Molecular screening of SHH, ZIC2, SIX3, and TGIF genes in patients with features of holoprosencephaly spectrum: Mutation review and genotype-phenotype correlations

Hedgehog-GLI mediated control of renal formation and malformation

CAKUT is the leading cause of end-stage kidney disease in children and comprises a broad spectrum of phenotypic abnormalities in kidney and ureter development. Molecular mechanisms underlying the pathogenesis of CAKUT have been elucidated in genetic models, predominantly in the mouse, a paradigm for human renal development. Hedgehog (Hh) signaling is critical to normal embryogenesis, including kidney development. Hh signaling mediates the physiological development of the ureter and stroma and has adverse pathophysiological effects on the metanephric mesenchyme, ureteric, and nephrogenic lineages. Further, disruption of Hh signaling is causative of numerous human developmental disorders associated with renal malformation; Pallister-Hall Syndrome (PHS) is characterized by a diverse spectrum of malformations including CAKUT and caused by truncating variants in the middle-third of the Hh signaling effector GLI3. Here, we outline the roles of Hh signaling in regulating murine kidney development, and review human variants in Hh signaling genes in patients with renal malformation.

Solitary Median Maxillary Central Incisor Syndrome: An Exploration of the Pathogenic Mechanism

This study aimed to identify the genetic cause of one Chinese family with solitary median maxillary central incisor (SMMCI) and explore the relationship between genotype and its phenotype. One Chinese family with clinical diagnosis of SMMCI was collected. Single Nucleotide Polymorphism (SNP) array was performed and identified variation was confirmed by whole-genome sequencing (WGS). The reported chromosomal abnormalities and pathogenic genes in patients with SMMCI in literature were reviewed and summarized. The proband was an 8-year-old boy presenting a typical solitary median maxillary central incisor with a range of other phenotypic anomalies, including ptosis. SNP array revealed a 14.3 Mbp heterozygous deletion at chromosome 18p11.32-p11.21 in the proband but not in the unaffected parents. WGS further confirmed the identified deletion. 194 genes were involved in the chromosome region. Among them, 12 genes had been shown to be associated with diseases, including TGIF1, a reported SMMCI gene. The de novo 18p deletion resulted in SMMCI in the present study. Our results provide new genetic evidence that structural abnormality in chromosome 18p contributes to solitary median maxillary central incisor.

Deletion of Six3 in post-proliferative neurons produces weakened SCN circadian output, improved metabolic function, and dwarfism in male mice

OBJECTIVE

The increasing prevalence of obesity makes it important to increase the understanding of the maturation and function of the neuronal integrators and regulators of metabolic function.

METHODS

Behavioral, molecular, and physiological analyses of transgenic mice with Sine oculis 3 (Six3) deleted in mature neurons using the Synapsin^creallele.

RESULTS

Conditional deletion of the homeodomain transcription factor Six3 in mature neurons causes dwarfism and weakens circadian wheel-running activity rhythms but increases general activity at night, and improves metabolic function, without impacting pubertal onset or fertility in males. The reduced growth in 6-week-old Six3^fl/fl:Synapsin^cre (Six3^syn) males correlates with increased somatostatin (SS) expression in the hypothalamus and reduced growth hormone (GH) in the pituitary. In contrast, 12-week-old Six3^syn males have increased GH release, despite an increased number of the inhibitory SS neurons in the periventricular nucleus. GH is important in glucose metabolism, muscle function, and bone health. Interestingly, Six3^syn males have improved glucose tolerance at 7, 12, and 18 weeks of age, which, in adulthood, is associated with increased % lean mass and increased metabolic rates. Further, 12-week-old Six3^syn males have reduced bone mineralization and a lower bone mineral density, indicating that reduced GH levels during early life cause a long-term reduction in bone mineralization.

CONCLUSION

Our study points to the novel role of Six3 in post-proliferative neurons to regulate metabolic function through SS neuron control of GH release.

The Role of Sonic Hedgehog in Human Holoprosencephaly and Short-Rib Polydactyly Syndromes

The Hedgehog (HH) signalling pathway is one of the major pathways controlling cell differentiation and proliferation during human development. This pathway is complex, with HH function influenced by inhibitors, promotors, interactions with other signalling pathways, and non-genetic and cellular factors. Many aspects of this pathway are not yet clarified. The main features of Sonic Hedgehog (SHH) signalling are discussed in relation to its function in human development. The possible role of SHH will be considered using examples of holoprosencephaly and short-rib polydactyly (SRP) syndromes. In these syndromes, there is wide variability in phenotype even with the same genetic mutation, so that other factors must influence the outcome. SHH mutations were the first identified genetic causes of holoprosencephaly, but many other genes and environmental factors can cause malformations in the holoprosencephaly spectrum. Many patients with SRP have genetic defects affecting primary cilia, structures found on most mammalian cells which are thought to be necessary for canonical HH signal transduction. Although SHH signalling is affected in both these genetic conditions, there is little overlap in phenotype. Possible explanations will be canvassed, using data from published human and animal studies. Implications for the understanding of SHH signalling in humans will be discussed.

New SHH and Known SIX3 Variants in a Series of Latin American Patients with Holoprosencephaly

Holoprosencephaly (HPE) is the failure of the embryonic forebrain to develop into 2 hemispheres promoting midline cerebral and facial defects. The wide phenotypic variability and causal heterogeneity make genetic counseling difficult. Heterozygous variants with incomplete penetrance and variable expressivity in the SHH, SIX3, ZIC2, and TGIF1 genes explain ∼25% of the known causes of nonchromosomal HPE. We studied these 4 genes and clinically described 27 Latin American families presenting with nonchromosomal HPE. Three new SHH variants and a third known SIX3 likely pathogenic variant found by Sanger sequencing explained 15% of our cases. Genotype-phenotype correlation in these 4 families and published families with identical or similar driver gene, mutated domain, conservation of residue in other species, and the type of variant explain the pathogenicity but not the phenotypic variability. Nine patients, including 2 with SHH pathogenic variants, presented benign variants of the SHH, SIX3, ZIC2, and TGIF1 genes with potential alteration of splicing, a causal proposition in need of further studies. Finding more families with the same SIX3 variant may allow further identification of genetic or environmental modifiers explaining its variable phenotypic expression.

Hedgehog proteins create a dynamic cholesterol interface

During formation of the Hedgehog (Hh) signaling proteins, cooperative activities of the Hedgehog INTein (Hint) fold and Sterol Recognition Region (SRR) couple autoproteolysis to cholesterol ligation. The cholesteroylated Hh morphogens play essential roles in embryogenesis, tissue regeneration, and tumorigenesis. Despite the centrality of cholesterol in Hh function, the full structure of the Hint-SRR ("Hog") domain that attaches cholesterol to the last residue of the active Hh morphogen remains enigmatic. In this work, we combine molecular dynamics simulations, photoaffinity crosslinking, and mutagenesis assays to model cholesterolysis intermediates in the human Sonic Hedgehog (hSHH) protein. Our results provide evidence for a hydrophobic Hint-SRR interface that forms a dynamic, non-covalent cholesterol-Hog complex. Using these models, we suggest a unified mechanism by which Hh proteins can recruit, sequester, and orient cholesterol, and offer a molecular basis for the effects of disease-causing hSHH mutations.

