A novel mutation of the KAL1 gene in monozygotic twins with Kallmann syndrome

Genetic analysis of failed male puberty using whole exome sequencing

OBJECTIVES

Although at least 598 genes are involved in the development of the hypothalamo-pituitary-testicular (HPT) axis, mutations in only 75 genes have so far been shown to cause delayed puberty.

METHODS

Six male patients with failed puberty, manifested as absence of pubertal changes by 18 years of age, underwent whole exome sequencing of genomic DNA with subsequent bioinformatics analysis and confirmation of selected variants by Sanger sequencing. Genes having plausibly pathogenic non-synonymous variants were characterized as group A (previously reported to cause delayed puberty), group B (expressed in the HPT-axis but no mutations therein were reported to cause delayed puberty) or group C (not reported previously to be connected with HPT-axis).

RESULTS

We identified variants in genes involved in GnRH neuron differentiation (2 in group A, 1 in group C), GnRH neuron migration (2 each in groups A and C), development of GnRH neural connections with supra-hypothalamic and hypothalamic neurons (2 each in groups A and C), neuron homeostasis (1 in group C), molecules regulating GnRH neuron activity (2 each in groups B and C), receptors/proteins expressed on GnRH neurons (1 in group B), signaling molecules (3 in group C), GnRH synthesis (1 in group B), gonadotropins production and release (1 each in groups A, B, and C) and action of the steroid hormone (1 in group A).

CONCLUSIONS

Non-synonymous variants were identified in 16 genes of the HPT-axis, which comprised 4 in group A that contains genes previously reported to cause delayed puberty, 4 in group B that are expressed along HPT-axis but no mutations therein were reported previously to cause delayed puberty and 8 in group C that contains novel candidate genes, suggesting wider genetic causes of failed male puberty.

Genetics of congenital olfactory dysfunction: a systematic review of the literature

Olfaction, as one of our 5 senses, plays an important role in our daily lives. It is connected to proper nutrition, social interaction, and protection mechanisms. Disorders affecting this sense consequently also affect the patients' general quality of life. Because the underlying genetics of congenital olfactory disorders (COD) have not been thoroughly investigated yet, this systematic review aimed at providing information on genes that have previously been reported to be mutated in patients suffering from COD. This was achieved by systematically reviewing existing literature on 3 databases, namely PubMed, Ovid Medline, and ISI Web of Science. Genes and the type of disorder, that is, isolated and/or syndromic COD were included in this study, as were the patients' associated abnormal features, which were categorized according to the affected organ(-system). Our research yielded 82 candidate genes/chromosome loci for isolated and/or syndromic COD. Our results revealed that the majority of these are implicated in syndromic COD, a few accounted for syndromic and isolated COD, and the least underly isolated COD. Most commonly, structures of the central nervous system displayed abnormalities. This study is meant to assist clinicians in determining the type of COD and detecting potentially abnormal features in patients with confirmed genetic variations. Future research will hopefully expand this list and thereby further improve our understanding of COD.

Normosmic idiopathic hypogonadotrophic hypogonadism due to a rare KISS1R gene mutation

Hypogonadotrophic hypogonadism is due to impaired or reduced gonadotrophin secretion from the pituitary gland. In the absence of any anatomical or functional lesions of the pituitary or hypothalamic gland, the hypogonadotrophic hypogonadism is referred to as idiopathic hypogonadotrophic hypogonadism (IHH). We present a case of a young lady born to consanguineous parents who was found to have IHH due to a rare gene mutation.

Congenital Hypogonadotropic Hypogonadism and Kallmann Syndrome: Past, Present, and Future

The proper development and coordination of the hypothalamic-pituitary-gonadal (HPG) axis are essential for normal reproductive competence. The key factor that regulates the function of the HPG axis is gonadotrophin-releasing hormone (GnRH). Timely release of GnRH is critical for the onset of puberty and subsequent sexual maturation. Misregulation in this system can result in delayed or absent puberty and infertility. Congenital hypogonadotropic hypogonadism (CHH) and Kallmann syndrome (KS) are genetic disorders that are rooted in a GnRH deficiency but often accompanied by a variety of non-reproductive phenotypes such as the loss of the sense of smell and defects of the skeleton, eye, ear, kidney, and heart. Recent progress in DNA sequencing technology has produced a wealth of information regarding the genetic makeup of CHH and KS patients and revealed the resilient yet complex nature of the human reproductive neuroendocrine system. Further research on the molecular basis of the disease and the diverse signal pathways involved will aid in improving the diagnosis, treatment, and management of CHH and KS patients as well as in developing more precise genetic screening and counseling regime.

Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease

Some individuals with a particular disease-causing mutation or genotype fail to express most if not all features of the disease in question, a phenomenon that is known as 'reduced (or incomplete) penetrance'. Reduced penetrance is not uncommon; indeed, there are many known examples of 'disease-causing mutations' that fail to cause disease in at least a proportion of the individuals who carry them. Reduced penetrance may therefore explain not only why genetic diseases are occasionally transmitted through unaffected parents, but also why healthy individuals can harbour quite large numbers of potentially disadvantageous variants in their genomes without suffering any obvious ill effects. Reduced penetrance can be a function of the specific mutation(s) involved or of allele dosage. It may also result from differential allelic expression, copy number variation or the modulating influence of additional genetic variants in cis or in trans. The penetrance of some pathogenic genotypes is known to be age- and/or sex-dependent. Variable penetrance may also reflect the action of unlinked modifier genes, epigenetic changes or environmental factors. At least in some cases, complete penetrance appears to require the presence of one or more genetic variants at other loci. In this review, we summarize the evidence for reduced penetrance being a widespread phenomenon in human genetics and explore some of the molecular mechanisms that may help to explain this enigmatic characteristic of human inherited disease.

X-linked GnRH deficiency: role of KAL-1 mutations in GnRH deficiency

The gene for X-linked Kallmann's syndrome (KAL-1, encoding anosmin-1) was cloned in 1991. Over a decade elapsed before autosomal forms of KS and most of other genetic forms of isolated hypogonadotrophic hypogonadism (IHH) became characterized, and the genetic diversity of these disorders fully appreciated. Although KAL-1 mutations appear to cause a more severe reproductive phenotype than other IHH genes, the biology of this multidomain extracellular matrix protein has only been partially characterized. Initial studies suggested a central role of anosmin-1, in GnRH neuron ontogeny - specifically in GnRH neuronal migration from the cribriform plate area into the brain - as well as in olfactory bulb development. Anosmin-1 is expressed extracellularly, with high affinity binding to cell membrane heparan sulphate proteoglycans. It is expressed in the outer layers of the developing olfactory bulb, the neuroretina, the cerebellum, spinal cord and developing kidney. Recent observations have demonstrated an anosmin-1 heparan sulphate dependent functional interaction with the product of the autosomal dominant KAL-2 (FGFR1: anosmin-2) gene, thereby modulating FGFR1 signalling. Although these genes are frequently co-expressed in developing tissues, this may not represent the sole mode of action of anosmin-1, and FGFR1 independent actions of the protein have also been identified. Structural and in vitro functional studies have shown that anosmin-1 may have complex biological actions. Anosmin-1 interactions with FGFR1 have however been best characterized and represent the dominant focus of this chapter.

Biochemical dissection of Anosmin-1 interaction with FGFR1 and components of the extracellular matrix

Anosmin-1, defective in Kallmann's syndrome, participates in the adhesion, migration and differentiation of different cell types in the CNS. Although not fully understood, the mechanisms of action of Anosmin-1 involve the interaction with different proteins, being the interaction with fibroblast growth factor receptor 1 (FGFR1) and the modulation of its signalling the best studied to date. Using glutathione-S-transferase pull-down assays we demonstrate that the FnIII.3 (Fibronectin-like type III) domain and the combination whey acidic protein-FnIII.1, but not each of them individually, interact with FGFR1. The interaction of the whey acidic protein-FnIII.1 domains is substantially reduced when the cysteine-rich region is present, suggesting a likely regulatory role for this domain. The introduction in FnIII.3 of any of the two missense mutations found in Kallmann's syndrome patients, E514K and F517L, abolished the interaction with FGFR1, what suggests an important role for these residues in the interaction. Interestingly, the chemoattraction of Anosmin-1 on rat neuronal precursors (NPs) via FGFR1 is retained by the N-terminal region of Anosmin-1 but not by FnIII.3 alone, and is lost in proteins carrying either one of the missense mutations, probably because of a highly reduced binding capacity to FGFR1. We also describe homophilic interaction Anosmin-1/Anosmin-1 via the FnIII repeats 1 and 4, and the interaction of FnIII.1 and FnIII.3 with Fibronectin and of FnIII.3 with Laminin.

