Altered mandibular development precedes the time of palate closure in mice homozygous for disproportionate micromelia: an oral clefting model supporting the Pierre-Robin sequence

Additional diagnoses in children with cleft lip and palate up to five years of age

Cleft lip and palate (CL/P) is the most common congenital craniofacial malformation and is often associated with additional diagnoses. The purpose of this study was to explore the cumulative five-year incidence of additional diagnoses for patients with cleft lip and palate. Further aims were, type of cleft and type of additional diagnose and to validate CLP registry data on additional diagnoses. Data from the CLP registry regarding children with CL/P in the Southern Health Care Region were retrieved and based on the registry, participants were selected. A review of medical records of participants born 2006-2016 was performed and data regarding participant characteristics and additional diagnoses were collected. Of the 250 participants included in the review of medical records, 90 participants (36%) had an additional diagnosis. Of the total number of identified additional diagnoses (n = 137), cardiovascular system (20.4%) and extremities and skeletal system (17.5%) were the most prevalent categories. The comparison between medical records and the CLP registry of all children showed a 14.4 percentage points higher incidence of additional diagnoses in the medical records. Roughly every third child received an additional diagnosis and diagnoses related to the cardiovascular system were the most frequent. This study also shows that additional diagnoses were under-reported in the CLP registry. Future research is necessary to strengthen associations of additional diagnoses to CL/P.

PRDM paralogs antagonistically balance Wnt/β-catenin activity during craniofacial chondrocyte differentiation

Cranial neural crest cell (NCC)-derived chondrocyte precursors undergo a dynamic differentiation and maturation process to establish a scaffold for subsequent bone formation, alterations in which contribute to congenital birth defects. Here, we demonstrate that transcription factor and histone methyltransferase proteins Prdm3 and Prdm16 control the differentiation switch of cranial NCCs to craniofacial cartilage. Loss of either paralog results in hypoplastic and disorganized chondrocytes due to impaired cellular orientation and polarity. We show that these proteins regulate cartilage differentiation by controlling the timing of Wnt/β-catenin activity in strikingly different ways: Prdm3 represses whereas Prdm16 activates global gene expression, although both act by regulating Wnt enhanceosome activity and chromatin accessibility. Finally, we show that manipulating Wnt/β-catenin signaling pharmacologically or generating prdm3-/-;prdm16-/- double mutants rescues craniofacial cartilage defects. Our findings reveal upstream regulatory roles for Prdm3 and Prdm16 in cranial NCCs to control Wnt/β-catenin transcriptional activity during chondrocyte differentiation to ensure proper development of the craniofacial skeleton.

Phenotypes, Developmental Basis, and Genetics of Pierre Robin Complex

The phenotype currently accepted as Pierre Robin syndrome/sequence/anomalad/complex (PR) is characterized by mandibular dysmorphology, glossoptosis, respiratory obstruction, and in some cases, cleft palate. A causative sequence of developmental events is hypothesized for PR, but few clear causal relationships between discovered genetic variants, dysregulated gene expression, precise cellular processes, pathogenesis, and PR-associated anomalies are documented. This review presents the current understanding of PR phenotypes, the proposed pathogenetic processes underlying them, select genes associated with PR, and available animal models that could be used to better understand the genetic basis and phenotypic variation of PR.

Control of skeletal morphogenesis by the Hippo-YAP/TAZ pathway

The Hippo-YAP/TAZ pathway is an important regulator of tissue growth, but can also control cell fate or tissue morphogenesis. Here, we investigate the function of the Hippo pathway during the development of cartilage, which forms the majority of the skeleton. Previously, YAP was proposed to inhibit skeletal size by repressing chondrocyte proliferation and differentiation. We find that, in vitro, Yap/Taz double knockout impairs murine chondrocyte proliferation, whereas constitutively nuclear nls-YAP5SA accelerates proliferation, in line with the canonical role of this pathway in most tissues. However, in vivo, cartilage-specific knockout of Yap/Taz does not prevent chondrocyte proliferation, differentiation or skeletal growth, but rather results in various skeletal deformities including cleft palate. Cartilage-specific expression of nls-YAP5SA or knockout of Lats1/2 do not increase cartilage growth, but instead lead to catastrophic malformations resembling chondrodysplasia or achondrogenesis. Physiological YAP target genes in cartilage include Ctgf, Cyr61 and several matrix remodelling enzymes. Thus, YAP/TAZ activity controls chondrocyte proliferation in vitro, possibly reflecting a regenerative response, but is dispensable for chondrocyte proliferation in vivo, and instead functions to control cartilage morphogenesis via regulation of the extracellular matrix.

