fras1 shapes endodermal pouch 1 and stabilizes zebrafish pharyngeal skeletal development

Abstract

SUMMARYLesions in the epithelially expressed human gene FRAS1 cause Fraser syndrome, a complex disease with variable symptoms, including facial deformities and conductive hearing loss. The developmental basis of facial defects in Fraser syndrome has not been elucidated. Here we show that zebrafish fras1 mutants exhibit defects in facial epithelia and facial skeleton. Specifically, fras1 mutants fail to generate a late-forming portion of pharyngeal pouch 1 (termed late-p1) and skeletal elements adjacent to late-p1 are disrupted. Transplantation studies indicate that fras1 acts in endoderm to ensure normal morphology of both skeleton and endoderm, consistent with well-established epithelial expression of fras1. Late-p1 formation is concurrent with facial skeletal morphogenesis, and some skeletal defects in fras1 mutants arise during late-p1 morphogenesis, indicating a temporal connection between late-p1 and skeletal morphogenesis. Furthermore, fras1 mutants often show prominent second arch skeletal fusions through space occupied by late-p1 in wild type. Whereas every fras1 mutant shows defects in late-p1 formation, skeletal defects are less penetrant and often vary in severity, even between the left and right sides of the same individual. We interpret the fluctuating asymmetry in fras1 mutant skeleton and the changes in fras1 mutant skeletal defects through time as indicators that skeletal formation is destabilized. We propose a model wherein fras1 prompts late-p1 formation and thereby stabilizes skeletal formation during zebrafish facial development. Similar mechanisms of stochastic developmental instability might also account for the high phenotypic variation observed in human FRAS1 patients.

Analyzing Fluctuating Asymmetry with Geometric Morphometrics: Concepts, Methods, and Applications

“…Additional studies have found pronounced effects of mutations in various genes on the fluctuating asymmetry in the size of structures and, even though they have not specifically investigated shape, sometimes found evident asymmetries of shape [333,339,340]. The observation that some of these effects depend on the genetic background [177] suggests that interactions among multiple components of the developmental system are important.…”

barx1 represses joints and promotes cartilage in the craniofacial skeleton

“…Development 140 (13) To examine chondrocyte arrangement, and the orientation of cartilage elements in live animals, we used the transgenic line sox9a:EGFP, which labels all chondrocytes (Talbot et al, 2012). These studies revealed gaps in sox9a-expressing cells at the jaw joint, as well as the Meckel's and ceratohyal cartilage gaps in mutants (Fig.…”

AMACO Is a Component of the Basement Membrane–Associated Fraser Complex

“…Both morpholinos led to a marked decrease in AMACO protein levels (Figure 5a and b,), while Fras1 appeared largely normal (Figure 5a and b, insets). Nevertheless, vwa2 morphants exhibited normal morphology both at 48 (Figure 5c and d) and 80 hpf (Figure 5g and h), including regions that normally display high AMACO levels (Gebauer et al , 2010) and that are, at least partly, affected in fras1 mutant zebrafish (Carney et al , 2010; Talbot et al , 2012), such as the body fins (Figure 5c and d), the pronephros (data not shown), the somitic myosepta and their connection to the myotomes (Figure 5e and f) and the craniofacial cartilage (Figure 5g and h). This suggests that AMACO per se is dispensable for early zebrafish development.…”