Clinical recognition and evaluation of patients with inherited serum thyroid hormone-binding protein mutations.

There are three important thyroid hormone-binding proteins in human serum, thyroxine-binding globulin, transthyretin, and albumin. Genetic variation in these proteins can lead to altered thyroid hormone binding and abnormalities in serum tests of thyroid hormone. Importantly, patients harboring these mutations are euthyroid; thus, the recognition of these conditions is crucial to prevent unnecessary repeated testing and treatment. This article provides an updated overview of serum thyroid hormone transport biology and reviews the underlying genetic alterations, clinical presentation, and appropriate evaluation of patients with suspected mutations in serum thyroid hormone-binding proteins.

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.

INSUFFICIENCY OF LEVOTHYROXINE THERAPY IN AUTOIMMUNE HYPOTHYROIDISM: EFFECT OF GLUCOCORTICOID ADMINISTRATION.

OBJECTIVE: The non-effectiveness of levothyroxine administration in hypothyroidism depends on many factors and mechanisms influencing its absorption in small intestins or bounding of circulating hormone with different active molecules. METHODS: Thyroid hormones, TSH, rT3, TGl, TPO-Ab, TG-Ab, were measured using commercially available assays. For anti-T4ab, radioiodine-labeled T4 was added to the patient's serum and the IgG fraction subsequently precipitated by addition of 15% polyethylene glycol. Background was determined by testing 100 control sera from individuals without autoimmune thyroid disease. RESULTS: A 42-year old woman (71.5 kg) with Hashimoto thyroiditis receiving levothyroxine (L-T4) 150 µg and liothyronine (L-T3) 37.5 µg was admitted to the hospital with clinical data of hypothyroidism, TSH-23.8 mU/L, FT4-6.18 pmol/L (n.range 9-19 pmol/L), TPO-Ab 696 IU/mL, TG-Ab 818 IU/mL, circulating T4- antibodies positive. She has a good adherence to medication, malabsorption or administration of other drugs were excluded. L-T4 absorption test revealed 44% increase of serum FT4 at 120 min after ingestion of 150 mcg L-T4 (2.1 mcg/kg). Methylprednisolone pulses of 500 mg i.v. administered in three consecutive days at equal doses of L-T4/L-T3 resulted in a rapid increase of FT4 to 14.5 pmol/L, fall of TSH to 0.18 mU/L and decrease of anti-T4 antibodies to referent range; TPO-Ab and TG-Ab also decreased significantly. Monotherapy by 150 mcg L-T4 was continued in the next three months. A recurrence of hypothyroidism with increase of circulating T4-Ab was observed 100 days later. New administrations of methylprednisolone two pulses of 500 mg revealed a similar normalization of thyroid hormones and anti-T4 antibodies. CONCLUSION: The data showed that T4-antibodies might be a cause of insufficient effects of levothyroxine therapy in autoimmune hypothyroidism. This could be overcome by glucocorticoid administration probably resulting in FT4 release from circulating immune complexes.

Identical mutations in unrelated families with generalized resistance to thyroid hormone occur in cytosine-guanine-rich areas of the thyroid hormone receptor beta gene. Analysis of 15 families.

Generalized resistance to thyroid hormone (GRTH) is a syndrome of variable reduction of tissue responsiveness to thyroid hormone. 28 different point mutations in the human thyroid hormone receptor beta (TR beta) gene have been associated with GRTH. These mutations are clustered in two regions of the T3 binding domain of the TR beta (codons 310-347 and 417-453). We now report point mutations in the TR beta gene of six additional families with GRTH and show that three mutations occurred each in three families with GRTH, and that three other mutations were each present in two families. In 11 of these 15 families, lack of a common ancestor could be confirmed by genetic analysis. 28 of the 38 point mutations so far identified, including all those occurring in more than one family, are located in cytosine-guanine-rich areas of the TR beta gene. Differences in clinical and laboratory findings in unrelated families harboring the same TR beta mutation suggest that genetic variability of other factors modulate the expression of thyroid hormone action.

Mutations of CpG dinucleotides located in the triiodothyronine (T3)-binding domain of the thyroid hormone receptor (TR) beta gene that appears to be devoid of natural mutations may not be detected because they are unlikely to produce the clinical phenotype of resistance to thyroid hormone.

