Defective thyroglobulin synthesis and secretion causing goiter and hypothyroidism.

The integrity of the Tg structure as a protein is essential for adequate synthesis of thyroid hormone. Also a large supply of iodine and of thyroid hormone is stored into the Tg molecule and available for secretion on demand. Mutations in Tg gene or hyposialylated Tg due to a defective sialyltransferase activity would cause a structurally defective protein and severely impair the functional ability of Tg. In this review we attempt to cover the abnormalities in the synthesis of Tg described in both animals and man. Hereditary congenital goiter with or without hypothyroidism is the phenotypic major clinical finding in these species. Affected animals include sheep, bovine cattle, bongo antelope, goats, and mice. As in man the inheritance mode is autosomal recessive. In most animal studies structurally abnormal Tg is present. The molecular basis for the defective Tg synthesis was attributable to nonsense mutation in exon 9 (Afrikander cattle) and in exon 8 (Dutch goats). In man the Tg defective synthesis has been reported in 89 subjects and frequently more than one sibling is affected in a given generation. Characteristically these patients exhibit hereditary congenital goiter with relatively low Tg levels that do not increase after stimulation with bovine TSH. High PBI concentrations with low serum T4 values indicate the serum presence of iodinated proteins (mainly iodoalbumin). Also iodinated peptides are frequently excreted into the urine. Tissue studies confirm that there is an absent Tg peak at gel filtration, and virtually no immunoassayable Tg is present in the tissue extracts. The molecular basis of these defects have been recently reported in a patient and includes low tissue Tg mRNA probably due to premature degradation of a defective Tg mRNA. The responsible mutation is a cytosine to thymine transition creating a stop codon at position 1510. The point mutation is removed by the preferential accumulation of a 171 nt deleted Tg mRNA. In another subject molecular studies revealed that exon 4 was missing from the major Tg transcript due to a cytosine to guanine transversion at position minus 3 in the acceptor splice site of intron 3. It is anticipated that other mutations responsible for these defects will be identifiable in the near future.

Inherited disorders of thyroid metabolism.

In summary, we have presented a brief survey of the current state of knowledge of inherited disorders of thyroid metabolism. Analysis of cases shows that the biochemical classification covers a wide range of abnormalities and it is likely that further biochemical studies will increase this heterogeneity as well as refining it. Genetic studies are often incomplete, and few in number compared with the classical study by Hutchison and McGirr of Scottish tinker families. Most important, this survey indicates that further research is needed to elucidate the precise molecular mechanisms of the working of the iodide pump, the oxidation and iodination and coupling mechanisms. Study of animal models and DNA sequencing and hybridization work will continue to expand our understanding of abnormalities of thyroglobulin metabolism. We urgently need to find the key to resistance of peripheral and pituitary tissues to thyroid hormone. Subtle dyshormonogenetic abnormalities may await discovery in the field of multinodular goiter and intrathyroidal calcification with goiter. Neonatal screening for hypothyroidism is likely to expand the number of cases for investigation and detailed study. There is an important relationship of dyshormonogenesis to follicular carcinoma. It is hoped that in time we will be able to transform inborn errors into areas of understanding in the realm of the thyroid gland.

Oral ingestion of a hydrolyzed gelatin meal in subjects with normal weight and in obese patients: Postprandial effect on circulating gut peptides, glucose and insulin.

Gut hormones [ghrelin, peptide YY (PYY) and glucagon-like peptide-1 (GLP-1)] are an important group of hormones that target appetite control. They are released from endocrine L cells of the small bowel in proportion to the volume, components and calories in a meal. In the current study, 20 g of gelatin (flavored and sweetened) were given to obese patients (n=12) and lean subjects (n=10). Subsequently, plasma samples were collected at-30- minute intervals up to 180 minutes and glucose, insulin, PYY, GLP-1 and ghrelin were assayed using specific and sensitive immunofluorometric and radioimmunoassays. As expected, obese patients had normal serum glucose levels, higher serum insulin, and lower plasma concentration of ghrelin at all times compared to lean subjects. GLP-1 plasma levels were significantly elevated at 60 minutes, peaking at 120 minutes in obese patients and lean subjects. As a consequence, there was a significant rise in serum insulin levels with a significantly higher peak level at 60 min (obese) and 30 min (lean). There were no significant changes in PYY plasma concentrations and no correlation was found between body mass index and concentrations of ghrelin, PYY and GLP-1 in the group of obese patients. In conclusion, a single gelatin meal induces a rise in plasma GLP-1 followed by an increase in serum levels of insulin. These findings may be applied to maximize satiety in obese patients as a means of improving adherence to calorie-controlled diets as well as provide better control of diabetic patients.

