Craniofacial Dysmorphology of Carpenter Syndrome: Lessons from Three Affected Siblings:

Revisiting Crouzon syndrome: reviewing the background and management of a multifaceted disease

PURPOSE

Crouzon syndrome is a complex craniosynostosis of autosomal dominant transfer, with a highly variable phenotypic appearance. Some patients are afflicted with a mild form of the disease and able to live a fully functional lifestyle, whereas many patients suffer from a severe form of the disease causing a significant impact on their quality of life. Although several case reports and genetic studies have been performed, there has been no recent review of the literature to collate the information on this disorder. In this paper, we seek to unify the findings of this disorder to provide the pediatric provider with a succinct, but complete discussion of this disease.

METHODS

Articles from 1994 to 2014 were queried on PubMed and reviewed for utility by all three researchers involved in the project. Further literature review was done on relevant articles found within references of selected articles.

RESULTS

Crouzon syndrome, although of variable penetrance, is thought to be caused in part by a mutation in the fibroblast growth factor receptor-2 (FGFR2) on chromosome 10. Aside from craniofacial malformations, the disease can also cause hearing loss and airway challenges due to malformations in the nasal cavity and nasopharyngeal airway. Management options from the perspective of the otorhinolaryngologist are diverse and revolve around craniosynostectomy, offsetting midfacial hypoplasia, addressing obstructive sleep apnea and a compromised airway, psychosocial and esthetic interventions, and ultimately requiring a multidisciplinary team approach.

CONCLUSION

Mutations in the FGFR2 are responsible for 50 % of mutations within this multifaceted syndrome. Crouzon syndrome is an autosomal dominant disorder with a number of distinguishing characteristics, including craniosynostosis, maxillary hypoplasia, exophthalmos, and multiple other features. Early intervention, both medically and surgically, as well as disciplined follow-up with the pediatric provider are crucial to the management of this disorder. In particular, management should address cranial suture release, midfacial advancement, evaluation for hearing deficits, and obstructive sleep apnea, with expedient intervention for airway compromise and increased intracranial pressure.

Rab proteins: the key regulators of intracellular vesicle transport

Vesicular/membrane trafficking essentially regulates the compartmentalization and abundance of proteins within the cells and contributes in many signalling pathways. This membrane transport in eukaryotic cells is a complex process regulated by a large and diverse array of proteins. A large group of monomeric small GTPases; the Rabs are essential components of this membrane trafficking route. Most of the Rabs are ubiquitously expressed proteins and have been implicated in vesicle formation, vesicle motility/delivery along cytoskeleton elements and docking/fusion at target membranes through the recruitment of effectors. Functional impairments of Rabs affecting transport pathways manifest different diseases. Rab functions are accompanied by cyclical activation and inactivation of GTP-bound and GDP-bound forms between the cytosol and membranes which is regulated by upstream regulators. Rab proteins are characterized by their distinct sub-cellular localization and regulate a wide variety of endocytic, transcytic and exocytic transport pathways. Mutations of Rabs affect cell growth, motility and other biological processes.

Caesarean section in a parturient with Carpenter syndrome and corrected Tetralogy of Fallot

Carpenter syndrome is an extremely rare congenital disorder characterized by craniofacial malformations, syndactyly, cardiac defects and obesity. Even after surgical correction of cardiac abnormalities, intrapartum care of a parturient with this condition can be challenging because of progression of residual cardiac defects compounded by the haemodynamic changes of pregnancy. We describe the anaesthetic management for caesarean section of a parturient with Carpenter syndrome and corrected Tetralogy of Fallot. Additional complicating issues included worsening cardiac function, asthma, needle-phobia and osteoarthritis.

Genetic causes of syndromic craniosynostoses

Syndromic craniosynostose exhibit variable clinical and genetic heterogeneity. Many of this disorders are caused by mutations in the fibroblast growth factor receptor genes: FGFR2, FGFR3 (encoding fibroblast growth factor receptors), TWIST1 (functions as an upstream regulator of FGFRs) and EFNB1 (gene encoding fibrillin1). However recent advances in molecular genetics have led to a discover of other genes implicated in different craniosynostosis syndromes.

A Novel Aberrant Splice Site Mutation in RAB23 Leads to an Eight Nucleotide Deletion in the mRNA and Is Responsible for Carpenter Syndrome in a Consanguineous Emirati Family

Carpenter syndrome is caused by mutations in the RAB23 gene that encodes a small GTPase of the Rab subfamily of proteins. Rab proteins are known to be involved in the regulation of cellular trafficking and signal transduction. Currently, only few mutations in RAB23 have been reported in patients with Carpenter syndrome. In this paper, we report the clinical features, molecular and functional analysis of 2 children from an Emirati consanguineous family with this syndrome. The affected children exhibit the typical features including craniosynostosis, typical facial appearance, polysyndactyly, and obesity. Molecular analysis of the RAB23 gene revealed a homozygous mutation affecting the first nucleotide of the acceptor splice site of exon 5 (c.482-1G>A). This mutation affects the authentic mRNA splicing and activates a cryptic acceptor site within exon 5. Thus, the erroneous splicing results in an eight nucleotide deletion, followed by a frameshift and premature termination codon at position 161 (p.V161fsX3). Due to the loss of the C-terminally prenylatable cysteine residue, the truncated protein will probably fail to associate with the target cellular membranes due to the absence of the necessary lipid modification. The p.V161fsX3 extends the spectrum of RAB23 mutations and points to the crucial role of prenylation in the pathogenesis of Carpenter syndrome within this family.

