KLB, encoding ß-Klotho, is mutated in patients with congenital hypogonadotropic hypogonadism.

Congenital hypogonadotropic hypogonadism (CHH) is a rare genetic form of isolated gonadotropin-releasing hormone (GnRH) deficiency caused by mutations in > 30 genes. Fibroblast growth factor receptor 1 (FGFR1) is the most frequently mutated gene in CHH and is implicated in GnRH neuron development and maintenance. We note that a CHH FGFR1 mutation (p.L342S) decreases signaling of the metabolic regulator FGF21 by impairing the association of FGFR1 with ß-Klotho (KLB), the obligate co-receptor for FGF21. We thus hypothesized that the metabolic FGF21/KLB/FGFR1 pathway is involved in CHH Genetic screening of 334 CHH patients identified seven heterozygous loss-of-function KLB mutations in 13 patients (4%). Most patients with KLB mutations (9/13) exhibited metabolic defects. In mice, lack of Klb led to delayed puberty, altered estrous cyclicity, and subfertility due to a hypothalamic defect associated with inability of GnRH neurons to release GnRH in response to FGF21. Peripheral FGF21 administration could indeed reach GnRH neurons through circumventricular organs in the hypothalamus. We conclude that FGF21/KLB/FGFR1 signaling plays an essential role in GnRH biology, potentially linking metabolism with reproduction.

Genetic aspects of hypothalamic and pituitary gland development.

Hypothalamo-pituitary development during embryogenesis is a highly complex process involving the interaction of a network of spatiotemporally regulated signaling molecules and transcription factors. Mutations in any of the genes encoding these components can lead to congenital hypopituitarism, which is often associated with a wide spectrum of defects affecting craniofacial/midline development. In turn, these defects can be incompatible with life, or lead to disorders encompassing holoprosencephaly (HPE) and cleft palate, and septo-optic dysplasia (SOD). In recent years, there has been increasing evidence of an overlapping genotype between this spectrum of disorders and Kallmann syndrome (KS), defined as the association of hypogonadotropic hypogonadism (HH) and anosmia. This is consistent with the known phenotypic overlap between these disorders and opens a new avenue of identifying novel genetic causes of the hypopituitarism spectrum. This chapter reviews the genetic and molecular events leading to the successful development of the hypothalamo-pituitary axis during embryogenesis, and focuses on genes in which variations/mutations occur, leading to congenital hypopituitarism and associated defects.

Human GnRH deficiency: a unique disease model to unravel the ontogeny of GnRH neurons.

Evolutionary survival of a species is largely a function of its reproductive fitness. In mammals, a sparsely populated and widely dispersed network of hypothalamic neurons, the gonadotropin-releasing hormone (GnRH) neurons, serve as the pilot light of reproduction via coordinated secretion of GnRH. Since it first description, human GnRH deficiency has been recognized both clinically and genetically as a heterogeneous disease. A spectrum of different reproductive phenotypes comprised of congenital GnRH deficiency with anosmia (Kallmann syndrome), congenital GnRH deficiency with normal olfaction (normosmic idiopathic hypogonadotropic hypogonadism), and adult-onset hypogonadotropic hypogonadism has been described. In the last two decades, several genes and pathways which govern GnRH ontogeny have been discovered by studying humans with GnRH deficiency. More importantly, detailed study of these patients has highlighted the emerging theme of oligogenicity and genotypic synergism, and also expanded the phenotypic diversity with the documentation of reversal of GnRH deficiency later in adulthood in some patients. The underlying genetic defect has also helped understand the associated nonreproductive phenotypes seen in some of these patients. These insights now provide practicing clinicians with targeted genetic diagnostic strategies and also impact on clinical management.

Congenital idiopathic hypogonadotropic hypogonadism: evidence of defects in the hypothalamus, pituitary, and testes.

CONTEXT: Idiopathic hypogonadotropic hypogonadism (IHH) with normal smell (normosmic IHH) or anosmia (Kallmann syndrome) is associated with defects in the production or action of GnRH. Accordingly, most IHH patients respond to physiological pulsatile GnRH replacement by normalizing serum LH, FSH, and testosterone (T) levels and achieving gametogenesis; some patients, however, show atypical responses. Interestingly, several IHH-associated genes are expressed in multiple compartments of the hypothalamic-pituitary-gonadal axis. OBJECTIVE: The aim of the study was to investigate whether the clinical, biochemical, or genetic characteristics of IHH men with atypical responses to GnRH indicate alternative or additional defects in the hypothalamic-pituitary-gonadal axis. SUBJECTS: We studied 90 IHH men undergoing long-term pulsatile GnRH treatment over 30 yr. DESIGN AND SETTING: We conducted a retrospective study of response to GnRH at a Clinical Research Center. INTERVENTIONS: Physiological regimens of pulsatile s.c. GnRH were administered for at least 12 months. Dose-response studies using i.v. GnRH pulses assessed the pituitary LH response. MAIN OUTCOME MEASURES: We measured serum T, LH, FSH, and inhibin B levels, sperm in ejaculate, and determined the sequence of IHH-associated genes. RESULTS: Twenty-six percent of subjects displayed atypical responses to GnRH: 1) 10 remained hypogonadotropic and hypogonadal, demonstrating pituitary and testicular defects; 2) eight achieved spermatogenesis and normal T but only with hypergonadotropism, indicating impaired testicular responsiveness to gonadotropins; and 3) five remained azoospermic despite achieving adult testicular volumes and normal hormonal profiles, suggesting primary defects in spermatogenesis. Mutations were identified only in KAL1 across groups. CONCLUSION: In addition to hypothalamic GnRH deficiency, IHH men can have primary pituitary and/or testicular defects, which are unmasked by GnRH replacement.

