Endocrine Reviews 19 (5): 521-539
Copyright © 1998 by The Endocrine Society
Gonadotropin-Releasing Hormone Deficiency in the Human (Idiopathic Hypogonadotropic Hypogonadism and Kallmann’s Syndrome): Pathophysiological and Genetic Considerations
Stephanie B. Seminara1,
Frances J. Hayes1 and
William F. Crowley, Jr.
Reproductive Endocrine Unit of the Department of Medicine and the
National Center for Infertility Research, Massachusetts General
Hospital, Boston, Massachusetts 02114
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Abstract
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- I. Introduction
- II. Ontogeny of Normal GnRH Secretion
- A. Fetal life
- B. Neonatal and childhood periods
- C. Puberty
- D. Adulthood
- III. Historical Perspective
- IV. Clinical Presentation of GnRH Deficiency
- A. Congenital GnRH deficiency
- B. Adult-onset GnRH deficiency
- C. Variant or partial forms of GnRH deficiency
- D. Associated anomalies in KS
- E. Differential diagnosis
- V. Approaches to Studying GnRH Secretion
- A. Animal and in vitro studies
- B. Human studies
- VI. Patterns of GnRH Secretion in Men with GnRH Deficiency
- VII. Patterns of GnRH Secretion in Women with GnRH Deficiency
- VIII. Genetic Studies
- IX. X-Linked Genes Controlling GnRH Secretion
- A. Kallmann’s syndrome gene
- B. Adrenal hypoplasia congenita (AHC) gene product
- X. Autosomal Genes Controlling GnRH Secretion
- A. The genetics of autosomal inheritance
- B. GnRH gene
- C. GnRH receptor gene
- D. Other autosomal genes
- XI. Conclusion
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I. Introduction
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INITIATION and maintenance of the reproductive axis in the
human are contingent upon the pulsatile secretion of GnRH from the
hypothalamus. Isolated GnRH deficiency is the clinical syndrome that
results from failure of this normal pattern of episodic GnRH secretion
to occur. It is characterized by 1) complete or partial absence of any
endogenous GnRH-induced LH pulsations (1, 2, 3, 4, 5); 2) normalization of
pituitary and gonadal function in response to physiological regimens of
exogenous GnRH replacement (3, 4, 6, 7, 8, 9); 3) normal findings on
radiographic imaging of the hypothalamic-pituitary region, and; 4)
normal baseline and reserve testing of the remainder of the
hypothalamic-pituitary axes. Typically, patients with hypogonadotropic
hypogonadism and anosmia have been given the diagnosis of Kallmann’s
syndrome (KS), and those with normal olfaction have been diagnosed as
idiopathic hypogonadotropic hypogonadism (IHH). However, both of these
presentations (i.e., with and without anosmia) can occur in
the same family with this disorder, thus demonstrating the variability
of expression of this trait. Isolated GnRH deficiency is remarkable for
its heterogeneity of clinical presentation (1, 3, 4, 5, 10, 11), variable
modes of inheritance (11, 12, 13, 14, 15, 16, 17), and inconsistent association with other
anomalies (11, 12, 16, 18, 19, 20).
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II. Ontogeny of Normal GnRH Secretion
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In the human, the pattern of GnRH-induced gonadotropin secretion
is constantly changing across sexual development. Hence, making the
diagnosis of isolated GnRH deficiency on the basis of serum
gonadotropins and sex steroids is critically dependent upon a clear
understanding of the normal developmental timing and sequence of
GnRH-induced activation of the reproductive axis, which provides an
ever changing backdrop of normative data.
A. Fetal life
GnRH has been detected in human embryonic brain extracts as early
as 4.5 weeks (21). By 9 weeks gestation, GnRH neurons have been
demonstrated in the fetal hypothalamus, although functional connections
between these neurons and the portal system are not established until
16 weeks (21). LH and FSH are first detectable in the pituitary at 10
weeks, are measurable in peripheral blood by 12 weeks, reach a peak in
midgestation, and then decrease toward term with the development of
functioning gonadal negative feedback mechanisms (for review see Ref.
22). The fact that gonadotropins are undetectable in the serum and
pituitaries of anencephalic infants (22), as well as the ability of
GnRH to induce LHß mRNA synthesis in pituitary cells from normal
second trimester fetuses (23), suggests a role for GnRH in gonadotropin
regulation during early fetal life. Further evidence for an operative
hypothalamic GnRH "pulse generator" activity at this time is
provided by studies in the ovine fetus, which demonstrate a pulsatile
mode of LH secretion as early as midgestation that can be blocked by
chronic administration of a GnRH agonist (24).
B. Neonatal and childhood periods
During the neonatal period there is clear evidence of GnRH
secretion as evidenced by the persistence of pulsatile secretion of
gonadotropins (25), which then decrease by approximately 6 months in
boys and 1–2 yr in girls to the low levels that are present until the
onset of puberty (26, 27). For many years, the childhood period was
considered a time of complete hypothalamic-pituitary quiescence.
However, newer ultrasensitive LH assays have revealed that pulsatile
GnRH secretion continues throughout this period, albeit at a markedly
reduced amplitude (28). The precise mechanism responsible for
reversibly restraining the hypothalamic GnRH pulse generator at this
time has not yet been elucidated. However, it is likely to involve a
process that inhibits GnRH release rather than its synthesis, based on
the demonstration in primates that abundant GnRH mRNA and protein are
present within the appropriate hypothalamic neurons at an equivalent
developmental stage (29). While the mechanism that subsequently
triggers the onset of augmented GnRH secretion at puberty is still
unclear, potential candidates include metabolizing enzymes responsible
for GnRH secretion (30, 31), neurotransmitters such as norepinephrine
(32), neuropeptide Y (33), aspartate (34), or 
-aminobutyric acid
(35), glial growth factors such as transforming growth
factor-
(36, 37), and/or metabolic signals such as leptin
(38, 39, 40).
C. Puberty
Commensurate with the onset of puberty, there is a sleep-entrained
reactivation of the reproductive axis characterized by a marked
increase in the amplitude of GnRH-induced LH pulses with much more
modest changes in frequency (41, 42, 43, 44, 45, 46, 47). This nocturnal augmentation of LH
secretion stimulates secretion of sex steroids and inhibin from the
gonads at night with a subsequent decrease to prepubertal levels during
the day (41). As puberty progresses, secretion of gonadotropins occurs
during both day and night, thus completing sexual development.