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.

A circulating, biologically inactive thyrotropin caused by a mutation in the beta subunit gene.

Mutation of a critical carboxy-terminal cysteine residue (C105V) in the thyrotropin-beta (TSH-beta) subunit gene was found in two related families with central hypothyroidism. Affected patients had low thyroid hormone levels and radioactive iodine uptake in the thyroid gland associated with measurable serum TSH. Thyrotropin-releasing hormone-stimulated TSH secretion did not increase thyroid hormone production in these patients as compared to their unaffected siblings, suggesting that the mutant TSH was biologically inactive in vivo. Recombinant TSH harboring this mutation was confirmed to be biologically inactive in an in vitro bioassay. Based on crystallographic structure of chorionic gonadotropin, a disulfide bond between C19 and C105 in the TSH-beta subunit is predicted to form the "buckle" of a "seat belt" that surrounds the common alpha subunit and maintains the conformation and bioactivity of the hormone. This natural mutation of the TSH-beta subunit confirms the importance of the seat belt in the family of pituitary and placental glycoprotein hormones.

Peripheral blood levels of thyroglobulin mRNA and serum thyroglobulin concentrations after radioiodine ablation of multinodular goiter with or without pre-treatment with recombinant human thyrotropin.

We investigated the effect of therapeutic doses of radioiodine (RAI) on peripheral serum messenger thyroglobulin RNA (Tg mRNA) and serum thyroglobulin (sTg) in patients with multinodular goiter (MNG) preceded or not by treatment with recombinant human TSH (rhTSH). Fourteen patients with large MNG (91-542 ml) received RAI (550-2960 MBq). Half of the patients received 0.45 mg of rhTSH prior to the treatment (RAI+rhTSH group) and half did not (RAI group). Patients' blood samples were collected before and 24, 48, and 72 h; 7 and 30 days; and 6, 9, and 12 months after RAI treatment. Serum Tg was measured by immunoradiometric assay, serum anti-Tg by radioimmunoassay, and quantification of circulating Tg mRNA was performed by real-time PCR. The shrinkage of MNG volume was documented by serial computed tomography (CT) scans before, 6 and 12 months after RAI. Peak Tg mRNA and sTg were reached earlier in the RAI+rhTSH group (24 h and 48 h) than in the RAI group (7 days). Both declined after the peak and the lowest levels were observed at 12 months. The mean reduction of the thyroid volume was 19.8% (RAI group) and 30.3% (RAI+rhTSH group) at 6 months (ns) and 32.8% RAI and 52.5% (RAI+rhTSH group) at 12 months (p<0.05). After RAI treatment there was a significant and positive correlation between goiter volume and sTg only in the RAI group (r=0.7; p=0.032). Serum anti-Tg had a transitory and relatively small elevation in 3 and 2 patients, respectively, in the RAI and RAI+rhTSH groups. We concluded that after RAI ablation of MNG there is a rapid release of Tg into the serum possibly from the colloid, which is followed by an elevation of serum Tg mRNA that may be due to an increased release of follicular cells into the blood stream. Both phenomena are enhanced by the use of rhTSH before RAI treatment as a consequence of a more effective and prolonged radiation exposure of the thyroid follicles.

Familial Congenital Hypothyroidism Caused by Abnormal and Bioinactive TSH due to Mutations in the beta-Subunit Gene.

