The structure of the human thyrotropin beta-subunit gene

A RECURRENT MUTATION IN TSHB GENE UNDERLYING CENTRAL CONGENITAL HYPOTHYROIDISM UNDETECTABLE IN NEONATAL SCREENING

Objective:

To describe the case of a patient with central congenital hypothyroidism (CCH) due to a recurrent mutation in the TSHB gene, as well as to conduct a genetic study of his family.

Case description:

It is presented a case report of a 5-month-old boy with a delayed diagnosis of isolated CCH in whom the molecular analysis was performed 12 years later and detected a recurrent mutation (c.373delT) in TSHB gene. The parents and sister were carriers of the mutant allele.

Comments:

The c.373delT mutation has previously been reported in patients from Brazil, Germany, Belgium, United States, Switzerland, Argentina, France, Portugal, United Kingdom and Ireland. In summary, our case and other ones reported in the literature support the theory that this mutation may be a common cause of isolated TSH deficiency. Isolated TSH deficiency is not detected by routine TSH-based neonatal screening, representing a clinical challenge. Therefore, when possible, molecular genetic study is indicated. Identification of affected and carriers allows the diagnosis, treatment and adequate genetic counseling.

Functional divergence of thyrotropin beta-subunit paralogs gives new insights into salmon smoltification metamorphosis

Smoltification is a metamorphic event in salmon life history, which initiates downstream migration and pre-adapts juvenile salmon for seawater entry. While a number of reports concern thyroid hormones and smoltification, few and inconclusive studies have addressed the potential role of thyrotropin (TSH). TSH is composed of a α-subunit common to gonadotropins, and a β-subunit conferring hormone specificity. We report the presence and functional divergence of duplicated TSH β-subunit paralogs (tshβa and tshβb) in Atlantic salmon. Phylogeny and synteny analyses allowed us to infer that they originated from teleost-specific whole genome duplication. Expression profiles of both paralogs in the pituitary were measured by qPCR throughout smoltification in Atlantic salmon from the endangered Loire-Allier population raised in a conservation hatchery. This revealed a striking peak of tshβb expression in April, concomitant with downstream migration initiation, while tshβa expression remained relatively constant. In situ hybridization showed two distinct pituitary cell populations, tshβa cells in the anterior adenohypophysis, and tshβb cells near to the pituitary stalk, a location comparable to the pars tuberalis TSH cells involved in seasonal physiology and behaviour in birds and mammals. Functional divergence of tshβ paralogs in Atlantic salmon supports a specific role of tshβb in smoltification.

Novel Splicing of Immune System Thyroid Stimulating Hormone β-Subunit-Genetic Regulation and Biological Importance

Thyroid stimulating hormone (TSH), a glycoprotein hormone produced by the anterior pituitary, controls the production of thyroxine (T₄) and triiodothyronine (T₃) in the thyroid. TSH is also known to be produced by the cells of the immune system; however, the physiological importance of that to the organism is unclear. We identified an alternatively-spliced form of TSHβ that is present in both humans and mice. The TSHβ splice variant (TSHβv), although produced at low levels by the pituitary, is the primary form made by hematopoietic cells in the bone marrow, and by peripheral leukocytes. Recent studies have linked TSHβv functionally to a number of health-related conditions, including enhanced host responses to infection and protection against osteoporosis. However, TSHβv also has been associated with autoimmune thyroiditis in humans. Yet to be identified is the process by which the TSHβv isoform is produced. Here, a set of genetic steps is laid out through which human TSHβv is generated using splicing events that result in a novel transcript in which exon 2 is deleted, exon 3 is retained, and the 3' end of intron 2 codes for a signal peptide of the TSHβv polypeptide.

Minireview: Insights Into the Structural and Molecular Consequences of the TSH-β Mutation C105Vfs114X

