Neurocristopathies: Enigmatic Appearances of Neural Crest Cell-derived Abnormalities.

The neural crest is an important transient structure that develops during embryogenesis in vertebrates. Neural crest cells are multipotent progenitor cells that migrate and develop into a diverse range of cells and tissues throughout the body. Although neural crest cells originate from the ectoderm, they can differentiate into mesodermal-type or endodermal-type cells and tissues. Some of these tissues include the peripheral, autonomic, and enteric nervous systems; chromaffin cells of the adrenal medulla; smooth muscles of the intracranial blood vessels; melanocytes of the skin; cartilage and bones of the face; and parafollicular cells of the thyroid gland. Neurocristopathies are a group of diseases caused by the abnormal generation, migration, or differentiation of neural crest cells. They often involve multiple organ systems in a single person, are often familial, and can be associated with the development of neoplasms. As understanding of the neural crest has advanced, many seemingly disparate diseases, such Treacher Collins syndrome, 22q11.2 deletion syndrome, Hirschsprung disease, neuroblastoma, neurocutaneous melanocytosis, and neurofibromatosis, have come to be recognized as neurocristopathies. Neurocristopathies can be divided into three main categories: dysgenetic malformations, neoplasms, and combined dysgenetic and neoplastic syndromes. In this article, neural crest development, as well as several associated dysgenetic, neoplastic, and combined neurocristopathies, are reviewed. Neurocristopathies often have clinical manifestations in multiple organ systems, and radiologists are positioned to have significant roles in the initial diagnosis of these disorders, evaluation of subclinical associated lesions, creation of treatment plans, and patient follow-up. Online supplemental material is available for this article. ©RSNA, 2019.

Tissue-selective effects of nucleolar stress and rDNA damage in developmental disorders.

Many craniofacial disorders are caused by heterozygous mutations in general regulators of housekeeping cellular functions such as transcription or ribosome biogenesis. Although it is understood that many of these malformations are a consequence of defects in cranial neural crest cells, a cell type that gives rise to most of the facial structures during embryogenesis, the mechanism underlying cell-type selectivity of these defects remains largely unknown. By exploring molecular functions of DDX21, a DEAD-box RNA helicase involved in control of both RNA polymerase (Pol) I- and II-dependent transcriptional arms of ribosome biogenesis, we uncovered a previously unappreciated mechanism linking nucleolar dysfunction, ribosomal DNA (rDNA) damage, and craniofacial malformations. Here we demonstrate that genetic perturbations associated with Treacher Collins syndrome, a craniofacial disorder caused by heterozygous mutations in components of the Pol I transcriptional machinery or its cofactor TCOF1 (ref. 1), lead to relocalization of DDX21 from the nucleolus to the nucleoplasm, its loss from the chromatin targets, as well as inhibition of rRNA processing and downregulation of ribosomal protein gene transcription. These effects are cell-type-selective, cell-autonomous, and involve activation of p53 tumour-suppressor protein. We further show that cranial neural crest cells are sensitized to p53-mediated apoptosis, but blocking DDX21 loss from the nucleolus and chromatin rescues both the susceptibility to apoptosis and the craniofacial phenotypes associated with Treacher Collins syndrome. This mechanism is not restricted to cranial neural crest cells, as blood formation is also hypersensitive to loss of DDX21 functions. Accordingly, ribosomal gene perturbations associated with Diamond-Blackfan anaemia disrupt DDX21 localization. At the molecular level, we demonstrate that impaired rRNA synthesis elicits a DNA damage response, and that rDNA damage results in tissue-selective and dosage-dependent effects on craniofacial development. Taken together, our findings illustrate how disruption in general regulators that compromise nucleolar homeostasis can result in tissue-selective malformations.

Restoration of polr1c in Early Embryogenesis Rescues the Type 3 Treacher Collins Syndrome Facial Malformation Phenotype in Zebrafish.

