotracer uptake. One brother had CH and both affected siblings have been clinically and biochemically euthyroid on levothyroxine replacement. Another sibling had normal thyroid function. Both Sudanese parents reported non-consanguinity. Peripheral blood DNA from the proposita was subjected to whole exome sequencing (WES). WES identified a novel homozygous missense mutation of the TG gene: c.7021G > A, p.Gly2322Ser, which was subsequently confirmed by Sanger sequencing and present in one allele of both parents. DNA samples from 354 alleles in four Sudanese ethnic groups (Nilotes, Darfurians, Nuba, and Halfawien) failed to demonstrate the presence of the mutant allele. Haplotyping showed a 1.71 centiMorgans stretch of homozygosity in the TG locus suggesting that this mutation occurred identical by descent and the possibility of common ancestry of the parents. The mutation is located in the cholinesterase-like (ChEL) domain of TG.

Conclusions

A novel rare missense mutation in the TG gene was identified. The ChEL domain is critical for protein folding and patients with CH due to misfolded TG may present without low serum TG despite the TG gene mutations.

Electronic supplementary material

The online version of this article (10.1186/s12881-018-0588-7) contains supplementary material, which is available to authorized users.

Mild thyroid peroxidase deficiency caused by TPO mutations with residual activity: Correlation between clinical phenotypes and enzymatic activity

Thyroid peroxidase (TPO) deficiency, caused by biallelic TPO mutations, is a well-established genetic form of congenital hypothyroidism (CH). More than 100 patients have been published, and the patients have been diagnosed mostly in the frame of newborn screening (NBS) programs. Correlation between clinical phenotypes and TPO activity remains unclear. Here, we report clinical and molecular findings of two unrelated TPO mutation-carrying mildly hypothyroid patients. The two patients were born at term after an uneventful pregnancy and delivery, and were NBS negative. They sought medical attention due to goiter at age 8 years. Evaluation of the thyroid showed mild elevation of serum TSH levels, normal or slightly low serum T₄ levels, high serum T₃ to T₄ molar ratio, high serum thyroglobulin levels, and high thyroidal ¹²³I uptake. We performed next-generation sequencing-based genetic screening, and found that one patient was compound heterozygous for two novel TPO mutations (p.Asp224del; c.820-2A>G), and the other was homozygous for a previously known mutation (p.Trp527Cys). In vitro functional analyses using HEK293 cells showed that the two amino acid-altering mutations (p.Asp224del and p.Trp527Cys) caused partial loss of the enzymatic activity. In conclusion, we report that TPO mutations with residual activity are associated with mild TPO deficiency, which is clinically characterized by marked goiter, mild TSH elevation, high serum T₃ to T₄ molar ratio, and high serum thyroglobulin levels. Our findings illuminate the hitherto under-recognized correlation between clinical phenotypes and residual enzymatic activity among patients with TPO deficiency.

Endoplasmic reticulum stress inhibits expression of genes involved in thyroid hormone synthesis and their key transcriptional regulators in FRTL-5 thyrocytes

Endoplasmic reticulum (ER) stress is characterized by the accumulation of misfolded proteins due to an impairment of ER quality control pathways leading to the activation of a defense system, called unfolded protein response (UPR). While thyrocytes are supposed to be highly susceptible to environmental conditions that cause ER stress due to the synthesis of large amounts of secretory proteins required for thyroid hormone synthesis, systematic investigations on the effect of ER stress on expression of key genes of thyroid hormone synthesis and their transcriptional regulators are lacking. Since the aim of the ER stress-induced UPR is to restore ER homeostasis and to facilitate cell survival through transient shutdown of ribosomal protein translation, we hypothesized that the expression of genes involved in thyroid hormone synthesis and their transcriptional regulators, all of which are not essential for cell survival, are down-regulated in thyrocytes during ER stress, while sterol regulatory element-binding proteins (SREBPs) are activated during ER stress in thyrocytes. Treatment of FRTL-5 thyrocytes with the ER stress inducer tunicamycin (TM) dose-dependently increased the mRNA and/or protein levels of known UPR target genes, stimulated phosphorylation of the ER stress sensor protein kinase RNA-like ER kinase (PERK) and of the PERK target protein eukaryotic initiation factor 2α (eIF2α) and caused splicing of the ER stress-sensitive transcription factor X-box binding protein (XBP-1) (P < 0.05). The mRNA levels and/or protein levels of genes involved in thyroid hormone synthesis, sodium/iodide symporter (NIS), thyroid peroxidase (TPO) and thyroglobulin (TG), their transcriptional regulators and thyrotropin (TSH) receptor and the uptake of Na125I were reduced at the highest concentration of TM tested (0.1 μg/mL; P < 0.05). Proteolytic activation of the SREBP-1c pathway was not observed in FRTL-5 cells treated with TM, whereas TM reduced proteolytic activation of the SREBP-2 pathway at 0.1 μg TM/mL (P < 0.05). In conclusion, the expression of key genes involved in thyroid hormone synthesis and their critical regulators and of the TSH receptor as well as the uptake of iodide is attenuated in thyrocytes during mild ER stress. Down-regulation of NIS, TPO and TG during ER stress is likely the consequence of impaired TSH/TSHR signaling in concert with reduced expression of critical transcriptional regulators of these genes.

The burden of trisomy 21 disrupts the proteostasis network in Down syndrome

Down syndrome (DS) is a genetic disorder caused by trisomy of chromosome 21. Abnormalities in chromosome number have the potential to lead to disruption of the proteostasis network (PN) and accumulation of misfolded proteins. DS individuals suffer from several comorbidities, and we hypothesized that disruption of proteostasis could contribute to the observed pathology and decreased cell viability in DS. Our results confirm the presence of a disrupted PN in DS, as several of its elements, including the unfolded protein response, chaperone system, and proteasomal degradation exhibited significant alterations compared to euploid controls in both cell and mouse models. Additionally, when cell models were treated with compounds that promote disrupted proteostasis, we observed diminished levels of cell viability in DS compared to controls. Collectively our findings provide a cellular-level characterization of PN dysfunction in DS and an improved understanding of the potential pathogenic mechanisms contributing to disrupted cellular physiology in DS. Lastly, this study highlights the future potential of designing therapeutic strategies that mitigate protein quality control dysfunction.

