Recessive Stargardt macular degeneration (STGD1) is an inherited blinding disease of children and young adults caused by mutations in the ABCA4 gene. ABCA4 is located in the membranous outer segments (OS) of photoreceptors, and transports the condensation product of retinaldehyde and phosphatidylethanolamine (N-ret-PE) into the cytoplasmic space. The pathologic hallmark of STGD1 is deposition of fluorescent, vitamin A-containing pigments (bisretinoids) in cells of the retinal pigment epithelium (RPE). Despite detailed knowledge about the genetics of STGD1 and the biochemistry of ABCA4, the mechanism of photoreceptor degeneration leading to visual loss in this disease is not well understood. It is known that the health and viability of photoreceptors depends upon a healthy RPE. Hence, RPE cells are thought to play an important role in the pathophysiology of STGD1. One possibility is that bisretinoids formed in OS during light exposure are directly cytotoxic to RPE cells, leading to photoreceptor degeneration. More recent observations suggest a more complex etiology for photoreceptor cell-death in STGD1. The RPE contributes significantly to the ocular immune response. For example, RPE cells express multiple complement negative-regulatory proteins, including complement-factor H (CFH), as protection against aberrant complement activation. In the Abca4-/- mouse-model for STGD1, accumulation of bisretinoids caused complement activation on RPE cells. The central hypothesis of this application is that complement dysregulation, caused by bisretinoid-buildup in the RPE, plays a central role in the pathogenesis of STGD1. We will evaluate bisretinoid-dependent complement activation in induced pluripotent-stem (iPS) cell-derived RPE cells from STGD1 patients and age-matched normal humans, all genetically characterized at the ABCA4 and CFH loci. Particularly, using these iPS-derived RPE cells, we will investigate the influence of ABCA4 genotypes on complement-system reactivity in the presence of normal CFH (Specific Aim 1). We present preliminary data showing that ABCA4 is expressed in RPE cells, in addition to photoreceptors. Using new transgenic and conditional knockout mouse lines, we will test the hypothesis that ABCA4 functions in RPE cells to transport retinaldehydes formed during proteolysis of rhodopsin and cone opsins from the phagolysosomes to the cytoplasm for recycling. Loss of this function in Abca4-/- mice and STGD1 patients may explain bisretinoids accumulation and complement dysregulation (Specific Aim 2). Finally, we will test a therapeutic strategy involving gene therapy to inhibit complement activation by increasing expression of complement receptor 1-related protein y in RPE cells of the STGD1 mouse (Specific Aim 3). Promising results here would provide an important 'proof-of-principle' that could open the door to a clinical trial of gen therapy for patients with STGD1. This alterative gene therapy approach may have broader applicability to other macular dystrophies caused by complement-mediated inflammation.
The mechanism of photoreceptor degeneration in recessive Stargardt maculopathy (STGD1) is unknown, and this disease has no effective treatment. This study will test the hypothesis that photoreceptor death in STGD1 results from chronic insult by the complement system on an affected patient's own retinal pigment epithelial (RPE) cells. Moreover, the study will validate a gene therapy approach to protect RPE cells from complement attack in a mouse model of STGD1 that could lead to clinical trials in STGD1 patients.
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