Treacher Collins syndrome (TCS) is a rare congenital birth disorder (1 in 50,000 live births) characterized by severe craniofacial defects. Recently, the authors' group unfolded the pathogenesis of polr1c Type 3 TCS by using the zebrafish model. Facial development depends on the neural crest cells, in which polr1c plays a role in regulating their expression. In this study, the authors aimed to identify the functional time window of polr1c in TCS by the use of photo-morpholino to restore the polr1c expression at different time points. Results suggested that the restoration of polr1c at 8 hours after fertilization could rescue the TCS facial malformation phenotype by correcting the neural crest cell expression, reducing the cell death, and normalizing the p53 mRNA expression level in the rescued morphants. However, such recovery could not be reproduced if the polr1c is restored after 30 hours after fertilization.

Sf3b4-depleted Xenopus embryos: A model to study the pathogenesis of craniofacial defects in Nager syndrome.

Mandibulofacial dysostosis (MFD) is a human developmental disorder characterized by defects of the facial bones. It is the second most frequent craniofacial malformation after cleft lip and palate. Nager syndrome combines many features of MFD with a variety of limb defects. Mutations in SF3B4 (splicing factor 3b, subunit 4) gene, which encodes a component of the pre-mRNA spliceosomal complex, were recently identified as a cause of Nager syndrome, accounting for 60% of affected individuals. Nothing is known about the cellular pathogenesis underlying Nager type MFD. Here we describe the first animal model for Nager syndrome, generated by knocking down Sf3b4 function in Xenopus laevis embryos, using morpholino antisense oligonucleotides. Our results indicate that Sf3b4-depleted embryos show reduced expression of the neural crest genes sox10, snail2 and twist at the neural plate border, associated with a broadening of the neural plate. This phenotype can be rescued by injection of wild-type human SF3B4 mRNA but not by mRNAs carrying mutations that cause Nager syndrome. At the tailbud stage, morphant embryos had decreased sox10 and tfap2a expression in the pharyngeal arches, indicative of a reduced number of neural crest cells. Later in development, Sf3b4-depleted tadpoles exhibited hypoplasia of neural crest-derived craniofacial cartilages, phenocopying aspects of the craniofacial skeletal defects seen in Nager syndrome patients. With this animal model we are now poised to gain important insights into the etiology and pathogenesis of Nager type MFD, and to identify the molecular targets of Sf3b4.

Prevention of Treacher Collins syndrome craniofacial anomalies in mouse models via maternal antioxidant supplementation.

Craniofacial anomalies account for approximately one-third of all birth defects and are a significant cause of infant mortality. Since the majority of the bones, cartilage and connective tissues that comprise the head and face are derived from a multipotent migratory progenitor cell population called the neural crest, craniofacial disorders are typically attributed to defects in neural crest cell development. Treacher Collins syndrome (TCS) is a disorder of craniofacial development and although TCS arises primarily through autosomal dominant mutations in TCOF1, no clear genotype-phenotype correlation has been documented. Here we show that Tcof1 haploinsufficiency results in oxidative stress-induced DNA damage and neuroepithelial cell death. Consistent with this discovery, maternal treatment with antioxidants minimizes cell death in the neuroepithelium and substantially ameliorates or prevents the pathogenesis of craniofacial anomalies in Tcof1(+/-) mice. Thus maternal antioxidant dietary supplementation may provide an avenue for protection against the pathogenesis of TCS and similar neurocristopathies.

A quantitative method for defining high-arched palate using the Tcof1(+/-) mutant mouse as a model.