Diverse Fate of an Enigmatic Structure: 200 Years of Meckel's Cartilage

Meckel's cartilage was first described by the German anatomist Johann Friedrich Meckel the Younger in 1820 from his analysis of human embryos. Two hundred years after its discovery this paper follows the development and largely transient nature of the mammalian Meckel's cartilage, and its role in jaw development. Meckel's cartilage acts as a jaw support during early development, and a template for the later forming jaw bones. In mammals, its anterior domain links the two arms of the dentary together at the symphysis while the posterior domain ossifies to form two of the three ear ossicles of the middle ear. In between, Meckel's cartilage transforms to a ligament or disappears, subsumed by the growing dentary bone. Several human syndromes have been linked, directly or indirectly, to abnormal Meckel's cartilage formation. Herein, the evolution, development and fate of the cartilage and its impact on jaw development is mapped. The review focuses on developmental and cellular processes that shed light on the mechanisms behind the different fates of this cartilage, examining the control of Meckel's cartilage patterning, initiation and maturation. Importantly, human disorders and mouse models with disrupted Meckel's cartilage development are highlighted, in order to understand how changes in this cartilage impact on later development of the dentary and the craniofacial complex as a whole. Finally, the relative roles of tissue interactions, apoptosis, autophagy, macrophages and clast cells in the removal process are discussed. Meckel's cartilage is a unique and enigmatic structure, the development and function of which is starting to be understood but many interesting questions still remain.

Understanding chondrodysplasia (cho): A comprehensive review of cho as an animal model of birth defects, disorders, and molecular mechanisms

BACKGROUND

The mutant chondrodysplasia (cho) is a cartilage-targeting disorder in C57BL mice that results in dwarfing and other malformations stemming from this collagenopathy. Clarke Fraser made the discovery of the mutation accidentally in the early 1960s during the thalidomide tragedy.

METHODS

For this review we identified key research on cho as since its discovery. Relevant data were compiled to make a comprehensive review that details discoveries associated with the cho mutation, that describes the associated phenotypes and molecular mechanisms, and that provides a discussion surrounding its current clinical relevance.

RESULTS

Mechanistically, cho acts by hindering chondrogenesis and endochondral bone formation. The phenotype results from a 1-nt deletion in the gene encoding the alpha 1 chain of type XI collagen. For more than half a century, researchers have studied the pathogenesis of the cho mutation in relation to a variety of mouse models of human birth defects and disease. These studies have resulted in several discoveries linking cho with such human disorders as dwarfism, tracheal stenosis, cleft palate, pulmonary hypoplasia, and osteoarthritis (OA).

CONCLUSION

The study of cho has led to numerous advances in understanding human birth defects, congenital disorders, and adult human disease. The most recent studies have suggested a role for the TGF-Beta, HtrA1, Ddr2, and Mmp-13 pathway in the degradation of articular cartilage and the development of OA in cho/+ mice. We have shown that the anti-hypertension drug Losartan is a TGF-Beta blocker that could be used to treat OA in Stickler syndrome, and thereby rescue the WT phenotype.

Transferrin receptor facilitates TGF-β and BMP signaling activation to control craniofacial morphogenesis

The Pierre Robin Sequence (PRS), consisting of cleft palate, glossoptosis and micrognathia, is a common human birth defect. However, how this abnormality occurs remains largely unknown. Here we report that neural crest cell (NCC)-specific knockout of transferrin receptor (Tfrc), a well known transferrin transporter protein, caused micrognathia, cleft palate, severe respiratory distress and inability to suckle in mice, which highly resemble human PRS. Histological and anatomical analysis revealed that the cleft palate is due to the failure of palatal shelves elevation that resulted from a retarded extension of Meckel's cartilage. Interestingly, Tfrc deletion dramatically suppressed both transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) signaling in cranial NCCs-derived mandibular tissues, suggesting that Tfrc may act as a facilitator of these two signaling pathways during craniofacial morphogenesis. Together, our study uncovers an unknown function of Tfrc in craniofacial development and provides novel insight into the etiology of PRS.

Cephalometrics in Stickler syndrome: Objectification of the typical facial appearance

INTRODUCTION

Stickler syndrome is a connective tissue disorder characterized by orofacial, ocular, skeletal and auditory symptoms. The orofacial phenotype mainly consists of midfacial hypoplasia, micrognathia and cleft palate. Large phenotypic variability is evident though. Few studies have tried to substantiate the typical facial appearance in Stickler syndrome patients.

METHODS

Molecularly confirmed Stickler patients were invited to undergo cephalometric analysis based on a lateral radiograph in standardized conditions. Angular and linear measurements were performed according to Steiner's and Sassouni's analysis and compared with age- and gender-matched reference values.

RESULTS

Thirteen patients aged 10-62y were included, twelve of whom had type 1 Stickler syndrome (COL2A1 mutation) and one type 2 Stickler syndrome (COL11A1 mutation). The position of maxilla and mandible relative to the cranial base was not significantly different from the reference population (S-N-A: p = 0.73, S-N-B: p = 0.43). The mandibular plane and y-axis showed an elevated angle with the cranial base in most patients, although not significant for the total group (S-N to Go-Me: p = 0.20, S-N to S-Gn: p = 0.18). Dental analysis was normal, except for a higher overjet value (p = 0.006) and a higher angle between occlusal plane and Frankfort plane (p = 0.022).

CONCLUSION

Cephalometric analysis was not able to thoroughly prove the abnormal facial appearance in Stickler syndrome. The majority of patients had normal dentofacial proportions. The most frequently observed anomaly in our series is a rather short and posteriorly rotated mandible, but clinical variability is high.

