Holoprosencephaly: from Homer to Hedgehog

Sonic hedgehog signaling in craniofacial development

Mutations in SHH and several other genes encoding components of the Hedgehog signaling pathway have been associated with holoprosencephaly syndromes, with craniofacial anomalies ranging in severity from cyclopia to facial cleft to midfacial and mandibular hypoplasia. Studies in animal models have revealed that SHH signaling plays crucial roles at multiple stages of craniofacial morphogenesis, from cranial neural crest cell survival to growth and patterning of the facial primordia to organogenesis of the palate, mandible, tongue, tooth, and taste bud formation and homeostasis. This article provides a summary of the major findings in studies of the roles of SHH signaling in craniofacial development, with emphasis on recent advances in the understanding of the molecular and cellular mechanisms regulating the SHH signaling pathway activity and those involving SHH signaling in the formation and patterning of craniofacial structures.

Gli2-Mediated Shh Signaling Is Required for Thalamocortical Projection Guidance

The thalamocortical projections are part of the most important higher level processing connections in the vertebrates and follow a highly ordered pathway from their origin in the thalamus to the cerebral cortex. Their functional complexities are not only due to an extremely elaborate axon guidance process but also due to activity-dependent mechanisms. Gli2 is an intermediary transcription factor in the Sonic hedgehog (Shh) pathway. During neural early development, Shh has an important role in dorsoventral patterning, diencephalic anteroposterior patterning, and many later developmental processes, such as axon guidance and cell migration. Using a Gli2 knockout mouse line, we have studied the role of Shh signaling mediated by Gli2 in the development of the thalamocortical projections during embryonic development. In wild-type brains, we have described the normal trajectory of the thalamocortical axons into the context of the prosomeric model. Then, we have compared it with the altered thalamocortical axons course in Gli2 homozygous embryos. The thalamocortical axons followed different trajectories and were misdirected to other territories probably due to alterations in the Robo/Slit signaling mechanism. In conclusion, the alteration of Gli2-mediated Shh signaling produces an erroneous specification of several territories related with the thalamocortical axons. This is translated into a huge modification in the pathfinding signaling mechanisms needed for the correct wiring of the thalamocortical axons.

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.

Hedgehog Signaling and Embryonic Craniofacial Disorders

Since its initial discovery in a Drosophila mutagenesis screen, the Hedgehog pathway has been revealed to be instrumental in the proper development of the vertebrate face. Vertebrates possess three hedgehog paralogs: Sonic hedgehog (Shh), Indian hedgehog (Ihh), and Desert hedgehog (Dhh). Of the three, Shh has the broadest range of functions both in the face and elsewhere in the embryo, while Ihh and Dhh play more limited roles. The Hedgehog pathway is instrumental from the period of prechordal plate formation early in the embryo, until the fusion of the lip and secondary palate, which complete the major patterning events of the face. Disruption of Hedgehog signaling results in an array of developmental disorders in the face, ranging from minor alterations in the distance between the eyes to more serious conditions such as severe clefting of the lip and palate. Despite its critical role, Hedgehog signaling seems to be disrupted through a number of mechanisms that may either be direct, as in mutation of a downstream target of the Hedgehog ligand, or indirect, such as mutation in a ciliary protein that is otherwise seemingly unrelated to the Hedgehog pathway. A number of teratogens such as alcohol, statins and steroidal alkaloids also disrupt key aspects of Hedgehog signal transduction, leading to developmental defects that are similar, if not identical, to those of Hedgehog pathway mutations. The aim of this review is to highlight the variety of roles that Hedgehog signaling plays in developmental disorders of the vertebrate face.

Facial Morphogenesis: Physical and Molecular Interactions Between the Brain and the Face

Morphogenesis of the brain and face is intrinsically linked by a number of factors. These include: origins of tissues, adjacency allowing their physical interactions, and molecular cross talk controlling growth. Neural crest cells that form the facial primordia originate on the dorsal neural tube. In the caudal pharyngeal arches, a Homeobox code regulates arch identity. In anterior regions, positional information is acquired locally. Second, the brain is a structural platform that influences positioning of the facial primordia, and brain growth influences the timing of primordia fusion. Third, the brain helps induce a signaling center, the frontonasal ectodermal zone, in the ectoderm, which participates in patterned growth of the upper jaw. Similarly, signals from neural crest cells regulate expression of fibroblast growth factor 8 in the anterior neural ridge, which controls growth of the anterior forebrain. Disruptions to these interactions have significant consequences for normal development of the craniofacial complex, leading to structural malformations and birth defects.

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.

Agnathia Holoprosencephaly and Situs Inversus in A Neonate Born to an Alcoholic Mother

Agnathia, holoprosencephaly and situs inversus complex is an extremely rare form of congenital malformation. Though a few cases have been reported from other parts of the world, to the best of our knowledge none has been reported from India so far. Maternal alcoholism is regarded as an important factor causing holoprosencephaly. Disruption of the Shh gene signaling pathway is also said to be a factor for the occurrence of holoprosencephaly as well as left right asymmetry. Though several factors are suspected as a cause of this deformity, the precise aetiopathogenesis is still under debate. Lack of knowledge might be due to paucity of data from cases due to its rarity. Hereby, we are presenting a case of agnathia, holoprosencephaly and situs inversus born at 32 wk of gestation by an alcoholic mother. Externally the child had agnathia and cyclopia. There was no mandible or any oral cavity. It was accompanied by noticeable limb deformity. Internally there was holoprosencephaly, situs inversus totalis with several visceral abnormalities. To the best of our knowledge this is the first case of agnathia, holoprosencephaly and situs inversus complex to be reported in an indexed literature from India. This report also strengthens the association of maternal alcoholism with occurrence of holoprosencephaly.