The palate functions as the roof of the mouth in mammals, separating the oral and nasal cavities. Its complex embryonic development and assembly poses unique susceptibilities to intrinsic and extrinsic disruptions. Such disruptions may cause failure of the developing palatal shelves to fuse along the midline resulting in a cleft. In other cases the palate may fuse at an arch, resulting in a vaulted oral cavity, termed high-arched palate. There are many models available for studying the pathogenesis of cleft palate but a relative paucity for high-arched palate. One condition exhibiting either cleft palate or high-arched palate is Treacher Collins syndrome, a congenital disorder characterized by numerous craniofacial anomalies. We quantitatively analyzed palatal perturbations in the Tcof1(+/-) mouse model of Treacher Collins syndrome, which phenocopies the condition in humans. We discovered that 46% of Tcof1(+/-) mutant embryos and new born pups exhibit either soft clefts or full clefts. In addition, 17% of Tcof1(+/-) mutants were found to exhibit high-arched palate, defined as two sigma above the corresponding wild-type population mean for height and angular based arch measurements. Furthermore, palatal shelf length and shelf width were decreased in all Tcof1(+/-) mutant embryos and pups compared to controls. Interestingly, these phenotypes were subsequently ameliorated through genetic inhibition of p53. The results of our study therefore provide a simple, reproducible and quantitative method for investigating models of high-arched palate.

Treacher Collins syndrome: unmasking the role of Tcof1/treacle.

Treacher Collins syndrome (TCS) is a rare congenital birth disorder characterized by severe craniofacial defects. The syndrome is associated with mutations in the TCOF1 gene which encodes a putative nucleolar phosphoprotein known as treacle. An animal model of the severe form of TCS, generated through mutation of the mouse homologue Tcof1 has recently revealed significant insights into the etiology and pathogenesis of TCS (Dixon and Dixon, 2004; Dixon et al., 2006; Jones et al 2008). During early embryogenesis in a TCS individual, an excessive degree of neuroepithelial apoptosis diminishes the generation of neural crest cells. Neural crest cells are a migratory stem and progenitor cell population that generates most of the tissues of the head including much of the bone, cartilage and connective tissue. It has been hypothesized that mutations in Tcof1 disrupt ribosome biogenesis to a degree that is insufficient to meet the proliferative needs of the neuroepithelium and neural crest cells. This causes nucleolar stress activation of the p53-dependent apoptotic pathway which induces neuroepithelial cell death. Interestingly however, chemical and genetic inhibition of p53 activity can block the wave of apoptosis and prevent craniofacial anomalies in Tcof1 mutant mice [Jones NC, Lynn ML, Gaudenz K, Sakai D, Aoto K, Rey JP, et al. Prevention of the neurocristopathy Treacher Collins syndrome through inhibition of p53 function. Nat Med 2008;14:125-33]. These findings shed new light on potential therapeutic avenues for the prevention of not only TCS but also other congenital craniofacial disorders which share a similar etiology and pathogenesis.

Craniofacial malformations: intrinsic vs extrinsic neural crest cell defects in Treacher Collins and 22q11 deletion syndromes.

The craniofacial complex is anatomically the most sophisticated part of the body. It houses all the major sensory organ systems and its origins are synonymous with vertebrate evolution. Of fundamental importance to craniofacial development is a specialized population of stem and progenitor cells, known as the neural crest, which generate the majority of the bone, cartilage, connective and peripheral nerve tissue in the head. Approximately one third of all congenital abnormalities exhibit craniofacial malformations and consequently, most craniofacial anomalies are considered to arise through primary defects in neural crest cell development. Recent advances however, have challenged this classical dogma, underscoring the influence of tissues with which the neural crest cells interact as the primary origin of patterning defects in craniofacial morphogenesis. In this review we discuss these neural crest cell interactions with mesoderm, endoderm and ectoderm in the head in the context of a better understanding of craniofacial malformations such as in Treacher Collins and 22q11 deletion syndromes.

[Musculoskeletal connections. Study of two cases of oto-mandibular dysplasia]. / Connexions musculo-squelettiques. Etude de deux cas de dysplasies oto-mandibulaires.

The current genetic data stress the importance of musculo-skeletal connections in the development of a coherent system connecting the tendons and aponeurosis muscles with the osseous parts. The observations in tomodensitometry of musculo-skeletal connections in otomandibular dysostosis make it possible qualitatively to observe the development of the muscles and their functions.

