The isolation of identical thyroxine containing amino acid sequences from bovine, ovine and porcine thyroglobulins

Congenital hypothyroidism caused by a novel homozygous mutation in the thyroglobulin gene

Congenital hypothyroidism (CH) due to thyroglobulin (TG) deficit is an autosomal recessive disease (OMIM #274700) characterized by hypothyroidism, goiter, low serum TG, and a negative perchlorate discharge test. The aim of this study was to perform the genetic analysis of the TG gene in two sisters born from consanguineus parents and affected by CH and low serum TG levels. The index patient and her sister were identified at neonatal screening for CH and treated with L-thyroxine (L-T4). After discontinuation of L-T4 therapy, hypothyroidism was confirmed, serum TG was undetectable, and no organification defect after (123)I scintigraphy and perchlorate test was shown; thyroid ultrasound showed a eutopic gland of normal size. DNA was extracted from peripheral white blood cells of the two sisters and the father. All 48 exons of TG gene were amplified by polymerase chain reaction and subjected to direct sequencing. A novel homozygous point mutation in exon 10 of TG gene was identified in the patient and her sister. The mutation determined a stop codon at position 768 (R768X) resulting in an early truncated protein or in the complete absence of the protein. The father (euthyroid) was heterozygous carrier of the mutation.

CONCLUSION

Genetic analysis of TG gene was performed in two sisters affected by CH. A novel point mutation of the TG gene determining a stop codon at position 768 of the protein was identified. The early truncated nonfunctioning protein or the absence of the protein due to the premature degradation of abnormal mRNA may be responsible of the observed phenotype.

Naturally occurring mutations in the thyroglobulin gene

Thyroglobulin (Tg) is a large glycoprotein dimer secreted into the follicular lumen. It serves as the matrix for the synthesis of thyroxine (T4) and triiodothyronine (T3), and the storage of thyroid hormone and iodide. In response to demand for thyroid hormone secretion, Tg is internalized into the follicular cell and digested in lysosomes. Subsequently, the thyronines T4 (approximately 80%) and T3 (approximately 20%) are released into the blood stream. Biallelic mutations in the Tg gene have been identified in several animal species and human patients presenting with goiter and overt or compensated hypothyroidism. In untreated patients, goiters are often remarkably large and display continuous growth. In most instances, the affected individuals have related parents and are homozygous for inactivating mutations in the Tg gene. More rarely, compound heterozygous mutations lead to a loss of function of both alleles. Molecular analyses indicate that at least some of these alterations result in a secretory defect and an endoplasmic reticulum storage disease (ERSD). This review discusses the nature and consequences of naturally occurring Tg gene mutations in humans and several animal species. Recent recommendations for the nomenclature of mutations have led to different numbering systems, an aspect that is discussed in order to clarify discrepancies between different publications.

Tyrosine 130 is an important outer ring donor for thyroxine formation in thyroglobulin

The thyroid couples two iodotyrosine molecules to produce thyroid hormone at the acceptor site in thyroglobulin, leaving dehydroalanine or pyruvate at the donor position. Previous work has located the acceptors but not the principal iodotyrosine donors. We incorporated [14C]tyrosine into beef thyroid slices, isolated and iodinated the [14C]thyroglobulin (Tg I), separated its N-terminal approximately 22-kDa hormone-rich peptide, and digested the latter with trypsin and endoproteinase Glu-C (EC 3.4.21.19). Nonlabeled thyroglobulin (Tg II) was isolated from the same glands and processed similarly, without iodination in vitro. Tg I was used to initially recognize pyruvate in peptide fractions, and Tg II was used to then identify its location in the thyroglobulin polypeptide chain. Sequencing of a tryptic peptide by mass spectrometry and Edman degradation showed a cleavage after Val129. An endoproteinase Glu-C-generated peptide had the predicted molecular mass of a fragment containing residues 130-146 with Tyr130 replaced by pyruvate; the identification of this peptide was supported by obtaining the expected shortened fragment after tryptic digestion. 14C-labeled pyruvate was identified in the same fraction as this peptide. We conclude that Tyr130 is an important donor of the outer iodothyronine ring. Its likely acceptor is Tyr5, the most important hormonogenic site of thyroglobulin, because Tyr5 and Tyr130 are proximate, because they are the most prominent early iodination sites in this part of thyroglobulin, and because the N-terminal region was previously found capable of forming T4 by itself.