De novo ZIC2 frameshift variant associated with frontonasal dysplasia in a Limousin calf

Background

Bovine frontonasal dysplasias like arhinencephaly, synophthalmia, cyclopia and anophthalmia are sporadic congenital facial malformations. In this study, computed tomography, necropsy, histopathological examinations and whole genome sequencing on an Illumina NextSeq500 were performed to characterize a stillborn Limousin calf with frontonasal dysplasia. In order to identify private genetic and structural variants, we screened whole genome sequencing data of the affected calf and unaffected relatives including parents, a maternal and paternal halfsibling.

Results

The stillborn calf exhibited severe craniofacial malformations. Nose and maxilla were absent, mandibles were upwardly curved and a median cleft palate was evident. Eyes, optic nerve and orbital cavities were not developed and the rudimentary orbita showed hypotelorism. A defect centrally in the front skull covered with a membrane extended into the intracranial cavity. Aprosencephaly affected telencephalic and diencephalic structures and cerebellum. In addition, a shortened tail was seen. Filtering whole genome sequencing data revealed a private frameshift variant within the candidate gene ZIC2 in the affected calf. This variant was heterozygous mutant in this case and homozygous wild type in parents, half-siblings and controls.

Conclusions

We found a novel ZIC2 frameshift mutation in an aprosencephalic Limousin calf. The origin of this variant is most likely due to a de novo mutation in the germline of one parent or during very early embryonic development. To the authors’ best knowledge, this is the first identified mutation in cattle associated with bovine frontonasal dysplasia.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07350-y.

Developmental Genes and Malformations in the Hypothalamus

The hypothalamus is a heterogeneous rostral forebrain region that regulates physiological processes essential for survival, energy metabolism, and reproduction, mainly mediated by the pituitary gland. In the updated prosomeric model, the hypothalamus represents the rostralmost forebrain, composed of two segmental regions (terminal and peduncular hypothalamus), which extend respectively into the non-evaginated preoptic telencephalon and the evaginated pallio-subpallial telencephalon. Complex genetic cascades of transcription factors and signaling molecules rule their development. Alterations of some of these molecular mechanisms acting during forebrain development are associated with more or less severe hypothalamic and pituitary dysfunctions, which may be associated with brain malformations such as holoprosencephaly or septo-optic dysplasia. Studies on transgenic mice with mutated genes encoding critical transcription factors implicated in hypothalamic-pituitary development are contributing to understanding the high clinical complexity of these pathologies. In this review article, we will analyze first the complex molecular genoarchitecture of the hypothalamus resulting from the activity of previous morphogenetic signaling centers and secondly some malformations related to alterations in genes implicated in the development of the hypothalamus.

Of mitogens and morphogens: modelling Sonic Hedgehog mechanisms in vertebrate development

Sonic Hedgehog (Shh) Is a critical protein in vertebrate development, orchestrating patterning and growth in many developing systems. First described as a classic morphogen that patterns tissues through a spatial concentration gradient, subsequent studies have revealed a more complex mechanism, in which Shh can also regulate proliferation and differentiation. While the mechanism of action of Shh as a morphogen is well understood, it remains less clear how Shh might integrate patterning, proliferation and differentiation in a given tissue, to ultimately direct its morphogenesis. In tandem with experimental studies, mathematical modelling can help gain mechanistic insights into these processes and bridge the gap between Shh-regulated patterning and growth, by integrating these processes into a common theoretical framework. Here, we briefly review the roles of Shh in vertebrate development, focusing on its functions as a morphogen, mitogen and regulator of differentiation. We then discuss the contributions that modelling has made to our understanding of the action of Shh and highlight current challenges in using mathematical models in a quantitative and predictive way. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

Pituitary stalk interruption syndrome broadens the clinical spectrum of the TTC26 ciliopathy

Ciliopathies are a heterogeneous group of disorders, related to abnormal ciliary function. Severe biliary ciliopathy, caused by bi-allelic mutations in TTC26, has been recently described in the context of a syndrome of polydactyly and severe neonatal cholestasis, with brain, kidney and heart involvement. Pituitary involvement has not been previously reported for patients with this condition. Pituitary stalk interruption syndrome (PSIS) is a congenital anomaly of the pituitary gland, diagnosed by characteristic MRI findings. We now describe four patients with TTC26 ciliopathy due to a homozygous c.695A>G p.Asn232Ser mutation and delineate PSIS as a novel clinical feature of this disorder, highlighting an important role of TTC26 in pituitary development.

Roles for urothelium in normal and aberrant urinary tract development

Congenital anomalies of the kidney and urinary tract (CAKUTs) represent the leading cause of chronic kidney disease and end-stage kidney disease in children. Increasing evidence points to critical roles for the urothelium in the developing urinary tract and in the genesis of CAKUTs. The involvement of the urothelium in patterning the urinary tract is supported by evidence that CAKUTs can arise as a result of abnormal urothelial development. Emerging evidence indicates that congenital urinary tract obstruction triggers urothelial remodelling that stabilizes the obstructed kidney and limits renal injury. Finally, the diagnostic potential of radiological findings and urinary biomarkers derived from the urothelium of patients with CAKUTs might aid their contribution to clinical care.

Nationwide epidemiological survey of holoprosencephaly in Japan

BACKGROUND

Holoprosencephaly (HPE) is a congenital malformation with an estimated prevalence of 0.10-6.06 per 10 000 births but with no nationwide data specific to Japan.

METHODS

This nationwide retrospective questionnaire survey was conducted from 2011 to 2013. All 467 training hospitals for perinatal and neonatal care certified by the Japan Society of Perinatal and Neonatal Medicine were contacted. The birth prevalence rate (BPR) was assessed from the primary survey and clinical characteristics from the secondary survey.

RESULTS

We received valid responses from 253 hospitals in the primary survey (54.6%). Of 390 342 live births, 60 were diagnosed with HPE (23 males and 37 females), resulting in an actual BPR of 1.54 per 10 000 live births. The point estimate for HPE cases was 100 (95% confidence interval [CI]: 80.7-120), and the estimated BPR of HPE was calculated to be 0.32 per 10 000 live births (95% CI: 0.26-0.38) based on 3 117 853 live births according to Japanese national statistics during the study period. In the secondary survey, we obtained data for 49 cases (19 males and 30 females). Of these, 20 were alobar (40.8%), 20 were semilobar (40.8%), five were lobar (10.4%), and four were of unknown type. Genetic examination was performed in 37 of the 49 HPE patients and revealed that chromosomes 13, 18, and 7 were affected in eight, six, and four patients, respectively.

CONCLUSIONS

This is the most extensive survey on holoprosencephaly to date in Japan. The estimated BPR was consistent with that reported in previous research.

