DOMINANT-NEGATIVE INHIBITION BY MUTANT THYROID-HORMONE RECEPTORS IS THYROID-HORMONE RESPONSE ELEMENT AND RECEPTOR ISOFORM-SPECIFIC

The heterogeneity of tissue-specific manifestations of generalized resistance to thyroid hormone (GRTH) could result from differential interactions between the mutant thyroid hormone (T3) receptor-beta (TRbeta) on T3 response elements (TREs) in different T3-responsive genes. To explore this hypothesis, the mutant TRbeta associated with kindred A, P448H; a TRbeta mutant, P448L; and a comparable TRalpha mutant (P398H) were tested for intrinsic function and for inhibition of wild-type TRalpha- and -beta-induced expression from four structurally distinct TREs, the rGH ABC*, the rGH palindrome (PAL), the rat malic enzyme (ME), and the chicken lysozyme silencer F2 (F2). The relative function of the mutants was similarly reduced on the four TREs studied and was T3 concentration dependent. The TRalpha mutant retained the intrinsically greater potency characteristic of this isoform, but remained impaired with respect to wild-type TRalpha even at 500 nm T3. In general, dominant negative inhibition of wild-type TRalpha and -beta function was dependent upon the T3 concentration, as expected from the decreased affinity for ligand conferred by this mutation. A T3 concentration sufficient to relieve the inhibition of wild-type TR function on the ABC*, PAL, and ME TREs (50 nm) had no effect on inhibition of the F2 TRE by the mutant TRs. Receptor isoform preferential inhibition was observed on the ABC*, PAL, and ME TREs by the mutant TRs. Thus, both TRE structure and the isoform of endogenously active receptor could determine the degree of inhibition of a specific gene in GRTH individuals. Further, the lack of dominant negative potential does not explain the absence of TRalpha mutations in GRTH kindreds.