Fibroblast growth factor 8 signaling through fibroblast growth factor receptor 1 is required for the emergence of gonadotropin-releasing hormone neurons

Defective FGFR1 Signaling Disrupts Glucose Regulation: Evidence From Humans With FGFR1 Mutations

Context

Activation of fibroblast growth factor receptor 1 (FGFR1) signaling improves the metabolic health of animals and humans, while inactivation leads to diabetes in mice. Direct human genetic evidence for the role of FGFR1 signaling in human metabolic health has not been fully established.

Objective

We hypothesized that individuals with naturally occurring FGFR1 variants ("experiments of nature") will display glucose dysregulation.

Methods

Participants with rare FGFR1 variants and noncarrier controls. Using a recall-by-genotype approach, we examined the β-cell function and insulin sensitivity of 9 individuals with rare FGFR1 deleterious variants compared to 27 noncarrier controls, during a frequently sampled intravenous glucose tolerance test at the Reproductive Endocrine Unit and the Harvard Center for Reproductive Medicine, Massachusetts General Hospital. FGFR1-mutation carriers displayed higher β-cell function in the face of lower insulin sensitivity compared to controls.

Conclusion

These findings suggest that impaired FGFR1 signaling may contribute to an early insulin resistance phase of diabetes pathogenesis and support the candidacy of the FGFR1 signaling pathway as a therapeutic target for improving the human metabolic health.

The initiation and maintenance of gonadotropin-releasing hormone neuron identity in congenital hypogonadotropic hypogonadism

Neurons that secrete gonadotropin-releasing hormone (GnRH) drive vertebrate reproduction. Genetic lesions that disrupt these neurons in humans lead to congenital hypogonadotropic hypogonadism (CHH) and reproductive failure. Studies on CHH have largely focused on the disruption of prenatal GnRH neuronal migration and postnatal GnRH secretory activity. However, recent evidence suggests a need to also focus on how GnRH neurons initiate and maintain their identity during prenatal and postnatal periods. This review will provide a brief overview of what is known about these processes and several gaps in our knowledge, with an emphasis on how disruption of GnRH neuronal identity can lead to CHH phenotypes.

Mendelian inheritance revisited: dominance and recessiveness in medical genetics

Understanding the consequences of genotype for phenotype (which ranges from molecule-level effects to whole-organism traits) is at the core of genetic diagnostics in medicine. Many measures of the deleteriousness of individual alleles exist, but these have limitations for predicting the clinical consequences. Various mechanisms can protect the organism from the adverse effects of functional variants, especially when the variant is paired with a wild type allele. Understanding why some alleles are harmful in the heterozygous state - representing dominant inheritance - but others only with the biallelic presence of pathogenic variants - representing recessive inheritance - is particularly important when faced with the deluge of rare genetic alterations identified by high throughput DNA sequencing. Both awareness of the specific quantitative and/or qualitative effects of individual variants and the elucidation of allelic and non-allelic interactions are essential to optimize genetic diagnosis and counselling.

The great migration: how glial cells could regulate GnRH neuron development and shape adult reproductive life

In mammals, reproductive function is under the control of hypothalamic neurons named Gonadotropin-Releasing Hormone (GnRH) neurons. These neurons migrate from the olfactory placode to the brain, during embryonic development. For the past 40 years, these neurons have been considered an example of tangential migration, i.e., dependent on the olfactory/vomeronasal/terminal nerves. Numerous studies have highlighted the factors involved in the migration of these neurons but thus far overlooked the cellular microenvironment that produces them. Many of these factors are dysregulated in hypogonadotropic hypogonadism, resulting in subfertility/infertility. Nevertheless, over the past ten years, several papers have reported the influence of glial cells (named olfactory ensheathing cells [OECs]) in the migration and differentiation of GnRH neurons. This review will describe the atypical origins, migration, and differentiation of these neurons, focusing on the latest discoveries. There will be a more specific discussion on the involvement of OECs in the development of GnRH neurons, during embryonic and perinatal life; as well as on their potential implication in the development of congenital or idiopathic hypogonadotropic hypogonadism (such as Kallmann syndrome).

Time and dose-dependent effects of FGF8-FGFR1 signaling in GnRH neurons derived from human pluripotent stem cells

Fibroblast growth factor 8 (FGF8), acting through the fibroblast growth factor receptor 1 (FGFR1), has an important role in the development of gonadotropin-releasing hormone-expressing neurons (GnRH neurons). We hypothesized that FGF8 regulates differentiation of human GnRH neurons in a time- and dose-dependent manner via FGFR1. To investigate this further, human pluripotent stem cells were differentiated during 10 days of dual-SMAD inhibition into neural progenitor cells, followed either by treatment with FGF8 at different concentrations (25 ng/ml, 50 ng/ml or 100 ng/ml) for 10 days or by treatment with 100 ng/ml FGF8 for different durations (2, 4, 6 or 10 days); cells were then matured through DAPT-induced inhibition of Notch signaling for 5 days into GnRH neurons. FGF8 induced expression of GNRH1 in a dose-dependent fashion and the duration of FGF8 exposure correlated positively with gene expression of GNRH1 (P<0.05, Rs=0.49). However, cells treated with 100 ng/ml FGF8 for 2 days induced the expression of genes, such as FOXG1, ETV5 and SPRY2, and continued FGF8 treatment induced the dynamic expression of several other genes. Moreover, during exposure to FGF8, FGFR1 localized to the cell surface and its specific inhibition with the FGFR1 inhibitor PD166866 reduced expression of GNRH1 (P<0.05). In neurons, FGFR1 also localized to the nucleus. Our results suggest that dose- and time-dependent FGF8 signaling via FGFR1 is indispensable for human GnRH neuron ontogeny.

This article has an associated First Person interview with the first author of the paper.

Summary: This article demonstrates the essential role FGF8–FGFR1 signaling has in the development of gonadotropin-releasing hormone (GnRH)-expressing neurons by using a human stem cell model.

p140Cap Controls Female Fertility in Mice Acting via Glutamatergic Afference on Hypothalamic Gonadotropin-Releasing Hormone Neurons

p140Cap, encoded by the gene SRCIN1 (SRC kinase signaling inhibitor 1), is an adaptor/scaffold protein highly expressed in the mouse brain, participating in several pre- and post-synaptic mechanisms. p140Cap knock-out (KO) female mice show severe hypofertility, delayed puberty onset, altered estrus cycle, reduced ovulation, and defective production of luteinizing hormone and estradiol during proestrus. We investigated the role of p140Cap in the development and maturation of the hypothalamic gonadotropic system. During embryonic development, migration of Gonadotropin-Releasing Hormone (GnRH) neurons from the nasal placode to the forebrain in p140Cap KO mice appeared normal, and young p140Cap KO animals showed a normal number of GnRH-immunoreactive (-ir) neurons. In contrast, adult p140Cap KO mice showed a significant loss of GnRH-ir neurons and a decreased density of GnRH-ir projections in the median eminence, accompanied by reduced levels of GnRH and LH mRNAs in the hypothalamus and pituitary gland, respectively. We examined the number of kisspeptin (KP) neurons in the rostral periventricular region of the third ventricle, the number of KP-ir fibers in the arcuate nucleus, and the number of KP-ir punctae on GnRH neurons but we found no significant changes. Consistently, the responsiveness to exogenous KP in vivo was unchanged, excluding a cell-autonomous defect on the GnRH neurons at the level of KP receptor or its signal transduction. Since glutamatergic signaling in the hypothalamus is critical for both puberty onset and modulation of GnRH secretion, we examined the density of glutamatergic synapses in p140Cap KO mice and observed a significant reduction in the density of VGLUT-ir punctae both in the preoptic area and on GnRH neurons. Our data suggest that the glutamatergic circuitry in the hypothalamus is altered in the absence of p140Cap and is required for female fertility.

