A new compound heterozygous for c.886C>T/c.2206C>T [p.R277X/p.Q717X] mutations in the thyroglobulin gene as a cause of foetal goitrous hypothyroidism

Curating the gnomAD database: Report of novel variants in the thyrogobulin gene using in silico bioinformatics algorithms

Thyroglobulin (TG) is a large glycosylated protein of 2767 amino acids, secreted by the thyrocytes into the follicular lumen. It plays an essential role in the process of thyroid hormone synthesis. TG gene variants lead to permanent congenital hypothyroidism. In the present work, we report a detailed population and bioinformatic prediction analyses of the TG variants indexed in the Genome Aggregation Database (gnomAD). The results showed a clear predominance of nonsense variants in the European (Finnish), European (Non-Finnish) and Ashkenazi Jewish ethnic groups, whereas the splice site variants predominate in South Asian and African/African-American populations. In total, 282 novel TG variants were described (47 missense involving the wild-type cysteine residues, 177 missense located in the ChEL domain and 58 splice site variants) which were not reported in the literature and that would have deleterious effects in prediction programs. In the gnomAD population, the estimated prevalence of heterozygous carriers of the potentially damaging variants was 1:320. In conclusion, we provide an updated and curated reference source for the diagnosis of thyroid disease, mainly to congenital hypothyroidism due to TG deficiency. The identification and characterization of TG variants is undoubtedly a valuable approach to study the TG structure/function relations and an important tool for clinical diagnosis and genetic counseling.

Structure and genetic variants of thyroglobulin: Pathophysiological implications

Thyroglobulin (TG) plays a main role in the biosynthesis of thyroid hormones (TH), and, thus, it is involved in a wide range of vital functions throughout the life cycle of all vertebrates. Deficiency of TH production due to TG genetic variants causes congenital hypothyroidism (CH), with devastating consequences such as intellectual disability and impaired growth if untreated. To this day, 229 variations in the human TG gene have been identified while the 3D structure of TG has recently appeared. Although TG deficiency is thought to be of autosomal recessive inheritance, the introduction of massive sequencing platforms led to the identification of a variety of monoallelic TG variants (combined with mutations in other thyroid gene products) opening new questions regarding the possibility of oligogenic inheritance of the disease. In this review we discuss remarkable advances in the understanding of the TG architecture and the pathophysiology of CH associated with TG defects, providing new insights for the management of congenital disorders as well as counseling benefits for families with a history of TG abnormalities. Moreover, we summarize relevant aspects of TH synthesis within TG and offer an updated analysis of animal and cellular models of TG deficiency for pathophysiological studies of thyroid dyshormonogenesis while highlighting perspectives for new investigations. All in all, even though there has been sustained progress in understanding the role of TG in thyroid pathophysiology during the past 50 years, functional characterization of TG variants remains an important area of study for future advancement in the field.

A novel mutation in intron 11 donor splice site, responsible of a rare genotype in thyroglobulin gene by altering the pre-mRNA splincing process. Cell expression and bioinformatic analysis

Thyroglobulin (TG) is a homodimeric glycoprotein synthesized by the thyroid gland. To date, two hundred twenty-seven variations of the TG gene have been identified in humans. Thyroid dyshormonogenesis due to TG gene mutations have an estimated incidence of approximately 1 in 100,000 newborns. The clinical spectrum ranges from euthyroid to mild or severe hypothyroidism. The purpose of the present study was to identify and characterize new variants in the TG gene. We report an Argentine patient with congenital hypothyroidism, enlarged thyroid gland and low levels of serum TG. Sequencing of DNA, expression of chimeric minigenes as well as bioinformatics analysis were performed. DNA sequencing identified the presence of compound heterozygous mutations in the TG gene: the maternal mutation consists of a c.3001+5G > A, whereas the paternal mutation consists of p.Arg296*. Minigen analysis of the variant c.3001+5A performed in HeLa, CV1 and Hek293T cell lines, showed a total lack of transcript expression. So, in order to validate that the loss of expression was caused by such variation, site-directed mutagenesis was performed on the mutated clone, which previously had a pSPL3 vector change, to give rise to a wild-type clone c.3001+5G, endorsing that the mutation c.3001+5G > A is the cause of the total lack of expression. In conclusion, we demonstrate that the c.3001+5G > A mutation causes a rare genotype, altering the splicing of the pre-mRNA. This work contributes to elucidating the molecular bases of TG defects associated with congenital hypothyroidism and expands our knowledge in relation to the pathologic roles of the position 5 in the donor splice site.

