A novel variant in Serpina7 gene in a family with thyroxine-binding globulin deficiency.

Thyroxine-binding globulin (TBG) carries approximately 75% of serum T4 and T3. This protein is encoded by serpina7 gene, formerly known as TBG gene, localized on X-chromosome (Xq22.2). A deficiency in TBG is suspected when abnormally low serum total T4 and T3 are encountered in clinically euthyroid subjects in the presence of normal serum TSH. This condition has been associated with different serpina7 gene mutations resulting in amino acid substitutions or truncations in the mature protein. Herein, we report a new serpina7 gene variant in three members of the same family. It results in the replacement of the normal asparagine 233 by isoleucine and, subsequently, in disruption of a glycosylation site. Co-segregation of this new variant with undetectable levels of TBG in the hemizygous man studied and failure to recognize the same variant in 100 alleles at random, made us to consider it as the underlying cause of the TBG deficiency.

A novel mutation (del 1711 G) in the TBG gene as a cause of complete TBG deficiency.

OBJECTIVE: Inherited thyroxine-binding globulin (TBG) deficiency is caused by mutations in the TBG gene (locus: Xq22.2), which result in defective synthesis or changes in the physical properties or biological function of a protein. DESIGN: We report a novel mutation of the TBG gene causing a complete TBG deficiency in three brothers of Polish origin. DNA was extracted from all of the family members and subjected to sequence analysis. We analyzed the family with a heterozygous mother, a normal father, their three hemizygous affected sons, and their two normal sons. MAIN OUTCOME: Our studies revealed a novel mutation, a single nucleotide deletion (guanine) at position 1711, codon 201 (Asp) in exon 2 (GAC --> AC). This mutation led to a frame shift and premature termination at codon 206, causing a short TBG protein of 205 amino acids (AA) compared to 395 AA of the normal TBG. This new TBG-CD variant was found in the mother and her three affected sons. CONCLUSION: This is a new variant of TBG-CD (TBG-CD-PL Poland) containing 205 AA.

TBG deficiency: description of two novel mutations associated with complete TBG deficiency and review of the literature.

Thyroxine-binding globulin (TBG) is the main thyroid hormone transport protein in serum. Inherited TBG defects lead to a complete (TBG-CD) or a partial (TBG-PD) deficiency and have a diagenic transmission, being clinically fully expressed only in hemizygous males and in homozygous females. In the present study, seven patients from two unrelated families with TBG-CD were studied and two novel TBG mutations were documented. In particular, a T insertion at the 5' donor splice site of exon 0, between nucleotides 2 and 3 at the beginning of intron 1 (g.IVS1+2_3insT) was found in one family and was named TBG-Milano. The other novel mutation is a T deletion at nucleotide 214 of exon 1, which leads to a frameshift at codon 50 with a premature stop codon at position 51 (c.214delT, P50fsX51) and was named TBG-Nikita. According to the X-linked transmission of the defect, females harboring the mutation showed a reduction in TBG levels with normal TSH and total thyroid hormone values at the lower limit of normal. Males harboring either TBG-Milano or TBG-Nikita, showed normal TSH values and low levels of total thyroid hormones and lacked TBG. In conclusion, we report two novel mutations of the TBG gene associated with a complete TBG defect. The first mutation lies at the 5' donor splice site of exon 0 and probably alters the start of translation, while the second is a single nucleotide deletion and leads to a premature stop codon.

[Problems with interpretation of low levels of thyroidal hormones conditioned by changes in transport proteins]. / Problémy s interpretací nízkých hladin tyreoidálních hormonu podmínené zmenami v transportních proteinech.

In clinical practice, we often observe conditions accompanied by secondary drop of binding proteins that bind, more or less specifically, thyroidal hormones. This is usually considered as normal situation that is often not properly interpreted from clinical point of view. In other words, we tolerate such conditions because we build on values of free hormones FT3 and FT4. However, it is very rare to observe significant decrease or even absence of thyroxin binding globulin (TBG) due to inborn error of metabolism. In such situations, the overall level of thyroidal hormones becomes a part of evaluated laboratory profile. Unusual laboratory constellation is in sharp contrast to the so-called "healthy patient". Due to increased migration of persons, we had an opportunity to take care of a patient of this kind.

C-terminal amino acid alteration rather than late termination causes complete deficiency of thyroxine-binding globulin CD-NeuIsenburg.