Physiological Characterization and Transcriptomic Properties of GnRH Neurons Derived From Human Stem Cells

Gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus play a key role in the regulation of reproductive function. In this study, we sought an efficient method for generating GnRH neurons from human embryonic and induced pluripotent stem cells (hESC and hiPSC, respectively). First, we found that exposure of primitive neuroepithelial cells, rather than neuroprogenitor cells, to fibroblast growth factor 8 (FGF8), was more effective in generating GnRH neurons. Second, addition of kisspeptin to FGF8 further increased the efficiency rates of GnRH neurogeneration. Third, we generated a fluorescent marker mCherry labeled human embryonic GnRH cell line (mCh-hESC) using a CRISPR-Cas9 targeting approach. Fourth, we examined physiological characteristics of GnRH (mCh-hESC) neurons: similar to GnRH neurons in vivo, they released the GnRH peptide in a pulsatile manner at ~60 min intervals; GnRH release increased in response to high potassium, kisspeptin, estradiol, and neurokinin B challenges; and injection of depolarizing current induced action potentials. Finally, we characterized developmental changes in transcriptomes of GnRH neurons using hESC, hiPSC, and mCh-hESC. The developmental pattern of transcriptomes was remarkably similar among the 3 cell lines. Collectively, human stem cell-derived GnRH neurons will be an important tool for establishing disease models to understand diseases, such as idiopathic hypothalamic hypogonadism, and testing contraceptive drugs.

The Differential Roles for Neurodevelopmental and Neuroendocrine Genes in Shaping GnRH Neuron Physiology and Deficiency

Gonadotropin releasing hormone (GnRH) neurons are hypothalamic neuroendocrine cells that control sexual reproduction. During embryonic development, GnRH neurons migrate from the nose to the hypothalamus, where they receive inputs from several afferent neurons, following the axonal scaffold patterned by nasal nerves. Each step of GnRH neuron development depends on the orchestrated action of several molecules exerting specific biological functions. Mutations in genes encoding for these essential molecules may cause Congenital Hypogonadotropic Hypogonadism (CHH), a rare disorder characterized by GnRH deficiency, delayed puberty and infertility. Depending on their action in the GnRH neuronal system, CHH causative genes can be divided into neurodevelopmental and neuroendocrine genes. The CHH genetic complexity, combined with multiple inheritance patterns, results in an extreme phenotypic variability of CHH patients. In this review, we aim at providing a comprehensive and updated description of the genes thus far associated with CHH, by dissecting their biological relevance in the GnRH system and their functional relevance underlying CHH pathogenesis.

[Molecular genetics and phenotypic features of congenital isolated hypogonadotropic hypogonadism]

Congenital isolated hypogonadotropic hypogonadism includes a group of diseases related to the defects of secretion and action of gonadotropin-releasing hormone (GNRH) and gonadotropins. In a half of cases congenital hypogonadism is associated with an impaired sense of smell. It's named Kallmann syndrome. Now 40 genes are known to be associated with function of hypothalamus pituitary gland and gonads. Phenotypic features of hypogonadism and therapy effectiveness are related to different molecular defects. However clinical signs may vary even within the same family with the same molecular genetic defect. Genotype phenotype correlation in patients with congenital malformations prioritizes the search for mutations in candidate genes. There are data of significant contribution of oligogenicity into the phenotype of the disease are presented in the review. Moreover, an issue of current isolated hypogonadotropic hypogonadism definition and classification revision is raised in the review due to hypogonadotropic hypogonadism development while there are mutations in genes not associated with GNRH neurons secretion and function.

Identification and validation of key miRNAs and miRNA-mRNA regulatory network associated with uterine involution in postpartum Kazakh sheep

MicroRNAs (miRNAs) are widely expressed in different mammalian tissues and exert their biological effects through corresponding target genes. miRNA target genes can be rapidly and efficiently identified and screened by combining bioinformatics prediction and experimental validation. To investigate the possible molecular regulatory mechanisms involving miRNAs during uterine involution in postpartum ewes, we used Illumina HiSeq sequencing technology to screen for the number and characteristics of miRNAs in faster uterine involution and normal uterine involution group. A total of 118 differentially expressed miRNAs, including 33 known miRNAs and 85 new miRNAs, were identified in the hypothalamic library, whereas 54 miRNAs, including 5 known miRNAs and 49 new miRNAs, were identified in the uterine library. Screening with four types of gene prediction software revealed 73 target genes associated with uterine involution, and subsequently, GO annotation and KEGG pathway analysis were performed. The results showed that, in the hypothalamic–uterine axis, uterine involution in postpartum ewes might primarily involve two miRNA-target gene pairs, namely, miRNA-200a–PTEN and miRNA-133–FGFR1, which can participate in GnRH signal transduction in the upstream hypothalamus and in the remodeling process at the downstream uterus, through the PI3K–AKT signaling pathway to influence the recovery of the morphology and functions of the uterus during the postpartum period in sheep. Therefore, identification of differentially expressed miRNAs in this study fills a gap in the research related to miRNAs in uterine involution in postpartum ewes and provides an important reference point for a comprehensive understanding of the molecular mechanisms underlying the regulation of postpartum uterine involution in female livestock.

Mechanisms underlying pre- and postnatal development of the vomeronasal organ

The vomeronasal organ (VNO) is sensory organ located in the ventral region of the nasal cavity in rodents. The VNO develops from the olfactory placode during the secondary invagination of olfactory pit. The embryonic vomeronasal structure appears as a neurogenic area where migratory neuronal populations like endocrine gonadotropin-releasing hormone-1 (GnRH-1) neurons form. Even though embryonic vomeronasal structures are conserved across most vertebrate species, many species including humans do not have a functional VNO after birth. The vomeronasal epithelium (VNE) of rodents is composed of two major types of vomeronasal sensory neurons (VSNs): (1) VSNs distributed in the apical VNE regions that express vomeronasal type-1 receptors (V1Rs) and the G protein subunit Gαi2, and (2) VSNs in the basal territories of the VNE that express vomeronasal type-2 receptors (V2Rs) and the G subunit Gαo. Recent studies identified a third subclass of Gαi2 and Gαo VSNs that express the formyl peptide receptor family. VSNs expressing V1Rs or V2Rs send their axons to distinct regions of the accessory olfactory bulb (AOB). Together, VNO and AOB form the accessory olfactory system (AOS), an olfactory subsystem that coordinates the social and sexual behaviors of many vertebrate species. In this review, we summarize our current understanding of cellular and molecular mechanisms that underlie VNO development. We also discuss open questions for study, which we suggest will further enhance our understanding of VNO morphogenesis at embryonic and postnatal stages.

Prenatal exposure of female mice to perfluorononanoic acid delays pubertal activation of the reproductive endocrine axis through enhanced hepatic FGF21 production

The developmental toxicity of perfluorononanoic acid (PFNA), a ubiquitous environmental contaminant, has been associated with the activation of PPARα. This study investigated influence of prenatal exposure to PFNA in pubertal activation of reproductive endocrine axis in female mice and explored underlying molecular mechanisms. Herein, we show that when PFNA (3 mg kg^-1 body weight) was orally administered during gestational days 1-18, dams showed an increase in liver weight and hepatic FGF21 synthesis via PPARα activation, and their female offspring (PFNA mice) showed an increase in liver weight and hepatic FGF21 synthesis from postnatal day (PND) 1 to PND21, which were corrected by the administration of the PPARα antagonist GW6471 from PND1-14 (pup-GW). Expression of vasopressin (VAP) in the hypothalamic suprachiasmatic nucleus (SCN) was reduced in PND14-30 PFNA mice, and could be rescued by pup-GW. Pubertal activation of kisspeptin neurons in anteroventral periventricular nucleus (AVPV) and hypothalamic GnRH neurons in PND21-30 PFNA mice was obviously suppressed, but were recovered by pup-GW or PND21-30 application of VAP. The times of vaginal opening and first estrus were delayed in PFNA mice with a decrease in ovary size and the numbers of primary, secondary and antral follicles, and corpora lutea, which were relieved by pup-GW or application of VAP. The findings indicate that prenatal exposure to PFNA through increased FGF21 production in postnatal female offspring impedes postnatal activation of SCN-VAP neurons, which suppresses pubertal onset in AVPV-kisspeptin neurons and reproductive endocrine axis, leading to delayed puberty and dysfunction of ovaries.