A ciliopathy with hydrocephalus, isolated craniosynostosis, hypertelorism, and clefting caused by deletion of Kif3a

Malformations of the facial midline are a consistent feature among individuals with defects in primary cilia. Here, we provide a framework in which to consider how these primary cilia-dependent facial anomalies occur. We generated mice in which the intraflagellar transport protein Kif3a was deleted in cranial neural crest cells. The Kif3a phenotypes included isolated metopic craniosynostosis, delayed closure of the anterior fontanelles, and hydrocephalus, as well as midline facial anomalies including hypertelorism, cleft palate, and bifid nasal septum. Although all cranial neural crest cells had truncated primary cilia as a result of the conditional deletion, only those in the midline showed evidence of hyper-proliferation and ectopic Wnt responsiveness. Thus, cranial neural crest cells do not rely on primary cilia for their migration but once established in the facial prominences, midline cranial neural crest cells require Kif3a function in order to integrate and respond to Wnt signals from the surrounding epithelia.

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.

A review of hedgehog signaling in cranial bone development

During craniofacial development, the Hedgehog (HH) signaling pathway is essential for mesodermal tissue patterning and differentiation. The HH family consists of three protein ligands: Sonic Hedgehog (SHH), Indian Hedgehog (IHH), and Desert Hedgehog (DHH), of which two are expressed in the craniofacial complex (IHH and SHH). Dysregulations in HH signaling are well documented to result in a wide range of craniofacial abnormalities, including holoprosencephaly (HPE), hypotelorism, and cleft lip/palate. Furthermore, mutations in HH effectors, co-receptors, and ciliary proteins result in skeletal and craniofacial deformities. Cranial suture morphogenesis is a delicate developmental process that requires control of cell commitment, proliferation and differentiation. This review focuses on both what is known and what remains unknown regarding HH signaling in cranial suture morphogenesis and intramembranous ossification. As demonstrated from murine studies, expression of both SHH and IHH is critical to the formation and fusion of the cranial sutures and calvarial ossification. SHH expression has been observed in the cranial suture mesenchyme and its precise function is not fully defined, although some postulate SHH to delay cranial suture fusion. IHH expression is mainly found on the osteogenic fronts of the calvarial bones, and functions to induce cell proliferation and differentiation. Unfortunately, neonatal lethality of IHH deficient mice precludes a detailed examination of their postnatal calvarial phenotype. In summary, a number of basic questions are yet to be answered regarding domains of expression, developmental role, and functional overlap of HH morphogens in the calvaria. Nevertheless, SHH and IHH ligands are integral to cranial suture development and regulation of calvarial ossification. When HH signaling goes awry, the resultant suite of morphologic abnormalities highlights the important roles of HH signaling in cranial development.

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.

Brother of cdo (umleitung) is cell-autonomously required for Hedgehog-mediated ventral CNS patterning in the zebrafish

The transmembrane protein Brother of Cdo (Boc) has been implicated in Shh-mediated commissural axon guidance, and can both positively and negatively regulate Hedgehog (Hh) target gene transcription, however, little is known about in vivo requirements for Boc during vertebrate embryogenesis. The zebrafish umleitung (uml(ty54)) mutant was identified by defects in retinotectal axon projections. Here, we show that the uml locus encodes Boc and that Boc function is cell-autonomously required for Hh-mediated neural patterning. Our phenotypic analysis suggests that Boc is required as a positive regulator of Hh signaling in the spinal cord, hypothalamus, pituitary, somites and upper jaw, but that Boc might negatively regulate Hh signals in the lower jaw. This study reveals a role for Boc in ventral CNS cells that receive high levels of Hh and uncovers previously unknown roles for Boc in vertebrate embryogenesis.

Management of children with holoprosencephaly

Holoprosencephaly (HPE) is the most common malformation of the embryonic forebrain in humans. Although HPE occurs along a continuous spectrum, it has been categorized into four types from most severe to least severe: alobar, semilobar, lobar, and middle interhemispheric (MIH) variant. Facial malformations are often associated with HPE and usually correlate with the severity of brain malformation. With the most severely affected newborns, there is a high mortality rate in the first month of life, however, with milder forms of HPE, the majority survive beyond infancy. The Carter Centers for Brain Research in Holoprosencephaly and Related Malformations have enrolled 182 living children in a prospective research study. Based on previously published reports using this database, reports from other investigators, as well as our experience and personal observations, the range of developmental, neurological, and medical problems found in children with HPE is described in this article. Virtually all children with HPE have some developmental disability and the severity correlates with the severity of the brain malformation on neuroimaging. Common medical problems include hydrocephalus, seizures, motor impairment, oromotor dysfunction with risk of poor nutrition and aspiration, chronic lung disease, gastroesophageal reflux, constipation, hypothalamic dysfunction with disturbed sleep-wake cycles and temperature dysregulation, as well as endocrine dysfunction. Diabetes insipidus in particular is found in about 70% of children with classic HPE. Recommendations for management of these problems are given based on experiences of the authors and familiarity with the literature.