Thyroid hormone receptor (TR) beta gene mutations identified in patients with resistance to thyroid hormone (RTH) revealed two clusters ("hot" areas) of mutations (RTHmut) in the triiodothyronine (T3)-binding domain. Furthermore, 45% of RTHmuts and 90% of recurring mutations are located in CpG dinucleotides ("hot spots"). To investigate why the region between the two hot areas lacks RTHmuts, we produced 10 artificial mutant TR beta s (ARTmut) in this "cold" region according to the hot spot rule (C-->T or G-->A substitutions in CpGs). The properties of ARTmuts were compared with those of six RTHmuts. Among all RTHmuts, R320H manifesting a mild form of RTH showed the least impairment of T3-binding affinity (Ka). In contrast, Ka was normal in six ARTmuts (group A), reduced to a lesser extent than R320H in three (group B), and one that was truncated (R410X) did not bind T3. All RTHmuts had impaired ability to transactivate T3-responsive elements and exhibited a strong dominant negative effect on cotransfected wild-type TR beta. Group B and A ARTmuts had minimally impaired or normal transactivation and weak or no dominant negative effect, respectively. R410X showed neither transactivation nor dominant negative effect. Natural mutations expected to occur in the cold region of TR beta should fail to manifest as RTH (group A) or should escape detection (group B) since the serum thyroid hormone levels required to compensate for the reduced binding affinity should be inferior to those found in subjects with R320H. R410X would manifest RTH only in the homozygote state. The cold region of the putative T3-binding domain is relatively insensitive to amino acid changes and, thus, may not be involved in a direct interaction with T3.

Two new inherited defects of the thyroxine-binding globulin (TBG) molecule presenting as partial TBG deficiency.

Serum-denatured TBG (dnTBG) measured in 32 families deficient in native TBG (nTBG) was undetectable in all subjects with complete nTBG deficiency and was high in 2 of 16 families with partial nTBG deficiency. nTBG (in mean micrograms per decaliter +/- SD) in members of the Quebec and Montreal families, respectively were: 258 +/- 54 and 230 in affected men, 747 +/- 190 and 927 +/- 90 in affected women, and 1568 +/- 151 and 1300 +/- 195 in unaffected relatives. Corresponding mean dnTBG levels were: 14.3 +/- 2.9 and 21.3 in affected men, 8.6 +/- 1.0 and 11.6 +/- 3.1 in affected women, and less than 2.1 and less than 2.6 in unaffected relatives. All were euthyroid with normal free thyroxine and thyrotropin levels. In comparison to common type TBG, TBG-Quebec was more heat labile by 10 degrees C and TBG-Montreal by 12 degrees C. The degree of dnTBG elevation and nTBG lability at 37 degrees C were correlated (r = 0.99). Isoelectric focusing showed cathodal shift of all TBG bands: TBG-Quebec by 0.06 isoelectric points (pI) and TBG-Montreal by 0.02 pI. These two TBG variants represent different mutations most likely affecting the polypeptide chain of the molecule. Their inheritance is X-chromosome linked. The instability of these TBGs at 37 degrees C may lead to more rapid degradation in vivo resulting in low nTBG and high dnTBG concentrations in serum.

Study of four new kindreds with inherited thyroxine-binding globulin abnormalities. Possible mutations of a single gene locus.

Five families with inherited thyroxine-binding globulin (TBG) abnormalities were studied. On the basis of serum thyroxine (T(4))- binding capacity of TBG in affected males, three family types were identified: TBG deficiency, low TBG, and high TBG capacity. In all families evidence for X-linked inheritance was obtained and in one family all criteria establishing this mode of inheritance were met. Only females were heterozygous, exhibiting values intermediate between affected males and normals. Overlap in heterozygotes was most commonly encountered in families with low TBG. QUANTITATIVE VARIATION IN THE SERUM CONCENTRATION OF FUNCTIONALLY NORMAL TBG WAS DEMONSTRATED BY: (a) failure of serum from TBG-deficient subjects to react with anti-TBG antibodies; (b) normal kinetics of T(4) and triiodothyronine-binding to TBG in sera from subjects with low TBG and high TBG capacity; (c) concordance of estimates of TBG concentration by T(4) saturation and by immunological methods; and (d) normal rate of heat inactivation of TBG. No abnormalities in serum transport of cortisol, testosterone, aldosterone, or thyroxine bound to prealbumin could be detected. These observations suggest that all the TBG abnormalities thus far observed reflect mutations at a single X-linked locus involved in the control of TBG synthesis.

