REGULATION OF THE HUMAN TRH (HTRH) GENE BY HUMAN THYROID-HORMONE RECEPTOR BETA-1 (HTR-BETA-1) MUTANTS

TRH is negatively regulated by T3 both in the hypothalamic paraventric ular nucleus and transient transfection models. Mutations in hTR beta 1 genes are associated with the syndrome of generalized resistance to thyroid hormone. To investigate potential effects of mutant TRs on T3 regulation of the hTRH gene, transient gene expression assays were per formed in human neuroblastoma (HTB-11) cells with an hTRH promoter-luc iferase construct, wild type (WT) hTR beta 1, and three qualitatively distinct hTR beta 1 mutant forms (ED, OK and PV). In the presence of T 3 (10(-9) M), liganded WT-hTR beta 1 inhibited hTRH promoter activity significantly (40%). Co-transfection of each of the two mutants (ED an d OK) achieved similar levels of inhibition only at 10 to 100 fold inc reased T3 concentrations. Of interest, a 10x excess of mutant ED or OK could also exert dominant negative effects upon WT hTR beta 1-T3 medi ated inhibitory actions on the hTRH promoter. In contrast, mutant TR-P V exerted neither inhibitory nor dominant negative effects at even hig her concentrations of T3. Moreover, all three unliganded mutant forms stimulated TRH promoter activity significantly in the absence of T3, d espite their different mutations in the ligand-binding domain (LBD). T hese data demonstrate that thyroid hormone resistance at the level of TRH gene regulation, due to reduced inhibitory actions of mutant TR-T3 complexes, as well as dominant negative effects upon WT hTR beta 1 me diated inhibition, likely contribute to elevated TSH values observed i n the syndrome of thyroid hormone resistance.