Thyroxine-binding globulin variant (TBG-Kumamoto): identification of a point mutation and genotype analysis of its family.
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Thyroxine-binding globulin (TBG) is the major thyroid hormone transport protein. Several inherited TBG variants resulting in partial or complete TBG deficiencies have been shown to be caused by either one or two nucleotide substitutions, or one nucleotide deletion in the coding regions of the TBG gene. In this report, a Japanese female patient (proband) with hyperthyroid state, whose lower TBG levels did not return to normal under the euthyroid state after treatment was examined. Genomic DNA samples from the proband with thyroxine-binding globulin deficiency (termed TBG-Kumamoto) and her family were subjected to the polymerase chain reaction, and the generated DNA fragments were sequenced. A single nucleotide substitution in the codon for the amino acid 363 of native TBG molecule (CCT to CTT) was found, resulting in the replacement of proline by leucine. It was revealed that the proband was a heterozygote and her father was a hemizygote. The mutation was confirmed by the allele-specific amplification of genomic DNAs from the proband and her father using oligonucleotide primers of normal or mutant residues at the 3' position in the polymerase chain reaction. These results indicate that the abnormality of TBG-Kumamoto is the consequence of this mutation. Genetically, this point mutation observed in TBG-Kumamoto might be classified as a new type of TBG deficiency. (+info)
In vitro expression of thyroxine-binding globulin (TBG) variants. Impaired secretion of TBGPRO-227 but not TBGPRO-113.
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Thyroxine-binding globulin (TBG) is a glycoprotein that transports thyroid hormones in blood. Of two naturally occurring variants in man that harbor single proline substitutions (TBG-CD5 and TBG-Montreal), only TBG-CD5 manifests as complete TBG deficiency. In order to determine the pathophysiology of these TBG disorders, we expressed TBG-CD5 and TBG-Montreal (TBG-M), as well as the common type TBG (TBG-C) in reticulocyte lysate and Xenopus oocytes. Vectors encoding the three TBG types were constructed, transcribed in vitro, and their products of cell-free translation and processing by canine microsomal membranes were analyzed. TBG-C and TBG-M had identical mobility on denaturing polyacrylamide gel electrophoresis but could be distinguished by differences in thyroxine (T4) binding. TBG-CD5 had altered electrophoretic mobility and did not bind T4. TBG-C and TBG-M expressed in microinjected Xenopus oocytes showed properties similar to their respective serum forms, whereas TBG-CD5 was found in small amounts only intracellularly. Our results confirm that the previously described alanine 113 to proline substitution is responsible for the altered properties of TBG-M. The substitution of leucine 227 by proline in TBG-CD5 appears to impair its cotranslational processing and secretion. (+info)
Co-occurrence of thyroid binding globulin excess and a pituitary macroadenoma containing thyroid stimulating hormone-producing cells.
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A patient is described in whom thyroid binding globulin (TBG) excess was found in association with a pituitary macroadenoma containing thyroid stimulating hormone (TSH)-producing cells, and the potential for diagnostic confusion arising from this unusual combination of endocrine disorders is discussed. The patient presented with visual field disturbance, and magnetic resonance imaging led to the diagnosis of a pituitary tumour. Raised levels of total thyroxine and triiodothyronine along with normal TSH levels suggested the possibility of a TSH-secreting pituitary adenoma. Immunostaining of the tumour removed at transsphenoidal surgery confirmed the presence of TSH-producing cells. When raised total thyroxine and triiodothyronine levels persisted postoperatively, the diagnosis of TBG excess was confirmed. Use of free, rather than total, thyroid hormone assays would almost certainly have prevented the diagnostic confusion concerning possible TSH-secreting pituitary adenoma, but may have resulted in the TBG excess being overlooked entirely. (+info)
Effect of thyroid hormone binding proteins on insulin receptor binding of B1-thyronine-insulin analogues.
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Certain thyronine-insulin analogues, which form non-covalent complexes with plasma proteins, have been shown to act preferentially in the liver. We hypothesized that this property may be dependent on the ability of the analogue to bind to the insulin receptor without prior dissociation from the binding protein. NaB1-L-thyroxyl-insulin, NaB1-3,3',5'-triiodothyronine-insulin, NaB1-D-thyroxyl-insulin and NaB1-L-thyroxyl-aminolauroyl-insulin were compared with insulin for their capacity to inhibit the binding of [125I]TyrA14-insulin to rat liver plasma membrane in albumin-free buffer. Effective doses at 50% maximum inhibition of binding (ED50) were calculated with and without addition of the thyroid hormone binding proteins transthyretin, thyroxine binding globulin and human serum albumin. The binding of thyronine-insulin analogues to insulin receptors was inhibited in a dose-dependent manner by the addition of thyroid hormone binding proteins at concentrations in the physiological range. Complexes of thyronine-insulin analogues with thyroid hormone binding proteins exhibit impaired insulin receptor binding affinities compared with those of the analogues in their free form. Hepatoselectivity in vivo may not depend on binding of the intact complexes to hepatocytes. These results have implications for the physiological role of hormone binding proteins and the in vivo properties of other insulin analogues which bind to plasma proteins. (+info)
Structural and functional microheterogeneity of rat thyroxine-binding globulin during ontogenesis.