Conditional Fgfr1 Deletion in GnRH Neurons Leads to Minor Disruptions in the Reproductive Axis of Male and Female Mice

In humans and mice, inactivating mutations in fibroblast growth factor receptor 1 (Fgfr1) lead to gonadotropin-releasing hormone (GnRH) deficiency and a host of downstream reproductive disorders. It was unclear if Fgfr1 signaling directly upon GnRH neurons critically drove the establishment of a functional GnRH system. To answer this question, we generated a mouse model with a conditional deletion of Fgfr1 in GnRH neurons using the Cre/loxP approach. These mice, called Fgfr1cKO mice, were examined along with control mice for their pubertal onset and a host of reproductive axis functions. Our results showed that Fgfr1cKO mice harbored no detectable defects in the GnRH system and pubertal onset, suffered only subtle changes in the pituitary function, but exhibited significantly disrupted testicular and ovarian morphology at 25 days of age, indicating impaired gametogenesis at a young age. However, these disruptions were transient and became undetectable in older mice. Our results suggest that Fgfr1 signaling directly on GnRH neurons supports, to some extent, the reproductive axis function in the period leading to the early phase of puberty, but is not critically required for pubertal onset or reproductive maintenance in sexually mature animals.

Gonadal Cycle-Dependent Expression of Genes Encoding Peptide-, Growth Factor-, and Orphan G-Protein-Coupled Receptors in Gonadotropin- Releasing Hormone Neurons of Mice

Rising serum estradiol triggers the surge release of gonadotropin-releasing hormone (GnRH) at late proestrus leading to ovulation. We hypothesized that proestrus evokes alterations in peptidergic signaling onto GnRH neurons inducing a differential expression of neuropeptide-, growth factor-, and orphan G-protein-coupled receptor (GPCR) genes. Thus, we analyzed the transcriptome of GnRH neurons collected from intact, proestrous and metestrous GnRH-green fluorescent protein (GnRH-GFP) transgenic mice using Affymetrix microarray technique. Proestrus resulted in a differential expression of genes coding for peptide/neuropeptide receptors including Adipor1, Prokr1, Ednrb, Rtn4r, Nmbr, Acvr2b, Sctr, Npr3, Nmur1, Mc3r, Cckbr, and Amhr2. In this gene cluster, Adipor1 mRNA expression was upregulated and the others were downregulated. Expression of growth factor receptors and their related proteins was also altered showing upregulation of Fgfr1, Igf1r, Grb2, Grb10, and Ngfrap1 and downregulation of Egfr and Tgfbr2 genes. Gpr107, an orphan GPCR, was upregulated during proestrus, while others were significantly downregulated (Gpr1, Gpr87, Gpr18, Gpr62, Gpr125, Gpr183, Gpr4, and Gpr88). Further affected receptors included vomeronasal receptors (Vmn1r172, Vmn2r-ps54, and Vmn1r148) and platelet-activating factor receptor (Ptafr), all with marked downregulation. Patch-clamp recordings from mouse GnRH-GFP neurons carried out at metestrus confirmed that the differentially expressed IGF-1, secretin, and GPR107 receptors were operational, as their activation by specific ligands evoked an increase in the frequency of miniature postsynaptic currents (mPSCs). These findings show the contribution of certain novel peptides, growth factors, and ligands of orphan GPCRs to regulation of GnRH neurons and their preparation for the surge release.

Functional Characteristics of Novel FGFR1 Mutations in Patients with Isolated Gonadotropin-Releasing Hormone Deficiency

BACKGROUND

Isolated gonadotropin-releasing hormone (GnRH) deficiency (IGD) has a wide phenotypic spectrum including Kallmann syndrome (KS) and normosmic idiopathic hypogonadotropic hypogonadism (nIHH). FGFR1 mutations have been identified in 3-10% of patients with KS or nIHH. This study was performed to investigate clinical phenotypes and functional characteristics of FGFR1 mutations in patients with IGD.

METHODS

This study included 8 patients (from 7 families) with FGFR1 mutations identified by targeted gene panel sequencing or whole exome sequencing (WES). The impact of the identified mutations on FGFR1 function was assessed using in vitro studies.

RESULTS

Seven heterozygous mutations in FGFR1 were identified in 8 patients from 7 independent families. The patients exhibited a wide spectrum of pubertal development, including anosmia in a prepubertal boy (n=1), delayed puberty (n=2), nIHH (n=3), and KS (n=2). Four of the mutations were classified as likely pathogenic, and the other three were variants of uncertain significance. FGF8-FGFR1 signaling activities for the novel FGFR1 variants (p.Y339H, p.S681I, and p.N185Kfs*16) were reduced by in vitro functional assay, indicating loss-of-function mutations.

CONCLUSIONS

This study identified seven rare sequence variants in FGFR1 in patients with KS and nIHH. Probands with an FGFR1 mutations displayed a wide phenotypic spectrum ranging from KS to anosmia. A prepubertal male with anosmia should be followed up to assess pubertal development because they can manifest hypogonadotropic hypogonadism after puberty. These results expand the phenotypic spectrum of FGFR1 mutations and suggest a broader biologic role of FGFR1 in reproduction.

Gonadotropin-releasing hormone neuron development in vertebrates

Gonadotropin-releasing hormone (GnRH) neurons are master regulators of the reproductive axis in vertebrates. During early mammalian embryogenesis, GnRH1 neurons emerge in the nasal/olfactory placode. These neurons undertake a long-distance migration, moving from the nose to the preoptic area and hypothalamus. While significant advances have been made in understanding the functional importance of the GnRH1 neurons in reproduction, where GnRH1 neurons come from and how are they specified during early development is still under debate. In addition to the GnRH1 gene, most vertebrate species including humans have one or two additional GnRH genes. Compared to the GnRH1 neurons, much less is known about the development and regulation of GnRH2 neuron and GnRH3 neurons. The objective of this article is to review what is currently known about GnRH neuron development. We will survey various cell autonomous and non-autonomous factors implicated in the regulation of GnRH neuron development. Finally, we will discuss emerging tools and new approaches to resolve open questions pertaining to GnRH neuron development.

Epigenomic control of gonadotrophin-releasing hormone neurone development and hypogonadotrophic hypogonadism

Mammalian reproductive success depends on gonadotrophin-releasing hormone (GnRH) neurones to stimulate gonadotrophin secretion from the anterior pituitary and activate gonadal steroidogenesis and gametogenesis. Genetic screening studies in patients diagnosed with Kallmann syndrome (KS), a congenital form of hypogonadotrophic hypogonadism (CHH), identified several causal mutations, including those in the fibroblast growth factor (FGF) system. This signalling pathway regulates neuroendocrine progenitor cell proliferation, fate specification and cell survival. Indeed, the GnRH neurone system was absent or abrogated in transgenic mice with reduced (ie, hypomorphic) Fgf8 and/or Fgf receptor (Fgfr) 1 expression, respectively. Moreover, we found that GnRH neurones were absent in the embryonic olfactory placode of Fgf8 hypomorphic mice, the putative birthplace of GnRH neurones. These observations, together with those made in human KS/CHH patients, indicate that the FGF8/FGFR1 signalling system is a requirement for the ontogenesis of the GnRH neuronal system and function. In this review, we discuss how epigenetic factors control the expression of genes such as Fgf8 that are known to be critical for GnRH neurone ontogenesis, fate specification, and the pathogenesis of KS/CHH.