D. Adulthood
Both sexes secrete gonadotropins in a pulsatile fashion during
adulthood but in very different patterns. In the adult male, wide
variations in LH interpulse interval have been reported with parallel
changes in sex steroids (48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62). However, in the majority of studies
in which intensive frequent sampling has been employed, LH pulses have
been shown to occur at an approximately 2-h frequency (49, 50, 51, 52, 53, 54, 56, 58, 59, 61, 62) (Fig. 1A
). In the female, the
reproductive axis is under more dynamic regulation, with a complex
series of changes in GnRH pulse frequency occurring throughout the
menstrual cycle (3, 63, 64, 65, 66, 67, 68). In the early follicular phase, the GnRH
pulse frequency starts at approximately 90 min, increases to 60 min in
the mid- and late-follicular phases and, with the appearance of
progesterone secretion in the luteal phase, slows to approximately
every 4–6 h. By the late luteal phase, the GnRH pulse generator is
active only at intervals of 6–8 h, but accelerates once again during
the luteal-follicular transition to approximately hourly intervals
during the day, acquiring the sleep-induced slowing typical of the
early follicular phase (69).
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Figure 1. Spectrum of GnRH-induced LH secretion in men with
GnRH deficiency. LH pulsations are indicated by
asterisks. Black bars indicate periods of
sleep. A, Normal adult male pattern of GnRH secretion with
high-amplitude regular LH pulsations and normal serum T and testicular
volume (TV). B, Apulsatile pattern of GnRH secretion in an IHH male
with complete absence of endogenous LH pulsations. C,
Sleep-entrained or developmental arrest pattern of LH secretion in
an IHH male characterized by relatively low amplitude LH pulsations
clustered during the nighttime hours analogous to the pattern that
normally occurs at puberty. D, Disordered amplitude pattern of LH
secretion in an IHH male. [Adapted with permission from N.
Santoro et al.: Endocr Rev 7:11–23, 1986
(3 ). © The Endocrine Society.]
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III. Historical Perspective
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Although Maestre de San Juan (70) first described the pathological
association of hypogonadism and anosmia in the 19th century, Kallmann
and Schoenfeld (12) were the first to ascribe a genetic basis to a
subset of patients with this disorder on the basis of three families
they observed with this condition. Coining the term "olfacto-genital
dysplasia," de Morsier (71) first noted the association of
hypogonadism and anosmia with agenesis of the olfactory bulb.
It has been difficult to ascertain the true incidence and mode of
transmission of GnRH deficiency with certitude. Since examination for
military service is required in some countries, reviewing military
records is a potential method (however limited) of ascertainment for
large male populations. Examining the medical records of 600,000
Sardinian conscripts over 37 yr identified 344 individuals rejected for
military service due to bilateral testicular atrophy. From 265 men
available for follow-up, 7 had a normal karyotype and were anosmic,
giving an incidence of KS of approximately 1 in 86,000 in this somewhat
isolated geographic population (72). In a separate review of 45,000
French men presenting for military service in 1 yr, 4 cases of
hypogonadotropic hypogonadism were identified, giving an incidence of
about 1 in 10,000 (73). The variations in these two studies portray the
difficulties of using military screening as a method of accurately
ascertaining the true incidence and prevalence of these conditions.
Moreover, given these crude estimates, it has also been challenging to
determine the true male to female ratio of reported cases, a ratio that
may shed light on the different modes of inheritance of this condition.
Our referral population at Massachusetts General Hospital of 250
consecutive cases over 20 yr demonstrates a ratio of male to female
subjects of 3.9:1. When familial cases are analyzed separately, the
ratio drops to 2.3:1. Thus, understanding the incidence of GnRH
deficiency remains a challenge; there may be differences in incidence
in different populations.
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IV. Clinical Presentation of GnRH Deficiency
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The clinical manifestation of isolated GnRH deficiency represents
a failure of GnRH secretion at puberty and possibly during the neonatal
period. Its phenotypic expression varies with its age of onset
(congenital vs. acquired) and its severity (complete
vs. partial).
A. Congenital GnRH deficiency
The diagnosis of isolated GnRH deficiency is occasionally made in
the neonatal period when male infants are found to have cryptorchidism
and microphallus in association with inappropriately low gonadotropin
and sex steroid levels, given the normal activation of the
hypothalamic-pituitary-gonadal axis that occurs in this window of
development. In utero, the early fetal testosterone (T)
production required for full sexual and external genital
differentiation is thought to have been stimulated by maternal hCG
alone. The resulting incomplete descent of the testes and growth of the
external genitalia occasionally observed in GnRH-deficient subjects at
this time implies failure of activation of the hypothalamic-pituitary
axis during the late fetal and early neonatal periods. While
cryptorchidism has been reported in up to 50% of patients with IHH or
KS in some small series (16, 74, 75), our experience at the
Massachusetts General Hospital suggests that this clinical presentation
occurs in only a minority of cases. Similarly, in our patient
population microphallus is rarely seen in association with GnRH
deficiency; however, approximately 30% of cases of microphallus in the
neonatal period may be attributed to hypogonadotropic hypogonadism
(76).
Even when cryptorchidism and microphallus are not present, some studies
have suggested that absence or anomalous morphology of the olfactory
bulbs on magnetic resonance imaging permits a presumptive diagnosis of
KS and is particularly useful for patients too young to undergo
meaningful testing of olfaction or of the
hypothalamic-pituitary-gonadal axis (77, 78). However, a more recent
study demonstrated normal olfactory bulbs in 25% of male KS patients
(79), suggesting either that this technique is not sufficiently
sensitive to differentiate KS from IHH in all cases or that the
phenotypic expression is wider than appreciated.
More typically, the diagnosis of GnRH deficiency is delayed until
adolescence when there is failure of pubertal development and absence
of appearance of secondary sex characteristics. Gynecomastia is rarely
seen in untreated IHH or KS patients. However, the clinical setting in
which it may occur is when patients are treated with either hCG or
suboptimal doses of T, in which case peripheral aromatization of T may
lead to a decrease in the T to estradiol ratio (75, 80, 81). Given that
adrenarche is independent of pubarche and therefore occurs normally in
patients with GnRH deficiency, it is theoretically possible that
peripheral aromatization of adrenal androgens may also play a role in
the origin of this gynecomastia.
Patients with isolated GnRH deficiency may be distinguished from
adolescents with constitutional delay by virtue of their growth
pattern, which indicates that they are of normal height for age (75),
while the latter tend to be short for chronological age (82). In
addition to the absence of sexual maturation, hypogonadism is also
associated with marked decreases in both cortical and trabecular bone
density compared with age-matched controls (83), which can be
normalized by long-term sex steroid replacement therapy (84, 85).