Fetal Goitrous Hypothyroidism and Polyhydramnios in a Patient with Compound Heterozygous DUOXA2 Mutations

BACKGROUND

Fetal goiter is only rarely observed in pregnant women without autoimmune thyroid disorders, and there is no epidemiological data on its pathophysiology. Dual oxidase maturation factor 2 (DUOXA2), together with dual oxidase 2, serves pivotal roles in thyroid hormone biosynthesis. To date, all reported patients with DUOXA2 mutations were diagnosed postnatally through newborn screening for congenital hypothyroidism.

CASE REPORT

The mother of a male fetus presented at 33 + 4 gestational weeks (GW) with a fetal goiter and polyhydramnios. Cordocentesis revealed fetal hypothyroidism (TSH 253.4 mU/L, FT4 0.29 ng/dL). Intra-amniotic levothyroxine injections were performed at GW 34 + 3 and 35 + 3. The patient was born after spontaneous vaginal delivery at 35 + 6 GW without obstetrical complications. He was treated with levothyroxine until the age of 6 years when reevaluation of his thyroid functions showed normal results (TSH 1.32 mU/L, FT4 1.81 ng/dL). Eleven causative genes of CH, including DUOXA2, were analyzed with use of a next-generation sequencing technique.

RESULTS

A next-generation sequencing-based mutation screen led us to find that he was compound heterozygous for 2 previously reported nonsense DUOXA2 mutations (p.[Tyr138*];[Tyr246*]).

CONCLUSION

The present case not only illustrates the phenotypic diversity of DUOXA2 mutation carriers but also implies that DUOXA2 is important in prenatal thyroid hormone production.

Molecular analysis of thyroglobulin mutations found in patients with goiter and hypothyroidism

Thyroid dyshormonogenesis due to thyroglobulin (TG) gene mutations have an estimated incidence of approximately 1 in 100,000 newborns. The clinical spectrum ranges from euthyroid to mild or severe hypothyroidism. Up to now, one hundred seventeen deleterious mutations in the TG gene have been identified and characterized. The purpose of the present study was to identify and characterize new mutations in the TG gene. We report eight patients from seven unrelated families with goiter, hypothyroidism and low levels of serum TG. All patients underwent clinical, biochemical and image evaluation. Sequencing of DNA, genotyping, as well as bioinformatics analysis were performed. Molecular analyses revealed three novel inactivating TG mutations: c.5560G>T [p.E1835*], c.7084G>C [p.A2343P] and c.7093T>C [p.W2346R], and four previously reported mutations: c.378C>A [p.Y107*], c.886C>T [p.R277*], c.1351C>T [p.R432*] and c.7007G>A [p.R2317Q]. Two patients carried homozygous mutations (p.R277*/p.R277*, p.W2346R/p.W2346R), four were compound heterozygous mutations (p.Y107*/p.R277* (two unrelated patients), p.R432*/p.A2343P, p.Y107*/p.R2317Q) and two siblings from another family had a single p.E1835* mutated allele. Additionally, we include the analysis of 48 patients from 31 unrelated families with TG mutations identified in our present and previous studies. Our observation shows that mutations in both TG alleles were found in 27 families (9 as homozygote and 18 as heterozygote compound), whereas in the remaining four families only one mutated allele was detected. The majority of the detected mutations occur in exons 4, 7, 38 and 40. 28 different mutations were identified, 33 of the 96 TG alleles encoded the change p.R277*. In conclusion, our results confirm the genetic heterogeneity of TG defects and the pathophysiological importance of the predicted TG misfolding and therefore thyroid hormone formation as a consequence of truncated TG proteins and/or missense mutations located within its ACHE-like domain.

