Incidental identification of a thyroid hormone receptor beta (THRB) gene variant in a family with autoimmune thyroid disease

Abstract

Background: Resistance to thyroid hormone (RTH) is a rare condition usually diagnosed in patients with classic thyroid function tests (TFTs) of elevated thyroid hormone levels with nonsuppressed TSH. The presence of autoimmune thyroid disease (AITD) can confound the clinical diagnosis of RTH. A family was evaluated because several members had elevated TSH and normal or low serum T4 concentrations with AITD. While these individuals were initially reported to have RTH, they were found to have a normal thyroid hormone receptor beta (THRB) gene sequence, and three other asymptomatic family members were found to harbor the variant TRβ G339S.

Methods: The THRB gene was sequenced in 19 members of a large Mexican/Aztec family. In vitro expression of the mutant TRβ protein was performed, as well as computer modeling of the variant compared to known mutations in the flanking codons.

Results: Investigation of an individual with AITD who was incorrectly diagnosed with RTH led to the fortuitous discovery of a THRB gene variant (G339S) in the proposita's father, paternal aunt, and cousin. This variant was not detected in analysis of 124 unrelated alleles. All individuals harboring G339S had normal TFTs. Normal in vitro expression and function of G339S and molecular modeling predicted that this variant would not have an effect on the hypothalamic-pituitary-thyroid axis as determined by thyroid hormone binding in vitro and thyroid function tests in vivo, despite profound effects seen in mutations in the adjacent codons 338 and 340.

Conclusion: We report an individual with normal TFTs and AITD harboring a novel THRB gene variant. In addition to illustrating the importance of accurate diagnosis of thyroid disease so that proper treatment and counseling can be given, TRβ codon 339 is not essential for normal TRβ function.

FIG. 1.

Pedigree and results of thyroid function tests. Square symbols indicate males, circles indicate females, Roman numeral to the left of the pedigree indicates the generation, and numerals on the right of each symbol indicate individual family member. Shaded symbols to the right indicate that a subject have the mutant TRβ G339S (I-3, I-6, II-10). Shaded symbols to the left indicate that a subject is positive for autoantibodies (I-1, I-2, I-3, II-1, II-2, II-3). Subject I-8 was not available for investigation but must also harbor the mutation because his son II-10 has the mutation. Abnormal values above the upper limit of normal are in red and abnormal values below the lower limit of normal are in blue. The arrow indicates the proposita. AITD, autoimmune thyroid disease; T₃, 3,3′,5-triiodothyronine; T₃r, 3,3′,5′-triiodothyronine; T₄, thyroxine; TT₃, TT₃r, and TT₄, total T₃, T₃r, and T₄, respectively; FT₄I, free T₄ index; TSH, thyrotropin; TG, thyroglobulin; TPO, thyroperoxidase; ab, autoantibodies; WT, wild type.

FIG. 2.

Modeling of the G339S mutation on the structure of TRβ. Modeling has been done using atomic coordinates of ligand binding domain of human TRβ bound to T3 (3GWS.pdb). The structure of TRβ is shown as a cartoon; some helices are indicated. The hormone is shown as a stick model with carbons in orange, iodines in magenta, oxygens in red, and nitrogens in blue. The surface presumably involved in homodimerization and heterodimerization with RXR is shown in blue. The position of mutation G339S is indicated using the red sphere. Lower inset: The area surrounding position 339 with Ser modeled in is shown in detail, and the neighboring residues are indicated. Upper inset: The superposition of 20 predicted conformations of the same area calculated using Rosetta Backrub is shown. The sidechains are shown as stick models with modeled Ser 339 being show as a thick stick model.

FIG. 3.

In vitro functional analysis of TRβ1. Analysis of 339S, and 338W compared with WT G339 and R338. Cos-7 cells were transfected with the empty vector to determine the background luciferase activity. Results are expressed relative to those obtained with the WT TRβ1. Data shown are from a single experiment. In contrast to the mutant 338W, the mutant 339S does not show a reduced stimulation with T₃ compared to the WT. Error bars represent standard error of the mean. RLU, relative luminescence units.