Hereditary TSH deficiency is a rare autosomal recessive disease described in inbred Japanese families and in Greek and Brazilian kindreds. The TSH-beta-subunit gene has been shown to be the site of mutations that will give rise to truncated proteins that cannot dimerize with the alpha subunit or, alternatively, will produce a mutated TSH that is present in the circulation of the affected patients, but it is biologically inactive. Characteristically, the patients with TSH-beta-subunit-defects are born with congenital hypothyroidism, with very low levels of serum thyroid hormones and serum thyroglobulin and, paradoxically, with serum TSH levels that are consistently undetectable or at very low levels. Goiter is not present at birth, but the low radioactive thyroid uptake will increase after bovine TSH stimulation. Other pituitary hormones responses to provocative tests are normal. The subunit levels are at high concentration and are significantly increased following TRH stimulation. In two kindreds, molecular biological studies have indicated mutations in two different sites of exon 2, generating a peptide that would not dimerize with subunits to synthesize TSH molecules. In one kindred, a truncated TSH-beta protein was translated that generated a biologically inactive but detectable serum TSH molecule. (c) 1997, Elsevier Science Inc. (Trends Endocrinol Metab 1997;8:15-20).

Immunosuppression of thyroiditis.

Immunization of mice with 50 micrograms human thyroglobulin (TG) in complete Freund's adjuvant leads to histological thyroiditis; production of IgG, IgA, and IgM anti-TG antibodies; and in vitro proliferative responses after incubation of lymphocytes with TG. Oral administration of 500 micrograms TG at four intervals before Tg immunization and once afterward causes up to 80% suppression of these responses. The effect is antigen specific and dose dependent. Feeding TG after immunization produces a 40% reduction in responses. We wished to define the mechanism of this antigen-specific oral tolerization. Popliteal lymph nodes (PLN) of orally tolerized animals (T) are reduced in size compared to those in immunized (I) animals not fed TG. PLN and mesenteric lymph nodes (MLN) of I animals produce interleukin-2 (IL-2) and interferon-gamma (IFN gamma) after in vitro incubation with TG, typical of an inflammatory immune response. PLN and MLN of tolerized animals do not proliferate in response to antigen, do not produce IL-2 or IFN gamma, but do not produce the cytokines IL-4 and transforming growth factor-beta (TGF beta). Mixing in vitro of spleen cells from T and I animals causes a reduction in the immune response when incubated with TG, but no reduction in response to purified protein derivative (PPD) (the antigen in complete Freund's adjuvant). When T splenocytes are incubated with TG and PPD together, the response to TG and PPD is suppressed. Partially purified CD8+ cells from tolerized animals produce IL-4 and TGF beta after exposure to human TG and induce suppression, whereas partially purified CD4+ cells produce IL-2 and IFN gamma and do not cause suppression. MLN cells do not proliferate in response to antigen, but do produce inhibitory cytokines. T animals appear to shift the immune response from a Th-1 helper cell subset response to a Th-2 helper cell immunosuppressive response. In this model, oral tolerization produces a dramatic reduction in the immune response. Exposure of MLN to oral TG appears to cause the production of regulatory cells that migrate to spleen and PLN. In vitro studies demonstrate that on exposure to antigen, these regulatory cells produce IL-4 and TGF beta, which suppress all aspects of specific immune responsiveness and nonspecifically suppress other ongoing immune responses (bystander effect). Oral tolerization may include some element of T cell deletion or anergy. This model defines an experimental system with possible relevance to immunosuppression of human autoimmune thyroid disease.

Thyroid growth immunoglobulins in large multinodular endemic goiters: effect of iodized oil.