Identification of hormonogenic tyrosines in fragment 1218-1591 of bovine thyroglobulin by mass spectrometry. Hormonogenic acceptor TYR-12donor TYR-1375

A fragment of bovine thyroglobulin encompassing residues 1218-1591 was prepared by limited proteolysis with thermolysin and continuous-elution polyacrylamide gel electrophoresis in SDS. The reduced and carboxymethylated peptide was digested with endoproteinase Asp-N and fractionated by reverse-phase high performance liquid chromatography. The fractions were analyzed by electrospray and fast atom bombardment mass spectrometry in combination with Edman degradation. The post-translational modifications of all seven tyrosyl residues of the fragment were characterized at an unprecedented level of definition. The analysis revealed the formation of: 1) monoiodotyrosine from tyrosine 1234; 2) monoiodotyrosine, diiodotyrosine, triiodothyronine (T3), and tetraiodothyronine (thyroxine, T4) from tyrosine 1291; and 3) monoiodotyrosine, diiodotyrosine, and dehydroalanine from tyrosine 1375. Iodothyronine formation from tyrosine 1291 accounted for 10% of total T4 of thyroglobulin (0.30 mol of T4/mol of 660-kDa thyroglobulin), and 8% of total T3 (0.08 mol of T3/mol of thyroglobulin). This is the first documentation of the hormonogenic nature of tyrosine 1291 of bovine thyroglobulin, as thyroxine formation at a corresponding site was so far reported only in rabbit, guinea pig, and turtle thyroglobulin. This is also the first direct identification of tyrosine 1375 of bovine thyroglobulin as a donor residue. It is suggested that tyrosyl residues 1291 and 1375 may support together the function of an independent hormonogenic domain in the mid-portion of the polypeptide chain of thyroglobulin.

An endoplasmic reticulum storage disease causing congenital goiter with hypothyroidism

In humans, deficient thyroglobulin (Tg, the thyroid prohormone) is an important cause of congenital hypothyroid goiter; further, homozygous mice expressing two cog/cog alleles (linked to the Tg locus) exhibit the same phenotype. Tg mutations might affect multiple different steps in thyroid hormone synthesis; however, the microscopic and biochemical phenotype tends to involve enlargement of the thyroid ER and accumulation of protein bands of M(r) < 100. To explore further the cell biology of this autosomal recessive illness, we have examined the folding and intracellular transport of newly synthesized Tg in cog/cog thyroid tissue. We find that mutant mice synthesize a full-length Tg, which appears to undergo normal N-linked glycosylation and glucose trimming. Nevertheless, in the mutant, Tg is deficient in the folding that leads to homodimerization, and there is a deficiency in the quantity of intracellular Tg transported to the distal portion of the secretory pathway. Indeed, we find that the underlying disorder in cog/cog mice is a thyroid ER storage disease, in which a temperature-sensitive Tg folding defect, in conjunction with normal ER quality control mechanisms, leads to defective Tg export. In relation to quality control, we find that the physiological response in this illness includes the specific induction of five molecular chaperones in the thyroid ER. Based on the pattern of chaperone binding, different potential roles for individual chaperones are suggested in glycoprotein folding, retention, and degradation in this ER storage disease.

Preferential sites of proteolytic cleavage of bovine, human and rat thyroglobulin. The use of limited proteolysis to detect solvent-exposed regions of the primary structure

The sites and the sequence of the proteolytic cleavages of bovine, human and rat thyroglobulin, during the limited proteolysis with thermolysin and trypsin, were determined by sequencing the NH2 termini of the peptides produced and comparing them to the cDNA-derived sequences of bovine, human and rat thyroglobulin. Major cleavage sites of bovine thyroglobulin included residues 240, 502, 993, 1218, 1784 with thermolysin, and 240, 520, 1142, 1783, 2515 with trypsin. Cleavage sites of human thyroglobulin included residues 503, 982, 990, 1405, 1831 with thermolysin, and 522, 1627, 2513 with trypsin. Those of rat thyroglobulin included residues 501, 1776, 1784 with thermolysin, and 522, 1771, 1825, 2515 with trypsin (numbered as in bovine thyroglobulin). Thus, thyroglobulin from various species presents well localized and conserved regions particularly sensitive to proteolysis. The most sensitive region extended for 30 residues after residue 500. Another major cluster of cleavages was centered around residue 1800; this region was only partially sensitive in human thyroglobulin. A conserved tryptic site lay at the COOH terminus of the molecule. Most cleavage sites occurred within the inserted sequences that disrupt the Cys-rich, tandem repeats of thyroglobulin and either contain or are located near exon-intron junctions. Several cleavage sites lay in proximity of early iodinated or hormonogenic tyrosyl residues or of putative N-linked glycosylation sites. While a predominantly beta-type secondary structure and a rigid three-dimensional structure were predicted for the Cys-rich repeats, stretches of predicted alpha-helices, beta-strands and irregular structure were interspersed in the regions surrounding the cleavage sites. These data demonstrate the existence of conserved regions of thyroglobulin inherently sensitive to proteolysis, which most likely represent solvent-exposed regions of the primary structure, possibly forming loops at the surface of thyroglobulin.

