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Pediatric Endocrinology, Childrens Hospital, Charité-Virchow-Klinikum, Humboldt-Universität zu Berlin (A.G., H.B., H.K.), and the Institute of Pharmacology, Freie Universität (T.S., T.G.), Berlin; Reinhard Nieter Childrens Hospital (H.P.K.), Wilhelmshaven; and the Department of Surgery, University of Halle (H.D.), Halle, Germany
Address all correspondence and requests for reprints to: Dr. Annette Grüters, University Childrens Hospital, Humboldt University, Augustenburger Platz 1, D-13353 Berlin, Germany. E-mail: grueters{at}ukrv.de
| Abstract |
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| Introduction |
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| Case Report |
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Methods
Neonatal TSH, serum TSH, serum T4, and serum T3 were measured with an immunofluorometric assay (Delfia, Wallac, Freiburg, Germany) or RIA. Serum fT4, fT3, antithyroperoxidase, antithyroglobulin, and TSHR antibodies were measured by RIA (Dynotest, Brahms Diagnostica, Berlin, Germany).
For mutation analyses, genomic DNA, PCR, single strand conformation polymorphism (SSCP), and sequencing were performed as described previously (8).
Construction of mutant TSHR genes
To characterize functional properties of the novel mutant TSHRs
(Fig. 1
) detected in this study, mutations were created by standard PCR
mutagenesis techniques (9) using the human TSHR expression plasmid,
TSHR-pcD-PS (8), as a template in which the
Bsu36I/BstEII segment had been replaced by a
corresponding fragment amplified from genomic DNA. PCR fragments
containing the S281N and R528H mutations were digested and used to
replace the corresponding KpnI/MscI and
MscI/Bsu36I fragments, respectively. The
identities of the different constructs and the correctness of all
PCR-derived sequences were confirmed by restriction analysis and
dideoxy sequencing.
Cell culture, expression, and functional characterization of TSHR constructs
COS-7 cells were grown and transiently transfected as described previously (8).
For inositol phosphate (IP) and cAMP measurements, cells were split into 12-well plates (2 x 105 cells/well) and transfected on the subsequent day. Approximately 24 h after transfection, 2 µCi/mL myo-[3H]inositol (18.6 Ci/mmol; Amersham, Aylesbury, UK) or [3H]adenine (28.8 Ci/mmol; DuPont-New England Nuclear, Brussels, Belgium) were added to the growth medium. After a 20- to 24-h labeling period, IP and cAMP accumulation assays were performed as described previously (10, 11).
For radioligand binding studies, cells were harvested 72 h after transfection, and displacement binding assays were performed with membranes prepared as preivously described (12). Incubations were carried out in a volume of 0.25 mL for 1 h at 22 C in the presence of 40,000 cpm [125I]bovine (b) TSH (54 µCi/µg; Brahms Diagnostica) and bTSH ranging from 0.001100 mU/mL. Nonspecific binding was defined as binding in the presence of 100 mU/mL bTSH. Membranes and bound ligand were separated from unbound ligand by centrifugation (10,000 x g for 10 min) through a silicon oil layer according to the procedure of McArdle et al. (13). The protein content of samples was determined by the method of Bradford (14). Binding data were analyzed by a nonlinear least squares curve-fitting procedure using the program Ligand (15).
Ribonucleic acid (RNA) preparation and RT-PCR
Tissue obtained from subtotal thyroidectomy was subjected to RT-PCR. About 250 mg thyroid tissue were homogenized with a mortar and pestle under liquid nitrogen. RNA was prepared using the Trizol method (Life Technologies, Eggenstein, Germany). First strand complementary DNA was synthesized by RT (Stratagene, Heidelberg, Germany), using an oligo(deoxythymidine) primer as recommended. Then, a DNA fragment spanning the entire nucleotide sequence of exons 9 and 10 was amplified by PCR with the sense primer 5'-G GAC GTG TCT CAA ACC AG-3' and the antisense primer 5'-GCA GCA GAC GAT GAC GAA GG-3'. The purified PCR product was cloned into the pCR-TOPO vector (Invitrogen, Leek, The Netherlands), and 10 clones were sequenced.
| Results |
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Five DNA fragments encompassing the entire exon 10 of the TSHR of
the patient were amplified by PCR. As indicated in Fig. 3B
, the PCR products representing a
401-bp segment (nucleotide position 13971797) showed marked
differences in electrophoretic mobility compared with amplification
products from her healthy brother or mother (see Fig. 3B
). Further
investigation of all family members available identified the sister,
father, paternal aunt, and paternal grandmother as heterozygote
carriers of the abnormality in exon 10. Direct sequencing of the
segment was performed, and a G to A transition in codon 528 was
identified, leading to an R to H exchange. The mutation in exon 10
within one allele led to the loss of a HhaI restriction site
in the 401-bp fragment that could now be used to identify this mutation
in other family members (see Fig. 3C
).