Lineage-specific roles of hedgehog-GLI signaling during mammalian kidney development

Aberrant hedgehog (Hh) signaling during embryogenesis results in various severe congenital abnormalities, including renal malformations. The molecular mechanisms that underlie congenital renal malformations remain poorly understood. Here, we review the current understanding of the lineage-specific roles of Hh signaling during renal morphogenesis and how aberrant Hh signaling during embryonic kidney development contributes to renal malformation.

Development of the Pituitary Gland

The development of the anterior pituitary gland occurs in distinct sequential developmental steps, leading to the formation of a complex organ containing five different cell types secreting six different hormones. During this process, the temporal and spatial expression of a cascade of signaling molecules and transcription factors plays a crucial role in organ commitment, cell proliferation, patterning, and terminal differentiation. The morphogenesis of the gland and the emergence of distinct cell types from a common primordium are governed by complex regulatory networks involving transcription factors and signaling molecules that may be either intrinsic to the developing pituitary or extrinsic, originating from the ventral diencephalon, the oral ectoderm, and the surrounding mesenchyme. Endocrine cells of the pituitary gland are organized into structural and functional networks that contribute to the coordinated response of endocrine cells to stimuli; these cellular networks are formed during embryonic development and are maintained or may be modified in adulthood, contributing to the plasticity of the gland. Abnormalities in any of the steps of pituitary development may lead to congenital hypopituitarism that includes a spectrum of disorders from isolated to combined hormone deficiencies including syndromic disorders such as septo-optic dysplasia. Over the past decade, the acceleration of next-generation sequencing has allowed for rapid analysis of the patient genome to identify novel mutations and novel candidate genes associated with hypothalmo-pituitary development. Subsequent functional analysis using patient fibroblast cells, and the generation of stem cells derived from patient cells, is fast replacing the need for animal models while providing a more physiologically relevant characterization of novel mutations. Furthermore, CRISPR-Cas9 as the method for gene editing is replacing previous laborious and time-consuming gene editing methods that were commonly used, thus yielding knockout cell lines in a fraction of the time. © 2020 American Physiological Society. Compr Physiol 10:389-413, 2020.

The Homeodomain Transcription Factors Vax1 and Six6 Are Required for SCN Development and Function

The brain's primary circadian pacemaker, the suprachiasmatic nucleus (SCN), is required to translate day-length and circadian rhythms into neuronal, hormonal, and behavioral rhythms. Here, we identify the homeodomain transcription factor ventral anterior homeobox 1 (Vax1) as required for SCN development, vasoactive intestinal peptide expression, and SCN output. Previous work has shown that VAX1 is required for gonadotropin-releasing hormone (GnRH/LHRH) neuron development, a neuronal population controlling reproductive status. Surprisingly, the ectopic expression of a Gnrh-Cre allele (Gnrh^cre) in the SCN confirmed the requirement of both VAX1 (Vax1^flox/flox:Gnrh^cre, Vax1^Gnrh-cre) and sine oculis homeobox protein 6 (Six6^flox/flox:Gnrh^cre, Six6^Gnrh-cre) in SCN function in adulthood. To dissociate the role of Vax1 and Six6 in GnRH neuron and SCN function, we used another Gnrh-cre allele that targets GnRH neurons, but not the SCN (Lhrh^cre). Both Six6^Lhrh-cre and Vax1^Lhrh-cre were infertile, and in contrast to Vax1^Gnrh-cre and Six6^Gnrh-cre mice, Six6^Lhrh-cre and Vax1^Lhrh-cre had normal circadian behavior. Unexpectedly, ~ 1/4 of the Six6^Gnrh-cre mice were unable to entrain to light, showing that ectopic expression of Gnrh^cre impaired function of the retino-hypothalamic tract that relays light information to the brain. This study identifies VAX1, and confirms SIX6, as transcription factors required for SCN development and function and demonstrates the importance of understanding how ectopic CRE expression can impact the results.

Hedgehog Signal and Genetic Disorders

The hedgehog (Hh) family comprises sonic hedgehog (Shh), Indian hedgehog (Ihh), and desert hedgehog (Dhh), which are versatile signaling molecules involved in a wide spectrum of biological events including cell differentiation, proliferation, and survival; establishment of the vertebrate body plan; and aging. These molecules play critical roles from embryogenesis to adult stages; therefore, alterations such as abnormal expression or mutations of the genes involved and their downstream factors cause a variety of genetic disorders at different stages. The Hh family involves many signaling mediators and functions through complex mechanisms, and achieving a comprehensive understanding of the entire signaling system is challenging. This review discusses the signaling mediators of the Hh pathway and their functions at the cellular and organismal levels. We first focus on the roles of Hh signaling mediators in signal transduction at the cellular level and the networks formed by these factors. Then, we analyze the spatiotemporal pattern of expression of Hh pathway molecules in tissues and organs, and describe the phenotypes of mutant mice. Finally, we discuss the genetic disorders caused by malfunction of Hh signaling-related molecules in humans.

Pituitary Transcription Factor Mutations Leading to Hypopituitarism

Congenital pituitary hormone deficiency is a disabling condition. It is part of a spectrum of disorders including craniofacial midline developmental defects ranging from holoprosencephaly through septo-optic dysplasia to combined and isolated pituitary hormone deficiency. The first genes discovered in the human disease were based on mouse models of dwarfism due to mutations in transcription factor genes. High-throughput DNA sequencing technologies enabled clinicians and researchers to find novel genetic causes of hypopituitarism for the more than three quarters of patients without a known genetic diagnosis to date. Transcription factor (TF) genes are at the forefront of the functional analysis of novel variants of unknown significance due to the relative ease in in vitro testing in a research lab. Genetic testing in hypopituitarism is of high importance to the individual and their family to predict phenotype composition, disease progression and to avoid life-threatening complications such as secondary adrenal insufficiency.This chapter aims to highlight our current understanding about (1) the contribution of TF genes to pituitary development (2) the diversity of inheritance and phenotype features in combined and select isolated pituitary hormone deficiency and (3) provide an initial assessment on how to approach variants of unknown significance in human hypopituitarism. Our better understanding on how transcription factor gene variants lead to hypopituitarism is a meaningful step to plan advanced therapies to specific genetic changes in the future.

Haploinsufficiency of SIX3 Abolishes Male Reproductive Behavior Through Disrupted Olfactory Development, and Impairs Female Fertility Through Disrupted GnRH Neuron Migration

Mating behavior in males and females is dependent on olfactory cues processed through both the main olfactory epithelium (MOE) and the vomeronasal organ (VNO). Signaling through the MOE is critical for the initiation of male mating behavior, and the loss of MOE signaling severely compromises this comportment. Here, we demonstrate that dosage of the homeodomain gene Six3 affects the degree of development of MOE but not the VNO. Anomalous MOE development in Six3 heterozygote mice leads to hyposmia, specifically disrupting male mounting behavior by impairing detection of volatile female estrus pheromones. Six3 is highly expressed in the MOE, main olfactory bulb (MOB), and hypothalamus; all regions essential in the proper migration of the gonadotropin-releasing hormone (GnRH) neurons, a key reproductive neuronal population that migrates along olfactory axons from the developing nose into the brain. Interestingly, we find that the reduction in Six3 expression in Six3 heterozygote mice compromises development of the MOE and MOB, resulting in mis-migration of GnRH neurons due to improper olfactory axon targeting. This reduction in the hypothalamic GnRH neuron population, by 45% in adulthood, leads to female subfertility, but does not impact male hormone levels, suggesting that male infertility is not related to GnRH neuron numbers, but exclusively linked to abnormal olfaction. We here determine that Six3 is haploinsufficient for MOE development, GnRH neuron migration, and fertility, and represents a novel candidate gene for Kallmann syndrome, a form of inherited infertility.