Iodized oil (IO) was administered to 10 goitrous patients recently emigrated to São Paulo (SP) from iodine deficiency areas and to 42 goitrous patients from 2 Brazilian chronic iodine deficiency regions, Loreto and Luziania (L). Thyroid growth-promoting immunoglobulin G (IgG) thyroid-stimulating antibody, serum thyroglobulin (Tg), TSH, and thyroid hormones were measured before and 1 yr after IO administration. In all patients there was a remarkable reduction of gland mass associated with a significant decrease (P less than 0.01) in both basal serum Tg and peak Tg levels after bovine TSH administration. The mean percent Tg increase after bovine TSH treatment was reduced to 82% above basal levels compared with 224% before IO. Mean serum TSH levels, elevated only in the L group [7.3 +/- 11 (+/- SD) microU/ml] decreased to the normal range after IO (2.5 +/- 2.1 microU/ml). Serum T3 and T4 concentrations did not change greatly. Tests for microsomal antibodies were negative before and after IO. IgG concentrates of serum obtained before and after IO were tested for their ability to stimulate incorporation of [3H]thymidine into DNA or to increase intracellular generation of cAMP in FRTL-5 cells. Thymidine incorporation activity was found in 8 of 10 patients from SP [316 +/- 37% (+/- SEM); range, 140-480%] and 25 of 42 patients in the L group (mean, 206 +/- 14; range, 120-500%) before IO. Stimulation of thymidine incorporation reflected true growth-promoting activity, as confirmed by experiments measuring cell number, was not accounted for by TSH in the preparation, and reflected IgG action because it was abolished by absorption with antihuman IgG. IgG from only 1 patient in group SP and 4 patients in group L stimulated intracellular production of cAMP in FRTL-5 cells. All patients except 1 in both groups had no IgG stimulation (less than 120%) of growth-promoting activity 1 yr after IO treatment. There was a significant positive correlation between thyroid growth-promoting activity and serum Tg concentrations (r = 0.58; P less than 0.001), but no significant correlation was found with other parameters (TSH, T4, and T3). We conclude that growth-promoting IgGs lacking ability to stimulate cAMP production may play a role in the large multinodular goiters due to chronic iodine deficiency.

A nonsense mutation causes human hereditary congenital goiter with preferential production of a 171-nucleotide-deleted thyroglobulin ribonucleic acid messenger.

Defective or impaired thyroglobulin (Tg) synthesis usually results in congenital goitrous hypothyroidism, virtual absence of Tg in thyroid tissue, and the presence of an elevated concentration of iodoalbumin. The final result of these abnormalities is a decreased rate of T3 and T4 synthesis. We have previously reported two siblings with this syndrome that was attributable to decreased levels of thyroid tissue Tg mRNA, resulting in decreased translation of a fully mature Tg. Further molecular studies in this family are the subject of this report. The Tg mRNA from normal and goitrous thyroid tissue was first reverse transcribed and divided into five overlapping portions from positions 57-8448, and the resulting cDNAs were amplified by polymerase chain reaction and analyzed by agarose gel electrophoresis. The amplification of nucleotides (nt) 4502-5184 from control thyroid tissue Tg mRNA showed a predominant fragment of 683 basepairs (bp) and a minor fragment of 512 bp. This latter fragment contained a 171-nt deletion that mapped between positions 4567 and 4737 of the Tg mRNA. In contrast, the fragment predominantly present in the congenital goiter was 512 bp. The sequencing of the 683-bp fragment revealed that the responsible mutation is a cytosine to thymine transition, creating a stop codon at position 1510. This results in loss of a TaqI restriction site. The point mutation is, thus, removed from a portion of the transcripts by the preferential accumulation in the goiter of a 171-nt-deleted Tg mRNA. The reading frame is maintained and is potentially fully translatable into a Tg polypeptide chain shorter by 57 residues. The presence of the deleted Tg mRNA in normal thyroid tissue, albeit at a low level, strongly suggests that the deleted mRNA sequence corresponds to a complete exon. Our studies suggest that the shorter, alternatively spliced Tg mRNA predominates in the goitrous tissue and probably has a shorter half-life. This would explain the tissue's low Tg mRNA levels, previously reported. Moreover, translation of the mutated transcript would generate a severely truncated Tg polypeptide with limited ability to generate thyroid hormone, resulting in congenital goitrous hypothyroidism.

Hyposialylated thyroglobulin in a patient with congenital goiter and hypothyroidism.