Thyroglobulin From Molecular and Cellular Biology to Clinical Endocrinology

Thyroglobulin (Tg) is a vertebrate secretory protein synthesized in the thyrocyte endoplasmic reticulum (ER), where it acquires N-linked glycosylation and conformational maturation (including formation of many disulfide bonds), leading to homodimerization. Its primary functions include iodide storage and thyroid hormonogenesis. Tg consists largely of repeating domains, and many tyrosyl residues in these domains become iodinated to form monoiodo- and diiodotyrosine, whereas only a small portion of Tg structure is dedicated to hormone formation. Interestingly, evolutionary ancestors, dependent upon thyroid hormone for development, synthesize thyroid hormones without the complete Tg protein architecture. Nevertheless, in all vertebrates, Tg follows a strict pattern of region I, II-III, and the cholinesterase-like (ChEL) domain. In vertebrates, Tg first undergoes intracellular transport through the secretory pathway, which requires the assistance of thyrocyte ER chaperones and oxidoreductases, as well as coordination of distinct regions of Tg, to achieve a native conformation. Curiously, regions II-III and ChEL behave as fully independent folding units that could function as successful secretory proteins by themselves. However, the large Tg region I (bearing the primary T4-forming site) is incompetent by itself for intracellular transport, requiring the downstream regions II-III and ChEL to complete its folding. A combination of nonsense mutations, frameshift mutations, splice site mutations, and missense mutations in Tg occurs spontaneously to cause congenital hypothyroidism and thyroidal ER stress. These Tg mutants are unable to achieve a native conformation within the ER, interfering with the efficiency of Tg maturation and export to the thyroid follicle lumen for iodide storage and hormonogenesis.

Autism-associated R451C mutation in neuroligin3 leads to activation of the unfolded protein response in a PC12 Tet-On inducible system

The expression of the autism-related mutant R451C neuroligin3 activates the three branches of the unfolded protein response in neuronal-like PC12 cells and leads to up-regulation of the response target genes BiP and CHOP.

Several forms of monogenic heritable autism spectrum disorders are associated with mutations in the neuroligin genes. The autism-linked substitution R451C in neuroligin3 induces local misfolding of its extracellular domain, causing partial retention in the ER (endoplasmic reticulum) of expressing cells. We have generated a PC12 Tet-On cell model system with inducible expression of wild-type or R451C neuroligin3 to investigate whether there is activation of the UPR (unfolded protein response) as a result of misfolded protein retention. As a positive control for protein misfolding, we also expressed the mutant G221R neuroligin3, which is known to be completely retained within the ER. Our data show that overexpression of either R451C or G221R mutant proteins leads to the activation of all three signalling branches of the UPR downstream of the stress sensors ATF6 (activating transcription factor 6), IRE1 (inositol-requiring enzyme 1) and PERK [PKR (dsRNA-dependent protein kinase)-like endoplasmic reticulum kinase]. Each branch displayed different activation profiles that partially correlated with the degree of misfolding caused by each mutation. We also show that up-regulation of BiP (immunoglobulin heavy-chain-binding protein) and CHOP [C/EBP (CCAAT/enhancer-binding protein)-homologous protein] was induced by both mutant proteins but not by wild-type neuroligin3, both in proliferative cells and cells differentiated to a neuron-like phenotype. Collectively, our data show that mutant R451C neuroligin3 activates the UPR in a novel cell model system, suggesting that this cellular response may have a role in monogenic forms of autism characterized by misfolding mutations.

A mouse model suggests two mechanisms for thyroid alterations in infantile cystinosis: decreased thyroglobulin synthesis due to endoplasmic reticulum stress/unfolded protein response and impaired lysosomal processing

Thyroid hormones are released from thyroglobulin (Tg) in lysosomes, which are impaired in infantile/nephropathic cystinosis. Cystinosis is a lysosomal cystine storage disease due to defective cystine exporter, cystinosin. Cystinotic children develop subclinical and then overt hypothyroidism. Why hypothyroidism is the most frequent and earliest endocrine complication of cystinosis is unknown. We here defined early alterations in Ctns(-/-) mice thyroid and identified subcellular and molecular mechanisms. At 9 months, T4 and T3 plasma levels were normal and TSH was moderately increased (∼4-fold). By histology, hyperplasia and hypertrophy of most follicles preceded colloid exhaustion. Increased immunolabeling for thyrocyte proliferation and apoptotic shedding indicated accelerated cell turnover. Electron microscopy revealed endoplasmic reticulum (ER) dilation, apical lamellipodia indicating macropinocytic colloid uptake, and lysosomal cystine crystals. Tg accumulation in dilated ER contrasted with mRNA down-regulation. Increased expression of ER chaperones, glucose-regulated protein of 78 kDa and protein disulfide isomerase, associated with alternative X-box binding protein-1 splicing, revealed unfolded protein response (UPR) activation by ER stress. Decreased Tg mRNA and ER stress suggested reduced Tg synthesis. Coordinated increase of UPR markers, activating transcription factor-4 and C/EBP homologous protein, linked ER stress to apoptosis. Hormonogenic cathepsins were not altered, but lysosome-associated membrane protein-1 immunolabeling disclosed enlarged vesicles containing iodo-Tg and impaired lysosomal fusion. Isopycnic fractionation showed iodo-Tg accumulation in denser lysosomes, suggesting defective lysosomal processing and hormone release. In conclusion, Ctns(-/-) mice showed the following alterations: 1) compensated primary hypothyroidism and accelerated thyrocyte turnover; 2) impaired Tg production linked to ER stress/UPR response; and 3) altered endolysosomal trafficking and iodo-Tg processing. The Ctns(-/-) thyroid is useful to study disease progression and evaluate novel therapies.