[Hypogonadotropic hypogonadism: new aspects in the regulation of hypothalamic-pituitary-gonadal axis]. / Hypogonadisme hypogonadotrope : notions récentes sur la régulation de l'axe hypothalamo-hypophyso-gonadique.

Hypogonadotropic hypogonadism (HH) is defined by the absence of sex steroid synthesis associated with the lack of appropriate gonadotrophin secretion. This leads to a variable degree of impuberism, often diagnosed during childhood or adolescence. Genetics of HH involve many genes. However, molecular defects have been identified in only 30 % of patients. Kallmann syndrome (KS) is defined by the association of HH and anosmia. Six genes are involved in KS (KAL1, FGFR1, FGF8, PROK2, PROKR2 and CHD7). However, genetics of KS is complex, because of the variability of the phenotype for a similar molecular defect. Otherwise, heterozygous anomalies are frequently described. Identification in the same patient of several mutations in some of these genes (digenism) could account for this variability. Autosomal recessive transmission is frequently observed in familial cases of HH without anosmia. Molecular alterations have been identified for several neuropeptides or their corresponding receptors, which are involved in the physiology of the gonadotropic axis : GNRHR, KISS1R/GPR54, neurokinin B (TAC3), TACR3 and GNRH1 (and PROK2, PROKR2 and CHD7). Anomalies of leptin or its receptor are also involved in HH cases. A new negative regulating element has been recently identified in humans : RFRP3, which is ortholog of the avian GnIH (gonadotrophin inhibitory hormone). Recent progress about these neuropeptides leads to a new model of comprehension of the gonadotropic axis physiology, from a linear model to a network model, which regulates the central element of regulation of the gonadotropic axis, represented by the GnRH neurons.

An unusual case of autonomous hyperparathyroidism in a patient with X-linked hypophosphatemic rickets and Kallmann syndrome.

We are reporting an unusual patient who presented to our medical center at 18 years of age for evaluation of disabling bilateral lower extremity deformity and delayed puberty. Extensive clinical, laboratory, and radiologic evaluation confirmed the coexistence of 2 X-linked inherited disorders, X-linked hypophosphatemic rickets (XLH) and Kallmann syndrome (KS). Treatment with oral phosphate and calcitriol along with intramuscular testosterone injections was initiated. Despite a dramatic response, the course of treatment was complicated by secondary hyperparathyroidism and, 13 years later, by the development of an autonomous parathyroid adenoma that was surgically resected. Furthermore, the coexistence of XLH and KS has not been reported before. We believe that the proximity of the KAL-1 gene (Xp 22.3), involved in the pathogenesis of KS, to the phosphate regulating endopeptidase on the X chromosome gene (Xp 22.1-22.2), involved in XLH, might be responsible for this association.

Forebrain gonadotropin-releasing hormone neuronal development: insights from transgenic medaka and the relevance to X-linked Kallmann syndrome.

Neurons that synthesize and release GnRH are essential for the central regulation of reproduction. Evidence suggests that forebrain GnRH neurons originate in the olfactory placode and migrate to their final destinations, although this is still a matter of controversy. X-linked Kallmann syndrome (X-KS), characterized by failed gonadal function secondary to deficient gonadotropin secretion, is caused by a mutation in KAL1, which is suggested to regulate the migration of forebrain GnRH neurons. Because rodents lack Kal1 in their genome and have GnRH neurons scattered throughout their forebrain, the development of forebrain GnRH neurons and the pathogenesis of X-KS have been difficult to study. In the present study, we generated transgenic medaka that expressed green fluorescent protein under the control of the gnrh1 and gnrh3 promoters for analyzing forebrain GnRH neuronal development. Our data revealed the presence of the following four gnrh1 neuronal populations: an olfactory region-derived ventral preoptic population, a dorsal preoptic population that migrates from the dorsal telencephalon, a medial ventral telencephalic population that migrates from the anterior telencephalon, and a nonmigratory ventral hypothalamic population. We found that all forebrain gnrh3 neurons, extending from the terminal nerve ganglion to the anterior mesencephalon, arise from the olfactory region and that trigeminal ganglion neurons express gnrh3. Maternal gnrh3 expression was also observed in oocytes and early embryos. We subsequently identified a KAL1 ortholog and its paralogous form in the medaka. Consistent with the X-KS phenotype, antisense knockdown of the medaka KAL1 ortholog resulted in the disruption of forebrain GnRH neuronal migration. Thus, these transgenic medaka provide a useful model system for studying GnRH neuronal development and disorders of GnRH deficiency.