Clinical manifestations of impaired GnRH neuron development and function

Gonadotropin-releasing hormone (GnRH) and olfactory neurons migrate together in embryologic development, and disruption of this process causes idiopathic hypogonadotropic hypogonadism (IHH) with anosmia (Kallmann syndrome (KS)). Patients with IHH/KS generally manifest irreversible pubertal delay and subsequent infertility due to deficient pituitary gonadotropins or GnRH. The molecular basis of IHH/KS includes genes that: (1) regulate GnRH and olfactory neuron migration; (2) control the synthesis or secretion of GnRH; (3) disrupt GnRH action upon pituitary gonadotropes, or (4) interfere with pituitary gonadotropin synthesis or secretion. KS patients may also have midline facial defects indicating the diverse developmental functions of genes involved. Most causative genes cause either normosmic IHH or KS except FGFR1, which may cause either phenotype. Recently, several balanced chromosomal translocations have been identified in IHH/KS patients, which could lead to the identification of new disease-producing genes. Although there are two cases reported who have digenic disease, this awaits confirmation in future larger studies. The challenge will be to determine the importance of these genes in the 10-15% of couples with normal puberty who have infertility.

Síndrome de Kallmann: uma revisão histórica, clínica e molecular

A síndrome de Kallmann (SK) é a associação de hipogonadismo hipogonadotrófico (HH) e anosmia descrita por Maestre de San Juan, em 1856, e caracterizada como condição hereditária por Franz Josef Kallmann, em 1944. Muitos aspectos de sua patogenia, variabilidade fenotípica e genotípica foram desvendados nos últimos 15 anos. Conseqüentemente, tem sido difícil manter-se atualizado frente à rapidez que o conhecimento dessa condição é gerado. Nesta revisão, resgatamos aspectos históricos pouco conhecidos sobre a síndrome e seus descobridores; incorporamos novas descobertas relacionadas à embriogênese dos neurônios olfatórios e produtores de GnRH. Esse processo é fundamental para compreender a associação de hipogonadismo e anosmia; descrevemos a heterogeneidade fenotípica e genotípica, incluindo mutações em cinco genes (KAL-1, FGFR1, PROKR2, PROK2 e NELF). Para cada gene, discutimos a função da proteína codificada na migração e maturação dos neurônios olfatórios e GnRH a partir de estudos in vitro e modelos experimentais e descrevemos características clínicas dos portadores dessas mutações.

Diversity in fibroblast growth factor receptor 1 regulation: learning from the investigation of Kallmann syndrome

The unravelling of the genetic basis of the hypogonadotrophic hypogonadal disorders, including Kallmann syndrome (KS), has led to renewed interest into the developmental biology of gonadotrophin-releasing hormone (GnRH) neurones and, more generally, into the molecular mechanisms of reproduction. KS is characterised by the association of GnRH deficiency with diminished olfaction. Until recently, only two KS-associated genes were known: KAL1 and KAL2. KAL1 encodes the cell membrane and extracellular matrix-associated secreted protein anosmin-1 which is implicated in the X-linked form of KS. Anosmin-1 shows high affinity binding to heparan sulphate (HS) and its function remains the focus of ongoing investigation, although a role in axonal guidance and neuronal migration, which are processes essential for normal GnRH ontogeny and olfactory bulb histogenesis, has been suggested. KAL2, identified as the fibroblast growth factor receptor 1 (FGFR1) gene, has now been recognised to be the underlying genetic defect for an autosomal dominant form of KS. The diverse signalling pathways initiated upon FGFR activation can elicit pleiotropic cellular responses depending on the cellular context. Signalling through FGFR requires HS for receptor dimerisation and ligand binding. Current evidence supports a HS-dependent interaction between anosmin-1 and FGFR1, where anosmin-1 serves as a co-ligand activator enhancing the signal activity, the finer details of whose mechanism remain the subject of intense investigation. Recently, mutations in the genes encoding prokineticin 2 (PK2) and prokineticin receptor 2 (PKR2) were reported in a cohort of KS patients, further reinforcing the view of KS as a multigenic trait involving divergent pathways. Here, we review the historical and current understandings of KS and discuss the latest findings from the molecular and cellular studies of the KS-associated proteins, and describe the evidence that suggests convergence of several of these pathways during normal GnRH and olfactory neuronal ontogeny.

Two cases of Kallmann syndrome associated with empty sella

Kallmann syndrome (KS) is a developmental disease characterized by the association of isolated hypogonadotropic hypogonadism and anosmia/hyposmia. We report an unusual presentation of two females with KS and empty sella. These females, aged at 20 and 29-year-old, presented primary amenorrhea with prepubertal estradiol and low gonadotropin levels. No other significant clinical signs were observed. Empty sella was observed on MRI in both cases. Sequencing of FGFR1 gene, recently implicated in autosomal form of KS, was performed and one splicing mutation (IVS14 + 1G > A) was identified in one patient.

