The candidate gene for the X-linked Kallmann syndrome encodes a protein related to adhesion molecules

Kallmann syndrome associates hypogonadotropic hypogonadism and anosmia and is probably due to a defect in the embryonic migration of olfactory and GnRH-synthesizing neurons. The Kallmann gene had been localized to Xp22.3. In this study 67 kb of genomic DNA, corresponding to a deletion interval containing at least part of the Kallmann gene, were sequenced. Two candidate exons, identified by multiparameter computer programs, were found in a cDNA encoding a protein of 679 amino acids. This candidate gene (ADMLX) is interrupted in its 3' coding region in the Kallmann patient, in which the proximal end of the KAL deletion interval was previously defined. A 5' end deletion was detected in another Kallmann patient. The predicted protein sequence shows homologies with the fibronectin type III repeat. ADMLX thus encodes a putative adhesion molecule, consistent with the defect of embryonic neuronal migration.     

The Kallmann syndrome gene homolog in C. elegans is involved in epidermal morphogenesis and neurite branching

Kallmann syndrome is an inherited disorder defined by the association of anosmia and hypogonadism, owing to impaired targeting and migration of olfactory axons and gonadotropin-releasing hormone secreting neurons. The gene responsible for the X-linked form of Kallmann syndrome, KAL-1, encodes a secreted protein of still elusive function. It has been proposed that KAL-1 might be involved in some aspects of olfactory axon guidance. However, the unavailability of a mouse model, and the difficulties in studying cellular and axonal migration in vertebrates have hampered an understanding of its function. We have identified the C. elegans homolog, kal-1, and document its function in vivo. We show that kal-1 is part of a mechanism by which neurons influence migration and adhesion of epidermal cells undergoing morphogenesis during ventral enclosure and male tail formation. We also show that kal-1 affects neurite outgrowth in vivo by modulating branching. Finally, we find that human KAL-1 cDNA can compensate for the loss of worm kal-1 and that overexpression of worm or human KAL-1 cDNAs in the nematode results in the same phenotypes. These data indicate functional conservation between the human and nematode proteins and establish C. elegans as a powerful animal in which to investigate KAL function in vivo. Our findings add a new player to the set of molecules, which appear to underlie both morphogenesis and axonal/neuronal navigation in vertebrates and invertebrates.     

Kallmann De Morsier syndrome: FGF-signaling insufficiency?

Kallmann syndrome (KAL) associates hypogonadotropic hypogonadism and anosmia, i.e. a deficiency of the sense of smell. Anosmia is related to the absence or the hypoplasia of the olfactory bulbs. Hypogonadism is due to GnRH deficiency, and is likely to result from the failed embryonic migration of GnRH-synthesizing neurons. These cells normally migrate from the olfactory epithelium to the forebrain along the olfactory nerve pathway. Kallmann syndrome is genetically heterogeneous. The gene responsible for the X-chromosome linked form of the disease, KAL-1, has been identified in 1991. KAL1 encodes a ~95 kDa glycoprotein of unknown function, which is present locally in various extracellular matrices during the period of organogenesis. The recent finding that FGFR1 mutations are involved in an autosomal dominant form of Kallmann syndrome (KAL-2), combined to the analysis of mutant mouse embryos that no longer express Fgfr1 in the telencephalon, suggests that the disease results from a deficiency in FGF-signaling at the earliest stage of olfactory bulb morphogenesis. We propose that the role of the KAL1 gene product, the extracellular matrix protein anosmin-1, is to enhance FGF-signaling, and suggest that the gender difference in anosmin-1 dosage (because KAL1 partially escapes X-inactivation) explains the higher prevalence of the disease in males.     

Kallmann syndrome 1 gene is expressed in the marsupial gonad

Kallmann syndrome is characterized by hypogonadotrophic hypogonadism and anosmia. The syndrome can be caused by mutations in several genes, but the X-linked form is caused by mutation in the Kallmann syndrome 1 (KAL1). KAL1 plays a critical role in gonadotropin-releasing hormone (GnRH) neuronal migration that is essential for the normal development of the hypothalamic-pituitary-gonadal axis. Interestingly, KAL1 appears to be missing from the rodent X, and no orthologue has been detected as yet. We investigated KAL1 during development and in adults of an Australian marsupial, the tammar wallaby, Macropus eugenii. Marsupial KAL1 maps to an autosome within a group of genes that was added as a block to the X chromosome in eutherian evolution. KAL1 expression was widespread in embryonic and adult tissues. In the adult testis, tammar KAL1 mRNA and protein were detected in the germ cells at specific stages of differentiation. In the adult testis, the protein encoded by KAL1, anosmin-1, was restricted to the round spermatids and elongated spermatids. In the adult ovary, anosmin-1 was not only detected in the oocytes but was also localized in the granulosa cells throughout folliculogenesis. This is the first examination of KAL1 mRNA and protein localization in adult mammalian gonads. The protein localization suggests that KAL1 participates in gametogenesis not only through the development of the hypothalamic-pituitary-gonadal axis by activation of GnRH neuronal migration, but also directly within the gonads themselves. Because KAL1 is autosomal in marsupials but is X-linked in eutherians, its conserved involvement in gametogenesis supports the hypothesis that reproduction-related genes were actively recruited to the eutherian X chromosome.     