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To functionally characterize this novel TSHR mutation, we
expressed wild-type and mutant TSHRs in COS-7 cells and performed cAMP
and IP accumulation as well as [125I]TSH binding assays
in parallel. As shown in Fig. 4A
, COS-7 cells transfected with the
R528H mutant displayed lower basal cAMP levels compared with the
wild-type TSHR. Maximum increases in cAMP levels after bTSH stimulation
(100 mU/mL) were reduced by about 40% in cells expressing TSHR-R528H
(Fig. 4A
and Table 2
). As functional expression of TSHR
R528H could not offer an explanation for the occurrence of severe
congenital hyperthyroidism in our patient, we extended sequence and
SSCP analyses to exons 19 of all family members. As shown in Fig. 3D
, a heterozygous, abnormally migrating fragment was found in the
patients exon 9, but not in other family members. Sequence analysis
revealed a G to A transition in codon 281, resulting in an S to N
exchange. Sequencing of the other TSHR exons revealed no additional
mutations in the TSHR gene of the investigated family members.
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For characterization of ligand binding properties, wild-type and mutant
TSHRs were expressed in COS-7 cells, and [125I]TSH
displacement studies were performed. No significant differences between
the Ki values were observed among the three mutants
compared with the wild-type TSHR (Table 2
). Membrane expression of all
mutant TSHRs was reduced to 3554% of the wild-type receptor
expression level (binding capacity, 315 fmol/mg membrane protein;
n = 4; Table 2
).
Furthermore, we constructed a recombinant TSHR containing both
mutations. Expression studies revealed similar functional properties of
the R528H/S281N mutant and the wild-type receptor (Table 2
). To
unequivocally clarify the allelic localization of both mutations in our
patient, we extracted RNA from thyroid tissue that had been collected
at the time of thyroidectomy and performed RT-PCR studies. Therefore,
PCR fragments spanning the entire exons 9 and 10 were subcloned, and 10
individual clones were sequenced. As previously indicated by functional
studies, the patient was found to be a compound heterozygote,
i.e. individual clones contained either the R528H or the
S281N mutation. Thus, a de novo germ-line mutation (S281N)
on the maternal allele was identified as the cause of severe congenital
hyperthyroidism in this patient, whereas the paternal R528H mutation
identified in several family members affected by thyroid disorders had
to be regarded as a polymorphism.
| Discussion |
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In the present case of congenital hyperthyroidism, the grandmothers hyperthyroidism leading to thyroidectomy in adulthood and the aunts hyperthyroidism requiring antithyroid drug treatment since adolescence first suggested inheritance with variable onset of disease. The R528H mutation that was identified in these individuals showed no increased constitutive receptor activation in expression experiments. An alignment of the TSHR sequence of the second intracellular loop with corresponding amino acid sequences of other glycoprotein hormone receptors showed that R528 in the TSHR is naturally replaced by the amino acid H at the corresponding position of the receptor for LH. Therefore, the R528H mutation could not account for congenital hyperthyroidism in our patient.
It has been demonstrated that a deletion mutation in the extracellular portion of the TSHR results in constitutive receptor activity (16), and in a recent study, a mutation in exon 9 (S281N) has been found in a thyrotoxic nodule. The identical de novo mutation (S281N) was found in our patient and constitutively activated the Gs/adenylyl cyclase. Thus, we describe the first case of congenital nonautoimmune hyperthyroidism caused by a de novo germ-line mutation (S281N) in the extracellular domain of the TSHR.
The current concept of TSHR activation has to be extended to a possible involvement of the receptors ectodomain. As the TSH receptor displays significant constitutive activity even in the unliganded state, it is possible that a negative constraint exerted by the extracellular unliganded domain usually maintains receptor quiescence. A mutation within the extracellular domain disrupting a crucial three-dimensional structure necessary for receptor silencing may then lead to constitutive activity.
In our patient, the decision for thyroidectomy was based on clinical grounds before mutational analysis and in vitro characterization. An early time point for thyroidectomy in patients with persisting nonautoimmune hyperthyroidism regardless of the results of sequencing and in vitro studies appears to be mandatory because three of the published cases remained hyperthyroid despite prolonged antithyroid drug treatment (3, 4) and suffered from irreversible sequelae. Analysis of the TSHR gene may help to justify the decision of thyroidectomy, but it has to be taken into account that the results of in vitro findings cannot be directly transferred to the in vivo situation, which is modulated by genetic and epigenetic factors. However, in any case of novel TSHR mutations, thorough functional characterization has to prove the causal relationship to the clinical picture. In this case, the detection and functional analysis of the additional mutation in exon 9 of the TSHR allowed us to conclude that there is no increased risk of developing hyperthyroidism for the patients father and sister.
| Acknowledgments |
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| Footnotes |
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Received October 31, 1997.
Revised January 13, 1998.
Accepted January 20, 1998.
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T3-1 cells. Mol Cell Endocrinol. 87:95103.[CrossRef][Medline]
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