PTTG-binding factor (PBF) is a novel regulator of the thyroid hormone transporter MCT8

Within the basolateral membrane of thyroid follicular epithelial cells, two transporter proteins are central to thyroid hormone (TH) biosynthesis and secretion. The sodium iodide symporter (NIS) delivers iodide from the bloodstream into the thyroid, and after TH biosynthesis, monocarboxylate transporter 8 (MCT8) mediates TH secretion from the thyroid gland. Pituitary tumor-transforming gene-binding factor (PBF; PTTG1IP) is a protooncogene that is up-regulated in thyroid cancer and that binds NIS and modulates its subcellular localization and function. We now show that PBF binds MCT8 in vitro, eliciting a marked shift in MCT8 subcellular localization and resulting in a significant reduction in the amount of MCT8 at the plasma membrane as determined by cell surface biotinylation assays. Colocalization and interaction between PBF and Mct8 was also observed in vivo in a mouse model of thyroid-specific PBF overexpression driven by a bovine thyroglobulin (Tg) promoter (PBF-Tg). Thyroidal Mct8 mRNA and protein expression levels were similar to wild-type mice. Critically, however, PBF-Tg mice demonstrated significantly enhanced thyroidal TH accumulation and reduced TH secretion upon TSH stimulation. Importantly, Mct8-knockout mice share this phenotype. These data show that PBF binds and alters the subcellular localization of MCT8 in vitro, with PBF overexpression leading to an accumulation of TH within the thyroid in vivo. Overall, these studies identify PBF as the first protein to interact with the critical TH transporter MCT8 and modulate its function in vivo. Furthermore, alongside NIS repression, PBF may thus represent a new regulator of TH biosynthesis and secretion.     

Novel mutation in MCT8 gene in a Brazilian boy with thyroid hormone resistance and severe neurologic abnormalities

MCT8 is a cellular transporter of thyroid hormones important in their action and metabolization. We report a male patient with the novel inactivating mutation 630insG in the coding region in exon 1 of MCT8. He was characterized clinically by severe neurologic impairment (initially with global hypotonia, later evolving with generalized hypertonia), normal growth during infancy, reduced weight gain, and absence of typical signs and symptoms of hypothyroidism, while the laboratory evaluation disclosed elevated T3, low total and free T4, and mildly elevated TSH serum levels. Treatment with levothyroxine improved thyroid hormone profile but was not able to alter the clinical picture of the patient. These data reinforce the concept that the role of MCT8 is tissue-dependent: while neurons are highly dependent on MCT8, bone tissue, adipose tissue, muscle, and liver are less dependent on MCT8 and, therefore, may suffer the consequences of the exposition to high serum T3 levels.     

甲状腺激素受体β基因V458A点突变所致甲状腺激素抵抗综合征

目的研究1例甲状腺激素抵抗综合征患者的甲状腺激素受体β(THRB)的基因型。方法收集1例甲状腺激素抵抗综合征患者及其父母外周血标本,提取基因组DNA后,应用PCR技术和直接测序法了解患者及其父母THRB基因有无突变。结果患者THRB基因外显子1～9无突变,第10号外显子在第458个密码子处有点突变(V458A),且为杂合子错义突变。患者父母THRB基因无突变。结论经基因诊断证实患者THRB基因存在V458A突变,突变位于THRB配体结合区。     

Retarded bone growth in thyroid hormone resistance. A clinical study of a large family with a novel thyroid hormone receptor mutation

OBJECTIVE: Thyroid hormone resistance (RTH) is characterised by variable tissue hyporesponsiveness to thyroid hormone. The disorder is usually caused by mutations in the thyroid hormone receptor beta (TR beta). We describe a large family with this disorder. SUBJECTS AND MEASUREMENT: We identified 36 family members with RTH in four generations by screening relatives of patients with the diagnosis. The diagnosis was verified by identification of a mutation in the thyroid hormone receptor beta (TR beta) gene. Symptoms, clinical findings and laboratory tests of 29 affected individuals were compared with those of 16 first-degree relatives. RESULTS: Bone maturation in children with RTH was delayed. The height was lower both in children and in adults with RTH than in the controls. Children with RTH had lower birth weight than the controls, particularly when the condition was inherited from the father. We did not observe increased prevalence of neuropsychological symptoms associated with RTH in this family. Palpitations and increased pulse rate indicated mild cardiac hyperthyroidism. Direct sequence analysis of the TR beta gene revealed a novel point mutation, a heterozygous transition c.1031G>C in exon 9 theoretically substituting Gly344Ala. CONCLUSIONS: We found evidence of skeletal tissue hypothyroidism that resulted in permanent growth retardation from prenatal to adult life. We found substantial variations in thyroid hormone levels and clinical presentation, but most individuals were without symptoms of thyroid disorder.     