A large family (14 children) with congenital goiter whose parents are first cousins was studied. Thyroid tissue was obtained, after 125I in vivo labeling, from one of the siblings (JBM). Gel filtration of thyroid proteins indicated that thyroglobulin (Tg) eluted as a single symmetrical peak in the same position as authentic 19S Tg. Gel electrophoresis in a 7.5% sodium dodecyl sulfate-polyacrylamide gel revealed a major band with the same mobility and immunoreactivity as normal 19S Tg. Hydrolysis of the patient's Tg indicated that most of the radioactivity was mono- and diiodotyrosines. The yield of T4 from JBM Tg (26 pmol/mg protein) was 5-fold less than normal thyroid tissue (140 pmol/mg protein) and approximately half of that in thyroid tissue from endemic goiter (51 pmol/mg). Total T3 released from JBM Tg was similar to the other two tissues. When the carbohydrate content of normal and patient Tg was analyzed, there was no differences in glucosamine, galactose or mannose content. However, unlike normal and endemic-goiter Tg, that had a mean sialic acid content of 7.3 and 5.6 micrograms/mg protein, respectively, the sialic acid concentration of the patients Tg was only 0.3 microgram/mg. Sialyltransferase activity was readily demonstrated in homogenate from normal thyroid or endemic goiter, but no sialyltransferase activity was detectable in a homogenate of JBM-thyroid tissue. We conclude that the finding of severely hyposialylated Tg is linked to a defect in iodotyrosine coupling seen in this patient with a possibly abnormal migration of Tg into the follicular lumen.

Structural studies of the thyrotropin receptor and Gs alpha in human thyroid cancers: low prevalence of mutations predicts infrequent involvement in malignant transformation.

The genes for either the TSH receptor (TSH-R) or the stimulatory guanine nucleotide-binding protein subunit (Gs alpha) can undergo somatic mutations in thyroid cells, leading to constitutive activation of adenylyl cyclase and the formation of clonal hyperfunctioning thyroid adenomas. Autonomously functioning thyroid adenomas are thought not to be common precursors of thyroid cancer. If this is the case, mutations of the TSH-R or Gs alpha would not be expected to be highly prevalent in thyroid carcinomas. In this paper we report the results of a screen for structural defects in exon 10 of the TSH-R (which includes the whole serpentine structure, but not the extracellular domain) and of Gs alpha in 30 thyroid carcinomas. Five of these were from patients with functioning metastasis, as we hypothesized that if mutations of these genes were to play a role in the progression to malignancy, they would be more likely to manifest in thyroid cancers that retain unusual differentiated function (i.e. capable of synthesizing enough thyroid hormone to render patients euthyroid or hyperthyroid after total thyroidectomy). None of the 30 tumors had activating point mutations of Gs alpha. Only 2 of 30 had somatic mutations of the TSH-R (codon 632: ACC to GCC, Thr to Ala; and ACC to ATC, Thr to Ile, respectively), the latter in a patient with a thyroid hormone-producting follicular carcinoma. These results suggest that events leading to constitutive activation of the adenylate cyclase signal transduction cascade are not a frequent event in the progression toward differentiated thyroid carcinomas.

Triac (3,5,3'-triiodothyroacetic acid) partially inhibits the thyrotropin response to synthetic thyrotropin-releasing hormone in normal and thyroidectomized hypothyroid patients.

The effects of a daily oral dose (1.4 mg) of 3,5,3'-Triiodothyroacetic acid (Triac) on thyroid hormone levels (T4, T3 and rT3) and on the TSH and PRL responses to TRH were studied in 15 normal subjects and 5 hypothyroid patients. There were no significant changes in weight, heart rate, reflex time, or serum concentration of either cholesterol or triglycerides after 6 weeks of Triac administration. However, T4 was significantly reduced to a lower mean level (mean +/- SEM, 7.3 +/- 0.7 to 4.3 +/- 0.6 microgram/dl) in the control group. T3 and rT3 concentrations increased, possibly due to a cross-reaction with Triac in their respective RIAs. The peak TSH response to TRH in the normal subjects was 17.6 +/- 3.4 muU/ml and fell significantly to 2.0 +/- 0.8 muU/ml after Triac administration. In the hypothyroid subjects the mean serum TSH level was significantly reduced from 136 +/- 66 to 12.6, 10.5, and 11.6 muU/ml in the weeks after Triac administration. The mean peak response of both TSH and PRL after TRH (206 muU and 44.8 ng/ml, respectively) declined significantly to 63.4 muU/ml and 24 ng/ml. It was concluded that this dose of Triac partially inhibits the synthesis and secretion of TSH and PRL without any major peripheral metabolic effects.

The effect of iodized oil on the TSH response to TRH in endemic goiter patients.

