Novel insight from transgenic mice into thyroid hormone resistance and the regulation of thyrotropin
E. Dale Abel, … , Douglas Forrest, Fredric E. Wondisford
E. Dale Abel, … , Douglas Forrest, Fredric E. Wondisford
Published January 15, 1999
Citation Information: J Clin Invest. 1999;103(2):271-279. https://doi.org/10.1172/JCI5205.
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Article

Novel insight from transgenic mice into thyroid hormone resistance and the regulation of thyrotropin

Abstract

Patients with resistance to thyroid hormone (RTH) exhibit elevated thyroid hormone levels and inappropriate thyrotropin (thyroid-stimulating hormone, or TSH) production. The molecular basis of this disorder resides in the dominant inhibition of endogenous thyroid hormone receptors (TRs) by a mutant receptor. To determine the relative contributions of pituitary versus hypothalamic resistance to the dysregulated production of thyroid hormone in these patients, we developed a transgenic mouse model with pituitary-specific expression of a mutant TR (Δ337T). The equivalent mutation in humans is associated with severe generalized RTH. Transgenic mice developed profound pituitary resistance to thyroid hormone, as demonstrated by markedly elevated baseline and non–triodothyronine (T3)-suppressible serum TSH and pituitary TSH-β mRNA. Serum thyroxine (T4) levels were only marginally elevated in transgenic mice and thyrotropin-releasing hormone (TRH) gene expression in the paraventricular hypothalamus was downregulated. After TRH administration, T4 concentrations increased markedly in transgenic, but not in wild-type mice. Transgenic mice rendered hypothyroid exhibited a TSH response that was only 30% of the response observed in wild-type animals. These findings indicate that pituitary expression of this mutant TR impairs both T3-mediated suppression and T3-independent activation of TSH production in vivo. The discordance between basal TSH and T4 levels and the reversal with TRH administration demonstrates that resistance at the level of both the thyrotroph and the hypothalamic TRH neurons are required to elevate thyroid hormone levels in patients with RTH.

Authors

E. Dale Abel, Helen C. Kaulbach, Angel Campos-Barros, Rexford S. Ahima, Mary-Ellen Boers, Koshi Hashimoto, Douglas Forrest, Fredric E. Wondisford

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Transgenic construct, pituitary expression of the transgene, and impact ...
Transgenic construct, pituitary expression of the transgene, and impact on basal thyroid hormone concentrations. (a) Schematic representation of the transgene. The vector contains 4.6 kb of the mouse glycoprotein subunit α gene cloned upstream of the human TR-β1 cDNA harboring the mutation Δ337T and ligated to a 400-bp fragment containing the SV40 polyadenylation signal. (b) RT-PCR scheme for detecting transgene expression and expression of the mutant and wild-type TR transcripts in mouse pituitaries and other tissues. RNA was obtained from pituitary, hypothalamus, liver, kidney, and heart, and analyzed by RT-PCR using primers (denoted a and b) that are homologous both to the human and mouse TR sequences. These primers amplify 261 bp of the carboxyl-terminus of the human and mouse TR-β gene. A polymorphism in the human gene results in the presence of a Sma1 restriction site that is not present in the mouse gene. Restriction digestion with this enzyme results in the appearance of 126/137-bp overlapping fragments indicating transgene expression. Gels shown were performed after Sma1 digestion of the RT-PCR products. Note that the smaller transcript appears only in the pituitaries of TG mice but is absent from WT mouse pituitaries and from multiple other tissues, including the hypothalamus in TG mice. Data is shown for line 1 mice; similar results were obtained for line 2 mice, which had 50% lower pituitary transgene expression vs. line 1. (c) Basal total T4, TSH concentrations, and TSH/T4 ratios (means ± SEM) in the WT and TG mice from two independent transgenic founder lines (1 and 2). The number of animals used for T4 determinations are WT: n = 89; line 1: n = 84; and line 2: n = 39, respectively. The number of animals used for TSH determinations are WT: n = 51; line 1: n = 40; and line 2: n = 46. The number of samples on which T4 and TSH were simultaneously measured for the purpose of calculating the TSH/T4 ratio are WT: n = 24; line 1: n = 26; and line 2: n = 26. Note the small increase in T4 concentration in line 1 and the lack of T4 elevation in line 2, despite large increases in TSH concentration. **P < 0.01 vs. WT; ***P < 0.0001 vs. WT. RT, reverse transcription; T4, thyroxine; TG, transgenic; TSH, thyroid-stimulating hormone; WT, wild-type.