IDENTIFICATION OF A POINT MUTATION IN THE THYROTROPIN RECEPTOR OF THEHYT HYT HYPOTHYROID MOUSE/

The hyt/hyt-hypothyroid mouse has an autosomal recessive, fetal-onset, severe hypothyroidism related to TSH hyporesponsiveness and associate d with elevated TSH. Our previous work has suggested that the hypothyr oidism and TSH hyporesponsiveness may result from a mutation in the hy t/hyt TSH receptor (TSHr) of the thyroid gland. Based on DNA sequencin g of the entire coding region of the TSHr gene from the wild-type BALB /cBY +/+ mouse, the +/+ TSHr is 92% and 94% identical at the nucleotid e and amino acid residue levels, respectively, compared to the rat TSH r gene. The coding region of the hyt/hyt TSHr, compared to that of the +/+ TSHr, has a single base change, CCG to CTG, at nucleotide positio n 1666, which leads to the replacement of a highly conserved proline a t amino acid position 556 with a leucine in transmembrane domain IV. T his mutation was introduced by site-directed mutagenesis into the wild -type human TSHr and transiently expressed in COS-7 cells. Although th e size and abundance of the mutant TSHr mRNA suggested that there was no effect on the nature of the mRNA, TSH binding and the response to T SH in transfected cells were abolished. Further studies are necessary to clarify how the Pro to Leu replacement interferes with receptor exp ression on the cell surface or influences TSH binding. These functiona l consequences of the mutation appear to account for the observed TSH hyporesponsiveness and hypothyroidism in the hyt/hyt mouse.