Molecular testing in holoprosencephaly

Holoprosencephaly (HPE) is a structural brain anomaly characterized by failure of the forebrain to separate during early embryogenesis. Both genetic and environmental etiologies of HPE have been discovered over the last three decades. Traditionally, the genetic workup for HPE has been a karyotype, chromosomal microarray, and/or Sanger sequencing of select genes. The recent increased availability of next-generation sequencing has changed the molecular diagnostic landscape for HPE, associating new genes with this disorder such as FGFR1. We conducted a systematic review of the medical literature for the molecular testing of HPE for studies published in the last 20 years. We also queried known commercial diagnostic laboratories and used information on their websites to construct a list of available commercial testing. Our group released its first recommendations in 2010 and this update incorporates the technology shifts and gene discoveries over the last decade. These recommendations provide a guide for genetic diagnosis of HPE, which is paramount for patients and their families for prognosis, treatment, and genetic counseling.

Extracephalic manifestations of nonchromosomal, nonsyndromic holoprosencephaly

Nonchromosomal, nonsyndromic holoprosencephaly (NCNS-HPE) has traditionally been considered as a condition of brain and craniofacial maldevelopment. In this review, we present the results of a comprehensive literature search supporting a wide spectrum of extracephalic manifestations identified in patients with NCNS-HPE. These manifestations have been described in case reports and in large cohorts of patients with "single-gene" mutations, suggesting that the NCNS-HPE phenotype can be more complex than traditionally thought. Likely, a complex network of interacting genetic variants and environmental factors is responsible for these systemic abnormalities that deviate from the usual brain and craniofacial findings in NCNS-HPE. In addition to the systemic consequences of pituitary dysfunction (as a direct result of brain midline defects), here we describe a number of extracephalic findings of NCNS-HPE affecting various organ systems. It is our goal to provide a guide of extracephalic features for clinicians given the important clinical implications of these manifestations for the management and care of patients with HPE and their mutation-positive relatives. The health risks associated with some manifestations (e.g., fatty liver disease) may have historically been neglected in affected families.

Clinical and experimental evidence suggest a link between KIF7 and C5orf42-related ciliopathies through Sonic Hedgehog signaling

Acrocallosal syndrome (ACLS) is an autosomal recessive neurodevelopmental disorder caused by KIF7 defects and belongs to the heterogeneous group of ciliopathies related to Joubert syndrome (JBTS). While ACLS is characterized by macrocephaly, prominent forehead, depressed nasal bridge, and hypertelorism, facial dysmorphism has not been emphasized in JBTS cohorts with molecular diagnosis. To evaluate the specificity and etiology of ACLS craniofacial features, we performed whole exome or targeted Sanger sequencing in patients with the aforementioned overlapping craniofacial appearance but variable additional ciliopathy features followed by functional studies. We found (likely) pathogenic variants of KIF7 in 5 out of 9 families, including the original ACLS patients, and delineated 1000 to 4000-year-old Swiss founder alleles. Three of the remaining families had (likely) pathogenic variants in the JBTS gene C5orf42, and one patient had a novel de novo frameshift variant in SHH known to cause autosomal dominant holoprosencephaly. In accordance with the patients' craniofacial anomalies, we showed facial midline widening after silencing of C5orf42 in chicken embryos. We further supported the link between KIF7, SHH, and C5orf42 by demonstrating abnormal primary cilia and diminished response to a SHH agonist in fibroblasts of C5orf42-mutated patients, as well as axonal pathfinding errors in C5orf42-silenced chicken embryos similar to those observed after perturbation of Shh signaling. Our findings, therefore, suggest that beside the neurodevelopmental features, macrocephaly and facial widening are likely more general signs of disturbed SHH signaling. Nevertheless, long-term follow-up revealed that C5orf42-mutated patients showed catch-up development and fainting of facial features contrary to KIF7-mutated patients.

Multisystem Involvement in a Patient with a PTCH1 Mutation: Clinical and Imaging Findings

In this article, we report on a Brazilian female patient born to consanguineous parents and presenting with alobar holoprosencephaly, severe eye involvement, and unusual skin hyperpigmented lesions. She was found to have a mutation (c.2240T > C; p.Val751Gly) in exon 15 of the PTCH1 gene. Mutations in this gene are associated with the nevoid basal cell carcinoma syndrome (NBCCS, OMIM 109400) and, in other instances, with holoprosencephaly (holoprosencephaly-7, OMIM 610828). Severe eye involvement ranging from orbital coloboma to microphthalmia has been seldom reported in patients with NBCCS with PTCH1 mutations. To our knowledge, this is the first report of an individual with central nervous system, skin, and eye manifestations due to a PTCH1 mutation. Mechanisms involved in these multisystem manifestations are discussed.

Conserved genetic pathways associated with microphthalmia, anophthalmia, and coloboma

The human eye is a complex organ whose development requires extraordinary coordination of developmental processes. The conservation of ocular developmental steps in vertebrates suggests possible common genetic mechanisms. Genetic diseases involving the eye represent a leading cause of blindness in children and adults. During the last decades, there has been an exponential increase in genetic studies of ocular disorders. In this review, we summarize current success in identification of genes responsible for microphthalmia, anophthalmia, and coloboma (MAC) phenotypes, which are associated with early defects in embryonic eye development. Studies in animal models for the orthologous genes identified overlapping phenotypes for most factors, confirming the conservation of their function in vertebrate development. These animal models allow for further investigation of the mechanisms of MAC, integration of various identified genes into common developmental pathways and finally, provide an avenue for the development and testing of therapeutic interventions.

Adrenocortical growth and cancer

The adrenal gland consists of two distinct parts, the cortex and the medulla. Molecular mechanisms controlling differentiation and growth of the adrenal gland have been studied in detail using mouse models. Knowledge also came from investigations of genetic disorders altering adrenal development and/or function. During embryonic development, the adrenal cortex acquires a structural and functional zonation in which the adrenal cortex is divided into three different steroidogenic zones. Significant progress has been made in understanding adrenal zonation. Recent lineage tracing experiments have accumulated evidence for a centripetal differentiation of adrenocortical cells from the subcapsular area to the inner part of the adrenal cortex. Understanding of the mechanism of adrenocortical cancer (ACC) development was stimulated by knowledge of adrenal gland development. ACC is a rare cancer with a very poor overall prognosis. Abnormal activation of the Wnt/β-catenin as well as the IGF2 signaling plays an important role in ACC development. Studies examining rare genetic syndromes responsible for familial ACT have played an important role in identifying genetic alterations in these tumors (like TP53 or CTNNB1 mutations as well as IGF2 overexpression). Recently, genomic analyses of ACT have shown gene expression profiles associated with malignancy as well as chromosomal and methylation alterations in ACT and exome sequencing allowed to describe the mutational landscape of these tumors. This progress leads to a new classification of these tumors, opening new perspectives for the diagnosis and prognostication of ACT. This review summarizes current knowledge of adrenocortical development, growth, and tumorigenesis.

