RESEARCH ARTICLE Long-term preservation of retinal function in the RCS rat model of retinitis pigmentosa following lentivirus-mediated gene therapy M Tschernutter 1 , FC Schlichtenbrede 1 , S Howe 2 , KS Balaggan 1 , PM Munro 1 , JWB Bainbridge 1 , AJ Thrasher 2 , AJ Smith 1 and RR Ali 1,2 1 Division of Molecular Therapy, Institute of Ophthalmology, University College London, UK; and 2 Molecular Immunology Unit, Institute of Child Health, University College London, UK The Royal College of Surgeons (RCS) rat is a well- characterized model of autosomal recessive retinitis pigmen- tosa (RP) due to a defect in the retinal pigment epithelium (RPE). It is homozygous for a null mutation in the gene encoding , a receptor tyrosine kinase found in RPE cells, that is required for phagocytosis of shed photoreceptor outer segments. The absence of Mertk results in accumulation of outer segment debris. This subsequently leads to progres- sive loss of photoreceptor cells. In order to evaluate the efficacy of lentiviral-mediated gene replacement therapy in the RCS rat, we produced recombinant VSV-G pseudotyped HIV-1-based lentiviruses containing a murine Mertk cDNA driven by a spleen focus forming virus (SFFV) promoter. The vector was subretinally injected into the right eye of 10-day- old RCS rats; the left eye was left untreated as an internal control. Here, we present a detailed assessment of the duration and extent of the morphological rescue and the resulting functional benefits. We examined animals at various time points over a period of 7 months by light and electron microscopy, and electroretinography. We observed correction of the phagocytic defect, slowing of photoreceptor cell loss and preservation of retinal function for up to 7 months. This study demonstrates the potential of gene therapy approaches for the treatment of retinal degenera- tions caused by defects specific to the RPE and supports the use of lentiviral vectors for the treatment of such disorders. Gene Therapy (2005) 12, 694–701. doi:10.1038/sj.gt.3302460 Published online 20 January 2005 Keywords: retinitis pigmentosa; lentivirus; RCS rat; MERTK; retinal pigment epithelium Introduction Inherited retinal degenerations, a heterogeneous group of diseases that include conditions such as retinitis pigmentosa (RP), are one of the leading causes of blindness in the Western world, affecting approximately 1 in 3500 people. The visual deficits are caused by the progressive dysfunction and degeneration of photore- ceptor cells, triggered by mutations in any one of 130 different genes. To date, over 90 of these genes have been identified and the majority cause photoreceptor defects when mutated. Many of these genes are expressed specifically in photoreceptor cells and encode enzymes of the phototransduction cascade or photoreceptor- specific structural proteins or transcription factors; 1 others are ubiquitously expressed, but appear to be essential only for photoreceptor cell physiology. Some gene defects, however, cause photoreceptor dysfunction and degeneration by affecting retinal pigment epithelium (RPE) function. Since the neuroretina is devoid of blood vessels, proper functioning of photoreceptors depends on the RPE. The two cell types are intimately connected: the inner membrane of the RPE cell is folded into microvilli, which surround the photoreceptor outer segment tips. Besides delivering oxygen and nutrients from the choroid to photoreceptor cells and recycling the chromophore, the RPE is also involved in the phagocy- tosis of waste material, which is mainly composed of shed outer segment tips. 2 The lack of effective treatments for inherited retinal degenerations has prompted the development of novel therapeutic approaches including gene therapy. To date, most of the studies aimed at developing gene therapy for inherited retinal degenerations have focused on the treatment of photoreceptor cell defects. There are, however, a number of reasons why RPE defects may generally prove to be more amenable to treatment. The transduction efficiency of RPE cells is usually much higher than that of photoreceptor cells with efficient transduction following subretinal injection of a variety of viral vectors, including those based on adenovirus, lentivirus and adeno-associated virus (AAV). In addi- tion, one RPE cell is in contact with a number of photoreceptors, and therefore a single transduced RPE cell might protect or restore function to several photoreceptor cells. Furthermore, partial correction of RPE function may have a significant impact on photo- receptor function and survival. Various forms of retinal degeneration are caused by RPE defects and there are Received 27 August 2004; accepted 27 October 2004; published online 20 January 2005 Correspondence: Dr RR Ali, Division of Molecular Therapy, Institute of Ophthalmology, University College London, Bath Street, London EC1V 9EL, UK Gene Therapy (2005) 12, 694–701 & 2005 Nature Publishing Group All rights reserved 0969-7128/05 $30.00 www.nature.com/gt