Endocrinological Features of Hartsfield Syndrome in an Adult Patient With a Novel Mutation of FGFR1

Hartsfield syndrome (HS: OMIM 615465) is a rare congenital disease associated with a mutation of the fibroblast growth factor receptor 1 gene (FGFR1) with the main features of holoprosencephaly and ectrodactyly. Patients with HS also present with endocrinological deficits, such as isolated hypogonadotropic hypogonadism and central diabetes insipidus. Although there are several studies on infancy/childhood history, there is no study of infant/childhood/adolescent/young adult HS natural history and endocrinological findings. Here, we report a male patient with HS associated with a novel de novo FGFR1 mutation (c. 1868A > C). The endocrinological profile was evaluated at ages 1 and 31 years. This long-term follow-up study highlights functional changes in the posterior pituitary gland and features of bone metabolism disorder. We also describe the anterior pituitary function. To our knowledge this is the first description of the natural history of an HS patient through birth to young adult age. Although the HS infants reported in the literature develop central diabetes insipidus, little is known about the serial changes in pituitary gland function during growth in HS patients. In this study we describe an adult patient with HS who showed improvement of hypernatremia during early adulthood. In addition, we emphasize the importance of prevention and treatment of osteoporosis in HS.

Characterization of the human GnRH neuron developmental transcriptome using a GNRH1-TdTomato reporter line in human pluripotent stem cells

Gonadotropin-releasing hormone (GnRH) neurons provide a fundamental signal for the onset of puberty and subsequent reproductive functions by secretion of gonadotropin-releasing hormone. Their disrupted development or function leads to congenital hypogonadotropic hypogonadism (CHH). To model the development of human GnRH neurons, we generated a stable GNRH1-TdTomato reporter cell line in human pluripotent stem cells (hPSCs) using CRISPR-Cas9 genome editing. RNA-sequencing of the reporter clone, differentiated into GnRH neurons by dual SMAD inhibition and FGF8 treatment, revealed 6461 differentially expressed genes between progenitors and GnRH neurons. Expression of the transcription factor ISL1, one of the top 50 most upregulated genes in the TdTomato-expressing GnRH neurons, was confirmed in 10.5 gestational week-old human fetal GnRH neurons. Among the differentially expressed genes, we detected 15 genes that are implicated in CHH and several genes that are implicated in human puberty timing. Finally, FGF8 treatment in the neuronal progenitor pool led to upregulation of 37 genes expressed both in progenitors and in TdTomato-expressing GnRH neurons, which suggests upstream regulation of these genes by FGF8 signaling during GnRH neuron differentiation. These results illustrate how hPSC-derived human GnRH neuron transcriptomic analysis can be utilized to dissect signaling pathways and gene regulatory networks involved in human GnRH neuron development.This article has an associated First Person interview with the first author of the paper.

Expanding the mutational spectrum of monogenic hypogonadotropic hypogonadism: novel mutations in ANOS1 and FGFR1 genes

BACKGROUND

Congenital hypogonadotropic hypogonadism (CHH) is a rare disease, triggered by defective GnRH secretion, that is usually diagnosed in late adolescence or early adulthood due to the lack of spontaneous pubertal development. To date more than 30 genes have been associated with CHH pathogenesis with X-linked recessive, autosomal dominant, autosomal recessive and oligogenic modes of inheritance. Defective sense of smell is present in about 50-60% of CHH patients and called Kallmann syndrome (KS), in contrast to patients with normal sense of smell referred to as normosmic CHH. ANOS1 and FGFR1 genes are all well established in the pathogenesis of CHH and have been extensively studied in many reported cohorts. Due to rarity and heterogenicity of the condition the mutational spectrum, even in classical CHH genes, have yet to be fully characterized.

METHODS

To address this issue we screened for ANOS1 and FGFR1 variants in a cohort of 47 unrelated CHH subjects using targeted panel sequencing. All potentially pathogenic variants have been validated with Sanger sequencing.

RESULTS

Sequencing revealed two ANOS1 and four FGFR1 mutations in six subjects, of which five are novel and one had been previously reported in CHH. Novel variants include a single base pair deletion c.313delT in exon 3 of ANOS1, three missense variants of FGFR1 predicted to result in the single amino acid substitutions c.331C > T (p.R111C), c.1964 T > C (p.L655P) and c.2167G > A (p.E723K) and a 15 bp deletion c.374_388delTGCCCGCAGACTCCG in exon 4 of FGFR1. Based on ACMG-AMP criteria reported variants were assigned to class 5, pathogenic or class 4, likely pathogenic. Protein structural predictions, the rarity of novel variants and amino acid conservation in case of missense substitutions all provide strong evidence that these mutations are highly likely to be deleterious.

CONCLUSIONS

Despite the fact that ANOS1 and FGFR1 are classical CHH genes and were thoroughly explored in several CHH cohorts we identified new, yet undescribed variants within their sequence. Our results support the genetic complexity of the disorder. The knowledge of the full genetic spectrum of CHH is increasingly important in order to be able to deliver the best personalised medical care to our patients.

Genetic Evaluation of Patients With Delayed Puberty and Congenital Hypogonadotropic Hypogonadism: Is it Worthy of Consideration?

Delayed puberty is a common reason of pediatric endocrinological consultation. It is often a self-limited (or constitutional) condition with a strong familial basis. The type of inheritance is variable but most commonly autosomal dominant. Despite this strong genetic determinant, mutations in genes implicated in the regulation of hypothalamic-pituitary-gonadal axis have rarely been identified in cases of self-limited delayed puberty and often in relatives of patients with congenital hypogonadotropic hypogonadism (i.e., FGFR1 and GNRHR genes). However, recently, next-generation sequencing analysis has led to the discovery of new genes (i.e., IGSF10, HS6ST1, FTO, and EAP1) that are implicated in determining isolated self-limited delayed puberty in some families. Despite the heterogeneity of genetic defects resulting in delayed puberty, genetic testing may become a useful diagnostic tool for the correct classification and management of patients with delayed puberty. This article will discuss the benefits and the limitations of genetic testing execution in cases of delayed puberty.

Deletion of the Homeodomain Protein Six6 From GnRH Neurons Decreases GnRH Gene Expression, Resulting in Infertility

Hypothalamic GnRH (luteinizing hormone-releasing hormone) neurons are crucial for the hypothalamic-pituitary-gonadal (HPG) axis, which regulates mammalian fertility. Insufficient GnRH disrupts the HPG axis and is often associated with the genetic condition idiopathic hypogonadotropic hypogonadism (IHH). The homeodomain protein sine oculis-related homeobox 6 (Six6) is required for the development of GnRH neurons. Although it is known that Six6 is specifically expressed within a more mature GnRH neuronal cell line and that overexpression of Six6 induces GnRH transcription in these cells, the direct role of Six6 within the GnRH neuron in vivo is unknown. Here we find that global Six6 knockout (KO) embryos show apoptosis of GnRH neurons beginning at embryonic day 14.5 with 90% loss of GnRH neurons by postnatal day 1. We sought to determine whether the hypogonadism and infertility reported in the Six6KO mice are generated via actions within the GnRH neuron in vivo by creating a Six6-flox mouse and crossing it with the LHRHcre mouse. Loss of Six6 specifically within the GnRH neuron abolished GnRH expression in ∼0% of GnRH neurons. We further demonstrated that deletion of Six6 only within the GnRH neuron leads to infertility, hypogonadism, hypogonadotropism, and delayed puberty. We conclude that Six6 plays distinct roles in maintaining fertility in the GnRH neuron vs in the migratory environment of the GnRH neuron by maintaining expression of GnRH and survival of GnRH neurons, respectively. These results increase knowledge of the role of Six6 in the brain and may offer insight into the mechanism of IHH.

TET1 regulates fibroblast growth factor 8 transcription in gonadotropin releasing hormone neurons

Fibroblast growth factor 8 (FGF8) is a potent morphogen that regulates the ontogenesis of gonadotropin-releasing hormone (GnRH) neurons, which control the hypothalamus-pituitary-gonadal (HPG) axis, and therefore reproductive success. Indeed, FGF8 and FGFR1 deficiency severely compromises vertebrate reproduction in mice and humans and is associated with Kallmann Syndrome (KS), a congenital disease characterized by hypogonadotropic hypogonadism associated with anosmia. Our laboratory demonstrated that FGF8 signaling through FGFR1, both of which are KS-related genes, is necessary for proper GnRH neuron development in mice and humans. Here, we investigated the possibility that non-genetic factors, such as the epigenome, may contribute to KS onset. For this purpose, we developed an embryonic explant model, utilizing the mouse olfactory placode (OP), the birthplace of GnRH neurons. We show that TET1, which converts 5-methylcytosine residues (5mC) to 5-hydroxymethylated cytosines (5hmC), controls transcription of Fgf8 during GnRH neuron ontogenesis. Through MeDIP and ChIP RT-qPCR we found that TET1 bound to specific CpG islands on the Fgf8 promoter. We found that the temporal expression of Fgf8 correlates with not only TET1 binding, but also with 5hmC enrichment. siRNA knockdown of Tet1 reduced Fgf8 and Fgfr1 mRNA expression. During this time period, Fgf8 also switched histone status, most likely via recruitment of EZH2, a major component of the polycomb repressor complex-2 (PRC2) at E13.5. Together, these studies underscore the significance of epigenetics and chromatin modifications to temporally regulated genes involved in KS.