Cell adhesion to anosmin via α5β1, α4β1, and α9β1 integrins

Anosmin is an extracellular matrix protein, and genetic defects in anosmin result in human Kallmann syndrome. It functions in neural crest formation, cell adhesion, and neuronal migration. Anosmin consists of multiple domains, and it has been reported to bind heparan sulfate, FGF receptor, and UPA. In this study, we establish cell adhesion/spreading assays for anosmin and use them for antibody inhibition analyses to search for an integrin adhesion receptor. We find that α5β1, α4β1, and α9β1 integrins are needed for effective adhesive receptor function in cell adhesion and cell spreading on anosmin; adhesion is inhibited by both RGD and α4β1 CS1-based peptides. This identification of anosmin-integrin adhesion receptors should facilitate studies of anosmin function in cell and developmental biology.     

A novel nonsense mutation of the KAL gene in two brothers with Kallmann syndrome

Kallmann syndrome (KS), defined by the association of hypogonadotropic hypogonadism and anosmia or hyposmia, can be caused by mutations in the KAL gene on Xp 22.3. This gene encodes an extracellular matrix glycoprotein called anosmin-1, which belongs to the class of cell adhesion molecules. In the absence of a functional KAL protein, migration of both olfactory and gonadotropin-releasing hormone neurons is arrested. A defective anosmin-1 molecule may also play a role in the development of synkinesia and renal agenesis, which are exclusively seen in the X-linked form of KS. We describe the clinical presentation and molecular diagnosis of the defect in two brothers with KS. An X-linked mode of transmission was assumed on the basis of synkinesia and the presence of oligomenorrhoea in the mother. A novel nonsense mutation was found in exon 13 of the KAL gene, encoding the region of the fourth fibronectin type III repeat of anosmin-1, which results in an apparently nonfunctional truncated protein.     

Le syndrome de kallmann de morsier aspect génétique

Kallmann syndrome (KS) is a rare, heterogeneous disorder consisting of congenital hypogonadotropic hypogonadism, associated with anosmia (or hyposmia) and other clinical manifestations such as mirror movements, and renal, urological and neurosensory disorders. The presence of anosmia with micropenis in boys is suggestive of the diagnostic of KS. In KS, the GnRH neurons do not migrate correctly from the olfactory placode to the hypothalamus during development and olfactory bulbs also fail to form, leading to anosmia. Mutations in KAL1 which encodes Anosmin-1, are responsible for the X-linked form of KS. Anosmin-1 is normally expressed in the brain, facial mesenchyme, mesonephros and metanephros. It is required to promote migration of GnRH neurons into the hypothalamus. It also allows migration of olfactory neurons from the olfactory bulbs to the hypothalamus. The loss of function mutations in FGFR1 “fibroblast growth factor” were identified in 2003 as a cause of autosomal forms of this disease. An additional autosomal cause of Kallmann syndrome was recently identified by a mutation in the prokineticin receptor-2 gene (PROKR2) (KAL-3) and its ligand prokineticin 2 (PROK2) (KAL-4). Mutations in these genes induce various degrees of olfactory and reproductive dysfunction, but not the other symptoms seen in KAL-1 and KAL-2 forms of KS. Neuropilin2, which has an important role in migration of GnRH neurons, is a recent candidate gene for KS. The authors describe the genetic features and recent findings of KS, necessary to understand this disease.     

PEX11 beta deficiency is lethal and impairs neuronal migration but does not abrogate peroxisome function

Zellweger syndrome is a lethal neurological disorder characterized by severe defects in peroxisomal protein import. The resulting defects in peroxisome metabolism and the accumulation of peroxisomal substrates are thought to cause the other Zellweger syndrome phenotypes, including neuronal migration defects, hypotonia, a developmental delay, and neonatal lethality. These phenotypes are also manifested in mouse models of Zellweger syndrome generated by disruption of the PEX5 or PEX2 gene. Here we show that mice lacking peroxisomal membrane protein PEX11 beta display several pathologic features shared by these mouse models of Zellweger syndrome, including neuronal migration defects, enhanced neuronal apoptosis, a developmental delay, hypotonia, and neonatal lethality. However, PEX11 beta deficiency differs significantly from Zellweger syndrome and Zellweger syndrome mice in that it is not characterized by a detectable defect in peroxisomal protein import and displays only mild defects in peroxisomal fatty acid beta-oxidation and peroxisomal ether lipid biosynthesis. These results demonstrate that the neurological pathologic features of Zellweger syndrome can occur without peroxisomal enzyme mislocalization and challenge current models of Zellweger syndrome pathogenesis.     