Studies on human thyroxine-binding globulin (TBG). IX. Some physical, chemical, and biological properties of radioiodinated TBG and partially desialylated TBG.

Thyroxine-binding globulin (TBG) and partially desialylated or slow TBG (STBG) were purified from human serum by affinity chromatography. Purified TBG was identical to TBG present in serum by the criteria of electrophoretic mobility, affinity for thyroxine (T4), and heat-inactivation response. Purified STBG had slower electrophoretic mobility and lower affinity for T4. Both bound T4 in an equimolar ratio, were immunoprecipitable, and had similar inactivation t1/2 at 61 degrees C. TBG and STBG were iodinated by the chloramine-T-catalyzed reaction. An average of from 0.02 to 6 atoms I could be incorporated per molecule of the protein by adjusting the conditions of the reaction (time, protein and iodide concentrations). 125-I, 131-I, and 127-I were used. Iodination increased the anodal mobility of TBG but did not affect the reversible T4-binding, precipitation by antiserum, or the heat-inactivation properties. "Heavily" and "lightly" iodinated TBG had identical disappearance half-times from serum in the rabbit. 15 min after the intravenous administration of [131-I]-STBG and [125-I]TBG mixture to rats, more than 90% of the injected 131-I dose was in the liver, and the liver 131-I/125-I ratio was 32-fold that of serum. Selective uptake of STBG by the liver was also observed in the rabbit and in man. The serum [125-I]STBG/[131-I]TBG ratio declined from 1 to 0.2 in 10 min in the intact rabbit but remained unchanged for 1 h in the acutely hepatectomized animal. In the rabbit, t 1/2 was approximately 3 min for STBG and 0.8-3.4 days for TBG. The radioiodine derived from the iodinated proteins is partly excreted in bile but the bulk was precipitable with specific antibodies. Some isotope in the form of iodide appeared in blood and was excreted in the urine. Since radioiodinated TBG and STBG preserve their biologic and immunologic properties they are useful as tracer materials for metabolic studies. In rat, rabbit, and man STBG is rapidly cleared from serum by the liver. Conversion of TBG to STBG may be the limiting step in the regulation of TBG metabolism.

Growth hormone responses to thyroid hormone in the neonatal rat: resistance and anamnestic response.

Differences in the growth hormone (GH) responses to primary and to secondary stimulation with triiodothyronine (T3) were studied in rats deprived of thyroid hormone from birth. Neonatal hypothyroidism was induced in pups by feeding pregnant rats an iodine-deficient, propylthiouracil-containing diet. T3 stimulation was carried out in pups by subcutaneous injection of a single dose of 50 mug T3/100 g body wt. Pituitary GH content, rate of GH synthesis in vitro, and GH messenger (m)RNA activity in a cellfree translation system were measured.No significant differences in body weight and in pituitary GH content were observed between hypothyroid and normal pups at ages 1, 3, and 6 d. 10- and 28-d-old hypothyroid pups showed a significant arrest of growth, decreased pituitary GH content, and development of GH responsiveness to T3. In contrast, serum thyroxine concentration in hypothyroid pups was <0.15 mug/dl, significantly lower than normal at all ages.GH synthesis and GH mRNA activity studied in pituitaries of 28-d-old rats were expressed as percent total protein synthesis and percent mRNA activity, respectively. GH synthesis and mRNA activity were 3.0 and 2.6% in hypothyroid rats, 3.3 and 2.9% in hypothyroid rats given a single T3 injection 14 d earlier (T3-withdrawn rats), and 26.8 and 27.1% in normal rats. Administration of T3 to hypothyroid rats induced an increase in GH synthesis and GH mRNA activity, reaching 5.8 and 5.6% 12 h after primary stimulation and 12.2 and 16.1% 12 h after secondary stimulation. The response rates were linear but 2.5-fold more rapid after secondary stimulation. The latter response was similar to that observed after T3 stimulation of rats rendered hypothyroid during adulthood. The responses of GH synthesis and mRNA activity were concordant after both primary and secondary T3 stimulation. A twofold increase in both parameters was observed as early as 2 h after T3 injection. Four conclusions can be drawn from these experiments. First, during neonatal life, GH accumulation in rat pituitaries is independent of thyroid hormone and is insensitive to T3. Second, GH dependence on and sensitivity to thyroid hormone is acquired between the 6th and 10th d of neonatal life. Third, secondary T3 stimulation produces an anamnestic response manifested by an increased rate of GH synthesis and mRNA activity. Fourth, primary T3 stimulation is not associated with a lag in the endogenous translation of the newly accumulated GH mRNA.