Naturally occurring thyrotropin (TSH) mutations are rare, which is also the case for the homologous heterodimeric glycoprotein hormones (GPHs) follitropin (FSH), lutropin (LH), and choriogonadotropin (CG). Patients with TSH-inactivating mutations present with central congenital hypothyroidism. Here, we summarize insights into the most frequent loss-of-function β-subunit of TSH mutation C105Vfs114X, which is associated with isolated TSH deficiency. This review will address the following question. What is currently known on the molecular background of this TSH variant on a protein level? It has not yet been clarified how C105Vfs114X causes early symptoms in affected patients, which are comparably severe to those observed in newborns lacking any functional thyroid tissue (athyreosis). To better understand the mechanisms of this mutant, we have summarized published reports and complemented this information with a structural perspective on GPHs. By including the ancestral TSH receptor agonist thyrostimulin and pathogenic mutations reported for FSH, LH, and choriogonadotropin in the analysis, insightful structure function and evolutionary restrictions become apparent. However, comparisons of immunogenicity and bioactivity of different GPH variants is hindered by a lack of consensus for functional analysis and the diversity of used GPH assays. Accordingly, relevant gaps of knowledge concerning details of GPH mutation-related effects are identified and highlighted in this review. These issues are of general importance as several previous and recent studies point towards the high impact of GPH variants in differential signaling regulation at GPH receptors (GPHRs), both endogenously and under diseased conditions. Further improvement in this area is of decisive importance for the development of novel targeted therapies.

A novel deletion in the thyrotropin Beta-subunit gene identified by array comparative genomic hybridization analysis causes central congenital hypothyroidism in a boy originating from Turkey

BACKGROUND

Isolated central congenital hypothyroidism (ICCH) is rare but important. Most ICCH patients are diagnosed later, which results in severe growth failure and intellectual disability.

OBJECTIVE

We describe a boy with ICCH due to a large homozygous TSHβ gene deletion.

RESULTS

A 51-day-old male Turkish infant, whose parents were first cousins, was admitted for evaluation of prolonged jaundice. His clinical appearance was compatible with hypothyroidism. Venous thyrotropin (TSH) was undetectably low, with a subsequent low free T4 and a low free T3, suggestive of central hypothyroidism. Using different PCR protocols, we could not amplify both coding exons of the boy's TSHβ gene, which suggested a deletion. An array comparative genomic hybridization (aCGH) using specific probes around the TSHβ gene locus showed him to be homozygous for a 6-kb deletion spanning all exons and parts of the 5' untranslated region of the gene.

CONCLUSIONS

Infants who are clinically suspected of having hypothyroidism should be evaluated thoroughly, even if their TSH-based screening result is normal. In cases with ICCH and undetectably low TSH serum concentrations, a TSHβ gene deletion should be considered; aCGH should be performed when gene deletions are suspected. In such cases, PCR-based sequencing techniques give negative results.

Historical note: many steps led to the 'discovery' of thyroid-stimulating hormone

Finding thyroid-stimulating hormone was a process rather than a circumscribed event, and many talented persons participated over many years. Key early participants were Bennet M. Allen and Philip E. Smith who had the misfortune just prior to World War I of independently and simultaneously starting very similar experiments with tadpoles. This led to a series of back and forth publications attempting to establish priority for finding evidence of a thyrotropic factor in the anterior pituitary. Decades of work by others would be required before sophisticated biochemical techniques would bring us to our modern understanding.

Zebrafish as a model for monocarboxyl transporter 8-deficiency

Allan-Herndon-Dudley syndrome (AHDS) is a severe psychomotor retardation characterized by neurological impairment and abnormal thyroid hormone (TH) levels. Mutations in the TH transporter, monocarboxylate transporter 8 (MCT8), are associated with AHDS. MCT8 knock-out mice exhibit impaired TH levels; however, they lack neurological defects. Here, the zebrafish mct8 gene and promoter were isolated, and mct8 promoter-driven transgenic lines were used to show that, similar to humans, mct8 is primarily expressed in the nervous and vascular systems. Morpholino-based knockdown and rescue experiments revealed that MCT8 is strictly required for neural development in the brain and spinal cord. This study shows that MCT8 is a crucial regulator during embryonic development and establishes the first vertebrate model for MCT8 deficiency that exhibits a neurological phenotype.

Congenital isolated central hypothyroidism caused by a "hot spot" mutation in the thyrotropin-beta gene

Two adult siblings presented to our practice with a known history of congenital central isolated hypothyroidism. Their growth, development, and general health had been normal. Although the disorder was known to result from thyrotropin (TSH) deficiency, providers in the past had made multiple adjustments in their levothyroxine replacement doses in attempts to normalize serum TSH levels. This suggests a need for better education of providers who care for patients with central hypothyroidism. We performed DNA sequencing of the TSHbeta gene and identified a homozygous single base deletion in codon 105, on exon 3, resulting in a frameshift and a premature termination signal at codon 114. This same mutation (C105FS114X) has been previously reported in South America and Europe and appears to be the most common genetic mutation associated with congenital isolated TSH deficiency. The identification of this mutation for the first time in the United States suggests that this disorder, now described in patients from countries on multiple continents, is more common than previously appreciated and may be a mutational "hot spot."

