Identification of preferentially expressed cochlear genes by systematic sequencing of a rat cochlea cDNA library

107 expressed sequence tags (ESTs) from a rat cochlea cDNA library were identified by systematic sequencing coupled to database selection and RT-PCR analysis of novel sequences. This approach led us to select a clone, pCO8, showing no significant homology with any database sequence, that corresponds to a mRNA whose expression is restricted to the cochlea, except for traces detected in brain. Additional clones with novel sequences enriched in the cochlea were also found. ESTs bearing significant homologies with database sequences (63 out of 107) were classified according to the putatively encoded protein. They include tissue-specific genes not previously described in the cochlea as well as known genes from other species. We performed in situ hybridization in cochlear tissues to localize the pCO8 mRNA and that of clone pCO6 which is 100% homologous to the delayed rectifier potassium channel drk1. We found that both mRNAs were exclusively expressed in the cellular body of the primary auditory neurons from the spiral ganglion of the cochlea. These results indicate that this approach is an efficient way to identify novel genes that could be of importance in cochlear function.