A TSHβ Variant with Impaired Immunoreactivity but Intact Biological Activity and Its Clinical Implications

Abstract

Background: Thyrotropin (TSH) deficiency caused by TSHβ gene mutations is a rare form of congenital central hypothyroidism. Nine different TSHβ gene mutations have been reported, all with clinical manifestations. The aim was to identify the genetic cause of undetectable TSH levels in two siblings with clinical euthyroidism.

Methods: Two brothers born to consanguineous Pakistani parents presented with undetectable serum TSH but normal iodothyronine concentrations and no clinical signs of hypothyroidism. Direct sequencing of the TSHβ gene, functional and immunological studies, protein homology modeling, and population frequency analysis were performed to characterize the cause of undetectable TSH in this family.

Results: Direct sequencing of the TSHβ gene revealed that the two brothers were homozygous for a single nucleotide substitution (c.223A>G) resulting in the replacement of arginine 55 with glycine (R55G). This variant was found in 12 out of 5008 alleles in the 1000 Genomes project (all South Asian). Serum TSH of the two brothers was undetectable in two of five platforms, both produced by Siemens, whereas TSH levels of the heterozygous brother and mother were half compared to the other three platforms (Roche Elecsys, Abbott Architect, and Beckman Coulter DxI). The falsely low TSH concentration was caused by the monoclonal antibody not recognizing the region containing the variant amino acid. This is supported by the fact that arginine modification--following phenylglyoxal treatment--led to a significant (96%) decrease in the TSH measurement with the Siemens platforms. Predictions based on PolyPhen-2 and in silico modeling revealed no functional impairment of the variant TSH.

Conclusions: A TSHβ variant with impaired immunoreactivity, but not bioactivity, is reported, and its biochemical impact in the homo- and heterozygous state is demonstrated. It is also shown that failure to bind to the monoclonal antibody is a direct consequence of the amino acid substitution.

FIG. 1.

Pedigree of the family and results of thyroid function tests and genetic analysis. (A) Results of thyroid function tests are aligned with each symbol representing a member of the family. Abnormal values are in bold numbers. (B) Thyrotropin (TSH) values obtained by five different platforms using immunometric assays. (C) Chromatograms showing sequences for a normal (WT/WT), heterozygous (Mut/WT), and homozygous (Mut/Mut) member of the family for the R55G TSHβ gene variant. Corresponding symbols are open, half-filled, and fully filled. The symbol in brackets indicates a deduced genotype. The proband is indicated with an arrow. Color images available online at www.liebertpub.com/thy

FIG. 2.

Partial representation of the 3D model of the TSH molecule (the TSH receptor in gray and the α-subunit in brown, model generated using the knowledge-based method within Prime v3.2). The normal (WT) molecule is compared to the variant R55G and the functionally impaired mutation G29R, common in Japanese (6,7). Amino acids at positions 55 and 29 are highlighted with yellow and indicated with the red arrows. All modeling images were generated using The PyMOL Molecular Graphics System v1.5.0.4. Color images available online at www.liebertpub.com/thy