Holoprosencephaly: new models, new insights

CNKSR2, a downstream mediator of retinoic acid signaling, modulates the Ras/Raf/MEK pathway to regulate patterning and invagination of the chick forebrain roof plate

During embryonic development, the forebrain roof plate undergoes invagination, leading to separation of the cerebral hemispheres. Any defects in this process, in humans, lead to middle interhemispheric holoprosencephaly (MIH-HPE). In this study, we have identified a previously unreported downstream mediator of retinoic acid (RA) signaling, CNKSR2, which is expressed in the forebrain roof plate in the chick embryo. Knockdown of CNKSR2 affects invagination, cell proliferation and patterning of the roof plate, similar to the phenotypes observed upon inhibition of RA signaling. We further demonstrate that CNKSR2 functions by modulating the Ras/Raf/MEK signaling. This appears to be crucial for patterning of the forebrain roof plate and its subsequent invagination, leading to the formation of the cerebral hemispheres. Thus, a set of novel molecular players have been identified that regulate the morphogenesis of the avian forebrain.

Is There a Prechordal Region and an Acroterminal Domain in Amphioxus?

This essay re-examines the singular case of the supposedly unique rostrally elongated notochord described classically in amphioxus. We start from our previous observations in hpf 21 larvae [Albuixech-Crespo et al.: PLoS Biol. 2017;15(4):e2001573] indicating that the brain vesicle has rostrally a rather standard hypothalamic molecular configuration. This correlates with the notochord across a possible rostromedian acroterminal hypothalamic domain. The notochord shows some molecular differences that specifically characterize its pre-acroterminal extension beyond its normal rostral end under the mamillary region. We explored an alternative interpretation that the putative extension of this notochord actually represents a variant form of the prechordal plate in amphioxus, some of whose cells would adopt the notochordal typology, but would lack notochordal patterning properties, and might have some (but not all) prechordal ones instead. We survey in detail the classic and recent literature on gastrulation, prechordal plate, and notochord formation in amphioxus, compare the observed patterns with those of some other vertebrates of interest, and re-examine the literature on differential gene expression patterns in this rostralmost area of the head. We noted that previous literature failed to identify the amphioxus prechordal primordia at appropriate stages. Under this interpretation, a consistent picture can be drawn for cephalochordates, tunicates, and vertebrates. Moreover, there is little evidence for an intrinsic capacity of the early notochord to grow rostralwards (it normally elongates caudalwards). Altogether, we conclude that the hypothesis of a prechordal nature of the elongated amphioxus notochord is consistent with the evidence presented.

Concepts in Multifactorial Etiology of Developmental Disorders: Gene-Gene and Gene-Environment Interactions in Holoprosencephaly

Many common developmental disorders are thought to arise from a complex set of genetic and environmental risk factors. These factors interact with each other to affect the strength and duration of key developmental signaling pathways, thereby increasing the possibility that they fail to achieve the thresholds required for normal embryonic patterning. One such disorder, holoprosencephaly (HPE), serves as a useful model system in understanding various forms of multifactorial etiology. Genomic analysis of HPE cases, epidemiology, and mechanistic studies of animal models have illuminated multiple potential ways that risk factors interact to produce adverse developmental outcomes. Among these are: 1) interactions between driver and modifier genes; 2) oligogenic inheritance, wherein each parent provides predisposing variants in one or multiple distinct loci; 3) interactions between genetic susceptibilities and environmental risk factors that may be insufficient on their own; and 4) interactions of multiple genetic variants with multiple non-genetic risk factors. These studies combine to provide concepts that illuminate HPE and are also applicable to additional disorders with complex etiology, including neural tube defects, congenital heart defects, and oro-facial clefting.

Δ9-Tetrahydrocannabinol inhibits Hedgehog-dependent patterning during development

Many developmental disorders are thought to arise from an interaction between genetic and environmental risk factors. The Hedgehog (HH) signaling pathway regulates myriad developmental processes, and pathway inhibition is associated with birth defects, including holoprosencephaly (HPE). Cannabinoids are HH pathway inhibitors, but little is known of their effects on HH-dependent processes in mammalian embryos, and their mechanism of action is unclear. We report that the psychoactive cannabinoid Δ9-tetrahydrocannabinol (THC) induces two hallmark HH loss-of-function phenotypes (HPE and ventral neural tube patterning defects) in Cdon mutant mice, which have a subthreshold deficit in HH signaling. THC therefore acts as a 'conditional teratogen', dependent on a complementary but insufficient genetic insult. In vitro findings indicate that THC is a direct inhibitor of the essential HH signal transducer smoothened. The canonical THC receptor, cannabinoid receptor-type 1, is not required for THC to inhibit HH signaling. Cannabis consumption during pregnancy may contribute to a combination of risk factors underlying specific developmental disorders. These findings therefore have significant public health relevance.

