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Endocrinologie Cellulaire et Moléculaire de la Reproduction, Centre National de la Recherche Scientifique, Université Paris VI (M.L.K., S.C., R.C.), 75005 Paris, France; Laboratoire de Biologie Hormonale, Hôpital Saint Vincent de Paul (N.L.), 75014 Paris, France; Department of Endocrinology, Hemel Hempstead General Hospital (C.E.H., C.J.J., N.R.F.), Watford, Hertfordshire HP2 4AD, United Kingdom; Service dEndocrinologie, Hôpital Saint Antoine (P.B.), 75012 Paris, France; and Unité de Génétique Moléculaire, Service de Biochimie Médicale, Hôpital Pitié-Salpêtrière, Assistance Publique-Hôpitaux de Paris (M.L.K., J.P.L.), 75013 Paris, France
Address all correspondence and requests for reprints to: Dr. Marie-Laure Kottler, Department of Génétique et Reproduction, Hôpital Clémenceau, Centre Hospitalo Universitaire, 14033 CAEN Cedex, France. E-mail: mlkottle{at}snv.jussieu.fr
| Abstract |
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The present study shows an absence of LH and FSH response to pulsatile
GnRH administration (20 µg/pulse, sc, every 90 min). However, GnRH
triggered free
-subunit (FAS) pulses of small amplitude,
demonstrating partial resistance to pharmacological doses of GnRH. FSH,
LH, and FAS concentrations were evaluated under chronic estrogen
treatment and repeat administration of GnRH. Not only were plasma FSH,
LH, and FAS concentrations decreased, but FAS responsiveness was
reduced.
This new case emphasizes the implication of the GnRH receptor mutations in the etiology of idiopathic hypogonadotropic hypogonadism. We also have evidence for a direct negative estrogen effect on gonadotropin secretion at the pituitary level, dependent on the GnRHR signaling pathway.
| Introduction |
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The mechanisms by which gonadal steroids regulate gonadotropin
secretion are only partially understood, showing gender (7, 8) and
species variations (9). There is evidence for a pituitary site for
estradiol action, but the exact mechanism of this action remain elusive
(10). Therefore, we have used this patient with complete
hypogonadotropic hypogonadism displaying GnRH resistance to study
in vivo the effect of chronic estrogen administration on the
gonadotropin and free
-subunit (FAS) release.
| Subject and Methods |
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The propositus came to medical attention in 1991 at the age of 16.8 yr with primary amenorrhea and lack of secondary sexual characteristics. Her initial laboratory data revealed low estradiol and gonadotropin concentrations: estradiol was 17 pmol/L (normal range, 3792), basal serum LH was 0.44 IU/L (normal ranges, 512 IU/L), and FSH was 0.50 IU/L (normal range, 26 IU/L). Computed tomography scan indicated normal pituitary and infundibulum. Bone age was 13 yr. Transabdominal ultrasound examination and laparoscopy showed an infantile uterus, healthy fallopian tubes, and two small, apparently unstimulated ovaries.
The patient was treated with a combination of estrogen and progestogen. Shortly afterward, she started to have withdrawal bleeding and to develop secondary sexual characteristics. Her subsequent clinical progress was satisfactory.
She was reinvestigated 7 yr later at age 24 yr. The uterus was of
normal size ultrasonically, with good endometrial lining. Only the left
ovary was identified and measured 11 x 6 mm with few small
follicles. GnRH (100 µg, iv) stimulation was repeated 5 weeks after
sex steroid replacement therapy was withdrawn (Table 1
). During this period the patient
remained amenorrheic, but did not experience any hot flushes.
|
Analysis of gonadotropin secretion
Protocol. The patient gave written informed consent for the studies.
Endogenous LH, FSH, and FAS release was evaluated by 20-min sampling over a 6-h period under pulsatile administration of GnRH (20 µg/pulse every 90 min, sc) during two different admissions separated by 3 months. In each case the first sample was taken 10 min after the first GnRH bolus.
During the first admission, the patient had been off sex steroid replacement therapy for 10 weeks. Then the patient received estrogen treatment (Premarin, Wyeth, Taylow, UK; 0.625 mg/day, orally) for 10 weeks and was readmitted under the estrogen treatment, for a repeat pulsatile administration of GnRH at a dose and rate identical to those used in her initial admission.