KAL1 mutations are not a common cause of idiopathic hypogonadotrophic hypogonadism in humans

Hypogonadotrophic hypogonadism results in the absence of puberty and if left untreated leads to infertility. Mutations in KAL1 are known to account for some of the cases of Kallmann syndrome. The aim of this study was to determine the prevalence of KAL1 mutations in a large number of patients with idiopathic hypogonadotrophic hypogonadism (IHH). One hundred and thirty eight patients (109 males and 29 females) with IHH were studied for mutations in KAL1. DNA from these patients was subjected to denaturing gradient gel electrophoresis or single strand conformation polymorphism to identify mutations. Sequencing was performed to confirm mutations detected. Four mutations were found in 109 males (3.7%). All four mutations were in anosmic/hyposmic men making the prevalence 4/63 (6.3%) in this group of patients. No mutations were found in the 29 female patients. KAL1 mutations are an uncommon cause of Kallmann syndrome.

Molecular pathogenesis of Kallmann's syndrome

Hypogonadotrophic hypogonadism (HH) is characterized by delayed or absent pubertal development secondary to gonadotrophin deficiency. HH can result from mutations of the gonadotrophin-releasing hormone receptor 1, the gonadotrophin beta-subunits, or various transcription factors involved in pituitary gland development. HH occurs in DAX1 mutations when associated with adrenal insufficiency (adrenal hypoplasia congenita), and is also linked with obesity in patients with mutations of leptin and its receptor, as well as mutations in prohormone convertase 1. Rarely, HH has resulted from kisspeptin receptor (GPR54) mutations, a gene implicated in the regulation of pubertal onset. When occurring with anosmia (a lack of sense of smell), HH is referred to as Kallmann's syndrome (KS). Two KS-related loci are currently known: KAL1, encoding anosmin-1, responsible for X-linked KS, and KAL2, encoding the fibroblast growth factor receptor 1 (FGFR1), mutated in autosomal dominant KS. Anosmin-1 is an extracellular glycoprotein with some unique structural characteristics; it interacts with both urokinase-type plasminogen activator and FGFR1. It has previously been shown that anosmin-1 enhances FGFR1 signalling in a heparan sulphate-dependent manner, and proposed that anosmin-1 fine-tunes FGFR1 signalling during olfactory and GnRH neuronal development. Here, we review the known normosmic causes of HH, and discuss novel developmental and molecular mechanisms underlying KS; finally, we introduce three novel genes (NELF, PKR2, and CHD7) that may be associated with some phenotypic features of KS.

Mechanisms of Disease: insights into X-linked and autosomal-dominant Kallmann syndrome

Kallmann syndrome (KS) is a disorder characterized by hypogonadotropic hypogonadism and anosmia. Although KS is genetically heterogeneous, only two causal genes have been identified to date. These include an X-linked gene that encodes anosmin 1 and an autosomal gene that encodes fibroblast growth factor receptor 1. Mutations in these two genes result in disorders that often include, but are not limited to, severe defects in olfactory and reproductive functions. In this respect, KS can be regarded as a 'human model' for understanding critical factors that regulate olfactory and reproductive development. Here we give an overview of the disorders that stem from mutations in these two genes, with special emphasis on the cellular mechanisms underlying olfactory and reproductive anomalies. Other, less well-known aspects of KS, such as the convergence of symptoms in patients with different genetic forms of KS and the unpredictable manifestation of KS symptoms, are also discussed.

Epigenetic contributors to the discordance of monozygotic twins

Human monozygotic (MZ) twins estimated to occur once in 250 live births, result from an errant decision by embryonic cell(s) to develop as separate embryos. They are considered genetically identical and any phenotypic discordance between them has been used to implicate the role of environment. More recent literature, however, has questioned these assumptions but the frequency and the nature of any genetic discordance between MZ twins remains poorly understood. We will review published cases of phenotypic and genetic discordance between monozygotic twins to argue that not all discordance between such twins is due to differences in environment. The causes of reduced concordance between MZ twins remains poorly understood. They represent among the challenging aspects of the genetics of complex multi-factorial traits and diseases. A number of questions regarding the published results on MZ twins merit a re-assessment in the light of modern molecular insight of the human genome. Such an assessment is needed in directing future studies on MZ twins. In particular, we will deal with the origin, development, genetic and epigenetic factors that may have implications in discordance of the MZ twin pairs.