TSH-induced galactose incorporation at the NH2 terminus of thyroglobulin secreted by FRTL-5 cells

FRTL-5 cells were cultured in media containing standard growth factors with or without TSH, plus labeled precursors of N-linked oligosaccharide chains. The thyroglobulin secreted in the medium was purified and fragmented with CNBr. Three peptides were identified by NH2-terminal sequencing, that were labeled mainly with D-[2-3H]mannose, independent of TSH. One of them, corresponding to the NH2-terminus of thyroglobulin, incorporated both more D-[2-3H]mannose and more D-[1-3H]galactose upon TSH addition. These data likely reflect a TSH-induced increment of N-linked glycosylation at the NH2-terminus of thyroglobulin, mostly with the maturation of high-mannose to complex chains.

The origin of the electrophoretic doublet of thyroglobulin

Bovine and human thyroglobulin show two closely migrating bands in reducing SDS-PAGE. Limited digestion with chymotrypsin, trypsin and thermolysin converted the slower band of the doublet into a peptide identical to the faster band, with an apparent mass of 270 kDa, in both species. The starting point of the faster band of the doublet was established at Ileu 520 with native bovine Tg and at Ser 503 with native human Tg, and at Ser 503 and Ser 504 with chymotrypsin-digested bovine and human Tg, respectively. These data explain the electrophoretic heterogeneity of thyroglobulin and unveil a region highly susceptible to proteolysis at about 500 residues from the NH2-terminus of the molecule.

Hormone-containing peptides from normal and goiter human thyroglobulins

A series of low iodine human thyroglobulin samples derived from colloid-rich goiter tissue was examined by HPLC mapping of tryptic digests and compared to normal human thyroglobulin. These samples ranged in iodine content from 2 to 8 gram-atoms of iodine (g.a. I) per mole and were not further iodinated in vitro. Peptides containing the principal hormonogenic sequence were detected using the long wavelength absorbance of the iodotyrosine derivatives at 325 nm. Two such peptides were isolated and sequenced. Their thyroxine content was confirmed by radioimmunoassay. The number of 325-nm-absorbing peaks was significantly lower in the normally iodinated human thyroglobulin than that observed the thyroglobulins of cattle and dog. This suggests a more restricted iodination in the human protein. Sodium dodecyl sulfate gel patterns of the reduced and alkylated proteins showed significant molecular size heterogeneity in all of the samples. Polypeptide fragments ranged in molecular size from approximately 330 to 45 kDa in the goiter derived material and from approximately 330 to 15 kDa in the normal human material. This difference between the proteins is consistent with earlier observations that peptides less than 45 kDa appear concomitantly with hormone formation. These data confirm that the human thyroglobulin molecule is capable of forming at least limited amounts of thyroid hormone at iodine levels as low as 4 g.a. I per mole. The hormone detected in this study was located at residue 5 near the amino terminus of the thyroglobulin molecule.

Glutathione deficiency and peripheral metabolism of thyroid hormones during dietary cysteine deprivation in rats

1. For a period of 24 d, young rats received a diet containing 120 g casein/kg or the same basic diet supplemented with 1.93 g cysteine/kg. 2. The thyroxine (T4) turnover was decreased in rats receiving the cysteine-deficient diet compared with that of rats on the supplemented diet. Moreover, the extrathyroidal T4 pool and T4 disposal rate decreased. 3. Cysteine deprivation also decreased the peripheral metabolism of 3,5,3'-triiodothyronine (T3). The T3 distribution space, extrathyroidal pool of T3 and T3 disposal rate were diminished. 4. In vitro, deiodination of T4 in liver homogenate assayed with endogenous glutathione (GSH) demonstrated decreased T3 production rates in the case of cysteine deficiency. This difference was minimized by the addition of GSH in amounts sufficient to saturate the reaction kinetics. In the light of this finding, GSH is probably involved in the promotion of certain thyroidal problems induced by a cysteine-deficient diet.