Thyroid function in a uremic rat model. Evidence suggesting tissue hypothyroidism.

The main objective of this study was to determine whether the principal abnormality of thyroid function observed in patients with chronic renal failure, low serum triiodothyronine (T(3)) concentration, causes hypothyroidism at the tissue level. A partially nephrectomized (Nx) uremic rat model was developed and the following parameters of thyroid function were assessed: serum total thyroxine (TT(4)), total T(3) (TT(3)), and thyrotropin and liver T(3) content, and activity of two thyroid hormone-dependent enzymes, mitochondrial alpha-glycerophosphate dehydrogenase (alpha-GPD) and cytosol malate dehydrogenase (MDH). The results were compared to those of intact control (C), thyroidectomized (Tx), and nephrectomized-thyroidectomized (NxTx) littermates.Results expressed as mean+/-SEM showed that Nx rats had a fivefold increase in blood urea nitrogen, (112+/-20 mg/dl in Nx, and 22+/-1 mg/dl in C) and manifested all the changes of of thyroid function observed in uremic men, including a low serum TT(3) level (30+/-7 ng/dl in Nx and 50+/-6 ng/dl in C). In the liver, T(3) was significantly reduced (18+/-2 ng/total liver in Nx and 35+/-3 ng/total liver in C) as well as the activities of alphaGPD (8.8+/-1.0 and 16.1+/-1.5 DeltaOD/min per total liver in Nx and C, respectively) and MDH (6.3+/-1.6 and 12.6+/-2.2 U/total liver in Nx and C, respectively). The reduction in liver enzyme activities correlated significantly with the decrease in T(3) content. The changes in Tx rats were as expected, showing a profound reduction in serum hormone levels, liver T(3) content, and liver enzyme activities. Serum thyrotropin was markedly elevated to 2,390+/-212 ng/ml as compared to 703+/-61 in C and 441+/-87 ng/ml in Nx rats. The NxTx rats showed the combined effects of nephrectomy and thyroidectomy; blood urea nitrogen was elevated to 203, and serum levels of TT(4), TT(3), and thyrotropin were 0.4, <10, and 2,525, respectively. Total liver T(3) and alphaGPD and MDH were strikingly low; the corresponding values were 3.5, 2.4, and 2.5.l-triiodothyronine replacement (0.4 mug/100 g body wt/d) for 4 wk in the Nx rats resulted in significant increases in liver enzyme activities, alphaGPD and MDH rose by 70 and 60% over their respective basal values without alteration in the severity of azotemia. From these data, we conclude that the reduction of liver T(3) content in the uremic rats, accompanied by a decrease in alphaGPD and MDH activity, indicates the presence of hypothyroidism at the tissue level. Restoration of enzyme activities toward normal levels after T(3) administration provided further supporting evidence that the diminution in liver enzyme activity was causally related to tissue T(3) deficiency.

New insights on the mechanism(s) of the dominant negative effect of mutant thyroid hormone receptor in generalized resistance to thyroid hormone.

Generalized resistance to thyroid hormone (GRTH) is a syndrome of hyposensitivity to triiodothyronine (T3) that displays autosomal dominant inheritance. The genetic defect commonly lies in the ligand-binding domain of one of the TR beta alleles. Since there are two major thyroid hormone receptor (TR) isoforms, TR alpha and TR beta, it is not known how the mutant receptor mediates a dominant negative effect. Previously, we showed that T3 caused dissociation of TR homodimers and TR alpha/TR beta dimers from several thyroid hormone response elements (TREs). Hence, we used the electrophoretic mobility shift assay to compare the effect of T3 on the DNA binding of mutant TR beta-1 (Mf-1) from a kindred with GRTH with normal TR beta. Mf-1 bound better as a homodimer than TR beta, but dissociated from DNA only at high T3 concentrations. Both receptors heterodimerized with nuclear auxiliary proteins. They also dimerized with TR alpha and with each other. Surprisingly, T3 disrupted the DNA binding of the Mf-1/TR isoform dimers. Thus, mechanisms for the dominant negative effect by mutant TRs likely involve either increased binding to TREs by mutant homodimers that cannot bind T3 (hence cannot dissociate from DNA) and/or the formation of inactive mutant TR/nuclear protein heterodimers.

