The thyrotropin (TSH) receptor (TSHR) is the
only 7-transmembrane G-protein coupled receptor (GPCR) for glycosylated
hormones that undergoes cleavage after its primary formation; the amino
terminal extracellular domain is cleaved at/near amino acid 289 (subunit
A) leaving a short residual extracellular amino acid sequence, the 7
transmembrane domains and extracellular and intracellular connecting
loops, and the intracellular carboxyl terminal domain (subunit B).
Subunit A then circulates and can serve as an immunogen. The role of
subunit A of the TSHR in the pathogenesis of autoimmune
hyperthyroidism and the development of TSHR stimulating immunoglobulin (TSIg)
was examined by the present investigators. They constructed within
adenovirus cDNA transcripts of the amino terminal 289 amino acid sequence
(subunit A), the wild-type (wt) TSHR from which amino acids
317-366 had been removed rendering the truncated TSHR resistant to
cleavage, and the intact wt TSHR.Adenoviruses expressing
different forms of the TSHR were then administered to female mice who
subsequently developed abnormalities of thyroid function and antibodies of
variable biologic activity in response to these proteins. In animals
receiving TSHR 1-289, clinical, biochemical, and thyroid histologic
evidence (thyromegaly, hyperthyroxinemia, and follicular hyperplasia) of
thyrotoxicosis developed. These animals also developed TSIg (assessed by
increase in cyclic AMP formation in CHO cells expressing TSHR). In only a
few mice receiving cleavage resistant TSHR or wt TSHR were serum thyroxine
levels increased and thyroid follicular hyperplasia present. In contrast,
all mice, regardless of the form of TSHR received, developed high but
approximately equal titers of immunoglobulins that bound to TSHR or
inhibited radiolabeled TSH from binding to TSHR. TSIg did not develop in
animals receiving cleavage resistant TSHR, but did appear in 30% of those
injected with wt TSHR. Higher titers of thyroid blocking antibodies
(assessed by their effect on TSH mediated increase in cyclic AMP
generation in CHO cells expressing TSHR) were present in mice receiving
the cleavage resistant form of the TSHR than in those receiving TSHR
1-289. The authors conclude that it is the extracellular segment of the
TSH receptor that is ordinarily shed that serves as the immunogen for the
development of TSIg in this experimental model of hyperthyroidism (and by
analogy in patients with Graves disease).
Chen C-R, et al. J Clin Invest 2003;111:1897-1904.
First Editor’s Comment: This extremely interesting manuscript
provides significant insight into the pathogenesis not only of
thyrotoxicosis, but of autoimmune thyroid disease itself. Thus, when the
ectodomain of the TSHR is cleaved, it provokes the production of TSHR
stimulating immunoglobulins (as well as low titers blocking antibodies) in
genetically susceptible individuals. In other at-risk patients, the intact
TSHR (or perhaps other sequences or epitopes of the TSHR) or TSH itself,
serves as the immunogen for development of TSHR function-blocking
antibodies. Other components of the thyroid gland serve as immunogens for
antibodies that are injurious to the thyroid cell. A human monoclonal
antibody has been recently isolated from a patient with Graves disease,
but the epitope of the TSHR to which it is directed has not been
identified to date.1,2 It would be of interest if it were directed to the
ectodomain of the human TSHR.
While a number of tyrosine kinase receptors shed their extracellular
domains (growth hormone binding protein, prolactin binding protein, many
cytokines), it is apparently unusual for G-protein coupled receptors to do
so. This is an area that merits further examination.
Allen W. Root, MD
Second Editor’s Comment: In Dr. Root’s editorial comment, he refers
to the recent identification of a monoclonal antibody that stimulates the
TSH receptor in the thyroid cell to release thyroxin.1,2 This also was no
small accomplishment in helping us understand Graves’ disease more fully.
As pointed out by Dayan, who states:
"So, is the final proof of the existence of thyroid-stimulating
immunoglobulin after a journey of 47 years of anything more than academic
interest? Almost certainly the answer is "yes." First, this finding might
lead to a new generation of assays for thyroid-stimulating immunoglobulin
in which competition for labeled TSH is replaced by competition for
specific monoclonal antibodies. If a sensitive assay can be developed, it
should have close to 100% specificity for Grave’s disease and replace all
other antibody tests, such as antithyroid peroxidase and
antithyroglobulin, in this condition. Second, it should finally allow us
to understand how such antibodies, even in the monomeric Fab form, can
activate the TSH receptor. Such understanding of the biology of
glycoprotein-hormone receptors may lead to new small-molecule agonists and
antagonists not only for thyroid disease but also for hypogonadism and
infertility (via the closely related receptors for luteinising and
follicle-stimulating hormones). And it may prove possible to clone a
potent human TSH-receptor-blocking antibody which might provide a rapid
initial treatment for thyrotoxicosis. Third, the finding may lead to a
better understanding of the pathogenesis of Graves’ disease. How is it
that the spontaneous development of such agonist antibodies, unique in
autoimmune diseases, occurs so frequently (almost 1 in 100 of the
population)? Does the agonist activity itself, once it appears, promote
autoimmunity in a positive feedback loop? Most intriguingly, cloning of
agonist TSH-receptor autoantibodies might reveal antibodies that
contribute to thyroid eye-disease, the most mysterious manifestation of
Graves’ disease, and perhaps lead to inhibitors for these antibodies. And
finally, agonist antibodies may prove a useful therapeutic agent in their
own right, such as to enhance iodine-131 uptake in thyroid cancers. Many
of the holy grails of biological science, from the structure of DNA to the
nature of the T-cell antigen receptor, have been found. Thankfully, once
in hand, they change into pointers to the many more waiting to be
discovered."
The findings of Chen and those of Sanders et al are linked closely and
the almost simultaneous reporting of these factors which are linked should
permit a logarithmic advance in our understanding of how antibodies and
receptor structure and function can relate and, consequently, provide
better therapy of immunological diseases.
Robert M. Blizzard, MD
References
1.
Sanders J, et al. Lancet
2003;362:126-128.
2.
Dayan CM. Lancet
2003;362:92-93.
