Molecular description of non-autoimmune hyperthyroidism at a neonate caused by a new thyrotropin receptor germline mutation

Abstract

BackgroundConstitutively activating germline mutations in the thyrotropin receptor (TSHR) gene result in non-autoimmune hyperthyroidism and can be transmitted as a dominant trait or occur sporadically. These mutations are mostly located in the serpentine part of this G-protein coupled receptor.MethodsSequencing exon 9 and 10 of the thyrotropin receptor gene in a two months old patient identified a mutation which was functionally characterized after transient transfection into COS-7 cells. Cell surface localization was investigated by an ELISA approach and for signalling properties we measured cAMP by alpha screen technology for Gs/adenylyl cyclase activation and use a reporter gene assay for determination of Gq/11 phospholipase C-β activation.ResultsWe detected a heterozygous mutation in the first extracellular loop of the TSHR gene leading to an exchange of an isoleucine residue for asparagine at amino acid position 486 (I486N). Cell surface localization was reduced to 51% of wild-type TSHR. Functional characterization of the mutant receptor revealed constitutive activation of the Gs/adenylyl cyclase pathway, in contrast basal activity of the Gq/11 pathway was comparable to the wild-type. The bovine TSH-induced cAMP accumulation was slightly reduced, but IP3 signaling was impaired.ConclusionWe identified a new TSHR germline mutation (I486N) in a neonate with non-autoimmune sporadic hyperthyroidism. The mutation is located at the extracellular loop 1 and exhibits an increase in basal cAMP accumulation, but unexpectedly impairs the capability for TSH induced Gq mediated signaling. The TSHR homology model suggests isoleucine 486 as a potential key-player for induction of signal transduction by an interplay with further activation sensitive extracellular parts.

The Pathogenic TSH β-Subunit Variant C105Vfs114X Causes a Modified Signaling Profile at TSHR

“…It is known that the TSHR is a promiscuous G protein-coupled receptor (GPCR) and initiates several distinct signaling pathways by binding of TSH [29,30]. We here found that activation of the Gs/adenylyl cyclase pathway through TSHR is diminished for the patient mutation as well as for all TSHB mutants designed to unravel the underlying molecular mechanism of the patients mutation (Figure 5).…”

“…Consequently, the pathogenic TSHB mutation may modify the TSHB structure, and possibly the heterodimeric hormone complex, which potentially leads to inactivation of the hormone or to modified signaling in interplay with the TSHR. The TSHR is able to activate all four G protein families [29,30,31,32,33]. For the Gq activation, it is known that higher concentrations of TSH are needed [29,30].…”

“…The TSHR is able to activate all four G protein families [29,30,31,32,33]. For the Gq activation, it is known that higher concentrations of TSH are needed [29,30]. Although the mutation C105Vfs114X was functionally characterized through cAMP signaling induction at the TSHR, the mutation’s effect on the activation of other TSHR-specific signaling pathways [11] has not been investigated.…”

What is hot in experimental thyroidology?

“…A report on a patient characterized by a novel germline TSH-receptor mutation links this case of non-autoimmune hyperthyroidism to new impliations in TSH-receptor structure [ 2 ]. The study highlights the importance of introducing novel diagnostic approaches to the clinics, because this particular mutation in the TSH-receptor is correlated to altered G q -signalling pathways, thereby pointing to an important role of Gq signalling in the thyroid…”

2012 European Thyroid Association Guidelines for the Management of Familial and Persistent Sporadic Non-Autoimmune Hyperthyroidism Caused by Thyroid-Stimulating Hormone Receptor Germline Mutations

“…To date, 27 families with FNAH [3,4,5,6,7,9,10,11,12,13,14,15,16,17,18,19,20,32,33,34,35,36,37,38,39,40] and 15 cases with PSNAH have been published [8,22,23,24,25,26,27,28,29,41,42,43,44,45,46,47,48,49]. Women are affected more frequently by the familial form (83 women compared to 69 men), but not by the sporadic form (TSH Receptor Mutation Database III, OMIM 609152).…”