Cases of BorderlineIn VitroConstitutive Thyrotropin Receptor Activity: How to Decide Whether a Thyrotropin Receptor Mutation Is Constitutively Active or Not?

Abstract

Our findings indicate that current methods to precisely classify mutants with only a slight increase of the basal activity as constitutively active are limited. The results concerning the level of the basal activity can be influenced by the vector and/or the cell system. A comprehensive clinical characterization of the respective patients appears as a necessary and promising adjunct for the activity classification of these borderline mutations.

Genetic hyperthyroidism: hyperthyroidism due to activating TSHR mutations

“…A systematic study of the strength of all TSHR activating mutations as well as their effects on the G q -PLC cascade would constitute the best basis to correlate biology and phenotype (as done in (47,55)). The hypothesis that a population of normal people with relatively low TSH level would correspond to defined genetic polymorphisms in TSHR sequences could be looked for as well as the possibility of mild FNAH in old patients developing hyperthyroidism with negative TSAb (62). Finally, the possibility of new TSHR strong activating neomutations in cases of spontaneous abortions may be examined.…”

Database(d) Guidelines: A Small World

“…Because there has been some controversy about a few TSH receptor mutants that were described as constitutively active in initial reports, and found not to be constitutively active upon further assessment by another group [8,9,10]. The methods were not exactly the same, the cell lines were different, the way of assessing the membrane expression was different, .... To make it clear if not short, a mutant is said to be constitutively active when it leads, in absence of stimulation by TSH, to a higher production of a second messenger (mainly cAMP) than the wild-type receptor.…”

“…It was claimed that a significantly steeper slope was the hallmark of a constitutive mutant (fig. 1 curve 1), and that mutants having a similar slope with wild-type receptor are not constitutively active mutants [10], and the guidelines recommend using this slope for borderline mutants. This way of analyzing the in vitro results, however, may largely depend on the way the cell surface expression is measured.…”

“…Some of the mutants analyzed this way display such a low binding that the reality of it is highly questionable (fig. 1, curve 2), and was even considered to be absent for one [9,10]. …”

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

“…The reason for this recommendation is that besides large variations in the extent of constitutive activity for several mutations characterized in different laboratories, 2 TSHR germline mutations and 4 somatic TSHR germline mutations all previously published as constitutively active did not reveal constitutive activity in COS cells when later analysed by the linear regression approach to determine constitutive activity as a function of TSHR expression [54,55]. Therefore, TSHR mutations with only slightly increased constitutive activity in particular should be characterized by a reproducible method which controls for one of the most important variables, e.g.…”