Congenital euthyroid goitre with impaired thyroglobulin transport

Proceedings of the 2013 Joint JSTP/NTP Satellite Symposium

The first joint Japanese Society of Toxicologic Pathology (JSTP) and National Toxicology Program (NTP) Satellite Symposium, entitled “Pathology Potpourri,” was held on January 29^th at Okura Frontier Hotel in Tsukuba, Ibaraki, Japan, in advance of the JSTP’s 29^th Annual Meeting. The goal of this Symposium was to present current diagnostic pathology or nomenclature issues to the toxicologic pathology community. This article presents summaries of the speakers’ presentations, including diagnostic or nomenclature issues that were presented, select images that were used for audience voting or discussion, and the voting results. Some lesions and topics covered during the symposium include: treatment-related atypical hepatocellular foci of cellular alteration in B6C3F1 mice; purulent ventriculoencephalitis in a young BALB/c mouse; a subcutaneous malignant schwannoma in a RccHan:WIST rat; spontaneous nasal septum hyalinosis/eosinophilic substance in B6C3F1 mice; a rare pancreatic ductal cell adenoma in a young Lewis rat; eosinophilic crystalline pneumonia in a transgenic mouse model; hyaline glomerulopathy in two female ddY mice; treatment-related intrahepatic erythrocytes in B6C3F1 mice; treatment-related subendothelial hepatocytes in B6C3F1 mice; spontaneous thyroid follicular cell vacuolar degeneration in a cynomolgus monkey; congenital hepatic fibrosis in a 1-year-old cat; a spontaneous adenocarcinoma of the middle ear in a young Crl:CD(SD) rat; and finally a series of cases illustrating some differences between cholangiofibrosis and cholangiocarcinoma in Sprague Dawley and F344 rats.

Spontaneous vacuolar degeneration of the thyroid follicular epithelium in cynomolgus monkeys

Vacuolar degeneration of the thyroid follicular epithelium was observed in two untreated female cynomolgus monkeys assigned to control groups. In light microscopy, large vacuoles containing a homogenous substance occupied the basal region of the epithelium, and the nuclei had shifted toward the apical region. The vacuoles showed negative reactions to PAS and thyroglobulin. Electron microscopic observation revealed dilatation of the rough endoplasmic reticulum corresponding to the vacuoles. The plasma TSH, T3 and T4 levels determined for the samples kept frozen were within the normal ranges, suggesting that the thyroid function was kept intact.

Metabolic pathways of tetraidothyronine and triidothyronine production by thyroid gland: a review of articles

Tetraidothyronine (T4) and Triiodothyronine (T3) are the two vital hormones in human metabolism produced by thyroid gland. The major pathways in thyroid hormone biosynthesis begin with iodine metabolism which occurs in three sequential steps: active iodide transport into thyroid followed by iodide oxidation and subsequent iodination of tyrosyl residues of thyroglobulin (Tg) to produce idotyrosines monoidotyrosine (MIT) and diiodothyrosine (DIT) on Tg. Oxidized iodine and tyrosyle residues which are an aromatic amino acids are integral part of T4 and T3. The thyroid iodine deficiency of either dietary, thyroid malfunction, or disorder of hypothalamus and pituitary to produce enough Thyroid Stimulating Hormone (TSH), eventually lead to hypothyroidism with sever side effects. Iodine oxidation is the initial step for thyroid hormone synthesis within thyroid, is mediated by thyroperoxidase enzyme (TPO), which itself is activated by TSH required for production of MIT and DIT. T4 and T3 are subsequently are synthesized on Tg following MIT and DIT coupling reaction. Thyroid hormones eventually produced and released into circulation through Tg pinocytosis from follicular space and subsequent lysozomal function, a process again stimulated by TSH. The production of T4 and T3 are highly regulated externally by a negative feed-back interrelation between serum T4, T3 and TSH and internally by the elevated iodine within thyroid gland. It is believed the extra iodine concentration within thyroid gland control thyroid hormones synthesis by inhibition of the TPO and hydrogen peroxide (H2O2) formation which is also an essential factor of iodine oxidation, via a complex mechanism. In healthy subjects the entire procedures of T4 and T3 synthesis re-start again following a drop in serum T4 and T3 concentration. On conditions of thyroid disorders, which caused by the distruption of either of above mechanisms, thyroid hormone deficiency and related clinical manifestations eventually begin to show themselves.

Thyroglobulin gene is associated with premature ovarian failure

Variants of the thyroglobulin gene were significantly associated with premature ovarian failure in a Korean population. Three single nucleotide polymorphisms and one haplotype were found to be associated with a significant increase in the risk for premature ovarian failure.

