Differential expression of thyroid hormone receptor isoforms dictates the dominant negative activity of mutant β receptor

Abstract

Mutations in the thyroid hormone receptor β gene (TRβ) cause resistance to thyroid hormone (RTH). Genetic analyses indicate that phenotypic manifestation of RTH is due to the dominant negative action of mutant TRβ. 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 (TRβPV mouse) that faithfully reproduces human RTH to address these questions. We demonstrated directly that TRβ1 protein was approximately 3-fold higher than TRα1 in the liver of TRβ+/+ mice but was not detectable in the heart of wild-type and TRβPV mice. The abundance of PV in the liver of TRβPV/PV was more than TRβPV/+ mice but not detectable in the heart. TRα1 in the liver was approximately 6-fold higher than that in the heart of wild-type and TRβPV 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 (T3)-positive regulated genes in the liver. In the heart, however, PV was significantly lower and thus could not effectively compete with TRα1 for binding to TRE, resulting in activation of the T3-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 β receptor, thereby resulting in variable phenotypic expression in RTH.