Vertebrate photoreceptor cells are the basic sensory apparatus of the retina, capable of converting the energy of absorbed photons into neuronal signals. The proximal portions of mammalian photoreceptor outer segments are synthesized daily by cell bodies, and outer segment tips are shed with a circadian rhythm, resulting in a complete turnover of outer segments about every 9 days. The shed outer segments are phagocytosed by adjacent retinal pigment epithelial (RPE) cells, and metabolites are recycled to photoreceptors. The Royal College of Surgeons (RCS) rat is a widely studied, classic model of recessively inherited retinal degeneration in which the RPE fails to phagocytose shed outer segments, and photoreceptor cells subsequently die. We have used a positional cloning approach to study the rdy (retinal dystrophy) locus of the RCS rat. Within a 0.3 cM genetic inclusion interval, we have discovered a small deletion of RCS DNA that disrupts the gene encoding the receptor tyrosine kinase Mertk. The deletion includes the splice acceptor site upstream of the second coding exon of Mertk and results in a shortened transcript that lacks this exon. The aberrant transcript joins the first and third coding exons, leading to a frameshift and a translation termination signal 20 codons after the AUG. The concordance of these and other data indicate that Mertk is probably the gene for rdy. Our results provide genetic evidence for an essential role of a receptor tyrosine kinase in a specialized form of phagocytosis and suggest a molecular model for ingestion of outer segments by RPE cells.

The Royal College of Surgeons (RCS) rat is a widely studied animal model of retinal degeneration in which the inability of the retinal pigment epithelium (RPE) to phagocytize shed photoreceptor outer segments leads to a progressive loss of rod and cone photoreceptors. We recently used positional cloning to demonstrate that the gene Mertk likely corresponds to the retinal dystrophy (rdy) locus of the RCS rat. In the present study, we sought to determine whether gene transfer of Mertk to a RCS rat retina would result in correction of the RPE phagocytosis defect and preservation of photoreceptors. We used subretinal injection of a recombinant replication-deficient adenovirus encoding rat Mertk to deliver the gene to the eyes of young RCS rats. Electrophysiological assessment of animals 30 days after injection revealed an increased sensitivity of treated eyes to low-intensity light. Histologic and ultrastructural assessment demonstrated substantial sparing of photoreceptors, preservation of outer segment structure, and correction of the RPE phagocytosis defect in areas surrounding the injection site. Our results provide definitive evidence that mutation of Mertk underlies the RCS retinal dystrophy phenotype, and that the phenotype can be corrected by treatment of juvenile animals. To our knowledge, this is the first demonstration of complementation of both a functional cellular defect (phagocytosis) and a photoreceptor degeneration by gene transfer to the RPE. These results, together with the recent discovery of MERTK mutations in individuals with retinitis pigmentosa, emphasize the importance of the RCS rat as a model for gene therapy of diseases that arise from RPE dysfunction.T he Royal College of Surgeons (RCS) rat strain displays an unusual retinal phenotype in which shed photoreceptor (PR) outer segment (OS) debris accumulates in the subretinal space (1). PR disk shedding normally commences at postnatal day (P)12 in the rat (2), and an OS debris layer is readily apparent in the eyes of RCS animals at P20. Genetic chimera studies suggest that the defect is a cell-autonomous function of the RPE, not the PRs (3). Indeed, in vivo (4) and RPE cell culture studies (5) revealed a defect in the ability of RCS RPE to phagocytize shed OS membranes. Despite the RPE-specific nature of the defect, death strikes the PRs first, through the process of apoptosis (6). The time course of PR degeneration is rapid, beginning around P20, with few PR nuclei remaining in the outer nuclear layer (ONL) by P60. Thinning and atrophy of the RPE begins after onset of PR degeneration. The cause of PR cell death is not well understood; the debris zone acting as a diffusion barrier to metabolites (7), disruption of the interphotoreceptor matrix (8), and PR hypoxia due to diminished oxygen diffusion (9) have been proposed as explanations.In contrast to the extensively studied retinal phenotype of the RCS rat, the nature of the genetic defect has remained obscure until recently. Early genetic linkage studies showed that the phenotype is complet...