G-Protein α-Subunit Gsα Is Required for Craniofacial Morphogenesis

The heterotrimeric G protein subunit Gsα couples receptors to activate adenylyl cyclase and is required for the intracellular cAMP response and protein kinase A (PKA) activation. Gsα is ubiquitously expressed in many cell types; however, the role of Gsα in neural crest cells (NCCs) remains unclear. Here we report that NCCs-specific Gsα knockout mice die within hours after birth and exhibit dramatic craniofacial malformations, including hypoplastic maxilla and mandible, cleft palate and craniofacial skeleton defects. Histological and anatomical analysis reveal that the cleft palate in Gsα knockout mice is a secondary defect resulting from craniofacial skeleton deficiencies. In Gsα knockout mice, the morphologies of NCCs-derived cranial nerves are normal, but the development of dorsal root and sympathetic ganglia are impaired. Furthermore, loss of Gsα in NCCs does not affect cranial NCCs migration or cell proliferation, but significantly accelerate osteochondrogenic differentiation. Taken together, our study suggests that Gsα is required for neural crest cells-derived craniofacial development.

Visualization of Chondrocyte Intercalation and Directional Proliferation via Zebrabow Clonal Cell Analysis in the Embryonic Meckel's Cartilage

Development of the vertebrate craniofacial structures requires precise coordination of cell migration, proliferation, adhesion and differentiation. Patterning of the Meckel's cartilage, a first pharyngeal arch derivative, involves the migration of cranial neural crest (CNC) cells and the progressive partitioning, proliferation and organization of differentiated chondrocytes. Several studies have described CNC migration during lower jaw morphogenesis, but the details of how the chondrocytes achieve organization in the growth and extension of Meckel's cartilage remains unclear. The sox10 restricted and chemically induced Cre recombinase-mediated recombination generates permutations of distinct fluorescent proteins (RFP, YFP and CFP), thereby creating a multi-spectral labeling of progenitor cells and their progeny, reflecting distinct clonal populations. Using confocal time-lapse photography, it is possible to observe the chondrocytes behavior during the development of the zebrafish Meckel's cartilage. Multispectral cell labeling enables scientists to demonstrate extension of the Meckel's chondrocytes. During extension phase of the Meckel's cartilage, which prefigures the mandible, chondrocytes intercalate to effect extension as they stack in an organized single-cell layered row. Failure of this organized intercalating process to mediate cell extension provides the cellular mechanistic explanation for hypoplastic mandible that we observe in mandibular malformations.

Pierre Robin sequence: review of diagnostic and treatment challenges

Pierre Robin sequence is not a rare condition and paediatric specialists caring for respiratory related issues are likely to encounter cases in their practice. There have been a few recent reviews on the topic, mostly focusing on the surgical interventions performed for cases with severe airway obstruction. In the present review, we will highlight the different challenges that remain today in the global evaluation of infants afflicted with this condition through a thorough review of the medical literature, giving the clinician a full scope of the disease and of the various management options. The need for an improved objective evaluation of airway obstruction and for a better classification will be emphasized. We are therefore proposing a novel classification scheme that will better account for respiratory and feeding difficulties in these infants. Finally, many knowledge gaps persist regarding this condition, underlining the necessity for further research both in the genetic field and regarding the outcome of therapy.

Analysis of the Relationship Between Micrognathia and Cleft Palate: A Systematic Review

Objective To gather data from relevant experimental and observational studies to determine the relationship between micrognathia and cleft palate. The goal is to raise awareness and motivate clinicians to consider the cause and effect relationship when confronted with patients with cleft palate, even if there is no clearly noticeable mandibular abnormality. Design Several electronic databases were systematically examined to find articles for this review, using search terms including "cleft palate," "micrognathia," "tongue," and "airway obstruction." PubMed was the source of all the articles chosen to be included. Exclusion criteria included case reports, articles focused on treatment options, and articles only tangentially related to cleft palate and/or micrognathia. Results A total of 930 articles were screened for relevance, and 82 articles were chosen for further analysis. Evidence gathered in this review includes a variety of etiological factors that are causative or associated with both micrognathia and cleft palate. Observational studies relating the two abnormalities are also included. Much of the included literature recognizes a cause-and-effect relationship between micrognathia and cleft palate. Conclusion On the basis of the published data, we suggest that micrognathia does induce cleft palate in humans and animals. With knowledge of this causative relationship, clinicians should consider the importance of gathering cephalometric data on the mandibles and tongues of patients presenting with isolated cleft palate to determine whether they have micrognathia as well. With more data, patterns may emerge that could give insight into the complex etiology of nonsyndromic cleft palate.

A LINE-1 insertion in DLX6 is responsible for cleft palate and mandibular abnormalities in a canine model of Pierre Robin sequence

Cleft palate (CP) is one of the most commonly occurring craniofacial birth defects in humans. In order to study cleft palate in a naturally occurring model system, we utilized the Nova Scotia Duck Tolling Retriever (NSDTR) dog breed. Micro-computed tomography analysis of CP NSDTR craniofacial structures revealed that these dogs exhibit defects similar to those observed in a recognizable subgroup of humans with CP: Pierre Robin Sequence (PRS). We refer to this phenotype in NSDTRs as CP1. Individuals with PRS have a triad of birth defects: shortened mandible, posteriorly placed tongue, and cleft palate. A genome-wide association study in 14 CP NSDTRs and 72 unaffected NSDTRs identified a significantly associated region on canine chromosome 14 (24.2 Mb–29.3 Mb; p_raw = 4.64×10⁻¹⁵). Sequencing of two regional candidate homeobox genes in NSDTRs, distal-less homeobox 5 (DLX5) and distal-less homeobox 6 (DLX6), identified a 2.1 kb LINE-1 insertion within DLX6 in CP1 NSDTRs. The LINE-1 insertion is predicted to insert a premature stop codon within the homeodomain of DLX6. This prompted the sequencing of DLX5 and DLX6 in a human cohort with CP, where a missense mutation within the highly conserved DLX5 homeobox of a patient with PRS was identified. This suggests the involvement of DLX5 in the development of PRS. These results demonstrate the power of the canine animal model as a genetically tractable approach to understanding naturally occurring craniofacial birth defects in humans.