Loss-of-function mutations in FGF8 can be independent risk factors for holoprosencephaly

The utilization of next generation sequencing has been shown to accelerate gene discovery in human disease. However, our confidence in the correct disease-associations of rare variants continues to depend on functional analysis. Here, we employ a sensitive assay of human FGF8 variants in zebrafish to demonstrate that the spectrum of isoforms of FGF8 produced by alternative splicing can provide key insights into the genetic susceptibility to human malformations. In addition, we describe novel mutations in the FGF core structure that have both subtle and profound effects on ligand posttranslational processing and biological activity. Finally, we solve a case of apparent digenic inheritance of novel variants in SHH and FGF8, two genes known to functionally coregulate each other in the developing forebrain, as a simpler case of FGF8 diminished function.

RNF216 Regulates the Migration of Immortalized GnRH Neurons by Suppressing Beclin1-Mediated Autophagy

RNF216, encoding an E3 ubiquitin ligase, has been identified as a causative gene for Gordon Holmes syndrome, characterized by ataxia, dementia, and hypogonadotropic hypogonadism. However, it is still elusive how deficiency in RNF216 leads to hypogonadotropic hypogonadism. In this study, by using GN11 immature GnRH neuronal cell line, we demonstrated an important role of RNF216 in the GnRH neuron migration. RNA interference of RNF216 inhibited GN11 cell migration, but had no effect on the proliferation of GN11 cells or GnRH expression. Knockdown of RNF216 increased the protein levels of its targets, Arc and Beclin1. RNAi of Beclin1, but not Arc, normalized the suppressive effect caused by RNF216 knockdown. As Beclin1 plays a critical role in the autophagy regulation, we further demonstrated that RNAi of RNF216 led to increase in autophagy, and autophagy inhibitor CQ and 3-MA rescued the GN11 cell migration deficit caused by RNF216 knockdown. We further demonstrated that pharmacological increase autophagy by rapamycin could suppress the GN11 cell migration. We thus have identified that RNF216 regulates the migration of GnRH neuron by suppressing Beclin1 mediated autophagy, and indicated a potential contribution of autophagy to the hypogonadotropic hypogonadism.

Fibroblast growth factor receptor 1 signaling transcriptionally regulates the axon guidance cue slit1

Axons sense molecular cues in their environment to arrive at their post-synaptic targets. While many of the molecular cues have been identified, the mechanisms that regulate their spatiotemporal expression remain elusive. We examined here the transcriptional regulation of the guidance gene slit1 both in vitro and in vivo by specific fibroblast growth factor receptors (Fgfrs). We identified an Fgf-responsive 2.3 kb slit1 promoter sequence that recapitulates spatiotemporal endogenous expression in the neural tube and eye of Xenopus embryos. We found that signaling through Fgfr1 is the main regulator of slit1 expression both in vitro in A6 kidney epithelial cells, and in the Xenopus forebrain, even when other Fgfr subtypes are present in cells. These data argue that a specific signaling pathway downstream of Fgfr1 controls in a cell-autonomous manner slit1 forebrain expression and are novel in identifying a specific growth factor receptor for in vivo control of the expression of a key embryonic axon guidance cue.

KLB, encoding β-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 β‐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.

β-Klotho sustains postnatal GnRH biology and spins the thread of puberty

Hypogonadotropic hypogonadism is a syndrome found to be isolated (IHH) or associated with anosmia, corresponding to the Kallmann syndrome (KS). It comprises a defect in gonadotropin‐releasing hormone (GnRH) secretion and absent or delayed puberty. Genetic causes have been identified with a high genetic heterogeneity. Fibroblast growth factor receptor 1 (FGFR1), a tyrosine kinase receptor, was one of the first genes whose mutations were identified as causative in KS. FGFR1 is responsible for the formation of the GnRH neuron system. Studying patients has not only allowed the identification of new etiologies for this syndrome but also helped to unravel the signaling pathways involved in the development of GnRH neurons and in GnRH control and function. The FGF21/FGFR1/Klotho B (KLB) signaling pathway mediates the response to starvation and other metabolic stresses. Preventing reproduction during nutritional deprivation is an adaptive process that is essential for the survival of species. In this work, Xu et al (2017), using a candidate gene approach, provide a description of the essential role played by this pathway in GnRH biology and in the pathogenesis of IHH and KS. They establish a novel link between metabolism and reproduction in humans.

GENETICS IN ENDOCRINOLOGY: Genetic counseling for congenital hypogonadotropic hypogonadism and Kallmann syndrome: new challenges in the era of oligogenism and next-generation sequencing

Congenital hypogonadotropic hypogonadism (CHH) and Kallmann syndrome (KS) are rare, related diseases that prevent normal pubertal development and cause infertility in affected men and women. However, the infertility carries a good prognosis as increasing numbers of patients with CHH/KS are now able to have children through medically assisted procreation. These are genetic diseases that can be transmitted to patients' offspring. Importantly, patients and their families should be informed of this risk and given genetic counseling. CHH and KS are phenotypically and genetically heterogeneous diseases in which the risk of transmission largely depends on the gene(s) responsible(s). Inheritance may be classically Mendelian yet more complex; oligogenic modes of transmission have also been described. The prevalence of oligogenicity has risen dramatically since the advent of massively parallel next-generation sequencing (NGS) in which tens, hundreds or thousands of genes are sequenced at the same time. NGS is medically and economically more efficient and more rapid than traditional Sanger sequencing and is increasingly being used in medical practice. Thus, it seems plausible that oligogenic forms of CHH/KS will be increasingly identified making genetic counseling even more complex. In this context, the main challenge will be to differentiate true oligogenism from situations when several rare variants that do not have a clear phenotypic effect are identified by chance. This review aims to summarize the genetics of CHH/KS and to discuss the challenges of oligogenic transmission and also its role in incomplete penetrance and variable expressivity in a perspective of genetic counseling.

Evaluating CHARGE syndrome in congenital hypogonadotropic hypogonadism patients harboring CHD7 variants

PURPOSE

Congenital hypogonadotropic hypogonadism (CHH), a rare genetic disease caused by gonadotropin-releasing hormone deficiency, can also be part of complex syndromes (e.g., CHARGE syndrome). CHD7 mutations were reported in 60% of patients with CHARGE syndrome, and in 6% of CHH patients. However, the definition of CHD7 mutations was variable, and the associated CHARGE signs in CHH were not systematically examined.

METHODS

Rare sequencing variants (RSVs) in CHD7 were identified through exome sequencing in 116 CHH probands, and were interpreted according to American College of Medical Genetics and Genomics guidelines. Detailed phenotyping was performed in CHH probands who were positive for CHD7 RSVs, and genotype-phenotype correlations were evaluated.

RESULTS

Of the CHH probands, 16% (18/116) were found to harbor heterozygous CHD7 RSVs, and detailed phenotyping was performed in 17 of them. Of CHH patients with pathogenic or likely pathogenic CHD7 variants, 80% (4/5) were found to exhibit multiple CHARGE features, and 3 of these patients were reclassified as having CHARGE syndrome. In contrast, only 8% (1/12) of CHH patients with nonpathogenic CHD7 variants exhibited multiple CHARGE features (P = 0.01).