B. Adult-onset GnRH deficiency
We recently described an acquired form of GnRH deficiency in men
termed adult-onset IHH, which appears to be irreversible (86). In these
patients, there is a history of an age-appropriate normal puberty
followed by a postpubertal decrease in libido and fertility with
near-normal testicular size. Their biochemical profile is
indistinguishable from men with congenital GnRH deficiency in that they
have an apulsatile pattern of LH secretion on frequent blood sampling,
low serum T concentrations, and normal restoration of the
pituitary-gonadal axis in response to a physiological regimen of
exogenous GnRH replacement in the majority of cases (90%). The
acquired nature of the defect is supported by the historical and
physical evidence of normal spontaneous sexual maturation, documented
fertility in several cases, and the results of serial neuroendocrine
studies in one individual that clearly document the progression from
normal to apulsatile LH secretion over a 4-yr period (Fig. 2). Unlike patients with functional GnRH
deficiency, such as hypothalamic amenorrhea, we were unable to identify
any factors typically known to impair GnRH secretion transiently in
these men, e.g., stress, exercise, or weight loss. The
neuroendocrine abnormality in adult-onset hypogonadotropic hypogonadism
appears to be permanent based upon serial observation in these cases
without any evidence of spontaneous waxing and waning of the
hypothalamic-pituitary-gonadal axis, as is typically seen in women with
hypothalamic amenorrhea.
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Figure 2. Serial studies of GnRH-induced LH secretion in a
man with adult-onset hypogonadotropic hypogonadism. The patient
initially presented with oligospermia, but at the time of the second
evaluation 4 yr later had symptoms of hypogonadism. The shaded
areas represent the mean LH levels ± 2 SD in
29 normal men sampled at 10-min intervals. [Adapted with permission
from L. B. Nachtigall et al.: N Engl J
Med 336:410–415, 1997 (86 ). © 1997 Massachusetts Medical
Society. All rights reserved.]
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C. Variant or partial forms of GnRH deficiency
1. Fertile eunuch syndrome. In 1950, Pasqualini and Bur (87)
described a patient with eunuchoid body proportions and secondary
sexual characteristics in the presence of normal sized testes and
preservation of spermatogenesis, a clinical syndrome to which the term
" fertile eunuch" was subsequently applied (88). While initially
thought to represent an isolated pituitary defect in LH release, the
site of the abnormality was subsequently shown to be in the
hypothalamus, based on the lack of response to clomiphene citrate
(89, 90, 91) (which has a hypothalamic site of action) combined with the
normal response to exogenous GnRH attesting to intact pituitary
gonadotrope function (91, 92, 93). Frequent sampling of LH secretion in two
men with the fertile eunuch syndrome demonstrated augmented LH and T
secretory activity synchronous with sleep (1). This pattern of
sleep-entrained GnRH-induced LH pulses is characteristically seen in
puberty (41) but has also been described in a subset of our patients
with isolated GnRH deficiency (3, 4) (Fig. 1C
). The fertile eunuch
syndrome is thus now thought to represent an incomplete form of GnRH
deficiency, most characteristically seen in those patients with larger
testicular sizes (1, 94). The enfeebled endogenous GnRH secretion in
this condition appears sufficient to stimulate Leydig cell secretion of
T locally to support spermatogenesis and testicular growth, but
insufficient to achieve the systemic levels required for full
virilization. In this regard, the clinical and biochemical picture of
some fertile eunuchs appears similar to that of midpuberty. Hence, we
have termed this biochemical phenotype the "developmental arrest"
pattern of disordered GnRH secretion. The fertile eunuch syndrome also
bears some clinical similarity to adult-onset IHH in that both are
characterized by the presence of GnRH deficiency in association with
normal or near-normal testicular size. However, "fertile eunuchs"
are distinguished by the preservation of spermatogenesis and the
achievement of fertility with T or hCG therapy alone (87, 88, 94, 95).
2. Delayed puberty. Patients with isolated GnRH deficiency
often relate a history of partial progression through puberty followed
by a permanent arrest of sexual maturation and an apulsatile pattern of
GnRH activity (4, 86). However, what is less well appreciated is that
frequently there is a history of delayed, but otherwise normal, puberty
in the families of patients with IHH (11, 19). In the general
population the incidence of delayed puberty is less than 1% (96).
However, in our series of 106 patients with isolated GnRH deficiency,
12% had relatives with a history of delayed puberty in the absence of
any other phenotypic features of IHH (11). A similar estimate was
provided by Schwankhaus et al. (19) who reported that
approximately one-third of their series of 41 men with IHH related a
history of delayed puberty in at least 1 family member. Only half these
cases were subsequently confirmed to have hypogonadotropic
hypogonadism. Taken together, these data suggest that delay in
initiating puberty, but subsequent normal progression of sexual
development, may represent the mildest end of the phenotypic spectrum
of GnRH deficiency.
D. Associated anomalies in KS
In addition to GnRH deficiency and anosmia, KS may be variably
associated with a variety of anomalies, including midline facial
defects such as cleft lip and/or palate, short metacarpals, and renal
agenesis (11, 16, 20). Neurological manifestations of this disorder
include sensorineural hearing loss (11, 12, 14, 16, 19, 81), mirror
movements (synkinesia) (12, 18, 19, 97), oculomotor abnormalities (11, 19), and cerebellar ataxia (19, 98). To date, the occurrence of renal
agenesis and synkinesia has been limited to the X-linked forms of KS
(99). The relationship between these somatic abnormalities and
expression of the KAL gene will be elaborated in Section
IX.A.3.
E. Differential diagnosis
1. Functional GnRH deficiency. Classic isolated GnRH
deficiency must be distinguished from functional forms of GnRH
deficiency where the defect in GnRH secretion is clearly transient
rather than permanent. This latter presentation is most commonly seen
in women in the setting of hypothalamic amenorrhea (HA). In susceptible
individuals, HA may be precipitated by factors such as significant
weight loss, exercise, or stress (100, 101, 102, 103, 104). In this situation, the
defect in GnRH secretion is reversible such that menstruation resumes
when the underlying abnormality is corrected. While in females the
presence or absence of menses serves as an obvious clinical marker of
the functioning of the hypothalamic-pituitary-gonadal axis, there is no
comparable clinical marker in the male. Moderate to severe dietary
restriction in otherwise healthy males has also been shown to decrease
T levels due to compromised GnRH secretion (105, 106). However, the
existence of a syndrome of functional GnRH deficiency in men analogous
to HA in females has yet to be definitively confirmed.
2. Structural lesions. Structural lesions of the hypothalamus
can interfere with the normal pattern of GnRH synthesis, secretion, or
stimulation of gonadotropes. These patients can be distinguished from
those with IHH or KS by the presence of multiple pituitary hormone
deficiencies and by the demonstration of a neuroanatomic lesion on
magnetic resonance imaging of the hypothalamic-pituitary region (107).
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V. Approaches to Studying GnRH Secretion
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Defining the physiology of GnRH in the human to understand the
clinical heterogeneity of IHH has proven a clinical investigational
challenge. One of the main obstacles to studying GnRH secretion is its
relatively complete confinement within the hypophyseal-portal blood
supply. As direct sampling of GnRH in the hypophyseal portal blood is
not feasible in the human and measurements of GnRH in the peripheral
circulation do not accurately reflect its secretion due to its rapid
half-life of 2–4 min (108, 109, 110), studies of GnRH secretion have had to
rely on inferential approaches.
