Thyroid hormone receptor mutants that cause resistance to thyroid hormone. Evidence for receptor competition for DNA sequences in target genes.
Several distinct mutations in the ligand-binding domain of the beta form of the thyroid hormone receptor have been reported in kindreds with the autosomal dominant syndrome of generalized resistance to thyroid hormone (GRTH). GRTH receptor mutants are functionally inactive but capable of inhibiting normal receptor function in transient expression studies. We examined the possibility that this dominant negative activity of the GRTH mutants involves competition for receptor binding to DNA. Mutations introduced into either the T3 ligand-binding domain (LBD) or into the DNA-binding domain (DBD) of the receptor eliminated the transcriptional activity of the receptor. In cotransfection experiments, the LBD mutants, but not the DBD mutants, inhibited the transcriptional activity of the normal receptor. The inhibitory activity of the LBD mutants was abolished by the introduction of an additional mutation into the DBD, suggesting that the DBD is required for dominant negative activity. A chimeric receptor, in which the DNA-binding domain of the thyroid hormone receptor was exchanged with the homologous region in the glucocorticoid receptor (GTG), was used to study thyroid hormone receptor competition for GTG interactions with thyroid receptor target sequences. In the absence of thyroid hormone, the normal thyroid hormone receptor inhibited dexamethasone stimulated transcription by GTG. The transcriptional activity of GTG was also inhibited by the LBD mutants but not by a DBD mutant of the thyroid hormone receptor. These results indicate that the thyroid hormone receptor mutations that occur in GRTH compete with normal receptors at DNA-binding sites in target genes to block normal receptor function.