CONTEXT: T(4)-binding globulin (TBG) is the main transport protein for T(4) in blood and a member of the superfamily of serine proteinase inhibitors. So far, 14 mutations leading to familial complete TBG deficiency have been reported. Eleven of these are caused by mutations leading to truncation of the molecule, and three are caused by single amino acid substitutions. OBJECTIVE: We report and study the complete deficiency TBG variant found in a patient from NeuIsenburg, Germany (TBG-CDNI). METHODS: Direct DNA sequencing was used to identify the TBG-CDNI mutation in the propositus, which was confirmed by allele-specific amplification. Site-directed mutagenesis and expression in Xenopus oocytes was used to study the secretion defect of TBG-CDNI and several variants by Western blot and T(4)-binding assay. RESULTS: The deletion of two nucleotides in codon 384 (1211_1212delTC) causes a frameshift altering the last 11 residues, introduces a new glycosylation site, and elongates the molecule by seven new amino acids. In contrast to normal TBG, TBG-CDNI was not secreted by Xenopus oocytes. Elongation of normal TBG by seven alanines did not affect its secretion or binding properties. On the other hand, neither disruption of its new glycosylation site nor termination of TBG-CDNI at the normal length repaired its secretion defect. CONCLUSIONS: In this first late termination variant of complete TBG deficiency, alteration of beta-strand 5B, located in the core of the molecule, rather than elongation of the molecule or introduction of a new glycosylation site, suffices to disrupt secretion of TBG-CDNI.

Thyroid hormone binding protein abnormalities in patients referred for thyroid disorders.

BACKGROUND & OBJECTIVES: Thyroid hormone binding protein (THBP) abnormalities are the major cause of discordance in commonly performed total thyroxine (T4) and thyrotropin (TSH) estimations, though these do not interfere with thyroid hormone action. Determination of such abnormalities in patients showing discordant thyroid function tests (TFTs) is diagnostically important as it eliminates equivocal assessment of thyroid function and treatment especially where proper methodology for free T4 (FT4) estimation is not available. This study was undertaken to analyse the THBP abnormalities in the population attending thyroid clinic. Family members of affected patients were also screened to study the inheritance of quantitative TBG abnormalities. METHODS: Blood samples of 15000 consecutive patients over a period of 4 years (1994-1997) were tested for thyroid function. THBP abnormalities were studied using polyacrylamide gel electrophoresis autoradiography. Serum thyroxine binding globulin (TBG), free and total T4, total tri-iodothyronine (TT3) were assayed by radioimmunoassay methods. RESULTS: In our screening of 15,000 thyroid patients over a four year period, we found the presence of complete and partial TBG deficiency and TBG excess to be 1:2,500, 1:200 and 1:15,000 respectively. Our study on the families of three affected patients revealed X-chromosome linked inheritance pattern of TBG deficiency in two families and TBG excess in one family. INTERPRETATION & CONCLUSION: Our study suggests that it would be beneficial to rule out THBP abnormalities before interpreting results of TFTs, particularly when there is large discrepancy between T4 and TSH levels. In case of inherited THBP abnormalities, the family members of the affected individual should also be screened to avoid misdiagnosis and erroneous treatment in case they develop thyroid dysfunction in future.

Partial deficiency of thyroxine-binding globulin-Allentown is due to a mutation in the signal peptide.

We present an unusual variant of T(4)-binding globulin (TBG) found in a family from Allentown, Pennsylvania (TBG-AT). The heterozygous proposita presented serum total T(4) and TBG levels ranging from low to normal. TBG gene sequencing revealed a C-to-T substitution in codon -2 (CAC to TAC) leading to the substitution of the normal histidine by a tyrosine within the signal peptide. No mutation within the mature peptide was found. Allele-specific PCR confirmed the H(-2)Y mutation in the propositas mother and son. T(4)-binding analysis of TBG in serum from the proposita and son showed normal affinity but reduced capacity when compared with the unaffected father. Heat stability and isoelectric focusing of TBG-AT were normal. In vitro expression of a recombinant TBG-AT in Xenopus oocytes revealed a diminished secretory efficiency and confirmed the normal binding affinity and heat stability of the small amount of secreted TBG-AT. This study has defined impaired cotranslational processing as a hitherto unrecognized cause of hereditary TBG deficiency.

Two novel mutations in the gene encoding thyroxine-binding globulin (TBG) as a cause of complete TBG deficiency in Taiwan.