Identification of disease-relevant modulators of the SHH pathway in the developing brain

Pathogenic gene variants in humans that affect the sonic hedgehog (SHH) pathway lead to severe brain malformations with variable penetrance due to unknown modifier genes. To identify such modifiers, we established novel congenic mouse models. LRP2-deficient C57BL/6N mice suffer from heart outflow tract defects and holoprosencephaly caused by impaired SHH activity. These defects are fully rescued on a FVB/N background, indicating a strong influence of modifier genes. Applying comparative transcriptomics, we identified Pttg1 and Ulk4 as candidate modifiers upregulated in the rescue strain. Functional analyses showed that ULK4 and PTTG1, both microtubule-associated proteins, are positive regulators of SHH signaling, rendering the pathway more resilient to disturbances. In addition, we characterized ULK4 and PTTG1 as previously unidentified components of primary cilia in the neuroepithelium. The identification of genes that powerfully modulate the penetrance of genetic disturbances affecting the brain and heart is likely relevant to understanding the variability in human congenital disorders.

CDON gene contributes to pituitary stalk interruption syndrome associated with unilateral facial and abducens nerve palsy

The relationship between congenital defects of the brain and facial anomalies was proven. The Hedgehog signaling pathway plays a fundamental role in normal craniofacial development in humans. Mutations in the sonic hedgehog (SHH) signaling gene CDON have been recently reported in patients with holoprosencephaly and with pituitary stalk interruption syndrome (PSIS). This study’s aim was an elucidation of an 18-year-old patient presenting PSIS, multiple pituitary hormone deficiency, and congenital unilateral facial and abducens nerve palsy. Additionally, bilateral sensorineural hearing loss, dominating at the right site, was diagnosed. From the second year of life, growth deceleration was observed, and from the age of eight, anterior pituitary hormone deficiencies were gradually confirmed and substituted. At the MRI, characteristic triad for PSIS (anterior pituitary hypoplasia, interrupted pituitary stalk and ectopic posterior lobe) was diagnosed. We performed a comprehensive genomic screening, including microarrays for structural rearrangements and whole-exome sequencing for a monogenic defect. A novel heterozygous missense variant in the CDON gene (c.1814G > T; p.Gly605Val) was identified. The variant was inherited from the mother, who, besides short stature, did not show any disease symptoms. The variant was absent in control databases and 100 healthy subjects originating from the same population. We report a novel variant in the CDON gene associated with PSIS and congenital cranial nerve palsy. The variant revealed autosomal dominant inheritance with incomplete penetrance in concordance with previous studies reporting CDON defects.

The hedgehog co-receptor BOC differentially regulates SHH signaling during craniofacial development

The Hedgehog (HH) pathway controls multiple aspects of craniofacial development. HH ligands signal through the canonical receptor PTCH1, and three co-receptors: GAS1, CDON and BOC. Together, these co-receptors are required during embryogenesis to mediate proper HH signaling. Here, we investigated the individual and combined contributions of GAS1, CDON and BOC to HH-dependent mammalian craniofacial development. Notably, individual deletion of either Gas1 or Cdon results in variable holoprosencephaly phenotypes in mice, even on a congenic background. In contrast, we find that Boc deletion results in facial widening that correlates with increased HH target gene expression. In addition, Boc deletion in a Gas1 null background partially ameliorates the craniofacial defects observed in Gas1 single mutants; a phenotype that persists over developmental time, resulting in significant improvements to a subset of craniofacial structures. This contrasts with HH-dependent phenotypes in other tissues that significantly worsen following combined deletion of Gas1 and Boc Together, these data indicate that BOC acts as a multi-functional regulator of HH signaling during craniofacial development, alternately promoting or restraining HH pathway activity in a tissue-specific fashion.

Cdon mutation and fetal alcohol converge on Nodal signaling in a mouse model of holoprosencephaly

Holoprosencephaly (HPE), a defect in midline patterning of the forebrain and midface, arises ~1 in 250 conceptions. It is associated with predisposing mutations in the Nodal and Hedgehog (HH) pathways, with penetrance and expressivity graded by genetic and environmental modifiers, via poorly understood mechanisms. CDON is a multifunctional co-receptor, including for the HH pathway. In mice, Cdon mutation synergizes with fetal alcohol exposure, producing HPE phenotypes closely resembling those seen in humans. We report here that, unexpectedly, Nodal signaling is a major point of synergistic interaction between Cdon mutation and fetal alcohol. Window-of-sensitivity, genetic, and in vitro findings are consistent with a model whereby brief exposure of Cdon mutant embryos to ethanol during gastrulation transiently and partially inhibits Nodal pathway activity, with consequent effects on midline patterning. These results illuminate mechanisms of gene-environment interaction in a multifactorial model of a common birth defect.

Recent advances in understanding inheritance of holoprosencephaly

Holoprosencephaly (HPE) is a complex genetic disorder of the developing forebrain characterized by high phenotypic and genetic heterogeneity. HPE was initially defined as an autosomal dominant disease, but recent research has shown that its mode of transmission is more complex. The past decade has witnessed rapid development of novel genetic technologies and significant progresses in clinical studies of HPE. In this review, we recapitulate genetic epidemiological studies of the largest European HPE cohort and summarize the novel genetic discoveries of HPE based on recently developed diagnostic methods. Our main purpose is to present different inheritance patterns that exist for HPE with a particular emphasis on oligogenic inheritance and its implications in genetic counseling.