[GnRH deficiency: new insights from genetics]. / Déficience en GnRH: le nouvel eclairage apporté par la génétique.

The acquisition of a sexually dimorphic phenotype is a critical event in mammalian development. Hypogonadotropic hypogonadism (HH) results from impaired secretion of GnRH. The patients display with delayed puberty, micropenis and cryptorchidism in the male reflecting gonadotropin insufficiency, and amenorrhea in the female. Kallmann's syndrome (KS) is defined by the association of HH and anosmia or hyposmia (absent smelling sense). Segregation analysis in familial cases has demonstrated diverse inheritance patterns, suggesting the existence of several genes regulating GnRH secretion. The X-linked form of the disease was associated with a genetic defect in the KALI gene located on the Xp22.3 region. KAL1 gene encodes an extracellular matrix glycoprotein anosmin-1, which facilitates neuronal growth and migration. Abnormalities in the migratory processes of the GnRH neurons with the olfactory neurons explain the association of HH with anosmia. Recently, mutations in the FGF recepteur 1 (FGFR1) gene were found in KS with autosomal dominant mode of inheritance. The role of FGFR1 in the function of reproduction requires further investigation. Besides HH with anosmia, there are isolated HH (IHH). No human GnRH mutations have been reported although hypogonadal mice due to a GnRH gene deletion exist. In patients with idiopathic HH and without anosmia an increasing number of GnRH receptor (GnRHR) mutations have been described which represent about 50% of familial cases. The clinical features are highly variable and there is a good relationship between genotype and phenotype. A complete loss of function is associated with the most severe phenotype with resistance to pulsatile GnRH treatment, absence of puberty and cryptorchidism in the male. In contrast, milder loss of function mutations causes incomplete failure of pubertal development. The preponderant role of GnRH in the secretion of LH by the gonadotrophs explains the difference of the phenotype between male and female with partial GnRH resistance. Affected females can have spontaneous telarche and normal breast development while affected males exhibit no pubertal development but normal testis volume, a feature described as "fertile-eunuch". High-dose pulsatile GnRH has been used to induce ovulation. Another gene, called GPR54, responsible for idiopathic HH has been recently described by segregation analysis in two different consanguineous families. The GPR54 gene is an orphan receptor, and its putative ligand is the product of the KISS-1 gene, called metastine. Their roles in the function of reproduction are still unknown.

Female hypogonadotropic hypogonadism. Hypothalamic amenorrhea syndrome.

The neuroendocrine-metabolic repertoire governing the reproductive cyclicity in women can be interrupted by a variety of social, environmental, nutritional, and psychological aberrations. Clinical conditions including exercise-related and psychogenic amenorrhea, and desynchronization of biological rhythms in the development of hypothalamic gonadotropin-releasing hormone dysfunction are discussed. Clinical and laboratory evaluations and modes of management are presented.

Clinical counterpoint: gonadotropin-releasing hormone deficiency: perspectives from clinical investigation.

Advances in our understanding of the pathophysiology of Kallmann syndrome have come from an interdisciplinary approach involving developmental biology, clinical investigation, and molecular biology. It is equally clear that progress to date is but the first chapter of what will be a fascinating biological story. It now seems likely that the full expression of reproductive potential from the neuroendocrine perspective is likely to be as complicated as other aspects of reproduction, such as the multigene control of external genital differentiation. An analogous story may well emerge for the neuroendocrine control of reproduction in which the GnRH gene is encoded on the eighth chromosome, the protein guiding the embryonic journey of the GnRH-producing neuron to the hypothalamus lies on the X chromosome, and many, as yet to be determined, other genetic loci collaborate in the full expression of reproductive potential. Such a detailed study is warranted not only because of the clinical and genetic implications for an individual patient with this disorder, but also from an organizational theme for the evolution of the species (and its potential regulation). Given the pressing nature of world population growth, obtaining such understanding and its applications to fertility and contraception is crucial. These advances will only come from enlightened interactions of clinical investigators, molecular geneticists, and developmental biologists in which interdisciplinary approaches should be fostered. This should be an exciting story to follow given the remarkable nature of the tools at hand to study these clinical conditions.