The syndromic craniosynostoses, usually involving multiple sutures, are hereditary forms of craniosynostosis associated with extracranial phenotypes such as limb, cardiac, CNS and tracheal malformations. The genetic etiology of syndromic craniosynostosis in humans is only partially understood. Syndromic synostosis has been found to be associated with mutations of the fibroblast growth factor receptor family (FGFR1, -R2, -R3), TWIST1, MSX2, and EFNB1. Apert, Pfeiffer, Crouzon, and Jackson-Weiss syndromes are due to gain-of-function mutations of FGFR2 in either the Ig II-III linker region (Apert) or Ig III domain. Loss of function mutations of TWIST1 and gain-of-function mutations of MSX2 lead to Saethre-Chotzen and Boston-type syndromes, respectively. The mutations in Pfeiffer (FGFR1), Muenke (FGFR3), and Apert syndrome (FGFR2) are caused by the same amino acid substitution in a highly conserved region of the Ig II-III linker region of these proteins, which suggests that these receptor tyrosine kinases have an overlapping function in suture biology. In this review we will discuss the historical descriptions, current phenotypes and molecular causes of the more common forms of syndromic craniosynostosis.

Craniosynostosis, the premature fusion of one or more cranial sutures, affects 1 in 2,500 live births. Isolated single-suture fusion is most prevalent, with sagittal synostosis occurring in 1/5,000 live births. The etiology of isolated (nonsyndromic) single-suture craniosynostosis is largely unknown. In syndromic craniosynostosis, there is a highly nonrandom pattern of causative autosomal dominant mutations involving TWIST1 and fibroblast growth factor receptors (FGFRs). Prior to our study, there were no published TWIST1 mutations in the anti-osteogenic C-terminus, recently coined the TWIST Box, which binds and inhibits RUNX2 transactivation. RUNX2 is the principal master switch for osteogenesis. We performed mutational analysis on 164 infants with isolated, single-suture craniosynostosis for mutations in TWIST1, the IgIIIa exon of FGFR1, the IgIIIa and IgIIIc exons of FGFR2, and the Pro250Arg site of FGFR3. We identified two patients with novel TWIST Box mutations: one with isolated sagittal synostosis and one with isolated coronal synostosis. Kress et al. [2006] reported a TWIST Box "nondisease-causing polymorphism" in a patient with isolated sagittal synostosis. However, compelling evidence suggests that their and our sequence alterations are pathogenic: (1) a mouse with a mutation of the same residue as our sagittal synostosis patient developed sagittal synostosis, (2) mutation of the same residue precluded TWIST1 interaction with RUNX2, (3) each mutation involved nonconservative amino acid substitutions in highly conserved residues across species, and (4) control chromosomes lacked TWIST Box sequence alterations. We suggest that genetic testing of patients with isolated sagittal or coronal synostosis should include TWIST1 mutational analysis.

R-spondins (RSPOs) are a recently characterized family of secreted proteins that activate WNT/␤-catenin signaling. In this study, we investigated the potential roles of the RSPO proteins during myogenic differentiation. Overexpression of the Rspo1 gene or administration of recombinant RSPO2 protein enhanced mRNA and protein expression of a basic helix-loophelix (bHLH) class myogenic determination factor, MYF5, in both C2C12 myoblasts and primary satellite cells, whereas MYOD or PAX7 expression was not affected. RSPOs also promoted myogenic differentiation and induced hypertrophic myotube formation in C2C12 cells. In addition, Rspo2 and Rspo3 gene knockdown by RNA interference significantly compromised MYF5 expression, myogenic differentiation, and myotube formation. Furthermore, Myf5 expression was reduced in the developing limbs of mouse embryos lacking the Rspo2 gene. Finally, we demonstrated that blocking of WNT/␤-catenin signaling by DKK1 or a dominant-negative form of TCF4 reversed MYF5 expression, myogenic differentiation, and hypertrophic myotube formation induced by RSPO2, indicating that RSPO2 exerts its activity through the WNT/␤-catenin signaling pathway. Our results provide strong evidence that RSPOs are key positive regulators of skeletal myogenesis acting through the WNT/␤-catenin signaling pathway.WNT signaling plays diverse roles in normal tissue development during embryogenesis and tissue function in adulthood. The importance of WNT signaling in skeletal myogenesis was initially demonstrated in embryonic skeletal myogenesis (1-4). The WNT1 and WNT3A ligands derived from the dorsal neural tube and the surface ectoderm positively regulate skeletal myogenesis within somites via the canonical WNT/␤-catenin signaling pathway (3, 4). Furthermore, noncanonical WNT signaling, which transmits signals through the RAC/RHO-dependent planar cell polarity and calcium-PKC pathways, regulates PAX3 and subsequently MYOD expression during myogenic differentiation (5) and a directional elongation of myofibers within the myotome (6).Recently, progress has been made in the understanding of the roles of WNT signaling in postnatal myogenesis. For instance, ␤-catenin was shown to promote self-renewal of satellite cells, the muscle stem/progenitor cells residing in adult skeletal muscle (7). In contrast, WNT/␤-catenin signaling was shown to initiate myogenic differentiation of satellite cells by replacing NOTCH signaling, which is critical for self-renewal of satellite cells, through the inhibition of GSK-3␤ (8). It was also demonstrated that the activation of the WNT/␤-catenin pathway occurs within the CD45-positive stem cell population present in regenerating skeletal muscles but not in satellite cells (9). In aged skeletal muscle, WNT10B regulates the balance between myogenic and adipogenic lineage determination (10). WNT signaling was also suggested to induce a fibroblastic phenotype in aged skeletal muscle (11). Furthermore, WNT7A regulates self-renewal of satellite cells via noncanonical WNT signaling (12). These ...