Characterization of five monoclonal antibodies obtained after immunization in vitro with a synthetic 19-amino acid peptide of thyroglobulin

A 19-amino acid synthetic peptide representing the highly conserved amino terminus of thyroglobulin was used for the production of monoclonal antibodies after immunization of splenocytes in vitro. The properties of five of the antibodies were studied. One reacted only with the synthetic peptide. The others reacted with both the synthetic peptide and with thyroglobulin from all species tested so far, confirming that the amino terminus of thyroglobulin is highly conserved. Two of the five antibodies showed a positive reaction when tested on frozen sections of thyroid tissue, but with different reaction patterns. Monoclonal antibody F4 gave a positive reaction in the colloid, which contains mainly 19S thyroglobulin. In contrast, monoclonal antibody G4 gave a positive reaction only in the follicular cells. Monoclonal antibody G4 binds primarily to low molecular weight compounds in thyroglobulin preparations, possibly representing breakdown products of the protein.

Primary structure of human thyroglobulin deduced from the sequence of its 8448-base complementary DNA

The mRNA encoding human thyroglobulin has been cloned and sequenced. It is made up of a 8301-nucleotide segment encoding a preprotein monomer of 2767 amino acids, flanked by non-coding 5' and 3' regions of 41 and 106 nucleotides, respectively. This preprotein consists of a leader sequence of 19 amino acids, followed by the sequence of the mature monomer, corresponding to a polypeptide of 2748 amino acids (Mr = 302773). On its amino-terminal side, 70% of the monomer is characterized by the presence of three types of repetitive units. In contrast, the remaining 30% of the protein is devoid of repetitive units. This last region however shows an interesting homology (up to 64%) with the acetylcholinesterase of Torpedo californica. The sites of thyroid hormones synthesis are clustered at both ends of the thyroglobulin monomer. By contrast, the potential glycosylation sites are scattered along the polypeptide chain.

Structural differences around hormonogenic sites on thyroglobulins from different species detected by monoclonal antibodies

Thyroxine remains attached to its synthetic site in thyroglobulin until it is released by proteolysis. Strong homology in the primary sequence surrounding thyroxine-forming residues in thyroglobulins from various species suggests a unique three-dimensional structure at hormonogenic sites. To examine this, two thyroxine-binding mouse anti-(chicken thyroglobulin) monoclonal antibodies, 1A10 and 5F6, were used as probes for this region in an enzyme-linked immunosorbent inhibition assay. The thyroxine content of thyroglobulins had a marked positive influence on the monoclonal antibody binding: when the thyroxine content of human thyroglobulin rose by 6.6-fold, cross-reactivities rose 25-fold for the 1A10 monoclonal antibody and 17.6-fold for the 5F6 monoclonal antibody. However, interspecies comparison of thyroglobulin preparations with similar thyroxine content showed lower than expected cross-reactivities for human, pig and sheep thyroglobulins when compared with chicken thyroglobulin. Only when the thyroxine content of heterologous thyroglobulin preparations was two or three times higher did the cross-reactivities equal or surpass that of chicken thyroglobulin. It is concluded that in thyroglobulin there are structural differences in the different animal species near the thyroxine-forming sites bound by these monoclonal antibodies. The known primary sequence similarity does not seem to result, therefore, in identical three-dimensional structures about this site. These differences may reflect species-specific variations in distant regions brought close as a result of chain folding to form the hormonogenic site, such as those around the donor diiodotyrosine residue or in polysaccharide structures. These monoclonal antibodies provide information about the structure of thyroglobulin, which cannot be obtained from knowledge of the amino acid sequence alone.

