Molecular markers for cell types of the inner ear and candidate genes for hearing disorders

To identify genes expressed in the vertebrate inner ear, we have established an assay that allows rapid analysis of the differential expression pattern of mRNAs derived from an auditory epithelium-specific cDNA library. We performed subtractive hybridization to create an enriched probe, which then was used to screen the cDNA library. After digoxigenin-labeled antisense cRNAs had been transcribed from hybridization-positive clones, we conducted in situ hybridization on slides bearing cryosections of late embryonic chicken heads, bodies, and cochleae. One hundred and twenty of the 196 clones analyzed encode 12 proteins whose mRNAs art specifically or highly expressed in the chicken's inner ear; the remainder encode proteins that occur more widely. We identified proteins that have been described previously as expressed in the inner ear, such as beta-tectorin, calbindin, and type II collagen. A second group of proteins abundant in the inner ear includes five additional types of collagens. A third group, including Coch-5B2 and an ear-specific connexin, comprises proteins whose human equivalents are candidates to account for hearing disorders. This group also includes proteins expressed in two unique cell types of the inner ear, homogene cells and cells of the tegmentum vasculosum.