Cleft palate is one of the most commonly occurring birth defects in children, and yet its cause is not completely understood. In order to better understand cleft palate we have turned to man's best friend, the domestic dog. Common breeding practices have made the dog a unique animal model to help understand the genetic basis of naturally occurring birth defects. A genome-wide association study of Nova Scotia Duck Tolling Retrievers with naturally occurring cleft palate led to the investigation of two homeobox genes, DLX5 and DLX6. Dogs with this mutation also have a shortened lower jaw, which resembles those who have Pierre Robin Sequence (PRS). Investigation into people with PRS identifies a mutation within a highly conserved and functional region of DLX5 that may contribute to the development of PRS. This exemplifies how the dog will help us better understand common birth defects.

Developmental and genetic perspectives on Pierre Robin sequence

Pierre Robin sequence (PRS) is a craniofacial anomaly comprising mandibular hypoplasia, cleft secondary palate and glossoptosis leading to life-threatening obstructive apnea and feeding difficulties during the neonatal period. The respiratory issues require careful management and in severe cases may require extended stays in neonatal intensive care units and surgical intervention such as lengthening the lower jaw or tracheotomy to relieve airway obstruction. These feeding and respiratory complications frequently continue well into childhood, affecting not only growth and development but also impacting on long term educational attainment. The diagnosis of PRS depends on readily recognizable clinical features but the phenotypic similarity of many PRS individuals conceals considerable etiological heterogeneity. Defects in the growth of the mandible sit at the core of PRS and the natural history of PRS can be classified into two major streams: primary defects of mandibular outgrowth and elongation and issues that are external to the mandibular skeleton but that secondarily impact on its growth. These altered developmental trajectories appear to be driven by a range of influences including defects in cartilage growth, neuromuscular function and fetal constraint. Various genetic and cytogenetic associations have been made with PRS and the diversity of these associations highlights the fact that there are numerous ways to arrive at this common phenotypic endpoint.

Inactivation of LAR family phosphatase genes Ptprs and Ptprf causes craniofacial malformations resembling Pierre-Robin sequence

Leukocyte antigen related (LAR) family receptor protein tyrosine phosphatases (RPTPs) regulate the fine balance between tyrosine phosphorylation and dephosphorylation that is crucial for cell signaling during development and tissue homeostasis. Here we show that LAR RPTPs are required for normal development of the mandibular and maxillary regions. Approximately half of the mouse embryos lacking both Ptprs (RPTPσ) and Ptprf (LAR) exhibit micrognathia (small lower jaw), cleft palate and microglossia/glossoptosis (small and deep tongue), a phenotype closely resembling Pierre-Robin sequence in humans. We show that jaw bone and cartilage patterning occurs aberrantly in LAR family phosphatase-deficient embryos and that the mandibular arch harbors a marked decrease in cell proliferation. Analysis of signal transduction in embryonic tissues and mouse embryonic fibroblast cultures identifies an increase in Bmp-Smad signaling and an abrogation of canonical Wnt signaling associated with loss of the LAR family phosphatases. A reactivation of β-catenin signaling by chemical inhibition of GSK3β successfully resensitizes LAR family phosphatase-deficient cells to Wnt induction, indicating that RPTPs are necessary for normal Wnt/β-catenin pathway activation. Together these results identify LAR RPTPs as important regulators of craniofacial morphogenesis and provide insight into the etiology of Pierre-Robin sequence.

Type 1 fibroblast growth factor receptor in cranial neural crest cell-derived mesenchyme is required for palatogenesis

Cleft palate is a common congenital birth defect. The fibroblast growth factor (FGF) family has been shown to be important for palatogenesis, which elicits the regulatory functions by activating the FGF receptor tyrosine kinase. Mutations in Fgf or Fgfr are associated with cleft palate. To date, most mechanistic studies on FGF signaling in palate development have focused on FGFR2 in the epithelium. Although Fgfr1 is expressed in the cranial neural crest (CNC)-derived palate mesenchyme and Fgfr1 mutations are associated with palate defects, how FGFR1 in palate mesenchyme regulates palatogenesis is not well understood. Here, we reported that by using Wnt1(Cre) to delete Fgfr1 in neural crest cells led to cleft palate, cleft lip, and other severe craniofacial defects. Detailed analyses revealed that loss-of-function mutations in Fgfr1 did not abrogate patterning of CNC cells in palate shelves. However, it upset cell signaling in the frontofacial areas, delayed cell proliferation in both epithelial and mesenchymal compartments, prevented palate shelf elevation, and compromised palate shelf fusion. This is the first report revealing how FGF signaling in CNC cells regulates palatogenesis.

