A novel mechanism of inherited TBG deficiency: mutation in a liver-specific enhancer

Mass spectrometry in measurement of thyroid biomarkers

In 2022, the number of patients with thyroid disease in China exceeded 200 million (10 million with hyperthyroidism, 90 million with hypothyroidism, and 100 million with other thyroid disease such as goiter, thyroid nodules, and thyroid cancer). Well-established markers include FT3, FT4, TT3, TT4, and TSH tested by a number of immunoassay methods. This approach is based on the primary binding of antigen with antibody and a subsequent secondary chemical reaction that provides an indirect measure. The use of traceable standards for quantitation remains an important factor to ensure inter-assay reliability and precision. Recently, mass spectrometry (MS) has received considerable attention as an analytic tool due to high resolution and quantitative accuracy. In addition, MS allows for sensitive determination of low-abundance markers making it ideal for development of traceable standards. Furthermore, this technology will allow for the development of highly accurate thyroid biomarker assays to facilitate diagnosis, enable early treatment and improve outcomes. Herein, we provide a systematic review and summary of MS in enhancing the analysis of thyroid biomarkers.

A cell type-aware framework for nominating non-coding variants in Mendelian regulatory disorders

Unsolved Mendelian cases often lack obvious pathogenic coding variants, suggesting potential non-coding etiologies. Here, we present a single cell multi-omic framework integrating embryonic mouse chromatin accessibility, histone modification, and gene expression assays to discover cranial motor neuron (cMN) cis-regulatory elements and subsequently nominate candidate non-coding variants in the congenital cranial dysinnervation disorders (CCDDs), a set of Mendelian disorders altering cMN development. We generated single cell epigenomic profiles for ~86,000 cMNs and related cell types, identifying ~250,000 accessible regulatory elements with cognate gene predictions for ~145,000 putative enhancers. Seventy-five percent of elements (44 of 59) validated in an in vivo transgenic reporter assay, demonstrating that single cell accessibility is a strong predictor of enhancer activity. Applying our cMN atlas to 899 whole genome sequences from 270 genetically unsolved CCDD pedigrees, we achieved significant reduction in our variant search space and nominated candidate variants predicted to regulate known CCDD disease genes MAFB, PHOX2A, CHN1, and EBF3 - as well as new candidates in recurrently mutated enhancers through peak- and gene-centric allelic aggregation. This work provides novel non-coding variant discoveries of relevance to CCDDs and a generalizable framework for nominating non-coding variants of potentially high functional impact in other Mendelian disorders.

Partial Thyroid Hormone-Binding Globulin Deficiency: A Case Report and Literature Review

Background

Thyroxine binding globulin (TBG) deficiency is a rare thyroid disease, mostly caused by genetic mutations and acquired by X-linked recessive inheritance. The clinical features of children with TBG deficiency and their family members were summarised and the Serpina7 gene mutation was analysed, providing a reference for the differentiation of TBG deficiency.

Methods

Thyroid function was detected in TBG deficient patients, and genetic analysis was performed using polymerase chain reaction (PCR) and direct DNA sequencing to detect the characteristics of TBG mutants. Using "thyroxine binding globulin, gene and mutation" as keywords, PubMed (biomedical literature database), Web of Science and other databases were searched for relevant studies to collect and summarise relevant information.

Results

The TBG (14.7 μg/mL), 70% triiodothyronine (T3) (<0.3 nmol/L), total T3 (Tr3) (<0.05 ng/mL) and thyroxine (T4) (14.72 nmol/L) values were lower than normal, while the thyrotropin (TSH) (2.33 uIU/mL), free T3 (FT3) (1.62 pmol/L), and free T4 (FT4) (11.39 pmol/L) values were normal. These values indicate a TBG partially deficient phenotype. Using PCR amplification and direct sequencing of the target gene, a missense mutation in exon 4 of the Serpina7 gene was found in the patient and the father, and the nucleic acid variant was C.909 (exon 4) g > T; the patient was heterozygous and the father was hemizygous. The literature search retrieved a total of 45 studies, most of which were related to mutations in the Serpina7 gene. The mutation locations included exons, introns, enhancers and promoters, with exons the predominant location. A total of 49 variants of the Serpina7 gene were identified.

