MUTATIONS OF CPG DINUCLEOTIDES LOCATED IN THE TRIIODOTHYRONINE (T-3)-BINDING DOMAIN OF THE THYROID-HORMONE RECEPTOR (TR) BETA-GENE THAT APPEARS TO BE DEVOID OF NATURAL MUTATIONS MAY NOT BE DETECTED BECAUSE THEY ARE UNLIKELY TO PRODUCE THE CLINICAL PHENOTYPE OF RESISTANCE TO THYROID-HORMONE

Thyroid hormone receptor (TR) beta gene mutations identified in patien ts with resistance to thyroid hormone (RTH) revealed two clusters (''h ot'' areas) of mutations (RTHmut) in the triiodothyronine (T-3)-bindin g domain. Furthermore, 45% of RTHmuts and 90% of recurring mutations a re located in CpG dinucleotides (''hot spots''). To investigate why th e region between the two hot areas lacks RTHmuts, we produced 10 artif icial mutant TR beta s (ARTmut) in this ''cold'' region according to t he hot spot rule (C --> T or G --> A substitutions in CpGs). The prope rties of ARTmuts were compared with those of six RTHmuts. Among all RT Hmuts, R320H manifesting a mild form of RTH showed the least impairmen t of T-3-binding affinity (K-a). In contrast, K-a was normal in six AR Tmuts (group A), reduced to a lesser extent than R320H in three (group B), and one that was truncated (R410X) did not bind T-3. All RTHmuts had impaired ability to transactivate T-3-responsive elements and exhi bited a strong dominant negative effect on cotransfected wild-type TR beta. Group B and A ARTmuts had minimally impaired or normal transacti vation and weak or no dominant negative effect, respectively. R410X sh owed neither transactivation nor dominant negative effect. Natural mut ations expected to occur in the cold region of TR beta should fail to manifest as RTH (group A) or should escape detection (group B) since t he serum thyroid hormone levels required to compensate for the reduced binding affinity should be inferior to those found in subjects with R 320H. R410X would manifest RTH only in the homozygote state. The cold region of the putative T-3-binding domain is relatively insensitive to amino acid changes and, thus, may not be involved in a direct interac tion with T-3.