Modeling the complex etiology of holoprosencephaly in mice

Holoprosencephaly (HPE) is a common developmental defect caused by failure to define the midline of the forebrain and/or midface. HPE is associated with heterozygous mutations in Nodal and Sonic hedgehog (SHH) pathway components, but clinical presentation is highly variable, and many mutation carriers are unaffected. It is therefore thought that such mutations interact with more common modifiers, genetic and/or environmental, to produce severe patterning defects. Modifiers are difficult to identify, as their effects are context-dependent and occur within the complex genetic and environmental landscapes that characterize human populations. This has made a full understanding of HPE etiology challenging. We discuss here the use of mice, a genetically tractable model sensitive to teratogens, as a system to address this challenge. Mice carrying mutations in human HPE genes often display wide variations in phenotypic penetrance and expressivity when placed on different genetic backgrounds, demonstrating the existence of silent HPE modifier genes. Studies with mouse lines carrying SHH pathway mutations on appropriate genetic backgrounds have led to identification of both genetic and environmental modifiers that synergize with the mutations to produce a spectrum of HPE phenotypes. These models favor a scenario in which multiple modifying influences-both genetic and environmental, sensitizing and protective-interact with bona fide HPE mutations to grade phenotypic outcomes. Despite the complex interplay of HPE risk factors, mouse models have helped establish some clear concepts in HPE etiology. A combination of mouse and human cohort studies should improve our understanding of this fascinating and medically important issue.

Ethanol itself is a holoprosencephaly-inducing teratogen

Ethanol is a teratogen, inducing a variety of structural defects in developing humans and animals that are exposed in utero. Mechanisms of ethanol teratogenicity in specific defects are not well understood. Oxidative metabolism of ethanol by alcohol dehydrogenase or cytochrome P450 2E1 has been implicated in some of ethanol's teratogenic effects, either via production of acetaldehyde or competitive inhibition of retinoic acid synthesis. Generalized oxidative stress in response to ethanol may also play a role in its teratogenicity. Among the developmental defects that ethanol has been implicated in is holoprosencephaly, a failure to define the midline of the forebrain and midface that is associated with a deficiency in Sonic hedgehog pathway function. Etiologically, holoprosencephaly is thought to arise from a complex combination of genetic and environmental factors. We have developed a gene-environment interaction model of holoprosencephaly in mice, in which mutation of the Sonic hedgehog coreceptor, Cdon, synergizes with transient in utero exposure to ethanol. This system was used to address whether oxidative metabolism is required for ethanol's teratogenic activity in holoprosencephaly. We report here that t-butyl alcohol, which is neither a substrate nor an inhibitor of alcohol dehydrogenases or Cyp2E1, is a potent inducer of holoprosencephaly in Cdon mutant mice. Additionally, antioxidant treatment did not prevent ethanol- or t-butyl alcohol-induced HPE in these mice. These findings are consistent with the conclusion that ethanol itself, rather than a consequence of its metabolism, is a holoprosencephaly-inducing teratogen.

Development of the Neuroendocrine Hypothalamus

The neuroendocrine hypothalamus is composed of the tuberal and anterodorsal hypothalamus, together with the median eminence/neurohypophysis. It centrally governs wide-ranging physiological processes, including homeostasis of energy balance, circadian rhythms and stress responses, as well as growth and reproductive behaviours. Homeostasis is maintained by integrating sensory inputs and effecting responses via autonomic, endocrine and behavioural outputs, over diverse time-scales and throughout the lifecourse of an individual. Here, we summarize studies that begin to reveal how different territories and cell types within the neuroendocrine hypothalamus are assembled in an integrated manner to enable function, thus supporting the organism's ability to survive and thrive. We discuss how signaling pathways and transcription factors dictate the appearance and regionalization of the hypothalamic primordium, the maintenance of progenitor cells, and their specification and differentiation into neurons. We comment on recent studies that harness such programmes for the directed differentiation of human ES/iPS cells. We summarize how developmental plasticity is maintained even into adulthood and how integration between the hypothalamus and peripheral body is established in the median eminence and neurohypophysis. Analysis of model organisms, including mouse, chick and zebrafish, provides a picture of how complex, yet elegantly coordinated, developmental programmes build glial and neuronal cells around the third ventricle of the brain. Such conserved processes enable the hypothalamus to mediate its function as a central integrating and response-control mediator for the homeostatic processes that are critical to life. Early indications suggest that deregulation of these events may underlie multifaceted pathological conditions and dysfunctional physiology in humans, such as obesity.

Signals from the brain induce variation in avian facial shape

BACKGROUND

How developmental mechanisms generate the phenotypic variation that is the raw material for evolution is largely unknown. Here, we explore whether variation in a conserved signaling axis between the brain and face contributes to differences in morphogenesis of the avian upper jaw. In amniotes, including both mice and avians, signals from the brain establish a signaling center in the ectoderm (the Frontonasal ectodermal zone or "FEZ") that directs outgrowth of the facial primordia.

RESULTS

Here we show that the spatial organization of this signaling center differs among avians, and these correspond to Sonic hedgehog (Shh) expression in the basal forebrain and embryonic facial shape. In ducks this basal forebrain domain is present almost the entire width, while in chickens it is restricted to the midline. When the duck forebrain is unilaterally transplanted into stage matched chicken embryos the face on the treated side resembles that of the donor.

CONCLUSIONS

Combined with previous findings, these results demonstrate that variation in a highly conserved developmental pathway has the potential to contribute to evolutionary differences in avian upper jaw morphology. Developmental Dynamics 244:1133-1143, 2015. © 2015 Wiley Periodicals, Inc.