Endocrinopathies in a boy with cryptic copy-number variations on 4q, 7q and Xp

We report a male patient with three copy-number variations (CNVs) and unique phenotype. He carried ~11.2 Mb terminal duplication on 4q, ~13.4 Mb terminal deletion on 7q and ~1.7 Mb interstitial duplication on Xp22.31, which were identified by array-based comparative genomic hybridization. He manifested mental retardation, mild brain anomalies and skeletal deformities ascribable to these CNVs, together with central precocious puberty and mild adrenocorticotropic hormone overproduction of unknown etiologies.

Holoprosencephaly: signaling interactions between the brain and the face, the environment and the genes, and the phenotypic variability in animal models and humans

Holoprosencephaly (HPE) is the most common developmental defect of the forebrain characterized by inadequate or absent midline division of the forebrain into cerebral hemispheres, with concomitant midline facial defects in the majority of cases. Understanding the pathogenesis of HPE requires knowledge of the relationship between the developing brain and the facial structures during embryogenesis. A number of signaling pathways control and coordinate the development of the brain and face, including Sonic hedgehog, Bone morphogenetic protein, Fibroblast growth factor, and Nodal signaling. Mutations in these pathways have been identified in animal models of HPE and human patients. Because of incomplete penetrance and variable expressivity of HPE, patients carrying defined mutations may not manifest the disease at all, or have a spectrum of defects. It is currently unknown what drives manifestation of HPE in genetically at-risk individuals, but it has been speculated that other gene mutations and environmental factors may combine as cumulative insults. HPE can be diagnosed in utero by a high-resolution prenatal ultrasound or a fetal magnetic resonance imaging, sometimes in combination with molecular testing from chorionic villi or amniotic fluid sampling. Currently, there are no effective preventive methods for HPE. Better understanding of the mechanisms of gene-environment interactions in HPE would provide avenues for such interventions.

Molecular analysis of holoprosencephaly in South America

Holoprosencephaly (HPE) is a spectrum of brain and facial malformations primarily reflecting genetic factors, such as chromosomal abnormalities and gene mutations. Here, we present a clinical and molecular analysis of 195 probands with HPE or microforms; approximately 72% of the patients were derived from the Latin American Collaborative Study of Congenital Malformations (ECLAMC), and 82% of the patients were newborns. Alobar HPE was the predominant brain defect in almost all facial defect categories, except for patients without oral cleft and median or lateral oral clefts. Ethmocephaly, cebocephaly, and premaxillary agenesis were primarily observed among female patients. Premaxillary agenesis occurred in six of the nine diabetic mothers. Recurrence of HPE or microform was approximately 19%. The frequency of microdeletions, detected using Multiplex Ligation-dependant Probe Amplification (MLPA) was 17% in patients with a normal karyotype. Cytogenetics or QF-PCR analyses revealed chromosomal anomalies in 27% of the probands. Mutational analyses in genes SHH, ZIC2, SIX3 and TGIF were performed in 119 patients, revealing eight mutations in SHH, two mutations in SIX3 and two mutations in ZIC2. Thus, a detailed clinical description of new HPE cases with identified genetic anomalies might establish genotypic and phenotypic correlations and contribute to the development of additional strategies for the analysis of new cases.

Transcriptional landscape of the prenatal human brain

The anatomical and functional architecture of the human brain is largely determined by prenatal transcriptional processes. We describe an anatomically comprehensive atlas of mid-gestational human brain, including de novo reference atlases, in situ hybridization, ultra-high resolution magnetic resonance imaging (MRI) and microarray analysis on highly discrete laser microdissected brain regions. In developing cerebral cortex, transcriptional differences are found between different proliferative and postmitotic layers, wherein laminar signatures reflect cellular composition and developmental processes. Cytoarchitectural differences between human and mouse have molecular correlates, including species differences in gene expression in subplate, although surprisingly we find minimal differences between the inner and human-expanded outer subventricular zones. Both germinal and postmitotic cortical layers exhibit fronto-temporal gradients, with particular enrichment in frontal lobe. Finally, many neurodevelopmental disorder and human evolution-related genes show patterned expression, potentially underlying unique features of human cortical formation. These data provide a rich, freely-accessible resource for understanding human brain development.

Rare nasal cleft in a patient with holoprosencephaly due to a mutation in the ZIC2 gene

BACKGROUND

Holoprosencephaly (HPE) is a spectrum of midline malformations of the prosencephalon generally reflected in a continuum of midline facial anomalies. Patients with mutation in the ZIC2 gene usually present a normal or mildly dysmorphic face associated with a severe brain malformation. Here we present a rare unilateral nasal cleft (Tessier cleft n. 1) with holoprosencephaly in a patient with a ZIC2 mutation.

CASE

The male newborn presented with alobar HPE, microcephaly, ocular hypertelorism, upslanting palpebral fissures, a bulky nose with a left paramedian alar cleft. Mutational screening for HPE genes revealed the occurrence of a frameshift mutation in the ZIC2 gene. The mutation was inherited from the father who presented only mild ocular hypotelorism but had an affected child with HPE from his first marriage.

CONCLUSION

The occurrence of oral clefts is common in patients with HPE, but unusual in patients with mutation in the ZIC2 gene. To our knowledge, clefts of the nasal alae have been reported only once or twice in patients with ZIC2 mutations. In documented patients from the literature, only 2% of individuals with described pathogenic mutations in the ZIC2 gene (3/171) presented facial clefts, one of them a nasal cleft, while common oral clefts were observed in 27% of individuals (7/26) described with nonpathogenic ZIC2 mutations or presenting a concomitant mutation in another HPE gene. When compared with the general population, nasal clefts are common in ZIC2 mutations and these mutations must be searched for in undiagnosed cases.

Sox2 is required for embryonic development of the ventral telencephalon through the activation of the ventral determinants Nkx2.1 and Shh

The Sox2 transcription factor is active in stem/progenitor cells throughout the developing vertebrate central nervous system. However, its conditional deletion at E12.5 in mouse causes few brain developmental problems, with the exception of the postnatal loss of the hippocampal radial glia stem cells and the dentate gyrus. We deleted Sox2 at E9.5 in the telencephalon, using a Bf1-Cre transgene. We observed embryonic brain defects that were particularly severe in the ventral, as opposed to the dorsal, telencephalon. Important tissue loss, including the medial ganglionic eminence (MGE), was detected at E12.5, causing the subsequent impairment of MGE-derived neurons. The defect was preceded by loss of expression of the essential ventral determinants Nkx2.1 and Shh, and accompanied by ventral spread of dorsal markers. This phenotype is reminiscent of that of mice mutant for the transcription factor Nkx2.1 or for the Shh receptor Smo. Nkx2.1 is known to mediate the initial activation of ventral telencephalic Shh expression. A partial rescue of the normal phenotype at E14.5 was obtained by administration of a Shh agonist. Experiments in Medaka fish indicate that expression of Nkx2.1 is regulated by Sox2 in this species also. We propose that Sox2 contributes to Nkx2.1 expression in early mouse development, thus participating in the region-specific activation of Shh, thereby mediating ventral telencephalic patterning induction.

