Type IX collagen is crucial for normal hearing

cDNA microarray analysis indicated that COL9A1 and COL9A3 are highly expressed in the human inner ear, suggesting that type IX collagen has a crucial functional role in the inner ear. This study further confirmed, by means of real-time PCR, the presence of collagen type IX genes in the mouse inner ear. Immunocytochemical analysis also revealed that type IX collagen is distributed in the tectorial membrane, where it co-localizes with type II collagen, indicating that type IX collagen may contribute to the three-dimensional integrated structure of type II collagen molecules. Mice with targeted disruption of the col9a1 gene were shown through assessment by auditory brain stem response to have hearing loss, suggesting an important role of type IX collagen in maintaining normal hearing. At the light microscopic level, the tectorial membrane of knock-out mice was found to be abnormal in shape, and electron microscopy confirmed disturbance of organization of the collagen fibrils. An antibody against type II collagen failed to detect type II collagen in the tectorial membrane of type IX collagen knock-out mice, suggesting that a lack of type IX collagen may affect the three-dimensional structure of type II collagen molecules. These findings indicate that genes encoding each chain of type IX collagen may fulfill an important function associated with the tectorial membrane in the auditory system.