Screening of nineteen unrelated families with generalized resistance to thyroid hormone for known point mutations in the thyroid hormone receptor beta gene and the detection of a new mutation.

Generalized resistance to thyroid hormone (GRTH) is a syndrome characterized by impaired tissue responsiveness to thyroid hormone. Two distinct point mutations in the hormone binding domain of the thyroid hormone receptor (TR) beta have recently been identified in two unrelated families with GRTH. One, Mf, involves a replacement of the normal glycine-345 for arginine in exon 7 and another, Mh, replaces the normal proline-453 for histidine in exon 8. To probe for the presence of the Mf and Mh defect in 19 unrelated families with GRTH, we applied separate polymerase chain reactions using allele-specific oligonucleotide primers containing the normal and each of the two mutant nucleotides at the 3'-position. A total of 24 affected subjects and 13 normal family members were studied. The mode of inheritance was dominant in 13 families, was unknown in 5 families, and was clearly recessive in 1 family in which only the consanguineous subjects were affected. Primers containing the substitutions specific for Mf and Mh amplified exons 7 and 8, respectively, only in affected members of each of the two index families. Primers containing the normal sequences amplified exons 7 and 8 of the TR beta gene in all subjects except affected members of one family. In this family with recessively inherited GRTH, neither exon could be amplified using any combinations of primers and DNA blot revealed absence of all coding exons. These results indicate a major deletion of the TR beta gene, including both DNA and hormone binding domains. Since heterozygous members of this family are not affected, the presence of a single normal allele is sufficient for normal function of the TR beta. These data also support the hypothesis that in the dominant mode of GRTH inheritance the presence of an abnormal TR beta interferes with the function of the normal TR beta. Distinct mutations are probably responsible for GRTH in unrelated families.

Thyroid dysfunction in chronic renal failure. A study of the pituitary-thyroid axis and peripheral turnover kinetics of thyroxine and triiodothyronine.

Thyroid function was evaluated in 46 patients with end-stage kidney disease and 42 normal subjects. Patients were studied before and after the institution of maintenance hemodialysis (HD) and after renal transplantation (RT). Serum total triiodothyronine concentrations (TT(3), ng/100 ml, mean+/-SD) were 63+/-17 and 83+/-22 in the non-HD and HD groups, respectively. Values from normal subjects were 128+/-25 and from RT patients 134+/-20. The TT(3) was in the hypothyroid range (<78 ng/100 ml; 2 SD below normal mean) in 80% of non-HD and 43% of HD patients. Mean serum total thyroxine concentration (TT(4)), although within the normal range, was lower than the control value. T(4)-binding globulin capacity was also slightly lower but the difference was not statistically significant. Among patients whose TT(4) was 1 SD below the normal mean, the free T(4) index was equally depressed, suggesting that factors other than decreased binding capacity might be responsible for the low TT(4). In addition, there was a 37% incidence of goiter. Mean serum thyroid-stimulating hormone (TSH) was not elevated and the TSH response to thyrotropin-releasing hormone (TRH) was distinctly blunted, suggesting the possibility of pituitary dysfunction as well. In vivo (125)I-l-T(4) and (131)I-l-T(3) kinetics during 0.2 mg/day of l-T(4) replacement showed marked reduction in T(3) turnover rate in the uremic patients, both before and during HD; the values (mug T(3)/day, mean+/-SD) for the different groups were as follows: normal, 33.8+/-6.1; non-HD, 13.5+/-2.6; HD, 12.9+/-3.1; and RT, 30.3+/-7.1. The low T(3) turnover rate was due to impaired extrathyroidal conversion of T(4) to T(3). The mean percent+/-SD of metabolized T(4) converted to T(3) was 37.2+/-5.8 in normal subjects, 15.7+/-3.1 in non-HD, 12.8+/-1.7 in HD, and 34.0+/-14.7 in RT patients. In contrast, thyroidal T(3) secretion rate was not different between the control and the three patient groups. Thus, it appears that uremia affects thyroid function at several levels: (a) subnormal pituitary TSH response to TRH; (b) possible intrathyroidal abnormalities as suggested by slightly decreased TT(4) and high incidence of goiter; and (c) abnormal peripheral generation of T(3) from T(4). Restoration of renal function with RT resulted in normalization of all parameters of thyroid function with the exception of blunted or absent TSH response to TRH. The latter may be a direct consequence of glucocorticoid administration.