Rescue of holoprosencephaly in fetal alcohol-exposed Cdon mutant mice by reduced gene dosage of Ptch1

Holoprosencephaly (HPE) is a commonly occurring developmental defect in which midline patterning of the forebrain and midface is disrupted. Sonic hedgehog (SHH) signaling is required during multiple stages of rostroventral midline development, and heterozygous mutations in SHH pathway components are associated with HPE. However, clinical presentation of HPE is highly variable, and carriers of heterozygous mutations often lack apparent defects. It is therefore thought that such mutations must interact with more common modifiers, genetic and/or environmental. We have modeled this scenario in mice. Cdon mutant mice have a largely subthreshold defect in SHH signaling, rendering them sensitive to a wide spectrum of HPE phenotypes by additional hits that are themselves insufficient to produce HPE, including transient in utero exposure to ethanol. These variable HPE phenotypes may arise in embryos that fail to reach a threshold level of SHH signaling at a specific developmental stage. To provide evidence for this possibility, here we tested the effect of removing one copy of the negative regulator Ptch1 from Cdon(-/-) embryos and compared their response to ethanol with that of Cdon(-/-);Ptch1(+/+) embryos. Ptch1 heterozygosity decreased the penetrance of HPE in this system by >75%. The major effect of reduced Ptch1 gene dosage was on penetrance, as those Cdon(-/-);Ptch1(+/-) embryos that displayed HPE did not show major differences in phenotype from Cdon(-/-);Ptch1(+/+) embryos with ethanol-induced HPE. Our findings are consistent with the notion that even in an etiologically complex model of HPE, the level of SHH pathway activity is rate-limiting. Furthermore, the clinical outcome of an individual carrying a SHH pathway mutation will likely reflect the sum effect of both deleterious and protective modifier alleles and their interaction with non-genetic risk factors like fetal alcohol exposure.

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.

Cdon mutation and fetal ethanol exposure synergize to produce midline signaling defects and holoprosencephaly spectrum disorders in mice

Holoprosencephaly (HPE) is a remarkably common congenital anomaly characterized by failure to define the midline of the forebrain and midface. HPE is associated with heterozygous mutations in Sonic hedgehog (SHH) pathway components, but clinical presentation is extremely variable and many mutation carriers are unaffected. It has been proposed that these observations are best explained by a multiple-hit model, in which the penetrance and expressivity of an HPE mutation is enhanced by a second mutation or the presence of cooperating, but otherwise silent, modifier genes. Non-genetic risk factors are also implicated in HPE, and gene-environment interactions may provide an alternative multiple-hit model to purely genetic multiple-hit models; however, there is little evidence for this contention. We report here a mouse model in which there is dramatic synergy between mutation of a bona fide HPE gene (Cdon, which encodes a SHH co-receptor) and a suspected HPE teratogen, ethanol. Loss of Cdon and in utero ethanol exposure in 129S6 mice give little or no phenotype individually, but together produce defects in early midline patterning, inhibition of SHH signaling in the developing forebrain, and a broad spectrum of HPE phenotypes. Our findings argue that ethanol is indeed a risk factor for HPE, but genetically predisposed individuals, such as those with SHH pathway mutations, may be particularly susceptible. Furthermore, gene-environment interactions are likely to be important in the multifactorial etiology of HPE.

Unique alterations of an ultraconserved non-coding element in the 3'UTR of ZIC2 in holoprosencephaly

Coding region alterations of ZIC2 are the second most common type of mutation in holoprosencephaly (HPE). Here we use several complementary bioinformatic approaches to identify ultraconserved cis-regulatory sequences potentially driving the expression of human ZIC2. We demonstrate that an 804 bp element in the 3′ untranslated region (3′UTR) is highly conserved across the evolutionary history of vertebrates from fish to humans. Furthermore, we show that while genetic variation of this element is unexpectedly common among holoprosencephaly subjects (6/528 or >1%), it is not present in control individuals. Two of six proband-unique variants are de novo, supporting their pathogenic involvement in HPE outcomes. These findings support a general recommendation that the identification and analysis of key ultraconserved elements should be incorporated into the genetic risk assessment of holoprosencephaly cases.

Comparison of mutation findings in ZIC2 between microform and classical holoprosencephaly in a Brazilian cohort

BACKGROUND

Holoprosencephaly is the most frequent congenital malformation of the forebrain in humans. It is anatomically classified by the relative degree of abnormal formation and separation of the developing central nervous system. Mutations of ZIC2 are the second most common heterozygous variations detected in holoprosencephaly (HPE) patients. Mutations in most known HPE genes typically result in variable phenotypes that rage from classic alobar HPE to microforms represented by hypotelorism, solitary central maxillary incisor (SCMI), and cleft lip/palate, among others. Patients with HPE owing to ZIC2 mutations have recently been described by a distinct phenotype compared with mutations in other HPE causative genes.

METHODS

We report the comparison of ZIC2 molecular findings by Sanger bidirectional DNA sequencing and ad hoc genotyping in a cohort of 105 Brazilian patients within the clinical spectrum of HPE, including classic and microform groups.

RESULTS

We detected a total of five variants in the ZIC2 gene: a common histidine tract expansion c.716_718dup (p.His239dup), a rare c.1377_1391del_homozygous (p.Ala466_470del, or Ala 15 to 10 contraction), a novel intronic c.1239+18G>A variant, a novel frameshift c.1215dupC (p.Ser406Glnfs*11), and a c.1401_1406dup (p.Ala469_470dup, or alanine tract expansion to 17 residues).