Iodide handling disorders (NIS, TPO, TG, IYD)

Iodide Handling Disorders lead to defects of the biosynthesis of thyroid hormones (thyroid dyshormonogenesis, TD) and thereafter congenital hypothyroidism (CH), the most common endocrine disease characterized by low levels of circulating thyroid hormones. The prevalence of CH is 1 in 2000-3000 live births. Prevention of CH is based on prenatal diagnosis, carrier identification, and genetic counseling. In neonates a complete diagnosis of TD should include clinical examination, biochemical thyroid tests, thyroid ultrasound, radioiodine or technetium scintigraphy and perchlorate discharge test (PDT). Biosynthesis of thyroid hormones requires the presence of iodide, thyroid peroxidase (TPO), a supply of hydrogen peroxide (DUOX system), an iodine acceptor protein, thyroglobulin (TG), and the rescue and recycling of iodide by the action of iodotyrosine deiodinase or iodotyrosine dehalogenase 1 (IYD or DEHAL1). The iodide transport is a two-step process involving transporters located either in the basolateral or apical membranes, sodium iodide symporter (NIS) and pendrin (PDS), respectively. TD has been linked to mutations in the solute carrier family 5, member 5 transporter (SLC5A5, encoding NIS), solute carrier family 26, member 4 transporter (SLC26A4, encoding PDS), TPO, DUOX2, DUOXA2, TG and IYD genes. These mutations produce a heterogeneous spectrum of CH, with an autosomal recessive inheritance. Thereafter, the patients are usually homozygous or compound heterozygous for the gene mutations and the parents, carriers of one mutation. In the last two decades, considerable progress has been made in identifying the genetic and molecular causes of TD. Recent advances in DNA sequencing technology allow the massive screening and facilitate the studies of phenotype variability. In this article we included the most recent data related to disorders caused by mutations in NIS, TPO, TG and IYD.

Thyroglobulin gene mutations in Chinese patients with congenital hypothyroidism

Mutations in Thyroglobulin (TG) are common genetic causes of congenital hypothyroidism (CH). But the TG mutation spectrum and its frequency in Chinese CH patients have not been investigated. Here we conducted a genetic screening of TG gene in a cohort of 382 Chinese CH patients. We identified 22 rare non-polymorphic variants including six truncating variants and 16 missense variants of unknown significance (VUS). Seven patients carried homozygous pathogenic variants, and three patients carried homozygous or compound heterozygous VUS. 48 out of 382 patients carried one of 18 heterozygous VUS which is significantly more often than their occurrences in control cohort (P < 0.0001). Unique to Asian population, the c.274+2T>G variant is the most common pathogenic variant with an allele frequency of 0.021. The prevalence of CH due to TG gene defect in Chinese population was estimated to be approximately 1/101,000. Our study uncovered ethnicity specific TG mutation spectrum and frequency.

Thyroglobulin From Molecular and Cellular Biology to Clinical Endocrinology

Thyroglobulin (Tg) is a vertebrate secretory protein synthesized in the thyrocyte endoplasmic reticulum (ER), where it acquires N-linked glycosylation and conformational maturation (including formation of many disulfide bonds), leading to homodimerization. Its primary functions include iodide storage and thyroid hormonogenesis. Tg consists largely of repeating domains, and many tyrosyl residues in these domains become iodinated to form monoiodo- and diiodotyrosine, whereas only a small portion of Tg structure is dedicated to hormone formation. Interestingly, evolutionary ancestors, dependent upon thyroid hormone for development, synthesize thyroid hormones without the complete Tg protein architecture. Nevertheless, in all vertebrates, Tg follows a strict pattern of region I, II-III, and the cholinesterase-like (ChEL) domain. In vertebrates, Tg first undergoes intracellular transport through the secretory pathway, which requires the assistance of thyrocyte ER chaperones and oxidoreductases, as well as coordination of distinct regions of Tg, to achieve a native conformation. Curiously, regions II-III and ChEL behave as fully independent folding units that could function as successful secretory proteins by themselves. However, the large Tg region I (bearing the primary T4-forming site) is incompetent by itself for intracellular transport, requiring the downstream regions II-III and ChEL to complete its folding. A combination of nonsense mutations, frameshift mutations, splice site mutations, and missense mutations in Tg occurs spontaneously to cause congenital hypothyroidism and thyroidal ER stress. These Tg mutants are unable to achieve a native conformation within the ER, interfering with the efficiency of Tg maturation and export to the thyroid follicle lumen for iodide storage and hormonogenesis.