Craniofacial development: the tissue and molecular interactions that control development of the head.

The molecular cascades that control craniofacial development have until recently been little understood. The paucity of data that exists has in part been due to the complexity of the head, which is the most intricate regions of the body. However, the generation of mouse mutants and the identification of gene mutations that cause human craniofacial syndromes, together with classical embryological approaches in other species, have given significant insight into how the head develops. These studies have emphasized how unique the head actually is, with each individual part governed by a distinct set of signalling interactions, again demonstrating the complexity of this region of the body. This review discussed the tissue and molecular interactions that control each region of the head. The processes that control neural tube closure together with correct development of the skull, midline patterning, neural crest generation and migration, outgrowth, patterning, and differentiation of the facial primordia and the branchial arches are thus discussed. Defects in these processes result in a number of human syndromes such as exencephaly, holoprosencephaly, musculoskeletal dysplasias, first arch syndromes such as Riegers and Treacher-Collins syndrome, and neural crest dysplasias such as DiGeorge syndrome. Our current knowledge of the genes responsible for these human syndromes together with how the head develops, is rapidly advancing so that we will soon understand the complex set of molecular and tissue interactions that build a head.

Prenatal diagnosis and confirmation of the acrofacial dysostosis syndrome type Rodriguez.

The group of acrofacial dysostosis (AFD) syndromes is very heterogeneous and contains many different entities. In 1990, Rodriguez et al. [1990: Am J Med Genet 35:484-489] described a new type of AFD characterized by severe mandibular hypoplasia, phocomelia and oligodactyly of the upper limbs, absence of fibulae, microtia, cleft palate, internal organ anomalies including arrhinencephaly and abnormal lung lobulation, and early lethality. We describe another case of AFD type Rodriguez, identified by prenatal ultrasonography at 25 weeks of gestation.

Increased levels of apoptosis in the prefusion neural folds underlie the craniofacial disorder, Treacher Collins syndrome.

Treacher Collins syndrome (TCS) is an autosomal dominant disorder of human craniofacial development that results from loss-of-function mutations in the gene TCOF1. Although this gene has been demonstrated to encode the nucleolar phosphoprotein treacle, the developmental mechanism underlying TCS remains elusive, particularly as expression studies have shown that the murine orthologue, Tcof1, is widely expressed. To investigate the molecular pathogenesis of TCS, we replaced exon 1 of Tcof1 with a neomycin-resistance cassette via homologous recombination in embryonic stem cells. Tcof1 heterozygous mice die perinatally as a result of severe craniofacial anomalies that include agenesis of the nasal passages, abnormal development of the maxilla, exencephaly and anophthalmia. These defects arise due to a massive increase in the levels of apoptosis in the prefusion neural folds, which are the site of the highest levels of Tcof1 expression. Our results demonstrate that TCS arises from haploinsufficiency of a protein that plays a crucial role in craniofacial development and indicate that correct dosage of treacle is essential for survival of cephalic neural crest cells.

Cranio-facial dysmorphism: experimental study in the mouse, clinical applications.

To obtain a better understanding of mandibulo-facial dysostosis and hemicraniofacial microsomia in man, the authors carried out a histologic and scanning electron microscope study of the facial malformations produced in mouse embryos by retinoic acid and methyl-triazene. The administration of 400 mg/kg 13 cis-retinoic acid (RA) to pregnant C57BL mice on day 9 of gestation produced anomalies of the cephalic extremity in the embryos resembling human mandibulo-facial dysostosis. The 64 embryos collected presented hypoplasia of the branchial arches or the snout in 79% of cases, auricular anomalies in 47% and ophthalmic anomalies in 12.5%. Fourteen NMRI mice on day 10.5 of gestation were treated with 1.5 mg (0.5 mg/kg) methyl-triazene (Methyl). The 126 embryos collected had developed a very high percentage of micromandibles and anomalies of both embryonic ears (94.6% to 100%). Finally, although the facial anomalies produced by retinoic acid resemble the human mandibulo-facial dysostosis syndrome, no correlation was found between hemicraniofacial microsomia and the administration of methyl-triazene.