The human thyroglobulin gene is over 300 kb long and contains introns of up to 64 kb

Thyroglobulin (Tg), the precursor of thyroid hormones, is a 660.000 Da dimeric glycoprotein synthesized exclusively in the thyroid gland. We have cloned the human thyroglobulin gene from cosmid and phage libraries and constructed a complete restriction map. The gene encodes an 8.7 kb mRNA, covers at least 300 kb DNA and contains at least 37 exons separated by large introns of up to 64 kb. A striking difference in structure between the 5' and 3' part of the gene suggests that it is composed of two evolutionarily different regions. The first 30 kb DNA encode 3 kb of the mRNA, yielding an exon:intron ratio of 1:10, whereas the remaining 270 kb encodes 5.7 kb of the mRNA with an exon:intron ratio of 1:47. In thyroid cells, the Tg gene is not rearranged and nuclear RNA homologous with sequences internal to the 64 kb intron is present, suggesting that the Tg gene is transcribed as a 300 kb RNA.

Primary structure of bovine thyroglobulin deduced from the sequence of its 8,431-base complementary DNA

In mammals, an adequate supply of thyroid hormones is essential for normal growth and neurological development. The biosynthesis of thyroid hormones involves an iodinated precursor protein, thyroglobulin, which may be considered an extreme example of a pro-hormone. Thyroglobulin is a dimeric glycoprotein of relative molecular mass (Mr) 660,000 (660K), which is secreted by the thyrocyte and stored in the lumen of the thyroid follicle. The hormonogenic reaction is extracellular, and involves iodination of tyrosyl residues of thyroglobulin and the intramolecular coupling of a subset of these into thyroxine (T4) and triiodothyronine (T3), which remain part of the polypeptide chain. Secretion of hormones results from the endocytosis of thyroglobulin followed by its complete hydrolysis in lysosomes. Considering that the maximum yield of hormones is approximately 6-8 per 660K protein, the whole process is apparently wasteful. However, the efficiency of thyroglobulin as a thyroid hormone precursor is extremely high when the supply of iodine is short; in such conditions, almost all the iodine incorporated is found in iodothyronine. Hence it is suggested that the thyroglobulin structure has evolved to allow for the preferential and efficient iodination and coupling of the hormonogenic tyrosines. Here we report the complete primary structure of bovine thyroglobulin, derived from the sequence of its 8,431-base-pair complementary DNA. The 2,769-amino-acid sequence is characterized by a pattern of imperfect repeats derived from three cysteine-rich motifs. Four hormonogenic tyrosines have been precisely localized near the amino and carboxyl ends of the protein.

The sequence of 967 amino acids at the carboxyl-end of rat thyroglobulin. Location and surroundings of two thyroxine-forming sites

The entire rat thyroglobulin mRNA sequence (about 8500 nucleotides) has been cloned in five recombinant plasmids containing overlapping cDNA inserts. The 3' end of the mRNA is precisely defined by the poly (A) tail found in the furthest 3' end clone. Evidence that most of the 5' end is cloned come from size considerations and from a primer extension experiment. At the 3' end of the mRNA only one long open reading frame is present in the sequence of 3018 nucleotides that has been established. In the deduced protein sequence we have localized two thyroxine-forming sites in a region containing a high concentration of tyrosine residues.

Sequence of the 5'-end quarter of the human-thyroglobulin messenger ribonucleic acid and of its deduced amino-acid sequence

Thyroglobulin, the dimeric glycoprotein (19 S, 2 X 330 kDa), specific to the thyroid gland, is the support for thyroid hormone synthesis. Elucidation of the mechanism for thyroid hormone synthesis requires the knowledge of the primary sequence of the protein. In this paper the sequence of the first coding 2190 nucleotides from the 5' end of the human mRNA is presented. This was obtained by sequencing two previously described overlapping clones and by construction and sequencing of a single-stranded cDNA corresponding to the 5' end of the mRNA. The nucleotide sequence represents a quarter of the human thyroglobulin mRNA, from which a polypeptide sequence of 730 amino acids at the NH2-terminal end of the monomer has been deduced. This sequence shows a repetition of five highly conserved motifs each of approximately 50 amino acids, the analysis of which allowed us to establish a consensus sequence. We have also demonstrated (a) the hormonogenic tyrosine residue recently described in the mature protein, which is located four amino acids after the NH2-terminal Asn; (b) a prepeptide signal of thyroglobulin secretion comprising 19 amino acids preceding the Asn residue, the NH2-terminal residue of the mature protein and (c) a six-signal tripeptide (Asn-Xaa-Thr or Ser) of N-glycosylation of the chain.