Conclusion

Serpina7 C.909G (P.L303F) is a mutation acquired from the father by X-linked recessive inheritance. The main clinical features of TBG deficiency patients are low serum T4, T3 and TBG levels, normal TSH, FT3 and FT4 levels, and no clinical manifestations.

Clinical recognition and evaluation of patients with inherited serum thyroid hormone-binding protein mutations

There are three important thyroid hormone-binding proteins in human serum, thyroxine-binding globulin, transthyretin, and albumin. Genetic variation in these proteins can lead to altered thyroid hormone binding and abnormalities in serum tests of thyroid hormone. Importantly, patients harboring these mutations are euthyroid; thus, the recognition of these conditions is crucial to prevent unnecessary repeated testing and treatment. This article provides an updated overview of serum thyroid hormone transport biology and reviews the underlying genetic alterations, clinical presentation, and appropriate evaluation of patients with suspected mutations in serum thyroid hormone-binding proteins.

Novel frameshift mutation causes early termination of the thyroxine-binding globulin protein and complete thyroxine-binding globulin deficiency in a Chinese family: A case report

BACKGROUND

Thyroxine-binding globulin (TBG; the gene product of SERPINA7) is the main transporter of thyroid hormones in humans. Mutations in the TBG gene may lead to inherited TBG deficiency. There have been 28 reported mutations that associate with complete TBG deficiency (TBG-CD). Here we identified a novel frameshift mutation causing early termination of the TBG protein and TBG-CD in a Chinese family.

CASE SUMMARY

A 46-year-old Chinese man was referred to our hospital with normal free thyroxine, free triiodothyronine, thyrotropin, but lower total thyroxine and total triiodothyronine, and undetectable serum TBG, indicative of TBG-CD. Blood samples were obtained from the patient’s family members and thyroid function and serum TBG were evaluated. Genomic DNA from peripheral blood was sequenced to detect possible TBG mutation(s). Quantitative PCR high-resolution melting curve analysis was used to screen TBG-Poly (L283F) among 117 Chinese men. A novel mutation of TBG (p.Phe135Alafs*21), a 19-nucleotide insertion in exon 1, was identified, which resulted in a truncated TBG protein product and caused TBG-CD. The other mutation, identified in the proband’s father, is a known polymorphism, TBG-Poly (L283F). The frequency of the TBG-Poly allele among 117 unrelated Han Chinese men from northeast China was 21.37%.

CONCLUSION

A novel mutation in the TBG gene associated with the TBG-CD phenotype was identified in a Chinese family. Additionally, it was found that 21.37% of Chinese males had TBG-Poly (L283F).

Thyroxine-binding globulin deficiency due to a novel SERPINA7 mutation: Clinical characterization, analysis of X-chromosome inactivation pattern and protein structural modeling

OBJECTIVE

Thyroxine-binding globulin (TBG) is the major human thyroid hormone transport protein, encoded by the SERPINA7 gene (Xq22.2). We aim to investigate the molecular basis of partial TBG deficiency (TBG-PD) in a female, by evaluating the X-chromosome inactivation pattern as well as the mutant protein structural modeling.

DESIGN AND METHODS

Sequencing of the coding region of the SERPINA7 gene was performed in a female with a TBG-PD phenotype and her first-degree relatives. The proband presented with low serum levels of total T3 (TT3) and total T4 (TT4), serum TSH level of 5.4 μUI/mL (normal range, 0.35-5.5), and serum TBG level of 5.5 mg/L (normal range, 13.6-27.2). X-chromosome inactivation pattern was evaluated by methylation analysis of the androgen receptor gene (Xq11.2). Structural analysis of the SERPIN family was performed using Pymol and Areaimol, and PFSTATS for conservation analysis and family-wide investigation of equivalent positions in human homologs. Modeller was used for point mutation structural modeling.

RESULTS

A novel missense SERPINA7 mutation (p.R35W; c.163C > T) was found in heterozygosity in the proband, and in hemizygosity in her affected siblings. The proband X-chromosome inactivation ratio was 20:80. The substitution of an arginine by a tryptophan is predicted to disrupt the protein surface and main electrostatic interactions. Tryptophans are extremely rare (0.1%) in this position.