CONCLUSION

Pathogenic or likely pathogenic CHD7 variants rarely cause isolated CHH. Therefore a detailed clinical investigation is indicated to clarify the diagnosis (CHH versus CHARGE) and to optimize clinical management.

On the descent of the epididymo-testicular unit, cryptorchidism, and prevention of infertility

This comprehensive review provides in-depth coverage of progress made in understanding the molecular mechanisms underlying cryptorchidism, a frequent pathology first described in about 1786 by John Hunter. The first part focuses on the physiology, embryology, and histology of epididymo-testicular descent. In the last 20 years epididymo-testicular descent has become the victim of schematic drawings with an unjustified rejection of valid histological data. This part also includes discussion on the roles of gonadotropin-releasing hormone, fibroblast growth factors, Müllerian inhibiting substance, androgens, inhibin B, and insulin-like 3 in epididymo-testicular descent. The second part addresses the etiology and histology of cryptorchidism as well as the importance of mini-puberty for normal fertility development. A critical view is presented on current clinical guidelines that recommend early orchidopexy alone as the best possible treatment. Finally, by combining classical physiological information and the output of cutting-edge genomics data into a complete picture the importance of hormonal treatment in preventing cryptorchidism-induced infertility is underscored.

The terminal nerve plays a prominent role in GnRH-1 neuronal migration independent from proper olfactory and vomeronasal connections to the olfactory bulbs

Gonadotropin-releasing hormone-1 (GnRH-1) neurons (GnRH-1 ns) migrate from the developing olfactory pit into the hypothalamus during embryonic development. Migration of the GnRH-1 neurons is required for mammalian reproduction as these cells control release of gonadotropins from the anterior pituitary gland. Disturbances in GnRH-1 ns migration, GnRH-1 synthesis, secretion or signaling lead to varying degrees of hypogonadotropic hypogonadism (HH), which impairs pubertal onset and fertility. HH associated with congenital olfactory defects is clinically defined as Kallmann Syndrome (KS). The association of olfactory defects with HH in KS suggested a potential direct relationship between defective olfactory axonal routing, lack of olfactory bulbs (OBs) and aberrant GnRH-1 ns migration. However, it has never been experimentally proven that the formation of axonal connections of the olfactory/vomeronasal neurons to their functional targets are necessary for the migration of GnRH-1 ns to the hypothalamus. Loss-of-function of the Arx-1 homeobox gene leads to the lack of proper formation of the OBs with abnormal axonal termination of olfactory sensory neurons ( Yoshihara et al., 2005). Our data prove that correct development of the OBs and axonal connection of the olfactory/vomeronasal sensory neurons to the forebrain are not required for GnRH-1 ns migration, and suggest that the terminal nerve, which forms the GnRH-1 migratory scaffold, follows different guidance cues and differs in gene expression from olfactory/vomeronasal sensory neurons.

Summary: Our work reveals that correct olfactory bulb development is not required for GnRH-1 neuronal migration. This study challenges the idea that GnRH-1 neuronal migration to the hypothalamus relies on correct routing of the olfactory and vomeronasal neurons and supports the existence of the TN in mammals.

Arachnoid cyst: a further anomaly associated with Kallmann syndrome?

BACKGROUND

Kallmann syndrome (KS) is defined by the association of hypogonadotropic hypogonadism and anosmia. It is characterized by a significant clinical and genetic heterogeneity; actually, it may present several non-reproductive non-olfactory anomalies, and all the ways of genetic transmission can be involved in the inheritance of the disease. Although six pathogenesis-related genes have been identified so far, KS remains sporadic in 70 % of the cases, and the genetic diagnosis is not available for all of them. The purpose of this paper is to present a further disease that can enrich the wide spectrum of KS variability, that is cerebral arachnoid cyst.

CASE DESCRIPTION

This 11-year-old boy presented with the typical characteristics of KS together with those related to a sylvian arachnoid cyst. He was admitted because of worsening headache. At the admission, the physical examination revealed eunuchoid aspect, micropenis, previous cryptorchidism, and anosmia. MRI pointed out a large, left sylvian arachnoid cyst, agenesia of the olfactory bulbs/tracts complex, and hypoplasia of the left olfactory sulcus. The child was operated on by endoscopic fenestration of the cyst, followed by transient external drainage for subdural hygroma and microscopic fenestration for recurrence of the cyst. His statural growth is normal but the sexual development still delayed in spite of hormone replacement therapy.

CONCLUSION

According to the present and the other four cases in the literature, arachnoid cyst should be included among the anomalies possibly accompanying KS date although this association seems to be occasional as far as embryogenesis and physiopathology are concerned.

Perinatal midline astrocyte development is impaired in fibroblast growth factor 8 hypomorphic mice

Our previous studies showed that Fgf8 mutations can cause Kallmann syndrome (KS), a form of congenital hypogonadotropic hypogonadism, in which patients do not undergo puberty and are infertile. Interestingly, some KS patients also have agenesis of the corpus callosum (ACC) suggesting that KS pathology is not limited to reproductive function. Here, we asked whether FGF8 dysfunction is the underlying cause of ACC in some KS patients. Indeed, early studies in transgenic mice with Fgf8 mutations reported the presence of failed or incomplete corpus callosum formation. Additional studies in transgenic mice showed that FGF8 function most likely prevents the prenatal elimination of glial fibrillary acidic protein (GFAP)-immunoreactive (IR) glial cells in the indusium griseum (IG) and midline zipper (MZ), two anterior-dorsal midline regions required for corpus callosum formation (i.e., between embryonic days (E) 15.5-18.5). Here, we tested the hypothesis that FGF8 function is critical for the survival of the GFAP-IR midline glial cells. First, we measured the incidence of apoptosis in the anterior-dorsal midline region in Fgf8 hypomorphic mice during embryonic corpus callosum formation. Second, we quantified the GFAP expression in the anterior-dorsal midbrain region during pre- and postnatal development, in order to study: 1) how Fgf8 hypomorphy disrupts prenatal GFAP-IR midline glial cell development, and 2) whether Fgf8 hypomorphy continues to disrupt postnatal GFAP-IR midline glial cell development. Our results indicate that perinatal FGF8 signaling is important for the timing of the onset of anterior-dorsal Gfap expression in midline glial cells suggesting that FGF8 function regulates midline GFAP-IR glial cell development, which when disrupted by Fgf8 deficiency prevents the formation of the corpus callosum. These studies provide an experimentally-based mechanistic explanation as to why corpus callosum formation may fail in KS patients with deficits in FGF signaling.

Fibroblast Growth Factor 8 Expression in GT1-7 GnRH-Secreting Neurons Is Androgen-Independent, but Can Be Upregulated by the Inhibition of DNA Methyltransferases

Fibroblast growth factor 8 (FGF8) is a potent morphogen that regulates the embryonic development of hypothalamic neuroendocrine cells. Indeed, using Fgf8 hypomorphic mice, we showed that reduced Fgf8 mRNA expression completely eliminated the presence of gonadotropin-releasing hormone (GnRH) neurons. These findings suggest that FGF8 signaling is required during the embryonic development of mouse GnRH neurons. Additionally, in situ hybridization studies showed that the embryonic primordial birth place of GnRH neurons, the olfactory placode, is highly enriched for Fgf8 mRNA expression. Taken together these data underscore the importance of FGF8 signaling for GnRH emergence. However, an important question remains unanswered: How is Fgf8 gene expression regulated in the developing embryonic mouse brain? One major candidate is the androgen receptor (AR), which has been shown to upregulate Fgf8 mRNA in 60-70% of newly diagnosed prostate cancers. Therefore, we hypothesized that ARs may be involved in the regulation of Fgf8 transcription in the developing mouse brain. To test this hypothesis, we used chromatin-immunoprecipitation (ChIP) assays to elucidate whether ARs interact with the 5'UTR region upstream of the translational start site of the Fgf8 gene in immortalized mouse GnRH neurons (GT1-7) and nasal explants. Our data showed that while AR interacts with the Fgf8 promoter region, this interaction was androgen-independent, and that androgen treatment did not affect Fgf8 mRNA levels, indicating that androgen signaling does not induce Fgf8 transcription. In contrast, inhibition of DNA methyltransferases (DNMT) significantly upregulated Fgf8 mRNA levels indicating that Fgf8 transcriptional activity may be dependent on DNA methylation status.

