Hearing and Balance Disorders

Hearing and balance disorders affect 30 million Americans in some form or another and cost the country 56 billion dollars annually. Hearing loss is no small matter, as we have no perfect cure for it. There are many hearing aids available, but this is nowhere near helping deaf persons hear as if they had no disorder.

Hearing loss can be caused by many factors, but approximately 50% of cases of deafness are believed to be genetic. Hearing loss can also be caused by acoustic trauma, such as prolonged exposure to high decibel noise levels. The sensory epithelial tissue can also be damaged by chemicals, such as specific antibiotics, certain chemotherapy agents, or other autotoxins. Hearing loss can also come with aging.

Our hearing comes from a complex pathway in our ears. Sound waves enter into the ear canal, travel to the eardrum, and vibrations are passed through the middle ear into the inner ear by three bones in the middle ear section. The last bone vibrates onto the oval window of the cochlea, and the vibrations are passed through fluid in the cochlea and they travel to the exit at the round window. By going through this coil in the cochlea pressure is released in the inner ear. The Organ of Corti is a very important part of the Cochlea, as this is where impulses are relayed to the brain. Stereocilia (hair-like structures) from Inner Hair Cells and Outer Hair Cells are inserted in the tectoral membrane. The tectoral membrane remains fairly still while the mechanism vibrates from the waves passed into the basilar membrane by the third bone in the middle ear. Ion channels are in turn opened within the auditory hair cells and impulses are sent to the brain via nerves at the back of these cells. These hair cells are very important, as they are the primary mechanoelectrical transducer of sound in our ears. The hair cells are also present in the Vestibular Organ, the area of the ear where changes in special orientation are detected. When human auditory hair cells are damaged, they do not regenerate. However, most non-mammalian species have the ability to regenerate these very important sensory cells.

In chickens, the hair cells in the utricle, which is part of the Vestibular Organ, are constantly proliferating as old hair cells die off. In the chicken’s cochlea, however, the hair cells normally remain quiescent unless they are damaged.

One goal of the Lovett Lab is to create a library of genes and transcription factors that are differentially expressed in the utricle and cochlea to discover how these regeneration genes are turned on and which genes they are.

This is being done by using custom gene chips that contain all the transcription factor genes, and by building subtracted cDNA libraries. My part in this project was to evaluate cDNA sequence from the subtracted cDNAs. I sequenced 120 chick utricle specific cDNAs. Of my finished sequences, 30% were homologous to genes such as Aconitase, Beta-tectorin, and MD1, and 70% of the sequences were completely novel. These new sequences will be used to investigate the regeneration process in the utricle.