A novel mutation in the thyroglobulin gene that causes goiter and dwarfism in Wistar Hannover GALAS rats

Outbred stocks of rats have been used extensively in biomedical, pharmaceutical and/or toxicological studies as a model of genetically heterogeneous human populations. One of such stocks is the Wistar Hannover GALAS rat. However, the colony of Wistar Hannover GALAS rat has been suspected of keeping a problematic mutation that manifests two distinct spontaneous abnormalities, goiter and dwarfism, which often confuses study results. We have successfully identified the responsible mutation, a guanine to thymine transversion at the acceptor site (3' end) of intron 6 in the thyroglobulin (Tg) gene (Tgc.749-1G>T), that induces a complete missing of exon 7 from the whole Tg transcript by mating experiments and subsequent molecular analyses. The following observations confirmed that Tgc.749-1G>T/Tgc.749-1G>T homozygotes manifested both dwarfism and goiter, while Tgc.749-1G>T/+ heterozygotes had only a goiter with normal appearance, suggesting that the mutant phenotypes inherit as an autosomal semi-dominant trait. The mutant phenotypes, goiter and dwarfism, mimicked those caused by typical endocrine disrupters attacking the thyroid. Hence a simple and reliable diagnostic methodology has been developed for genomic DNA-based genotyping of animals. The diagnostic methodology reported here would allow users of Wistar Hannover GALAS rats to evaluate their study results precisely by carefully interpreting the data obtained from Tgc.749-1G>T/+ heterozygotes having externally undetectable thyroidal lesions.

Electromagnetic fields at 2.45 GHz trigger changes in heat shock proteins 90 and 70 without altering apoptotic activity in rat thyroid gland

Non-ionizing radiation at 2.45 GHz may modify the expression of genes that codify heat shock proteins (HSP) in the thyroid gland. Using the enzyme-linked immunosorbent assay (ELISA) technique, we studied levels of HSP-90 and HSP-70. We also used hematoxilin eosin to look for evidence of lesions in the gland and applied the DAPI technique of fluorescence to search for evidence of chromatin condensation and nuclear fragmentation in the thyroid cells of adult female Sprague-Dawley rats. Fifty-four rats were individually exposed for 30 min to 2.45 GHz radiation in a Gigahertz transverse electromagnetic (GTEM) cell at different levels of non-thermal specific absorption rate (SAR), which was calculated using the finite difference time domain (FDTD) technique. Ninety minutes after radiation, HSP-90 and HSP-70 had decreased significantly (P<0.01) after applying a SAR of 0.046±1.10 W/Kg or 0.104±5.10⁻³ W/Kg. Twenty-four hours after radiation, HSP-90 had partially recovered and HSP-70 had recovered completely. There were few indications of lesions in the glandular structure and signs of apoptosis were negative in all radiated animals. The results suggest that acute sub-thermal radiation at 2.45 GHz may alter levels of cellular stress in rat thyroid gland without initially altering their anti-apoptotic capacity.

Protein disulfide isomerase in redox cell signaling and homeostasis

Thiol proteins may potentially act as redox signaling adaptor proteins, adjusting reactive oxygen species intermediates to specific signals and redox signals to cell homeostasis. In this review, we discuss redox effects of protein disulfide isomerase (PDI), a thioredoxin superfamily oxidoreductase from the endoplasmic reticulum (ER). Abundantly expressed PDI displays ubiquity, interactions with redox and nonredox proteins, versatile effects, and several posttranslational modifications. The PDI family contains >20 members with at least some apparent complementary actions. PDI has oxidoreductase, isomerase, and chaperone effects, the last not directly dependent on its thiols. PDI is a converging hub for pathways of disulfide bond introduction into ER-processed proteins, via hydrogen peroxide-generating mechanisms involving the oxidase Ero1α, as well as hydrogen peroxide-consuming reactions involving peroxiredoxin IV and the novel peroxidases Gpx7/8. PDI is a candidate pathway for coupling ER stress to oxidant generation. Emerging information suggests a convergence between PDI and Nox family NADPH oxidases. PDI silencing prevents Nox responses to angiotensin II and inhibits Akt phosphorylation in vascular cells and parasite phagocytosis in macrophages. PDI overexpression spontaneously enhances Nox activation and expression. In neutrophils, PDI redox-dependently associates with p47phox and supports the respiratory burst. At the cell surface, PDI exerts transnitrosation, thiol reductase, and apparent isomerase activities toward targets including adhesion and matrix proteins and proteases. Such effects mediate redox-dependent adhesion, coagulation/thrombosis, immune functions, and virus internalization. The route of PDI externalization remains elusive. Such multiple redox effects of PDI may contribute to its conspicuous expression and functional role in disease, rendering PDI family members putative redox cell signaling adaptors.

Revisiting the regulated secretory pathway: from frogs to human

The regulated secretory pathway is a hallmark of endocrine and neuroendocrine cells. This process comprises different sequential steps, including ER-associated protein synthesis, ER-to-Golgi protein transport, Golgi-associated posttranslational modification, sorting and packing of secretory proteins into carrier granules, cytoskeleton-based granule transport towards the plasma membrane and tethering, docking and fusion of granules with specialized releasing zones in the plasma membrane. Each one of these steps is tightly regulated by a large number of factors that function in a spatially and temporarily coordinated fashion. During the past three decades, much effort has been devoted to characterize the precise role of the yet-known proteins participating in the different steps of this process and to identify new regulatory factors in order to obtain a unifying picture of the secretory pathway. In spite of this and given the enormous complexity of the process, certain steps are not fully understood yet and many players remain to be identified. In this review, we offer a summary of the current knowledge on the main molecular mechanisms that govern and ensure the correct release of secretory proteins. In addition, we have integrated the advance on the field made possible by studies carried out in non-mammalian vertebrates, which, although not very numerous, have substantially contributed to acquire a mechanistic understanding of the regulated secretory pathway.

