Studies on the congenitally goitrous sheep. Iodoproteins of the goitre

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.

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.

Clinical and morphologic findings of familial goiter in bongo antelope (Tragelaphus eurycerus)

Inherited defects of thyroglobulin synthesis resulting in congenital goiter are well described in certain breeds of domestic ungulates and in human beings. Goiter associated with synthesis of an abnormal thyroglobulin and the presence of thyroidal albumin was identified in five closely related bongo antelopes (Tragelaphus eurycerus). The goiter had an adult onset, and the affected bongos appeared to remain euthyroid with normal serum T3 and T4 values, normal serum cholesterol concentrations, and nonelevated concentrations of circulating thyroid stimulating hormone (TSH). Goitrous bongos had significant reproductive difficulties, including reduced cyclic activity and prolonged gestations, but were otherwise normal. Over the course of the disease, the thyroid glands greatly enlarged (up to 10 x 20 cm) and became polycystic. Microscopically, there was an admixture of giant colloid-filled follicles and follicles of normal size lined with variable follicular epithelium ranging from squamoid to mildly to moderately hyperplastic. The pathogenesis of goiter in the bongo may reflect a mixture of genetic predisposition coupled with environmental factors, including a period of exposure to a goitrogen.

Evidence for and possible mechanisms of non-genotoxic carcinogenesis in the rodent thyroid

Thyroid tumours are a common finding in toxicity tests in rodents. It is known that prolonged administration of antithyroid drugs leads to the development of multiple thyroid tumours, and the role of genotoxic and non-genotoxic mechanisms in this needs definition. The role of drugs with an antithyroid action in thyroid carcinogenesis requires a knowledge of thyroid physiology. This review briefly discusses the anatomy and physiology of the thyroid before concentrating on the cellular pathology of the changes that take place in the transition from a normal to a neoplastic thyroid cell. The malignant cell is characterised by excess growth and invasiveness. The normal thyroid cell does not possess an unlimited growth potential because of a growth-desensitising mechanism (GDM) of the antioncogene type. Spontaneous thyroid carcinogenesis requires three key steps which are presumed to arise by mutation and clonal selection: the loss of the GDM, the acquisition of TSH-independent growth, and the acquisition of invasiveness. The sequence of the cell biological changes involved is not fully understood, but it has been shown that IGF-1 is a necessary co-factor for the growth-stimulating effect of TSH in the normal cell, and that autocrine production of IGF-1 is a feature of spontaneous thyroid adenomas. Another early change that has been shown in both experimental and human thyroid tumours is mutation of one of the ras oncogenes. In carcinogenesis due to the prolonged administration of an agent known to interfere with thyroid hormone metabolism and to induce a high TSH, two rather than three key steps will be required for carcinogenesis, as the development of TSH independent growth will not confer any selective advantage. We have shown that monoclonal lesions induced in this way regress when the goitrogen is withdrawn and therefore retain TSH dependence. The development of the other two key changes--the loss of the GDM and the acquisition of invasiveness--may be due to genotoxic or non-genotoxic mechanisms. They can occur in man in the absence of any known mutagenic agent. In patients with dyshormonogenesis a congenital defect in one of the steps of thyroid hormone synthesis is associated with multiple tumour production. It is reassuring that in these patients, exposed to decades of high TSH levels, benign lesions are common, but malignant thyroid tumours are very rare. The occurrence of thyroid tumours following the use of substances known to interfere with thyroid hormone metabolism does not itself exclude a genotoxic component to the carcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Afrikander cattle congenital goiter: size heterogeneity in thyroglobulin mRNA

The mRNA coding for thyroglobulin in cattle homozygous for an autosomal recessive defect of thyroglobulin synthesis was investigated using a recombinant plasmid containing bovine mRNA coding sequence. Total RNA preparations from goiter contained one third of the thyroglobulin mRNA sequences found in normal thyroid tissue. This mRNA was not translated into thyroglobulin by Xenopus oocytes. Northern transfer analysis revealed both a normal sized and a smaller thyroglobulin mRNA in the goiter.

Dog thyroid glands after chronic administration of antithyroid drugs

The effect of chronic administration of antithyroid drugs on the composition of thyroid proteins in dogs was investigated. After administration of thiourea for periods of 10 days to 4 months and also of ethylenethiourea for 6 months thyroid glands were examined morphologically by immunoperoxidase staining using anti-19S-thyroglobulin and anti-albumin antisera. After follicles were devoid of apparent colloid, the immunoreactivity of follicular cells to the 19S antiserum progressively increased with administration periods. A large number of follicular cells were also densely stained with the albumin antiserum. In contrast to the immunoreaction of 19S-thyroglobulin, which was detected in all follicular cells, the appearance of immunoreactive albumin was restricted to 60-80% of the follicular cell population even after several months' administration. Thyroid extracts after treatment with ethylenethiourea for 6 months were analyzed with gel chromatography, polyacrylamide gradient gel electrophoresis, and immunoblotting. Large-molecular-weight components, including 27S iodoprotein, were completely absent in the goitrous thyroids. On the contrary, small-molecular-weight components markedly increased in number and amount, although of these two or three bands alone were weakly immunoreactive to the 19S antiserum. Both 19S and 12S components were still significant products, showing an intense immunoreactivity for 19S-thyroglobulin after the chronic administration of goitrogen. Albumin markedly increased in amount and constituted 24.2% +/- 7.6% of total stained proteins, seven times as high as the value of normal controls (3.4% +/- 2.3%).

