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Curing human deafness with the help of zebrafish

By Katharine de Baun

Zebrafish assisting cures for human deafness

What do zebrafish have to do with understanding the causes -- and possible cures for -- human deafness?

Biochemistry and Biophysics researcher Colin Johnson and his team are finding out by using zebrafish to understand the role the protein otoferlin plays in human hearing. He was recently awarded a 5-year, $1.7M NIH grant to support this pioneering work.

It’s well known that mutations in the otoferlin gene are linked to profound hearing loss in humans. Otoferlin is involved in mediating neurotransmitter release from hair cells in the ear, translating sound waves into signals that the brain can “hear.” Progress in understanding on a molecular level exactly how the mutations cause deafness, and devising possible gene therapies to cure it, has been stymied, however, by the fact that studies were done exclusively in mice.

Johnson’s groundbreaking innovation was to switch from mice to zebrafish as a model, opening up several new research avenues. First, even though zebrafish and mammalian genomes diverged approximately 420 million years ago, the amino acid sequences in the otoferlin gene have been largely conserved over time, so that researchers can apply what they learn in zebrafish to humans.

Second, the zebrafish embryos develop externally and are transparent, making it easier to visually evaluate results from genetic changes. Last but not least, Johnson found it much easier to transfect zebrafish hair cells with mutant and truncated otoferlin genes.

Johnson’s NIH grant will build upon his earlier research, published in Molecular and Cellular Biology. There, he established zebrafish as a model for otoferlin studies, discovering that otoferlin is involved not only in the hearing mechanism but in embryonic development. Zebrafish embryos whose otoferlin genes have been suppressed or “knocked down” not only have hearing defects but balance and locomotion deficiencies as well as uninflated swim bladders. “Rescue experiments,” whereby full-length or truncated mouse otoferlin genes were transfected in zebrafish, restored hearing, balance, and inflation of the swim bladder.

The NIH support enables Johnson to further explore possible gene therapies by engineering and testing different truncated forms of mouse otoferlin to determine the minimal regions of the protein required for hearing. Determining the smallest region is critical since the full-length otoferlin gene is far too large for viral-mediated gene therapies. He and his team will also seek to understand on a molecular basis why otoferlin mutations result in hearing loss and explore its biophysical properties.

"We're quite optimistic that this strategy for studying hearing will both improve our understanding of how humans interpret sounds," says Johnson, "and aid in the development of treatments of hearing loss."