Silencing of the CHM Gene Alters Phagocytic and Secretory Pathways in the Retinal Pigment Epithelium

The mechanism of degeneration of the retinal pigment epithelium (RPE), photoreceptors, and choroid in choroideremia (CHM) remains largely unknown. This X-linked, monogenic disorder is caused by mutations in the ubiquitously expressed CHM gene, which encodes Rab escort-protein-1 (REP-1).1,2 REP-1 participates in the geranylgeranylation of ras-related GTPases, Rab proteins, that are integral to the trafficking of vesicles in endocytic and exocytic pathways.3–6 Patients with CHM have a relative or complete lack of REP-1,7–9 which conceivably cannot be compensated for in the eye by REP-2, a product of the CHM-like gene and the only other REP.10 Alternately, the relative affinity of some Rab/REP-2 complexes for geranylgeranyl transferase may be less than that of Rab/REP-1.11 The lack of REP-1 within cells of the eye has been hypothesized to affect the transport of opsin to the photoreceptor outer segment, the apical distribution of melanosomes in the RPE, and the phagocytosis of photoreceptor outer segments by the RPE.12 For example, the RPE in the zebrafish chm model fails to process outer segments within the phagolysosomes.13 The RPE is a monolayer of polarized pigmented cells that lies between the neuroretina and the choroid and that plays a crucial role in the function and survival of the retina by performing a number of essential functions such as the phagocytosis of photoreceptor outer segments and the maintenance of immune privilege within the eye by polarized balanced secretion of anti-inflammatory and proinflammatory cytokines, among them interleukin (IL)-8 and monocyte chemotactic protein (MCP)-1.14,15 Various lines of evidence point to the RPE as having a central role in the pathogenesis of CHM. Rodrigues et al.,16 studying the eye of a 19-year-old CHM affected male, identified a few pigment-filled macrophages within the retina that had attached outer segment structures, phagosomes, occasional melanin granules, and “curvilinear rod-like profiles.” The authors suggested the possibility of a defect in outer segment phagocytosis. Bonilha et al.,17 reporting the pathology on a 91-year-old female CHM carrier, noted the absence of RPE apical microvilli and basal infoldings. Moreover, the RPE donor basal surface was dominated by the presence of banded fibers composed of clumps of widely spaced collagen. Bruch's membrane and the space between the basal membrane of the RPE contained many smooth and bristle-like-coated vesicles. RPE ultrastructural changes were consistent with cells that could not carry out several nurturing functions. The aim of our study was to determine the effect of REP-1 depletion on cellular trafficking in endocytic and exocytic pathways in human RPE cells. Here we show that lack of REP-1 expression leads to reduced degradation of POS by RPE cells, most likely because of the inhibition of the phagosome-lysosome fusion events, and increased constitutive secretion of MCP-1 and IL-8 by RPE cells. These observations may explain the accumulation of unprocessed outer segments within the phagolysosomes of RPE cells and the finding of inflammatory cells in pathologic eye specimens from patients with CHM.