Loss of REP-1 in retinal pigment epithelial cells leads to impaired phagosome processing and altered lysosomal pathway function.

Choroideremia (CHM) is a rare form of retinal degeneration caused by mutations in the ubiquitously expressed CHM gene, encoding for Rab escort-protein-1 (REP-1). REP-1 is required for the prenylation of Rab GTPase proteins, regulators of intracellular membrane traffic, yet what specific cellular pathways are affected in CHM and how this contributes to disease progression remain unclear. Using both ARPE-19 and iPSC-derived retinal-pigmented epithelium (RPE) cells, where the CHM gene was knocked-out using CRISPR/Cas9, we show that CHM(-) cells have an increased number of lysosomes. Cathepsins and BSA-Gold were correctly delivered to the lysosomes, suggesting that lysosome organelle identity and targeting machinery are largely unaffected, yet, digestion of photoreceptor outer segments (POS)-derived-phagosomes is impaired, resulting in a doubling of undigested POS-derived autofluorescent material in CHM(-) cells. Delayed acquisition of LAMP1 by the phagosome was observed. These findings reveal that REP-1 loss leads to subtle lysosomal pathway dysfunction, resulting in defects in phagosome targeting and/or digestion. Our data are consistent with the idea that gradual lysosomal impairment is associated with a premature aging process, characterized by the accumulation of lipofuscin toxic material and eventual RPE and photoreceptor degeneration, which likely drives the progressive vision loss observed in patients with CHM.