Six new mutations of the thyroglobulin gene discovered in taiwanese children presenting with thyroid dyshormonogenesis

Identification of southern Taiwan genetic variants in thyroid dyshormonogenesis through whole-exome sequencing

Thyroid dyshormonogenesis (TDH) is responsible for 15%-25% of congenital hypothyroidism (CH) cases. Pathogenetic variants of this common inherited endocrine disorders vary geographically. Unraveling the genetic underpinnings of TDH is essential for genetic counseling and precise therapeutic strategies. This study aims to identify genetic variants associated with TDH in Southern Taiwan using whole exome sequencing (WES). We included CH patients diagnosed through newborn screening at a tertiary medical center from 2011 to 2022. Permanent TDH was determined based on imaging evidence of bilateral thyroid structure and the requirement for continuous medication beyond 3 years of age. Genomic DNA extracted from blood was used for exome library construction, and pathogenic variants were detected using an in-house algorithm. Of the 876 CH patients reviewed, 121 were classified as permanent, with 47 (40%) confirmed as TDH. WES was conducted for 45 patients, and causative variants were identified in 32 patients (71.1%), including DUOX2 (15 cases), TG (8 cases), TSHR (7 cases), TPO (5 cases), and DUOXA2 (1 case). Recurrent variants included DUOX2 c.3329G>A, TSHR c.1349G>A, TG c.1348delT, and TPO c.2268dupT. We identified four novel variants based on genotype, including TSHR c.1135C>T, TSHR c.1349G>C, TG c.2461delA, and TG c.2459T>A. This study underscores the efficacy of WES in providing definitive molecular diagnoses for TDH. Molecular diagnoses are instrumental in genetic counseling, formulating treatment, and developing management strategies. Future research integrating larger population cohorts is vital to further elucidate the genetic landscape of TDH.

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.

p.L571P in the linker domain of rat thyroglobulin causes intracellular retention

Thyroglobulin (TG), a large glycosylated protein secreted by thyrocytes into the thyroid follicular lumen, plays an essential role in thyroid hormone biosynthesis. Rattus norvegicus TG (rTG) is encoded by a large single copy gene, 186-kb long, located on chromosome 7 composed of 48 exons encoding a 8461-kb mRNA. Although the TG gene displays sequence variability, many missense mutations do not impose any adverse effect on the TG protein, whereas other nucleotide substitutions may affect its TG stability and/or TG intracellular trafficking. In order to gain a further understanding of the protein domains regulating its intracellular fate, we cloned a full-length cDNA from rTG into the pcDNA6/V5-His B expression vector. However, transient expression of the cDNA in HEK293T cells showed that the encoded protein was not a wild-type molecule, as it was unable to be secreted in the culture supernatant. Sequencing analyses revealed three random mutations, which accidentally emerged during the course of cloning: c.1712T>C [p.L571P] in the linker domain (amino acid positions 360 to 604), c.2027A>G [p.Q676R] in TG type 1-6 repeat and c.2720A>G [p.Q907R] in the TG type 1-7 repeat. Expression of cDNAs encoding a combination of two mutations [p.Q676R-p.Q907R], [p.L571P-p.Q907R] or [p.L571P-p.Q676R] indicated that any TG bearing the p.L571P substitution was trapped intracellularly. Indeed, we expressed the single point mutant p.L571P and confirmed that this point mutation was sufficient to cause intracellular retention of mutant TG in HEK293T cells. Endo H analysis showed that the p.L571P mutant is completely sensitive to the enzyme, whereas the will-type TG acquires full N-glycan modifications in Golgi apparatus. This data suggest that the p.L571P mutant contains the mannose-type N-glycan, that was added at the first stage of glycosylation. Complex-type N-glycan formation in the Golgi apparatus does not occur, consistent with defective endoplasmic reticulum exit of the mutant TG. Moreover, predictive analysis of the 3D linker domain showed that the p.L571P mutation would result in a significant protein conformational change. In conclusion, our studies identified a novel amino acid residue within the linker domain of TG associated with its conformational maturation and intracellular trafficking.