Identification of ERp29 as a biomarker for predicting nasopharyngeal carcinoma response to radiotherapy

Radioresistance continues to be a major problem in the treatment of nasopharyngeal carcinoma (NPC). This study aimed to identify novel proteins associated with NPC radio-resistance. We used a mass spectrometry driven-proteomic strategy to identify novel proteins associated with NPC radio-resistance, and differential proteins were subsequently processed by bio-informatic analysis. As a result, twelve proteins were identified with aberrant expression in radioresistant (RR) NPC tissues compare to radiosensitive (RS) NPC tissues. Among these proteins, ERp29, Mn-SOD, HSP27 and GST ω1 were found to be significantly up-regulated in RR NPC tissues, and ERp29 was selected for further validation. Immunohistochemistry analysis confirmed that ERp29 was overexpressed in RR NPC tissues compared with RS NPC tissues. To prove the role of ERp29 in the induction of NPC radioresistance, ERp29 was down-regulated in the ERp29 enriched NPC cells CNE-1 and 6-10B by specific shRNA. Radiosensitivity was measured using cell proliferation assay and clonogenic survival assay, and cell apoptosis was measured using flow cytometric analysis. We found that ERp29 knockdown attenuated CNE-1 and 6-10B cell radioresistance and enhanced cell apoptosis. These results suggest that ERp29 associates with radioresistance in NPC, and ERp29 could be a potential biomarker for predicting NPC response to radiotherapy.

Maturation of thyroglobulin protein region I

In vertebrates, the thyroglobulin (Tg) gene product must be exported to the lumen of thyroid follicles for thyroid hormone synthesis. In toto, Tg is composed of multiple type-1 repeats connected by linker and hinge (altogether considered as "region I," nearly 1,200 residues); regions II-III (~720 residues); and cholinesterase-like (ChEL) domain (~570 residues). Regions II-III and ChEL rapidly acquire competence for secretion, yet regions I-II-III require 20 min to become a partially mature disulfide isomer; stabilization of a fully oxidized form requires ChEL. Transition from partially mature to mature Tg occurs as a discrete "jump" in mobility by nonreducing SDS-PAGE, suggesting formation of at most a few final pairings of Cys residues that may be separated by significant intervening primary sequence. Using two independent approaches, we have investigated which portion of Tg is engaged in this late stage of its maturation. First, we demonstrate that this event is linked to oxidation involving region I. Introduction of the Tg-C1245R mutation in the hinge (identical to that causing human goitrous hypothyroidism) inhibits this maturation, although the Cys-1245 partner remains unidentified. Second, we find that Tg truncated after its fourth type-1 repeat is a fully independent secretory protein. Together, the data indicate that final acquisition of secretory competence includes conformational maturation in the interval between linker and hinge segments of region I.

Inhibiting ERp29 expression enhances radiosensitivity in human nasopharyngeal carcinoma cell lines

ERp29 is an endoplasmic reticulum (ER) stress-inducible protein. It was found that ERp29 was highly expressed in several cancers and associated with resistance to oxidative and radiation stress, which may serve as a novel target for nasopharyngeal carcinoma (NPC) anticancer approach. In this study, we used immunohistochemistry to detect ERp29 expression in radioresistant and radiosensitive NPC tissues. As a result, ERp29 was up-regulated in radioresistant NPC tissues compared to radiosensitive NPC tissues. We also found that ERp29 knockdown attenuated radioresistance of NPC CNE-1 cells and ERp29 overexpression enhanced radioresistance of NPC CNE-2 cells. When exposed to radiation, ERp29 knockdown CNE-1 cells increased radiation-induced cell apoptosis and ERp29 overexpression CNE-2 cells reduced radiation-induced cell apoptosis. Further, we demonstrated that ERp29 up-regulated the expression of Hsp27. In conclusion, our study supports ERp29 could potentiate resistance to radiation in NPC cells, targeting of ERp29 is a rational strategy in treating radioresistant NPC.

A patient with Pendred syndrome whose goiter progressed with normal serum thyrotropin and iodine organification

Biallelic mutations of SLC26A4 (encoding pendrin) cause Pendred syndrome (PS), an autosomal recessive genetic disorder with deafness and goiter. The mechanism underlying the development of the goiter is unknown. Here, we report clinical and molecular findings of a patient with PS. This 27-year-old woman was born to nonconsanguineous healthy parents. She was seen at our hospital due to hearing loss at age 3 years, and subsequently developed goiter at age 10 years. From age 15 years, her thyroid gland showed progressive enlargement accompanied by elevation of serum thyroglobulin reaching 10-fold the normal amount. Thyroidal iodine uptake was also increased during goiter progression ((123)I uptake at 24 hr: 20.2% at age 17 years; 69.4% at age 24 years; reference, 8-40), while serum thyrotropin (TSH) levels and iodine organification (examined by the perchrolate or thiocyanate discharge test) remained normal. We sequenced SLC26A4 using standard PCR-based technique, and found one novel (p.T537P) and one recurrent (p.H723R) mutations in a compound heterozygous state. Expression experiments using COS-7 cells showed that the two mutants were entrapped in the endoplasmic reticulum and were poorly localized at the plasma membrane. In summary, a molecularly confirmed PS patient showed goiter progression accompanied by elevated serum thyroglobulin and increased thyroidal iodine uptake, but normal serum TSH levels and normal iodine organification. This implies that some pendrin mutations may involve direct stimulation of thyroid cell proliferation with no TSH hyperstimulation and no iodine organification defect.

Biallelic p.R2223H mutation in the thyroglobulin gene causes thyroglobulin retention and severe hypothyroidism with subsequent development of thyroid carcinoma

CONTEXT

Dyshormonogenesis due to genetic defect in thyroglobulin (Tg) synthesis and secretion can lead to congenital hypothyroidism.

OBJECTIVES

The aim of the study was to analyze the TG gene for the presence of mutations and to study the underlying mechanisms leading to dyshormonogenesis.

CASES

Two siblings aged 25 and 31 yr presented with recurrent goitrous hypothyroidism with undetectable serum Tg. The older sibling was diagnosed with follicular variant of papillary thyroid carcinoma (FVPTC) at age 21 and metastatic FVPTC 8 yr later.

METHODS

The entire coding region of TG gene was sequenced. BRAF, RAS, and P53 mutations or PAX8/PPAR-gamma rearrangement were screened in the FVPTC. Tg expression was studied by immunohistochemistry.