Identification and functional characterization of zebrafish solute carrier Slc16a2 (Mct8) as a thyroid hormone membrane transporter

Most components of the thyroid system in bony fish have been described and characterized, with the notable exception of thyroid hormone membrane transporters. We have cloned, sequenced, and expressed the zebrafish solute carrier Slc16a2 (also named monocarboxylate transporter Mct8) cDNA and established its role as a thyroid hormone transport protein. The cloned cDNA shares 56-57% homology with its mammalian orthologs. The 526-amino-acid sequence contains 12 predicted transmembrane domains. An intracellular N-terminal PEST domain, thought to be involved in proteolytic processing of the protein, is present in the zebrafish sequence. Measured at initial rate and at the body/rearing temperature of zebrafish (26 C), T(3) uptake by zebrafish Slc16a2 is a saturable process with a calculated Michaelis-Menten constant of 0.8 μM T(3). The rate of T(3) uptake is temperature dependent and Na(+) independent. Interestingly, at 26 C, zebrafish Slc16a2 does not transport T(4). This implies that at a normal body temperature in zebrafish, Slc16a2 protein is predominantly involved in T(3) uptake. When measured at 37 C, zebrafish Slc16a2 transports T(4) in a Na(+)-independent manner. In adult zebrafish, the Slc16a2 gene is highly expressed in brain, gills, pancreas, liver, pituitary, heart, kidney, and gut. Beginning from the midblastula stage, Slc16a2 is also expressed during zebrafish early development, the highest expression levels occurring 48 h after fertilization. This is the first direct evidence for thyroid hormone membrane transporters in fish. We suggest that Slc16a2 plays a key role in the local availability of T(3) in adult tissues as well as during the completion of morphogenesis of primary organ systems.     

Resistance to thyroid hormone caused by a new mutation (V336M) in the thyroid hormone receptor beta gene.

Resistance to thyroid hormone (RTH), usually caused by an inherited defect of the thyroid hormone receptor, (TRbeta), results in a reduced responsiveness of target tissues to thyroid hormone. Until now, more than 600 cases with RTH have been identified. Although usually linked to the TRbeta gene, located on chromosome 3, RTH may also occur in the absence of mutations in the coding region of this gene. We report a 10-month-old boy who had laboratory findings typical of RTH. He was born prematurely on the 34th week of gestation and his thyrotropin (TSH) during neonatal screening was 121 microU/mL, a value very high for RTH or prematurity. Direct sequencing of the TRbeta gene from the patient's genomic DNA revealed a heterozygous substitution of the normal valine with a mutant methionine in codon 336 (V336M) that has not been previously reported. In vitro expression studies showed that this mutant TRbeta has an impaired triiodothyronine (T3)-dependent transactivation that reduces the activity of the wild-type TRbeta (dominant negative effect). While the functional impairment of V336M is not unusual compared to other TRbeta gene mutations, the very high TSH value in this prematurely born infant suggests that fetuses with RTH have altered maturation of the hypothalamo-pituitary-thyroid axis or actually may suffer from hypothyroidism.     

Novel mutations in SLC16A2 associated with a less severe phenotype of MCT8 deficiency

Metabolic Brain Disease - Mutations in the thyroid hormone transporter MCT8 cause severe intellectual and motor disability and abnormal serum thyroid function tests, a syndrome known as MCT8...     

From the Cover: A thyroid hormone receptor mutation that dissociates thyroid hormone regulation of gene expression in vivo