Defects in protein folding in congenital hypothyroidism

Primary congenital hypothyroidism (CH) is the most common endocrine disease in children and one of the most common preventable causes of both cognitive and motor deficits. CH is a heterogeneous group of thyroid disorders in which inadequate production of thyroid hormone occurs due to defects in proteins involved in the gland organogenesis (dysembryogenesis) or in multiple steps of thyroid hormone biosynthesis (dyshormonogenesis). Dysembryogenesis is associated with genes responsible for the development or growth of thyroid cells: such as NKX2-1, FOXE1, PAX8, NKX2-5, TSHR, TBX1, CDCA8, HOXD3 and HOXB3 resulting in agenesis, hypoplasia or ectopia of thyroid gland. Nevertheless, the etiology of the dysembryogenesis remains unknown for most cases. In contrast, the majority of patients with dyshormonogenesis has been linked to mutations in the SLC5A5, SLC26A4, SLC26A7, TPO, DUOX1, DUOX2, DUOXA1, DUOXA2, IYD or TG genes, which usually originate goiter. About 800 genetic mutations have been reported to cause CH in patients so far, including missense, nonsense, in-frame deletion and splice-site variations. Many of these mutations are implicated in specific domains, cysteine residues or glycosylation sites, affecting the maturation of nascent proteins that go through the secretory pathway. Consequently, misfolded proteins are permanently entrapped in the endoplasmic reticulum (ER) and are translocated to the cytosol for proteasomal degradation by the ER-associated degradation (ERAD) machinery. Despite of all these remarkable advances in the field of the CH pathogenesis, several points on the development of this disease remain to be elucidated. The continuous study of thyroid gene mutations with the application of new technologies will be useful for the understanding of the intrinsic mechanisms related to CH. In this review we summarize the present status of knowledge on the disorders in the protein folding caused by thyroid genes mutations.

High Diagnostic Yield of Targeted Next-Generation Sequencing in a Cohort of Patients With Congenital Hypothyroidism Due to Dyshormonogenesis

OBJECTIVE

To elucidate the molecular cause in a well-characterized cohort of patients with Congenital Hypothyroidism (CH) and Dyshormonogenesis (DH) by using targeted next-generation sequencing (TNGS).

STUDY DESIGN

We studied 19 well-characterized patients diagnosed with CH and DH by targeted NGS including genes involved in thyroid hormone production. The pathogenicity of novel mutations was assessed based on in silico prediction tool results, functional studies when possible, variant location in important protein domains, and a review of the recent literature.

RESULTS

TNGS with variant prioritization and detailed assessment identified likely disease-causing mutations in 10 patients (53%). Monogenic defects most often involved TG, followed by DUOXA2, DUOX2, and NIS and were usually homozygous or compound heterozygous. Our review shows the importance of the detailed phenotypic description of patients and accurate analysis of variants to provide a molecular diagnosis.

CONCLUSIONS

In a clinically well-characterized cohort, TNGS had a diagnostic yield of 53%, in accordance with previous studies using a similar strategy. TG mutations were the most common genetic defect. TNGS identified gene mutations causing DH, thereby providing a rapid and cost-effective genetic diagnosis in patients with CH due to DH.

Congenital Primary Hypothyroidism with the Homozygous Nonsense Mutation P.K1374* in the Thyroglobulin Gene and a Normal-sized Thyroid Gland on Levothyroxine Replacement

Thyroglobulin (TG) gene mutations cause thyroid dyshormonogenesis, which is typically associated with a congenital goiter. We herein report the case of a 64-year-old man with congenital primary hypothyroidism who had a normal-sized thyroid gland on levothyroxine replacement. He had short stature (-3.1 standard deviations) and mild intellectual impairment. Thyroid autoantibodies were all negative, and the serum TG levels were undetectable. Eventually, he was found to have the novel homozygous nonsense mutation p.K1374* in the TG gene. The possibility of TG mutation should be considered for patients with congenital primary hypothyroidism and a very low serum TG level, regardless of the thyroid size.

Targeted next-generation sequencing of thirteen causative genes in Chinese patients with congenital hypothyroidism