The etiology of cleft palate formation in BMP7-deficient mice

Palatogenesis is a complex process implying growth, elevation and fusion of the two lateral palatal shelves during embryogenesis. This process is tightly controlled by genetic and mechanistic cues that also coordinate the growth of other orofacial structures. Failure at any of these steps can result in cleft palate, which is a frequent craniofacial malformation in humans. To understand the etiology of cleft palate linked to the BMP signaling pathway, we studied palatogenesis in Bmp7-deficient mouse embryos. Bmp7 expression was found in several orofacial structures including the edges of the palatal shelves prior and during their fusion. Bmp7 deletion resulted in a general alteration of oral cavity morphology, unpaired palatal shelf elevation, delayed shelf approximation, and subsequent lack of fusion. Cell proliferation and expression of specific genes involved in palatogenesis were not altered in Bmp7-deficient embryos. Conditional ablation of Bmp7 with Keratin14-Cre or Wnt1-Cre revealed that neither epithelial nor neural crest-specific loss of Bmp7 alone could recapitulate the cleft palate phenotype. Palatal shelves from mutant embryos were able to fuse when cultured in vitro as isolated shelves in proximity, but not when cultured as whole upper jaw explants. Thus, deformations in the oral cavity of Bmp7-deficient embryos such as the shorter and wider mandible were not solely responsible for cleft palate formation. These findings indicate a requirement for Bmp7 for the coordination of both developmental and mechanistic aspects of palatogenesis.

1.9 Mb microdeletion of 21q22.11 within Braddock-Carey contiguous gene deletion syndrome region: dissecting the phenotype

Braddock-Carey syndrome is characterized by Pierre Robin sequence, agenesis of the corpus callosum, facial dysmorphisms, developmental delay, and congenital thrombocytopenia. Recently, Braddock-Carey syndrome was demonstrated to be caused by chromosomal microdeletion in 21q22 including the RUNX1 gene, whose haploinsufficiency is responsible for thrombocytopenia phenotype. Therefore, the syndrome has emerged as a contiguous gene deletion syndrome. Here, we describe an infant with Pierre Robin sequence, facial anomalies, congenital heart defects, hypotonia, and the absence of thrombocytopenia, who was found to have a 1.9 Mb microdeletion within the Braddock-Carey contiguous deletion syndrome region. This deletion spares the RUNX1 gene, narrowing the genomic region responsible for a part of the Braddock-Carey syndrome phenotype. Further studies are awaited to understand the role of the genes located within 21q22 in the pathogenesis of Braddock-Carey syndrome.

Specific inactivation of Twist1 in the mandibular arch neural crest cells affects the development of the ramus and reveals interactions with hand2

BACKGROUND

The basic helix-loop-helix (bHLH) transcription factor Twist1 fulfills an essential function in neural crest cell formation, migration, and survival and is associated with the craniosynostic Saethre-Chotzen syndrome in humans. However, its functions during mandibular development, when it may interact with other bHLH transcription factors like Hand2, are unknown because mice homozygous for the Twist1 null mutation die in early embryogenesis. To determine the role of Twist1 during mandibular development, we used the Hand2-Cre transgene to conditionally inactivate the gene in the neural crest cells populating the mandibular pharyngeal arch.

RESULTS

The mutant mice exhibited a spectrum of craniofacial anomalies, including mandibular hypoplasia, altered middle ear development, and cleft palate. It appears that Twist1 is essential for the survival of the neural crest cells involved in the development of the mandibular ramal elements. Twist1 plays a role in molar development and cusp formation by participating in the reciprocal signaling needed for the formation of the enamel knot. This gene is also needed to control the ossification of the mandible, a redundant role shared with Hand2.

CONCLUSION

Twist1, along with Hand2, is essential for the proximodistal patterning and development of the mandible and ossification.

Robin sequence: from diagnosis to development of an effective management plan

The triad of micrognathia, glossoptosis, and resultant airway obstruction is known as Robin sequence (RS). Although RS is a well-recognized clinical entity, there is wide variability in the diagnosis and care of children born with RS. Systematic evaluations of treatments and clinical outcomes for children with RS are lacking despite the advances in clinical care over the past 20 years. We explore the pathogenesis, developmental and genetic models, morphology, and syndromes and malformations associated with RS. Current classification systems for RS do not account for the heterogeneity among infants with RS, and they do not allow for prediction of the optimal management course for an individual child. Although upper airway obstruction for some infants with RS can be treated adequately with positioning, other children may require a tracheostomy. Care must be customized for each patient with RS, and health care providers must understand the anatomy and mechanism of airway obstruction to develop an individualized treatment plan to improve breathing and achieve optimal growth and development. In this article we provide a comprehensive overview of evaluation strategies and therapeutic options for children born with RS. We also propose a conceptual treatment protocol to guide the provider who is caring for a child with RS.