A. Animal and in vitro studies
Using the ablation-replacement model, pioneering studies by Knobil
and co-workers (111) in the 1970s were the first to demonstrate that
pulsatile GnRH secretion is a prerequisite for the maintenance of
physiological gonadotrope function. In rhesus monkeys rendered GnRH
deficient by radiofrequency lesions in the hypothalamus, episodic
stimulation of the gonadotrope by GnRH restores appropriate
gonadotropin levels, whereas continuous stimulation results in a
paradoxical desensitization of the gonadotrope response. Alternative
approaches to studying GnRH secretion include the "push-pull"
technique developed in sheep in which a direct readout of GnRH release
can be obtained by inserting a perfusion cannula into the brain
(112, 113, 114, 115). This technique confirmed the pulsatile mode of GnRH
secretion and also validated the concept that each LH pulse represents
a secondary secretory derivative of an antecedent GnRH pulse from the
hypothalamus (112, 113).
An important recent advance has been the development of immortalized
GnRH cell lines by genetically targeted tumorigenesis in transgenic
mice (116, 117). These cell lines maintain many differentiated
functions in vitro and thus provide an important
experimental model for the study of neuroendocrine regulation. While
all these models have made valuable contributions to the study of GnRH
physiology, they suffer from inherent limitations when applied to the
human in that the animal studies are limited by species specificity
while the GnRH cell lines are additionally "developmentally
frozen."
B. Human studies
The current armamentarium available to the clinical
investigator interested in studying GnRH secretion in the human
includes frequent sampling studies in both normal and disease models,
use of pharmacological probes (e.g., a GnRH antagonist), and
genetic studies, all of which provide complementary information.
Traditionally, LH has been used as a surrogate marker of GnRH pulse
generator activity in the human (62, 66, 118), based on its validation
in several animal models as a faithful mirror of GnRH secretion (112, 113). More recently, the pulsatile component of glycoprotein-free
-subunit (FAS) secretion has been shown to be closely correlated
with that of LH (119) and to be driven by GnRH, based on its
eradication by blockade of the GnRH receptor (120). Given its half-life
of 12–15 min, FAS is particularly useful in tracking GnRH when it is
being secreted at fast frequencies and at low amplitudes (121). In
addition, much information has been gained from the study of disease
models such as IHH, in which examining the baseline characteristics as
well as the ability of exogenously administered GnRH to restore normal
function allows one to determine the normal requirements for endogenous
GnRH secretion. GnRH antagonists also act as useful physiological
probes in that they can provide a semiquantitative estimate of
endogenous GnRH secretion (122), analogous to the previous use of
naloxone to quantitate endorphin tone (123). The principle employed is
that at submaximal levels of GnRH receptor blockade induced by the
administration of a pure GnRH antagonist, the amount of GnRH present
will be inversely proportional to the degree of LH inhibition. This
review will focus on the contribution made by both frequent sampling
and genetic studies to our understanding of the pathophysiology of
isolated GnRH deficiency in the human.
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VI. Patterns of GnRH Secretion in Men with GnRH Deficiency
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In isolated GnRH deficiency, a variety of aberrant
gonadotropin-secretory patterns have been observed, indicating a
spectrum of defects in GnRH secretion in keeping with the diverse
clinical presentation (1, 3, 4, 86) (Fig. 1).
In a well defined population of 50 men with isolated GnRH
deficiency, we examined pulsatile gonadotropin secretion during
frequent blood sampling at 10- to 20-min intervals for 12–24 h (3, 4).
The largest subset (84%) of patients exhibited no detectable LH pulses
(apulsatile pattern, Fig. 1B). In the majority, there was neither
historical nor physical evidence of sexual maturation, an indication of
the most severe form of GnRH deficiency. A second group demonstrated LH
pulsations that were evident predominantly during sleep (developmental
arrest pattern, Fig. 1C) similar to the pattern described by Boyar
et al. both in early puberty (41) and in the fertile eunuch
syndrome (1). When correlated with the clinical presentation, subjects
with this sleep-entrained pattern of GnRH secretion exhibited some
evidence of testicular growth and gave a history consistent with an
arrest of pubertal development. Therefore, it appears that there is an
abnormality in the program for maturation of the hypothalamic-pituitary
axis in these subjects resulting from a "developmental arrest" in
GnRH secretion. Repeat studies in these subjects using both a second
gonadotropin assay system as well as a second period of frequent
sampling confirmed the reproducibility of this pattern.
A third pattern of GnRH secretion was discernible in a family of
four brothers with IHH, three of whom had frequent sampling studies. In
these individuals, pulsatile GnRH secretion was evident, but individual
LH pulses were noted to be of diminished amplitude compared with normal
men, as well as their three unaffected siblings (decreased amplitude
pattern, Fig. 1D). This pattern of a normal pulse frequency but
decreased pulse amplitude is suggestive of either an enfeebled pattern
of endogenous GnRH secretion or a state of GnRH resistance as can occur
with a partial defect at the level of the GnRH receptor. Recently,
there have been a number of reports of IHH due to mutations of the GnRH
receptor gene (Refs. 124, 125, 126 ; see Section X). Finally, in a
single patient, pulsatile LH activity appeared normal by RIA; however,
LH bioactivity was absent when tested in the dispersed rat Leydig cell
assay (127).
In an interesting subset of GnRH-deficient men, an apulsatile pattern
of LH secretion was observed when measured by RIA, whereas distinct
pulses were observed for FAS (128). Whether this finding implies a
different threshold of sensitivity of the gonadotrope for stimulation
of LH and FAS by GnRH, or whether this finding is merely a reflection
of greater sensitivity of the FAS assay is unclear.
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VII. Patterns of GnRH Secretion in Women with GnRH Deficiency
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Unlike men in whom secretion of GnRH is maintained at a relatively
stable frequency of 2 h, the ovulatory menstrual cycle in the
female involves the above mentioned dynamic sequence of changes in the
hypothalamic-pituitary-ovarian axis (3, 63, 64, 65, 66, 67, 68, 118). Frequent sampling
studies of women with hypothalamic hypogonadism demonstrate a spectrum
of defects in pulsatile LH secretion analogous to those just described
in the male (3, 118, 129, 130). In 52 studies performed in 40 women
with hypogonadotropic amenorrhea, the early follicular phase of the
menstrual cycle was chosen as the most appropriate reference point for
comparison with normal women based upon its analogous sex steroid
milieu (3). Nineteen of the women had primary amenorrhea secondary to
either IHH (n = 12) or KS (n = 7). The remaining 21 women had
experienced a secondary loss of menstrual function of at least 6 months
duration and were classified as having hypothalamic amenorrhea.