Metabolism of thyroxine-binding globulin in man. Abnormal rate of synthesis in inherited thyroxine-binding globulin deficiency and excess.

It has been previously suggested that inherited thyroxine-binding globulin (TBG) abnormalities in man may be due to mutations at a single X-chromosome-linked locus controlling TBG synthesis. However, abnormalities in TBG degradation have not been excluded. The availability of purified human TBG and its successful labeling with radioiodide allowed us to examine such possibility. Human TBG was purified by affinity chromatography, labeled under sterile conditions with 131I or 125I,, and mixed with [125I]thyroxine (T4) or [131I]T4, respectively, before their intravenous injection. Blood and urine samples were collected over a 10-day period, and the turnover parameters were calculated. In eight normal volunteers mean values +/-SD for TBG and T4 respectively, were as follows: Half time (t1/2) 5.3 +/- 0.4 and 7.0 +/- 0.6 days; distribution space (DS) 7.2 +/- 1.0 and 10.8 +/- 1.2 liters; and total daily degradation (D) 0.211 +/- 0.053 and 0.088 +/- 0.011 mumol/day. In all subjects, t1/2 of TBG was shorter than that of T4; and the DS was smaller. 2.4 mol of TBG was degraded for each mole of T4. In five of six subjects from four families, comprising hemizygous and heterozygous carriers of TBG absence, decrease, and excess, the t1/2 and DS for TBG were within the normal range. The D of TBG was proportional to the serum concentration of the protein. Changes in the T4 kinetics in these patients were compatible with euthyroidism and with the known alterations in the extrathyroidal T4 pool associated with the changes in serum TBG concentration. A striking decrease in the t1/2 of TBG was found only in a patient with acquired diminution in TBG concentration and in patients with thyrotoxicosis or other conditions apparently unrelated to thyroid dysfunction. TBG t1/2 was 2.5 days in a patient with multiple myeloma and 3.6 days in two patients with thyrotoxicosis. Decreased TBG t1/2 was also observed in three of six patients with nonthyroidal pathology and was associated with an increase in TBG D disproportionate to their level of serum TBG. These studies indicate that changes in TBG concentration in patients with X-chromosome-linked TBG abnormalities are due to alterations in its rate of synthesis. In other conditions, abnormalities of TBG degradation and/or rate of synthesis may be found.

Regulation of glycosaminoglycan synthesis by thyroid hormone in vitro.

Human skin fibroblasts synthesize and accumulate glycosaminoglycans (GAG). Recently, we reported that fibroblasts incubated in thyroid hormone-deficient media accumulate more GAG than do cultures incubated in the same media enriched with 0.1 muM triiodothyronine (T(3)) (1981. Endocrinology. 108: 2397). The current study characterizes that enhanced accumulation. Confluent cultures were maintained in thyroid hormone-deficient media without or with added T(3), labeled with [(3)H]acetate and analyzed for total [(3)H]GAG and [(3)H]hyaluronic acid content. Addition of T(3) to thyroid hormone-depleted media consistently inhibited the incorporation of [(3)H]acetate into GAG by 28-60% in fibroblast cultures from four different normal human donors. Maximal inhibitory effect was observed within 3 d after hormone addition at concentrations > 1 nM. 73% of the maximal inhibitory effect was observed in the presence of physiologic concentrations of T(3) (0.16 nM total T(3) or 1.4 pM free T(3)). The following observations indicated that T(3) inhibition of [(3)H]GAG accumulation is most likely due to a decrease in GAG synthesis rather than to changes in the acetate pool or GAG degradation: (a) Addition of 0, 100, 500, and 2,500 muM unlabeled acetate progressively decreased [(3)H]acetate incorporation into GAG, up to 80%, without altering the further inhibitory effect of T(3) (35-40%); (b). A similar effect of T(3) on GAG (32% inhibition) was observed using [(3)H]glucosamine as substrate; (c) T(3) decreased hyaluronate synthetase activity by 32%; and (d) There was no effect of T(3) on GAG degradation in a pulse-chase experiment. The effect of T(3) on [(3)H]GAG accumulation appears to be quite specific, since the hormone had no effect on the incorporation of [(3)H]leucine into trichloroacetic acid-precipitable material.Thus, thyroid hormone inhibits GAG accumulation in a dose-, time-dependent, and reversible manner. This inhibition is apparently due to specific effects on the rate of macromolecular synthesis.