Prdm16 is required for normal palatogenesis in mice

Transcriptional cofactors are essential to the regulation of transforming growth factor beta (TGFbeta) superfamily signaling and play critical and widespread roles during embryonic development, including craniofacial development. We describe the cleft secondary palate 1 (csp1) N-ethyl-N-nitrosourea-induced mouse model of non-syndromic cleft palate (NSCP) that is caused by an intronic Prdm16 splicing mutation. Prdm16 encodes a transcriptional cofactor that regulates TGFbeta signaling, and its expression pattern is consistent with a role in palate and craniofacial development. The cleft palate (CP) appears to be the result of micrognathia and failed palate shelf elevation due to physical obstruction by the tongue, resembling human Pierre Robin sequence (PRS)-like cleft secondary palate. PRDM16 should be considered a candidate for mutation in human clefting disorders, especially NSCP and PRS-like CP.

Modulation of diabetes-induced palate defects by maternal immune stimulation

Maternal diabetes can induce a number of developmental abnormalities in both laboratory animals and humans, including deformities of the face and palate. The incidence of birth defects in newborns of women with diabetes is approximately 3 to 5 times higher than among nondiabetics. In mice, nonspecific activation of the maternal immune system can reduce fetal abnormalities caused by various etiologies including hyperglycemia. This study was conducted to determine whether nonspecific maternal immune stimulation could reduce diabetes-induced palate defects and orofacial clefts. Female ICR mice were immune stimulated before induction of hyperglycemia with Freund's complete adjuvant (FCA), granulocyte-macrophage colony-stimulating factor (GM-CSF), or interferon-gamma (IFNgamma). Streptozocin was used to induce hyperglycemia (26-35 mmol blood glucose) in females before breeding. Fetuses from 12 to 18 litters per treatment group were collected on Day 17 of gestation. Palate width and length were measured, and the incidence of orofacial clefts was determined. Palate length and width were both decreased by maternal hyperglycemia. Maternal immune stimulation with GM-CSF or FCA limited the degree of palate shortening from the hyperglycemia. Each of the three immune stimulants attenuated significant narrowing of the palate. Rates of orofacial clefts were not significantly different between treatment groups. Palatogenesis is a complex process driven by cellular signals, which regulate cell growth and apoptosis. Dysregulation of cellular signals by maternal hyperglycemia can result in fetal malformations. Maternal immune stimulation may prevent dysregulation of these signaling pathways thus reducing fetal malformations and normalizing palate growth.

The heterozygous disproportionate micromelia (dmm) mouse: morphological changes in fetal cartilage precede postnatal dwarfism and compared with lethal homozygotes can explain the mild phenotype

The disproportionate micromelia (Dmm) mouse has a mutation in the C-propeptide coding region of the Col2a1 gene that causes lethal dwarfism when homozygous (Dmm/Dmm) but causes only mild dwarfism observable approximately 1-week postpartum when heterozygous (Dmm/+). The purpose of this study was 2-fold: first, to analyze and quantify morphological changes that precede the expression of mild dwarfism in Dmm/+ animals, and second, to compare morphological alterations between Dmm/+ and Dmm/Dmm fetal cartilage that may correlate with the marked skeletal differences between mild and lethal dwarfism. Light and electron transmission microscopy were used to visualize structure of chondrocytes and extracellular matrix (ECM) of fetal rib cartilage. Both Dmm/+ and Dmm/Dmm fetal rib cartilage had significantly larger chondrocytes, greater cell density, and less ECM per unit area than +/+ littermates. Quantitative RT-PCR showed a decrease in aggrecan mRNA in Dmm/+ vs +/+ cartilage. Furthermore, the cytoplasm of chondrocytes in Dmm/+ and Dmm/Dmm cartilage was occupied by significantly more distended rough endoplasmic reticulum (RER) compared with wild-type chondrocytes. Fibril diameters and packing densities of +/+ and Dmm/+ cartilage were similar, but Dmm/Dmm cartilage showed thinner, sparsely distributed fibrils. These findings support the prevailing hypothesis that a C-propeptide mutation could interrupt the normal assembly and secretion of Type II procollagen trimers, resulting in a buildup of proalpha1(II) chains in the RER and a reduced rate of matrix synthesis. Thus, intracellular entrapment of proalpha1(II) seems to be primarily responsible for the dominant-negative effect of the Dmm mutation in the expression of dwarfism.

FGF signaling in mandibular skeletogenesis

OBJECTIVE

To examine the functions of FGF/FGFR signaling during mandibular skeletogenesis in ovo.

DESIGN

We examined the effects of inhibition of FGF signaling during mandibular skeletogenesis by overexpressing replication-competent RCAS virus encoding a truncated form of FGFR3 in the chicken mandibular process between stages 17 and 26.

RESULTS

Injection of RCAS-dnFGFR3 into the developing mandible resulted in abnormalities in a stage- and region-dependent manner. Injection at early stages of development resulted in the truncation of Meckel's cartilage, severely reduced outgrowth of the mandibular process and absence of five of the mandibular bones. Injection at later stages did not affect the outgrowth of the mandibular process and Meckel's cartilage but resulted in abnormalities in mandibular osteogenesis in a region-specific manner. The bones in the more caudal region were frequently truncated whereas bones in the more rostral regions such as dentary and splenial bones were frequently absent.

CONCLUSION

Together these experiments have revealed essential roles for FGF/FGFR signaling in the elongation of Meckel's cartilage, development of osteogenic condensations and appositional growth of mandibular bones.