Novel compound heterozygous Thyroglobulin mutations c.745+1G>A/c.7036+2T>A associated with congenital goiter and hypothyroidism in a Vietnamese family. Identification of a new cryptic 5' splice site in the exon 6

Several patients were identified with dyshormonogenesis caused by mutations in the thyroglobulin (TG) gene. These defects are inherited in an autosomal recessive manner and affected individuals are either homozygous or compound heterozygous for the mutations. The aim of the present study was to identify new TG mutations in a patient of Vietnamese origin affected by congenital hypothyroidism, goiter and low levels of serum TG. DNA sequencing identified the presence of compound heterozygous mutations in the TG gene: the maternal mutation consists of a novel c.745+1G>A (g.IVS6 + 1G>A), whereas the hypothetical paternal mutation consists of a novel c.7036+2T>A (g.IVS40 + 2T>A). The father was not available for segregation analysis. Ex-vivo splicing assays and subsequent RT-PCR analyses were performed on mRNA isolated from the eukaryotic-cells transfected with normal and mutant expression vectors. Minigene analysis of the c.745+1G>A mutant showed that the exon 6 is skipped during pre-mRNA splicing or partially included by use of a cryptic 5' splice site located to 55 nucleotides upstream of the authentic exon 6/intron 6 junction site. The functional analysis of c.7036+2T>A mutation showed a complete skipping of exon 40. The theoretical consequences of splice site mutations, predicted with the bioinformatics tool NNSplice, Fsplice, SPL, SPLM and MaxEntScan programs were investigated and evaluated in relation with the experimental evidence. These analyses predicted that both mutant alleles would result in the abolition of the authentic splice donor sites. The c.745+1G>A mutation originates two putative truncated proteins of 200 and 1142 amino acids, whereas c.7036+2T>A mutation results in a putative truncated protein of 2277 amino acids. In conclusion, we show that the c.745+1G>A mutation promotes the activation of a new cryptic donor splice site in the exon 6 of the TG gene. The functional consequences of these mutations could be structural changes in the protein molecule that alter the biosynthesis of thyroid hormones.

Novel mutational mechanism in the thyroglobulin gene: imperfect DNA inversion as a cause for hereditary hypothyroidism

The objective of this study was to perform genetic analysis in three brothers of Turkish origin born from consanguineus parents and affected by congenital hypothyroidism, goiter and low levels of serum TG. The combination of sequencing of DNA, PCR mapping, quantitative real-time PCR, inverse-PCR (I-PCR), multiplex PCR and bioinformatics analysis were used in order to detect TG mutations. We demonstrated that the three affected siblings are homozygous for a DNA inversion of 16,962bp in the TG gene associated with two deleted regions at both sides of the inversion limits. The inversion region includes the first 9bp of exon 48, 1015bp of intron 47, 191bp of exon 47, 1523bp of intron 46, 135bp of exon 46 and the last 14,089bp of intron 45. The proximal deletion corresponds to 27bp of TG intron 45, while the distal deletion spans the last 230bp of TG exon 48 and the first 588bp of intergenic region downstream TG end. The parents were heterozygous carriers of the complex rearrangement. In conclusion, a novel large imperfect DNA inversion within the TG gene was identified by the strategy of I-PCR. This aberration was not detectable by normal sequencing of the exons and exon/intron boundaries. Remarkably, the finding represents the first description of a TG deficiency disease caused by a DNA inversion.

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.