First and second branchial arch syndrome: aspects on the embryogenesis, elucidations, and rehabilitation using the osseointegration concept.

BACKGROUND: The osseointegration concept has dramatically changed the possibility of rehabilitating patients with craniofacial defects due to branchial arch syndromes. PURPOSE: This article describes some problems related to the investigative routines and rehabilitation of individuals with malformations of the first and second branchial arches of the craniofacial region. Animal model systems have increased the knowledge of basic embryonic processes that can explain the extent of the malformations. Though most clinical first and second branchial arch syndromes are likely to be caused by sporadic mutations, inherited syndromes occur and also teratogenically induced syndromes are known. Prenatal diagnosis ruling out heredity and exogenous influence seems possible in the future. The possibility of preventing and alleviating fulminant syndromes prenatally also could be conceivable in the future. PATIENTS AND METHODS: The rehabilitation process starts early after birth and should involve a team of specialists including clinical geneticists, pediatricians, audiologists, plastic surgeons, maxillofacial surgeons, otosurgeons, anaplastologists, speech pathologists, pedodontists, and orthodontists. With the development of the osseointegration concept in which craniofacial prostheses and hearing aids can be adapted on implants anchored in the craniofacial skeleton, a new field in the rehabilitation of these malformations has opened. RESULTS: Important aspects in the use of the osseointegration concept include determination of the lowest age for implant surgery, accessibility of adequate bone for implants, the growth of the craniofacial skeleton during childhood, and the possibility for the patient and his or her parents to care for the skin penetration. Adverse tissue reactions, durability of craniofacial prostheses, and the possibility of unknown adverse reactions to metal implants in the body over a long time are other aspects of concern. CONCLUSIONS: Patients with branchial arch syndromes benefit from a well-planned multidisciplinary rehabilitation process in which osseointegrated bone-anchored hearing aids and bone-anchored ear prostheses can be useful tools.

Clinical appearance of spontaneous and induced first and second branchial arch syndromes.

The clinical appearance was investigated of 29 patients with mandibulofacial dysostosis, 26 with hemifacial microsomia, and seven with thalidomide-induced malformations affecting derivatives of the first and second branchial arches. Malformations of the external ear, ear canal, middle ear, zygoma, maxilla, mandible, and lower eye lid were prominent features of the syndromes. Facial nerve and 6th cranial nerve paralysis as well as anophthalmia or microphthalmia were seen only in patients with hemifacial microsomia and in the thalidomide-induced syndrome. We compared the clinical results with those in an animal model in which an induced first and second branchial arch syndrome depends on disturbed migration of neural crest cell during early embryogenesis. The critical time for a similar process in humans would be between the 20th and 29th days of pregnancy.

Diamond-Blackfan anemia associated with Treacher-Collins syndrome.

We describe a 2-year-old girl with a rare combination of congenital red cell aplasia or Diamond-Blackfan anemia (DBA) and Treacher-Collins syndrome (TCS). The anemia is only marginally responsive to high-dose corticosteroid, and the child is transfusion dependent. There is no one in the family affected with either DBA or TCS. A hypothesis is advanced that the simultaneous occurrence of the dysmorphism and erythroid agenesis in this case may have been the consequences of an insult to the fetus at the critical stage of development of maxillomandibular structure and the stage of primitive erythroid cell migration from the yolk sac to the fetal liver and bone marrow.

Is Far a Hox mutation?