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.

An outline of inherited disorders of the thyroid hormone generating system

To date, various genetic defects impairing the biosynthesis of thyroid hormone have been identified. These congenital heterogeneous disorders result from mutations of genes involved in many steps of thyroid hormone synthesis, storage, secretion, delivery, or utilization. In contrast to thyroid dyshormonogenesis, the elucidation of the underlying etiology of most cases of thyroid dysgenesis is much less understood. It is suggested that genetic factors might play a role in some cases of thyroid dysgenesis and the best candidate genes involved are those encoding transcription factors known to play a role in the embryonic development of the thyroid gland. Moreover, discordance for thyroid dysgenesis is the rule for monozygotic twins as recently reported and this may result from epigenetic phenomena, early somatic mutations, or postzygotic events. In the final part of this review the molecular defects involved in proteins that transport thyroid hormone in the circulation are described: thyroxine-binding globulin (TBG), transtiretin and albumin, that may be associated with altered thyroid function tests and other pathologic conditions such as amyloidotic polyneuropathy.

Missing secretory granules, dilated endoplasmic reticulum, and nuclear dislocation in the thyroid gland of rdw rats with hereditary dwarfism

Previous studies on the rdw rat have suggested that its dwarfism is caused primarily by dysfunction of the thyroid gland. In this study, rat thyroid glands were analyzed endocrinologically and morphologically to clarify the primary cause of dwarfism in the rdw rat. The rdw rat showed lowered thyroid hormone (T4 and T3) levels but elevated TSH in serum. The rdw thyroid gland was almost proportional in size and it was not goiter in gross inspection. Our histological investigation produced three results that may lend important evidence in understanding the problem in the thyroid gland of rdw rats. First of all, secretory granules could not be detected in the follicular epithelial cells of the rdw. Secondly, thyroglobulin was found at very low levels in the follicular lumen by immunohistochemical analysis. In contrast, it could be detected in a substantial quantity inside the dilated rER and in the huge vacuoles that are formed by swelling of the rough endoplasmic reticulum (rER) at the basal side of the follicular epithelial cells. Additionally, the nucleus of the follicular epithelial cells was pressed to the luminal side by the enlarged rER. These morphological changes would indicate that the transport of thyroglobulin is stopped at or before the formation of the secretory granules and thyroglobulin is not secreted into the follicular lumen. The rdw characterization strongly supports that rdw dwarfism is induced by hypothyroidism due to some defect(s) in the thyroid gland.

Two novel cysteine substitutions (C1263R and C1995S) of thyroglobulin cause a defect in intracellular transport of thyroglobulin in patients with congenital goiter and the variant type of adenomatous goiter

We analyzed the thyroglobulin (Tg) gene of 2 unrelated patients with congenital goiter and the Tg gene of 2 siblings with the variant type of adenomatous goiter. The clinical characteristics of the patients with congenital goiter and the variant type of adenomatous goiter were very similar, except for serum Tg levels, which were less than 15 pmol/L in the patients with congenital goiter, but 117-181 pmol/L in the patients with the variant type of adenomatous goiter (normal, 15-50 pmol/L). The tissue content of Tg in the thyroid glands of all 4 patients was reduced at 0.9-3.8% of total protein (normal, 19-40%). The missense mutation C1263R was detected in the 2 unrelated patients with congenital goiter; the pedigree study showed an autosomal recessive pattern of inheritance. In the 2 siblings with the variant type of adenomatous goiter, the missense mutation C1995S was homozygously detected. In the Tg complementary DNA of 110 normal subjects, the allelic frequencies of the C1263R and C1995S mutations were each less than 0.5%. Also in the normal subjects were detected 35 nucleotide polymorphisms, the insertion of 3 nucleotides, and 1 alternative splicing, each of which was not associated with any specific thyroid disease. From these data, the molecular mechanism of the C1263R and C1995S mutations was elucidated. We first analyzed the carbohydrate residues of C1263R Tg and C1995S Tg. Sensitivity to treatment by endoglycosidase H suggests that C1263R Tg and C1995S Tg were retained in the endoplasmic reticulum (ER). Also, the presence of endoglycosidase H-resistant Tg as well as endoglycosidase H-sensitive Tg in the patients with the variant type of adenomatous goiter suggests that a fraction of C1995S Tg was transported to the Golgi and associated with the mildly increased serum Tg levels. Native PAGE and Western blot analysis with anti-Tg antibody showed that C1263R Tg and C1995S Tg form high mol wt aggregates in the ER. Our results suggest that missense mutations that replace cysteine with either arginine or serine cause an abnormal three-dimensional structure of Tg. Such misfolded Tg polypeptides are retained in the ER as high mol wt aggregates.