Assays. Serum FSH and LH concentrations were measured in
duplicate by means of sensitive fluoroimmunometric assays (AutoDelphia,
Wallac, Inc., Turku, Finland). In brief, 25 µL serum
were distributed in wells coated with anti-FSHß or anti-LHß
antibody and incubated for 2 h, then europium-labeled antibody was
added (anti-FSH
or anti-LHß, respectively). The detection was
based on the time-resolved fluorescent technique. Between- and
within-assay coefficients of variation (CVs) were 1.5% and
1.3%, respectively, for FSH at the level of 2 IU/L. The detection
limit was 0.05 IU/L. For LH at the level of 0.3 IU/L, between- and
within-assay CVs were 6.7% and 1.8%, respectively. The
detection limit was 0.05 IU/L. Samples were run in the same assay.
FAS was measured by RIA in duplicate as previously described (11) using
the First International Reference Preparation of hCG
-subunit as a
standard. The detection limit was 0.04 IU/L. At the level of 0.3 IU/L,
between- and within-assay CVs were 6.2% and 11.5%, respectively, and
at the level of 0.9 IU/L, they were 3.2% and 7.0%.
Inhibin B was measured as previously described (12).
Pulse analysis. Hormone secretion was analyzed using Van Cauters algorithm (13), which requires a 2 CV threshold above the preceding value to define a pulse.
DNA sequencing and functional analysis of the GnRHR in Chinese hamster ovary (CHO-K1) cells
Genomic DNA was sequenced as previously described (5). Construction of pMSG mammalian expression vector (Pharmacia Biotech, Orsay, France) encoding the wild-type (WT) receptor has been previously described (5). Genomic DNA (exon 3) of the propositus was amplified using primers I3S and E3R located in intron 2 and exon 3, respectively. I3S was previously described (14), and E3R was designed to contain a XhoI restriction site (5'-CAGCTCCAGCATTCCCAGATGGAGAG-ATTC-3'). Mutant DNA fragment was constructed by ex-changing the WT BsiHKA1/XhoI DNA segment with the mutated one.
Transfections and functional studies, binding, and inositol phosphate (IP) production were performed as previously described (5). For radioligand binding studies, CHO-K1 cells were transfected with increasing amounts of L314X GnRHR DNA vector (1.29 µg).
WT and mutant GnRHR messenger ribonucleic acids (mRNAs) were extracted from transfected CHO-K1 cells and quantified by dot blot as previously described (5).
| Results |
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Two germline mutations were found in the patients DNA (Fig. 1
). In exon 1, a guanine to adenine (G to
A) mutation at position 317 resulted in a glutamine to arginine
substitution at residue position 106 (Q106R). This amino acid belongs
to the first extracellular loop (e1) of the receptor (Fig. 2
). In exon 3, a thymidine to adenine (T
to A) transversion at position 941 resulted in a null mutation at
position 314 (L314X), belonging to the seventh transmembrane domain
(TM7). We also found at position 151 a previously described
Mae polymorphism (15). Segregation analysis provided
evidence that the propositus had inherited germline mutations and was
compound heterozygote. Her father was heterozygous for the L314X
mutation, as was her brother. Her mother carried the Q106R mutation
associated with the Mae polymorphism. Her sister was
homozygous for the WT receptor.
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The WT receptor exhibited high affinity binding
(Kd = 1.61 ± 0.43 nmol/L), and GnRH
(107 mol/L) induced a maximum 7-fold increase in
total IP production. In contrast, the L314X mutant showed neither
measurable binding nor IP production despite transfection with
increasing amounts of DNA construct (Table 2
).
|
Response to the pulsatile administration of GnRH
During the first 7-h sampling period in the absence of estrogen
treatment, only one LH pulse was identified (Fig. 3
). LH as well as FSH levels remained
low. However, pulsatile secretion of FAS was better preserved, and GnRH
triggered small FAS pulses with a 40- to 50-min lag time. During
treatment, estradiol levels remained low, and no ovarian follicular
development occurred, as demonstrated by ultrasonic examination.
Inhibin B concentration remain below the normal values (<15
pg/mL).
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| Discussion |
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We describe for the first time a nonsense mutation located at position Leu7.44 (314) in TM7 of the GnRHR, generating a mRNA encoding a protein of only 313 amino acids and a partial deletion of the TM7. This deletion includes Tyr7.53 (323), which is a highly conserved residue among the G protein-coupled receptors (GPCRs), and Asp7.49 (319). The latter Asp7.49 (319) has been previously shown to be in spatial relationship with Asn2.50 (87) located in TM2 (16, 17, 18, 19, 20) and contributes to GnRH receptor function (20, 21, 22). Our functional studies, which reveal a complete loss of function of the mutant receptor, are in agreement with a major role of this deleted C-terminal region (22). Other reports have also shown that C-terminal truncated GPCRs frequently tend to be trapped in the endoplasmic reticulum (23) and are not processed to the membrane. In addition, mRNA levels of the L314X mutant receptor were lower than those of the WT. Whether this mutant receptor is really translated in vivo remains uncertain. Nevertheless, whatever the mechanism, disruption of biosynthesis (transcription or translation), trafficking to the membrane, and/or misfolding, it appears evident that the mutant form results in a loss of function.
