Abstract
Objectives: Familial non-autoimmune hyperthyroidism is a rare condition in which constitutively activating mutations in the TSH receptor (TSHR) gene are one molecular cause of the disease. Identified naturally occurring mutations are good targets to study the activation mechanism of the receptor.
Patient and Methods: Here we describe a dystrophic patient in whom the diagnosis of non-autoimmune hyperthyroidism was made at the age of 5.5 years due to suppressed TSH and elevated thyroid hormone levels. The patient presented only with slight tachycardia and is not under thyreostatic therapy. Ultrasound examination revealed no enlargement of the thyroid gland. Familial thyroid disease was reported in the patient?s mother who is under carbimazol treatment but no further information of the family exist. Subsequently the TSHR gene was screened for mutations by direct sequencing. Functional characterization was performed after transient transfection of COS-7 cells and determination of b-TSH induced intracellular cAMP formation. Computer homology modelling was performed based on recently published GPCR crystal structures.
Results: A new heterozygous mutation in transmenbrane helix 6, p.Cys636Trp, was identified. Functional characterization revealed constitutive activation of the Gs/adenylyl cyclise pathway. The mutation is located in a “hot spot” for constitutively activating mutations (CAMs) in the TSHR. Homology modeling for this mutated TSHR was performed to obtain molecular insights in the pathogenic mechanism of constitutive activation caused by mutation Cys636Trp. Our studies suggest that introduction of the bulky and aromatic side chain of tryptophane instead of the highly conserved and less bulky cysteine at position 636 leads to a structural shift between transmembrane helix 6 and 7.
Conclusion: The conformational rearrangement between these two helices and in particular the resulting amino acid interaction(s) stabilize the TSHR towards the activated state and cause the constitutive activity. The investigated mutation gives further insight into the complex activation mechanism of the TSHR.