Constitutively activating mutations in the
thyrotropin (
TSH) receptor have been identified as a major molecular cause of hyperfunctioning
thyroid adenomas. A smaller subset of these benign
tumors is caused by constitutive activation of the
adenylyl cyclase cascade by somatic mutations in the Gsalpha gene. In this study, we analyzed hyperfunctioning
thyroid adenomas from seven Brazilian patients for
TSH receptor and
G(s)alpha gene mutations. Solitary autonomous
thyroid adenomas were identified by ultrasound and scintigraphy, and
DNA was extracted from adenomatous and periadenomatous tissue. Exons 9 and 10 of the
TSH receptor gene, and exons 8 and 9 of the
G(s)alpha gene, were amplified by polymerase chain reaction (PCR) and subjected to direct sequence analysis. Six of seven
adenomas harbored heterozygous mutations known to confer constitutive activity to the
TSH receptor. In one case,
aspartate 619 was substituted by
glycine (D619G). In four
adenomas,
alanine 623 was replaced by
valine (A623V). Both residues are located in the third intracellular loop. In one instance,
aspartate 633 located in the sixth transmembrane domain was replaced by
tyrosine (D633Y). In this patient, one allele also contained a change of
aspartate 727 to
glutamate (D727E). This substitution is thought to be a polymorphic variant of the wild-type but it has also been associated with toxic multinodular
goiters. Functional comparison of D727 with E727 did not reveal differences in basal or TSH-stimulated cyclic
adenosine monophosphate (cAMP)-dependent
luciferase activity in transiently transfected cells. These results demonstrate a high prevalence of activating
TSH receptor mutations in toxic
adenomas in this small series from Brazil (approximately 86%). These findings are in agreement with reports from other countries with a marginal
iodine intake but contrast with studies from regions with a high
iodine intake where these mutations appear to be less prevalent.