Dominant-negative kinase domain mutations in FGFR1 can explain the clinical severity of Hartsfield syndrome

Mutations in FGFR1 have recently been associated with Hartsfield syndrome, a clinically distinct syndromic form of holoprosencephaly (HPE) with ectrodactly, which frequently includes combinations of craniofacial, limb and brain abnormalities not typical for classical HPE. Unrelated clinical conditions generally without craniofacial or multi-system malformations include Kallmann syndrome and idiopathic hypogonadotropic hypogonadism. FGFR1 is a principal cause for these less severe diseases as well. Here we demonstrate that of the nine FGFR1 mutations recently detected in our screen of over 200 HPE probands by next generation sequencing, only five distinct mutations in the kinase domain behave as dominant-negative mutations in zebrafish over-expression assays. Three FGFR1 mutations seen in HPE probands behave identical to wild-type FGFR1 in rescue assays, including one apparent de novo variation. Interestingly, in one HPE family, a deleterious FGFR1 allele was transmitted from one parent and a loss-of-function allele in FGF8 from the other parent to both affected daughters. This family is one of the clearest examples to date of gene:gene synergistic interactions causing HPE in humans.

Involvement of Fibroblast Growth Factors and Their Receptors in Epididymo-Testicular Descent and Maldescent

Maldescent of the epididymo-testicular unit can occur as an isolated event or as a component of various syndromes. When part of a syndrome, crypto-epididymis is usually accompanied by other genital and/or extragenital features. Epididymis development is primarily regulated by androgens, and successful epididymo-testicular unit development and descent requires an intact hypothalamic-pituitary-gonadal axis. The developing gonadotropin-releasing hormone system is essential for epididymo-testicular descent and is highly sensitive to reduced fibroblast growth factor (FGF) signaling. Our understanding of the impact of FGFR1 in the process of epididymo-testicular descent has recently improved. At later stages of embryonic development, the undifferentiated epididymal mesenchyme is a specific domain for FGFR1 expression. The majority of individuals with syndromic crypto-epididymis, as well as individuals with isolated maldescent of the epididymo-testicular unit, exhibit some disturbance of FGF, FGFR1 and/or genes involved in hypothalamic-pituitary-gonadal axis regulation. However, the mechanisms underlying FGF dysregulation may differ between various syndromes.

The Regulation and Function of Fibroblast Growth Factor 8 and Its Function during Gonadotropin-Releasing Hormone Neuron Development

Over the last few years, numerous studies solidified the hypothesis that fibroblast growth factor (FGF) signaling regulates neuroendocrine progenitor cell proliferation, fate specification, and cell survival and, therefore, is critical for the regulation and maintenance of homeostasis of the body. One important example that underscores the involvement of FGF signaling during neuroendocrine cell development is gonadotropin-releasing hormone (GnRH) neuron ontogenesis. Indeed, transgenic mice with reduced olfactory placode (OP) Fgf8 expression do not have GnRH neurons. This observation indicates the requirement of FGF8 signaling for the emergence of the GnRH neuronal system in the embryonic OP, the putative birth place of GnRH neurons. Mammalian reproductive success depends on the presence of GnRH neurons to stimulate gonadotropin secretion from the anterior pituitary, which activates gonadal steroidogenesis and gametogenesis. Together, these observations are critical for understanding the function of GnRH neurons and their control of the hypothalamus-pituitary-gonadal (HPG) axis to maintain fertility. Taken together, these studies illustrate that GnRH neuron emergence and hence HPG function is vulnerable to genomic and molecular signals that abnormally modify Fgf8 expression in the developing mouse OP. In this short review, we focus on research that is aimed at unraveling how androgen, all-trans retinoic acid, and how epigenetic factors modify control mouse OP Fgf8 transcription in the context of GnRH neuronal development and mammalian reproductive success.

Fibroblast growth factor 8 regulates postnatal development of paraventricular nucleus neuroendocrine cells

Background

Fibroblast growth factors (FGFs) are crucial signaling molecules that direct the development of the vertebrate brain. FGF8 gene signaling in particular, may be important for the development of the hypothalamus–pituitary–adrenal (HPA)-axis. Indeed, newborn Fgf8 hypomorphic mice harbor a major reduction in the number of vasopressin (VP) neurons in the paraventricular nucleus (PVN), the central output component of the HPA-axis. Additionally, recent studies indicated that adult heterozygous (^+/neo) Fgf8 hypomorphic mice exhibit more anxiety-like behaviors than wildtype (WT) mice. These studies led us to investigate whether Fgf8 hypomorphy abrogated VP and/or corticotropin-releasing hormone (CRH) neuronal development in the postnatal day (PN) 21 and adult mouse PVN. Furthermore, we studied whether Fgf8 hypomorphy disrupted HPA responsiveness in these mice.

Methods

Using immunohistochemistry, we examined the development of VP and CRH neurons located in the PVN of PN 21 and adult Fgf8^+/neo mice. Moreover, we used a restraint stress (RS) paradigm and measured corticosterone levels with enzyme immunoassays in order to assess HPA axis activation.

Results

The number of VP neurons in the PVN did not differ between WT and Fgf8^+/neo mice on PN 21 and in adulthood. In contrast, CRH immunoreactivity was much higher in Fgf8^+/neo mice than in WT mice on PN 21, this difference was no longer shown in adult mice. RS caused a higher increase in corticosterone levels in adult Fgf8^+/neo mice than in WT mice after 15 min, but no difference was seen after 45 min.

Conclusions

First, Fgf8 hypomorphy did not eliminate VP and CRH neurons in the mouse PVN, but rather disrupted the postnatal timing of neuropeptide expression onset in PVN neurons. Second, Fgf8 hypomorphy may, in part, be an explanation for affective disorders involving hyperactivity of the HPA axis, such as anxiety.

Patterning, specification, and differentiation in the developing hypothalamus

Owing to its complex structure and highly diverse cell populations, the study of hypothalamic development has historically lagged behind that of other brain regions. However, in recent years, a greatly expanded understanding of hypothalamic gene expression during development has opened up new avenues of investigation. In this review, we synthesize existing work to present a holistic picture of hypothalamic development from early induction and patterning through nuclear specification and differentiation, with a particular emphasis on determination of cell fate. We will also touch on special topics in the field including the prosomere model, adult neurogenesis, and integration of migratory cells originating outside the hypothalamic neuroepithelium, and how these topics relate to our broader theme.

Fgf8-Deficient Mice Compensate for Reduced GnRH Neuronal Population and Exhibit Normal Testicular Function

Gonadotropin-releasing hormone (GnRH) is critical for the onset and maintenance of reproduction in vertebrates. The development of GnRH neurons is highly dependent on fibroblast growth factor (Fgf) signaling. Mice with a hypomorphic Fgf8 allele (Fgf8 Het) exhibited a ~50% reduction in GnRH neuron number at birth. Female Fgf8 Het mice were fertile but showed significantly delayed puberty. However, it was unclear if these mice suffered additional loss of GnRH neurons after birth, and if male Fgf8 Het mice had normal pubertal transition and testicular function. In this study, we examined postnatal GnRH neuron number and hypothalamic GnRH content in Fgf8 Het mice from birth to 120 days of age. Further, we examined seminal vesicle and testicular growth, testicular histology, and circulating luteinizing hormone (LH) around and after pubertal transition. Our results showed that GnRH neuron numbers were significantly and consistently reduced in Fgf8 Het mice of both sexes in all ages examined, suggesting these animals were born with an inherently defective GnRH system, and no further postnatal loss of GnRH neurons had occurred. Despite an innately compromised GnRH system, male and female Fgf8 mice exhibited normal levels of immunoassayable hypothalamic GnRH peptide at all ages examined except on 60 days of age, suggesting increased GnRH synthesis or reduced turnover as a compensatory mechanism. Fgf8 Het males also had normal seminal vesicle and testicular mass/body mass ratios, testicular histology, and circulating LH. Overall, our data speak to the extraordinary ability of a GnRH system permanently compromised by developmental defect to overcome pre-existing deficiencies to ensure pubertal progression and reproduction.