To identify the spectrum and prevalence of thirteen causative genes mutations in congenital hypothyroidism (CH) patients, we collected blood samples and extracted genomic DNA of 106 CH patients, and designed a customized targeted next-generation sequencing panel containing 13 CH-causing genes to detect mutations. A total of 132 mutations were identified in 65.09% of patients (69/106) on the following nine genes: DUOX2, TG, TPO, TSHR, TTF1, TTF2, NKX2-5, PAX8 and GNAS. 69.70% (92/132) mutations related to thyroid dyshormonogenesis genes, including DUOX2 (n = 49), TG (n = 35), and TPO (n = 8). 21.21% (28/132) mutations related to thyroid dysgenesis genes, including TSHR (n = 19), TTF1 (n = 5), TTF2 (n = 1), PAX8 (n = 2), and NKX2-5 (n = 1). 9.09% (12/132) mutations related to GNAS, which was associated with thyrotropin resistance. No mutation of THRA, TSHB, IYD or SLC5A5 was detected. Among 69 mutations detected patients, 41 (59.42%) patients were two or more mutations detected, and mutations of 30 (43.48%) patients related to two or three genes. According to the pathomechanism of the mutant genes, 57.97% CH patients were classified as thyroid dyshormonogenesis. Overall, DUOX2, TG and TSHR mutations were the most common genetic defects in Chinese CH patients, and thyroid dyshormonogenesis could be the first genetic etiology of CH in Chinese. Besides, multiple mutations accounts for a part of genetic pathogenesis.

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.

The genetic characteristics of congenital hypothyroidism in China by comprehensive screening of 21 candidate genes

OBJECTIVE

Congenital hypothyroidism (CH), the most common neonatal metabolic disorder, is characterized by impaired neurodevelopment. Although several candidate genes have been associated with CH, comprehensive screening of causative genes has been limited.

DESIGN AND METHODS

One hundred ten patients with primary CH were recruited in this study. All exons and exon-intron boundaries of 21 candidate genes for CH were analyzed by next-generation sequencing. And the inheritance pattern of causative genes was analyzed by the study of family pedigrees.

RESULTS

Our results showed that 57 patients (51.82%) carried biallelic mutations (containing compound heterozygous mutations and homozygous mutations) in six genes (DUOX2, DUOXA2, DUOXA1, TG, TPO and TSHR) involved in thyroid hormone synthesis. Autosomal recessive inheritance of CH caused by mutations in DUOX2, DUOXA2, TG and TPO was confirmed by analysis of 22 family pedigrees. Notably, eight mutations in four genes (FOXE1, NKX2-1, PAX8 and HHEX) that lead to thyroid dysgenesis were identified in eight probands. These mutations were heterozygous in all cases and hypothyroidism was not observed in parents of these probands.

CONCLUSIONS

Most cases of congenital hypothyroidism in China were caused by thyroid dyshormonogenesis rather than thyroid dysgenesis. This study identified previously reported causative genes for 57/110 Chinese patients and revealed DUOX2 was the most frequently mutated gene in these patients. Our study expanded the mutation spectrum of CH in Chinese patients, which was significantly different from Western countries.

[Advances in genetic research of congenital hypothyroidism in China]

Congenital hypothyroidism (CH), which results from insufficient thyroid hormone biosynthesis, is one of the most common neonatal endocrine disorders. Thyroid dysgenesis and thyroid dyshormonogenesis are the two causes of CH and either one will lead to deficiencies of enzymes during thyroid hormone biosynthesis and insufficient thyroid hormone biosynthesis. Recently, researchers have performed extensive studies on genetics of CH. This paper reviews genes reported to be associated with CH in China.

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 truncating thyroglobulin gene mutations associated with congenital hypothyroidism

Mutations in the thyroglobulin (TG) gene have been reported to cause congenital hypothyroidism (CH) and we have been investigating the genetic architecture of CH in a large cohort of consanguineous/multi-case families. Our aim in this study was to determine the genetic basis of CH in four affected individuals coming from two separate consanguineous families. Since CH is usually inherited in autosomal recessive manner in consanguineous/multi-case families, we adopted a two-stage strategy of genetic linkage studies and targeted sequencing of the TG gene. First we investigated the potential genetic linkage of families to any known CH locus using microsatellite markers and then determined the pathogenic mutations in linked-genes by Sanger sequencing. Both families showed potential linkage to TG locus and we detected two previously unreported nonsense TG mutations (p.Q630X and p.W637X) that segregated with the disease status in both families. This study highlights the importance of molecular genetic studies in the definitive diagnosis and classification of CH, and also adds up to the limited number of nonsense TG mutations in the literature. It also suggests a new clinical testing strategy using next-generation sequencing in all primary CH cases.

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.

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.