Targeting oncogenesis by introduction of a 5.2-kbp segment of the 5' regulatory region of the human thyrotropin beta-subunit gene

We produced transgenic mice carrying a fusion gene (TTP-5) consisting of a 5.2-kbp segment of the 5' flanking sequence of the human thyrotropin beta-subunit (TSH beta) gene linked to the simian virus 40 large T antigen (SVT) gene. These mice developed pituitary tumors 6 months after birth and wasted away. With the 5.2-kbp TSH beta 5' flanking region governing SVT expression, SVT mRNA was present in the pituitary and testis but not in other tissues, as detected by the reverse transcriptase-polymerase chain reaction. Histological and immunohistochemical analyses showed that the pituitary tumors of the transgenic mice were composed of moderately differentiated pituitary cells that expressed TSH, growth hormone, and prolactin. These results indicate that the 5.2-kbp segment of the human TSH beta 5' regulatory region is sufficient to drive expression of SVT and induce tumorigenesis of hormone-producing pituitary cells in transgenic mice.

Canine thyrotropin beta-subunit gene: cloning and expression in Escherichia coli, generation of monoclonal antibodies, and transient expression in the Chinese hamster ovary cells

The gene encoding the mature beta subunit of canine thyroid stimulating hormone (cTSH beta) was cloned, sequenced and expressed in Escherichia coli and in Chinese hamster ovary (CHO) cells, and monoclonal antibodies against the recombinant cTSH beta purified from E. coli were generated. The gene fragment that encodes mature TSH beta was cloned from the canine genomic DNA by direct polymerase chain reaction (PCR) using primers that were designed based on the consensus sequences from other species. The resulting 891 basepairs (bp) of genomic DNA consisted of two coding exons of the canine TSH beta gene and an intron of 450 bp. The two exons, which encode the mature cTSH beta subunit, was joined together by an overlap PCR and was expressed in E. coli as 6xHis-tagged protein. The purified recombinant cTSH beta with a molecular weight of about 15 kDa was recognized by the polyclonal antibodies prepared against the native canine TSH in Western blot. Monoclonal antibodies were raised against the purified cTSH beta and subsequently characterized. For transient expression in CHO cells that are permanently transfected with the bovine common alpha gene, a 60-oligonucleotide signal peptide coding sequence was added to the 5' end of the cTSH beta gene before it was cloned into the mammalian expression vector pRSV and used to transfect CHO cells. The medium from these transfected cells, presumably containing the bovine alpha and canine TSH beta in heterodimeric confirmation, exhibited TSH bioactivity as indicated by the stimulation of cAMP production in the cultured FRTL-5 thyrocytes.

Immortalized human pituitary cells express glycoprotein alpha-subunit and thyrotropin beta (TSH beta)

A major problem in the study of human pituitary cells is their lack of proliferative capacity in vitro. To address this issue, we have infected normal human, postmortem pituitary cells in monolayer culture with a temperature-sensitive (tsA58) mutant of SV40 large T antigen. Several epithelial-like colonies were isolated; and one, designated CHP2, has been studied in detail to identify its functional characteristics. CHP2 cells have undergone more than 150 culture passages and retain an epithelial morphology. They exhibit tight temperature-dependent growth, in the presence and absence of serum, with cell division at 33 C and growth inhibition at 39 C. CHP2 cells, at both temperatures, showed diffuse immunostaining for human alpha-subunit and focal staining for TSH beta. Gene expression was confirmed by RT-PCR and sequencing. TRH and GnRH receptors were not detectable, and their absence was confirmed by their lack of effects on intracellular calcium and inositol phospholipids. Cytogenetic analysis showed that the cells had a modal peak in the diploid range and a smaller peak in the tetraploid range. There was also a consistent loss of chromosome 22 and a normal chromosome 2 homologue, the latter being replaced by one of two chromosome 2 markers, M2A or M2B. In conclusion, we have immortalized human pituitary cells using SV40 tsT, from which we have cloned a cell line expressing alpha-subunit and TSH beta.