Resistance to thyroid hormone (RTH) is most often due to point mutations in the β-isoform of the thyroid hormone (TH) receptor (TR-β). The majority of mutations involve the ligand-binding domain, where they block TH binding and receptor function on both stimulatory and inhibitory TH response elements. In contrast, a few mutations in the ligand-binding domain are reported to maintain TH binding and yet cause RTH in certain tissues. We introduced one such naturally occurring human RTH mutation (R429Q) into the germline of mice at the TR-β locus. R429Q knock-in (KI) mice demonstrated elevated serum TH and inappropriately normal thyroid-stimulating hormone (TSH) levels, consistent with hypothalamic–pituitary RTH. In contrast, 3 hepatic genes positively regulated by TH (Dio1, Gpd1, and Thrsp) were increased in R429Q KI animals. Mice were then rendered hypothyroid, followed by graded T₃ replacement. Hypothyroid R429Q KI mice displayed elevated TSH subunit mRNA levels, and T₃ treatment failed to normally suppress these levels. T₃ treatment, however, stimulated pituitary Gh levels to a greater degree in R429Q KI than in control mice. Gsta, a hepatic gene negatively regulated by TH, was not suppressed in R429Q KI mice after T₃ treatment, but hepatic Dio1 and Thrsp mRNA levels increased in response to TH. Cardiac myosin heavy chain isoform gene expression also showed a specific defect in TH inhibition. In summary, the R429Q mutation is associated with selective impairment of TH-mediated gene repression, suggesting that the affected domain, necessary for TR homodimerization and corepressor binding, has a critical role in negative gene regulation by TH.     

A point mutation in the 3,5,3'-triiodothyronine-binding domain of thyroid hormone receptor-beta associated with a family with generalized resistance to thyroid hormone.

A tight linkage between generalized resistance to thyroid hormone (GRTH) and the thyroid hormone receptor-beta (TR beta) gene is indicated. We evaluated a family with GRTH for the TR beta gene. We found that a new point mutation, consisting of a cytosine to adenine replacement at nucleotide position 1642, resulted in substitution in codon 448 in the T3-binding domain of TR beta. This base substitution was found in only one allele of affected members, but not in unaffected members of the family. The in vitro translation products of this mutant TR beta gene demonstrated significantly reduced T3-binding affinity. Previously, others have reported a kindred with GRTH, in that the same codon was subjected to proline to histidine replacement due to a mutation consisting of a cytosine to adenine replacement at nucleotide position 1643. There appeared to be a significant phenotypic difference between our kindred and that described by others.     

Mice with a targeted mutation in the thyroid hormone beta receptor gene exhibit impaired growth and resistance to thyroid hormone

Patients with mutations in the thyroid hormone receptor beta (TRbeta) gene manifest resistance to thyroid hormone (RTH), resulting in a constellation of variable phenotypic abnormalities. To understand the molecular basis underlying the action of mutant TRbeta in vivo, we generated mice with a targeted mutation in the TRbeta gene (TRbetaPV; PV, mutant thyroid hormone receptor kindred PV) by using homologous recombination and the Cre/loxP system. Mice expressing a single PV allele showed the typical abnormalities of thyroid function found in heterozygous humans with RTH. Homozygous PV mice exhibit severe dysfunction of the pituitary-thyroid axis, impaired weight gains, and abnormal bone development. This phenotype is distinct from that seen in mice with a null mutation in the TRbeta gene. Importantly, we identified abnormal expression patterns of several genes in tissues of TRbetaPV mice, demonstrating the interference of the mutant TR with the gene regulatory functions of the wild-type TR in vivo. These results show that the actions of mutant and wild-type TRbeta in vivo are distinct. This model allows further study of the molecular action of mutant TR in vivo, which could lead to better treatment for RTH patients.     

Generalized resistance to thyroid hormone associated with a mutation in the ligand-binding domain of the human thyroid hormone receptor beta.

The syndrome of generalized resistance to thyroid hormone is characterized by elevated circulating levels of thyroid hormone in the presence of an overall eumetabolic state and failure to respond normally to triiodothyronine. We have evaluated a family with inherited generalized resistance to thyroid hormone for abnormalities in the thyroid hormone nuclear receptors. A single guanine----cytosine replacement in the codon for amino acid 340 resulted in a glycine----arginine substitution in the hormone-binding domain of one of two alleles of the patient's thyroid hormone nuclear receptor beta gene. In vitro translation products of this mutant human thyroid hormone nuclear receptor beta gene did not bind triiodothyronine. Thus, generalized resistance to thyroid hormone can result from expression of an abnormal thyroid hormone nuclear receptor molecule.     

