Differential sensitivity of thyroid hormone receptor isoform homodimers and mutant heterodimers to hormone-induced dissociation from deoxyribonucleic acid: its role in dominant negative action.

General resistance to thyroid hormone is an inheritable disease with resistance of peripheral tissues to elevated levels of thyroid hormone. Genetic studies have shown that it is due to interference in the functions of wild-type thyroid hormone nuclear receptors (wTRs) via the dominant negative effect of mutant TRs (mTRs). The present study compared the heterodimerization of the two TR isoforms, TR beta1 and TR alpha1, with mutant TRs to understand if mTRs had isoform-dependent dominant negative action. Using electrophoresis gel mobility shift assay, we have demonstrated that mutant PV, S, ED, and OK form heterodimers with wTR alpha1 and deltaTR beta1 (in which the A/B domain of wTR beta1 has been deleted), on the F2-thyroid hormone response element (TRE). In the presence of T3, both homo- and heterodimer complexes are dissociated in a T3 concentration dependent manner. The ED50 for deltaTR beta1 homodimers was 3-fold higher than that of wTR alpha1 homodimers. ED50s for deltaTR beta1/mTR heterodimers were 10- to 40-fold higher than the corresponding wTR alpha1/mTR heterodimers. Mutant ED and OK homodimers were only partially dissociated at the highest T3 concentrations used (100 nM), whereas no dissociation could be detected for PV and S homodimers, indicating differential sensitivity of the F2-bound TR dimers to the T3-induced dissociation. We presented a model that indicates the dissociation of any particular TR dimer from F2 is determined by competition of T3 for both of its constituent TRs. By transfection assays, we showed that the potency of the dominant negative action of PV on TR alpha1 and TR beta1 inversely correlated with the sensitivity of the appropriate mTR/wTR heterodimer to T3-induced dissociation from F2. The differential dominant negative action of mutants on the two TR isoforms could play an important role in the heterogeneity of tissue-specific manifestations in patients with resistance to thyroid hormone.