Frequent sampling of gonadotropins was performed at 10- to 20-min
intervals for a period of 12–24 h.
As described for the male, the LH secretory pattern varied from
an apulsatile pattern, to defects in pulse frequency and amplitude, to
sleep augmentation of LH secretion (Fig. 3). In the group with primary amenorrhea,
all but one subject had no endogenous LH pulsations (Fig. 3A); the
single exception exhibited only one LH pulse during a 24-h study.
However, in the subjects with HA, the most common abnormality was a
decrease in LH pulse frequency, which occurs in approximately 40% of
subjects (Fig. 3B). In almost a quarter of the HA patients studied, the
LH secretory pattern was indistinguishable from that of the early
follicular phase controls. Interestingly, several of the women in this
category resumed spontaneous menstrual activity within a 2-yr period of
follow-up, unlike women in any other category. Therefore, it would
appear that these women were sampled during a period of almost complete
recovery from more severely deranged patterns of GnRH secretory
activity.
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Figure 3. Patterns of GnRH secretion in women with
amenorrhea associated with hypogonadotropic hypogonadism. Detected LH
pulsations are indicated by asterisks. Black
bars indicate periods of sleep. A, Apulsatile pattern with no
endogenous LH pulsations. B, Disordered frequency pattern of LH
secretion. C, Decreased amplitude pattern of LH secretion. D,
Developmental arrest or sleep-entrained pattern of LH secretion with
augmentation of pulsatile LH activity during sleep. [Reproduced with
permission from J. E. Hall et al.: In: Nutrition
& Reproduction. Louisiana State University Press, Baton Rouge,
LA, in press (214 )].
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Repeat studies in one individual studied on three separate occasions
over a 12-month period demonstrated marked differences in the LH
secretory pattern over time, from a pattern indistinguishable from that
of normal women to varying abnormalities in LH pulse frequency (3).
This changing pattern of GnRH secretion over time would explain the
marked variability in both serum gonadotropin levels and responsiveness
to diagnostic and therapeutic maneuvers that is clinically observed in
this disorder.
Thus, an identical spectrum of abnormalities in GnRH secretion, which
is likely to contribute to the clinical and biochemical heterogeneity
of this disorder, occurs in both men and women with isolated GnRH
deficiency. The variety of patterns observed, as well as the
consistency of the abnormal pulse pattern within families, suggests
that differing genetic determinants may govern the expression of GnRH
secretion.
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VIII. Genetic Studies
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GnRH deficiency may be inherited via autosomal dominant, autosomal
recessive, and X-linked modes of inheritance, underscoring considerable
genetic heterogeneity in this syndrome (11, 12, 14, 15, 16, 81, 131, 132, 133)
(Figs. 4 and 5). Unique genetic mechanisms for both KS
and IHH have been described (124, 126, 134, 135). However, some
probands with KS have family members with congenital hypogonadotropic
hypogonadism but normal olfaction (11, 12, 14, 81) (Fig. 4). This
variable expressivity suggests that some individuals with isolated GnRH
deficiency may line a single diagnostic spectrum of hypogonadism rather
than represent discrete diagnostic subsets.
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Figure 4. Autosomal recessive inheritance of GnRH deficiency
in a family with evidence of consanguinity, multiple affected members
of the same generation, and an equal sex distribution. Note the
presence of GnRH deficiency both with and without anosmia in the same
family. [Adapted with permission from J. Waldstreicher et
al.: J Clin Endocrinol Metab 81:4388–4395,
1996 (11 ). © The Endocrine Society.]
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Figure 5. Father-to-son transmission of KS, which is
incompatible with X-linked inheritance. Autosomal dominant transmission
is most likely. [Adapted with permission from J. Waldstreicher
et al.: J Clin Endocrinol Metab81:4388–4395, 1996 (11 ). © The Endocrine Society.]
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IX. X-Linked Genes Controlling GnRH Secretion
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A. Kallmann’s syndrome gene
1. Isolation of the KAL gene. Large terminal and interstitial
deletions on Xp22.3 produce a "contiguous gene syndrome" including
short stature, chondroplasia punctata, mental retardation, ichthyosis,
and KS. In 1989, the genes for these diseases were ordered along the
short arm of the X chromosome, and an interval for KS was defined
proximal to the steroid sulfatase gene (136). The X chromosome map
assignment for KS was confirmed by linkage (137), and the KAL gene was
eventually isolated by two groups using positional cloning (134, 135).
Multiple lines of investigation have provided evidence confirming the
causative role of KAL in KS. A cDNA probe covering the KAL gene was
used to search for Xp22.3 deletions in 20 unrelated X-linked cases
(138). Two deletions, including the entire KAL gene, were identified.
Similar molecular analyses revealed a 3.3-kb intragenic deletion in
another series of patients (139). These deletions upheld the role of
KAL, but suggested that large deletions are uncommon in individuals
with the X-linked form of the disease.
Final validation of the KAL gene was established with the discovery of
point mutations in additional X-linked patients. A sequencing approach
was used to identify such mutations in the 14 coding exons and splice
junctions in 19 unrelated men with KS (140). Nine unique point
mutations were identified, eight occurring within 4 exons and one at a
splice site junction. The mutations consisted of five nonsense
mutations, a single-base deletion, a single-base insertion, a missense
mutation, and the mutation at the splice site (Fig. 6).
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Figure 6. The locations of the reported KAL mutations are
depicted relative to its domain structure by the arrows.
The distribution of exons encoding different domains of the protein is
indicated on top. The relative positions of the
fibronectin type III repeats (R1–R4) are indicated. [Adapted with
permission from N. A. Georgopoulos et al.: J Clin
Endocrinol Metab 82:213–217, 1997 (195 ). © The Endocrine
Society.]
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Although these mutations revealed considerable genetic heterogeneity in
KS, the DNA sequencing had failed to uncover any mutations in the
coding sequence of 10/19 X-linked patients, leading many to speculate
about the genetic etiology of KS in the remaining cases (140). However,
the authors tried to confirm X linkage in those pedigrees (n = 7)
in which the affected individuals were members of a single sibship by
using a polymorphic dinucleotide repeat just telomeric to the KAL gene
to eliminate non-X-linked forms of the disease. This method of
segregation analysis, applied to only three of the seven families, in
fact, ruled out X linkage in a single family with three affected
siblings. However, four other families were not tested, leaving
questions about their true mode of inheritance. The original assignment
of X linkage in the entire patient cohort had been based upon the
presence of at least one affected male in addition to the proband, the
absence of affected females, and the absence of consanguinity. By these
criteria, a family with two affected brothers would have been
classified as X linked. However, such a family could also represent any
monogenic mode of inheritance, including autosomal dominant and
recessive forms. Therefore, for the four families in which the pedigree
was compatible with X linkage but dinucleotide repeat analysis was not
performed, other modes of inheritance are still possible. For the two
families in which X linkage was supported by the segregation analysis
but in which no KAL mutation was identified, the authors postulated
that a mutation could lie 1) in the noncoding part of exon 1 or exon
14, 2) within an intron, or 3) within the promoter region (140). Thus,
the true incidence of noncoding defects in confirmed X-linked cases of
KS remains unclear.
