Craniosynostosis and Multiple Skeletal Anomalies in Humans and Zebrafish Result from a Defect in the Localized Degradation of Retinoic Acid

Abstract

Excess exogenous retinoic acid (RA) has been well documented to have teratogenic effects in the limb and craniofacial skeleton. Malformations that have been observed in this context include craniosynostosis, a common developmental defect of the skull that occurs in 1 in 2500 individuals and results from premature fusion of the cranial sutures. Despite these observations, a physiological role for RA during suture formation has not been demonstrated. Here, we present evidence that genetically based alterations in RA signaling interfere with human development. We have identified human null and hypomorphic mutations in the gene encoding the RA-degrading enzyme CYP26B1 that lead to skeletal and craniofacial anomalies, including fusions of long bones, calvarial bone hypoplasia, and craniosynostosis. Analyses of murine embryos exposed to a chemical inhibitor of Cyp26 enzymes and zebrafish lines with mutations in cyp26b1 suggest that the endochondral bone fusions are due to unrestricted chondrogenesis at the presumptive sites of joint formation within cartilaginous templates, whereas craniosynostosis is induced by a defect in osteoblastic differentiation. Ultrastructural analysis, in situ expression studies, and in vitro quantitative RT-PCR experiments of cellular markers of osseous differentiation indicate that the most likely cause for these phenomena is aberrant osteoblast-osteocyte transitioning. This work reveals a physiological role for RA in partitioning skeletal elements and in the maintenance of cranial suture patency.

Retinoic acid-induced premature osteoblast-to-preosteocyte transitioning has multiple effects on calvarial development

“…Human patients with a partial or complete loss of CYP26B1 function display severe defects in the calvarial plates of the skull roof, including a premature fusion of the coronal sutures (craniosynostosis) between the frontal and parietal plates in the hypomorphs, and calvarial hypoplasia, intercalvarial patencies and calvarial fragmentation in the amorph (Laue et al, 2011). Further studies with zebrafish cyp26b1 hypomorphic mutants led to the elaboration of a model, proposing that excess of RA induces a premature transition of osteoblasts to preosteocytes within the coronal suture.…”

“…Whereas osteoblasts ensure bone matrix (osteoid) production, preosteocytes stimulate its mineralization (Dallas and Bonewald, 2010;Franz-Odendaal et al, 2006). Accordingly, the premature accumulation of preosteocytes within the suture of the hypomorphs leads to ectopic mineralization of the sutural matrix, hence the seeming hyperossification (Laue et al, 2011). However, it remains unclear to what extent this mechanism might also contribute to the calvarial hypoplasia and fragmentation displayed by the human CYP26B1 amorph.…”

Osteoblast de- and redifferentiation is controlled by a dynamic response to retinoic acid during zebrafish fin regeneration

“…Retinoic acid (RA) has been shown to play multiple roles in bone development and repair by exerting pleiotropic effects on cells of the chondroblast, osteoblast and osteoclast lineages (Adams et al, 2007;Allen et al, 2002;Conaway et al, 2013;Dranse et al, 2011;Koyama et al, 1999;Laue et al, 2008Laue et al, , 2011Li et al, 2010;Lie and Moren, 2012;Lind et al, 2013;Nallamshetty et al, 2013;Song et al, 2005;Spoorendonk et al, 2008;Weston et al, 2003;Williams et al, 2009). Although partially conflicting results were reported, the general consensus for osteoblastogenesis is that RA signaling restricts osteoblast differentiation but promotes subsequent bone matrix synthesis by mature osteoblasts.…”

Cutaneous Retinoic Acid Levels Determine Hair Follicle Development and Downgrowth

“…Of the three cytochromes, Cyp26b1 is the only enzyme specifically expressed in the dermis surrounding the developing hair follicles (17,18). Human null and hypomorphic mutations in the gene encoding the RA-degrading enzyme CYP26B1 have been recently reported to lead to skeletal and craniofacial anomalies, including fusions of long bones, calvarial bone hypoplasia, and craniosynostosis (19).…”