Craniosynostosis genetics: The mystery unfolds

Craniosynsostosis syndromes exhibit considerable phenotypic and genetic heterogeneity. Sagittal synostosis is common form of isolated craniosynostosis. The sutures involved, the shape of the skull and associated malformations give a clue to the specific diagnosis. Crouzon syndrome is one of the most common of the craniosynostosis syndromes. Apert syndrome accounts for 4.5% of all craniosynostoses and is one of the most serious of these syndromes. Most syndromic craniosynostosis require multidisciplinary management. The following review provides a brief appraisal of the various genes involved in craniosynostosis syndromes, and an approach to diagnosis and genetic counseling.

Coronal synostosis syndrome (Muenke syndrome): the value of genetic testing versus clinical diagnosis

BACKGROUND

Muenke syndrome is a fibroblast growth factor receptor 3 (FGFR-3)-associated coronal craniosynostosis syndrome, which was first described in 1997.

CASE

We report an infant girl who was born to a 29-year-old primapara at 38 weeks' gestation. When evaluated at 3 days old, physical examination revealed a high forehead with frontal bossing, upturned nose, arched palate, shallow midface structures, and heavily ridged coronal sutures bilaterally. Clinically, the infant seemed to be neurologically normal. Skull radiographs and computed tomography confirmed the presence of bilateral coronal synostosis, with patency of all other sutures. Family history was remarkable, in that the infant's father, paternal grandmother, and a paternal cousin demonstrated subtle craniofacial features, which had not been previously identified. Mutation analysis of FGFR-3 revealed a missense mutation in exon 6, c.749 C>G, with a resultant amino acid change from proline to arginine at codon 250 (P250R), in keeping with Muenke syndrome (Am J Hum Genet 1997;60:555-564). The mutation was subsequently identified in her father, suggesting variable expression in this family, as he had only mild midfacial flattening. At 9 months of age, our patient underwent anterior cranial expansion, correction of orbital hypertelorism, intracranial orbital osteotomies, and advancement of the frontal bandeau. She tolerated the procedure well and has done well postoperatively.

CONCLUSIONS

We report the case of an infant with Muenke syndrome, with evidence of variable expressivity within the paternal family. The pertinent literature, in which only 2 prior Canadian cases were identified, is reviewed.

Craniosynostosis involving the squamous temporal sutures: a rare and possibly underreported etiology for cranial vault asymmetry

Craniosynostosis is a condition in which 1 or more cranial sutures fuse prematurely, often secondary to a fibroblast growth factor receptor (FGFR) mutation, typically involving FGFR2 or FGFR3. This mutation may occur sporadically or in the setting of a genetic syndrome and typically presents within the first few days of life or in early infancy. Most commonly, the sagittal and coronal sutures are involved, although involvement of the lambdoidal and/or metopic sutures is not uncommon. Surgical correction is undertaken both for cosmetic purposes and to relieve raised intracranial pressure, both of which can be severe, depending on the sutures involved. We report on 2 children who presented in their first year of life with synostosis involving: in one instance, a single squamous temporal suture, and in the other, both squamous temporal sutures. The initial presentation and clinical courses of these 2 patients are highly distinct from one another, although both ultimately did quite well after extensive cranial remodeling. To the best of our knowledge, only a handful of patients with squamous synostosis have been reported in the medical literature.

Sudden death in a child with Carpenter Syndrome. Case report and literature review

Carpenter syndrome (Acrocephalopolysyndactyly type II) is a rare disorder characterized by acrocephaly, mental retardation, congenital heart disease, syndactyly, preaxial polydactyly, obesity, cryptorchidism, hypogenitalism, bony abnormalities, and umbilical hernia. We present a case of unexpected death of a 7-year-old boy with Carpenter Syndrome complicated by twin and premature birth as well as repaired congenital heart disease.

RAB23 mutations in Carpenter syndrome imply an unexpected role for hedgehog signaling in cranial-suture development and obesity

Carpenter syndrome is a pleiotropic disorder with autosomal recessive inheritance, the cardinal features of which include craniosynostosis, polysyndactyly, obesity, and cardiac defects. Using homozygosity mapping, we found linkage to chromosome 6p12.1-q12 and, in 15 independent families, identified five different mutations (four truncating and one missense) in RAB23, which encodes a member of the RAB guanosine triphosphatase (GTPase) family of vesicle transport proteins and acts as a negative regulator of hedgehog (HH) signaling. In 10 patients, the disease was caused by homozygosity for the same nonsense mutation, L145X, that resides on a common haplotype, indicative of a founder effect in patients of northern European descent. Surprisingly, nonsense mutations of Rab23 in open brain mice cause recessive embryonic lethality with neural-tube defects, suggesting a species difference in the requirement for RAB23 during early development. The discovery of RAB23 mutations in patients with Carpenter syndrome implicates HH signaling in cranial-suture biogenesis--an unexpected finding, given that craniosynostosis is not usually associated with mutations of other HH-pathway components--and provides a new molecular target for studies of obesity.