2. KAL protein. The protein encoded by the KAL gene, termed
anosmin, has 680 amino acids and shares homologies with molecules
involved in neural development. The N terminus contains a domain with
similarities to a consensus sequence of the whey acidic protein family
and a 4-disulfide core motif (141, 142) found in protease inhibitors
(143) and neurophysins (144). The C terminus contains four contiguous
fibronectin type III repeats (145) found in neural cell adhesion
molecules such as N-CAM, N-CAM L1, TAG1, and contactin (146, 147),
receptor-linked protein kinases, and phosphatases (148). Although the
protein has a leader peptide, the absence of a transmembrane domain or
phosphoinositol linkage site suggests that KAL is an extracellular
matrix protein.
Both the human and chick KAL cDNAs have been expressed in transfected
Chinese hamster ovary (CHO) cells to characterize the KAL gene product.
The protein is N-glycosylated and bound to the plasma membrane (149).
Heparan-sulfate chains of heparan-sulfate proteoglycans are involved in
the binding of the protein to the cell membrane (149). In addition to
being localized on the cell surface, KAL is cleaved at a major
proteolytic site and secreted in culture medium (150).
3. Histopathology and spatiotemporal pattern of expression. As
a prerequisite to understanding the spatiotemporal expression of KAL,
it is first necessary to understand the migratory pathways of
GnRH-containing neurons. Studies in several species have demonstrated
that GnRH neurons originate outside the central nervous system (CNS)
from the olfactory placode (151, 152, 153). The placode is a discrete
thickening of ectoderm that subsequently develops into an epithelium
from which the GnRH and olfactory neurons differentiate. In the chick,
GnRH neurons can first be detected within the olfactory placode on
embryonic day (ED) 3.5 (152) and ED4 (153). By ED5, olfactory axons
have exited the olfactory placode, and GnRH neurons can be detected
after the extracranial course of the olfactory nerve to the developing
forebrain (154). As embryogenesis continues through ED7–9, the
majority of GnRH neurons can be detected in close apposition throughout
the length of the olfactory nerve (154). Once in the CNS, the GnRH
neurons disperse, reaching the olfactory bulb, nucleus accumbens,
preoptic area, and medial septal area by ED10–12 (154).
In mammalian systems, additional nerves also derive from the olfactory
epithelium, but the migration pathway of GnRH neurons is similar to the
chick. Immunohistochemistry and in situ hybridization in
mouse embryos have demonstrated that both GnRH and olfactory neurons
originate in the olfactory placode (151). In addition, the terminalis
and vomeronasal nerves also originate from the olfactory epithelium and
migrate to the developing forebrain to reach their final destinations
in the preoptic area and accessory olfactory bulbs, respectively (155, 156). GnRH neurons migrate along these "cranial" nerves to their
final destinations in the hypothalamus and preoptic areas (151).
"Zoo blot" analysis using a human cDNA clone has revealed sequence
homologies for KAL among many species, including monkey, cow, rabbit,
sheep, and chicken (157). Homology between the human and chicken KAL
genes is greater than 90% within the putative functional domains.
Curiously, despite this evidence for evolutionary conservation of the
gene, no mouse homolog has yet been identified, a feature apparently
common to genes from the Xp22.3 region (158).
The time course of KAL expression in the chicken has yielded valuable
insights into the migration defect of GnRH neurons in human KS and
possibly into the associated somatic anomalies seen in this condition
(i.e., renal agenesis). In situ hybridization
studies in chick embryos have revealed a wide range of ectodermal,
mesodermal, and endodermal derivatives that express KAL during two
distinctly different periods of embryonic development. During ED3–6,
KAL transcripts are found in developing limb buds, facial mesenchyme,
and selected neuronal populations such as the oculomotor nucleus
primordium, which contains neurons for innervation of the extrinsic eye
muscles (156, 157). Expression of KAL within the brain varies
temporally through development with an interesting discordance between
KAL expression and GnRH neuronal migration. By ED5, only background
levels of KAL mRNA are detected in the olfactory epithelium and
mesenchymal tissue, although the GnRH neurons have already embarked
upon their migration pathway along the olfactory nerve. KAL then
appears to be up-regulated in the olfactory bulb by ED7–8 (156, 157)
at a time when most GnRH neurons are penetrating the bulb region (152).
By ED9–10, KAL expression can be found in the mitral cells, coinciding
with the formation of synapses between the incoming olfactory nerves
and the mitral cell layer (156). Notably, studies of isolated olfactory
epithelium have shown preserved olfactory axon outgrowth, demonstrating
that early migration of these nerves is an intrinsic property of the
olfactory epithelium (159). Moreover, if the olfactory placode is
ablated in the chick, KAL expression is still maintained in the
olfactory bulb (160), suggesting that expression of this gene is
independent of incoming innervation by the olfactory nerve.
In the human, by 45 days of embryogenesis (approximately equal to ED8
in the chick), KAL transcripts cannot be detected in the brain (161),
but olfactory nerves have already begun their migration from the nasal
epithelium. These observations again strongly suggest that KAL
expression in the human is not necessary for early migration of nerves
from the olfactory epithelium. However, by 11 weeks gestation, KAL
expression can be found in the olfactory bulbs with greatest expression
over the outer olfactory nerve layer (161). By 19 weeks of development,
the highest levels of KAL are found in the granule cell and olfactory
nerve layers (160), distinctly different from the chick olfactory bulb
in which the mitral cells demonstrate the highest KAL expression.
In a 19-week human fetus with X-linked KS, the olfactory nerves were
shown to have passed through the cribiform plate but arrested
prematurely in a tangle of nerve fibers within the meninges (162). Use
of anti-GnRH antibodies revealed that the GnRH neurons also had
migrated from their origins in the olfactory placode and crossed the
perforations in the cribiform plate but were similarly arrested on the
dorsal surface of the plate below the forebrain, never reaching their
normal location in the hypothalamus (162). Thus, although GnRH and
olfactory neurons share a common migration route, a defect in neuronal
interaction, rather than migration, appears to be the primary defect in
KS. As KAL expression occurs both before and during the 19-week stage
of development, the fetus study demonstrates that KAL expression is
required neither for nerve outgrowth from the placode nor navigation
toward the meninges. Collectively, therefore, it appears that KAL may
play a role in the later events of neuronal migration including 1) the
entrance of GnRH neurons into the olfactory bulb (157) and/or 2) the
establishment of contact between the incoming olfactory axons and the
central neurons of the bulb (156, 161). In the absence of KAL and, by
extension, the absence of normal synaptic connections, it has been
hypothesized that the olfactory nerve might undergo retrograde
degeneration (156), with the GnRH migration defect secondary to this
defective scaffold.