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Thyroxine-binding globulin (TBG), the major carrier of thyroid hormones in human and murine sera, is in the rat a developmentally regulated protein, showing a large surge during post-natal growth followed by virtual disappearance in adults. Here we study as a function of age, from the 19-day embryo to 60 days after birth, the structural and binding characteristics of rat TBG microheterogeneity. Serum obtained throughout development, when pre-incubated with 125I-thyroxine (T4), was shown by isoelectric focusing (IEF; pH range 4-5) to contain six labelled isoforms of TBG, with isoelectric points between 4.25 and 4.55. These isoforms differ in their sialic acid content. The relative labelling densities of the isoforms show age-related changes: in neonates, the bulk of T4 is bound to the most alkaline (least sialylated) TBG isoforms; then, with advancing age, it shifts to the most acidic isoforms. To understand whether this progressive transfer of ligand reflects developmental changes in the relative abundance of isoforms, we submitted sera from rats of different ages to crossed immunoelectrofocusing analysis. We demonstrate that the relative proportions of the TBG isoforms remain fairly constant, independent of the level of total TBG. The most acidic forms always represented the majority (approximately 50%), with the most alkaline ones only representing 15% of total TBG. Experiments based on IEF of charcoal-treated sera, supplemented or not with lipidic serum extracts, further demonstrate that the paradoxical low labelling seen in the neonates for the most abundant highly sialylated isoforms is due to inhibition of their binding abilities by liposoluble components, which are particularly concentrated in the sera at the earlier post-natal ages. These studies represent the first analysis of concentration versus binding functions of rat TBG isoforms in the physiological conditions of normal ontogeny. Our results point to an important influence for the serum environment on the binding properties of TBG isoforms. The physiological significance of such interactions remains to be clarified. (+info)
Familial thyroxin-binding globulin excess with ichthyosis: a case report.
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Thyroxin (T4) binding globulin (TBG) the major thyroid hormone transport protein in humans. Congenital or acquired problems lead to TBG excess. Inheritance of TBG excess follows an X-linked pattern. A 21-month-old boy with ichthyosis was referred to the Pediatric Endocrinology Clinic with high levels of thyroid hormones (TT3 = 325 ng/dl, TT4 23 microg/dl, FT3 = 3.49 pg/dl, FT4 = 1.44 ng/dl, TSH = 2.48 microIU/ml). He was clinically euthyroidic. Thyroid gland was normal in size and homogeneous. Thyroid autoantibodies were negative. TSH responded normally to thyroid releasing hormone (TRH) stimulus. TBG was elevated (56 microg/ml). Family investigation revealed high levels of TBG in mother grandfather, and an uncle. To our knowledge, no other TBG excess with ichthyosis has been reported in the literature. (+info)
A variant of porcine thyroxine-binding globulin has reduced affinity for thyroxine and is associated with testis size.
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The field of genomics applies the dissection of genetic differences toward an understanding of the biology of complex traits. Quantitative trait loci (QTL) for testis size, plasma FSH in boars, and body composition (backfat) have been identified near the centromere on the X chromosome in a Meishan-White Composite resource population. Since thyroid function affects Sertoli cell development and adult testis size in rodents, and thyroxine-binding globulin (TBG) maps to this region on the porcine X chromosome, TBG was a positional candidate gene for testis size. We discovered a polymorphism in exon 2 of the porcine TBG gene that results in an amino acid change of the consensus histidine to an asparagine. This single nucleotide polymorphism (SNP) resides in the ligand-binding domain of the mature polypeptide, and the Meishan allele is the conserved allele found in human, bovine, sheep, and rodent TBG. Binding studies indicate altered binding characteristics of the allelic variants of TBG with the asparagine (White Composite) isoform having significantly greater affinity for thyroxine than the histidine (Meishan) isoform. Alternate alleles in boars from the resource population are also significantly associated with testis weight. Therefore, this polymorphism in TBG is a candidate for the causative variation affecting testis size in boars. (+info)
Is free thyroxine accurately measurable at room temperature?
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Two recently developed two-step, or "back-titration" assay kits for free thyroxine (FT4)--one based on a europium-labeled derivative of T4, the other on conventional radiolabeled T4--were compared with both symmetrical dialysis and an indirect FT4 radioimmunoassay. Discrepancies between the two-step and the other two methods were observed, particularly in samples that had very low to zero thyroxine-binding globulin (TBG) content. In those instances the two-step methods gave values almost twice as high as the other methods. This effect could be largely reversed in one of the two-step assays and completely reversed in the other by performing the first incubation step at 37 degrees C rather than at room temperature, as prescribed by the manufacturers. When symmetrical dialysis is performed at both temperatures, FT4 at room temperature is about 40% of the amount determined at 37 degrees C, except in zero-TBG samples, where it averages almost 80% of the value at 37 degrees C. Moreover, we demonstrated that the affinity of TBG for T4 is much more temperature-dependent than the affinity of transthyretin and albumin for T4, so that the net temperature effect on the FT4 in a sample depends on the relative contribution of TBG to total binding. We conclude that performing FT4 assays at room temperature is principally incorrect and leads to falsely increased values when samples with very low TBG concentrations are analyzed. (+info)