New pathogenic thyrotropin receptor mutations decipher differentiated activity switching at a conserved helix 6 motif of family A GPCR.

CONTEXT: In this paper we report two new TSH receptor (TSHR) mutations. One mutation (Pro639(6.50)Leu) was identified in two siblings with congenital hypothyroidism, and a second mutation (Cys636(6.47)Arg) was found in a patient suffering from nonautoimmune hyperthyroidism. Both mutations are located in transmembrane helix (TMH) 6 at the conserved Cys(6.47)-Trp(Met)(6.48)-Leu(Ala)(6.49)-Pro(6.50) motif of family A G protein-coupled receptors (GPCR).
OBJECTIVE: To study the pathogenic mechanisms, we tested patients' mutations and further side chain variations regarding their effects on TSHR signaling.
RESULTS: Substitution Pro639Leu fully inactivates the promiscuous TSHR for cAMP (Gs) and IP (Gq) signaling. In contrast, Cys636Arg leads to constitutive activation of Gs. Organization of TSHR in oligomers was not modified by mutations at position 636. Interestingly, it is known from crystal structures of GPCR that Pro(6.50) is located at a TMH6 kink-distortion, which is a pivot during activation-related helical movements. However, the cell surface expressions of all mutants at position 639 were comparable to wild type, indicating a helical conformation like wild type.
CONCLUSION: Until now, only naturally occurring constitutively activating mutations in TSHR TMH6 have been reported, but here we present the first pathogenic inactivating mutation (Pro639Leu). Our data are indicative of differentiated regulation of Gs and Gq signaling at particular TMH6 positions, but without any effects on TSHR oligomer constellation. Details of signaling modulation by each mutant at positions 636(6.47) and 639(6.50) help us to understand high conservation of these amino acids in family A GPCR. Described molecular (pathogenic) mechanisms are likely not unique for TSHR.