Roy E. Weiss

Different functions for the thyroid hormone receptors TRα and TRβ in the control of thyroid hormone production and post‐natal development

The biological activities of thyroid hormones are thought to be mediated by receptors generated by the TRα and TRβ loci. The existence of several receptor isoforms suggests that different functions are mediated by specific isoforms and raises the possibility of functional redundancies. We have inactivated both TRα and TRβ genes by homologous recombination in the mouse and compared the phenotypes of wild‐type, and single and double mutant mice. We show by this method that the TRβ receptors are the most potent regulators of the production of thyroid stimulating hormone (TSH). However, in the absence of TRβ, the products of the TRα gene can fulfill this function as, in the absence of any receptors, TSH and thyroid hormone concentrations reach very high levels. We also show that TRβ, in contrast to TRα, is dispensable for the normal development of bone and intestine. In bone, the disruption of both TRα and TRβ genes does not modify the maturation delay observed in TRα−/− mice. In the ileum, the absence of any receptor results in a much more severe impairment than that observed in TRα−/− animals. We conclude that each of the two families of proteins mediate specific functions of triiodothyronin (T3), and that redundancy is only partial and concerns a limited number of functions.

Mutations in SECISBP2 result in abnormal thyroid hormone metabolism

Incorporation of selenocysteine (Sec), through recoding of the UGA stop codon, creates a unique class of proteins. Mice lacking tRNASec die in utero, but the in vivo role of other components involved in selenoprotein synthesis is unknown, and Sec incorporation defects have not been described in humans. Deiodinases (DIOs) are selenoproteins involved in thyroid hormone metabolism. We identified three of seven siblings with clinical evidence of abnormal thyroid hormone metabolism. Their fibroblasts showed decreased DIO2 enzymatic activity not linked to the DIO2 locus. Systematic linkage analysis of genes involved in DIO2 synthesis and degradation led to the identification of an inherited Sec incorporation defect, caused by a homozygous missense mutation in SECISBP2 (also called SBP2). An unrelated child with a similar phenotype was compound heterozygous with respect to mutations in SECISBP2. Because SBP2 is epistatic to selenoprotein synthesis, these defects had a generalized effect on selenoproteins. Incomplete loss of SBP2 function probably causes the mild phenotype.

Mice deficient in the steroid receptor co‐activator 1 (SRC‐1) are resistant to thyroid hormone

Steroid receptor co‐activator 1 (SRC‐1) is a transcription co‐factor that enhances the hormone‐dependent action, mediated by the thyroid hormone (TH) receptor (TR) and other nuclear receptors. In vitro studies have shown that SRC‐1 is necessary for the full expression of TH effect. SRC‐1 knockout mice (SRC‐1−/−) provide a model to examine the role of this co‐activator on TH action in vivo. At baseline, SRC‐1−/− mice display resistance to TH (RTH) as evidenced by a 2.5‐fold elevation of serum TSH levels, despite a 50% increase in serum free TH levels as compared with wild‐type (SRC‐1+/+) mice. When mice were made hypothyroid, TSH levels increased, obliterating the difference between SRC‐1+/+ and SRC‐1−/− mice observed at baseline. In contrast, the decline of TSH by treatment with L‐triiodothyronine was severely blunted in SRC‐1−/− mice. These data indicate that SRC‐1 is not required for the upregulation of TSH in TH deficiency. However, SRC‐1 enhances the sensitivity of TSH downregulation by TH. This is the first demonstration of RTH caused by a deficient co‐factor other than TR. It supports the hypothesis that a putative defect in the SRC‐1 gene or another co‐factor could be the cause of RTH in humans without mutations in the TR genes.

Partial deficiency of Thyroid transcription factor 1 produces predominantly neurological defects in humans and mice

Three genes, TTF1, TTF2, and PAX8, involved in thyroid gland development and migration have been identified. Yet systematic screening for defects in these genes in thyroid dysgenesis gave essentially negative results. In particular, no TTF1 gene defects were found in 76 individuals with thyroid dysgenesis even though a deletion of this gene in the mouse results in thyroid and lung agenesis and defective diencephalon. We report a 6-year-old boy with predominant dyskinesia, neonatal respiratory distress, and mild hyperthyrotropinemia. One allele of his TTF1 gene had a guanidine inserted into codon 86 producing a nonsense protein of 407, rather than 371, amino acids. The mutant TTF1 did not bind to its canonical cis-element or transactivate a reporter gene driven by the thyroglobulin promoter, a natural target of TTF1. Failure of the mutant TTF1 to interfere with binding and transactivation functions of the wild-type TTF1 suggested that the syndrome was caused by haploinsufficiency. This was confirmed in mice heterozygous for Ttf1 gene deletion, heretofore considered to be normal. Compared with wild-type littermates, Ttf1(+/-) mice had poor coordination and a significant elevation of serum thyrotropin. Therefore, haploinsufficiency of the TTF1 gene results in a predominantly neurological phenotype and secondary hyperthyrotropinemia.

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.

