Using mutant zebra fish, researchers studying the earliest formation of cartilage of the mouth believe they may have gotten a look at a mechanism involved in a genetic defect linked to Fraser syndrome deafness in humans.
Reporting in the Aug. 1 issue of the journal Development, they identify a potential developmental pathway worthy of more scrutiny in future research into Fraser syndrome, a many-faceted and rare recessive genetic disease. In humans, a mutation in the gene FRAS1, which plays a role in skin epithelial formation during early development, has been linked to Fraser syndrome. A comparable version of the gene, fras1, in zebra fish is required for stable skeletal formation.
In the study -- done at the University of Oregon's Institute of Neuroscience -- researchers modeled craniofacial symptoms related to hearing loss in Fraser syndrome using mutant zebra fish, focusing on an endodermal pouch (known as p1), which in humans forms the Eustachian tube.
Using tissue labeling and time-lapse microscopy, the research team found "a previously unrecorded, late-forming portion of the first pharyngeal pouch in the zebra fish," said lead author Jared Coffin Talbot, now a postdoctoral researcher at Ohio State University. He earned his doctorate from the UO in 2011.
The newly seen component, researchers wrote, is a fras1-dependent "endodermal outpocket" -- referred to in the paper as a late-p1. "If this homology can be taken as a guide, then endodermal pouching defects might underlie some ear defects in Fraser patients," they concluded.
"In fras1-mutant fish, some skeletal elements near late-p1 do not form properly during this time period," Talbot said. "However, after this time period, two other skeletal elements that would have normally been separated by late-p1 fuse together in fras1 mutants that lack late-p1. We propose that in normal development late-p1 holds apart skeletal elements found fused in fras1 mutants."
To test that idea, the researchers -- in the lab of Charles B. Kimmel, professor emeritus of biology -- added healthy epithelia tissue from wild-type zebra fish, into fras1-mutant embryos. Doing so allowed for normal facial development in the mutants.
"To my knowledge, the connection between skeletal development and Fraser syndrome deafness has not yet been made in mammals," Talbot said. "The literature has been largely mute as to why skeletal defects are found in fras1 mutants; this is what made it an interesting topic to study, but it does make a direct zebrafish-human connection more difficult to make. We believe that the middle ear and Eustachian tube are the homologous structures to what we're studying in zebra fish."
Fraser syndrome is a poorly understood disorder that can affect multiple parts of the body, but the majority of cases involve the eyes. Canadian geneticist C.R. Fraser described the disease in 1962. The FRAS1 gene previously had been identified as an adhesive and signaling molecule in epithelial tissues. The new paper shows the gene also acts in epithelia to sculpt skeletal shape.