Thyrotropin expression in hypophyseal pars tuberalis-specific cells is 3,5,3'-triiodothyronine, thyrotropin-releasing hormone, and pit-1 independent

The expression of TSH subunit genes (TSH alpha and -beta) in pituitary thyrotropes is primarily regulated via circulating thyroid hormone levels (T3) and the hypothalamic TRH. Hypophyseal pars tuberalis (PT)-specific cells also express both hormonal subunits of TSH, but do not resemble thyrotropes of the pars distalis (PD) with respect to their distinct morphology, secretion, and direct modulation of TSH expression by photoperiodic inputs and melatonin. To investigate whether this distinct regulation of TSH is related to a different molecular structure or different signaling cascades, we analyzed PT-specific TSH and its transcriptional regulation in ovine PT-specific cells. After construction of PT- and PD-specific complementary DNA (cDNA) libraries, the cloning and sequencing of several TSH alpha and -beta subunit clones revealed identical sizes and sequences for the translated and untranslated regions in both hypophyseal compartments. Transcription start site analysis also displayed three identical start sites for the transcription of TSH beta in PT and PD. After cloning of the ovine TRH receptor cDNA and a partial T3 receptor cDNA, in situ hybridization. Northern blot analysis, and PCR experiments showed that TRH and T3 receptors are not expressed in specific cells of the PT. The transcription factor Pit-1 that is involved in TSH expression of thyrotropes could only be detected in the PD. In additional experiments rats were treated with T4 or TRH, and subsequent in situ hybridization studies showed that TSH beta messenger RNA (mRNA) formation was not altered in the PT. In the PD, however, TSH beta mRNA was significantly reduced in the T4-treated group, but was enhanced in the TRH-treated group. We conclude that PT-specific cells of the pituitary are characterized by the transcription of TSH subunits that are identical to TSH expressed in thyrotropes of the PD. The absence of TRH, T3 receptor mRNA, and Pit-1, respectively, as well as the different reactions compared to PD thyrotropes in in vivo experiments lead to the conclusion that the expression of TSH in PT-specific cells of the pituitary is not regulated via the classical thyrotrope receptors and their intracellular pathways, but through a novel, photoperiod-dependent mechanism.

The regulatory region and transcription factor required for the expression of rat and salmon pituitary hormone-encoding genes show cell-type and species specificity

The promoter regions of the genes encoding the rat and chum salmon growth hormones (GH) and rat prolactin (PRL) were combined with a reporter gene and introduced into GH- and/or PRL-producing cells from rat. The rat GH and PRL promoters (pGH and pPRL, respectively) were most active in cells producing GH and PRL, respectively. The activity of the salmon pGH was much less than that of the rat pGH in rat GH-producing cells. The regulatory region required for cell-type-specific gene expression of pituitary hormones thus contains information, not only for cell-type specificity, but possibly for species specificity as well. A reporter plasmid containing the GH or somatolactin (SL) promoter and an effector plasmid having a gene encoding transcription factor Pit-1 (rat or salmon) were cotransfected into HeLa (human) or EPC (carp) cells. Rat and salmon Pit-1 were more active in HeLa and EPC cells, respectively, indicating that Pit-1 appears to interact species specifically with the transcription machinery.

Targeted pituitary tumorigenesis using the human thyrotropin beta-subunit chain promoter in transgenic mice

We have generated transgenic mice that express the simian virus 40 (SV40) large T antigen under the control of a 1109 bp 5'-flanking sequence of the human thyrotropin beta-subunit (TSH beta) gene. The hybrid gene, termed TTP-1, was microinjected into fertilized mouse eggs and 11 transgenic mice were obtained. One of the transgenic mice, a female, a phenotypical dwarf, developed a pituitary tumor and wasted away from 7 to 9 weeks after birth. To establish the transgenic mouse line, her ovaries were transferred to a normal female, whose ovaries were removed beforehand. To examine the tissue specificity of transgene expression, mRNA of SV40 large T antigen was monitored in various tissues from the transgenic mice by the reverse transcriptase-polymerase chain reaction analysis, and was detected only in the pituitary. Histological and immunohistochemical analyses showed that the pituitary tumors of the transgenic mice were composed of poorly differentiated pituitary cells expressing SV40 large T antigen. These results indicated that the 1109 bp sequence of the human TSH beta 5'-flanking region is essential for pituitary-specific expression of SV40 large T antigen in transgenic mice, which exhibited a dwarf phenotype and developed pituitary tumors. The tumors were composed of undifferentiated cells and did not produce thyrotropin. These transgenic mice should provide a valuable animal model for studying the pathogenesis of anterior pituitary tumors.