Modulation of glucose regulation and insulin secretion by circadian rhythmicity and sleep.

To define the roles of circadian rhythmicity (intrinsic effects of time of day independent of the sleep or wake condition) and sleep (intrinsic effects of the sleep condition, irrespective of the time of day) on the 24-h variation in glucose tolerance, eight normal men were studied during constant glucose infusion for a total of 53 h. The period of study included 8 h of nocturnal sleep, 28 h of continuous wakefulness, and 8 h of daytime sleep. Blood samples for the measurement of glucose, insulin, C-peptide, cortisol, and growth hormone were collected at 20-min intervals throughout the entire study. Insulin secretion rates were derived from C-peptide levels by deconvolution. Sleep was polygraphically monitored. During nocturnal sleep, levels of glucose and insulin secretion increased by 31 +/- 5% and 60 +/- 11%, respectively, and returned to baseline in the morning. During sleep deprivation, glucose levels and insulin secretion rose again to reach a maximum at a time corresponding to the beginning of the habitual sleep period. The magnitude of the rise above morning levels averaged 17 +/- 5% for glucose and 49 +/- 8% for calculated insulin secretion. Serum insulin levels did not parallel the circadian variation in insulin secretion, indicating the existence of an approximate 40% increase in insulin clearance during the night. Daytime sleep was associated with a 16 +/- 3% rise in glucose levels, a 55 +/- 7% rise in insulin secretion, and a 39 +/- 5% rise in serum insulin. The diurnal variation in insulin secretion was inversely related to the cortisol rhythm, with a significant correlation of the magnitudes of their morning to evening excursions. Sleep-associated rises in glucose correlated with the amount of concomitant growth hormone secreted. These studies demonstrate previously underappreciated effects of circadian rhythmicity and sleep on glucose levels, insulin secretion, and insulin clearance, and suggest that these effects could be partially mediated by cortisol and growth hormone.

Genetic immunization of outbred mice with thyrotropin receptor cDNA provides a model of Graves' disease.

We performed genetic immunization of outbred NMRI mice, using a cDNA encoding the human thyrotropin receptor (TSHr). All mice produced antibodies capable of recognizing the recombinant receptor expressed at the surface of stably transfected Chinese hamster ovary (CHO) cells, and sera from most of the immunized mice blocked TSH-dependent stimulation of cAMP accumulation in cells expressing the TSHr. Five out of 29 female mice showed sign of hyperthyroidism including elevated total T4 and suppressed TSH levels. The serum of these mice contained thyroid-stimulating activity, as measured in a classic assay using CHO cells expressing recombinant TSHr. In contrast, only 1 male out of 30 had moderately elevated serum total T4 with undetectable TSH values. The hyperthyroid animals had goiters with extensive lymphocytic infiltration, characteristic of a Th2 immune response. In addition, these animals displayed ocular signs reminiscent of Graves' ophthalmopathy, including edema, deposit of amorphous material, and cellular infiltration of their extraocular muscles. Our results demonstrate that genetic immunization of outbred NMRI mice with the human TSHr provides the most convincing murine model of Graves' disease available to date.

Congenital hypothyroidism due to mutations in the sodium/iodide symporter. Identification of a nonsense mutation producing a downstream cryptic 3' splice site.