CONCLUSIONS

From these patients, only the latter two mutations found in classic HPE are likely to be medically significant. In contrast, variants detected in the microform group are not likely to be pathogenic. We show conclusively that the histidine tract expansion is a polymorphic alteration that demonstrates considerable differences in allele frequencies across different ethnic groups. Therefore, careful population studies of rare variants can improve genotype-phenotype correlations. Birth Defects Research (Part A) 2012.

Molecular analysis of the Noggin (NOG) gene in holoprosencephaly patients

Holoprosencephaly (HPE) is the most common structural anomaly of the human forebrain. Various genetic and teratogenic causes have been implicated in its pathogenesis. A recent report in mice described Noggin (NOG) as a candidate gene involved in the etiogenesis of microform HPE. Here, we present for the first time genetic analysis of a large HPE cohort for sequence variations in NOG. On the basis of our study, we conclude that mutations in the coding region of NOG are rare, and play at most an uncommon role in human HPE.

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 syndrome of congenital circumferential skin folds associated with multiple congenital anomalies

Congenital circumferential skin folds can be found in individuals with no additional defects, as well as in patients with multiple congenital anomalies and developmental abnormalities. Current data point to etiological heterogeneity of syndromic cases. We describe a 7-month-old girl with a novel combination of symmetrical congenital circumferential skin folds, dysmorphic features, and multiple congenital abnormalities. Examination of the patient revealed symmetrical congenital circumferential skin folds and dysmorphic features, as well as multiple congenital anomalies including nasal pyriform aperture stenosis, ventricular septal defect, absent spleen, camptodactyly, and severe psychomotor retardation. Skin biopsy demonstrated subcutaneous fat extending into the superficial and deep reticular dermis. Sequencing of the CDON, SHH, ZIC2, SIX3, and TGIF genes (associated with holoprosencephaly) did not disclose pathogenic alterations. Extensive review of previously described cases of syndromic congenital circumferential skin folds did not reveal a similar combination of clinical and histopathological findings.

Perfil de habilidades do desenvolvimento em crianças com holoprosencefalia e holoprosencefalia like

OBJETIVO: investigar e comparar o desempenho nas habilidades relacionadas ao desenvolvimento motor, cognitivo, linguístico, de socialização e autocuidados de indivíduos com holoprosencefalia e com holoprosencefalia-like. MÉTODO: participaram deste estudo 20 indivíduos com diagnóstico de holoprosencefalia, na faixa etária de 18 a 72 meses, de ambos os sexos, divididos em 2 grupos. O grupo 1 (G1) composto por 12 indivíduos com sinais clínicos do espectro da holoprosencefalia, e o grupo 2 (G2) com holoprosencefalia-like composto por 8 indivíduos com sinais clínicos do espectro da holoprosencefalia-like. A coleta de dados foi realizada por meio da aplicação do Inventário Portage Operacionalizado que avalia as áreas alvos deste estudo. Para a análise estatística utilizou-se análise descritiva da mediana e dos valores mínimos e máximos e foi aplicado o teste estatístico de Mann Whitney (< 0,05% para significância). RESULTADOS: os grupos 1 e 2 apresentaram alterações em todas as áreas do desenvolvimento avaliadas. Entretanto, os indivíduos do G1, com holoprosencefalia apresentaram maiores comprometimentos nas habilidades: motora, cognitiva, de linguagem, de socialização e autocuidados, quanto comparados aos indivíduos do G2, com holoprosencefalia-like. CONCLUSÃO: o desempenho nas áreas motoras, cognitivas, de linguagem, de socialização e autocuidados de indivíduos com holoprosencefalia e holoprosencefalia-like foi aquém do esperado, principalmente naqueles indivíduos com holoprosencefalia, que se justifica pelo maior comprometimento no sistema nervoso central. A natureza destas alterações pode estar associada ao universo de alterações neurológicas e craniofaciais descritas nestes quadros clínicos e também à influência do ambiente social.

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.

Boc modifies the holoprosencephaly spectrum of Cdo mutant mice

Holoprosencephaly (HPE) is caused by a failure to form the midline of the forebrain and/or midface. It is one of the most common human birth defects, but clinical expression is extremely variable. HPE is associated with mutations in the sonic hedgehog (SHH) pathway. Mice lacking the Shh pathway regulator Cdo (also called Cdon) display HPE with strain-dependent penetrance and expressivity, implicating silent modifier genes as one cause of the variability. However, the identities of potential HPE modifiers of this type are unknown. We report here that whereas mice lacking the Cdo paralog Boc do not have HPE, Cdo;Boc double mutants on a largely Cdo-resistant genetic background have lobar HPE with strong craniofacial anomalies and defects in Shh target gene expression in the developing forebrain. Boc is therefore a silent HPE modifier gene in mice. Furthermore, Cdo and Boc have specific, selective roles in Shh signaling in mammals, because Cdo;Boc double-mutant mice do not display the most severe HPE phenotype seen in Shh-null mice, nor do they have major defects in digit patterning or development of vertebrae, which are also Shh-dependent processes. This is in contrast to reported observations in Drosophila, where genetic removal of the Cdo and Boc orthologs Ihog and Boi results in a complete loss of response to the hedgehog ligand. Therefore, there is evolutionary divergence between mammals and insects in the requirement of the hedgehog pathway for Cdo/Ihog family members, with mammalian development involving additional factors and/or distinct mechanisms at this level of pathway regulation.

