A newly discovered TSHR variant (L665F) associated with non-autoimmune hyperthyroidism in an Austrian family induces constitutive TSHR activation by steric repulsion between TM1 and TM7

Objective: New in-vivo mutations in G-protein coupled receptors open opportunities for further insights into the mechanism of receptor activation. Here we describe the molecular mechanism of constitutive Thyroid-Stimulating Hormone Receptor (TSHR) activation in an Austrian family with three generations of familial non-autoimmune hyperthyroidism.

Patients: The index patient was diagnosed with hyperthyroidism during her first pregnancy. The first two of her four children were diagnosed with hyperthyroidism at the age of 11 and 10 years, respectively. TSH suppression was also observed in the third child at the age of 8 years, who has normal fT4 levels until now. TSH suppression in infancy was observed in the youngest child. The mother of the index patient was diagnosed with toxic multinodular goiter at the age of 36 years.

Methods: DNA was extracted from blood samples from the index patient, her mother and her four children. Screening for TSHR mutations was performed by high resolution melting (HRM) assays and subsequent sequencing. Elucidation of the underlying mechanism of TSHR activation was carried out by generation and structural analysis of a TSHR transmembrane homology model and verification of model predictions by functional characterization of targeted receptor mutations.

Results and conclusions: A newly discovered TSHR mutation L665F in transmembrane helix (TM) 7 of the receptor was detected in six members of this family. Functional characterization of L665F revealed constitutive activation for the Gs pathway and thus represents the molecular cause for hyperthyroidism in this family. The constitutive activation is possibly linked to a steric clash introduced by the L665F mutation between TMs 1, 2 and 7.