To date, 8 different mutations of the GnRHR leading to partial or complete loss of function have been reported in patients with IHH (2, 3, 4, 5, 6, 24) as an autosomal recessive trait as shown by pedigree analysis. In all cases, heterozygous patients for the GnRHR mutations have normal pubertal development and fertility. Screening 46 unrelated IHH patients without anosmia, Layman et al. (4) found GnRHR gene mutations in 2.2% (8), although in families with an affected female the incidence of GnRHR mutation was higher (7.1%). Such a low frequency was confirmed in our personal study, emphasizing the implication of a number of important and decisive factors other than GnRH in the modulation of gonadotropin secretion in humans. Mutations are distributed along the coding sequence, as reported for other GPCRs, localized mainly, however, within the transmembrane domain. In previous reports as well as the present study, two hot spots have been noted regardless of the geographic origin of the patients (North or South America and Europe): Q106R (2, 3), which affects the binding, and R262Q, (2, 4, 5), which affects IP production.
The phenotype of the patients appears to vary from partial hypogonadotropic deficiency with significant levels of gonadotropins and gonadal steroids (2, 3) to complete hypogonadism with the absence of pubertal development (4, 5, 6, 24). Our patient displays complete hypogonadism, with low gonadotropin levels and a poor response to GnRH administration, whereas the patient described by De Roux (2) exhibited partial hypogonadism with normal gonadotropin levels. Both share the same Q106R mutation, but the mutation in the second allele is different, L314X instead of R262Q. In vitro studies have shown that either Q106R or R262Q results in a partial loss of receptor function (2, 4). Thus, the difference between the phenotypes could be related to the allelic combination of the mutations, with the biological activity of the L314X receptor being more severely affected than that of the R262Q. However, there is also evidence for heterogeneity in the clinical (3) and biological (5) presentations of family members carrying the same molecular abnormalities, thus supporting the implication of other factors or mechanisms in the expression of the phenotype. In a recent experiment, Grosse et al. (25) found in vitro a possible inhibitory action of a truncated receptor on the functionality of the WT. However, at least two subjects (patients father and brother), heterozygous for the L314X GnRHR, have gone through puberty and exhibited normal pubertal development. Thus, a dominant negative effect of the L314X receptor could be excluded in vivo.
Response to pulsatile administration of GnRH
The present study shows an absence of response to pulsatile GnRH administration in term of gonadotropin release. FSH and LH concentrations were low, suggesting that several pathways may converge on gonadotropin to establish a basal level of secretion (26). However, exogenous GnRH stimulation elicits erratic FAS pulses of low amplitude. FAS release is under the dual control of GnRH and TRH; it is, however, a more sensitive index of GnRH administration (27), particularly in patients with IHH (28, 29, 30). This different threshold of sensitivity of gonadotrophs to the GnRH stimulation of LH and FAS was also described in patients with other mutated GnRHR (5). Taken together, these data suggest that FAS may be superior to LH to estimate GnRH sensitivity in patients with GnRHR mutation. As the L314X is nonfunctional, it serves as a null background to interpret results from the Q106R mutant, which was found to have reduced GnRH binding and which requires higher levels of GnRH to elicit responses (2).
Estrogen amplifies the depression of gonadotropin and FAS release
It has long been established that gonadal steroids act on both the hypothalamus and pituitary to regulate the synthesis and release of gonadotropins; however, the specific mechanisms involved are still unclear. Patients with GnRH resistance represent an interesting alternative model to dissect in vivo the effect of estrogen on gonadotropin secretion. We found that estrogen decreases gonadotropin and FAS levels as well as the response of FAS to pharmacological doses of GnRH.
As a clear relationship exists between GnRH secretion and the amplitude of the response to LH, an estradiol-induced decrease in the amount of GnRH released from the hypothalamus could account for the observed decrease in gonadotropin secretion (31). In the hypothalamus, ovariectomy increases, and estradiol replacement restrains both GnRH pulse frequency and amplitude (32). For example, in ovariectomized monkeys, estradiol induces a 4-fold reduction in the concentration of GnRH in cerebral fluid (33). Patients with an altered GnRHR are estrogen-deprived and would therefore, be expected to have a hypothalamic secretion of GnRH similar to that observed in castrates. Thus, the possibility exists that the decrease in the basal level of gonadotropins, due to estrogen, results from a primary effect on endogenous hypothalamic GnRH. Such an indirect action has been demonstrated in vivo to be responsible for the inhibition of the transcription of FAS (34). This mechanism, however, remains difficult to assess in our study, as the altered GnRHR responds poorly to GnRH. Another possibility is a direct action on the pituitary. There are numerous data that suggest that estrogens act at the pituitary level (9). For instance, in GnRH-deficient patients in whom the pattern of the GnRH administration (frequency and doses) can be kept constant without interference with endogenous GnRH, a significant decrease in gonadotropin levels as well as in LH pulse amplitude could be demonstrated under estradiol treatment (35). In addition, a dimorphic effect on the basal activity of the FAS promoter was observed in pituitary cells from rats, with females exhibiting lower basal activity but greater GnRH responses than males (7), reinforcing the hypothesis that estrogens play a role in FAS expression independent of the hypothalamic secretion.
