Introduction
Among the varied causes of craniosynostosis (premature fusion of one or more sutures of the skull vault), a monogenic etiology is most commonly identified in individuals with fusion of the coronal sutures, the major pair of transverse sutures crossing the vertex of the skull.1 Coronal synostosis, which can be present bilaterally (bicoronal) or unilaterally (unicoronal), affects approximately 1 in 10,000 children2 and is the type most commonly associated with an identifiable syndrome. Common monogenic disorders that characteristically present with coronal synostosis are Muenke (MIM: 602849) and Apert (MIM: 101200) syndromes, caused by localized gain-of-function mutations encoded by FGFR3 (MIM: 134934) and FGFR2 (MIM: 176943), respectively; Saethre-Chotzen syndrome (MIM: 101400) (TWIST1 [MIM: 601622] haploinsufficiency); TCF12-related craniosynostosis (MIM: 600480 and 615314) (also a haploinsufficiency); and craniofrontonasal syndrome (MIM: 304110) (cellular interference involving variants in the X-linked EFNB1 gene [MIM: 300035]).3,4
Even in the absence of an obvious syndromic diagnosis, a specific mutation can be identified in about 60% of individuals with bicoronal and 30% with unicoronal synostosis.1,4 The high monogenic load in coronal synostosis can be accounted for by the specific developmental origin of the coronal suture, which lies at an embryonic tissue boundary between neural-crest-derived frontal bone and mesoderm-derived parietal bone.5,6 Based on analysis of mouse models,7 coronal synostosis is frequently caused by disruption in the maintenance of the population of stem cells within the suture during early development (typically, embryonic days [E]12.5–14.5), caused, for example, by abnormalities in migration of neural crest cells8 or abnormal paracrine signaling through fibroblast growth factor receptors.9,10
An alternative possibility is that coronal synostosis could be caused by a primary failure of the suture to develop. Lineage tracing demonstrates that the cells of the future coronal suture originate from paraxial cephalic mesoderm at E7.5 and migrate laterally to locate above the developing eye.11 This region constitutes the supraorbital regulatory center and during E11.5–E13.5, cells from this zone migrate apically to form and populate the coronal suture.6,11,12 One of the genes characteristically expressed by these cells is engrailed 1 (En1), a homolog of the Drosophila engrailed segment polarity gene. Mice with homozygous loss of En1 function have generalized calvarial bone hypoplasia and persistent widening of the sutural gaps, which is associated with a posterior shift in the boundary between cells of neural crest and mesodermal origin.11,13 An orthologous mutation has not yet been described in humans.
Here, we report an additional genetic etiology for coronal synostosis, caused by heterozygous variants in the final exon of ZIC1 (zinc finger protein of cerebellum 1 [MIM: 600470]), identified in four simplex case subjects and a three-generation pedigree. ZIC1, located on chromosome 3q25.1, belongs to a family of five genes encoding Zn-finger transcription factors, which are arranged as one unpaired and two paired paralogs in the human and mouse genomes;14ZIC genes are homologous to the Drosophila pair-rule gene odd-paired, which is required for activation of embryonic engrailed expression.15 Vertebrate ZICs have important roles in multiple developmental processes, including neurogenesis, left-right axis formation, myogenesis, and skeletal patterning.16,17 Heterozygous complete deletions of ZIC1 were previously associated with Dandy-Walker malformation (DWM; hypoplasia and upward rotation of the cerebellar vermis and cystic dilatation of the fourth ventricle [MIM: 220200]);18 we now show that mutations affecting the highly conserved C terminus of the protein, which are likely to be associated with a gain of function, lead to a distinct phenotype of coronal suture fusion and learning disability. In addition to its previously established importance for neurogenesis,19,20 this work shows that ZIC1 is required for normal coronal suture development. We find that murine Zic1 is expressed in the supraorbital regulatory center, suggesting that this gene acts at a very early stage of coronal suture development,7 potentially (reflecting a similar epistatic relationship to that in Drosophila) by regulating En1.