The mouse First arch mutation, Far, causes a severe syndrome of craniofacial defects described previously. All of the known defects are derived from the anterior first arch, and to a very small extent, the dorsal second arch. Recently Far has been shown to be closely linked to Ulnaless on chromosome 2, and therefore in the vicinity of the Hox-4 gene cluster. This paper reports the results of several studies focused on the development origin of the most consistently expressed dominant effect caused by Far, an abnormal major bifurcation of the maxillary nerve. Nerve-stained whole-mount preparations of day 12 embryos showed that in Far mutants the maxillary nerve appears to have a central wedge missing from the normal single-stalked fan shape, and that the nerve defect in Far/Far and +/Far may be equally severe. The effect of retinoic acid on the development of the maxillary nerve was tested. Maternal treatment with 5 mg/kg retinoic acid on day 9 of gestation had no detectable effect on the maxillary nerve of +/Far embryos, and similar treatment with a teratogenic dosage (20 mg/kg) on day 8 or 9 produced no Far-like maxillary nerve defects in genetically normal embryos. The neural crest cells that give rise to nerves and mesenchyme of the first arch originate from specific rhombomeres, discrete segments of the developing head. The rhombomeres of 15 embryos at the 14-23 somite stages, of which 75% are expected to be +/Far or Far/Far, were examined. There was no detectable defect in segmentation or morphology of the rhombomeres compared with controls. The significance of ectopic cartilage in the palate of Far/Far mutants in relation to nerve bifurcation was explored. In histological studies, five out of six Far/Far day-15 fetuses had a rod of ectopic cartilage lateral to the posterior palate, running parallel to, and morphologically similar to, Meckel's cartilage, and lying between the two trunks of the abnormally bifurcated maxillary nerve. None of six +/Far day-15 fetuses examined had detectable ectopic cartilage in this region. We hypothesize that the maxillary nerve defects in Far mutants may be explained by the presence of an ectopic precartilaginous blastema that does not always further develop into detectable cartilage. The ectopic cartilage found in Far/Far resembles the epibranchial cartilage expressed in more posterior branchial arches and in the first arch of lower organisms, and therefore may represent an atavistic posteriorization of the anterior first arch in Far mutants.(ABSTRACT TRUNCATED AT 400 WORDS)

[Role of the neural crest in maxillofacial malformations: facts and hypotheses]. / Rôle des crêtes neurales dans les malformations maxillo-faciales: faits et hypothèses.

The occurrence of neural crest defects has been postulated in the genesis of several maxillo-facial malformations. It seems to be the case for mandibulo-facial dysostosis and holoprosencephaly.

Pathogenesis of cleft palate in Treacher Collins, Nager, and Miller syndromes.

Abnormalities of the secondary palate were studied in an animal model in which features of Treacher Collins syndrome (TCS) and Nager or Miller syndromes (both of which are facially similar to Treacher Collins, but include limb malformations) were induced by acute maternal exposure to 13-cis-retinoic acid (13-cis-RA, isotretinoin, Accutane). Previous work in our laboratory has illustrated that excessive cell death in the proximal aspect of the maxillary and mandibular prominences of the first visceral arch and in the apical ectodermal ridge of the limb bud probably accounts for the characteristic craniofacial and limb abnormalities observed (Sulik et al, 1987; Sulik and Dehart, 1988). The current study shows that maternal treatment with 400 mg per kilogram 13-cis-RA at 8 days 14 hours (8d14hr) or 9d6hr post fertilization results in abnormalities of the secondary palate that vary in incidence and severity. Following the earlier treatment time, 82 percent (68 of 74) of the 18d fetuses were affected, with, severely hypoplastic, unfused palatal shelves present in 34 percent (25 of 74). The less severely affected fetuses had malformations that involved primarily the posterior aspect of the palatal shelves. This malformation (foreshortening of the posterior portion of the palate) constituted the major developmental alteration that resulted from treatment at the later time, at which time a 52 percent (26 of 50) malformation incidence was seen. The change in pattern of malformations with treatment time is consistent with the changing pattern of programmed cell death, which was observed to occur in the first visceral arch.