Sonic hedgehog (SHH) mutation in patients within the spectrum of holoprosencephaly

Holoprosencephaly (HPE) is a malformation sequence where the cerebral hemispheres fail to separate into distinct left and right halves. It can be associated with midline structural anomalies of the central nervous system and/or face. SHH is the major gene implicated in HPE and it plays a critical role in early forebrain and central nervous system development. SHH is expressed in the human embryo in the notochord, the floorplate of the neural tube, and the posterior limb buds. In the present study we performed mutational analysis of the entire coding region of the SHH gene in 37 unrelated individuals with the HPE spectrum. Three different variants were found throughout the extent of the gene. No genotype-phenotype correlation is evident based on the type or position of the mutations. This study confirms the great genetic heterogeneity of the disease and the difficulty to establish genotype-phenotype correlations.

A primary cilia-dependent etiology for midline facial disorders

Human faces exhibit enormous variation. When pathological conditions are superimposed on normal variation, a nearly unbroken series of facial morphologies is produced. When viewed in full, this spectrum ranges from cyclopia and hypotelorism to hypertelorism and facial duplications. Decreased Hedgehog pathway activity causes holoprosencephaly and hypotelorism. Here, we show that excessive Hedgehog activity, caused by truncating the primary cilia on cranial neural crest cells, causes hypertelorism and frontonasal dysplasia (FND). Elimination of the intraflagellar transport protein Kif3a leads to excessive Hedgehog responsiveness in facial mesenchyme, which is accompanied by broader expression domains of Gli1, Ptc and Shh, and reduced expression domains of Gli3. Furthermore, broader domains of Gli1 expression correspond to areas of enhanced neural crest cell proliferation in the facial prominences of Kif3a conditional knockouts. Avian Talpid embryos that lack primary cilia exhibit similar molecular changes and similar facial phenotypes. Collectively, these data support our hypothesis that a severe narrowing of the facial midline and excessive expansion of the facial midline are both attributable to disruptions in Hedgehog pathway activity. These data also raise the possibility that genes encoding ciliary proteins are candidates for human conditions of hypertelorism and FNDs.

Pfeiffer-like syndrome with holoprosencephaly: a newborn with maternal smoking and alcohol exposure

We report the case of a female infant with Pfeiffer-like syndrome and holoprosencephaly. She had a cloverleaf skull, ocular proptosis, broad thumbs and halluces, and variable accompanying anomalies compatible with Pfeiffer syndrome. She also displayed microcephaly, short palpebral fissures, and a smooth philtrum, which are clinical signs consistent with fetal alcohol syndrome. She suffered from multiple congenital anomalies and died at 41 days of age. Cardio-pulmonary failure, brain abnormalities, prematurity, and multiple complications contributed to her death. The patient displayed normal chromosomal numbers and type. DNA analysis did not reveal fibrobtast growth factor receptor (FGFR) genes FGFR1, FGFR2, FGFR3 or TWIST gene mutations. We review the previous reports of Pfeiffer syndrome and holoprosencephaly and describe our infant patient with Pfeiffer-like syndrome, holoprosencephaly, and heavy in utero maternal alcohol and smoking exposures.

Sensory function in severe semilobar holoprosencephaly

We report a 4-year-old child with severe semi-lobar holoprosencephaly (HPE) not expected to survive after birth. Magnetic resonance imaging (MRI) revealed agenesis of the corpus callosum, absence of the third ventricle, fused thalami and basal ganglia. To investigate sensory function, visual, auditory and somatosensory evoked potential and imaging studies were carried out. The visual response evoked by human face stimuli evoked larger responses over the left side of the holosphere as compared to responses evoked by checkerboard pattern, while auditory evoked potentials were evident over the frontal regions to both pure tones and speech stimuli. No consistent scalp somatosensory evoked potentials were evident. This case demonstrates that electrophysiological measures are able to identify and quantify sensory processing not expected to be present based on the anatomical presentation of the cortex in a child with severe HPE.

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.

Gas1 is a modifier for holoprosencephaly and genetically interacts with sonic hedgehog

Holoprosencephaly (HPE) is a clinically heterogeneous developmental anomaly affecting the CNS and face, in which the embryonic forebrain fails to divide into distinct halves. Numerous genetic loci and environmental factors are implicated in HPE, but mutation in the sonic hedgehog (Shh) gene is an established cause in both humans and mice. As growth arrest-specific 1 (Gas1) encodes a membrane glycoprotein previously identified as a Shh antagonist in the somite, we analyzed the craniofacial phenotype of mice harboring a targeted Gas1 deletion. Gas1(-/-) mice exhibited microform HPE, including midfacial hypoplasia, premaxillary incisor fusion, and cleft palate, in addition to severe ear defects; however, gross integrity of the forebrain remained intact. These defects were associated with partial loss of Shh signaling in cells at a distance from the source of transcription, suggesting that Gas1 can potentiate hedgehog signaling in the early face. Loss of a single Shh allele in a Gas1(-/-) background significantly exacerbated the midline craniofacial phenotype, providing genetic evidence that Shh and Gas1 interact. As human GAS1 maps to chromosome 9q21.3-q22, a region previously associated with nonsyndromic cleft palate and congenital deafness, our results establish GAS1 as a potential locus for several human craniofacial malformations.

