Fishing the molecular bases of Treacher Collins syndrome

Treacher Collins syndrome (TCS) is an autosomal dominant disorder of craniofacial development, and mutations in the TCOF1 gene are responsible for over 90% of TCS cases. The knowledge about the molecular mechanisms responsible for this syndrome is relatively scant, probably due to the difficulty of reproducing the pathology in experimental animals. Zebrafish is an emerging model for human disease studies, and we therefore assessed it as a model for studying TCS. We identified in silico the putative zebrafish TCOF1 ortholog and cloned the corresponding cDNA. The derived polypeptide shares the main structural domains found in mammals and amphibians. Tcof1 expression is restricted to the anterior-most regions of zebrafish developing embryos, similar to what happens in mouse embryos. Tcof1 loss-of-function resulted in fish showing phenotypes similar to those observed in TCS patients, and enabled a further characterization of the mechanisms underlying craniofacial malformation. Besides, we initiated the identification of potential molecular targets of treacle in zebrafish. We found that Tcof1 loss-of-function led to a decrease in the expression of cellular proliferation and craniofacial development. Together, results presented here strongly suggest that it is possible to achieve fish with TCS-like phenotype by knocking down the expression of the TCOF1 ortholog in zebrafish. This experimental condition may facilitate the study of the disease etiology during embryonic development.     

Characterization of the nucleolar gene product, treacle, in Treacher Collins syndrome

Treacher Collins syndrome (TCS) is an autosomal dominant disorder of craniofacial development caused by mutations in the gene TCOF1. Its gene product, treacle, consists mainly of a central repeat domain, which shows it to be structurally related to the nucleolar phosphoprotein Nopp140. Treacle remains mostly uncharacterized to date. Herein we show that it, like Nopp140, is a highly phosphorylated nucleolar protein. However, treacle fails to colocalize with Nopp140 to Cajal (coiled) bodies. As in the case of Nopp140, casein kinase 2 appears to be responsible for the unusually high degree of phosphorylation as evidenced by its coimmunoprecipitation with treacle. Based on these and other observations, treacle and Nopp140 exhibit distinct but overlapping functions. The majority of TCOF1 mutations in TCS lead to premature termination codons that could affect the cellular levels of the full-length treacle. We demonstrate however, that the cellular amount of treacle varies less than twofold among a collection of primary fibroblasts and lymphoblasts and regardless of whether the cells were derived from TCS patients or healthy individuals. Therefore, cells of TCS patients possess a mechanism to maintain wild-type levels of full-length treacle from a single allele.     

Anthropometric evaluation of dysmorphology in craniofacial anomalies: Treacher Collins syndrome

Advances in surgical techniques for correction of craniofacial anomalies have necessitated the development of objective pre- and postoperative quantitative assessments. Standard anthropometric techniques, supplemented by additional methods oriented to specific clinical problems, have proved useful in defining surface dysmorphology in craniofacial patients. A series of 77 surface measurements of the head and face and 41 proportions were determined in 20 preoperative patients with Treacher Collins syndrome, a rare congenital defect of the first and second branchial arches. To permit comparison with age- and sex-specific data for healthy North American children, the patient data were converted to standard (Z) scores. To test the hypothesis Z = 0, Student's t-test was performed on all variables. The anthropometric findings verified many of the clinical findings in this syndrome. In addition, a number of previously unreported defects were found. The cranium was low and short with a low, narrow forehead and a narrow cranial base. The face was narrow and shallow, the mandible long and narrow, and the lower face receding. The eye fissures were short with an antimongoloid inclination, but the orbits were hyperteloric. The nasal root was high and wide, the nasofrontal angle open, and the bridge inclination low. The labial fissure was narrow, and the ears were microtic. Except in the nasal root the defects were hypoplastic. Most of these defects were either horizontal or anteroposterior. Recognition of the defective areas and their contribution to disproportions of the head and face is important in the development of surgical strategies.     

The skeletal anatomy of mandibulofacial dysostosis (Treacher Collins syndrome)

Three-dimensional osseous surface re-formation imaging from CT scans was used to examine the facial skeletons of 14 living patients with mandibulofacial dysostosis. Partial to complete aplasia of the zygomatic process of the temporal bone, mild hypoplasia to aplasia of the frontal process of the zygoma, antimongoloid slant of the transverse orbital axis, and hypoplasia of the medial pterygoid plates and muscles are common to all patients examined. Deformities of the zygoma, zygomatic process of the frontal bone, mandible, and lateral pterygoid plates and muscles vary from minimal to severe, including aplasia. The body of the zygoma is the least affected part of the bone. Right-left asymmetry characterizes these deformities in all patients. The most consistent skeletal aplasia (cleft) in mandibulofacial dysostosis involves the zygomatic process of the temporal bone rather than the zygoma itself.     

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.     