GnRH, anosmia and hypogonadotropic hypogonadism--where are we?

Gonadotropin releasing hormone (GnRH) neurons originate the nasal placode and migrate into the brain during prenatal development. Once within the brain, these cells become integral components of the hypothalamic-pituitary-gonadal axis, essential for reproductive function. Disruption of this system causes hypogonadotropic hypogonadism (HH). HH associated with anosmia is clinically defined as Kallman syndrome (KS). Recent work examining the developing nasal region has shed new light on cellular composition, cell interactions and molecular cues responsible for the development of this system in different species. This review discusses some developmental aspects, animal models and current advancements in our understanding of pathologies affecting GnRH. In addition we discuss how development of neural crest derivatives such as the glia of the olfactory system and craniofacial structures control GnRH development and reproductive function.

Exogenous kisspeptin administration as a probe of GnRH neuronal function in patients with idiopathic hypogonadotropic hypogonadism

CONTEXT

Idiopathic hypogonadotropic hypogonadism (IHH) results from defective synthesis, secretion, or action of GnRH. Kisspeptin is a potent stimulus for GnRH secretion.

OBJECTIVE

We probed the functional capacity of the GnRH neuronal network in patients with IHH.

PARTICIPANTS

Eleven subjects with congenital IHH (9 men and 2 women) and one male subject who underwent reversal of IHH were studied. Six of the twelve subjects had an identified genetic cause of their IHH: KAL1 (n = 1), FGFR1 (n = 3), PROKR2 (n = 1), GNRHR (n = 1).

INTERVENTION

Subjects underwent q10 min blood sampling to measure GnRH-induced LH secretion at baseline and in response to intravenous boluses of kisspeptin (0.24 nmol/kg) and GnRH (75 ng/kg) both pre- and post-six days of treatment with exogenous GnRH (25 ng/kg sc every 2 h).

RESULTS

All subjects with abiding IHH failed to demonstrate a GnRH-induced LH response to exogenous kisspeptin. In contrast, the subject who achieved reversal of his hypogonadotropism demonstrated a robust response to kisspeptin.

CONCLUSIONS

The functional capacity of the GnRH neuronal network in IHH patients is impaired, as evidenced by their inability to respond to the same dose of kisspeptin that effects a robust GnRH-induced LH response in healthy men and luteal-phase women. This impairment is observed across a range of genotypes, suggesting that it reflects a fundamental property of GnRH neuronal networks that have not been properly engaged during pubertal development. In contrast, a patient who had experienced reversal of his hypogonadotropism responded to exogenous kisspeptin.

The indirect role of fibroblast growth factor-8 in defining neurogenic niches of the olfactory/GnRH systems

Bone morphogenic protein-4 (BMP4) and fibroblast growth factor-8 (FGF8) are thought to have opposite roles in defining epithelial versus neurogenic fate in the developing olfactory/vomeronasal system. In particular, FGF8 has been implicated in specification of olfactory and gonadotropin releasing hormone-1 (GnRH) neurons, as well as in controlling olfactory stem cell survival. Using different knock-in mouse lines and Cre-lox-mediated lineage tracing, Fgf8 expression and cell lineage was analyzed in the developing nose in relation to the expression of Bmp4 and its antagonist Noggin (Nog). FGF8 is expressed by cells that acquire an epidermal, respiratory cell fate and not by stem cells that acquire neuronal olfactory or vomeronasal cell fate. Ectodermal and mesenchymal sources of BMP4 control the expression of BMP/TGFβ antagonist Nog, whereas mesenchymal sources of Nog define the neurogenic borders of the olfactory pit. Fgf8 hypomorph mouse models, Fgf8(neo/neo) and Fgf8(neo/null), displayed severe craniofacial defects together with overlapping defects in the olfactory pit including (1) lack of neuronal formation ventrally, where GnRH neurons normally form, and (2) altered expression of Bmp4 and Nog, with Nog ectopically expressed in the nasal mesenchyme and no longer defining the GnRH and vomeronasal neurogenic border. Together our data show that (1) FGF8 is not sufficient to induce ectodermal progenitors of the olfactory pit to acquire neural fate and (2) altered neurogenesis and lack of GnRH neuron specification after chronically reduced Fgf8 expression reflected dysgenesis of the nasal region and loss of a specific neurogenic permissive milieu that was defined by mesenchymal signals.

FGFR1 mutations cause Hartsfield syndrome, the unique association of holoprosencephaly and ectrodactyly

Background

Harstfield syndrome is the rare and unique association of holoprosencephaly (HPE) and ectrodactyly, with or without cleft lip and palate, and variable additional features. All the reported cases occurred sporadically. Although several causal genes of HPE and ectrodactyly have been identified, the genetic cause of Hartsfield syndrome remains unknown. We hypothesised that a single key developmental gene may underlie the co-occurrence of HPE and ectrodactyly.

Methods

We used whole exome sequencing in four isolated cases including one case-parents trio, and direct Sanger sequencing of three additional cases, to investigate the causative variants in Hartsfield syndrome.

Results

We identified a novel FGFR1 mutation in six out of seven patients. Affected residues are highly conserved and are located in the extracellular binding domain of the receptor (two homozygous mutations) or the intracellular tyrosine kinase domain (four heterozygous de novo variants). Strikingly, among the six novel mutations, three are located in close proximity to the ATP's phosphates or the coordinating magnesium, with one position required for kinase activity, and three are adjacent to known mutations involved in Kallmann syndrome plus other developmental anomalies.

Conclusions

Dominant or recessive FGFR1 mutations are responsible for Hartsfield syndrome, consistent with the known roles of FGFR1 in vertebrate ontogeny and conditional Fgfr1-deficient mice. Our study shows that, in humans, lack of accurate FGFR1 activation can disrupt both brain and hand/foot midline development, and that FGFR1 loss-of-function mutations are responsible for a wider spectrum of clinical anomalies than previously thought, ranging in severity from seemingly isolated hypogonadotropic hypogonadism, through Kallmann syndrome with or without additional features, to Hartsfield syndrome at its most severe end.

Mutations in FGF17, IL17RD, DUSP6, SPRY4, and FLRT3 are identified in individuals with congenital hypogonadotropic hypogonadism

Congenital hypogonadotropic hypogonadism (CHH) and its anosmia-associated form (Kallmann syndrome [KS]) are genetically heterogeneous. Among the >15 genes implicated in these conditions, mutations in FGF8 and FGFR1 account for ~12% of cases; notably, KAL1 and HS6ST1 are also involved in FGFR1 signaling and can be mutated in CHH. We therefore hypothesized that mutations in genes encoding a broader range of modulators of the FGFR1 pathway might contribute to the genetics of CHH as causal or modifier mutations. Thus, we aimed to (1) investigate whether CHH individuals harbor mutations in members of the so-called "FGF8 synexpression" group and (2) validate the ability of a bioinformatics algorithm on the basis of protein-protein interactome data (interactome-based affiliation scoring [IBAS]) to identify high-quality candidate genes. On the basis of sequence homology, expression, and structural and functional data, seven genes were selected and sequenced in 386 unrelated CHH individuals and 155 controls. Except for FGF18 and SPRY2, all other genes were found to be mutated in CHH individuals: FGF17 (n = 3 individuals), IL17RD (n = 8), DUSP6 (n = 5), SPRY4 (n = 14), and FLRT3 (n = 3). Independently, IBAS predicted FGF17 and IL17RD as the two top candidates in the entire proteome on the basis of a statistical test of their protein-protein interaction patterns to proteins known to be altered in CHH. Most of the FGF17 and IL17RD mutations altered protein function in vitro. IL17RD mutations were found only in KS individuals and were strongly linked to hearing loss (6/8 individuals). Mutations in genes encoding components of the FGF pathway are associated with complex modes of CHH inheritance and act primarily as contributors to an oligogenic genetic architecture underlying CHH.