A single amino acid change in the acetylcholinesterase-like domain of thyroglobulin causes congenital goiter with hypothyroidism in the cog/cog mouse: a model of human endoplasmic reticulum storage diseases

Newly synthesized thyroglobulin (Tg), the major secretory glycoprotein of the thyroid gland, folds and homodimerizes in the endoplasmic reticulum (ER) before its export to the site of iodination, where it serves as the precursor for thyroid hormone synthesis. In families with defective Tg export, affected individuals suffer from a thyroidal ER storage disease characterized by a distended thyrocyte ER containing misfolded Tg, along with induced ER molecular chaperones. Inherited as an autosomal recessive trait, deficient Tg causes congenital hypothyroidism in newborns that, if untreated, results in goiter along with serious cognitive and growth defects. Recently, a similar phenotype has been observed in inbred cog/cog mice, although the precise molecular defect has remained undefined. Here, we have isolated and cloned a full-length 8.5-kb Tg cDNA from cog/cog mice and unaffected isogenic AKR/J mice. Comparison of the complete sequences reveals that cog/cog mice express a Leu-2263 --> Pro missense mutation in the acetylcholinesterase-homology domain of Tg. Heterologous expression studies in COS cells indicate that cog Tg exhibits a severe defect in exit from the ER. Site-directed mutagenesis of cog Tg to convert the single amino acid back to Leu-2263 restores normal Tg secretion. We conclude that the cog mutation in Tg is responsible for this ER storage disease that causes thyroid dyshormonogenesis.

Intracellular protein transport to the thyrocyte plasma membrane: potential implications for thyroid physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments-apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T4 deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.

Congenital hypothyroid goiter with deficient thyroglobulin. Identification of an endoplasmic reticulum storage disease with induction of molecular chaperones

Recent advances in understanding the molecular pathogenesis of congenital hypothyroid goiter in cog/cog mice, have raised important questions concerning the maturation of thyroglobulin (the thyroid prohormone) in certain human kindreds with congenital goiter. We have now examined affected siblings from two unrelated families that synthesize an apparently normally glycosylated, > 300 kD immunoreactive thyroglobulin, yet have a reduced quantity of intraglandular thyroglobulin and that secreted into the circulation. From thyroid tissues of the four patients, light microscopic approaches demonstrated presence of intracellular thyroglobulin despite its absence in thyroid follicle lumina, while electron microscopy indicated abnormal distention of the endoplasmic reticulum (ER). We have confirmed biochemically that most intrathyroidal thyroglobulin fails to reach the (Golgi) compartment where complex carbohydrate modification takes place. Moreover, the disease in the affected patients is associated with massive induction of specific ER molecular chaperones including the hsp90 homolog, GRP94, and the hsp70 homolog, BiP. The data suggest that these patients synthesize a mutant thyroglobulin which is defective for folding/assembly, leading to a markedly reduced ability to export the protein from the ER. Thus, these kindreds suffer from a thyroid ER storage disease, a cell biological defect phenotypically indistinguishable from that found in cog/cog mice.

An endoplasmic reticulum storage disease causing congenital goiter with hypothyroidism

In humans, deficient thyroglobulin (Tg, the thyroid prohormone) is an important cause of congenital hypothyroid goiter; further, homozygous mice expressing two cog/cog alleles (linked to the Tg locus) exhibit the same phenotype. Tg mutations might affect multiple different steps in thyroid hormone synthesis; however, the microscopic and biochemical phenotype tends to involve enlargement of the thyroid ER and accumulation of protein bands of M(r) < 100. To explore further the cell biology of this autosomal recessive illness, we have examined the folding and intracellular transport of newly synthesized Tg in cog/cog thyroid tissue. We find that mutant mice synthesize a full-length Tg, which appears to undergo normal N-linked glycosylation and glucose trimming. Nevertheless, in the mutant, Tg is deficient in the folding that leads to homodimerization, and there is a deficiency in the quantity of intracellular Tg transported to the distal portion of the secretory pathway. Indeed, we find that the underlying disorder in cog/cog mice is a thyroid ER storage disease, in which a temperature-sensitive Tg folding defect, in conjunction with normal ER quality control mechanisms, leads to defective Tg export. In relation to quality control, we find that the physiological response in this illness includes the specific induction of five molecular chaperones in the thyroid ER. Based on the pattern of chaperone binding, different potential roles for individual chaperones are suggested in glycoprotein folding, retention, and degradation in this ER storage disease.