THYROID Volume 14, Number 5, 2004 © Mary Ann Liebert, Inc. Search for Genetic Variants in the Retinoid X Receptor-g Gene by Polymerase Chain Reaction–Single-Strand Conformation Polymorphism in Patients with Resistance to Thyroid Hormone without Mutations in Thyroid Hormone Receptor b Gene Stefano Romeo, 1 Claudia Menzaghi, 2 Rocco Bruno, 3 Federica Sentinelli, 1 Mara Fallarino, 1 Francesca Fioretti, 1 Sebastiano Filetti, 1,3 Armando Balsamo, 4 Umberto Di Mario, 1 and Marco G. Baroni 1 Resistance to thyroid hormone (RTH) is an inherited disease characterized by reduced tissue sensitivity to thy- roid hormone. Approximately 90% of subjects with RTH have mutation in the thyroid hormone receptor b (TRb) gene. Approximately 10% of subjects diagnosed as having RTH do not carry mutation in the TRb gene. A possible linkage was reported with the retinoid X receptor-g (RXR-g) gene in two families. The aim of this study is to search for mutation within the RXR-g gene in unrelated subjects with diagnosed RTH without mu- tations in the TRb gene. Four subjects with RTH were studied, and sequence variants in the RXR-g gene were searched by polymerase chain reaction–single-strand conformation polymorphism (PCR-SSCP). Analysis of all the 10 exons of the RXR-g gene, including intron-exon boundaries, promoter region and 39 untranslated region (UTR) reveled two variant bands in subjects II and III. Sequencing of these variants showed two single nucle- otide polymorphisms (SNPs): 447C . T in exon 3 for patients II and IVS9 1 6A . G for patient III. Both SNPs were also present at high frequency in a group of normal subjects and in nonaffected relatives of subject III. In conclusion, in patients with RTH we have found two SNPs in the RXR-g gene; these SNPS are common in the general population, thus excluding a role for the RXR-g gene in these patients. 355 Introduction R ESISTANCE TO THYROID HORMONE (RTH) is an inherited dis- ease characterized by reduced tissue sensitivity to thy- roid hormone. Persistent elevation of free thyroxine (T 4 ) and triiodothyronine (T 3 ) with elevated or nonsuppressed levels of thyrotropin (TSH), associated with goiter and sinus tachy- cardia are the most relevant clinical evidences. Two differ- ent clinical presentations may occur: some individuals may appear euthyroid or frankly hypothyroid (generalized RTH) while others may be clinically hyperthyroid (pituitary RTH). Even though this classification may be useful in clinical prac- tice it has no demonstrated pathophysiologic basis, as shown by ex vivo studies where physiologic binding and activity of thyroid hormones in target tissue has been demonstrated in RTH patients (1). RTH shows an autosomal dominant pat- tern of inheritance in the majority of subjects. Approximately 90% of subjects with RTH have mutations in the thyroid hor- mone receptor b (TRb) gene, mostly clustering within the li- gand-binding domain (LBD) (2,3). Mutant TRs are tran- scriptionally inactive, due either to reduced ligand binding or to impaired interaction with coactivators, resulting in the inhibition of wild-type TR action in a dominant-negative manner. Approximately 10% of subjects diagnosed as having RTH do not carry mutation in the TRb gene (4). Furthermore, anal- ysis in families did not demonstrate linkage with the TRa gene (5), suggesting the possibility that mutation in cofac- tors or dysregulation of their expression may be involved in the RTH phenotype in these patients. Recently, Reutrakul and coworkers (6) have performed a linkage study of can- didate genes for RTH, including genes for nuclear corepres- sors, coactivators and coregulators in families with RTH that were negative for TRb mutations. A possible link was 1 Department of Clinical Sciences, Division of Endocrinology, University of Rome “La Sapienza,” Rome, Italy. 2 Research Unit of Endocrinology, Scientific Institute “Casa Sollievo della Sofferenza,” S. Giovanni Rotondo, Italy. 3 Department of Clinical and Experimental Medicine, University of Catanzaro, Catanzaro, Italy. 4 Endocrinology Division, Mauriziano Hospital, Torino, Italy.