RESULTS

Biallelic c.6725G>A (p.R2223H) and c.6396C>T (p.S2113L) sequence variations were detected in both patients and monoallelic variations in their family members. The c.6396C>T (p.S2113L) sequence variation was found in 14% of 100 population controls, whereas c.6725G>A variation was not present in the controls. Two previously reported polymorphisms (c.2200T>G and c.3082A>G) were present in all the family members. Strong cytoplasmic immunostaining of Tg was observed in the hyperplastic thyroid epithelial cells and weak or no staining in the follicular lumen. Cytoplasmic staining was localized in the endoplasmic reticulum. Reduced staining was found in the FVPTC. Neither RAS, BRAF, or P53 gene mutation nor a PAX8/PPAR-gamma rearrangement was detected in the tumor tissue.

CONCLUSIONS

Biallelic c.6725G>A (p.R2223H) mutation causes Tg retention in the endoplasmic reticulum, resulting in dyshormonogenesis. Prolonged TSH stimulation may promote malignant transformation and development of thyroid cancer. The c.6396C>T (p.S2113L) is a novel polymorphism.

Abnormal expression of collagen IV in lens activates unfolded protein response resulting in cataract

Human diseases caused by mutations in extracellular matrix genes are often associated with an increased risk of cataract and lens capsular rupture. However, the underlying mechanisms of cataract pathogenesis in these conditions are still unknown. Using two different mouse models, we show that the accumulation of collagen chains in the secretory pathway activates the stress signaling pathway termed unfolded protein response (UPR). Transgenic mice expressing ectopic Col4a3 and Col4a4 genes in the lens exhibited activation of IRE1, ATF6, and PERK associated with expansion of the endoplasmic reticulum and attenuation of general protein translation. The expression of the transgenes had adverse effects on lens fiber cell differentiation and eventually induced cell death in a group of transgenic fiber cells. In Col4a1(+/Deltaex40) mutant mice, the accumulation of mutant chains also caused low levels of UPR activation. However, cell death was not induced in mutant lenses, suggesting that low levels of UPR activation are not proapoptotic. Collectively, the results provide in vivo evidence for a role of UPR in cataract formation in response to accumulation of terminally unfolded proteins in the endoplasmic reticulum.

The p.A2215D thyroglobulin gene mutation leads to deficient synthesis and secretion of the mutated protein and congenital hypothyroidism with wide phenotype variation

CONTEXT

Thyroglobulin (TG) is a large glycoprotein and functions as a matrix for thyroid hormone synthesis. TG gene mutations give rise to goitrous congenital hypothyroidism (CH) with considerable phenotype variation.

OBJECTIVES

The aim of the study was to report the genetic screening of 15 patients with CH due to TG gene mutations and to perform functional analysis of the p.A2215D mutation.

DESIGN

Clinical evaluation and DNA sequencing of the TG gene were performed in all patients. TG expression was analyzed in the goitrous tissue of one patient. Human cells were transfected with expression vectors containing mutated and wild-type human TG cDNA.

RESULTS

All patients had an absent rise of serum TG after stimulation with recombinant human TSH. Sequence analysis revealed three previously described mutations (p.A2215D, p.R277X, and g.IVS30+1G>T), and two novel mutations (p.Q2142X and g.IVS46-1G>A). Two known (g.IVS30+1G/p.A2215D and p.A2215D/p.R277X) and one novel (p.R277X/g.IVS46-1G>A) compound heterozygous constellations were also identified. Functional analysis indicated deficiency in TG synthesis, reduction of TG secretion, and retention of the mutant TG within the cell, leading to an endoplasmic reticulum storage disease, whereas small amounts of mutant TG were still secreted within the cell system.

CONCLUSION

All studied patients were either homozygous or heterozygous for TG gene mutations. Two novel mutations have been detected, and we show that TG mutation p.A2215D promotes the retention of TG within the endoplasmic reticulum and reduces TG synthesis and secretion, causing mild hypothyroidism. In the presence of sufficient iodine supply, some patients with TG mutations are able to compensate the impaired hormonogenesis and generate thyroid hormone.

Perchlorate and ethylenethiourea induce different histological and molecular alterations in a non-mammalian vertebrate model of thyroid goitrogenesis

Despite evidence for a conserved role of thyroid-stimulating hormone (TSH) in regulating vertebrate thyroid function, molecular data on thyroid responses to TSH are mainly limited to mammalian species. In this study, we examined histological and molecular changes in the thyroid of Xenopus laevis tadpoles during a 12-day treatment with 20mg/l perchlorate (PER) and 50mg/l ethylenethiourea (ETU). Inhibition of thyroid hormone (TH) synthesis by PER and ETU was evident from developmental retardation, reduced expression of TH-regulated genes and up-regulation of tshb-A mRNA. Thyroid histopathology revealed goiters with strikingly different follicular morphologies following PER and ETU treatment. Using real-time PCR, we analyzed thyroids sampled on day 12 for differential expression of 60 candidate genes. Further temporal analyses were performed for a subset of 14 genes. Relative to the control, PER and ETU treatment modulated the expression of 51 and 49 transcripts, respectively. Particularly genes related to TH synthesis and protein metabolism were similarly affected by PER and ETU. However, several genes were differentially expressed in PER- and ETU-treated tadpoles. Specifically, goiter formation in the PER treatment was associated with low expression of genes related to DNA replication but high expression of negative growth regulators. Results from this work provide for the first time a characterization of gene expression profiles during goitrogenesis in a non-mammalian vertebrate model. Overall, our data suggest that, in addition to TSH over-stimulation, further mechanisms related to the mode of goitrogen action contribute to the regulation of thyroid gene expression.