Holoprosencephaly and holoprosencephaly-like phenotype and GAS1 DNA sequence changes: Report of four Brazilian patients

Holoprosencephaly (HPE) is genetically heterogeneous. Variable phenotypic manifestations within families with normal and affected patients have been attributed to the number and type of HPE gene mutations. Environmental agents may also contribute to the severity as well as the requirement of multiple hits. Clinical expression is extremely variable ranging from minor facial signs to complex craniofacial anomalies such as cyclopia. Main genes involved include SHH, GLI2, PTCH1, TGIF, ZIC2, TDGF1, SIX3; however, several other candidates have been proposed. Recently it was established that the human growth arrest specific gene 1 (GAS1) is a potential locus for several human craniofacial malformations. Here, we report on four Brazilian patients with GAS1 DNA sequence change who presented variable phenotypical manifestations ranging from classic HPE to HPE-like signs. Two patients had single DNA sequence change in the GAS1 gene, while in other two, an additional mutation in the SHH gene was observed. Clinical manifestations presented by these patients suggest that GAS1 could be considered a candidate locus for one of the types of human HPE.

Analysis of genotype-phenotype correlations in human holoprosencephaly

Since the discovery of the first gene causing holoprosencephaly (HPE), over 500 patients with mutations in genes associated with non-chromosomal, non-syndromic HPE have been described, with detailed descriptions available in over 300. Comprehensive clinical analysis of these individuals allows examination for the presence of genotype-phenotype correlations. These correlations allow a degree of differentiation between patients with mutations in different HPE-associated genes and for the application of functional studies to determine intragenic correlations. These early correlations are an important advance in the understanding of the clinical aspects of this disease, and in general argue for continued analysis of the genetic and clinical findings of large cohorts of patients with rare diseases in order to better inform both basic biological insight and care and counseling for affected patients and families.

The molecular genetics of holoprosencephaly

Holoprosencephaly (HPE) has captivated the imagination of Man for millennia because its most extreme manifestation, the single-eyed cyclopic newborn infant, brings to mind the fantastical creature Cyclops from Greek mythology. Attempting to understand this common malformation of the forebrain in modern medical terms requires a systematic synthesis of genetic, cytogenetic, and environmental information typical for studies of a complex disorder. However, even with the advances in our understanding of HPE in recent years, there are significant obstacles remaining to fully understand its heterogeneity and extensive variability in phenotype. General lessons learned from HPE will likely be applicable to other malformation syndromes. Here we outline the common, and rare, genetic and environmental influences on this conserved developmental program of forebrain development and illustrate the similarities and differences between these malformations in humans and those of animal models.

Zebrafish zic2a patterns the forebrain through modulation of Hedgehog-activated gene expression

Holoprosencephaly (HPE) is the most common congenital malformation of the forebrain in human. Several genes with essential roles during forebrain development have been identified because they cause HPE when mutated. Among these are genes that encode the secreted growth factor Sonic hedgehog (Shh) and the transcription factors Six3 and Zic2. In the mouse, Six3 and Shh activate each other's transcription, but a role for Zic2 in this interaction has not been tested. We demonstrate that in zebrafish, as in mouse, Hh signaling activates transcription of six3b in the developing forebrain. zic2a is also activated by Hh signaling, and represses six3b non-cell-autonomously, i.e. outside of its own expression domain, probably through limiting Hh signaling. Zic2a repression of six3b is essential for the correct formation of the prethalamus. The diencephalon-derived optic stalk (OS) and neural retina are also patterned in response to Hh signaling. We show that zebrafish Zic2a limits transcription of the Hh targets pax2a and fgf8a in the OS and retina. The effects of Zic2a depletion in the forebrain and in the OS and retina are rescued by blocking Hh signaling or by increasing levels of the Hh antagonist Hhip, suggesting that in both tissues Zic2a acts to attenuate the effects of Hh signaling. These data uncover a novel, essential role for Zic2a as a modulator of Hh-activated gene expression in the developing forebrain and advance our understanding of a key gene regulatory network that, when disrupted, causes HPE.

The mutational spectrum of holoprosencephaly-associated changes within the SHH gene in humans predicts loss-of-function through either key structural alterations of the ligand or its altered synthesis

Mutations within either the SHH gene or its related pathway components are the most common, and best understood, pathogenetic changes observed in holoprosencephaly patients; this fact is consistent with the essential functions of this gene during forebrain development and patterning. Here we summarize the nature and types of deleterious sequence alterations among over one hundred distinct mutations in the SHH gene (64 novel mutations) and compare these to over a dozen mutations in disease-related Hedgehog family members IHH and DHH. This combined structural analysis suggests that dysfunction of Hedgehog signaling in human forebrain development can occur through truncations or major structural changes to the signaling domain, SHH-N, as well as due to defects in the processing of the mature ligand from its pre-pro-precursor or defective post-translation bi-lipid modifications with palmitate and cholesterol.

