The regulation of gene expression in hair cells

No genes have been discovered for which expression is limited only to inner ear hair cells. This is hardly surprising, since the number of mammalian genes is estimated to be 20-25,000, and each gene typically performs many tasks in various locations. Many genes are expressed in inner ear sensory cells and not in other cells of the labyrinth. However, these genes are also expressed in other locations, often in other sensory or neuronal cell types. How gene transcription is directed specifically to hair cells is unclear. Key transcription factors that act during development can specify cell phenotypes, and the hair cell is no exception. The transcription factor ATOH1 is well known for its ability to transform nonsensory cells of the developing inner ear into hair cells. And yet, ATOH1 also specifies different sensory cells at other locations, neuronal phenotypes in the brain, and epithelial cells in the gut. How it specifies hair cells in the inner ear, but alternate cell types in other locations, is not known. Studies of regulatory DNA and transcription factors are revealing mechanisms that direct gene expression to hair cells, and that determine the hair cell identity. The purpose of this review is to summarize what is known about such gene regulation in this key auditory and vestibular cell type.