The TSH and T3 response to synthetic TRH was evaluated in 4 groups of patients: normal controls and goitrous subjects from the urban area of Sao Paulo (urinary iodine excretion: 172.2 +/- 48.3 mug I/g creatinine) and nongoitrous and goitrous subjects from the endemic areas of Sao Bento (urinary iodine excretion: 53.8 +/- 17.1 mug I/g). Plasma T4 and T3 were within our normal range in all groups of patients. The mean plasma TSH was significantly higher (5.2 +/- 3.3 muU/ml) in goitrous subjects living in Sao Bento as compared to normal control groups both in urban or endemic areas, and after TRH these patients had an exaggerated and sustained TSH response with a significantly higher peak level (21.1 +/- 7.9 muU/ml). T3 concentration rose in all subjects following TRH and all patients from the Sao Bento endemic areas had a significantly higher proportionate increase in plasma T3 at 120 min. After an injection of iodized oil basal plasma TSH returned to the normal range in the goitrous subjects from Sao Bento. The mean peak TSH response to TRH was 9.1 +/- 3.8 muU/ml at 3 months after the iodized oil injection, and only at 6 months after the iodized oil TSH response was significantly reduced (peak level: 6.1 +/- 2.4 muU/ml). It is confirmed that plasma TSH levels are increased in endemic goitrous patients but not in normal controls living in the same endemic area and it is suggested that the pituitary threshold for inhibition of secretion of TSH by T4 and T3 has been reset in these goitrous subjects to achieve a persistently higher secretion rate of TSH.

Thyroid function and chronic hepatosplenic schistosomiasis: peripheral conversion of thyroxine to 3,5,3'-triiodothyronine and pituitary-thyroid relationship.

The relationship between chronic hepatosplenic schistosomiaisis (CHES) and circulating thyroid hormones as well as the TSH response to TRH were investigated in 41 hospitalized CHES patients and compared to those in 11 patients with non-CHES cirrhosis with severe hepatic failure. CHES patients were subdivided into 3 groups depending on the severity of parenchymal dysfunction, based upon a composite clinical and laboratory index. Angiographic and hemodynamic studies of CHES patients revealed altered hepatic arteriograms, suggesting a decreased arterial blood flow associated with an increased venous blood flow from the portal system. A significantly reduced serum concentration of total T4 (but not free T4) was only found in the cirrhotic patients. Compared to CHES groups I and II, CHES group III patients and the non-CHES cirrhotics had significantly lower mean serum T3 levels of 80 +/- 12 and 52 +/- 8 ng/dl, respectively. The serum rT3 concentration was elevated (69 +/- 6.2 ng/dl) only in the cirrhotic patients. Both basal and peak TSH levels after TRH were within the normal range for all 4 groups of patients. The basal (40.7 +/- 8.3 ng/ml) and peak (85.5 +/- 13.7 ng/ml) serum PRL levels T4-binding globulin after TRH administration were only elevated in the cirrhotic group. Although the mean T4-binding globulin values were lower in CHES group III (17.5 +/- 3.2 micrograms/ml) and in the non-CHES cirrhotic group (18.3 +/- 2.1 micrograms/ml) compared to those in groups I (21.8 +/- 2.2 micrograms/ml) and II (20.4 +/- 2.3 micrograms/ml), the differences between groups were not statistically significant. It was concluded that hemodynamic changes without parenchymal failure have little, if any, effect on the hepatic T4 5'-monodeiodination to T3, and that the low T3 and high rT3 state does not modify the pituitary secretion of TSH, presumably by a local (at the thyrotroph level) normal conversion of T4 to T3, even at very low peripheral T3 concentrations.

A novel mutation (Q40P) in PAX8 associated with congenital hypothyroidism and thyroid hypoplasia: evidence for phenotypic variability in mother and child.