Multiple functions of Snail family genes during palate development in mice

Palate development requires precise regulation of gene expression changes, morphogenetic movements and alterations in cell physiology. Defects in any of these processes can result in cleft palate, a common human birth defect. The Snail gene family encodes transcriptional repressors that play essential roles in the growth and patterning of vertebrate embryos. Here we report the functions of Snail (Snai1) and Slug (Snai2) genes during palate development in mice. Snai2(-/-) mice exhibit cleft palate, which is completely penetrant on a Snai1 heterozygous genetic background. Cleft palate in Snai1(+/-) Snai2(-/-) embryos is due to a failure of the elevated palatal shelves to fuse. Furthermore, while tissue-specific deletion of the Snai1 gene in neural crest cells does not cause any obvious defects, neural-crest-specific Snai1 deletion on a Snai2(-/-) genetic background results in multiple craniofacial defects, including a cleft palate phenotype distinct from that observed in Snai1(+/-) Snai2(-/-) embryos. In embryos with neural-crest-specific Snai1 deletion on a Snai2(-/-) background, palatal clefting results from a failure of Meckel's cartilage to extend the mandible and thereby allow the palatal shelves to elevate, defects similar to those seen in the Pierre Robin Sequence in humans.

[Genetic collagen disorders and the impact on craniofacial development]

Extracellular matrix molecules provide to tissues their mechanical properties and constitute a reservoir of local or regional signals that regulate cellular function. Collagens, the major components of osseous and collagenous matrices, have structural similarities, but are encoded by different genes. We describe here osteogenesis imperfecta, a collagen I, the principal constituent of bone, genetic disease, and its craniofacial implications. By comparing it with genetic disorders of cartilage collagen (Kniest and Stickler syndromes) we try to clarify the respective influences of these matrix molecules upon craniofacial development.

The genetic basis of the Pierre Robin Sequence

OBJECTIVE

The Pierre Robin Sequence (PRS) is subgroup of the cleft palate population. As with the etiology of cleft lip or palate, the etiology of PRS is generally unknown. Some factors are suggestive of a genetic basis for PRS. The purpose of this study was to compare genetic information on PRS available in the literature and in a cytogenetic database to facilitate focused genetic studies of PRS.

DESIGN

After searching Medline for "pierre robin and genetics," the Mendelian Cytogenetics Network database for "robin" and "pierre robin," and two reviews from the Human Cytogenetics Database for "cleft palate" and "micrognathia," a comparison of the data and a search in Online Mendelian Inheritance in Man (OMIM) Gene Map was performed to identify relevant candidate genes.

RESULTS

The findings revealed consistency to a certain degree to loci 2q24.1-33.3, 4q32-qter, 11q21-23.1, and 17q21-24.3. A search in the OMIM Gene Map provided many candidate genes for PRS in these regions. The GAD67 on 2q31, the PVRL1 on 11q23-q24, and the SOX9 gene on 17q24.3-q25.1 are suggested to be of particular importance.

CONCLUSION

Candidate loci and a few potential candidate genes for PRS are proposed from the present study. This may enable researchers to focus their effort in the studies of PRS.

Premature osteoarthritis in the Disproportionate micromelia (Dmm) mouse

OBJECTIVE

Degeneration of articular cartilage leads to the development of osteoarthritis (OA), but the molecular pathology of the disease is poorly understood. The Disproportionate micromelia (Dmm) mouse has a deletion mutation in the C-propeptide encoding region of Col2a1, which leads to a defective cartilage matrix. The objective of this study was to determine whether heterozygous (Dmm/+) mice develop premature OA, and could therefore serve as an animal model for studying the molecular pathways leading to OA.

DESIGN

Histological analysis was utilized to determine the state of articular cartilage degeneration in Dmm/+ mice at 3, 6, 9, 12, 15, and 22 months of age. Severity of OA was quantified with a modified Mankin scoring system. In addition, articular cartilage thickness, cell density, and the extracellular matrix (ECM) fraction of articular cartilage were quantified.

RESULTS

Articular cartilage erosion was significantly more severe in Dmm/+ than in wild-type (+/+) mice beginning at 9 months, and modified Mankin scoring revealed Dmm/+ articular cartilage to be in a more severe osteoarthritic state as early as 3 months. In addition, Dmm/+ articular cartilage was thinner than +/+ cartilage and showed increased cell density and decreased matrix fraction compared with +/+ from the earliest time points measured.

CONCLUSIONS

The present study demonstrates that Dmm/+ mice develop premature OA. The observed degenerative changes of Dmm/+ articular cartilage closely resemble those of human OA patients, with or without Col2a1 mutations, suggesting that Dmm/+ mice are a useful model for investigating mechanisms involved in OA.

Oral clefts with associated anomalies: methodological issues

OBJECTIVE

To discuss methodological factors that account for the wide variation in the reported prevalence rates of anomalies associated with oral clefts.

CONCLUSIONS

The published prevalences of associated anomalies vary considerably because of the following: (1) differences in case definition and inclusion/exclusion criteria; (2) length of time after birth that cases are examined; (3) variability of clinical expression of associated anomalies; (4) knowledge and technology available to produce syndrome delineation; (5) selection of patients, sources of ascertainment, and sample size; and (6) true population differences and changes in frequency over time.