ERp29 is a radiation-responsive gene in IEC-6 cell

ERp29 is a resident protein of the endoplasmic reticulum (ER) lumen, which is thought to be involved in the folding of secretory proteins. In our previous work, it was found that, when treated with ionizing radiation (IR), the ERp29 expression was increased in mouse intestinal epithelia and cultured IEC-6 cells, which suggested that ERp29 might be a radiation-induced gene. The current work is to confirm the induction of ERp29 by IR and to analyze its role in irradiated IEC-6 cells. Our results showed that ERp29 expression was elevated by IR in IEC-6 cells at mRNA and protein levels in a time-dependent manner. IEC-6 cells with different exogenous ERp29 expression were obtained by transfection with sense and antisense expression vectors of ERp29 coding region. As ERp29 expression was inhibited, these cells exhibited more serious radiation injury and more sensitivity to IR-induced apoptosis. To further elucidate the induction of ERp29, we analyzed the XBP1 expression after IR. Results showed that the spliced form of XBP1 mRNA rapidly reached a peak at 3 hours after irradiation, which indicated that UPR sensor was involved in radiation and might be a reason to induce ERp29 expression. Our results demonstrate that ERp29 is a radiation associated protein and plays an important role in protecting cells from IR.

The cholinesterase-like domain of thyroglobulin functions as an intramolecular chaperone

Thyroid hormonogenesis requires secretion of thyroglobulin, a protein comprising Cys-rich regions I, II, and III (referred to collectively as region I-II-III) followed by a cholinesterase-like (ChEL) domain. Secretion of mature thyroglobulin requires extensive folding and glycosylation in the ER. Multiple reports have linked mutations in the ChEL domain to congenital hypothyroidism in humans and rodents; these mutations block thyroglobulin from exiting the ER and induce ER stress. We report that, in a cell-based system, mutations in the ChEL domain impaired folding of thyroglobulin region I-II-III. Truncated thyroglobulin devoid of the ChEL domain was incompetent for cellular export; however, a recombinant ChEL protein ("secretory ChEL") was secreted efficiently. Coexpression of secretory ChEL with truncated thyroglobulin increased intracellular folding, promoted oxidative maturation, and facilitated secretion of region I-II-III, indicating that the ChEL domain may function as an intramolecular chaperone. Additionally, we found that the I-II-III peptide was cosecreted and physically associated with secretory ChEL. A functional ChEL domain engineered to be retained intracellularly triggered oxidative maturation of I-II-III but coretained I-II-III, indicating that the ChEL domain may also function as a molecular escort. These insights into the role of the ChEL domain may represent potential therapeutic targets in the treatment of congenital hypothyroidism.

Endoplasmic reticulum (ER) chaperone regulation and survival of cells compensating for deficiency in the ER stress response kinase, PERK

The activity of PERK, an endoplasmic reticulum (ER) transmembrane protein kinase, assists in an ER stress response designed to inhibit general protein synthesis while allowing upregulated synthesis of selective proteins such as the ATF4 transcription factor. PERK null mice exhibit phenotypes that especially affect secretory cell types. Although embryonic fibroblasts from these mice are difficult to transfect with high efficiency, we have generated 293 cells stably expressing the PERK-K618A dominant negative mutant. 293/PERK-K618A cells, in response to ER stress: (a) do not properly inhibit general protein synthesis, (b) exhibit defective/delayed induction of ATF4 and BiP, and (c) exhibit exuberant splice activation of XBP1 and robust cleavage activation of ATF6, with abnormal regulation of calreticulin levels. The data suggest compensatory mechanisms allowing for cell survival in the absence of functional PERK. Interestingly, although induction of CHOP (a transcription factor implicated in apoptosis) is notably delayed after onset of ER stress, 293/PERK-K618A cells eventually produce CHOP at normal or even supranormal levels and exhibit increased apoptosis either in response to general ER stress or, more importantly, to specific misfolded secretory proteins.

Differential requirement of unfolded protein response pathway for calreticulin expression in Caenorhabditis elegans

Accumulation of unfolded proteins in the endoplasmic reticulum triggers the unfolded protein response (UPR) pathway, which increases the expression of chaperones to maintain the homeostasis. Calreticulin is a calcium-binding chaperone located in the lumen of endoplasmic reticulum (ER). Here we show that in response to a UPR inducing reagent, tunicamycin, the expression of calreticulin (crt-1) is specifically up-regulated in Caenorhabditis elegans. Tunicamycin (TM) induced expression of the crt-1 requires IRE-1 and XBP-1 but is ATF-6 and PEK-1 independent. Analysis of the crt-1 promoter reveals a putative XBP-1 binding site at the -284 to -278 bp region, which was shown to be necessary for TM-mediated induction. Genetic analysis of crt-1 mutants and mutants of UPR pathway genes show various degrees of developmental arrest upon TM treatment. Our results suggest that the TM-induced UPR pathway culminates in the up-regulation of crt-1, which protects the worm from deleterious accumulation of unfolded proteins in the ER. Knockdown of the crt-1, pdi-2, or pdi-3 increased the crt-1 expression, whereas knockdown of the hsp-3 or hsp-4 did not have any effect on crt-1 expression, indicating the existence of complex compensatory networks to cope up with ER stress.

Thyroglobulin gene mutations producing defective intracellular transport of thyroglobulin are associated with increased thyroidal type 2 iodothyronine deiodinase activity

CONTEXT

Most patients with defective synthesis and/or secretion of thyroglobulin (Tg) present relatively high serum free T3 (FT3) concentrations with disproportionately low free T4 (FT4) resulting in a high FT3/FT4 ratio. The mechanism of this change in FT3/FT4 ratio remains unknown.

OBJECTIVE

We hypothesize that increased type 2 iodothyronine deiodinase (D2) activity in the thyroid gland may explain the higher FT3/FT4 ratio that is frequently observed in patients with abnormal Tg synthesis.

DESIGN

We recently identified a compound heterozygous patient (patient A) with a Tg G2356R mutation and one previously described (C1245R) that is known to cause a defect in intracellular transport of Tg. In the current study, after determining the abnormality caused by G2356R, we measured D2 activity as well as its mRNA level in the thyroid gland. We also measured the thyroidal D2 activity in three patients with Tg transport defect and in normal thyroid tissue.

RESULTS

Morphological and biochemical analysis of the thyroid gland from patient A, complemented by a pulse-chase experiment, revealed that G2356R produces a defect in intracellular Tg transport. D2 activity but not type 1 deiodinase in thyroid glands of patients with abnormal Tg transport was significantly higher than in normal thyroid glands, whereas D2 mRNA level in patient A was comparable with that in normal thyroid glands. Furthermore, there was a positive correlation between D2 activity and FT3/FT4 ratios.