In situ hybridization data in other nonolfactory tissues
suggest that KAL may have a more generalized role in development. In
the chick, strong expression of KAL occurs in the striatum by ED8, in
the Purkinje cells of the cerebellum by ED9–10, and in the optic
tectum by the last days of incubation (156). Expression of KAL in these
structures appears to persist into adult life (156, 157). Outside the
CNS, expression of KAL has also been detected in mesenchymal
derivatives, including facial mesenchyme, fibrous and perichondral
cells, small blood vessel walls, glomeruli of mesonephros and
metanephros, and developing limb buds (156, 157). In the human,
abnormal KAL expression in the cerebellum may be related to symptoms
such as nystagmus and ataxia seen in some patients with KS (19).
Likewise, correlations may also exist between expression in the facial
mesenchyme and cleft palate, and expression in the
mesonephros/metanephros and renal agenesis. Because not all KS patients
display the same constellation of symptoms, these correlations are
crude and suggest that different mechanisms may be at play between the
chick and human KAL expression or that alternative mechanisms affect
phenotypic expression (157). Moreover, since the frequency of these
anomalies has been best described in X-linked cases (140), it is not
clear whether the frequency and/or phenotypic expression of these
associated findings in autosomally transmitted forms is comparable.
An immunofluorescent double immunostaining technique with antisera
raised against human GnRH and the ubiquitin-processing enzyme Protein
Gene Product-9.5 (PGP-9.5) has been used to evaluate GnRH
immunoreactivity in the nasal epithelia of patients with both KS and
IHH (163). Colocalization of GnRH+ and PGP+ immunoreactivity has
indicated the presence of GnRH-synthesizing cells in the upper nasal
epithelia in two adult KS males, one KS female, one IHH male, and one
control eugonadal male. This finding of GnRH-synthesizing cells in the
nasal mucosa in human adult life in affected patients raises a number
of possibilities regarding the distribution of this cell population:
the GnRH neurons may represent remnant cells that simply failed to
migrate normally during embryogenesis or they may have been generated
postnatally de novo. In either case, the presence of cells
with GnRH-synthesizing capability in the nasal mucosa of an IHH
normosmic subject demonstrates that a GnRH synthetic defect is not
responsible for this subtype of GnRH deficiency.
Because the histopathology of KAL suggests involvement in early
morphogenetic events as well as neuronal differentiation, its
transcriptional control may also involve both developmental and
cell-specific mechanisms. Characterization of the human KAL promoter
(1.9-kb fragment upstream from the first exon) has revealed two sites
of gene transcription initiation in a quail embryonic neuroretina QNR/D
cell line (164). Although neural cell adhesion molecule is
controlled by homeobox transcription factors, no homology between the
KAL promoter and any homeodomain binding site has yet been found. The
KAL promoter does contain a typical CCAAT box that binds nuclear factor
Y, two GC boxes that bind SP1 transcription factor, and two AP2-binding
sites, but no TATA box (164). A deletion mutant containing nucleotides
+2 to -437 demonstrated weak but tissue-specific transcriptional
activity in quail neuroretina cell lines. No activity was detected
using longer segments, suggesting that upstream sequences in the KAL
promoter may contain negative regulatory elements.
4. Clinical paradoxes. By using increasingly sophisticated
tools to phenotype patients, investigators have expanded the list of
occasional symptoms and signs found in GnRH-deficient patients in
addition to hypogonadotropic hypogonadism and anosmia. Within this
ever-expanding roster, unilateral renal agenesis appears to have the
most important clinical implications for patients and may be quite
prevalent in X-linked individuals. Six of 11 men with an identified
mutation in the KAL gene were found to have unsuspected unilateral
renal aplasia (140), a finding of sufficient frequency and importance
to warrant screening with renal ultrasound in all such cases. Thus,
although the KAL gene defect is the best characterized of the familial
genes for GnRH deficiency to date, numerous questions regarding its
biology remain unanswered. Despite the wide range of KAL expression
during early mesenchymal development in the chicken (arterial walls,
limb buds, etc.), there is not a parallel range of clinical signs in
most human patients. Although the role of the KAL gene in
axonal migration may explain some of the neurological defects in
X-linked KS (e.g., synkinesia), the role of KAL in renal
development remains unknown.
Numerous clinical reports have demonstrated a puzzling lack of
genotype/phenotype correlation in KS. For example, despite having an
identical deletion spanning the STS and KAL genes, two brothers were
reported as demonstrating a marked discordance in phenotype (165). One
brother had hypogonadotropic hypogonadism and severe hyposmia while the
other had normal pubertal development and only a mild olfactory defect.
In another set of two brothers bearing the same point mutation within
KAL (G924 to A base substitution creating a premature stop
codon), different phenotypes were again observed (79). One had KS with
bilateral synkinesia, left renal agenesis, and cryptorchidism while his
brother also had KS but none of the associated findings. These
observations suggest that 1) other proteins with redundant function to
those of KAL can possibly compensate when the KAL gene is defective
and/or that 2) other epigenetic phenomena may be involved in the
phenotypic manifestations of KS. Compensatory mechanisms may also exist
in IHH as identical twins discordant only for GnRH deficiency have been
identified (Fig. 7).
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Figure 7. Example of identical twins discordant for IHH. The
fertile twin took diethylstilbestrol during her pregnancies and bore
two affected children. The pedigree is compatible with autosomal
dominant transmission with incomplete penetrance. [Adapted with
permission from J. Waldstreicher et al.: J Clin Endocrinol
Metab 81:4388–4395, 1996 (11 ). © The Endocrine
Society.]
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B. Adrenal hypoplasia congenita (AHC) gene product
1. Isolation of the DAX gene. At least one additional gene on
the X chromosome can cause hypogonadotropic hypogonadism in association
with AHC, a disorder that typically presents with primary adrenal
insufficiency in infancy. AHC may arise as part of either X-linked or
autosomal recessive syndromes. As treatment with adrenal steroids has
allowed patients to survive into adulthood, subsequent development of
hypogonadotropic hypogonadism has become a recognized feature of the
X-linked form. Numerous attempts to stimulate gonadotropin secretion in
patients with AHC using pulsatile GnRH have failed (166, 167, 168),
suggesting a pituitary origin for the hypogonadotropic hypogonadism.
However, other investigators (including ourselves) have been able to
induce normal LH, FSH, and T levels during both short- and long-term
pulsatile GnRH therapy in a subset of these patients (169, 170).