Mice deficient in MCT8 reveal a mechanism regulating thyroid hormone secretion

The mechanism of thyroid hormone (TH) secretion from the thyroid gland into blood is unknown. Humans and mice deficient in monocarboxylate transporter 8 (MCT8) have low serum thyroxine (T4) levels that cannot be fully explained by increased deiodination. Here, we have shown that Mct8 is localized at the basolateral membrane of thyrocytes and that the serum TH concentration is reduced in Mct8-KO mice early after being taken off a treatment that almost completely depleted the thyroid gland of TH. Thyroid glands in Mct8-KO mice contained more non-thyroglobulin-associated T4 and triiodothyronine than did those in wild-type mice, independent of deiodination. In addition, depletion of thyroidal TH content was slower during iodine deficiency. After administration of 125I, the rate of both its secretion from the thyroid gland and its appearance in the serum as trichloroacetic acid-precipitable radioactivity was greatly reduced in Mct8-KO mice. Similarly, the secretion of T4 induced by injection of thyrotropin was reduced in Mct8-KO in which endogenous TSH and T4 were suppressed by administration of triiodothyronine. To our knowledge, this study is the first to demonstrate that Mct8 is involved in the secretion of TH from the thyroid gland and contributes, in part, to the low serum T4 level observed in MCT8-deficient patients.

Attention-deficit hyperactivity disorder and thyroid function

Attention-deficit hyperactivity disorder (ADHD) is thought to have a biologic basis, but the precise cause is unknown. It is one of the neurodevelopmental abnormalities frequently observed in children with generalized resistance to thyroid hormone (GRTH), suggesting that thyroid abnormalities may be related to ADHD. We report a prospective screening study for thyroid abnormalities in 277 children with ADHD by measurement of serum levels of total thyroxine, free thyroxine index, and thyrotropin. Fourteen children with ADHD had thyroid function test abnormalities: six had a normal free thyroxine index and elevated thyroxine level (group 1); three had a high free thyroxine index and a normal thyrotropin level (group 2); and five had a low free thyroxine index with a normal thyrotropin level (group 3). GRTH could not be demonstrated in a detailed study of four of the subjects in whom it was suspected (groups 1 and 2). Although the prevalence of ADHD in subjects with GRTH has been reported to be 46%, the overall prevalence of GRTH must be less than 1:2500 because we failed to detect GRTH in the 277 children with ADHD studied. We conclude that the prevalence of thyroid abnormalities is higher (5.4%) in children with ADHD than in the normal population (< 1%).

Effects of ligand and thyroid hormone receptor isoforms on hepatic gene expression profiles of thyroid hormone receptor knockout mice.

Little is known about the overall patterns of thyroid hormone (Th)‐mediated gene regulation by the main Th receptor (Tr) isoforms, Tr‐α and Tr‐β, in vivo. We used 48 complementary DNA microarrays to examine hepatic gene expression profiles of wild‐type and Thra and Thrb knockout mice under different Th conditions: no treatment, treatment with 3,3′,5‐triiodothyronine (T3), Th‐deprivation using propylthiouracil (PTU), and treatment with a combination of PTU and T3. Hierarchical clustering analyses showed that positively regulated genes fit into three main expression patterns. In addition, only a subpopulation of target genes repressed basal transcription in the absence of ligand. Interestingly, Thra and Thrb knockout mice showed similar gene expression patterns to wild‐type mice, suggesting that these isoforms co‐regulate most hepatic target genes. Differences in the gene expression patterns of Thra/Thrb double‐knockout mice and Th‐deprived wild‐type mice show that absence of receptor and of hormone can have different effects. This large‐scale study of hormonal regulation reveals the functions of Th and of Tr isoforms in the regulation of gene expression patterns.

Thyrotropin Regulation by Thyroid Hormone in Thyroid Hormone Receptor β-Deficient Mice1

Thyroid hormone responsive genes can be both positively and negatively regulated by thyroid hormone. TSH is down-regulated by thyroid hormone and rises during thyroid hormone deprivation. Because both thyroid hormone receptor (TR) α and β genes are expressed in the pituitary gland, it is unclear what the relative roles of TRα and TRβ are in TSH regulation. Experiments using over expression of artificial genes have yielded conflicting results. The TRβ knock-out mouse that lacks both TRβ1 and TRβ2 isoforms provides a model to examine the role of these receptors in TSH regulation. TRβ deficient (TRβ−/−) and wild-type (TRβ+/+) mice of the same strain were deprived of thyroid hormone by feeding them a low iodine diet containing propylthiouracil and were then treated with different doses of L-T3 and L-T4. Thyroid hormone deprivation rapidly increased the serum TSH level in both TRβ+/+ and TRβ−/− mice, reaching a similar level in the absence of thyroid hormone. In contrast, the decline of serum TSH by treatment ...

EFFECT OF THYROID HORMONE ON GROWTH: Lessons from the Syndrome of Resistance to Thyroid Hormone

Thyroid hormone deprivation results in deleterious effects on bone growth. The delayed bone development is mediated by a direct effect of thyroid hormone on bone and an indirect effect of the hormone on GH release and IGF-1 action. Both TR alpha and TR beta are expressed in bone cells. To examine the role of TR beta on bone, we have reviewed the growth abnormalities in the human syndrome of RTH caused by mutations in the TR beta gene. The mutant TR beta reduces the tissue responsiveness to thyroid hormone, producing in some tissues variable degrees of thyroid hormone deprivation. With regard to bone, relative thyroid hormone deficiency caused by the mutant TR beta produces short stature and delayed bone growth but does not attenuate growth to the extent that absolute thyroid hormone deficiency does. These observations indicate that an intact TR beta is required for normal bone development and growth.