A mutation in the zebrafish choroideremia gene causes hair cell degeneration in the ear and lateral line organ

The mechanosensory receptor of the auditory system, the hair cell, is not well understood at a molecular level. In an effort to further our knowledge of this unique cell type, we have conducted a genetic screen in the zebrafish, Danio rerio, identifying 17 mutant lines defective in the development or function of the inner ear and lateral-line organ. Five-day-old larvae from 12 lines exhibited deficiencies in a simple behavioral test designed to assess the startle response elicited by a vibratory stimulus as well as the ability to maintain a consistent orientation with respect to gravity. Mutant larvae from one line, ru848, were unresponsive to vibratory stimuli; however, they could swim in response to touch stimuli although they were unable to maintain an upright orientation. The overall appearance of the five-day-old larvae was normal, apart from the absence of an inflated swim bladder and abnormal pigmentation in the eyes. We were unable to measure in mutant inner ears the extracellular microphonic potentials apparent in wild-type fish. When the inner ear or lateral-line organ of five-day-old larvae was exposed to the cationic fluorophore 4-Di-2-ASP, which is normally taken up by wild-type hair cells, few hair cells were visible. Signs of hair-cell degeneration were apparent from 42 hours after fertilization. The hair cells that remained in five-day-old larvae appeared morphologically abnormal; in the semicircular canals, the surviving kinocilia were significantly shorter than those of wild-type hair cells. Using positional cloning techniques, we localized the ru848 mutation to the zebrafish homolog of the human choroideremia gene. This gene encodes Rab escort protein 1, a protein necessary for the prenylation and membrane association of Rabs, small, Ras-related GTP-binding proteins involved in vesicular transport. Mutations in the human gene induce choroideremia, a disease marked by slow-onset degeneration of rod photoreceptor and retinal pigment epithelial cells. The degenerative phenotype resulting from the ru848 mutation in the zebrafish choroideremia gene indicates that hair cells and particular cell types in the retina are uniquely sensitive to the loss of Rab escort protein and depend on the presence of functional Rab molecules for survival.