Congenital hypothyroidism associated with thyroid hypoplasia can be caused by several genetic defects, including mutations in the TSHbeta-subunit, the TSH receptor, the G(s)alpha-subunit, and the transcription factor PAX8. Four girls with sporadic congenital hypothyroidism and hypoplastic thyroid glands were analyzed for mutations in PAX8 and TTF2 (FKHL15). Mutations in the coding region of the TSHbeta-subunit gene, the TSH receptor gene, and exons 8 and 9 of G(s)alpha had been excluded previously. Serum TSH concentrations were 150 mU/liter or more, TG levels were within normal limits, and thyroid autoantibodies were absent. Technetium scintigraphies did not reveal the presence of thyroid tissue, but ultrasonography documented hypoplastic, normally located glands. One patient was found to harbor a heterozygous transversion 119A-->C in exon 3 of PAX8 replacing a conserved glutamine by proline in the paired box domain (Q40P). Analysis of her family members revealed that her mother, who has a thyroid gland of normal size and mild, adult-onset autoimmune hypothyroidism, is also heterozygous for this mutation. Functional analyses of the PAX8 Q40P mutation showed impaired binding to a PAX8 response element and absent trans-activation of a thyroid peroxidase promoter luciferase reporter gene. These findings confirm the important role of PAX8 in the development of the thyroid, but they indicate that PAX8 gene mutations may have a variable penetrance or expressivity. The absence of mutations in the coding sequences of the analyzed genes in the three other patients supports the concept that the pathogenesis of congenital hypothyroidism associated with thyroid hypoplasia is diverse.

A 138-nucleotide deletion in the thyroglobulin ribonucleic acid messenger in a congenital goiter with defective thyroglobulin synthesis.

Two siblings (HSN and AcSN) with congenital goitrous hypothyroidism were investigated in terms of clinical, biochemical, and molecular biology. Diagnosis of defective thyroglobulin (Tg) was based on findings of low serum T4, low normal or normal serum T3, a negative percholate discharge test, and the virtual absence of the serum Tg response to challenge by bovine TSH. Only minute amounts of Tg-related antigens were detected by RIA in the goitrous tissue (HSN, 0.82 mg/g, compared to 70-90 mg/g in normal thyroid tissue), as confirmed by sodium dodecyl sulfate-agarose gel electrophoresis that indicated the virtual absence of Tg. The Tg messenger ribonucleic acids (mRNAs) from controls and HSN thyroid tissue were first reverse transcribed and then divided into several portions from positions 57-8448; the resulting complementary DNAs were, in turn, amplified by reverse polymerase chain reaction. The amplification of nucleotides 5165-6048 from control thyroid tissue Tg mRNA showed a fragment of 884 base pairs (bp). In contrast, the fragment present in the HSN was +/- 750 bp and lacked the normal fragment. The sequencing of the smaller fragment revealed that 138 bp were missing between positions 5590-5727 of the HSN Tg mRNA. This deletion does not affect the reading frame of the resulting mRNA and is potentially fully translatable into a Tg polypeptide chain that is shorter by 46 residues. A cysteine residue is maintained by the junction between the proximal T from leucine 1831 and the distal GT from cysteine 1877. DNA genomic polymerase chain reaction amplification excludes a deletion in the Tg gene and indicates that the deleted 138-nucleotide sequences lie in the same exon. The functional consequences of the deletion are not entirely clear, but it is conceivable that the excision of this segment of the Tg molecule could affect the protein structure, resulting in its premature degradation, very low colloid storage, and diminished thyroid hormone production rate.

Low levels of thyroglobulin messenger ribonucleic acid in congenital goitrous hypothyroidism with defective thyroglobulin synthesis.

