ABSTRACT
Thyroid hormone (T3) regulates bone turnover and mineralization in adults and is essential for skeletal development during childhood. Hyperthyroidism is an established risk factor for osteoporosis. Nevertheless, T3 actions in bone remain poorly understood. Patients with resistance to thyroid hormone, due to mutations of the T3-receptor beta (TRbeta) gene, display variable phenotypic abnormalities, particularly in the skeleton. To investigate the actions of T3 during bone development, we characterized the skeleton in TRbetaPV mutant mice. TRbetaPV mice harbor a targeted resistance to thyroid hormone mutation in TRbeta and recapitulate the human condition. A severe phenotype, which includes shortened body length, was evident in homozygous TRbetaPV/PV animals. Accelerated growth in utero was associated with advanced endochondral and intramembranous ossification. Advanced bone formation resulted in postnatal growth retardation, premature quiescence of the growth plates, and shortened bone length, together with increased bone mineralization and craniosynostosis. In situ hybridization demonstrated increased expression of fibroblast growth factor receptor-1, a T3-regulated gene in bone, in TRbetaPV/PV perichondrium, growth plate chondrocytes, and osteoblasts. Thus, the skeleton in TRbetaPV/PV mice is thyrotoxic and displays phenotypic features typical of juvenile hyperthyroidism.
Subject(s)
Bone Development/genetics , Receptors, Thyroid Hormone/genetics , Thyroid Hormone Resistance Syndrome/physiopathology , Animals , Animals, Newborn , Body Height/genetics , Bone Density , Bone and Bones/abnormalities , Craniosynostoses/genetics , Craniosynostoses/pathology , Female , Gene Expression Regulation, Developmental , Growth Plate , Hyperthyroidism/genetics , Limb Deformities, Congenital/genetics , Limb Deformities, Congenital/pathology , Male , Mice , Mice, Mutant Strains , Receptor Protein-Tyrosine Kinases/genetics , Receptor, Fibroblast Growth Factor, Type 1 , Receptors, Fibroblast Growth Factor/genetics , Receptors, Thyroid Hormone/metabolism , Thyroid Hormone Receptors beta , Thyroid Hormone Resistance Syndrome/genetics , Thyroxine/bloodABSTRACT
Thyroid hormone and the beta isoform of its receptor, Trb, are essential for normal development of the mammalian auditory system. We have analyzed auditory system function and structure in a mouse strain with a targeted Thrb mutation, Thrb(PV), which leads to the loss of binding of thyroid hormone (T3) to the Trb protein. Heterozygosity for the orthologous human THRB(PV) mutation and other similar mutations in human THRB cause resistance to thyroid hormone (RTH), which is occasionally associated with mild sensorineural hearing impairment. Auditory brainstem response analysis of heterozygous Thrb(PV)/+ mice demonstrates that they develop normal hearing. In contrast, Thrb(PV)/Thrb(PV) mice have severe hearing impairment that is already present at 3 weeks of age. This hearing loss is associated with disruption of postnatal morphogenesis of the tectorial membrane and organ of Corti. Comparison with the previously described phenotype of a Thrb -/- knockout strain suggests that Thrb(PV) disrupts the function of other genes that are critical for development and/or maintenance of these structures.
Subject(s)
Cochlea/abnormalities , Mutation/physiology , Thyroid Hormone Receptors beta/genetics , Triiodothyronine/physiology , Amino Acid Sequence/genetics , Animals , Animals, Newborn/growth & development , Cochlea/growth & development , Cochlea/physiopathology , Congenital Abnormalities/genetics , Congenital Abnormalities/physiopathology , Drug Resistance/genetics , Hearing/physiology , Humans , Mice , Mice, Transgenic/genetics , Molecular Sequence DataABSTRACT
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.