Cumulative ligand activity of NODAL mutations and modifiers are linked to human heart defects and holoprosencephaly

The cyclopic and laterality phenotypes in model organisms linked to disturbances in the generation or propagation of Nodal-like signals are potential examples of similar impairments resulting in birth defects in humans. However, the types of gene mutation(s) and their pathogenetic combinations in humans are poorly understood. Here we describe a mutational analysis of the human NODAL gene in a large panel of patients with phenotypes compatible with diminished NODAL ligand function. Significant reductions in the biological activity of NODAL alleles are detected among patients with congenital heart defects (CHD), laterality anomalies (e.g. left-right mis-specification phenotypes), and only rarely holoprosencephaly (HPE). While many of these NODAL variants are typical for family-specific mutations, we also report the presence of alleles with significantly reduced activity among common population variants. We propose that some of these common variants act as modifiers and contribute to the ultimate phenotypic outcome in these patients; furthermore, we draw parallels with strain-specific modifiers in model organisms to bolster this interpretation.

The full spectrum of holoprosencephaly-associated mutations within the ZIC2 gene in humans predicts loss-of-function as the predominant disease mechanism

Mutations of the ZIC2 transcription factor gene are among the most common heterozygous variations detected in holoprosencephaly (HPE) patients, a patient group who lack critical midline forebrain specification due to defective embryonic signaling during development. Recent studies indicate that complete deficiency of the related murine Zic2 transcription factor can also be a contributing factor to variable midline deficiencies, presenting during mid-gastrulation, that could explain similar forebrain anomalies in this model system. Here we collect and summarize all available mutations in the human ZIC2 gene detected in HPE patients (21 published and 62 novel). Our analysis corroborates this mechanism proposed in mice by predicting loss-of-function as the likely pathogenetic mechanism common to most, if not all, of these mutations in HPE.

3-D reconstruction of a human fetus with combined holoprosencephaly and cyclopia

Background

The purpose of this study was to examine a human fetus with combined holoprosencephaly and cyclopia by means of histology and 3-D reconstruction to determine the internal structure and extent of the malformation.

Methods

The head from a human fetus at 20 weeks gestation and a diagnosis of holoprosencephaly and cyclopia was investigated histologically and three-dimensionally reconstructed with CAD techniques. The cranial bones, blood vessels, nerves, eye and brain anlagen were reconstructed.

Results

The 3-D reconstruction revealed both severe malformation and absence of the facial midline bones above the maxilla, and a malformation of the maxilla and sphenoid bone. The mandible, posterior cranial bones, cranial nerves and blood vessels were normal. A synophthalmic eye with two lenses was found. The prosencephalon was a single small protrusion above the diencephalon. No nasal cavity was present. Above the single eye a proboscis was found.

Conclusion

The absence of the facial midline bones above the maxilla and the presence of a proboscis as a nose-like structure above the cyclopic eye both mean that there was a developmental defect in the fronto-nasal facial process of this fetus.

Pathogenesis of holoprosencephaly

Holoprosencephaly (HPE), the most common human forebrain malformation, occurs in 1 in 250 fetuses and 1 in 16,000 live births. HPE is etiologically heterogeneous, and its pathology is variable. Several mouse models of HPE have been generated, and some of the molecular causes of different forms of HPE and the mechanisms underlying its variable pathology have been revealed by these models. Herein, we summarize the current knowledge on the genetic alterations that cause HPE and discuss some important questions about this disease that remain to be answered.

Truncating loss-of-function mutations of DISP1 contribute to holoprosencephaly-like microform features in humans

Defective function of the Sonic Hedgehog (SHH) signaling pathway is the most frequent alteration underlying holoprosencephaly (HPE) or its various clinical microforms. We performed an extensive mutational analysis of the entire human DISP1 gene, required for secretion of all hedgehog ligand(s) and which maps to the HPE 10 locus of human chromosome 1q41, as a HPE candidate gene. Here, we describe two independent families with truncating mutations in human DISP1 that resemble the cardinal craniofacial and neuro-developmental features of a recently described microdeletion syndrome that includes this gene; therefore, we suggest that DISP1 function contributes substantially to both of these signs in humans. While these clinical features are consistent with common HPE microforms, especially those linked to defective signaling by Sonic Hedgehog, we have insufficient evidence so far that functionally abnormal DISP1 alleles will commonly contribute to the more severe features of typical HPE.

Mutations in the human SIX3 gene in holoprosencephaly are loss of function

Holoprosencephaly (HPE) is the most common developmental anomaly of the human forebrain; however, the genetics of this heterogeneous and etiologically complex malformation is incompletely understood. Heterozygous mutations in SIX3, a transcription factor gene expressed in the anterior forebrain and eyes during early vertebrate development, have been frequently detected in human HPE cases. However, only a few mutations have been investigated with limited functional studies that would confirm a role in HPE pathogenesis. Here, we report the development of a set of robust and sensitive assays of human SIX3 function in zebrafish and apply these to the analysis of a total of 46 distinct mutations (19 previously published and 27 novel) located throughout the entire SIX3 gene. We can now confirm that 89% of these putative deleterious mutations are significant loss-of-function alleles. Since disease-associated single point mutations in the Groucho-binding eh1-like motif decreases the function in all assays, we can also confirm that this interaction is essential for human SIX3 co-repressor activity; we infer, in turn, that this function is important in HPE causation. We also unexpectedly detected truncated versions with partial function, yet missing a SIX3-encoded homeodomain. Our data indicate that SIX3 is a frequent target in the pathogenesis of HPE and demonstrate how this can inform the genetic counseling of families.