Prevention of the neurocristopathy Treacher Collins syndrome through inhibition of p53 function

Treacher Collins syndrome (TCS) is a congenital disorder of craniofacial development arising from mutations in TCOF1, which encodes the nucleolar phosphoprotein Treacle. Haploinsufficiency of Tcof1 perturbs mature ribosome biogenesis, resulting in stabilization of p53 and the cyclin G1-mediated cell-cycle arrest that underpins the specificity of neuroepithelial apoptosis and neural crest cell hypoplasia characteristic of TCS. Here we show that inhibition of p53 prevents cyclin G1-driven apoptotic elimination of neural crest cells while rescuing the craniofacial abnormalities associated with mutations in Tcof1 and extending life span. These improvements, however, occur independently of the effects on ribosome biogenesis; thus suggesting that it is p53-dependent neuroepithelial apoptosis that is the primary mechanism underlying the pathogenesis of TCS. Our work further implies that neuroepithelial and neural crest cells are particularly sensitive to cellular stress during embryogenesis and that suppression of p53 function provides an attractive avenue for possible clinical prevention of TCS craniofacial birth defects and possibly those of other neurocristopathies.     

Mapping the Treacher Collins syndrome locus to 5q31.3----q33.3.

Treacher Collins syndrome is an autosomal dominant disorder of abnormal craniofacial development. Linkage analysis was performed in Treacher Collins families with restriction fragment length or microsatellite polymorphisms associated with eight loci previously mapped to 5q31----qter. Positive lod scores were obtained for four loci, D5S119, D5S207, D5S209, and D5S210, which map to 5q31.3----q33.3. The Treacher Collins syndrome locus was linked closest to locus D5S210, which is associated with microsatellite polymorphisms, with a maximum lod score of 8.65 at theta = 0.02. The Treacher Collins syndrome locus was excluded from locus ADRB2R, which maps to 5q31----q32, and loci D5S22, D5S61, and D5S43, which map to 5q34----qter. There was no evidence for genetic heterogeneity among eight families with variable expression of the condition.     

Clinical and molecular genetic features of ARC syndrome

Arthrogryposis, renal dysfunction and cholestasis (ARC) syndrome (MIM 208085) is an autosomal recessive multisystem disorder that may be associated with germline VPS33B mutations. VPS33B is involved in regulation of vesicular membrane fusion by interacting with SNARE proteins, and evidence of abnormal polarised membrane protein trafficking has been reported in ARC patients. We characterised clinical and molecular features of ARC syndrome in order to identify potential genotype-phenotype correlations. The clinical phenotype of 62 ARC syndrome patients was analysed. In addition to classical features described previously, all patients had severe failure to thrive, which was not adequately explained by the degree of liver disease and 10% had structural cardiac defects. Almost half of the patients who underwent diagnostic organ biopsy (7/16) developed life-threatening haemorrhage. We found that most patients (9/11) who suffered severe haemorrhage (7 post biopsy and 4 spontaneous) had normal platelet count and morphology. Germline VPS33B mutations were detected in 28/35 families (48/62 individuals) with ARC syndrome. Several mutations were restricted to specific ethnic groups. Thus p.Arg438X mutation was common in the UK Pakistani families and haplotyping was consistent with a founder mutation with the most recent common ancestor 900–1,000 years ago. Heterozygosity was found in the VPS33B locus in some cases of ARC providing the first evidence of a possible second ARC syndrome gene. In conclusion we state that molecular diagnosis is possible for most children in whom ARC syndrome is suspected and VPS33B mutation analysis should replace organ biopsy as a first line diagnostic test for ARC syndrome.     

The gene for Treacher Collins syndrome maps to the long arm of chromosome 5.

Treacher Collins syndrome (TCS) is an autosomal dominant disorder of craniofacial development, the features of which include conductive hearing loss and cleft palate. We have studied 12 unrelated TCS families with multiple affected individuals for linkage to five chromosome 5 markers. There is strong evidence demonstrating linkage to three of these markers. Multipoint linkage analysis places the mutation causing TCS in the interval between the gene for the glucocorticoid receptor and the anonymous marker D5S22, with a maximum multipoint lod score of 9.1.     

The use of a temporal osteoperiosteal flap for the reconstruction of malar hypoplasia in Treacher Collins syndrome

The clinical use of a temporal periosteal bone flap for the reconstruction of a malar bone in a patient with the Treacher Collins syndrome is presented. The temporal muscle functions as an axial carrier of the periosteum that induces osteogenesis in young children, whereas the bone segments may serve as a nucleus for further bone formation from the periosteum. Correction of the eyelid coloboma was obtained by the rotation and advancement of a temporopalpebral flap.     

