By Pranav Parikh
Usher syndrome type 3 is an inherited disease in which an individual is born with typical hearing and develops hearing loss in the stages of early childhood. They will most likely develop complete hearing loss by the time they are an adult. Though cases of Usher syndrome type 3 (and its subtypes) are quite infrequent, representing 2 percent of total Usher syndrome cases, the onset symptoms have damaging and often irreversible consequences that severely disrupt the lives of those living with the condition. There is currently no cure for the disease, but cochlear implants have seen some success in providing partial hearing function in patients.
A 3D model of the HARS enzyme, including the catalytic site (where the reaction occurs) and the anticodon site (the part that starts protein synthesis through RNA transcription).
Susan Robey-Bond, Ph.D., a 2012 Emerging Research Grants scientist, and her team at the University of Vermont College of Medicine were able to isolate a mechanism involved in the development of Usher syndrome. Histidyl-tRNA synthetase is an enzyme that is instrumental in protein synthesis. This enzyme, given the acronym HARS, is thought to be involved in the presentation of Usher syndrome type 3B in patients. The early symptoms of temporary hearing and vision loss, hallucinations, and sometimes sudden fatal buildup of fluid in the lungs may be triggered by a fever-causing illness. The hearing and vision loss are eventually severe and permanent.
A graphical representation depicting temperature variation between the wild-type and mutant version of the HARS enzyme.
Usher syndrome type 3B is autosomal recessive, meaning children of parents carrying the gene but who do not display symptoms have a likelihood of developing the disease. It is caused by a USH3B mutation, which substitutes a serine amino acid for a tyrosine amino acid in HARS. The team studying the biochemical properties of the gene compared the Y454S mutation in the HARS enzyme with its wild-type (non-mutated) form and found similar functional biochemical characteristics, as stated in the researchers’ recent paper in Biochemistry.
The amino acid activation, aminoacylation, and tRNA binding functions were all consistent between the mutation and wild-type genes. In later analysis, though, the team found that at an elevated temperature the Y454S substitution was less stable than the wild-type. More specifically, cells from patients containing the Y454S mutation displayed lower levels of protein synthesis, which could explain the onset of deafness these patients experience. How these proteins are implicated in the hearing processes will eventually help develop cures or better treatments for Usher syndrome.
Susan Robey-Bond, Ph.D., was a 2012 Emerging Research Grants recipient. For more, see her Biochemistry paper:, “The Usher Syndrome Type IIIB Histidyl-tRNA Synthetase Mutation Confers Temperature Sensitivity.”
