Human iPSC derived disease model of MERTK-associated retinitis pigmentosa

Abstract

Retinitis pigmentosa (RP) represents a genetically heterogeneous group of retinal dystrophies affecting mainly the rod photoreceptors and in some instances also the retinal pigment epithelium (RPE) cells of the retina. Clinical symptoms and disease progression leading to moderate to severe loss of vision are well established and despite significant progress in the identification of causative genes, the disease pathology remains unclear. Lack of this understanding has so far hindered development of effective therapies. Here we report successful generation of human induced pluripotent stem cells (iPSC) from skin fibroblasts of a patient harboring a novel Ser331Cysfs*5 mutation in the MERTK gene. The patient was diagnosed with an early onset and severe form of autosomal recessive RP (arRP). Upon differentiation of these iPSC towards RPE, patient-specific RPE cells exhibited defective phagocytosis, a characteristic phenotype of MERTK deficiency observed in human patients and animal models. Thus we have created a faithful cellular model of arRP incorporating the human genetic background which will allow us to investigate in detail the disease mechanism, explore screening of a variety of therapeutic compounds/reagents and design either combined cell and gene- based therapies or independent approaches.

Concise Review: Human Induced Pluripotent Stem Cell Models of Retinitis Pigmentosa

“…RPE dysfunction was modeled in several RP patient‐specific cells. Dysfunctional RPE in MERTK‐associated RP was successfully modeled using hiPSCs . MERTK, known to mediate OS phagocytosis, is disrupted in royal college of surgeons (RCS) rats , a classic model for retinal degeneration inherited as an autosomal recessive trait, and found to cause early‐onset RP in patients .…”

“…Mutations in visual cycle proteins: RPE‐specific retinaldehyde‐binding protein‐1 (RALB), RPE65, lecithin retinol acyltransferase (phosphatidylcholine‐retinol O‐acyltransferase, LRAT) and photoreceptor phagocytosis such as MERTK do not lead to degeneration of RPE cells themselves, but instead abrogate RPE cell functions related to photoreceptor OS turn over. This function is fully captured by introducing xeno‐derived purified photoreceptor outer segments or rodent derived neural retina, as for example in . In contrast, mutations in membrane frizzled‐related protein (MFRP), a type II transmembrane protein, induce defective RPE cell morphology, pigmentation, and cell junctions, and the disease can be recapitulated in the hiPSC‐RPE cell monolayer .…”

“…RPE cells are more readily obtained in vitro from pluripotent cells compared to photoreceptors. This is partly due to the default differentiation of pluripotent cells toward ectoderm upon fibroblast growth factor (bFGF) withdrawal, which induces the formation of pigmented patches that can be enriched by manual excision and enzymatic expansion . Others use exogenously added growth and transcription factors involved in retinogenesis such as transforming growth factor beta (TGF‐β) family protein Activin A, nicotinamide , in order to increase the target cell population purity.…”

“…Dysfunctional RPE in MERTK‐associated RP was successfully modeled using hiPSCs . MERTK, known to mediate OS phagocytosis, is disrupted in royal college of surgeons (RCS) rats , a classic model for retinal degeneration inherited as an autosomal recessive trait, and found to cause early‐onset RP in patients . As expected, the generated RPE cells from the affected patient failed to phagocytize the shed OS material; a circadian activity performed by RPE cells in contrast to healthy control's cells, highlighting the capacity of hiPSCs to mimic the disease.…”

Induced Pluripotent Stem Cell Therapies for Degenerative Disease of the Outer Retina: Disease Modeling and Cell Replacement

“…53 Adenovirus vectors have also been used to deliver BEST1 mutants to iPSC-derived RPE; these studies have defined amino acids needed for channel activity and correct protein localization. 54 iPSC-derived RPE from patients with mutations in the MERTK gene, which causes RP, exhibits defective phagocytosis 55 and from patients with mutations in RP2 shows defects in IFT20 localization, Golgi cohesion, and protein trafficking, which could be corrected either by protein overexpression or by translational read through inducing drugs. 56 iPSC lines from AMD-affected individuals have also been used to derive RPE cells, to model the disease in vitro or for drug screening.…”