Hereditary congenital goitre with thyroglobulin deficiency causing hypothyroidism

The thyroid glands of two hypothyroid goitrous siblings aged 13 and 14 and of a 21-year-old hypothyroid goitrous female were examined. In all three patients a very high thyroid uptake of iodide was observed in the presence of a negative perchlorate discharge test. An abnormally high serum protein bound iodine (12.9-20.0 micrograms/dl) and low serum T4 concentration suggested the presence of increased serum levels of iodoalbumin. Surprisingly, serum T3 levels were normal or low normal (80-220 ng/dl) in several determinations. Basal serum TSH was elevated and an exaggerated TSH response was observed after TRH. Serum thyroglobulin was undetectable in one patient, low normal in another and in the normal range for the third one. Except for the patient with undetectable Tg the two other subjects slightly increased the serum Tg levels after a bovine TSH injection. Plasma chromatography after a tracer dose of 125I disclosed only minute amounts of T3 + T4 and MIT + DIT. Studies performed in the homogenized thyroid tissues indicated that these goitrous glands had pronounced decrease of immunoreactive thyroglobulin. The total amount of Tg-like proteins (RIA) in the thyroid soluble protein extract was only 16-122 micrograms/g (normal: 50-70 mg/g of tissue). Ultracentrifugal studies were unable to demonstrate the presence of mature (18-20S) thyroglobulin. Only one peak (3.6-4.1S) was obtained in the pooled soluble proteins supernatants. Hydrolysis of the homogenates indicated, by subsequent column chromatography, very low relative concentrations of iodotyrosines and iodothyronines and that a relatively large amount of iodide remained associated with subcellular proteins and undigested. The predominant histological pattern was of the intermediary differentiated adenoma type, microfollicular or fetal, with several atypical features and capsular invasion which may suggest malignant change. We conclude that a defective Tg export from the cell to the lumen or an anomaly in the structural gene leading to inadequate translation of Tg mRNA finally results in deficient storage of normal, mature Tg in the colloid with subsequent goitrous hypothyroidism.

Familial goitre with partial iodine organification defect, lack of thyroglobulin, and high levels of thyroid peroxidase

From a sibship of three sisters having congenital goitre and normal hearing, two had impairment of organification of iodine. S1 (4 years old) had goitre since birth, euthyroidism, and a negative perchlorate test. S2 (15 years old) and S3 (13 years old) were hypothyroid, and had radioiodide discharge after potassium perchlorate administration of 19.8% and 26.1%, respectively. Thyroid tissue was obtained at thyroidectomy. Peroxidase activity, in the thyroidal subcellular particles, was found to be qualitatively normal, but quantitatively increased. In the triiodide assay, the activity was: S1 6912 u, S2 2590 u, and S3 3844 u (normal values 900-1700 u). In the tyrosine-iodinase assay, the activities, expressed as nmoles of iodide incorporation per gram of tissue, were S1 1046, S2 471 (normal values 220-410). The activity of the thyroidal NADPH-cytochrome c reductase, an enzyme possibly involved in hydrogen peroxide generation, was: S1 0.084, S2 0.047, and S3 0.005 (normal values 0.018 muEq/min/mg). No thyroglobulin was detected by analytical ultracentrifugation, polyacrylamide gel electrophoresis, or double immunodiffusion in agar of the supernatant fractions. In patient S2, whose gland was labelled in vivo with 125I, 60% of the total radioactivity of the gland (pooled nodular and paranodular specimens) was in a particulate iodoprotein that was solublilized by trypsin, deoxycholate or digitonin. In the soluble fraction there were two iodoproteins: iodalbumin, and a second iodoprotein similar to the solubilized particulate iodoprotein. It is postulated that absence of the normal thyroidal receptor protein might be in some cases a cause of iodine organification defect.

Studies on mono- and diiodohistidine. II. Congenital goitrous hypothyroidism with thyroglobulin defect and iodohistidine-rich iodoalbumin production

Butanol-insoluble iodinated compounds in the urine of patients with congenital goiters have been generally regarded as iodopeptides. Monoiodohistidine (MIH) and diiodohistidine (DIH) were identified from the urine of four patients with congenital goitrous hypothyroidism. From radioiodine studies, 40-70% of the urinary radioactivity was in the iodide-free fraction from which about 40% was identified as MIH and DIH by crystallizations to a constant specific activity. Iodotyrosines were simultaneously identified in the urine. However the presence of an iodotyrosine-deiodinase activity was demonstrated in the two removed goiters with a normal K(m) for MIT. In vivo iodotyrosine deiodination was normal for hypothyroid subjects. No thyroglobulin was identified in the thyroids from these patients. The major iodoprotein was iodoalbumin which, after in vivo labeling, contained 84-89% of the total soluble protein radioactivity. The thyroxine content of the goiter iodoalbumins and other iodoproteins was extremely low. Iodohistidines were identified in comparable proportions in the iodoalbumin and in the other iodoproteins isolated from each goiter. The average iodohistidine content of these proteins as crystallizable MIH and DIH was in the individual cases 15 and 4% of the in vivo incorporated radioiodine. DIH was identified in all iodoprotein fractions. The mean DIH/MIH ratios from the individual cases were 1.16 and 0.35. The corresponding DIT/MIT ratios were 3.19 and 1.45, respectively. The major consequence of this thyroglobulin defect is the iodination of inappropriate proteins (mainly albumin) resulting in low yields of thyroxine and high yields of iodohistidines. Iodohistidines from the goiter iodoproteins were not deiodinated and, at least for MIH, were quantitatively excreted in the urine of these patients. From the MIH iodoalbumin content and the MIH urinary excretion, goiter iodoalbumin turnover estimates were made and, although elevated, could not maintain a normal thyroxine secretion. The urinary excretion of iodohistidines easily demonstrated by column chromatography is offered as a test for detecting this variety of congenital goiter.