Novel insertion in exon 5 of the TCOF1 gene in twin sisters with Treacher Collins syndrome

Treacher Collins syndrome (TCS) is associated with an abnormal differentiation of the first and second pharyngeal arches during fetal development. This causes mostly craniofacial deformities, which require numerous corrective surgeries. TCS is an autosomal dominant disorder and it occurs in the general population at a frequency of 1 in 50,000 live births. The syndrome is caused by mutations in the TCOF1 gene, which encodes the serine/alanine-rich protein named Treacle. Over 120 mutations of the TCOF1 gene responsible for TCS have been described. About 70% of recognized mutations are deletions, which lead to a frame shift, formation of a termination codon, and shortening of the protein product of the gene. Herewith, a new heterozygotic insertion, c.484_668ins185bp, was described in two monozygotic twin sisters suffering from TCS. This mutation was absent in their father, brother, and uncle, indicating a de novo origin. The insertion causes a shift in the reading frame and premature termination of translation at 167 aa. The novel insertion is the longest ever found in the TCOF1 gene and the only one found among monozygotic twin sisters.     

A YAC contig encompassing the Treacher Collins syndrome critical region at 5q31.3-32.

Treacher Collins syndrome (TCOF1) is an autosomal dominant disorder of craniofacial development the features of which include conductive hearing loss and cleft palate. Previous studies have localized the TCOF1 locus between D5S519 (proximal) and SPARC (distal), a region of 22 centirays as estimated by radiation hybrid mapping. In the current investigation we have created a contig across the TCOF1 critical region, using YAC clones. Isolation of a novel short tandem repeat polymorphism corresponding to the end of one of the YACs has allowed us to reduce the size of the critical region to approximately 840 kb, which has been covered with three nonchimeric YACs. Restriction mapping has revealed that the region contains a high density of clustered rare-cutter restriction sites, suggesting that it may contain a number of different genes. The results of the present investigation have further allowed us to confirm that the RPS14 locus lies proximal to the critical region and can thereby be excluded from a role in the pathogenesis of TCOF1, while ANX6 lies within the TCOF1 critical region and remains a potential candidate for the mutated gene.     

Clinical and genetic features of Warts, Hypogammaglobulinemia, Infections and Myelokathexis (WHIM) syndrome

WHIM syndrome is a dominantly inherited primary immunodeficiency disorder representing the first identified example of human disease caused by mutations in the gene encoding for the chemokine receptor CXCR4. Pathogenesis is mediated by CXCR4 hyperfunction, leading to increased responsiveness to its unique ligand CXCL12 (also known as SDF-1). The altered CXCR4/CXCL12 interaction likely impairs cellular homeostasis and trafficking, resulting in immunological dysfunctions. The acronym WHIM resumes the main features of the syndrome: Warts, Hypogammaglobulinemia, Infections and Myelokathexis, which is abnormal retention of mature neutrophils in the bone marrow. WHIM patients suffer from recurrent bacterial infections since childhood and manifest a specific susceptibility to HPV infections. Hematological findings include neutropenia, lymphopenia and hypogammaglobulinemia. Because of the rarity of the disease and the heterogeneity in clinical presentation, diagnosis is often delayed. In the majority of patients, the phenotype is incomplete at the onset and WHIM syndrome is not suspected. Early identification may improve clinical and therapeutic management. Symptomatic treatments include G-CSF, substitutive immunoglobulins and antibiotic prophylaxis. A new therapeutic strategy might include the potent inhibitor of CXCR4 function plerixafor (Mozobil), as an agent specifically targeting the molecular defect in order to attenuate the phenotypic manifestations of the syndrome.     

CAD/CAM bilateral ear prostheses construction for Treacher Collins syndrome patients using laser scanning and rapid prototyping

Ear defects in patients affected by Treacher Collins syndrome necessitate the replacement of the existing anatomic residuals of the ears with custom-made prostheses. This paper describes a multidisciplinary protocol involving both medicine and computer-aided design/computer-aided manufacturing for manufacturing ear prostheses. Using innovative prototyping technologies together with conventional silicone processing procedures, a step-by-step procedure is presented. The complete workflow includes laser scanning of the defective regions of a patient's face, the use of 3D anatomic models from an ear digital library and rapid prototyping of both substructures for bar anchoring and moulds for silicone processing.     

The mutational spectrum in Treacher Collins syndrome reveals a predominance of mutations that create a premature-termination codon.

Treacher Collins syndrome (TCS) is an autosomal dominant disorder of craniofacial development, the features of which include conductive hearing loss and cleft palate. The TCS locus has been mapped to human chromosome 5q31.3-32 and the mutated gene identified. In the current investigation, 25 previously undescribed mutations, which are spread throughout the gene, are presented. This brings the total reported to date to 35, which represents a detection rate of 60%. Of the mutations that have been reported to date, all but one result in the introduction of a premature-termination codon into the predicted protein, treacle. Moreover, the mutations are largely family specific, although a common 5 bp deletion in exon 24 (seven different families) and a recurrent splicing mutation in intron 3 (two different families) have been identified. This mutational spectrum supports the hypothesis that TCS results from haploinsufficiency.