RESISTANCE TO THYROID-HORMONE CAUSED BY 2 MUTANT THYROID-HORMONE RECEPTOR-BETA, R243Q AND R243W, WITH MARKED IMPAIRMENT OF FUNCTION THAT CANNOT BE EXPLAINED BY ALTERED IN-VITRO 3,5,3'-TRIIODOTHYROININE BINDING-AFFINITY

Resistance to thyroid hormone (RTH) is a syndrome of reduced responsiv eness to thyroid hormone caused by mutations in the thyroid hormone re ceptor beta (TR beta) gene. Mutant TR beta s exhibit variable degrees of impaired T-3 binding resulting in reduced T-3-mediated function. Th e dominant mode of inheritance is attributed to the ability of mutant TR beta s to interfere with the function of the wild-type (WT) TR, a p henomenon known as dominant negative effect (DNE). We recently identif ied two families with RTH having mutations in amino acid 243 (R243Q an d R243W) in whom the mechanism of RTH appears to be distinct from that of other natural TR beta mutations. These mutations, which are locate d in the hinge domain of the TR beta, do not significantly alter the b inding affinity for T-3, measured in vitro. The present study was unde rtaken to characterize the properties of these mutant TR beta s to und erstand the molecular basis of the RTH phenotype. Two other mutant TR beta producing RTH with mild (320H) and severe (345R) impairment of T- 3 binding were studied in parallel. The results demonstrate that TR be ta s 243Q and 243W could be translocated into the nucleus where they e xerted normal ligand-independent repression of positively regulated th yroid hormone response elements. Yet, the addition of 10 nmol/L T-3 fa iled to normalize the transactivation (16-13% of WT) and revert the DN E exerted by the two TR beta mutants. In contrast, at this T-3 concent ration, the transactivation function of 320H was significantly higher (50% of WT), and the DNE was completely abolished, in keeping with the mild clinical form of RTH. Formation of 243Q and 243W homodimers on t hyroid hormone response elements could not be as readily prevented by T-3 as those formed by the WT and 320H TR beta s. These results sugges t that the substitution of R243 in TR beta produces RTH by increasing the propensity for the formation of tightly bound homodimers or by red uction of the receptor affinity for T-3 only after it binds to DNA.