Temporal perturbations in sonic hedgehog signaling elicit the spectrum of holoprosencephaly phenotypes

Abstract

One of the most perplexing questions in clinical genetics is why patients with identical gene mutations oftentimes exhibit radically different clinical features. This inconsistency between genotype and phenotype is illustrated in the malformation spectrum of holoprosencephaly (HPE). Family members carrying identical mutations in sonic hedgehog (SHH) can exhibit a variety of facial features ranging from cyclopia to subtle midline asymmetries. Such intrafamilial variability may arise from environmental factors acting in conjunction with gene mutations that collectively reduce SHH activity below a critical threshold. We undertook a series of experiments to test the hypothesis that modifying the activity of the SHH signaling pathway at discrete periods of embryonic development could account for the phenotypic spectrum of HPE. Exposing avian embryos to cyclopamine during critical periods of craniofacial development recreated a continuum of HPE-related defects. The craniofacial malformations included hypotelorism, midfacial hypoplasia, and facial clefting and were not the result of excessive crest cell apoptosis. Rather, they resulted from molecular reprogramming of an organizing center whose activity controls outgrowth and patterning of the mid and upper face. Collectively, these data reveal one mechanism by which the variable expressivity of a disorder such as HPE can be produced through temporal disruption of a single molecular pathway.

NODAL and SHH dose-dependent double inhibition promotes an HPE-like phenotype in chick embryos

“…Interestingly, in HPE patients with altered SHH signaling, midline malformation ranges from hypotelorism to complete cyclopia (Muenke and Beachy, 2000). The features of our model are consistent with the demonstration by Cordero et al of a relationship between the timing of Hedgehog disruption and the severity of the facial phenotype (Cordero et al, 2004). Interestingly, embryos subjected to cyclopamine treatment starting as late as stage 7 developed a cyclopic phenotype.…”

“…These experiments show that minor inhibition of both SHH and NODAL pathways can lead to abnormal development of the forebrain midline, whereas similar inhibition of one or other in isolation does not. The penetrance of the phenotypes obtained was incomplete and variable, which is a common feature of most HPE models described (Lowe et al, 2001;Allen et al, 2007;Zhang et al, 2011;Cordero et al, 2004).…”

“…Studies with model organisms demonstrate that SHH produced by the prechordal mesendoderm is required for initiating the development of the midline of the forebrain and the midface (Rubenstein and Beachy, 1998;Muenke and Beachy, 2000;Kiecker and Niehrs, 2001). Abnormal activity of the SHH signaling pathway at particular periods of embryonic development might account for the phenotypical spectrum of HPE (Cordero et al, 2004). There is a progressive mechanism whereby SHH produced by one midline structure induces Shh expression in successive midline structures.…”

Frontal nasal prominence expression driven by Tcfap2a relies on a conserved binding site for STAT proteins

“…Although an in vivo role for Stats downstream of FGFR2 has not yet been shown, mouse models of craniosynostosis containing altered FGFR2, display several phenotypes that overlap with those observed in FKO, Wnt1-Cre; Floxdel/KI, or Tcfap2a ϩ/Ϫ mice including shortened snouts, dental malocclusion, widespaced eyes, and cleft palate (Zhang et al, 1996;Nottoli et al, 1998;De Moerlooze et al, 2000;Hajihosseini et al, 2001;Eswarakumar et al, 2002Eswarakumar et al, , 2004Chen et al, 2003;Brewer et al, 2004;Nelson and Williams, 2004;Wang et al, 2005). Thus, the presence of a STAT site in the FNP/LBM-specific enhancer likely places AP-2␣ downstream of FGFR signaling in the FNP, which is consistent with other studies showing FGF-dependent changes of Tcfap2a in the craniofacial region (Shen et al, 1997;Cordero et al, 2004).…”

The function and regulation of TBX22 in avian frontonasal morphogenesis

“…The frontonasal mass gives rise to the facial midline skeleton and its development is partially dependent on retinoids produced by the brain (Niederreither et al, 1999;Schneider et al, 2001;Halilagic et al, 2007). The actions of sonic hedgehog produced by the foregut endoderm are an important inductive signal for the frontonasal mass (Benouaiche et al, 2008) and for midline specification (Cordero et al, 2004;Marcucio et al, 2005). Blocking FGF receptor signaling in the forebrain impedes growth of the frontonasal mass (Hu and Marcucio, 2009).…”