Thyroid hormone regulation of thyrotropin gene expression

Thyroid hormones suppress the synthesis and release of thyrotropin from thyrotropes in the anterior pituitary gland, a feature that is critical in the classic negative-feedback loop of the pituitary-thyroid endocrine axis. The major effect of thyroid hormones in this system is exerted at the transcriptional level. The molecular mechanisms by which there is negative regulation of TSH subunit gene expression by thyroid hormone have been elucidated. The TSH subunit genes have isolated and characterized. Structure-function analyses using fusion genes and DNA transfection approaches have defined the putative negative TREs among the promoters of the rat, mouse, and human alpha and TSH beta genes. These sequences are either largely overlapping direct TRE half-sites, TRE half-sites as direct repeats gapped by two nucleotides, or single TRE half-sites. These arrangements are distinct from those seen in positive TREs. Recent knowledge regarding the molecular mechanisms of thyroid action in general forces consideration of multiple TR isoforms, TR heterodimer partners (TRAPs), and thyroid hormones in the ultimate mechanisms of negative action. Several models have been proposed, but none has yet been proved. In addition, the role of thyroid hormone in the regulation of gene expression at the posttranscriptional level is beginning to be addressed. Future work should continue to illuminate these important facets of gene regulation.

Nucleotide sequence of the complementary DNA for human Pit-1/GHF-1

Human cDNA clones encoding Pit-1/GHF-1, a pituitary-specific DNA binding factor, were obtained by PCR following reverse transcription of human pituitary RNA. It is approx. 1.3 kb in size with 0.1 kb 5' non-coding region, 0.9 kb protein-coding region and 0.3 kb 3' non-coding region. The predicted human Pit-1/GHF-1 peptide structure has 291 amino acids and is highly conserved among mouse, rat and bovine. In addition, the 5' non-coding region is highly conserved with rat pit-1/GHF-1 sequence to the transcription start site.

Immunological and biological characteristics of recombinant human thyrotropin

Analyses of epitopes and biological activity were made on two preparations of recombinant human thyrotropin produced in Chinese hamster ovary cells. Seven monoclonal antibodies recognizing four epitopes on alpha subunit of hTSH (hTSH alpha) and three on beta subunit (hTSH beta) were used for the analysis. Binding activities of the two rhTSH with each antibody were almost identical with that of a pituitary-derived reference hTSH, except at one epitope on hTSH alpha. Their immunoreactivity measured by four commercial immunoassay kits and their bioactivity by thyrotropin dependent FRTL-5 cell system, however, agreed closely with those of the reference hTSH. From these results and its constant availability, rhTSH will be a good candidate for a future standard material in assays for hTSH.

Cloning and analysis of the 5'-flanking region of rat Na+/K(+)-ATPase alpha 1 subunit gene

We cloned a 13.3 kilobase (kb) fragment of genomic DNA spanning at least the first two exons of the rat Na+/K(+)-ATPase alpha 1 subunit gene (NKAA1) and 1.5 kb of the 5'-flanking region. S1 nuclease mapping analysis of the 5' end of the Na+/K(+)-ATPase mRNA indicated that the transcription initiation site was located 262 base pairs (bp) upstream of the translation initiation codon. The transcription initiation site of the Na+/K(+)-ATPase alpha 1 subunit gene was identical among six tissues of adult rat (kidney, brain, heart, thyroid, liver and lung). A TATA-box-like sequence (at position -32), two Sp1 factor binding sequences (-137, -56), an active transcription factor consensus binding sequence (-71) and two glucocorticoid-responsive element half consensus sequences (-750, -481) were found in the 5'-flanking region. The sequence of the first exon and the 5'-flanking region of the rat NKAA1 was 63% homologous to that of the horse equivalent. Maximum homology (82%) between the two genes was observed in the region from 361 bp upstream of the translation initiation site to the 3' end of the first exon. The TATA-like box, Sp1 binding site and the active transcriptional factor (ATF) consensus site in this region were conserved in both rat and horse.