Neuropathology of developmental abnormalities

Varieties of neuropathological disorders are caused by a perturbation of normal developmental processes, resulting from insults by heterogeneous etiologic factors. These factors trigger the sequence of molecular, biochemical, and morphologic alterations of the brain, resulting morphologically and/or functionally abnormal brain. The resulting brain contains basic components of the normal brain but is assembled in an abnormal way. The developmental stage when the insults occur appears to largely dictate the outcome of the pathological processes. Depending on the developmental stage involved, the morphology of the brain may be grossly abnormal or is apparently normal but functionally abnormal. The brain development progresses in an orderly fashion and can be divided into several major developmental stages; the neurulation (neural tube formation), ventral induction (formation of prosencephalon), neuroepithelial cell proliferation and migration, neuroglial differentiation and establishment of neuronal circuits. The perturbation of these developmental stages results in uniquely specific pathological outcome, regardless of the etiologic factors/agents. In this review, I will briefly discuss the normal pattern of brain development and neuropathology of the representative disorders resulting from the deviation of normal developmental processes in the individual developmental stage.

Functional analysis of mutations in TGIF associated with holoprosencephaly

Holoprosencephaly (HPE) is the most common structural malformation of the forebrain and face in humans. Our current understanding of the pathogenesis of HPE attempts to integrate genetic susceptibility, evidenced by mutations in the known HPE genes, with the epigenetic influence of environmental factors. Mutations or deletions of the human TGIF gene have been associated with HPE in multiple population cohorts. Here we examine the functional effects of all previously reported mutations, and describe four additional variants. Of the eleven sequence variations in TGIF, all but four can be demonstrated to be functionally abnormal. In contrast, no potentially pathogenic sequence alterations were detected in the related gene TGIF2. These results provide further evidence of a role for TGIF in HPE and demonstrate the importance of functional analysis of putative disease-associated alleles.

Links between abnormal brain structure and cognition in holoprosencephaly

Converging information on medical issues, motor ability, and cognitive outcomes is essential when addressing long-term clinical management in children with holoprosencephaly. This study considered whether adding more informative structural indices to classic holoprosencephaly categories would increase prediction of cognitive outcomes. Forty-two children with holoprosencephaly were examined to determine the association of deep gray nuclei abnormalities with cognitive abilities and the effect of motor skill deficits on cognitive performance. Additionally, a cognitive profile was described using the Carter Neurocognitive Assessment, an experimental diagnostic instrument designed specifically for young children with severe neurodevelopmental dysfunction. Findings indicated that nonseparation of the deep gray nuclei was significantly associated with the cognitive construct of vocal communication, but not with the cognitive constructs of social awareness, visual attention, or auditory comprehension. Importantly, motor skill deficits did not significantly affect performance on the Carter Neurocognitive Assessment. This study is the first investigation to provide a descriptive overview of specific cognitive skills in this group of children. The results also strongly suggest that this feature of the brain's structure does not predict all aspects of neurodevelopmental function. These findings contribute a critical component to the growing body of knowledge regarding the medical and clinical outcomes of children with holoprosencephaly.

Boc is a receptor for sonic hedgehog in the guidance of commissural axons

In the spinal cord, sonic hedgehog (Shh) is secreted by the floor plate to control the generation of distinct classes of ventral neurons along the dorsoventral axis. Genetic and in vitro studies have shown that Shh also later acts as a midline-derived chemoattractant for commissural axons. However, the receptor(s) responsible for Shh attraction remain unknown. Here we show that two Robo-related proteins, Boc and Cdon, bind specifically to Shh and are therefore candidate receptors for the action of Shh as an axon guidance ligand. Boc is expressed by commissural neurons, and targeted disruption of Boc in mouse results in the misguidance of commissural axons towards the floor plate. RNA-interference-mediated knockdown of Boc impairs the ability of rat commissural axons to turn towards an ectopic source of Shh in vitro. Taken together, these data suggest that Boc is essential as a receptor for Shh in commissural axon guidance.

Holoprosencephaly: clinical, anatomic, and molecular dimensions

Holoprosencephaly is addressed under the following headings: alobar, semilobar, and lobar holoprosencephaly; arrhinencephaly; agenesis of the corpus callosum; pituitary abnormalities; hindbrain abnormalities; syntelencephaly; aprosencephaly/atelencephaly; neural tube defects; facial anomalies; median cleft lip; minor facial anomalies; single maxillary central incisor; holoprosencephaly-like phenotype; epidemiology; genetic causes of holoprosencephaly; teratogenic causes of holoprosencephaly; SHH mutations; ZIC2 mutations; SIX3 mutations; TGIF mutations; PTCH mutations; GLI2 mutations; FAST1 mutations; TDGF1 mutations; and DHCR7 mutations.

