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 res istance to thyroid hormone (GRTH) could result from differential inter actions between the mutant thyroid hormone (T3) receptor-beta (TRbeta) on T3 response elements (TREs) in different T3-responsive genes. To e xplore this hypothesis, the mutant TRbeta associated with kindred A, P 448H; a TRbeta mutant, P448L; and a comparable TRalpha mutant (P398H) were tested for intrinsic function and for inhibition of wild-type TRa lpha- and -beta-induced expression from four structurally distinct TRE s, the rGH ABC, the rGH palindrome (PAL), the rat malic enzyme (ME), and the chicken lysozyme silencer F2 (F2). The relative function of th e mutants was similarly reduced on the four TREs studied and was T3 co ncentration dependent. The TRalpha mutant retained the intrinsically g reater potency characteristic of this isoform, but remained impaired w ith respect to wild-type TRalpha even at 500 nm T3. In general, domina nt negative inhibition of wild-type TRalpha and -beta function was dep endent upon the T3 concentration, as expected from the decreased affin ity for ligand conferred by this mutation. A T3 concentration sufficie nt 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 d egree of inhibition of a specific gene in GRTH individuals. Further, t he lack of dominant negative potential does not explain the absence of TRalpha mutations in GRTH kindreds.