OBJECTIVE: Thyroxine-binding globulin (TBG) encoded by the TBG gene on chromosome Xq22 is the major transport protein, carrying approximately 75% of circulating T4. Inherited defects in TBG are associated with three phenotypes based on the level of TBG in serum of affected hemizygous males: complete TBG deficiency (TBG-CD), partial TBG deficiency (TBG-PD) and TBG excess (TBG-E). In this study, we report two unrelated Han Chinese males with complete TBG deficiency who carry different mutations in the TBG gene. PATIENTS: Two index cases of Han males who were diagnosed as having TBG deficiency on the basis of undetectable serum TBG and an additional 75 (50 males and 25 females) normal Han Chinese. MEASUREMENT: Serum thyroid hormones were measured by chemiluminescent immunoassay, thyroid autoantibodies by an agglutination test, and TSH receptor antibody and TBG by radioimmunoassay. Genomic DNA extraction, polymerase chain reaction (PCR) and DNA sequence analysis of the TBG gene were performed with standard methods. RESULTS: One index case had one missense mutation in his copy of the gene, a G --> A transition in codon 52 that results in the replacement of serine by asparagine, and a known polymorphism in codon 283 (TTG --> TTt) that results in the replacement of leucine by phenylalanine. The allelic frequency of TBG-Poly allele in 75 normal Han Chinese (100 chromosomes) was 31%. A second index case was hemizygous for a nonsense mutation in codon 280 of exon 3 (TGG --> TGa). This mutation, located in the C-terminal of TBG, predicts a markedly truncated protein. CONCLUSIONS: This is the first report of complete thyroxine-binding globulin deficiency (TBG-CDT1 and TBG-CDT2) due to TBG gene mutations in Taiwan.

Impact of thyroxine-binding globulin on thyroid hormone economy during pregnancy.

Pregnancy-associated changes in thyroid hormone economy are well-established and are of significant clinical relevance to women with established hypothyroidism because they usually result in increased thyroxine dose requirements by these women. Studies suggest that elevations in serum thyroxine-binding globulin (TBG) have the most influence on this increased need for thyroxine, although the exact contributions by TBG rises and by other mechanisms is as yet unclear. We report the case of a 42-year-old woman, with both established primary hypothyroidism and TBG deficiency, who we have now managed through two full-term pregnancies. The patient was noted to have a baseline TBG that was approximately 30% of the average baseline level reported for non-TBG-deficient individuals. Her TBG levels were induced by pregnancy, although the absolute increase of 1.0 mg/dL was only half the increase usually associated with pregnancy. Despite the patient's low baseline TBG level and her blunted pregnancy-associated TBG induction, her absolute and relative pregnancy-associated increases in thyroxine replacement dosage mirrored those found in non-TBG-deficient, hypothyroid women. Thus, our limited study suggests that an increase in TBG concentration is not the key determinant for the increase in thyroxine requirement in pregnancy.

Complete thyroxine-binding globulin (TBG) deficiency in two families without mutations in coding or promoter regions of the TBG genes: in vitro demonstration of exon skipping.

Inherited thyroxine-binding globulin (TBG) deficiency is caused by mutations in the TBG gene located on the X-chromosome. We now describe two families (K and H) with X-linked complete TBG deficiency without mutations in the coding or promoter regions of the TBG gene. The propositi of both families presented with euthyroid hypothyroxinemia and were found to have undetectable TBG in serum. Affected females had approximately half the normal serum TBG concentration except for one woman who also had undetectable TBG (family H). All four of her children (two boys and two girls) were affected. Affected members of family K had no mutations in any of the five exons or in the minimal promoter region of the TBG gene. However, a G to A substitution, five base pairs downstream from exon 3, was associated to the phenotype of TBG deficiency (TBG-Jackson) and was not present in 100 normal alleles. In contrast to individuals without this mutation, no TBG mRNA could be detected in fibroblasts of the propositus, expressing solely TBG-Jackson. In vitro transcription of genomic DNA containing the mutant intron in an exon trapping system showed that this mutation, reducing the consensus value on the 5' donor splice site, affects the normal splicing process. The transcript of TBG-Jackson lacks exon 3 and is unstable. The deduced amino acid sequence has a frameshift and an early stop codon at position 325. Affected subject of family H had no mutations in the TBG gene including all exons, all introns, the minimal promoter, and the 3' untranslated sequence. However, an intragenic A/G polymorphism (125 bp upstream from exon 2) was identified. It allowed us to confirm a cosegregation of the phenotype to the TBG gene and to show that the single female with complete TBG deficiency was homozygous for the polymorphic TBG allele. The cause of TBG deficiency in this family remains unknown.

Two novel variants in the thyroxine-binding globulin (TBG) gene behind the diagnosis of TBG deficiency.