Factor analysis of neuroanatomical and clinical characteristics of holoprosencephaly

The objective of this study is to better understand the relationship between neuroradiologic and clinical characteristics in holoprosencephaly (HPE) using the multivariate analysis called factor analysis. HPE is a brain malformation characterized by incomplete cleavage of the cerebral hemispheres and deep gray structures. We performed evaluations on 89 children with HPE that included their history, developmental assessment, and physical examination. Ten clinical variables included in factor analysis were the grade of spasticity, dystonia, choreoathetosis, hypotonia, mobility, upper extremity/hand function, expressive language, feeding/swallowing difficulty, endocrinopathies, and temperature dysregulation. Five neuroimaging variables graded by pediatric neuroradiologists were the grade of HPE (from least to most severe: lobar, semilobar, and alobar) and the degree of non-separation of caudate, lentiform, thalamic, and hypothalamic nuclei. Factor analysis using principle component extraction and varimax rotation was utilized. Four significant factors were identified: (1) neuroimaging/developmental factor, (2) motor factor, (3) hypothalamic/oromotor factor, and (4) hypotonia factor. These four factors accounted for 65.2% of the variance. In this factor analysis of HPE patients, we were able to reduce the large number of clinical and radiological variables into four factors. These factors and the constructs underlying them provide structure to the data and provide key parameters for future studies involving neurodevelopmental outcomes in HPE.

The Tgif2 gene contains a retained intron within the coding sequence

Background

TGIF and TGIF2 are homeodomain proteins, which act as TGFβ specific Smad transcriptional corepressors. TGIF recruits general repressors including mSin3 and CtBP. The related TGIF2 protein functions in a similar manner, but does not bind CtBP. In addition to repressing TGFβ activated gene expression, TGIF and TGIF2 repress gene expression by binding directly to DNA. TGIF and TGIF2 share two major blocks of similarity, encompassing the homeodomain, and a conserved carboxyl terminal repression domain. Here we characterize two splice variants of the Tgif2 gene from mouse and demonstrate that the Tgif2 gene contains a retained intron.

Results

By PCR from mouse cDNA, we identified two alternate splice forms of the Tgif2 gene. One splice variant encodes the full length 237 amino acid Tgif2, whereas the shorter form results in the removal of 39 codons from the centre of the coding region. The generation of this alternate splice form occurs with the mouse RNA, but not the human, and both splice forms are present in all mouse tissues analyzed. Human and mouse Tgif2 coding sequences contain a retained intron, which in mouse Tgif2 is removed by splicing from around 25–50% of RNAs, as assessed by RT-PCR. This splicing event is dependent on sequences within the mouse Tgif2 coding sequence. Both splice forms of mouse Tgif2 encode proteins which are active transcriptional repressors, and can repress both TGFβ dependent and independent transcription. In addition, we show that human and mouse Tgif2 interact with the transcriptional corepressor mSin3.

Conclusion

These data demonstrate that the Tgif2 gene contains a retained intron, within the second coding exon. This retained intron is not removed from the human mRNA at a detectable level, but is spliced out in a significant proportion of mouse RNAs. This alternate splicing is dependent entirely on sequences within the mouse Tgif2 coding sequence, suggesting the presence of an exonic splicing enhancer. Both splice forms of mouse Tgif2 produce proteins which are functional transcriptional repressors.

Differential diagnosis in fetuses with absent septum pellucidum

OBJECTIVE

To elucidate the characteristic morphological features that may help in the prenatal differential diagnosis of absent septum pellucidum as demonstrated by ultrasound.

METHODS

Twenty-six fetuses were referred to the fetal neurology clinic due to mild to severe ventriculomegaly and a connection between the lateral ventricles. The following ultrasonographic features were evaluated: place and extent of the ventricular communication, non-cleavage of the hemispheres and deep gray nuclei, callosal anomalies, position of the choroid plexus, and other central nervous system and facial or body anomalies. A flowchart was created in order to facilitate the final diagnosis.

RESULTS

The presence of non-cleavage and/or characteristic facial anomalies prompted the diagnosis of holoprosencephaly (HPE) in 14 fetuses, including two fetuses with the middle interhemispheric variant. Ten fetuses were diagnosed as having hydrocephalus based on the lack of the same features and the observation that the communication between the lateral ventricles was at the level of the third ventricle with almost normal anterior and posterior segments. In two fetuses the diagnosis of septo-optic dysplasia vs. isolated agenesis of the cavum septi pellucidi was contemplated.

CONCLUSIONS

The use of the proposed flowchart enabled differentiation between hydrocephalus and HPE. The communication between the ventricles in hydrocephalic fetuses may be due to a disruption of the septum pellucidum or to a pathological enlargement of the foramen of Monro. Published by John Wiley & Sons, Ltd.

Phenotypic and molecular variability of the holoprosencephalic spectrum

Since 1996, a European network has been organized from Rennes, France and holoprosencephalic files were collected for clinical and molecular study. Familial instances of typical and atypical holoprosencephaly (HPE) were found in 30% of cases. All affected children had psychomotor delay with microcephaly, often associated with endocrine, digestive, and respiratory abnormalities, and thermal dysregulation. Among 173 subjects in the molecular study, 28 heterozygous mutations were identified (16%): 15 SHH mutations, 6 ZIC2 mutations, 5 SIX3 mutations, and 2 TGIF mutations.

Evaluation and management of children with holoprosencephaly

Recent advances in genetics and neuroimaging have greatly contributed to our understanding of the spectrum of midline brain and craniofacial malformations known as holoprosencephaly. Neuroradiologic studies have provided detailed characteristics of four major types of holoprosencephaly: alobar, semilobar, lobar, and middle interhemispheric variant. Clinical studies in children with these types of holoprosencephaly have revealed a wide range of survival and neurologic outcomes. Motor and developmental dysfunctions correlate with the severity of the brain malformation in holoprosencephaly. These findings have implications in the management of medical problems associated with holoprosencephaly and overall prognostication.