Differences in GnRH administration (i.e., nonphysiological
regimens) may partly explain some of the early failures of pulsatile
therapy. However, the heterogeneity of responses to GnRH suggests that
certain subsets of AHC patients may have distinct hypothalamic
vs. pituitary defects.
As with KAL, molecular studies of patients with contiguous gene
syndromes including AHC, Duchenne muscular dystrophy, and glycerol
kinase deficiency led to the localization and isolation of the gene
responsible for AHC at Xp21, referred to as DAX-1 [dosage-sensitive
sex-reversal (DSS) AHC critical region on human X chromosome, gene 1]
(171). DAX-1 is located within the DSS locus, which, when duplicated in
46XY males, causes development into phenotypic females (172). In AHC,
missense mutations in DAX-1 have been identified in the C-terminal
presumptive ligand-binding domain, whereas frameshift or nonsense
mutations have been described in the N-terminal domain, suggesting that
the C-terminal half of the protein may be more important for protein
function (172, 173, 174, 175, 176, 177, 178, 179) (Fig. 8). De
novo deletions of the DAX gene have also been reported in
patients, suggesting that molecular analysis of the DAX-1 gene may be
helpful in the genetic counseling of families (179, 180). Similar to
KAL, the regulation of DAX expression appears to be complex. Not every
suspected case of X-linked AHC has yielded DAX-1 mutations, suggesting
considerable genetic heterogeneity or differential expression of the
gene (173). In an X-linked pedigree of AHC and GnRH deficiency with a
proven DAX mutation, an affected 3-month-old demonstrated normal T and
LH levels, consistent with an intact hypothalamic-pituitary axis (178).
Therefore, the growing body of literature on DAX-1 suggests that a wide
range of phenotypes may be associated with the same DAX mutation.
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Figure 8. The locations of representative examples of DAX-1
mutations are depicted relative to its domain structure. The nuclear
receptor-like domain is blackened, and the amino-terminal
repeats are depicted by arrows. The junction of exons 1
and 2 is denoted by an arrowhead. Frameshift and
nonsense mutations leading to premature truncation of the protein are
shown in the upper two panels. Missense mutations and
the single codon deletion (dV269) are shown in the bottom
panel. The black bars below the figure indicate
protein domains that are involved in transcriptional silencing.
Nucleotides are numbered such that the A of the ATG initiation codon is
+1, as per the recommendation of the Nomenclature Working Group.
[Reprinted from R. N. Yu et al.: The role of DAX-1 in
reproduction. In: Trends in Endocrinology and
Metabolism, vol 9 (5 ):169–175, 1998 with permission from
Elsevier Science (215 ).]
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2. DAX protein. DAX-1 encodes a protein that appears to share
homology with the nuclear hormone receptor superfamily with a novel
DNA-binding domain (172). Detailed clinical investigations of two
kindreds with AHC and hypogonadotropic hypogonadism and
confirmed missense mutations in DAX-1 have helped illuminate the
functional role of this protein. Two probands underwent frequent blood
sampling to analyze their baseline gonadotropin dynamics (177). One
individual demonstrated erratic LH secretion and only a single FAS
surge (Fig. 9). The second individual
demonstrated low LH levels but FAS concentrations within the normal
range. In neither individual did pulsatile exogenous GnRH elicit a
gonadotropin response. These data suggest that DAX-1 mutations impair
gonadotropin production at both the hypothalamus and pituitary.
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Figure 9. Baseline secretory patterns of LH and free
-subunit in two patients with DAX mutations. Gonadotropin secretion
was determined by 10-min blood sampling. Shaded areasrepresent the mean ± 2 SD for each hormone
determined in 20 normal adult males studied using the same protocol
(62 119 ). Mean FSH and T levels determined from pools of samples are
indicated. Pulses were detected by a modified version of the Santen and
Bardin (54 ) procedure and are denoted by arrowheads.
Panel A shows the secretory patterns for the patient with a thr418asp
mutation. Panel B shows the secretory patterns for the patient with a
gly329glu mutation. [Adapted with permission from R. L. Habibyet al.: J Clin Invest 98:1055–1062, 1996
(177 ) by copyright permission of The American Society for Clinical
Investigation.]
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Similar to DAX-1, steroidogenic factor 1 (SF-1) is an orphan
nuclear hormone receptor with a critical regulatory role in the
development of the adrenals and the hypothalmic-pituitary-gonadal
axis. SF-1 has been shown to control the expression of the
P450 steroid hydroxylase genes in the gonads and adrenal
cortex (181) and to regulate the Mullerian-inhibiting substance (MIS)
gene (182, 183), the -subunit of gonadotropins (184), and the
ß-subunit of LH (185). Because 1) SF-1 and DAX-1 colocalize in
multiple cell lineages of the hypothalamic-pituitary axis (172), 2)
SF-1 is expressed earlier in mouse adrenal development than DAX-1 (186, 187), and 3) the SF-1 knockout mouse has a more severe phenotype than
patients with DAX-1 mutations (188), it was hypothesized that SF-1
regulates DAX-1 expression (i.e., is "upstream" of DAX-1
in the normal cascade of development). The findings that an SF-1
response element (SF-1-RE) has been identified in the DAX-1 promoter
(189) and that SF-1 has been shown to increase the transcriptional
activity of the DAX-1 promoter in adrenocortical cells (190) have
furthered this speculation. However, the control of SF-1 itself in
directing testis determination appears complicated, with SF-1 and WT1
(Wilms’ tumor 1) working synergistically to affect MIS expression and
DAX-1 inhibiting this interaction (191). These observations, combined
with the association to the duplicated dosage-sensitive sex reversal
locus in 46XY males, suggest that DAX-1, while necessary for normal
adrenal and hypothalamic/pituitary development, may also serve as a
"brake" in normal male maturation (191).
As observed with other members of the nuclear hormone receptor
superfamily, the C terminus of the DAX gene contains transcriptional
silencing activity (192). DAX-1 has been shown to bind to DNA hairpin
secondary structures and to block steroidogenesis in adrenal cells via
transcriptional repression of the steroidogenic acute regulatory
protein (StaR) promoter (193). DAX-1 is also able to repress the
transcriptional activity of SF-1 (194). Since the missense mutations
described for DAX-1 in AHC appear to be clustered in its C terminus
(ligand-binding domain), it is possible that such mutations impair the
potent transcriptional silencing properties of this protein. Taken
together, DAX-1 appears to be a transcriptional repressor whose loss of
function may be associated with a spectrum of adrenal, hypothalamic,
and pituitary abnormalities.
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X. Autosomal Genes Controlling GnRH Secretion
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A. The genetics of autosomal inheritance
Despite the biology revealed by the X-linked genes, considerable
evidence has accumulated to suggest that not only are there autosomal
genes 
Top
Abstract
I. Introduction