Pituitary stalk interruption syndrome and isolated pituitary hypoplasia may be caused by mutations in holoprosencephaly-related genes

CONTEXT

Holoprosencephaly (HPE) is a developmental defect characterized by wide phenotypic variability, ranging from minor midline malformations (eg, single central incisor) to severe deformities. In 10-15% of HPE patients, mutations in specific genes have been identified (eg, SHH, TGIF, SIX3). Pituitary stalk interruption syndrome (PSIS) constitutes a distinct abnormality of unknown pathogenesis, whereas isolated pituitary hypoplasia (IPH) has been linked to various developmental genes.

OBJECTIVE

Three of our patients with PSIS had a single central incisor, a malformation encountered in some HPE cases. Based on this observation, we initiated a search for mutations in HPE-associated genes in 30 patients with PSIS or IPH.

DESIGN AND PARTICIPANTS

The entire coding region of the TGIF, SHH, and SIX3 genes was sequenced in patients with combined pituitary hormone deficiency associated with either PSIS or IPH and in healthy controls.

RESULTS

Two novel mutations in the HPE-related genes were detected (ie, c.799 C>T, p.Q267X in the TGIF gene, and c.1279G>A, p.G427R in the SHH gene) in 2 of our patients. The overall incidence of HPE-related gene mutations in our nonsyndromic and nonchromosomal patients was 6.6%. No molecular defect in the SIX3 gene was detected in our cohort.

CONCLUSIONS

The data suggest that HPE-related gene mutations are implicated in the etiology of isolated pituitary defects (PSIS or IPH). Alternatively, PSIS or IPH may constitute mild forms of an expanded HPE spectrum.

The Xenopus homeobox gene pitx3 impinges upon somitogenesis and laterality

Pitx3 has been identified as the causative locus in a developmental eye mutation associated with mammalian anterior segment dysgenesis, congenital cataracts, and aphakia. In recent studies of frog eye development we discovered that pitx3 expresses symmetrically in the somites and lateral plate mesoderm and asymmetrically during cardiac and gut looping. We report that disruption of pitx3 activity on one side of an embryo relative to the other, either by over- or underexpression of pitx3, elicits a crooked dorsal axis in embryos that is a consequence of a retarded progression through somitogenesis. Unlike in amniotes, Xenopus somites form as cohorts of presomitic cells that rotate perpendicular to the dorsal axis. Since no vertebral anomalies have been reported in mouse and human Pitx3 mutants, we attempt to distinguish whether the segmentation clock is uniquely affected in frog or if the pitx3 perturbation inhibits the cellular changes that are necessary to rotation of presomitic cells. In Xenopus, pitx3 appears to inhibit the rotation of presomitic cell cohorts and to be necessary to the bilaterally symmetric expression of pitx2 in somites.

Single-nucleotide variants in two Hedgehog genes, SHH and HHIP, as genetic cause of combined pituitary hormone deficiency

OBJECTIVE

Combined pituitary hormone deficiency (CPHD) is characterized by deficiencies of two or more anterior pituitary hormones. Its genetic cause is unknown in the majority of cases. The Hedgehog (Hh) signalling pathway has been implicated in disorders associated with pituitary development. Mutations in Sonic Hedgehog (SHH) have been described in patients with holoprosencephaly (with or without pituitary involvement). Hedgehog interacting protein (HHIP) has been associated with variations in adult height in genome wide association studies. We investigated whether mutations in these two genes of the Hh pathway, SHH and HHIP, could result in 'idiopathic' CPHD.

DESIGN/PATIENTS

We directly sequenced the coding regions and exon - intron boundaries of SHH and HHIP in 93 CPHD patients of the Dutch HYPOPIT study in whom mutations in the classical CPHD genes PROP1, POU1F1, HESX1, LHX3 and LHX4 had been ruled out. We compared the expression of Hh genes in Hep3B transfected cells between wild-type proteins and mutants.

RESULTS

We identified three single-nucleotide variants (p.Ala226Thr, c.1078C>T and c.*8G>T) in SHH. The function of the latter was severely affected in our in vitro assay. In HHIP, we detected a new activating variant c.-1G>C, which increases HHIP's inhibiting function on the Hh pathway.

CONCLUSIONS

Our results suggest involvement of the Hedgehog pathway in CPHD. We suggest that both SHH and HHIP are investigated as a second screening in CPHD, after mutations in the classical CPHD genes have been ruled out.

NODAL and SHH dose-dependent double inhibition promotes an HPE-like phenotype in chick embryos

Holoprosencephaly (HPE) is a common congenital defect that results from failed or incomplete forebrain cleavage. HPE is characterized by a wide clinical spectrum, with inter- and intrafamilial variability. This heterogeneity is not well understood and it has been suggested that HPE involves a combination of multiple gene mutations. In this model, several mutated alleles or modifying factors are presumed to act in synergy to cause and determine the severity of HPE. This could explain the various clinical phenotypes. Screening for HPE-associated genes in humans suggests the involvement of NODAL or SHH signaling, or both. To test this multigenic hypothesis, we investigated the effects of chemical inhibition of these two main HPE signaling pathways in a chick embryo model. SB-505124, a selective inhibitor of transforming growth factor-B type I receptors was used to inhibit the NODAL pathway. Cyclopamine was used to inhibit the SHH pathway. We report that both inhibitors caused HPE-like defects that were dependent on the drug concentration and on the developmental stage at the time of treatment. We also investigated double inhibition of NODAL and SHH pathways from the onset of gastrulation by using subthreshold inhibitor concentrations. The inhibitors of the NODAL and SHH pathways, even at low concentration, acted synergistically to promote an HPE-like phenotype. These findings support the view that genetic heterogeneity is important in the etiology of HPE and may contribute to the phenotypic variability.

Utilizing prospective sequence analysis of SHH, ZIC2, SIX3 and TGIF in holoprosencephaly probands to describe the parameters limiting the observed frequency of mutant gene×gene interactions

Clinical molecular diagnostic centers routinely screen SHH, ZIC2, SIX3 and TGIF for mutations that can help to explain holoprosencephaly and related brain malformations. Here we report a prospective Sanger sequence analysis of 189 unrelated probands referred to our diagnostic lab for genetic testing. We identified 28 novel unique mutations in this group (15%) and no instances of deleterious mutations in two genes in the same subject. Our result extends that of other diagnostic centers and suggests that among the aggregate 475 prospectively sequenced holoprosencephaly probands there is negligible evidence for direct gene-gene interactions among these tested genes. We model the predictions of the observed mutation frequency in the context of the hypothesis that gene×gene interactions are a prerequisite for forebrain malformations, i.e. the "multiple-hit" hypothesis. We conclude that such a direct interaction would be expected to be rare and that more subtle genetic and environmental interactions are a better explanation for the clinically observed inter- and intra-familial variability.