We characterized the virtual absence of immunoassayable thyroglobulin (Tg) in the serum and thyroid gland of two siblings (MA, JNA) and one nephew (RSS) from a family without inbreeding or familial goiter. Diagnosis of defective Tg gene expression was based on findings of normal PBI and low serum T4, low or normal serum T3, negative perchlorate discharge test, and virtual absence of the serum Tg response to challenge by bovine TSH. This conclusion was confirmed by analysis of proteins in the goiter extracts. Only minute amounts of immunoassayable Tg were detected by RIA (MA, 0.11; JNA, 0.19 mg/g tissue; compared to 70-90 mg/g in normal thyroid tissue). Gel filtration in Sephacryl S300 showed the absence of a normal Tg peak at 280 nm and concentration of label mostly on albumin. A minor intermediate peak of radioactivity was also detected, with the size of, approximately, normal Tg. Sodium dodecyl sulfate-agarose gel electrophoresis indicated the absence of Tg dimer and monomer, and Western blotting and immunoelectrophoresis confirmed this finding. Dot blot quantification of Tg and thyroid peroxidase mRNA indicated decreased hybridization of the patients' mRNA (MA, 44%; JNA, 63%) with phTgM2 (Tg probe) and increased hybridization (MA, 191%; JNA, 182%) with the pM5 (thyroid peroxidase probe) compared with control thyroid tissue. Dot blot analysis of Tg mRNA from the two siblings weakly hybridized with 3' and 5' Tg probes. RNA analysis by means of Northern transfer showed a clear signal of hybridization with Tg probe (phTgM1) in the 8- to 9-kilobase range, corresponding to the normal size Tg mRNA. No major polymorphisms were noted in Southern blotting, using seven restriction endonucleases. We conclude that no gross alteration of the 5' region of Tg gene was present in these patients. Ultrastructural examination of the thyroid tissue indicated that the rough endoplasmic reticulum was not augmented, nor were the cisternae of rough endoplasmic reticulum dilated. The defect observed in these goiters is diminished tissue concentration of Tg mRNA with defective translation. However, small amounts of functionally active Tg could be synthesized, iodinated, and immediately hydrolized, yielding mostly T3, owing to the intense tissue stimulation by TSH.

Deficiency of superoxide dismutase in endemic goiter tissue.

Superoxide dismutase (SOD) activity and its concentration were measured in thyroid tissues obtained from patients with Graves' disease, Hashimoto's thyroiditis, differentiated thyroid cancer, and endemic goiter (before and after iodine supplementation) as well as in normal thyroid tissue (paranodular tissue) from patients with follicular adenomas. SOD activity was measured by pyrogallol assay in ethanol-chloroform extracts of the thyroid homogenates. The SOD concentration in the thyroid extract was measured as immunoreactive SOD by electroimmunoassay. Endemic goiter tissues (n = 10) contained significantly lower SOD activity [mean, 1.9 +/- 1.9 (+/- SD) vs. 7.5 +/- 3.9 ng purified SOD/micrograms DNA; P less than 0.02] and concentration (mean, 0.2 +/- 0.1 vs. 0.8 +/- 0.5 ng SOD/microgram DNA; P less than 0.01) compared with those of normal tissues. No other pathological thyroid tissues had such consistently low SOD levels. Lactate dehydrogenase activity, a marker of cytosolic enzyme, was not lower in endemic goiter tissues than in normal tissues, suggesting that both tissues possessed functioning cells capable of producing cytosolic enzyme. Thyroid tissue from endemic goiter patients previously treated with iodized oil injection also had low SOD activity and concentration. Western blot analysis indicated that SOD protein in the endemic goiter tissue did not differ from that in normal thyroid tissue. We conclude that there is deficiency of cytosolic SOD in endemic goiter tissue. Since the deficiency of cytosolic SOD causes more prolonged exposure to oxygen free radicals, the decrease in SOD might contribute to the degenerative changes frequently found in these tissues.

Metastatic thyroid carcinoma arising from congenital goiter due to mutation in the thyroperoxidase gene.

A very large cervical tumor that extended to the upper mediastinum was seen in a newborn after an uneventful pregnancy. The computed axial tomography scan confirmed the presence of a solid mass with precise limits and scattered foci of calcifications situated in the anterolateral region of the neck. The infant underwent thyroidectomy on the seventh day after birth. Pathological examination revealed a follicular carcinoma of the thyroid and probable dyshormonogenetic hyperplastic goiter. At 5 months of age, whole body scans indicated the presence of lung and bone metastases, which were treated with therapeutic doses of radioiodine. Genomic DNA was obtained from the newborn, her parents, her paternal aunt, and her paternal grandparents. Denaturing gradient gel electrophoresis analysis of PCR fragments corresponding to exon 14 of the thyroid peroxidase (TPO) gene indicated the presence of a mutant TPO allele present in the propositus, her father, and her paternal grandmother. Sequencing of the TPO gene demonstrated a mutation resulting from an insertion of a single extra cytosine in a stretch of seven cytosines at positions 2505-2511. The insertion caused a frame shift and a stop signal in exon 16. This sequence would translate into a structurally modified and probably inactive TPO protein. We conclude that the aggressive thyroid metastatic carcinoma arose from a dyshormonogenetic goiter caused by a defective TPO protein.