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Differential expression of thyroid hormone receptor isoforms dictates the dominant negative activity of mutant Beta
Xiao-Yong Zhang1, Masahiro Kaneshige, Yuji Kamiya
1Gene Regulation Section, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.
Abstract:
Mutations in the thyroid hormone receptor beta gene (TRbeta) cause resistance to thyroid hormone (RTH). Genetic analyses indicate that phenotypic manifestation of RTH is due to the dominant negative action of mutant TRbeta. However, the molecular mechanisms underlying the dominant negative action of mutants and how the same mutation results in marked variability of resistance in different tissues in vivo are not clear. Here we used a knock-in mouse (TRbetaPV mouse) that faithfully reproduces human RTH to address these questions. We demonstrated directly that TRbeta1 protein was approximately 3-fold higher than TRalpha1 in the liver of TRbeta(+/+) mice but was not detectable in the heart of wild-type and TRbetaPV mice. The abundance of PV in the liver of TRbeta(PV/PV) was more than TRbeta(PV/+) mice but not detectable in the heart. TRalpha1 in the liver was approximately 6-fold higher than that in the heart of wild-type and TRbetaPV mice. Using TR isoforms and PV-specific antibodies in gel shift assays, we found that in vivo, PV competed not only with TR isoforms for binding to thyroid hormone response elements (TRE) but also competed with TR for the retinoid X receptors in binding to TRE. These competitions led to the inhibition of the thyroid hormone (T(3))-positive regulated genes in the liver. In the heart, however, PV was significantly lower and thus could not effectively compete with TRalpha1 for binding to TRE, resulting in activation of the T(3)-target genes by higher levels of circulating thyroid hormones. These results indicate that in vivo, differential expression of TR isoforms in tissues dictates the dominant negative activity of mutant beta receptor, thereby resulting in variable phenotypic expression in RTH.
Insights
Thyroid hormone resistance (RTH) arises from TRbeta gene mutations. This study reveals tissue-specific TRbeta expression dictates mutant receptor activity, explaining variable RTH symptoms in different organs.
Area of Science:
- Endocrinology
- Molecular Biology
- Genetics
Background:
- Mutations in the thyroid hormone receptor beta (TRbeta) gene cause resistance to thyroid hormone (RTH), a condition where the body doesn't respond properly to thyroid hormones.
- The dominant negative action of mutant TRbeta is understood to cause RTH, but the precise molecular mechanisms and reasons for variable tissue-specific resistance remain unclear.
- Understanding these mechanisms is crucial for developing targeted therapies for RTH.
Purpose of the Study:
- To elucidate the molecular mechanisms underlying the dominant negative action of mutant TRbeta in resistance to thyroid hormone (RTH).
- To investigate how differential tissue expression of TR isoforms contributes to the variable phenotypic manifestation of RTH in vivo.
- To utilize a knock-in mouse model (TRbetaPV) that accurately replicates human RTH to address these questions.
Main Methods:
- Utilized a TRbetaPV knock-in mouse model to study RTH.
- Quantified the expression levels of TRbeta1, TRalpha1, and the mutant PV receptor in liver and heart tissues using Western blotting and specific antibodies.
- Performed gel shift assays with TR isoforms and PV-specific antibodies to analyze receptor binding to thyroid hormone response elements (TRE) and competition with retinoid X receptors (RXRs).
Main Results:
- TRbeta1 protein was significantly higher in the liver than in the heart of wild-type and TRbetaPV mice; PV was undetectable in the heart.
- In the liver, the mutant PV receptor competed with TR isoforms for TRE binding and with TR for RXR binding, inhibiting T(3)-regulated genes.
- In the heart, lower PV levels resulted in less competition, allowing TRalpha1 to activate T(3)-target genes, explaining tissue-specific resistance variability.
Conclusions:
- Differential expression of thyroid hormone receptor isoforms in various tissues dictates the dominant negative activity of mutant TRbeta.
- This tissue-specific differential expression explains the variable phenotypic expression observed in resistance to thyroid hormone (RTH) patients.
- The TRbetaPV mouse model is a valuable tool for studying the molecular basis of RTH and its variable clinical manifestations.