We also document in our study that acute FAS responsiveness to pharmacological doses of GnRH decreased under estrogen administration. A relationship between the dose of bolus GnRH and the pituitary response was found in normal subjects (31) and in patients with GnRH deficiency (35) or GnRH resistance (5). As discussed above, the amount of GnRH-inducing gonadotropin secretion represents, in patients with GnRH resistance, a summation of endogenous vs. exogenous GnRH. Thus, gonadotropin responsiveness may vary according to GnRH input. Data for GnRH levels in pituitary portal blood are unknown in humans, especially in ovariectomized females. Comparatively, in the rhesus monkey (33) or in the ewe (36), levels ranged from 11.6278 pg/mL and from 1535 pg/mL, respectively. Pharmacokinetic studies have shown that the concentration of GnRH obtained in response to sc administrations of GnRH pulses (20 µg/pulse or 400 ng/kg) represents a excessive dose compared with that normally present and thus could override the variations in endogenous secretion (37, 38). As in our study bolus GnRH remains at a dose and rate identical to those used in the initial experiment (20 µg/pulse), we suggest that the decrease in FAS response could be due to factors other than variations in GnRH input. Similar results were observed in GnRH-deficient men. LH pulse amplitude was lowered by estradiol, albeit in response to the same GnRH bolus administration (35). Thus, if the variation in gonadotropin secretion does not come from changes in GnRH input, this would suggest an alteration in some mechanism(s) involved in the GnRH signaling pathway at the pituitary level. Due to the complexity of the signaling in gonadotrophs (39), such an alteration could result from changes at a variety of target points within the cascade of reactions, including 1) receptor number, 2) elements of the transduction pathways (postreceptor signaling effect), and/or 3) gonadotropin synthesis and/or release.
Changes in the number of pituitary GnRHR have been implicated as an important mechanism underlying the regulation of gonadotropin secretion (40, 41, 42, 43). Gonadal steroids have been shown to regulate GnRHR expression and to act in concert with GnRH to produce the LH surge in female rats (41, 44) and sheep (45, 46, 47, 48), however with differences between species (9). Whether estrogens can also regulate the expression of GnRHR and alter GnRHR in humans remains unknown. In particular, no canonical estradiol response element has been identified in the human GnRHR promoter (49, 50).
Administration to rhesus monkeys of a potent estrogen receptor
antagonist that does not cross the blood-brain barrier blocks the
inhibitory action of estradiol on LH secretion (10). In
vitro studies have shown that estradiol inhibits the transcription
of FAS (51) and suppresses GnRH-stimulated LH release in monkey
pituitary cells, whereas it amplifies the action of GnRH in rats (9),
blocks LH secretion in ovine pituitary cells (52), and reduces, in the
mouse gonadotrope
T31 cell line, GnRH binding and coupling
efficiency to second messenger generation (53).
Finally, given the relationship between GnRH dose and gonadotropin response, the fact that estrogen treatment decreases or abolishes the FAS pulse amplitude strongly suggests that estrogen modulates receptor signaling in this patient. We cannot, however, exclude a specific action of the estrogen used in this study (conjugated equine estrogen vs. estradiol).
Taken together, these data support the down-regulation of gonadotropin and FAS by long term estrogen treatment via a direct action in the pituitary that is associated with and possibly also related to a modification of GnRHR expression and/or another postreceptor event involved in the neurohormonal regulation of gonadotropin secretion.
In conclusion, we have described a woman with IHH and compound heterozygote mutations in the GnRHR gene. The new L314X mutation described here totally abolished the biological activity of the receptor. The present study demonstrates the partial resistance of LH and FSH to the GnRH stimulation as a result of expression of the Q106R mutant. We also add arguments suggesting that estrogen has an inhibitory function at the pituitary level, through an alteration of the GnRHR signaling pathway.
| Acknowledgments |
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| Footnotes |
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Received December 3, 1999.
Revised April 20, 2000.
Accepted May 13, 2000.
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