New insights into thyroglobulin gene: molecular analysis of seven novel mutations associated with goiter and hypothyroidism

The thyroglobulin (TG) gene is organized in 48 exons, spanning over 270 kb on human chromosome 8q24. Up to now, 62 inactivating mutations in the TG gene have been identified in patients with congenital goiter and endemic or non-endemic simple goiter. The purpose of the present study was to identify and characterize new mutations in the TG gene. We report 13 patients from seven unrelated families with goiter, hypothyroidism and low levels of serum TG. All patients underwent clinical, biochemical and imaging evaluation. Single-strand conformation polymorphism (SSCP) analysis, endonuclease restriction analysis, sequencing of DNA, genotyping, population screening, and bioinformatics studies were performed. Molecular analyses revealed seven novel inactivating TG mutations: c.378C>A [p.Y107X], c.2359C>T [p.R768X], c.2736delG [p.R893fsX946], c.3842G>A [p.C1262Y], c.5466delA [p.K1803fsX1833], c.6000C>G [p.C1981W] and c.6605C>G [p.P2183R] and three previously reported mutations: c.886C>T [p.R277X], c.6701C>A [p.A2215D] and c.7006C>T [p.R2317X]. Six patients from two families were homozygous for p.R277X mutation, four were compound heterozygous mutations (p.Y107X/p.C1262Y, p.R893fsX946/p.A2215D, p.K1803fsX1832/p.R2317X), one carried three identified mutations (p.R277X/p.C1981W-p.P2183R) together with a hypothetical micro deletion and the remaining two siblings from another family with typical phenotype had a single p.R768X mutated allele. In conclusion, our results confirm the genetic heterogeneity of TG defects and the pathophysiological importance of altered TG folding as a consequency of truncated TG proteins and missense mutations located in ACHE-like domain or that replace cysteine.

Two novel mutations in the thyroglobulin gene as cause of congenital hypothyroidism: identification a cryptic donor splice site in the exon 19

Thyroglobulin (TG) is a homodimeric glycoprotein synthesized by the thyroid gland. To date, 52 mutations of the TG gene have been identified in humans. The purpose of the present study was to identify and characterize new mutations in the TG gene. We report a French patient with congenital hypothyroidism, mild enlarged thyroid gland and low levels of serum TG. Sequencing of DNA, genotyping, expression of chimeric minigenes as well as bioinformatics analysis were performed. DNA sequencing identified the presence of compound heterozygous mutations in the TG gene: the paternal mutation consists of a c.3788-3789insT or c.3788dupT, whereas the maternal mutation consists of g.IVS19+3_+4delAT. Minigene analysis of the g.IVS19+3_+4delAT mutant showed that the exon 19 is skipped during pre-mRNA splicing or partially included by use of cryptic 5' splice site located to 100 nucleotides downstream of the wild type exon-intron junction. The c.3788-3789insT mutation results in a putative truncated protein of 1245 amino acids, whereas g.IVS19+3_4delAT mutation originates two putative truncated proteins of 1330 and 1349 amino acids. In conclusion, we show that the g.IVS19+3_+4delAT mutation promotes the activation of a cryptic donor splice site in the exon 19 of the TG gene. These results open up new perspectives in the knowledge of the mechanism of splicing for the TG pre-mRNA.

Maturation of thyroglobulin protein region I

In vertebrates, the thyroglobulin (Tg) gene product must be exported to the lumen of thyroid follicles for thyroid hormone synthesis. In toto, Tg is composed of multiple type-1 repeats connected by linker and hinge (altogether considered as "region I," nearly 1,200 residues); regions II-III (~720 residues); and cholinesterase-like (ChEL) domain (~570 residues). Regions II-III and ChEL rapidly acquire competence for secretion, yet regions I-II-III require 20 min to become a partially mature disulfide isomer; stabilization of a fully oxidized form requires ChEL. Transition from partially mature to mature Tg occurs as a discrete "jump" in mobility by nonreducing SDS-PAGE, suggesting formation of at most a few final pairings of Cys residues that may be separated by significant intervening primary sequence. Using two independent approaches, we have investigated which portion of Tg is engaged in this late stage of its maturation. First, we demonstrate that this event is linked to oxidation involving region I. Introduction of the Tg-C1245R mutation in the hinge (identical to that causing human goitrous hypothyroidism) inhibits this maturation, although the Cys-1245 partner remains unidentified. Second, we find that Tg truncated after its fourth type-1 repeat is a fully independent secretory protein. Together, the data indicate that final acquisition of secretory competence includes conformational maturation in the interval between linker and hinge segments of region I.