A 12-yr-old hypothyroid girl was diagnosed at birth as athyreotic because her thyroid gland could not be visualized by isotope scanning. Goiter development due to incomplete thyrotropin suppression, a thyroidal radioiodide uptake of < 1%, and a low saliva to plasma ratio of 2.5 suggested iodide (I-) transport defect. mRNA isolated from her thyroid gland and injected into Xenopus oocytes failed to increase I- transport. Sequencing of the entire Na+/I- symporter (NIS) cDNA revealed a C to G transversion of nucleotide (nt) 1146 in exon 6, resulting in a Gln 267 (CAG) to Glu (GAG) substitution. This missense mutation produces an NIS with undetectable I- transport activity when expressed in COS-7 cells. Although only this missense mutation was identified in thyroid and lymphocyte cDNA, genotyping revealed that the proposita and her unaffected brother and father were heterozygous for this mutation. However, amplification of cDNA with a primer specific for the wild-type nt 1146 yielded a sequence lacking 67 nt. Genomic DNA showed a C to G transversion of nt 1940, producing a stop codon as well as a new downstream cryptic 3' splice acceptor site in exon 13, responsible for the 67 nt deletion, frameshift, and premature stop predicting an NIS lacking 129 carboxy-terminal amino acids. This mutation was inherited from the mother and present in the unaffected sister. Thus, although the proposita is a compound heterozygote, because of the very low expression (< 2.5%) of one mutant allele, she is functionally hemizygous for an NIS without detectable bioactivity.

Sequence of the variant thyroxine-binding globulin of Australian aborigines. Only one of two amino acid replacements is responsible for its altered properties.

A form of thyroxine-binding globulin (TBG) with reduced affinity for hormone and increased susceptibility to heat and acid denaturation has been identified in Australian Aborigines (TBG-A). Results of heat denaturation of TBG established that the TBGA allele is X linked and has a frequency of 50.9% in Western Australian Aborigines. The sequence of an isolated TBGA allele differed at two positions from that of the normal TBG allele (TBGC). One substitution was in codon 191, ACA (threonine) rather than GCA (alanine), and the other was in codon 283, TTT (phenylalanine) instead of TTG (leucine). These nucleotide substitutions resulted in the loss of sites for the enzymes Bgl 1 and Tth 111 II, respectively. The nucleotide substitutions in the TBG-A allele was confirmed by digestion of genomic DNA segments amplified using the polymerase chain reaction. The Bgl 1 and Tth 111 II sites were absent in the genes of two Aboriginal men expressing TBG-A and were present in those of three Aboriginal and six Caucasian males expressing TBG-C. The TBG gene of a seventh Caucasian male possessed the Bgl 1 site but had lost the Tth 111 II site; sequencing of this allele revealed only the substitution in codon 283 identical to that in the TBGA allele. As the biochemical properties of TBGPhe-283 expressed by this individual were indistinguishable from normal TBGLeu-283, we believe that the abnormal properties of TBG-A are due to substitution of alanine for threonine at residue 191.

Genetic analysis reveals different functions for the products of the thyroid hormone receptor alpha locus.

Thyroid hormone receptors are encoded by the TRalpha (NR1A1) and TRbeta (NR1A2) loci. These genes are transcribed into multiple variants whose functions are unclear. Analysis by gene inactivation in mice has provided new insights into the functional complexity of these products. Different strategies designed to modify the TRalpha locus have led to strikingly different phenotypes. In order to analyze the molecular basis for these alterations, we generated mice devoid of all known isoforms produced from the TRalpha locus (TRalpha(0/0)). These mice are viable and exhibit reduced linear growth, bone maturation delay, moderate hypothermia, and reduced thickness of the intestinal mucosa. Compounding TRalpha(0) and TRbeta(-) mutations produces viable TRalpha(0/0)beta(-/-) mice, which display a more severe linear growth reduction and a more profound hypothermia as well as impaired hearing. A striking phenotypic difference is observed between TRalpha(0/0) and the previously described TRalpha(-/-) mice, which retain truncated TRDeltaalpha isoforms arising from a newly described promoter in intron 7. The lethality and severe impairment of the intestinal maturation in TRalpha(-/-) mice are rescued in TRalpha(0/0) animals. We demonstrate that the TRDeltaalpha protein isoforms, which are natural products of the TRalpha locus, are the key determinants of these phenotypical differences. These data reveal the functional importance of the non-T3-binding variants encoded by the TRalpha locus in vertebrate postnatal development and homeostasis.