Reduced NODAL signaling strength via mutation of several pathway members including FOXH1 is linked to human heart defects and holoprosencephaly

Abnormalities of embryonic patterning are hypothesized to underlie many common congenital malformations in humans including congenital heart defects (CHDs), left-right disturbances (L-R) or laterality, and holoprosencephaly (HPE). Studies in model organisms suggest that Nodal-like factors provide instructions for key aspects of body axis and germ layer patterning; however, the complex genetics of pathogenic gene variant(s) in humans are poorly understood. Here we report our studies of FOXH1, CFC1, and SMAD2 and summarize our mutational analysis of three additional components in the human NODAL-signaling pathway: NODAL, GDF1, and TDGF1. We identify functionally abnormal gene products throughout the pathway that are clearly associated with CHD, laterality, and HPE. Abnormal gene products are most commonly detected in patients within a narrow spectrum of isolated conotruncal heart defects (minimum 5%-10% of subjects), and far less commonly in isolated laterality or HPE patients (approximately 1% for each). The difference in the mutation incidence between these groups is highly significant. We show that apparent gene dosage discrepancies between humans and model organisms can be reconciled by considering a broader combination of sequence variants. Our studies confirm that (1) the genetic vulnerabilities inferred from model organisms with defects in Nodal signaling are indeed analogous to humans; (2) the molecular analysis of an entire signaling pathway is more complete and robust than that of individual genes and presages future studies by whole-genome analysis; and (3) a functional genomics approach is essential to fully appreciate the complex genetic interactions necessary to produce these effects in humans.

Early mammillary pouch specification in the course of prechordal ventralization of the forebrain tegmentum

The mammillary body, a ventral specialization of the caudal hypothalamus, lies close to the transition between epichordal and prechordal parts of the forebrain (Puelles and Rubenstein, 2003). This report examines its presumed causal connection with either prechordal or notochordal mesodermal induction, as well as the timing of its specification, in the context of early ventral forebrain patterning. It was recently found that the ephrin receptor gene EphA7 is selectively expressed in the mammillary pouch from early stages of development (HH14: García-Calero et al., 2006). We used mammillary EphA7 expression as well as ventral hypothalamic expression of the gene markers Nkx2.1 and Shh to analyze experimental effects on mammillary specification and morphogenesis after axial mesoderm ablation at stages HH4+ to HH6. Progressively delayed ablation of the prechordal plate revealed its sequential implication in molecular specification of the entire ventral forebrain, including the mammillary and tuberal regions of the hypothalamus. We observed differential contact requirements for induction by the prechordal plate of all the forebrain regions expressing Shh and Nkx2.1, including distant subpallial ones. In contrast, ablation of the anterior notochordal tip at these stages did not elicit significant patterning changes, particularly no effects on mammillary EphA7 expression or mammillary pouch development.

The ups and downs of holoprosencephaly: dorsal versus ventral patterning forces

Holoprosencephaly (HPE), characterized by incomplete separation of forebrain and facial components into left and right sides, is a common developmental defect in humans. It is caused by both genetic and environmental factors and its severity covers a wide spectrum of phenotypes. The genetic interactions underlying inherited forms of HPE are complex and poorly understood. Animal models, in particular mouse mutants, are providing a growing understanding of how the forebrain develops and how the cerebral hemispheres become split into left and right sides. These insights, along with the characterization to date of some of the genes involved in human HPE, suggest that two distinct mechanisms underlie the major classes of HPE, 'classic' and midline interhemispheric (MIH). Disruption either directly or indirectly of the ventralizing effect of sonic hedgehog signaling appears central to all or most forms of classic HPE, while disruption of the dorsalizing effect of bone morphogenetic protein signaling may be key to cases of MIH HPE.

Murine models of holoprosencephaly

Holoprosencephaly (HPE), the most common developmental defect of the forebrain and midface, is caused by a failure to delineate the midline in these structures. Both genetic and environmental etiologies exist for HPE, and clinical presentation is highly variable. HPE occurs in sporadic and inherited forms, and even HPE in pedigrees is characterized by incomplete penetrance and variable expressivity. Heterozygous mutations in eight different genes have been identified in human HPE, and disruption of Sonic hedgehog expression and/or signaling in the rostroventral region of the embryo is a major common effect of these mutations. An understanding of the mechanisms whereby genetic defects and teratogenic exposures become manifest as developmental anomalies of varying severity requires experimental models that accurately reproduce the spectrum of defects seen in human HPE. The mouse has emerged as such a model, because of its ease of genetic manipulation and similarity to humans in development of the forebrain and face. HPE is generally observed in mice homozygous for mutations in orthologs of human HPE genes though, unlike humans, rarely in mice with heterozygous mutations. Moreover, reverse genetics in the mouse has provided a wealth of new candidate human HPE genes. Construction of hypomorphic alleles, interbreeding to produce double mutants, and analysis of these mutations on different genetic backgrounds has generated multiple models of HPE and begun to provide insight into the conundrum of the HPE spectrum. Here, we review forebrain development with an emphasis on the pathways known to be defective in HPE and describe the strengths and weaknesses of various murine models of HPE.