Experimental aspects of the pathogenesis of Robin sequence

OBJECTIVE

The Pierre Robin Sequence (PRS) is a good example of disturbed embryonic development of the secondary palate involving insufficient mandibular growth, failed forward tongue movement, and, in the case of a cleft, impeded fusion of the secondary palate. Discussion continues regarding which of the involved pathogenetic factors is the primary cause of the induced cascade of signs: insufficient mandibular growth or failed descent of the tongue.

DESIGN

Forty-five randomly selected, 18-day-old formalin-fixed A/WySn mouse fetuses were investigated. The strain is known to have a basic genetic defect and as much as 44% clefts in the offspring. Twenty-four fetuses in the group had a cleft palate. Mandible position was measured relative to the head and to the presence or absence of a cleft. Cleft width and tongue position were also determined. Thirty-eight NMRI mouse fetuses of the same age served as controls.

RESULTS

All A/WySn fetuses showed marked mandibular retrognathia, which was more severe in the cleft group (p < .05), but there was no correlation between the degree of retrognathia and cleft width. The median cleft width was 3.4 mm (1.6 through 6.3 mm). The tongue was in the cleft in all 12 fetuses with wide clefts (>3.4 mm wide), and free in the oral cavity in the other 12. Tongue position did not influence the degree of retrognathia (p < .05). Moreover, the tongue was free in all fetuses with severe retrognathia.

CONCLUSION

The results support the primary role of retroposition of the mandible in the development of cardinal symptoms of Pierre Robin Sequence.

Targeted disruption of the Mn1 oncogene results in severe defects in development of membranous bones of the cranial skeleton

Fusion of the MN1 gene to TEL (ETV6) results in myeloid leukemia. The fusion protein combines the transcription activating domain of MN1 and the DNA binding domain of TEL and is thought to act as a deranged transcription factor. In addition, disruption of the large first exon of the MN1 gene is thought to inactivate MN1 function in a meningioma. To further investigate the role of MN1 in cancer, we generated Mn1 knockout mice. Mn1(+/-) animals were followed for 30 months, but they had no higher incidence of tumor formation than wild-type littermates. Mn1 null mice, however, were found to die at birth or shortly thereafter as the result of a cleft palate. Investigation of newborn or embryonic day 15.5 (E15.5) to E17.5 null mice revealed that the development of several bones in the skull was abnormal. The affected bones are almost exclusively formed by intramembranous ossification. They are either completely agenic at birth (alisphenoid and squamosal bones and vomer), hypoplastic, deformed (basisphenoid, pterygoid, and presphenoid), or substantially thinner (frontal, parietal, and interparietal bones). In heterozygous mice hypoplastic membranous bones and incomplete penetrance of the cleft palate were observed. We conclude that Mn1 is an important factor in development of membranous bones.

MR imaging of fetal head and neck anomalies

This article describes an approach to imaging of fetal head and neck anomalies. Topics include cleft lip and palate, facial clefts, amniotic band sequence, micrognathia and retrognathia, ocular and orbital abnormalities, craniosynostosis, posterior nuchal translucency, cephaloceles, vascular anomalies, and tumors. Some of the more common syndromes are also described.

Genetically altered mouse models: the good, the bad, and the ugly

Targeted gene disruption in mice is a powerful tool for generating murine models for human development and disease. While the human genome program has helped to generate numerous candidate genes, few genes have been characterized for their precise in vivo functions. Gene targeting has had an enormous impact on our ability to delineate the functional roles of these genes. Many gene knockout mouse models faithfully mimic the phenotypes of the human diseases. Because some models display an unexpected or no phenotype, controversy has arisen about the value of gene-targeting strategies. We argue in favor of gene-targeting strategies, provided they are used with caution, particularly in interpreting phenotypes in craniofacial and oral biology, where many genes have pleiotropic roles. The potential pitfalls are outweighed by the unique opportunities for developing and testing different therapeutic strategies before they are introduced into the clinic. In the future, we believe that genetically engineered animal models will be indispensable for gaining important insights into the molecular mechanisms underlying development, as well as disease pathogenesis, diagnosis, prevention, and treatment.

Targeted deletion of a branchial arch-specific enhancer reveals a role of dHAND in craniofacial development

The basic helix-loop-helix transcription factor dHAND is expressed in the mesenchyme of branchial arches and the developing heart. Mice homozygous for a dHAND (Hand2) null mutation die early in embryogenesis from cardiac abnormalities, precluding analysis of the potential role of dHAND in branchial arch development. Two independent enhancers control expression of dHAND in the heart and branchial arches. Endothelin-1 (ET-1) signaling regulates the branchial arch enhancer and is required for dHAND expression in the branchial arches. To determine the potential role of dHAND in branchial arch development and to assess the role of the ET-1-dependent enhancer in dHAND regulation in vivo, we deleted this enhancer by homologous recombination. Mice lacking the dHAND branchial arch enhancer died perinatally and exhibited a spectrum of craniofacial defects that included cleft palate, mandibular hypoplasia and cartilage malformations. Expression of dHAND was abolished in the ventolateral regions of the first and second branchial arches in these mutant mice, but expression was retained in a ventral domain where the related transcription factor eHAND is expressed. We conclude that dHAND plays an essential role in patterning and development of skeletal elements derived from the first and second branchial arches and that there are heterogeneous populations of cells in the branchial arches that rely on different cis-regulatory elements for activation of dHAND transcription.