J. Parma et al., DIVERSITY AND PREVALENCE OF SOMATIC MUTATIONS IN THE THYROTROPIN RECEPTOR AND G(S)ALPHA GENES AS A CAUSE OF TOXIC THYROID ADENOMAS, The Journal of clinical endocrinology and metabolism, 82(8), 1997, pp. 2695-2701
A total of 33 different autonomous hot nodules from 31 patients, origi
nating mainly from Belgium, were investigated for the presence of soma
tic mutations in the TSH receptor and G(s) alpha genes. This constitut
es an extension of our previous study, including the first 11 nodules
of the series. The complete coding sequence of the TSH receptor gene a
nd the segments of G(s) alpha known to harbor mutations impairing guan
osinetriphosphotase activity were studied by direct sequencing of geno
mic DNA extracted from the nodules. DNA from the juxtanodular tissue o
r peripheral white blood cells was analyzed in all patients to confirm
that the mutations identified were somatic. Twenty-seven mutations (8
2%) were found in the TSH receptor gene, affecting a total of 12 diffe
rent residues or locations. All these mutations but 2 (see below) have
been identified previously as activating mutations. Only 2 mutations
were found in G(s) alpha (6%). In 4 nodules, no mutation was detected.
Five residues (Ser281, Ile486, Ile568, Phe631, and Asp633) were found
mutated in 3 or 4 different nodules, making them hot spots for activa
ting mutations. Phe631 and Asp633 belong to a cluster of 5 consecutive
residues (629-633) in the N-terminal half of transmembrane segment VI
, which harbor together 44% of the mutations identified in this cohort
. Two novel mutations were identified: a point mutation causing substi
tution of Phe for Leu at position 629 (L629F); and a deletion of 12 ba
ses removing residues 658-661 at the C-terminal portion of exoloop 3 (
del658-661). When tested by transfection in COS-7 cells, both mutant r
eceptors display increase in constitutive stimulation of basal cAMP ac
cumulation. Although it is still capable of binding TSH, the del658-66
1 mutant has completely lost the ability to respond to the stimulation
by the hormone. Our results demonstrate that, in a cohort of patients
from a moderately iodine deficient area, somatic mutations increasing
the constitutive activity of the TSH receptor are the major cause of
autonomous hot nodules.