A de novo mutation in an already mutant nucleotide of the thyroid hormone receptor beta gene perpetuates resistance to thyroid hormone

Resistance to thyroid hormone (RTH) is a syndrome of reduced sensitivity to thyroid hormone, most commonly caused by mutations in the thyroid hormone receptor (TR) beta gene. Mutations are mostly located in the ligand-binding domain of the TRbeta, decreasing T(3) binding to the mutant TRbeta molecule, which in turn interferes with the function of the wild-type (WT) TR. A total of 122 different TRbeta gene mutations have been identified so far, with 46 occurring in more than one family. We now report a family with two novel TRbeta mutations occurring in the same nucleotide. The proposita had two children from each of her two marriages. One daughter and one son from each marriage had severe RTH with free T(4) and T(3) levels 3- to 4-fold the mean normal values and unsuppressed TSH, mental retardation, and deafness. The proposita had a missense mutation (GTG to GGG) in codon 458 of the TRbeta gene, resulting in the replacement of the normal valine with glycine (V458G). Although this mutation was transmitted to her affected son, the mutated codon in her affected daughter was GAG, encoding glutamic acid (V458E). Haplotype analysis showed that this de novo mutation occurred on the already mutant allele of the proposita. Cotransfection of each of these mutant TRbetas with the wild-type TRbeta showed a potent dominant negative effect. Large amounts of T(3) were required to dissociate homodimers of the mutant TRbeta bound to DNA. In addition, and in contrast to other mutant TRbetas with severe T(3)-binding defects, homodimer release failed to recruit the steroid receptor coactivator. No defects in heterodimerization with retinoid X receptor-alpha or association with a nuclear receptor corepressor, were identified. These in vitro data are in agreement with the in vivo phenotype of severe RTH. Unique and previously unreported in human inherited diseases is the occurrence of a de novo mutation at an already mutant nucleotide. Because the occurrence by chance is extremely unlikely, it is postulated that the presence of three guanines in the sequence created by the mutant nucleotide of the proposita results in a mutagenic site prone to de novo mutation.     

A novel loss-of-function mutation in the proton-coupled folate transporter from a patient with hereditary folate malabsorption reveals that Arg 113 is crucial for function

Hereditary folate malabsorption (HFM) patients harbor inactivating mutations including R113S in the proton-coupled folate transporter (PCFT), an intestinal folate transporter with optimal activity at acidic pH. Here we identified and characterized a novel R113C mutation residing in the highly conserved first intracellular loop of PCFT. Stable transfectants overexpressing a Myc-tagged wild-type (WT) and mutant R113C PCFT displayed similar transporter targeting to the plasma membrane. However, whereas WT PCFT transfectants showed a 22-fold increase in [(3)H]folic acid influx at pH 5.5, R113C or mock transfectants showed no increase. Moreover, WT PCFT transfectants displayed a 50% folic acid growth requirement concentration of 7 nM, whereas mock and R113C transfectants revealed 24- to 27-fold higher values. Consistently, upon fluorescein-methotrexate labeling, WT PCFT transfectants displayed a 50% methotrexate displacement concentration of 50 nM, whereas mock and R113C transfectants exhibited 12- to 14-fold higher values. Based on the crystal structure of the homologous Escherichia coli glycerol-3-phosphate transporter, we propose that the cationic R113 residue of PCFT is embedded in a hydrophobic pocket formed by several transmembrane helices that may be part of a folate translocation pore. These findings establish a novel loss of function mutation in HFM residing in an intracellular loop of PCFT crucial for folate transport.     

Delayed puberty and primary amenorrhea associated with a novel mutation of the human follicle-stimulating hormone receptor: clinical,histological, and molecular studies

Inactivating mutations of the FSH receptor have been described in rare cases of premature ovarian failure. Only one mutation was associated with a complete phenotype, including delayed puberty, primary amenorrhea, and small ovaries. We describe here a new patient presenting a similar complete phenotype of premature ovarian failure, with high plasma FSH levels associated with very low estrogen and inhibin B levels. No biological response to high doses of recombinant FSH was detected. A novel homozygous Pro(519)Thr mutation was found in this patient. This mutation is located in the second extracellular loop of the FSH receptor, within a motif highly conserved in gonadotropin and TSH receptors. The mutation totally impairs adenylate cyclase stimulation in vitro. FSH binding experiments and confocal microscopy showed that this mutation alters the cell surface targeting of the mutated receptor, which remains trapped intracellularly. Histological studies of the ovaries of the patient showed an increase in the density of small follicles compared with age-matched normal women. A complete block in follicular maturation after the primary stage was also observed. Immunocytochemical studies allowed detection of the expression of c-Kit and proliferation cellular nuclear antigen, whereas no apoptosis was shown by the 3'-end-labeling method. This observation supports the concept that in humans FSH seems mandatory for the initiation of follicular growth only after the primary stage. In our patient complete FSH resistance yields infertility, which is remarkably associated with the persistence of a high number of small follicles.