A novel mutation of the human 7-dehydrocholesterol reductase gene reduces enzyme activity in patients with holoprosencephaly

Defects in cholesterol biosynthesis genes are recognized as a leading cause for holoprosencephaly (HPE). Previous reports suggest that mutations of human 7-dehydrocholesterol reductase (Dhcr7), which catalyzes the final step of cholesterol biosynthesis, may cause HPE [Clin. Genet. 53 (1998) 155]. To determine whether Dhcr7 mutations are involved in HPE pathogenesis, we analyzed the sequence of exon 9, which contains both a catalytic domain and a mutational hot spot. We examined 36 prematurely terminated fetuses with HPE at their gestation ages in the range from 21 to 33 weeks by single strand conformation polymorphism analysis and DNA sequencing. A novel missense mutation was identified: G344D. Dhcr7 enzyme assays using overexpressed recombinant mutant proteins revealed altered enzyme activity. Mutant G344D harbored less than 50% of enzyme activity compared with the control. Two previously reported mutations, R404C and G410S, abolished enzyme activity. These results suggest that mutation of the Dhcr7 gene is involved in HPE pathogenesis.

Abnormal serotonergic development in a mouse model for the Smith-Lemli-Opitz syndrome: implications for autism

The Smith-Lemli-Opitz syndrome (SLOS) is a malformation/mental retardation syndrome resulting from an inborn error in 3beta-hydroxysteroid Delta7-reductase (DHCR7), the terminal enzyme required for cholesterol biosynthesis. Using a targeting strategy designed to virtually eliminate Dhcr7 activity, we have created a SLOS mouse model that exhibits commissural deficiencies, hippocampal abnormalities, and hypermorphic development of serotonin (5-HT) neurons. The latter is of particular interest with respect to current evidence that serotonin plays a significant role in autism spectrum disorders and the recent clinical observation that 50% of SLOS patients present with autistic behavior. Immunohistochemical analyses have revealed a 306% increase in the area of 5-HT immunoreactivity (5-HT IR) in the hindbrains of mutant (Dhcr7-/-) mice as compared to age-matched wild type animals. Amount of 5-HT IR was measured as total area of IR per histological section. Additionally, a regional increase as high as 15-fold was observed for the most lateral sagittal hindbrain sections. In Dhcr7-/- mice, an expansion of 5-HT IR into the ventricular zone and floor plate region was observed. In addition, the rostral and caudal raphe groups exhibited a radial expansion in Dhcr7-/- mice, with 5-HT IR cells present in locations not seen in wild type mice. This increase in 5-HT IR appears to represent an increase in total number of 5-HT neurons and fibers. These observations may help explain the behavioral phenotype seen in SLOS, and provide clues for future therapeutic interventions that utilize pharmacological modulation of the serotonergic system.

Drosophila TGIF proteins are transcriptional activators

The information carried by transforming growth factor beta (TGF-beta) signaling molecules induces profound responses in target cells. To restrict this information to appropriate cells, TGF-beta signaling pathways are tightly regulated by dynamic interactions with transcriptional activators and repressors. Numerous cross-species experiments have shown that TGF-beta family members and their signal transduction machinery (receptors and Smad signal transducers) are functionally conserved between vertebrates and invertebrates. TG-interacting factor (TGIF) is a homeodomain-containing transcriptional corepressor of TGF-beta-dependent gene expression in mammals that is associated with holoprosencephaly in humans. Here we report a biochemical analysis of TGIF from zebra fish and Drosophila. Our study reveals an unprecedented role reversal between vertebrate and invertebrate TGIF proteins. Zebra fish TGIF, like its mammalian relative, interacts with general corepressors and represses TGF-beta-responsive gene expression. We identified a tandem duplication of TGIF genes in Drosophila. In contrast to vertebrate TGIFs, both Drosophila TGIFs strongly activate transcription. We also demonstrate that Drosophila TGIF proteins physically interact with both Mad and dSmad2, suggesting a role in Dpp and activin signaling. Thus, dTGIF may be the first transcription factor in the Drosophila activin pathway. Overall, our study suggests that assumptions about the functional equivalence of conserved proteins must be validated experimentally.

Congenital abnormalities of body patterning: embryology revisited

Many of the developmental mechanisms and molecular pathways that underlie fundamental features of body patterning are shared by all vertebrates, and some have even been conserved across evolution from invertebrates to vertebrates. Defects in such processes are a common cause of congenital malformation syndromes, and rapid progress is being made in elucidating their embryological and genetic basis. Here, I focus on three examples, each of which has been the subject of recent advances, and which together illustrate many of the most interesting and important aspects of these disorders. The first example is the development of the pharyngeal apparatus and its perturbation in DiGeorge's syndrome; the second is the induction and differentiation of the forebrain and its perturbation in holoprosencephaly; and the third is the role played by the human HOX genes in congenital malformations.