New findings for phenotype-genotype correlations in a large European series of holoprosencephaly cases

BACKGROUND

Holoprosencephaly (HPE) is the most common forebrain defect in humans. It results from incomplete midline cleavage of the prosencephalon.

METHODS

A large European series of 645 HPE probands (and 699 relatives), consisting of 51% fetuses and 49% liveborn children, is reported.

RESULTS

Mutations in the four main genes involved in HPE (SHH, ZIC2, SIX3, TGIF) were identified in 25% of cases. The SHH, SIX3, and TGIF mutations were inherited in more than 70% of these cases, whereas 70% of the mutations in ZIC2 occurred de novo. Moreover, rearrangements were detected in 22% of the 260 patients screened by array comparative genomic hybridisation. 15 probands had two mutations providing additional support for the 'multiple-hit process' in HPE. There was a positive correlation between the severity of the brain malformation and facial features for SHH, SIX3, and TGIF, but no such correlation was found for ZIC2 mutations. The most severe HPE types were associated with SIX3 and ZIC2 mutations, whereas microforms were associated with SHH mutations. The study focused on the associated brain malformations, including neuronal migration defects, which predominated in individuals with ZIC2 mutations, and neural tube defects, which were frequently associated with ZIC2 (rachischisis) and TGIF mutations. Extracraniofacial features were observed in 27% of the individuals in this series (up to 40% of those with ZIC2 mutations) and a significant correlation was found between renal/urinary defects and mutations of SHH and ZIC2.

CONCLUSIONS

An algorithm is proposed based on these new phenotype-genotype correlations, to facilitate molecular analysis and genetic counselling for HPE.

Control of mammalian kidney development by the Hedgehog signaling pathway

The kidney is the most common site of congenital malformations that result in impaired renal function. Yet, the molecular mechanisms that control renal malformations are poorly understood. The Hedgehog signaling pathway plays critical roles during mammalian organogenesis. Aberrant Hedgehog signaling results in severe congenital abnormalities, including renal malformations. Here, we review the current body of knowledge on Hedgehog signaling during renal morphogenesis and highlight the gaps in our understanding. Furthermore, we propose mechanisms by which Hedgehog signaling contributes to both normal and abnormal renal development.

Zic2 hypomorphic mutant mice as a schizophrenia model and ZIC2 mutations identified in schizophrenia patients

ZIC2 is a causal gene for holoprosencephaly and encodes a zinc-finger-type transcriptional regulator. We characterized Zic2^kd/+ mice with a moderate (40%) reduction in Zic2 expression. Zic2^kd/+ mice showed increased locomotor activity in novel environments, cognitive and sensorimotor gating dysfunctions, and social behavioral abnormalities. Zic2^kd/+ brain involved enlargement of the lateral ventricle, thinning of the cerebral cortex and corpus callosum, and decreased number of cholinergic neurons in the basal forebrain. Because these features are reminiscent of schizophrenia, we examined ZIC2 variant-carrying allele frequencies in schizophrenia patients and in controls in the Japanese population. Among three novel missense mutations in ZIC2, R409P was only found in schizophrenia patients, and was located in a strongly conserved position of the zinc finger domain. Mouse Zic2 with the corresponding mutation showed lowered transcription-activating capacity and had impaired target DNA-binding and co-factor-binding capacities. These results warrant further study of ZIC2 in the pathogenesis of schizophrenia.

Minimal evidence for a direct involvement of twisted gastrulation homolog 1 (TWSG1) gene in human holoprosencephaly

Holoprosencephaly (HPE) is the most common disorder of human forebrain and facial development. Presently understood etiologies include both genetic and environmental factors, acting either alone, or more likely, in combination. The majority of patients without overt chromosomal abnormalities or recognizable associated syndromes have unidentified etiologies. A potential candidate gene, Twisted Gastrulation Homolog 1 (TWSG1), was previously suggested as a contributor to the complex genetics of human HPE based on (1) cytogenetic studies of patients with 18p deletions, (2) animal studies of TWSG1 deficient mice, and (3) the relationship of TWSG1 to bone morphogenetic protein (BMP) signaling, which modulates the primary pathway implicated in HPE, Sonic Hedgehog (SHH) signaling. Here we present the first analysis of a large cohort of patients with HPE for coding sequence variations in TWSG1. We also performed fine mapping of 18p for a subset of patients with partial 18p deletions. Surprisingly, minimal evidence for alterations of TWSG1 was found, suggesting that sequence alterations of TWSG1 are neither a common direct cause nor a frequent modifying factor for human HPE pathologies.

NOTCH, a new signaling pathway implicated in holoprosencephaly

Genetics of Holoprosencephaly (HPE), a congenital malformation of the developing human forebrain, is due to multiple genetic defects. Most genes that have been implicated in HPE belong to the sonic hedgehog signaling pathway. Here we describe a new candidate gene isolated from array comparative genomic hybridization redundant 6qter deletions, DELTA Like 1 (DLL1), which is a ligand of NOTCH. We show that DLL1 is co-expressed in the developing chick forebrain with Fgf8. By treating chick embryos with a pharmacological inhibitor, we demonstrate that DLL1 interacts with FGF signaling pathway. Moreover, a mutation analysis of DLL1 in HPE patients revealed a three-nucleotide deletion. These various findings implicate DLL1 in early patterning of the forebrain and identify NOTCH as a new signaling pathway involved in HPE.

Holoprosencephaly: recommendations for diagnosis and management

PURPOSE OF REVIEW

This review presents recent advances in our understanding and clinical management of holoprosencephaly (HPE). HPE is the most common developmental disorder of the human forebrain and involves incomplete or failed separation of the cerebral hemispheres. The epidemiology, clinical features, causes, diagnostic approach, management, and outcomes of HPE are discussed.

RECENT FINDINGS

Chromosomal abnormalities account for the most commonly identified cause of HPE. However, there are often unidentifiable causes in patients with nonsyndromic, nonchromosomal forms of HPE. The prevalence of HPE may be underestimated given that patients with mild forms often are not diagnosed until they present with severely affected children. Pregestational maternal diabetes mellitus is the most recognized risk factor for HPE, as supported by recent large-scale epidemiological studies. Genetic studies using microarray-based comparative genomic hybridization technology have resulted in better characterization of important HPE loci.

SUMMARY

HPE encompasses a wide spectrum of forebrain and midline defects, with an accompanying wide spectrum of clinical manifestations. A coordinated, multidisciplinary care team is required for clinical management of this complex disorder. Further research will enable us to better understand the pathogenesis and causes of HPE, and thus to improve the genetic counseling of patients and their families.

