Functional properties of a mutant T3 receptor beta (R338W) identified in a subject with pituitary resistance to thyroid hormone.

Previously, we identified a point mutation of the T3 receptor (TR) beta gene (R338W) in a patient with pituitary resistance to thyroid hormone (PRTH). The mutation existed in one of two hot spot areas in TRbeta gene where clusters of mutations have been found in subjects with generalized resistance to thyroid hormone (GRTH). Interestingly, R338W induces the phenotypical features responsible for PRTH. In the present study, we examined the functional properties of R338W in comparison with those of a GRTH-mutant, K443E. The levels of thyroid hormones and inappropriately elevated TSH (SITSH) were similar between subjects with K443E and R338W. Transcriptional activities and dominant negatives potencies were measured by CAT assay in CV1 cells transfected with each mutant TRbeta1 or along with wild-type TR. When a reporter gene containing T3-responsive elements (TRE), TRE-pal2, DR4 or myosin heavy chain alpha subunit, was used, transcriptional activation induced by R338W was higher than that by K443E. At 50 nM T3, K443E decreased the transcriptional activity of wild-type TRbeta1 on TRE-pal2 by 31.5%, while R338W reduced by 13.6% (n = 15, P < 0.05). Co-expression of retinoid X receptor (RXR) alpha increased transcriptional activity of R338W and K443E, but not of wild-type TRbeta1. Dominant negative activity on TRE-TSHalpha subunit of R338W was milder than that of K443E. When T3-binding activities of mutant TRbeta1s expressed in the cells were assayed under the same cell conditions for CAT assay, both mutant TRbeta1 showed remarkably reduced activity with no difference between the two. Gel mobility shift assay using TRE-DR4 showed poor homodimer formation of R338W. Heterodimerization with RXRalpha was similar between R338W, K443E and wild-type TRbeta1. The result of the present study suggested that R338W had relatively mild transcriptional and dominant negative activities on several TREs including TRE-TSHalpha subunit. We also showed poor homodimerization of R338W, which might be related to its weak dominant negative potency.