Deletions of the steroid sulphatase gene in “classical” X-linked ichthyosis and in X-linked ichthyosis associated with Kallmann syndrome

Summary We have studied 16 men, from 10 unrelated Italian families, affected by steroid suphatase (STS) deficiency, which is the basic defect of X-linked ichthyosis (XLI). The patients' clinical diagnoses were of either isolated ichthyosis or ichthyosis associated with Kallmann syndrome (KS) (hypogonadotropic hypogonadism and anosmia). DNA from patients and their relatives was analysed by Southern blotting followed by hydridization with an STS cDNA probe. None of the patients affected by either XLI or XLI/KS showed any hybridization signal, thus revealing a deletion in the STS gene. We suggest that a gene deletion may be the most common molecular defect involved in XLI and that the syndrome XLI/KS may be due to a deletion of both the STS and the KS loci.     

Molecular heterogeneity of steroid sulfatase deficiency: a multicenter study on 57 unrelated patients, at DNA and protein levels

Steroid sulfatase (STS) deficiency is the biochemical defect of X-linked ichthyosis (XLI), one of the most common X-linked disorders. We studied 57 European unrelated patients affected by STS deficiency. Twenty-eight patients were from Italy, 24 from the United Kingdom, 4 from The Netherlands, and 1 from Denmark. In two families XLI was associated with Kallmann syndrome (hypogonadotropic hypogonadism and anosmia). STS enzymatic activity was profoundly deficient in all cases. Direct DNA analysis, using cDNA and genomic probes from the STS gene and linked regions, demonstrated heterogeneity of the molecular defect. Forty-eight patients (84%) showed a deletion of the STS gene. In 44 cases the deletion also involved the STS flanking locus DXS237. In 1 patient a partial deletion of the STS gene was detected and in 9 patients no evidence of deletion was found. Locus DXS31 (probe M1A), previously mapped to Xp22.3-pter, was not deleted either in 24 patients with X-linked ichthyosis or in two families with X-linked ichthyosis associated with Kallmann syndrome. Consequently, the following loci order could be suggested: telomere--DXS31--(DXS237, STS)--Kallmann--centromere. Immunoblotting experiments, performed using anti-STS polyclonal antibodies, revealed the absence of cross-reacting material to STS in all cases tested, including 4 patients without evidence of deletions.     

Specification of GnRH-1 neurons by antagonistic FGF and retinoic acid signaling

A small population of neuroendocrine cells in the rostral hypothalamus and basal forebrain is the key regulator of vertebrate reproduction. They secrete gonadotropin-releasing hormone (GnRH-1), communicate with many areas of the brain and integrate multiple inputs to control gonad maturation, puberty and sexual behavior. In humans, disruption of the GnRH-1 system leads to hypogonadotropic gonadism and Kallmann syndrome. Unlike other neurons in the central nervous system, GnRH-1 neurons arise in the periphery, however their embryonic origin is controversial, and the molecular mechanisms that control their initial specification are not clear. Here, we provide evidence that in chick GnRH-1 neurons originate in the olfactory placode, where they are specified shortly after olfactory sensory neurons. FGF signaling is required and sufficient to induce GnRH-1 neurons, while retinoic acid represses their formation. Both pathways regulate and antagonize each other and our results suggest that the timing of signaling is critical for normal GnRH-1 neuron formation. While Kallmann's syndrome has generally been attributed to a failure of GnRH-1 neuron migration due to impaired FGF signaling, our findings suggest that in at least some Kallmann patients these neurons may never be specified. In addition, this study highlights the intimate embryonic relationship between GnRH-1 neurons and their targets and modulators in the adult.     

Mutation of the Variant α-Tubulin TUBA8 Results in Polymicrogyria with Optic Nerve Hypoplasia

The critical importance of cytoskeletal function for correct neuronal migration during development of the cerebral cortex has been underscored by the identities of germline mutations underlying a number of human neurodevelopmental disorders. The proteins affected include TUBA1A, a major α-tubulin isoform, and microtubule-associated components such as doublecortin, and LIS1. Mutations in these genes are associated with the anatomical abnormality lissencephaly, which is believed to reflect failure of neuronal migration. An important recent observation has been the dependence of cortical neuronal migration upon acetylation of α-tubulin at lysine 40 by the histone acetyltransferase Elongator complex. Here, we describe a recognizable autosomal recessive syndrome, characterized by generalized polymicrogyria in association with optic nerve hypoplasia (PMGOH). By autozygosity mapping, we show that the molecular basis for this condition is mutation of the TUBA8 gene, encoding a variant α-tubulin of unknown function that is not susceptible to the lysine 40 acetylation that regulates microtubule function during cortical neuron migration. Together with the unique expression pattern of TUBA8 within the developing cerebral cortex, these observations suggest a role for this atypical microtubule component in regulating mammalian brain development.     

