Craniosynostosis, Philadelphia type: a new autosomal dominant syndrome with sagittal craniosynostosis and syndactyly of the fingers and toes

Ciliary Signalling and Mechanotransduction in the Pathophysiology of Craniosynostosis

Craniosynostosis (CS) is the second most prevalent inborn craniofacial malformation; it results from the premature fusion of cranial sutures and leads to dimorphisms of variable severity. CS is clinically heterogeneous, as it can be either a sporadic isolated defect, more frequently, or part of a syndromic phenotype with mendelian inheritance. The genetic basis of CS is also extremely heterogeneous, with nearly a hundred genes associated so far, mostly mutated in syndromic forms. Several genes can be categorised within partially overlapping pathways, including those causing defects of the primary cilium. The primary cilium is a cellular antenna serving as a signalling hub implicated in mechanotransduction, housing key molecular signals expressed on the ciliary membrane and in the cilioplasm. This mechanical property mediated by the primary cilium may also represent a cue to understand the pathophysiology of non-syndromic CS. In this review, we aimed to highlight the implication of the primary cilium components and active signalling in CS pathophysiology, dissecting their biological functions in craniofacial development and in suture biomechanics. Through an in-depth revision of the literature and computational annotation of disease-associated genes we categorised 18 ciliary genes involved in CS aetiology. Interestingly, a prevalent implication of midline sutures is observed in CS ciliopathies, possibly explained by the specific neural crest origin of the frontal bone.

The epidemiology, genetics and future management of syndactyly

Syndactyly is a condition well documented in current literature due to it being the most common congenital hand defect, with a large aesthetic and functional significance.There are currently nine types of phenotypically diverse non-syndromic syndactyly, an increase since the original classification by Temtamy and McKusick(1978). Non-syndromic syndactyly is inherited as an autosomal dominant trait, although the more severe presenting types and sub types appear to have autosomal recessive and in some cases X-linked hereditary.Gene research has found that these phenotypes appear to not only be one gene specific, although having individual localised loci, but dependant on a wide range of genes and subsequent signalling pathways involved in limb formation. The principal genes so far defined to be involved in congenital syndactyly concern mainly the Zone of Polarizing Activity and Shh pathway.Research into the individual phenotypes appears to complicate classification as new genes are found both linked, and not linked, to each malformation. Consequently anatomical, phenotypical and genotypical classifications can be used, but are variable in significance, depending on the audience.Currently, management is surgical, with a technique unchanged for several decades, although future development will hopefully bring alternatives in both earlier diagnosis and gene manipulation for therapy.

Copy-number variations, noncoding sequences, and human phenotypes

Whereas single-nucleotide polymorphisms and their role in predisposition to disease have been studied extensively, the analysis of structural variants--genomic changes such as insertions, deletions, inversions, duplications, and translocations--is still in its infancy. Changes in copy number, also known as copy-number variations (CNVs), constitute one such group of these structural variants. CNVs are structural genomic variants that arise from deletions (loss) or duplications (gain), and as a consequence result in a copy-number change of the respective genomic region. CNVs may include entire genes or regions of transcribed sequence, or, indeed, comprise only nontranscribed sequences. Whereas the duplication or deletion of a gene can be expected to have an effect on gene dosage, the consequences of CNVs in nontranscribed sequences are less obvious. Here we review CNVs that involve regulatory nontranscribed regions of the genome, describe the associated human phenotypes, and discuss possible disease mechanisms.

Copy-number variations involving the IHH locus are associated with syndactyly and craniosynostosis

Indian hedgehog (IHH) is a secreted signaling molecule of the hedgehog family known to play important roles in the regulation of chondrocyte differentiation, cortical bone formation, and the development of joints. Here, we describe that copy-number variations of the IHH locus involving conserved noncoding elements (CNEs) are associated with syndactyly and craniosynostosis. These CNEs are able to drive reporter gene expression in a pattern highly similar to wild-type Ihh expression. We postulate that the observed duplications lead to a misexpression and/or overexpression of IHH and by this affect the complex regulatory signaling network during digit and skull development.

FGFs, their receptors, and human limb malformations: clinical and molecular correlations

Fibroblast growth factors (FGFs) comprise a family of 22 distinct proteins with pleiotropic signaling functions in development and homeostasis. These functions are mediated principally by four fibroblast growth factor receptors (FGFRs), members of the receptor tyrosine kinase family, with heparin glycosaminoglycan as an important cofactor. Developmental studies in chick and mouse highlight the critical role of FGF-receptor signaling in multiple phases of limb development, including the positioning of the limb buds, the maintenance of limb bud outgrowth, the detailed patterning of the limb elements, and the growth of the long bones. Corroborating these important roles, mutations of two members of the FGFR family (FGFR1 and FGFR2) are associated with human disorders of limb patterning; in addition, mutations of FGFR3 and FGF23 affect growth of the limb bones. Analysis of FGFR2 mutations in particular reveals a complex pattern of genotype/phenotype correlation, which will be reviewed in detail. Circumstantial evidence suggests that the more severe patterning abnormalities are mediated by illegitimate paracrine signaling in the mesoderm, mediated by FGF10 or by a related FGF, and this is beginning to gain some experimental support. A further test of this hypothesis is provided by a unique family segregating two FGFR2 mutations in cis (S252L; A315S), in which severe syndactyly occurs in the absence of the craniosynostosis that typically accompanies FGFR2 mutations.

Oral findings in Carpenter syndrome

Acrocephalopolysyndactyly Type II (Carpenter Syndrome) is determined by autosomal recessive inheritance. Only some 40 cases have been described. Variable clinical signs have been described including prolonged retention of primary teeth and hypodontia. This paper describes the oral and dental findings in a family containing two affected brothers. The family pedigree is informative, as the mother has had children by three partners. The two affected individuals are full brothers. The first affected brother has delayed dental development, severe hypodontia and small tooth crown size. Mesio-distal and bucco-lingual dimensions were measured on the study models and compared with population data. The younger brother also has delayed dental development but only mild hypodontia. Their half sister has severe hypodontia but no signs of Carpenter Syndrome. This family study demonstrates two affected individuals with typical clinical features and a pedigree compatible with autosomal recessive inheritance. Small tooth crown size has been shown by standardized measurement and evidence advanced that hypodontia is not part of the syndrome but a coincidental finding which segregates independently. We have also shown that the marked delay in emergence of teeth is associated more with problems of tooth eruption, possibly related to the bony abnormalities, than to a generalized delay in dental development.