Substitutions of tyrosine 601 in the human thyrotropin receptor result in increase or loss of basal activation of the cyclic adenosine monophosphate pathway and disrupt coupling to G(q/11)

Constitutively activating mutations of the thyrotropin (TSH) receptor have been identified as a molecular cause of toxic adenomas, nonautoimmune famil ial hyperthyroidism, and sporadic congenital hyperthyroidism. By analyzing genomic DNA from a toxic adenoma, we detected a novel somatic mutation in c odon 601, tyrosine to asparagine (Y601N), a residue located in the carboxyt erminal part of the fifth transmembrane helix. This codon is also notable f or the presence of a polymorphic variant, Y601H. These two naturally occurr ing substitutions (Y601N and Y601H) were analyzed together with an artifici al mutation, Y601F, to study the role of this residue for receptor function further. Transient transfection assays revealed that the Y601N mutation re sults in constitutive activation of the cyclic adenosine monophosphate (cAM P) pathway, but that it is unable to couple to G(q/11) Y601H and Y601F do n ot display basal activity while retaining responsiveness to TSH, but also l ose the ability to induce inositol phosphate accumulation in response to TS H. These studies define Y601N as a mutation that selectively activates the cAMP pathway, and they confirm that Y601H is not a silent polymorphism. In conclusion, residue Y601 has an important role for the characteristic const itutive basal activity of the TSH receptor and coupling to G(q/11).