OBJECTIVE: Search for germline mutations in the thyroxine-binding globulin (TBG) gene of two unrelated Portuguese females of Caucasian origin in whom the diagnosis of TBG deficiency was suspected because of suppressed TSH despite marginally low total thyroxine and tri-iodothyronine. DESIGN AND METHODS: Screening for germline mutations was conducted by non-radioactive PCR-SSCP analysis. The variants documented by this approach were characterized by sequencing. Moreover, in order to define whether they were mutations or polymorphisms we looked for the same variants analysing 100 alleles at random. To achieve this goal we used, alternatively, restriction analysis and the minisequencing method with an automated capillary electrophoresis system and fluorescent-labelled dideoxynucleotides. RESULTS AND CONCLUSIONS: Two novel variants, one in each patient, were identified. One, involved codon 23 (TCA-->TAA) and the other, codon 223 (CAA-->TAA). Analysis of 50 DNA samples, randomly chosen, revealed that all were homozygous for the wild variant at codon 23. One of them was heterozygous for the variant CAA-->TAA at codon 223. This sample was found to correspond to a Caucasian female in whom serum TBG proved to be not detected. Since both variants identified result in stop codons likely to induce truncated TBG proteins, they are probably responsible for the TBG phenotype observed in the individuals studied.

Three novel mutations causing complete T(4)-binding globulin deficiency.

Inherited T(4)-binding globulin deficiency is caused by mutations in the T(4)-binding globulin gene located on the X chromosome. We describe herein three novel mutations in three different families producing complete T(4)-binding globulin deficiency. The proposita of a family from Harwichport is a female with XO Turner's syndrome who expressed only the mutant T(4)-binding globulin allele. Her T(4)-binding globulin sequence has a 19-nucleotide deletion in the distal portion of exon 4. This causes a frameshift and a premature stop at codon 384 of the mature protein. Structure analysis with the Swiss PDB-Viewer revealed that this mutation removes beta-strand s5B from the core of the T(4)-binding globulin molecule, leading to a severe folding defect that is likely to prevent synthesis and secretion. The propositi of complete T(4)-binding globulin deficiency 7 and 8 were 7-month-old and 3-wk-old male infants who were identified because of low serum T(4) levels detected during neonatal screening. Sequencing of complete T(4)-binding globulin deficiency 7 revealed a single nucleotide deletion, a G at position 2690 in exon 3. This leads to an alteration of the amino acid sequence starting at codon 283 and a premature stop at codon 301. Complete T(4)-binding globulin deficiency 8 also has a deletion of the first nucleotide of exon 4, a G at position 3358. This leads to a frameshift and a premature stop at codon 374. As in the case of complete T(4)-binding globulin deficiency J, which has also a nucleotide deletion but downstream (position 3421) and a stop at codon 374, these two T(4)-binding globulin mutants undoubtedly have a defect in intracellular transport and therefore fail to be secreted. This explains the lack of T(4)-binding globulin in the hemizygous affected subjects.

[Hereditary thyroxin-binding globulin deficiency--changed thyroid function tests]. / Hereditaer tyroksinbindande globulinmangel--endringar i thyreoideafunksjonsprøver.

BACKGROUND: Variation in concentrations of carrier proteins of hormones may influence the effect of the hormones and may cause confusion in the interpretation of laboratory results. MATERIAL AND METHOD: A Caucasian family with a hereditary thyroxin-binding globulin (TBG) deficiency was investigated. 22 persons in two generations had blood tests for TBG, thyrotropin (TSH), three-iodothyronin (T3), thyroglobulin (TG), thyroxin and for free thyroxin (FT4) by two different commercial tests, Delfia and IMx Abbott (IMx). Relevant health information was collected of all persons. RESULTS: Six males had very low T4 values, non-detectable TBG, increased FT4 values on the Delfia test and within normal range on the IMx test. Six females had lower borderline T4 and TBG. All persons were clinical euthyroid. INTERPRETATION: The condition is considered to represent X-chromosome linked inheritance with hemizygote affected males and heterozygote female carriers with intermediate values for T4 and TBG. Commercial test kits for FT4 may present considerably different results in conditions with TBG deficiency. When a high level of measured FT4 combined with normal TSH is found; TBG deficiency should be considered.

A novel mutation causing complete thyroxine-binding globulin deficiency (TBG-CD-Negev) among the Bedouins in southern Israel.

T4-binding globulin (TBG) is the major thyroid hormone transport protein in human serum. Inherited TBG abnormalities do not usually alter the metabolic status and are transmitted in X-linked inheritance. A high prevalence of complete TBG deficiency (TBG-CD) has been reported among the Bedouin population in the Negev (southern Israel). In this study we report a novel single mutation causing complete TBG deficiency due to a deletion of the last base of codon 38 (exon 1), which led to a frame shift resulting in a premature stop at codon 51 and a presumed truncated peptide of 50 residues. This new variant of TBG (TBG-CD-Negev) was found among all of the patients studied. We conclude that a single mutation may account for TBG deficiency among the Bedouins in the Negev. This report is the first to describe a mutation in a population with an unusually high prevalence of TBG-CD.