Objective: Choroideremia (CHM) is a rare inherited retinal disease (IRD), accounting for ~2% of IRDs. It causes night blindness in childhood and central vision loss by 40–50 years. Although the causal gene (CHM) and its product, Rab Escort Protein 1 (REP1), are known, the molecular basis underlying the clinical symptoms remains unclear. This indicates gaps in our understanding of the disease mechanism, which could inform new treatments. REP1 prenylates Rab GTPases and direct them to cellular membranes; its absence leads to the build-up of unprenylated Rabs, especially Rab27a, in retinal cells. Rab27a regulates the L-type calcium channel Cav1.3 in the rodent retinal pigment epithelium (RPE). We previously identified functional L-type channels in iPSC-derived RPE. We now aim to characterise changes in the localization and functionality of Cav1.3 in CHM patient iPSC-derived RPE compared to healthy RPE to gain a deeper understanding of dysregulated pathways.

Methods: iPSCs derived from healthy and CHM individuals were differentiated into RPE in parallel. Characterisation of the localisation of Cav 1.3 was performed by immunofluorescence and confocal microscopy. Functional studies were carried by calcium imaging. Complementary transcriptomic studies were performed by sequencing and analysis of bulk RNA extracts.

Results: In iPSC-derived RPE from healthy individuals, Cav1.3 was found mainly localised at the plasma membrane, whereas a strong cytosolic localisation was also observed in iPSC-derived RPE from CHM patients, suggesting impaired transport of Cav1.3 to its target membrane. Accordingly, dysregulated calcium signalling in iPSC-derived RPE from CHM patients was found. This dysregulation was related to L-type calcium channels, as Nifedipine, a specific channel blocker, failed to inhibit ATP-induced calcium responses in CHM RPE as compared to control. Transcriptomic analysis further identified altered expression of calcium channel subunits in CHM RPE.

Conclusion: Localisation of the L-type calcium channel Cav1.3 is impaired in iPSC-derived RPE from CHM patients. At the functional level, calcium signalling related to L-type calcium channel function is affected in CHM RPE. Furthermore, transcriptomic analysis will help identify novel pathways altered in CHM patient-derived RPE.