Four families with loss of function mutations of the thyrotropin receptor.

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.