CONCLUSION

Increased thyroidal D2 activity in the thyroid gland is responsible for the higher FT3/FT4 ratios in patients with defective intracellular Tg transport.

Proteomic profiling of cold thyroid nodules

Cold thyroid nodules (CTNs) represent a frequent endocrine disorder accounting for up to 85% of thyroid nodules in a population living in an iodine-deficient area. Benign CTNs need to be distinguished from thyroid cancer, which is relatively rare. The molecular etiology of benign CTNs is unresolved. To obtain novel insights into their pathogenesis, protein expression profiling was performed in a series of 27 solitary CTNs (10 follicular adenoma and 20 adenomatous nodules) and surrounding normal thyroid tissues using two-dimensional gel electrophoresis combined with mass spectrometry analysis, Western blotting, and immunohistochemistry. The proteome analysis revealed a specific fingerprint of CTNs with up-regulation of three functional systems: 1) thyroid cell proliferation, 2) turnover of thyroglobulin, and 3) H2O2 detoxification. Western blot analysis and immunohistochemistry confirmed the proteome data and showed that CTNs exhibit significant up-regulation of proteins involved in thyroid hormone synthesis yet are deficient in T4-containing thyroglobulin. This is consequential to intranodular iodide deficiency, mainly due to cytoplasmic sodium iodide symporter localization, and portrays the CTN as an activated proliferating lesion with an intranodular hypothyroid milieu. Furthermore, we provide preliminary evidence that up-regulation of H2O2 generation in CTNs could override the antioxidative system resulting in oxidative stress, which is suggested by the finding of raised 8-oxo-guanidine DNA adduct formation in CTNs.

Oxidoreductase interactions include a role for ERp72 engagement with mutant thyroglobulin from the rdw/rdw rat dwarf

Newly synthesized thyroglobulin (Tg), the secretory glycoprotein that serves as precursor in thyroid hormone synthesis, normally forms transient covalent protein complexes with oxidoreductases of the endoplasmic reticulum (ER). The Tg-G2320R mutation is responsible for congenital hypothyroidism in rdw/rdw rats, in which a lack of secondary thyroid enlargement (goiter) implicates death of thyrocytes as part of disease pathogenesis. We found that mutant Tg-G2320R was retained within the ER with no detectable synthesis of thyroxine, had persistent exposure of free cysteine thiols, and was associated with activated ER stress response but incomplete ER-associated degradation (ERAD). Tg-G2320R associated with multiple ER resident proteins, most notably ERp72, including covalent Tg-ERp72 interactions. In PC Cl3 thyrocytes, inducible overexpression of ERp72 increased the ability of cells to maintain Tg cysteines in a reduced state. Noncovalent interactions of several ER chaperones with newly synthesized Tg-G2320R diminished over time in parallel with ERAD of the mutant protein, yet a small ERAD-resistant Tg fraction remained engaged in covalent association with ERp72 even 2 days post-synthesis. Such covalent protein aggregates may set the stage for apoptotic thyrocyte cell death, preventing thyroid goiter formation in rdw/rdw rats.

Molecular advances in thyroglobulin disorders

Synthesis of tri-iodothyronine (T(3)) and thyroxine (T(4)) follows a metabolic pathway that depends on the integrity of the thyroglobulin structure. This large glycoprotein is a homodimer of 660 kDa synthesized and secreted by the thyroid cells into the lumen of thyroid follicle. In humans it is coded by a single copy gene, 270 kb long, that maps on chromosome 8q24 and contains an 8.5 kb coding sequence divided into 48 exons. The preprotein monomer is composed of a 19-amino acid signal peptide followed by a 2749-amino acid polypeptide. In the last decade, several mutations in the thyroglobulin gene were reported. In animals, four of them have been observed in Afrikander cattle (p.R697X), Dutch goats (p.Y296X), cog/cog mouse (p.L2263P) and rdw rats (p.G2300R). Mutations in the human thyroglobulin gene are associated with congenital goiter or endemic and nonendemic simple goiter. Thirty-five inactivating mutations have been identified and characterized in the human thyroglobulin gene: 20 missense mutations (p.C175G, p.Q310P, p.Q851H, p.S971I, p.R989C, p.P993L, p.C1058R, p.C1245R, p.S1447N, p.C1588F, p.C1878Y, p.I1912V, p.C1977S, p.C1987Y, p.C2135Y, p.R2223H, p.G2300D, p.R2317Q, p.G2355V, p.G2356R), 8 splice site mutations (g.IVS3-3C>G, g.IVS5+1G>A, g.IVS10-1G>A, g.IVS24+1G>C, g.IVS30+1G>T, g.IVS30+1G>A, g.IVS34-1G>C, g.IVS45+2T>A) 5 nonsense mutations (p.R277X, p.Q692X, p.W1418X, p.R1511X, p.Q2638X) and 2 single nucleotide deletions (p.G362fsX382, p.D1494fsX1547). The thyroglobulin gene has been also identified as the major susceptibility gene for familial autoimmune thyroid diseases (AITD) by linkage analysis using highly informative polymorphic markers. In conclusion the identification of mutations in the thyrogobulin gene has provided important insights into structure-function relationships.

Docosahexaenoic acid in combination with celecoxib modulates HSP70 and p53 proteins in prostate cancer cells

The role of cyclooxygenase-2 (COX-2) and the mechanism by which it influences the development and behavior of prostate cancer is unclear. Selective COX-2 inhibitors may be effective against prostate cancer via COX-2-independent mechanisms. But administration of high doses of COX-2 inhibitors over longer period of time may not be devoid of side effects. There is increasing interest in using COX-2 inhibitors in combination with other chemopreventive agents to overcome the issue of toxicity. However, the molecular mechanisms underlying their combined actions are not well understood. Therefore, the present study was designed to determine the effects of low doses of docosahexaenoic acid (DHA) in combination with celecoxib on the molecular targets at the proteins level in rat prostate cancer cells. Two-dimensional gel electrophoresis, in combination with mass spectrometry analysis, was used for protein identification. Western blot analysis confirmed the proteins identified. Paraffin-embedded tissue sections from the rat prostate tumor were used to detect base level expression of heat shock protein 70 (HSP70) and p53. The rate of cancer cell growth was inhibited more effectively (p < 0.01) by DHA in combination with celecoxib at lower doses (2.5 microM each). A total number of twelve proteins were differentially expressed by the combined action of DHA and celecoxib at low doses. It was interesting to note that these agents activated both HSP70 and p53 proteins. Activation of HSP70 by the combined actions of DHA and celecoxib in the presence of wild-type p53 reveals a unique COX-2 independent mode of action against prostate cancer.