Definitive localization of X-linked Kallman syndrome (hypogonadotropic hypogonadism and anosmia) to Xp22.3: close linkage to the hypervariable repeat sequence CRI-S232.

Kallmann syndrome is a genetically heterogeneous disease characterized by hypogonadotropic hypogonadism and anosmia. Six families in which the disorder followed an X-linked inheritance were investigated by linkage analysis. Diagnostic criteria were uniformly applied and included tests for hypogonadotropic hypogonadism and anosmia. Close linkage was found by using the hypervariable repeated sequence CRI-S232 (DXS278) previously mapped to Xp22.3. At a maximum lod score of 6.5, the recombination fraction was calculated as .03. Of 30 fully informative meioses, one recombination between the disease locus and the loci recognized by probe CRI-S232 was observed. When an independent approach is used, these results confirm the X-linked Kallmann syndrome assignment previously made by deletion mapping, and allow definitive localization of the syndrome assignment previously made by deletion mapping, and allow definitive localization of the syndrome to the Xp22.3 region. This opens the way to carrier detection and to the identification of a gene responsible for this disorder.     

A de novo interstitial deletion of 8p11.2 including ANK1 identified in a patient with spherocytosis, psychomotor developmental delay, and distinctive facial features

The contiguous gene syndrome involving 8p11.2 is recognized as a combined phenotype of both Kallmann syndrome and hereditary spherocytosis, because the genes responsible for these 2 clinical entities, the fibroblast growth factor receptor 1 (FGFR1) and ankyrin 1 (ANK1) genes, respectively, are located in this region within a distance of 3.2Mb. We identified a 3.7Mb deletion of 8p11.2 in a 19-month-old female patient with hereditary spherocytosis. The identified deletion included ANK1, but not FGFR1, which is consistent with the absence of any phenotype or laboratory findings of Kallmann syndrome. Compared with the previous studies, the deletion identified in this study was located on the proximal end of 8p, indicating a pure interstitial deletion of 8p11.21. This patient exhibited mild developmental delay and distinctive facial findings in addition to hereditary spherocytosis. Thus, some of the genes included in the deleted region would be related to these symptoms.     

Treacher Collins syndrome—an example of a craniofacial development defect

Treacher Collins syndrome is an autosomal dominant disorder of craniofacial development the features of which include conductive hearing loss and cleft palate; diagnosis is complicated by marked variability in expression. Whilst the underlying genetic defect is unknown it is thought to be due to an abnormality of neural crest cell migration. The mutated gene has been mapped to chromosome 5q32-33.1, a high resolution genetic and radiation hybrid map surrounding the locus created, and flanking markers identified. The critical region has subsequently been cloned in yeast artificial chromosome and attempts to isolate the mutated gene are in progress.     

Invertebrate Models of Kallmann Syndrome: Molecular Pathogenesis and New Disease Genes

Kallmann Syndrome is a heritable disorder characterized by congenital anosmia, hypogonadotropic hypogonadism and, less frequently, by other symptoms. The X-linked form of this syndrome is caused by mutations affecting the KAL1 gene that codes for the extracellular protein anosmin-1. Investigation of KAL1 function in mice has been hampered by the fact that the murine ortholog has not been identified. Thus studies performed in other animal models have contributed significantly to an understanding of the function of KAL1. In this review, the main results obtained using the two invertebrate models, the nematode worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster, are illustrated and the contribution provided by them to the elucidation of the molecular pathogenesis of Kallmann Syndrome is discussed in detail. Structure-function dissection studies performed in these two animal models have shown how the different domains of anosmin-1 carry out specific functions, also suggesting a novel intramolecular regulation mechanism among the different domains of the protein. The model that emerges is one in which anosmin-1 plays different roles in different tissues, interacting with different components of the extracellular matrix. We also describe how the genetic approach in C. elegans has allowed the discovery of the genes involved in KAL1-heparan sulfate proteoglycans interactions and the identification of HS6ST1 as a new disease gene.     

Hypogonadotroper Hypogonadismus aufgrund eines IHH oder Kallmann-Syndroms beim Mann

The prevalence of isolated hypogonadotropic hypogonadism (IHH)/Kallmann syndrome in males is about 1:10,000. IHH is characterized by absent sexual development and low levels of gonadotropins and testosterone. Kallmann syndrome is characterized by the association of HH and total or partial loss of olfaction (in 60%). IHH/Kallmann syndrome are very heterogeneous disorders. Clinical and genetic diagnosis as well as therapy will be discussed.