Nearly two decades ago, scientists found that mutations in the otoferlin gene lead to near absolute deafness in humans and is responsible for profound congenital deafness in children. Otoferlin is present in the cochlear inner hair cells and mediates neurotransmitter release of the contents of sound to the auditory nerves and eventually to the brain.
Due to its large size and low solubility, scientists have been able to study the protein in only limited ways. This has prevented “a molecular level explanation for otoferlin-related forms of deafness,” according to an article published in the Proceedings of the National Academy of Sciences (PNAS) by researchers in the Department of Biochemistry and Biophysics at Oregon State University.
Associate biochemistry professor Colin Johnson, doctoral biochemistry student Nicole Hams, former biochemistry doctoral student Murugesh Padmanaryana and biophysicist Weihong Qiu collaborated on the project.
The team has come up with a novel method to overcome major challenges in the study of the otoferlin protein and understanding of its function and molecular level explanations of its defective mutations. Johnson and his group developed a single molecule assay to quantitatively characterize full-length otoferlin from mammalian cultured cells.
"A major challenge in the hearing and deafness field has been in identifying a truncated form of otoferlin small enough to be packaged into delivery vehicles for gene therapy," Johnson said.
The large size of otoferlin has up to recently prevented “rescue experiments,” from being conducted. In a rescue experiment, a modified mRNA for otoferlin is transfected in an animal model to replace a suppressed or knocked down otoferlin gene responsible for deafness.
The study by Johnson and colleagues determined a truncated form of otoferlin that can function in the encoding of sound. In addition, they also discovered that there are significant differences between otoferlin and other synaptic proteins that highlight the former’s unique interactions with the calcium channel responsible for hearing.
The study also demonstrates that a deafness-causing mutation in otoferlin weakens the binding between otoferlin and a calcium synapse in the ear, “suggesting that deficiencies in this interaction may form the basis for otoferlin-related hearing loss.”
This pathbreaking study by Johnson and his team advances scientific understanding of otoferlin as a calcium sensitive linker protein and its role in the transport of neurotransmitters in response to sound.
Johnson was recently awarded a 5-year $1.7 M NIH grant to support his pioneering work.
Read more on Johnson's research: Curing human deafness with the help of zebrafish.