SONIC HEDGEHOG mutations causing human holoprosencephaly impair neural patterning activity

Holoprosencephaly (HPE) is a common forebrain malformation associated with mental retardation and craniofacial anomalies. Multiple lines of evidence indicate that loss of ventral neurons is associated with HPE. The condition is etiologically heterogeneous, and abnormalities in any of several genes can cause human HPE. Among these genes, mutations in SONIC HEDGEHOG ( SHH) are the most commonly identified single gene defect causing human HPE. SHH mediates a number of processes in central nervous system development and is required for the normal induction of ventral cell types in the brain and spinal cord. Although a number of missense mutations in SHH have been identified in patients with HPE, the functional significance of these mutations has not yet been determined. We demonstrate that two SHH mutations that cause human HPE result in decreased in vivo activity of SHH in the developing nervous system. These mutant forms of SHH fail to regulate genes properly that are normally responsive to SHH signaling and do not induce ventrally expressed genes. In addition, the immunoreactivity of the mutant proteins is altered, suggesting that the conformation of the SHH protein has been disrupted. These studies are the first demonstration that mutations in SHH associated with human HPE perturb the in vivo patterning function of SHH in the developing nervous system.

Microform holoprosencephaly in mice that lack the Ig superfamily member Cdon

Holoprosencephaly (HPE), the most common developmental defect of the forebrain and midface, is caused by a failure to delineate the midline in these structures. Despite the identification of several HPE genes, its genetic basis is largely unknown. Furthermore, the phenotype of affected individuals is highly variable, even within pedigrees. Facial defects in HPE range from cyclopia and proboscis in severe cases to solitary median maxillary central incisor in individuals with microforms of HPE. Cdon (also known as Cdo), an Ig superfamily member, is a component of a cell surface receptor that positively regulates skeletal myogenesis. Cdon is also highly expressed in the frontonasal and maxillary processes (FNP and MXP, respectively) of the developing mouse embryo, structures that contain signaling centers that pattern the face. We report here that mice homozygous for targeted mutations of Cdon display the hallmark facial defects associated with microforms of HPE. This is the first example of a mouse mutant with this phenotype, and this finding implicates a new family of receptors in development of the facial midline and suggests a potential role for Cdon in the pathogenesis and expressivity of HPE in humans.

The interplay of genetic and environmental factors in craniofacial morphogenesis: holoprosencephaly and the role of cholesterol

Cyclopia, the paradigmatic "face [that] predicts the brain" in severe holoprosencephaly (HPE) (DeMyer et al., 1964), has been recognized since ancient times. Descriptive embryologists and pathologists have noted the continuum of defective separation of the forebrain and loss of central nervous system (CNS) midline structures for more than a century. It has been recognized more recently that inhibitors of cholesterol biosynthesis, whether consumed in native plants by range sheep, or experimentally applied to early embryos, could phenocopy the natural malformation, as could a variety of other teratogens (maternal diabetes, alcohol). Yet it has been less than a decade that the genomic knowledge base and powerful analytic methods have brought the sciences of descriptive, molecular, and genetic embryology within range of each other. In this review, we discuss the clinical presentations and pathogenesis of HPE. We will outline various genetic and teratogenic mechanisms leading to HPE. Lastly, we will attempt to examine the pivotal role of cholesterol and the Sonic Hedgehog (Shh) pathway in this disorder and in normal embryonic forebrain development.

Previously undescribed nonsense mutation in SHH caused autosomal dominant holoprosencephaly with wide intrafamilial variability

Holoprosencephaly (HPE) is the most common developmental defect of the forebrain and midface in humans, with a frequency of 1/16,000 live births. Different genes are implicated in the pathogenesis of HPE; these include SHH, ZIC2, SIX3, TGIF, and human DKK1. We describe here a family with recurrence of autosomal dominant HPE in different members showing a wide clinical variability. The mother presents a single central maxillary incisor and mild hypotelorism as signs of the diseases, while three of her sons were affected by HPE. By direct sequencing and restriction analysis of exon 2 of the SHH gene, we have identified a previously undescribed nonsense mutation at codon 128 (W128X). The identification of this mutation allowed us to give a prenatal diagnosis in this family and confirms a wide intrafamilial variability in the phenotypic spectrum.

