INTRODUCTION: Non-autoimmune hyperthyroidism (NAH) is a rare genetic disorder caused by germline-activating variants in the TSH receptor (TSHR) gene. While most NAH-related TSHR variants are located in the seven-transmembrane domain (7TMD), three variants (p.Leu267Phe, p.Ser281Asn, and p.Asn406Ser) have been identified in the extracellular domain (ECD), with p.Leu267Phe previously not showing constitutively active in vitro. METHODS: We searched for TSHR variants in a Japanese family with NAH using PCR-based direct sequencing. We created three ECD variants (p.Leu267Phe, p.Ser281Asn, and p.Asn406Ser) and a series of variants in which Leu267 was mutated to 18 amino acids other than Leu and Phe. Based on the cryo-electron microscopic structures, we evaluated the structure-function relationship of TSHR. We compared their cAMP-producing capacities to WT-TSHR in the luciferase activity assay using HEK293 cells. Western blot and fluorescence immunostaining were performed using HA-tagged TSHR vectors to compare Leu267Phe-TSHR with WT-TSHR. RESULTS: A heterozygous TSHR variant (p.Leu267Phe) was identified. Comparison of the cryo-electron microscopy structures of the activated and inactivated TSHRs revealed a significant change in the ECD structure around Leu267. The ligand-independent cAMP-producing capacities compared to WT-TSHR were 238 �� 20% (mean �� SEM) for Leu267Phe. Of all 19 possible variants created through systematic mutagenesis, only Leu267Phe-TSHR and Leu267Tyr-TSHR exhibited significantly higher ligand-independent cAMP-producing capacities. Immunoblotting and fluorescence immunostaining showed that the Leu267Phe variant did not affect protein expression levels and intracellular localization of TSHR. DISCUSSION/CONCLUSION: Leu267Phe-TSHR causes NAH. Substituting Leu267 to aromatic amino acids may shift the equilibrium of the TSHR state toward activation.
