IDENTIFICATION AND FUNCTIONAL-CHARACTERIZATION OF 2 NEW SOMATIC MUTATIONS CAUSING CONSTITUTIVE ACTIVATION OF THE THYROTROPIN RECEPTOR IN HYPERFUNCTIONING AUTONOMOUS ADENOMAS OF THE THYROID

It has recently been shown that somatic and germ line mutations of the TSH receptor gene cause autonomous hyperfunctioning thyroid adenomas and nonautoimmune toxic thyroid hyperplasia by constitutive activation of the TSH receptor. A ''saturated'' map of these mutations is a prer equisite for a systematic screening for these clinically important mut ations. In this context, it is also of interest to determine whether d ifferent amino acid substitutions at the same residue cause constituti ve activation of the TSH receptor, as suggested by site-directed mutag enesis of the alpha(1) beta-adrenergic receptor. We, therefore, screen ed further hyperfunctioning autonomous adenomas of the thyroid for con stitutively activating mutations. We identified two new somatic mutati ons, changing alanine in position 623 to valine (A623V) and threonine in position 632 to isoleucine (T632I). Both mutations constitutively a ctivated cAMP when transiently expressed in COS cells. Together with n eighboring mutations, the T632I mutation demonstrates the importance o f transmembrane domain VI for the activation of the TSH receptor and c haracterizes it as a hot spot for constitutively activating mutations. The previously identified A623I and the newly identified A623V mutati ons demonstrate that several amino acid substitutions at the same resi due can cause constitutive activation of the TSH receptor.