Haplotype analysis reveals founder effects of thyroglobulin gene mutations C1058R and C1977S in Japan

CONTEXT

Thyroglobulin (Tg) mutations were previously believed to be rare, resulting in congenital goitrous hypothyroidism. However, an increasing number of patients with Tg mutations, who are euthyroid to mildly hypothyroid, have been identified in Japan.

OBJECTIVES

The purpose of this study was to investigate whether the three frequently found Tg mutations, namely C1058R, C1245R, and C1977S, were caused by a founder effect.

RESULTS

We found 26 different mutations within the Tg gene in 52 patients from 41 families. Thirty-five patients were homozygous for the mutations, whereas the others were compound heterozygous. The occurrence of Tg mutation within the general Japanese population is one in 67,000. Patients with the C1245R mutation were found throughout Japan, whereas those with the C1058R mutation were confined to a small village on a southern island, and those with the C1977S mutation were restricted to a city. The eight patients with the C1058R mutation and the seven patients with the C1977S mutation all showed the same combinations of 18 single-nucleotide polymorphisms in the coding region of the Tg gene, which would appear in one in 810 million and one in 37 billion, respectively, control subjects.

CONCLUSIONS

The frequently found mutations, C1058R and C1977S, were caused by founder effects. This result suggests that Tg mutations may provide a genetic basis for the cause of familial euthyroid goiter.

ERp29, an unusual redox-inactive member of the thioredoxin family

Oxidative folding in the endoplasmic reticulum is accomplished by a group of oxidoreductases where the protein disulfide isomerase (PDI) plays a key role. Structurally, redox-active PDI domains, like many other enzymes utilizing cysteine chemistry, adopt characteristic thioredoxin folds. However, this structural unit is not necessarily associated with the redox function and the current review focuses on the interesting example of a loss-of-function PDI-like protein from the endoplasmic reticulum, ERp29. ERp29 shares a common predecessor with PDI; however in the course of divergent evolution it has lost a hallmark active site motif of redox enzymes but retained the characteristic structural fold in one of its domains. Although the functional characterization of ERp29 is far from completion, all available data point to its important role in the early secretory pathway and allow tentative categorization as a secretion factor/escort protein of a broad profile.

ERp29 is an essential endoplasmic reticulum factor regulating secretion of thyroglobulin

ERp29 is a ubiquitously expressed endoplasmic reticulum (ER) protein, which is found in the folding complexes of several secretory proteins in the ER. In our previous work, it was suggested that ERp29 function is critical for the folding/secretion of thyroglobulin (Tg), a major secretory product of thyroid cells. Current work is an attempt to substantiate this assumption by answering the question whether the secretion of Tg can be regulated through the manipulation of ERp29 expression in the FRTL-5 rat thyroid cells. Indeed, transient overexpression of ERp29 resulted in twofold enhancement of the Tg secretion whereas the RNAi-mediated ERp29 silencing led to the attenuation of the Tg export. Mutational analysis has suggested two loci that might be involved in the ERp29-Tg interactions: the interdomain linker including Cys157, an amino acid, which is important for the structural integrity of the C-terminal domain and an uncharged surface on the N-terminal domain flanked by Tyr64 and Gln70.

High incidence of thyroid cancer in long-standing goiters with thyroglobulin mutations

In this paper, we report the high prevalence of thyroid malignancy in patients with thyroglobulin mutations in Japan. Mutations of the thyroglobulin gene cause defective thyroid hormone synthesis, resulting in congenital hypothyroidism. Since our report in 1999 on the thyroglobulin mutations C1264R and C1996S, we have identified 14 adult patients (7 males and 7 females) from 9 unrelated families. They visited hospitals for treatment of huge goiters that first appeared in childhood. Persistent growth of the thyroid gland, probably caused by thyrotropin (TSH) stimulation, partially compensated thyroid hormone production, resulting in lowered serum TSH concentrations in turn. Consequently, many patients had to undergo multiple operations. Of 11 patients who had undergone surgery, 7 had thyroid cancers. Of five patients whose thyroid tissue was available, we found a heterozygous activating mutation, either V599E or K600E, in cancerous tissue from each of 2 patients. From these observations, we conclude that goiter resulting from thyroglobulin mutations is associated with thyroid cancer.

Naturally occurring mutations in the thyroglobulin gene

Thyroglobulin (Tg) is a large glycoprotein dimer secreted into the follicular lumen. It serves as the matrix for the synthesis of thyroxine (T4) and triiodothyronine (T3), and the storage of thyroid hormone and iodide. In response to demand for thyroid hormone secretion, Tg is internalized into the follicular cell and digested in lysosomes. Subsequently, the thyronines T4 (approximately 80%) and T3 (approximately 20%) are released into the blood stream. Biallelic mutations in the Tg gene have been identified in several animal species and human patients presenting with goiter and overt or compensated hypothyroidism. In untreated patients, goiters are often remarkably large and display continuous growth. In most instances, the affected individuals have related parents and are homozygous for inactivating mutations in the Tg gene. More rarely, compound heterozygous mutations lead to a loss of function of both alleles. Molecular analyses indicate that at least some of these alterations result in a secretory defect and an endoplasmic reticulum storage disease (ERSD). This review discusses the nature and consequences of naturally occurring Tg gene mutations in humans and several animal species. Recent recommendations for the nomenclature of mutations have led to different numbering systems, an aspect that is discussed in order to clarify discrepancies between different publications.