Hedgehog signaling update

In vertebrate hedgehog signaling, hedgehog ligands are processed to become bilipidated and then multimerize, which allows them to leave the signaling cell via Dispatched 1 and become transported via glypicans and megalin to the responding cells. Hedgehog then interacts with a complex of Patched 1 and Cdo/Boc, which activates endocytic Smoothened to the cilium. Patched 1 regulates the activity of Smoothened (1) via Vitamin D3, which inhibits Smoothened in the absence of hedgehog ligand or (2) via oxysterols, which activate Smoothened in the presence of hedgehog ligand. Hedgehog ligands also interact with Hip1, Patched 2, and Gas1, which regulate the range as well as the level of hedgehog signaling. In vertebrates, Smoothened is shortened at its C-terminal end and lacks most of the phosphorylation sites of importance in Drosophila. Cos2, also of importance in Drosophila, plays no role in mammalian transduction, nor do its homologs Kif7 and Kif27. The cilium may provide a function analogous to that of Cos2 by linking Smoothened to the modulation of Gli transcription factors. Disorders associated with the hedgehog signaling network follow, including nevoid basal cell carcinoma syndrome, holoprosencephaly, Smith-Lemli-Opitz syndrome, Greig cephalopolysyndactyly syndrome, Pallister-Hall syndrome, Carpenter syndrome, and Rubinstein-Taybi syndrome.

The canonical Wnt/β-catenin signaling pathway regulates Fgf signaling for early facial development

The canonical Wnt/β-catenin signaling pathway has implications in early facial development; yet, its function and signaling mechanism remain poorly understood. We report here that the frontonasal and upper jaw primordia cannot be formed after conditional ablation of β-catenin with Foxg1-Cre mice in the facial ectoderm and the adjacent telencephalic neuroepithelium. Gene expression of several cell-survival and patterning factors, including Fgf8, Fgf3, and Fgf17, is dramatically diminished in the anterior neural ridge (ANR, a rostral signaling center) and/or the adjacent frontonasal ectoderm of the β-catenin conditional mutant mice. In addition, Shh expression is diminished in the ventral telencephalon of the mutants, while Tcfap2a expression is less affected in the facial primordia. Apoptosis occurs robustly in the rostral head tissues following inactivation of Fgf signaling in the conditional mutants. Consequently, the upper jaw, nasal, ocular and telencephalic structures are absent, but the tongue and mandible are relatively developed in the conditional mutants at birth. Using molecular biological approaches, we demonstrate that the Fgf8 gene is transcriptionally targeted by Wnt/β-catenin signaling during early facial and forebrain development. Furthermore, we show that conditional gain-of-function of β-catenin signaling causes drastic upregulation of Fgf8 mRNA in the ANR and the entire facial ectoderm, which also arrests facial and forebrain development. Taken together, our results suggest that canonical Wnt/β-catenin signaling is required for early development of the mammalian face and related head structures, which mainly or partly acts through the initiation and modulation of balanced Fgf signaling activity.

Clinical features and outcomes of holoprosencephaly in Korea

The clinical spectrum of holoprosencephaly is broad, and its etiology is heterogeneous. To investigate the clinical spectrum of holoprosencephaly in Korea, we performed a database analysis of 55 cases of holoprosencephaly, including 12 diagnosed postnatally, all from a single institution. The 55 patients were categorized into several types: 37 alobar, eight semilobar, eight lobar, and two middle interhemispheric variant. Associated brain (41.8%) and craniofacial (74.5%) features varied substantially. Of 40 patients studied according to karyotype, chromosomal aberrations were detected in 18 (45.0%). Twenty-seven (49.1%) patients, diagnosed postnatally, exhibited milder types of holoprosencephaly and less profound craniofacial malformations than in prenatal diagnoses. Moreover, in postnatally diagnosed patients, the subgroup surviving longer than 1 month also exhibited a milder holoprosencephaly type and lower incidence of associated craniofacial malformations. The most frequent clinical signs in living children with holoprosencephaly included microcephaly, global developmental delay, and seizures. Holoprosencephaly represents a heterogeneous entity with different clinical manifestations and etiologies. A high index of suspicion, coupled with appropriate imaging studies, can enable accurate diagnoses and prognoses of holoprosencephaly.

Aberrant forebrain signaling during early development underlies the generation of holoprosencephaly and coloboma

In this review, we highlight recent literature concerning the signaling mechanisms underlying the development of two neural birth defects, holoprosencephaly and coloboma. Holoprosencephaly, the most common forebrain defect, occurs when the cerebral hemispheres fail to separate and is typically associated with mispatterning of embryonic midline tissue. Coloboma results when the choroid fissure in the eye fails to close. It is clear that Sonic hedgehog (Shh) signaling regulates both forebrain and eye development, with defects in Shh, or components of the Shh signaling cascade leading to the generation of both birth defects. In addition, other intercellular signaling pathways are known factors in the incidence of holoprosencephaly and coloboma. This review will outline recent advances in our understanding of forebrain and eye embryonic pattern formation, with a focus on zebrafish studies of Shh and retinoic acid pathways. Given the clear overlap in the mechanisms that generate both diseases, we propose that holoprosencephaly and coloboma can represent mild and severe aspects of single phenotypic spectrum resulting from aberrant forebrain development. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

The unfolding clinical spectrum of holoprosencephaly due to mutations in SHH, ZIC2, SIX3 and TGIF genes

Holoprosencephaly is a severe malformation of the brain characterized by abnormal formation and separation of the developing central nervous system. The prevalence is 1:250 during early embryogenesis, the live-born prevalence is 1:16 000. The etiology of HPE is extremely heterogeneous and can be teratogenic or genetic. We screened four known HPE genes in a Dutch cohort of 86 non-syndromic HPE index cases, including 53 family members. We detected 21 mutations (24.4%), 3 in SHH, 9 in ZIC2 and 9 in SIX3. Eight mutations involved amino-acid substitutions, 7 ins/del mutations, 1 frame-shift, 3 identical poly-alanine tract expansions and 2 gene deletions. Pathogenicity of mutations was presumed based on de novo character, predicted non-functionality of mutated proteins, segregation of mutations with affected family-members or combinations of these features. Two mutations were reported previously. SNP array confirmed detected deletions; one spanning the ZIC2/ZIC5 genes (approx. 100 kb) the other a 1.45 Mb deletion including SIX2/SIX3 genes. The mutation percentage (24%) is comparable with previous reports, but we detected significantly less mutations in SHH: 3.5 vs 10.7% (P=0.043) and significantly more in SIX3: 10.5 vs 4.3% (P=0.018). For TGIF1 and ZIC2 mutation the rate was in conformity with earlier reports. About half of the mutations were de novo, one was a germ line mosaic. The familial mutations displayed extensive heterogeneity in clinical manifestation. Of seven familial index patients only two parental carriers showed minor HPE signs, five were completely asymptomatic. Therefore, each novel mutation should be considered as a risk factor for clinically manifest HPE, with the caveat of reduced clinical penetrance.