SOMATIC MUTATIONS CAUSING CONSTITUTIVE ACTIVITY OF THE THYROTROPIN RECEPTOR ARE THE MAJOR CAUSE OF HYPERFUNCTIONING THYROID ADENOMAS - IDENTIFICATION OF ADDITIONAL MUTATIONS ACTIVATING BOTH THE CYCLIC ADENOSINE-3',5'-MONOPHOSPHATE AND INOSITOL PHOSPHATE-CA2+ CASCADES

A series of somatic mutations of the TSH receptor gone have been demon strated in hyperfunctioning thyroid adenomas. The mutations studied up to now cause constitutive (i.e. TSH-independent) activation of the cA MP- regulatory cascade only. As a follow-up to our original study, we have now completely sequenced exon number 10 of the TSH receptor gene in the same series of toxic adenomas. An activating mutation was found in nine of 11 tumors; In addition to the mutations already described, two isoleucine residues belonging to the first and second extracellul ar loops of the receptor (Ile(486) and Ile(568)) were found mutated. T wo different adenomas were found to harbor a different amino acid subs titution at residue 486 (Ile(486)Phe, Ile(486)Met). Ile(568) was mutat ed to threonine in one. When studied by transfection in COS-7 cells, a ll three mutations caused very strong activation of the cAMP-regulator y cascade. In addition, the Ile(486)Phe and, to a lesser extent, the I le(486)Met and Ile(568)Thr mutants stimulated constitutively the inosi tol phosphate-diacylglycerol cascade. Our results demonstrate-that 1) the first and second extracellular loops contribute to the silencing o f the unliganded TSH receptor; 2) the two regulatory cascades normally under TSH control can be constitutively activated by somatic mutation s of the receptor; 3) the TSH receptor can be activated by mutation of a large number of residues distributed over the first and second extr acellular loops, the third intracellular loop, and the third, sixth, a nd seventh transmembrane segments; 4) activating mutations of the TSH receptor constitute the major cause of toxic adenomas, accounting for about 80% of the cases.