Bioinformatics and mutations leading to exon skipping

Our knowledge about human genes and the consequences of mutations leading to human genetic diseases has drastically improved over the last few years. It has been recognized that many mutations are indeed pathogenic because they impact the mRNA rather than the protein itself. With our better understanding of the very complex mechanism of splicing, various bioinformatics tools have been developed. They are now frequently used not only to search for sequence motifs corresponding to splicing signals (splice sites, branch points, ESE, and ESS) but also to predict the impact of mutations on these signals. We now need to address the impact of mutations that affect the splicing process, as their consequences could vary from the activation of cryptic signals to the skipping of one or multiple exons. Despite the major developments of the bioinformatics field coupled to experimental data generated on splicing, it is today still not possible to efficiently predict the consequences of mutations impacting splicing signals, especially to predict if they will lead to exon skipping or to cryptic splice site activation.

Clinical, biochemical, and molecular findings in Argentinean patients with goitrous congenital hypothyroidism

We describe the clinical, biochemical, and molecular findings of a cohort of Argentinean patients with congenital hypothyroidism (CH) and goiter studied to characterize iodide organification and thyroglobulin (TG) defects. 20 CH patients (16 unrelated) were grouped according to serum TG levels and a perchlorate discharge test (PDT) in: group 1 (G1): nine patients with high TG and PDT > 10% who were studied for tiroperoxidase (TPO), dual oxidase 2 (DUOX2), and dual oxidase A2 (DUOXA2) defects and group 2 (G2): 11 patients with low TG and PDT < 10% studied for TG defects. Goiter characteristics, outcome, and TT₄ and TT₃ levels without treatment were compared between groups. 6/9 G1 patients harbored mutations in TPO gene and 3/9 in DUOX2 gene. In G2, mutations of TG gene were found in 3/11 homozygous, 5/11 compound heterozygous, and 3/11 heterozygous patients. Goiter was only evidenced by thyroid scan in the neonatal period in both groups; was moderately enlarged in patients diagnosed during infancy. In the late detected patients, goiter was big and nodular in G1 while diffuse and moderate in G2. Early detected patients grew and developed normally while those diagnosed late were severely mentally retarded in G1 and only mildly retarded in G2. Thyroid hormone levels of G1 were significantly lower than those of G2 P < 0.01. Molecular approach to characterize defects in organification and TG defects was optimized by TG measurements and PDT. Clinical and biochemical differences based on molecular findings will allow further investigations on genotype-phenotype relationships.

Biallelic p.R2223H mutation in the thyroglobulin gene causes thyroglobulin retention and severe hypothyroidism with subsequent development of thyroid carcinoma

CONTEXT

Dyshormonogenesis due to genetic defect in thyroglobulin (Tg) synthesis and secretion can lead to congenital hypothyroidism.

OBJECTIVES

The aim of the study was to analyze the TG gene for the presence of mutations and to study the underlying mechanisms leading to dyshormonogenesis.

CASES

Two siblings aged 25 and 31 yr presented with recurrent goitrous hypothyroidism with undetectable serum Tg. The older sibling was diagnosed with follicular variant of papillary thyroid carcinoma (FVPTC) at age 21 and metastatic FVPTC 8 yr later.

METHODS

The entire coding region of TG gene was sequenced. BRAF, RAS, and P53 mutations or PAX8/PPAR-gamma rearrangement were screened in the FVPTC. Tg expression was studied by immunohistochemistry.

RESULTS

Biallelic c.6725G>A (p.R2223H) and c.6396C>T (p.S2113L) sequence variations were detected in both patients and monoallelic variations in their family members. The c.6396C>T (p.S2113L) sequence variation was found in 14% of 100 population controls, whereas c.6725G>A variation was not present in the controls. Two previously reported polymorphisms (c.2200T>G and c.3082A>G) were present in all the family members. Strong cytoplasmic immunostaining of Tg was observed in the hyperplastic thyroid epithelial cells and weak or no staining in the follicular lumen. Cytoplasmic staining was localized in the endoplasmic reticulum. Reduced staining was found in the FVPTC. Neither RAS, BRAF, or P53 gene mutation nor a PAX8/PPAR-gamma rearrangement was detected in the tumor tissue.

CONCLUSIONS

Biallelic c.6725G>A (p.R2223H) mutation causes Tg retention in the endoplasmic reticulum, resulting in dyshormonogenesis. Prolonged TSH stimulation may promote malignant transformation and development of thyroid cancer. The c.6396C>T (p.S2113L) is a novel polymorphism.