Binderoid complete cleft lip/palate

A small subset of infants with complete cleft lip/palate look different because they have nasolabiomaxillary hypoplasia and orbital hypotelorism. The authors' purpose was to define the clinical and radiographic features of these patients and to comment on operative management, classification, and terminology. The authors reviewed 695 patients with all forms of incomplete and complete cleft lip/palate and identified 15 patients with nasolabiomaxillary hypoplasia and orbital hypotelorism. All 15 patients had complete labial clefting (5 percent of 320 patients with complete cleft lip/palate), equally divided between bilateral and unilateral forms. The female-to-male ratio was 2:1. Of the seven infants with unilateral complete cleft lip/palate, one had an intact secondary palate and all had a hypoplastic septum, small alar cartilages, narrow basilar columella, underdeveloped contralateral philtral ridge, ill-defined Cupid's bow, thin vermilion-mucosa on both sides of the cleft, and a diminutive premaxilla. Of the eight infants with bilateral complete cleft lip, one had an intact secondary palate. The features were the same as in patients with unilateral cleft, but with a more severely hypoplastic nasal tip, conical columella, tiny prolabium, underdeveloped lateral labial elements, and small/mobile premaxilla. Central midfacial hypoplasia and hypotelorism did not change during childhood and adolescence. Intermedial canthal measurements remained 1.5 SD below normal age-matched controls. Skeletal analysis (mean age, 10 years; range, 4 months to 19 years) documented maxillary retrusion (mean sagittal maxillomandibular discrepancy, 13.7 mm; range, 3 to 17 mm), absent anterior nasal spine, and a class III relationship. The mean sella nasion A point (S-N-A) angle of 74 degrees (range, 65 to 79 degrees) and sella nasion B point (S-N-B) angle of 81 degrees (range, 71 to 90 degrees) were significantly different from age-matched norms ( = 0.0007 and = 0.004, respectively). The ipsilateral central and lateral incisors were absent in all children with unilateral cleft, whereas a single-toothed premaxilla was typically found in the bilateral patients. Several modifications were necessary during primary nasolabial repair because of the diminutive bony and soft-tissue elements. All adolescent patients had Le Fort I maxillary advancement and construction of an adult nasal framework with costochondral or cranial graft. Other often-used procedures were bony augmentation of the anterior maxilla; cartilage grafts to the nasal tip and columella; and dermal grafting to the median tubercle, philtral ridge, and basal columella. Infants with complete unilateral or bilateral cleft lip/palate in association with nasolabiomaxillary hypoplasia and orbital hypotelorism do not belong on the holoprosencephalic spectrum because they have normal head circumference, stature, and intelligence, nor should they be referred to as having Binder anomaly. The authors propose the term cleft lip/palate for these children. Early recognition of this entity is important for counseling parents and because alterations in standard operative methods and orthodontic protocols are necessary.

Prenatal diagnosis of holoprosencephaly in two fetuses with der (7)t(1;7)(q32;q32)pat inherited from the father with double translocations

The presence of two independent translocations in one person is rare. Herein, we report the prenatal diagnosis of two sibling fetuses with holoprosencephaly, whose father is a carrier of double translocations. The karyotype of the father is 46,XY, t(1;7) (q32;q32), t(14,15) (q32.1;q26.3). The two fetuses had variable facial dysmorphisms and identical cytogenetic abnormality-a derivative (7) t(1;7) (q32;q32) inherited from the father. The proband 1 showed a small mouth, a single median eye and a proboscis above the eye, while the proband 2 showed hypotelorism, a flat nose, cleft lip and cleft palate. Both fetuses also had alobar holoprosencephaly. Haploinsufficiency of the sonic hedgehog gene at 7q36 does account for the occurrence of holoprosencephaly in the two fetuses with a deletion of distal 7q (7q32 --> qter).

Malformations of the craniofacial region: evolutionary, embryonic, genetic, and clinical perspectives

Malformations of the craniofacial region are reviewed with respect to evolutionary, embryonic, genetic, and clinical perspectives under the following headings: How Old Is Our Head?, Head Organization Genes, Genetics of Craniofacial Anomalies, Craniofacial Derivatives, Anencephaly, Cephalocele, Holoprosencephaly, Craniosynostosis, Hypertelorism, Branchial Arch Anomalies, and Orofacial Clefting.

Holoprosencephaly: genetic, neuroradiological, and clinical advances

Recent advances in genetics and neuroimaging have greatly contributed to our understanding of the spectrum of midline brain and craniofacial malformations known as holoprosencephaly. This review summarizes our current understanding of the epidemiology and molecular-genetic bases of these malformations, as well as recent neuroradiological and clinical studies, which have revealed that the manifestations of holoprosencephaly are far more variable than previously appreciated. We also discuss the implications for and importance of accurate diagnosis, prognosis, management of common medical problems, and counseling for affected families.

Chordin and noggin promote organizing centers of forebrain development in the mouse

In this study we investigate the roles of the organizer factors chordin and noggin, which are dedicated antagonists of the bone morphogenetic proteins(BMPs), in formation of the mammalian head. The mouse chordin and noggin genes(Chrd and Nog) are expressed in the organizer (the node) and its mesendodermal derivatives, including the prechordal plate, an organizing center for rostral development. They are also expressed at lower levels in and around the anterior neural ridge, another rostral organizing center. To elucidate roles of Chrd and Nog that are masked by the severe phenotype and early lethality of the double null, we have characterized embryos of the genotype Chrd-/-;Nog+/-. These animals display partially penetrant neonatal lethality, with defects restricted to the head. The variable phenotypes include cyclopia,holoprosencephaly, and rostral truncations of the brain and craniofacial skeleton. In situ hybridization reveals a loss of SHH expression and signaling by the prechordal plate, and a decrease in FGF8 expression and signaling by the anterior neural ridge at the five-somite stage. DefectiveChrd-/-;Nog+/- embryos exhibit reduced cell proliferation in the rostral neuroepithelium at 10 somites, followed by increased cell death 1 day later. Because these phenotypes result from reduced levels of BMP antagonists, we hypothesized that they are due to increased BMP activity. Ectopic application of BMP2 to wild-type cephalic explants results in decreased FGF8 and SHH expression in rostral tissue, suggesting that the decreased expression of FGF8 and SHH observed in vivo is due to ectopic BMP activity. Cephalic explants isolated from Chrd;Nog double mutant embryos show an increased sensitivity to ectopic BMP protein, further supporting the hypothesis that these mutants are deficient in BMP antagonism. These results indicate that the BMP antagonists chordin and noggin promote the inductive and trophic activities of rostral organizing centers in early development of the mammalian head.