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Four families with loss of function mutations of the thyrotropin receptor
N de Roux1, M Misrahi, R Brauner
1INSERM U-135 and Laboratoire d'Hormonologie et Biologie Moléculaire Hôpital de Bicêtre, Le Kremlin, France.
Abstract:
We observed four families with loss of function mutations of the TSH receptor gene. One patient had a homozygous Pro162 Ala substitution. The three other were compound heterozygotes: 1) Gln324-->Stop and Asp410 Asn2), Cys41 Ser and Phe525 Leu, 3) Cys390 Trp and Trp546-->Stop. In all patients, the plasma TSH concentration was increased, whereas T3 and T4 concentrations were normal. The TSH levels were normal in the heterozygous parents. These results confirmed the recessive character of TSH receptor defects. Expression of the various mutated receptors in transfected COS-7 cells demonstrated the impairment of their function. We studied the expression of the receptors on the cell surface by immunofluorescence, their ability to bind hormone, and their capacity to activate adenylate cyclase. Some mutations allowed us to identify sites that are especially important for receptor function. The substitution Cys390 Trp abolished high affinity hormone binding. Receptor mutated at Asp410 Asn bound the hormone normally, but failed to activate adenylate cyclase. This result underscores the role of this acidic extracellular residue, close to the first transmembrane segment, in signal transmission. The Phe525 Leu substitution also markedly impaired adenylate cyclase activation, underlining the importance of the second intracellular loop in receptor signaling.
Insights
Thyroid-stimulating hormone (TSH) receptor gene mutations cause resistance to thyroid hormones. These loss-of-function mutations, confirmed as recessive, lead to elevated TSH levels with normal thyroid hormone concentrations.
Area of Science:
- Genetics
- Endocrinology
- Molecular Biology
Background:
- Thyroid-stimulating hormone (TSH) receptor (TSHR) gene mutations can lead to thyroid hormone resistance.
- Understanding these mutations is crucial for diagnosing and managing related endocrine disorders.
Purpose of the Study:
- To investigate the functional consequences of novel TSHR loss-of-function mutations.
- To identify specific regions within the TSHR critical for hormone binding and signal transduction.
Main Methods:
- Genetic analysis of four families with TSHR mutations.
- Functional characterization of mutated TSHR in transfected COS-7 cells.
- Assessment of cell surface expression, hormone binding, and adenylate cyclase activity.
Main Results:
- Identified four distinct TSHR loss-of-function mutations, including homozygous and compound heterozygous cases.
- Demonstrated impaired receptor function, including reduced hormone binding and/or defective signal activation.
- Specific mutations highlighted the importance of extracellular residues for hormone binding and intracellular loops for signal transmission.
Conclusions:
- TSHR defects exhibit recessive inheritance.
- Specific amino acid substitutions critically impact TSHR function, affecting hormone binding and/or signal transduction.
- This study elucidates key functional domains of the TSH receptor.