UCLA

Recessive Stargardt disease (STGD1) is a blinding juvenile macular degeneration caused by mutations in ATP-binding cassette subfamily A member 4 (ABCA4), a membrane protein thought to be exclusively expressed in photoreceptor outer segment (OS) discs. Its proposed function is an inward-directed flippase for N-retinylidene-phosphatidylethanolamine (N-ret-PE), the conjugate of retinaldehyde and phosphatidylethanolamine, across OS discs. In STGD1 patients and mice with a null mutation in ABCA4 (Abca4-/-), excess free retinaldehyde irreversibly reacts with N-ret-PE to form toxic bisretinoids, which accumulate as lipofuscin pigments in the retinal pigment epithelium (RPE). Chapter 1 of this dissertation provides the first evidence that ABCA4 is expressed in human and murine RPE, with subcellular localization to endo-phagolysosomal membranes, thus suggesting that bisretinoids may be forming directly in the RPE, in addition to being secondarily deposited from phagocytosed OS. Chapter 2 reveals that transgenic mice expressing ABCA4 in the RPE, but not in the retina, had decreased autofluorescence and lipofuscin granules, greater photoreceptor preservation, and significant reduction of bisretinoid and complement factor C3b accumulation, confirming that RPE-specific ABCA4 plays a functional role in RPE internal membranes, as it does in photoreceptor OS discs. Since bisretinoids in the RPE result in susceptibility to complement-mediated injury and failure to support the neurosensory retina, we attempted in Chapter 3 to rescue the phenotype in Abca4-/- mice by increasing expression of a major complement negative-regulatory protein (CRP), complement receptor 1-like protein y (CRRY), in the RPE. Subretinal injection of recombinant adeno-associated virus containing CRRY (AAV-CRRY) in Abca4-/- mice significantly increased RPE CRRY levels, while reducing the accumulation of C3b, total bisretinoids and RPE lipofuscin. Rescue of Abca4-/- mice using AAV-CRRY gene therapy suggests that modulation of the complement system by increasing expression of CRPs in the RPE may be a potential treatment strategy for retinopathies associated with complement dysregulation. Chapter 4 discusses the implications of this dissertation which 1) presents a previously undiscovered role for RPE-expressed ABCA4 to limit excess toxic bisretinoid formation directly in the RPE and thereby maintain RPE health, and 2) demonstrates that the pathophysiology of STGD1 involves cell-autonomous, bisretinoid-induced susceptibility to complement, which can be reduced by complement modulation in the RPE.