CONCLUSIONS

We report a new SERPINA7 variant associated with TBG-PD in three siblings. We named this variant TBG-Brasilia. The X-chromosome inactivation pattern may have accounted for the rare phenotypic expression in a female. The hydrophobic nature of the mutant is predicted to create an apolar patch at the surface, which results in protein aggregation and/or misfolding, potentially responsible for thyroid hormone transport defect.

Directional thyroid hormone distribution via the blood stream to target sites

Thyroid hormones are bound to three major serum transport proteins, thyroxin-binding globulin (TBG), transthyretin (TTR) and human serum albumin (HSA). TBG has the strongest affinity for thyroid hormones, TTR is also found in the cerebrospinal fluid and HSA is the most abundant protein in plasma. Combination defects of either a high affinity TTR or HSA variant do not compensate TBG deficiency, underscoring the dominant role of TBG among the thyroid hormone transport proteins. On the other hand, coexistence of raised affinity TTR and HSA variants causes an augmented hyperthyroxinemia. Variations in thyroid hormone transport proteins may alter thyroid function tests to mimic hypo- or hyperthyroidism. As affected individuals are clinically euthyroid and do not require treatment, identification of thyroid hormone transport protein defects is important to avoid unnecessary diagnostic and therapeutic interventions. Mammals share the multilayered system of thyroid hormone binding proteins with humans. Some of them, especially carnivores, do not express TBG. In dogs, this defect has been shown to be caused by a defective hepatocyte nuclear factor-1 binding site in the TBG promoter, preventing TBG synthesis in the liver. The major endogenous thyroid hormone metabolite 3-iodothyronamine (3-T1AM) exerts marked cryogenic, metabolic, cardiac and central nervous system actions. It is bound to apolipoproteinB-100 (ApoB100), possibly facilitating its cellular uptake via interaction with the low density lipoprotein-receptor. This review summarizes the handling of hydrophobic charged thyroid hormone signaling molecules and their metabolite 3-T1AM in aqueous body fluids and the advantages and limits of their serum distributor proteins.

First report of inherited thyroxine-binding globulin deficiency in Iran caused by a known de novo mutation in SERPINA7

Background

Thyroxine-binding globulin (TBG) is the main transporter of thyroid hormones in human serum, encoded by the gene TBG (SERPINA7), located in long arm of X-chromosome (Xq21-q22). Deficiency of SERPINA7 (serum protease inhibitor, clade A [alpha-1 antiproteinase, antitrypsin], member 7) leads to inherited TBG deficiency. Several mutations have been reported in the coding and noncoding regions of SERPINA7 in association with TGB deficiency.

Methods

Automated chemiluminescence immunoassays were used to determine TSH, free and total T4 and T3 (fT4, TT4, TT3) and TBG. Direct DNA sequencing identified the mutation in SERPINA7.

Results

We present a 3 and 4/12 year old boy, born premature, who was mismanaged as hypothyroidism before referral to our center, and was diagnosed with TBG deficiency at our center with a hemizygous substitution in exon 1, position c.347T > A, leading to replacement of isoleucine for arginine in position 96 (considering the first 20 amino acid signal peptide).

Conclusion

This known mutation, reported as the first SERPINA7 mutation in Iran, emphasizes the point that endocrinologists should pay more attention to inherited TBG to prevent unnecessary treatment.

Inherited defects of thyroxine-binding proteins

Thyroid hormones (TH) are bound to three major serum transport proteins, thyroxine-binding globulin (TBG), transthyretin (TTR) and human serum albumin (HSA). TBG has the strongest affinity for TH, whereas HSA is the most abundant protein in plasma. Individuals harboring genetic variations in TH transport proteins present with altered thyroid function tests, but are clinically euthyroid and do not require treatment. Clinical awareness and early recognition of these conditions are important to prevent unnecessary therapy with possible untoward effects. This review summarizes the gene, molecular structure and properties of these TH transport proteins and provides an overview of their inherited abnormalities, clinical presentation, genetic background and pathophysiologic mechanisms.