Retinitis Pigmentosa (RP) Cell Models for Research

Retinitis Pigmentosa (RP) is a group of inherited retinal dystrophies characterized by progressive degeneration of photoreceptor cells, leading to severe vision loss and eventual blindness. The global prevalence is approximately 1 in 4,000 individuals, affecting over 1.5 million people worldwide (WHO, 2020). RP can be inherited in autosomal dominant, autosomal recessive, or X-linked patterns, with over 100 genes implicated. The disease typically presents with night blindness in adolescence, followed by peripheral vision loss and central vision loss in later stages. There is currently no cure, and available treatments are limited to vitamin A supplementation and gene therapy for specific subtypes (e.g., RPE65-associated RP). The clinical heterogeneity and genetic complexity make RP a significant challenge for drug development, highlighting the need for robust research models.

RP serves as an excellent model for studying photoreceptor biology, retinal degeneration, and gene therapy approaches. The availability of well-characterized genetic mutations allows for the creation of isogenic cell lines with specific disease-causing variants, enabling mechanistic studies and drug screening. Public datasets, such as those from the Foundation Fighting Blindness and the Retinal Information Network, provide extensive genetic and clinical data. Open questions include the role of oxidative stress, inflammation, and mitochondrial dysfunction in disease progression, as well as the development of effective therapies for various genetic subtypes.

The pathogenesis of RP involves several interconnected pathways:

• • Photoreceptor apoptosis: Mutations in genes such as RHO, RPGR, and PDE6B lead to protein misfolding, ER stress, and activation of apoptotic cascades.
• • Ciliary dysfunction: Many RP genes encode proteins involved in the photoreceptor connecting cilium, and mutations disrupt intraflagellar transport, leading to cell death.
• • Oxidative stress: Impaired antioxidant defenses and increased reactive oxygen species (ROS) contribute to retinal degeneration.
• • Inflammation: Chronic activation of microglia and complement system exacerbates photoreceptor loss.

These pathways provide targets for therapeutic intervention, including gene editing, pharmacological chaperones, and antioxidant therapy.

Data from ClinVar and NCBI Gene.

Key signaling networks implicated in RP include:

• • cGMP-PKG signaling: Mutations in PDE6B or CNGA1 lead to elevated cGMP, activating PKG and causing apoptosis.
• • MAPK/ERK pathway: Stress-induced activation of JNK and p38 MAPK promotes photoreceptor death.
• • PI3K/AKT pathway: Reduced survival signaling contributes to neurodegeneration.
• • Wnt signaling: Altered Wnt/β-catenin activity affects photoreceptor differentiation and survival.

These networks offer opportunities for pharmacological intervention and gene therapy.

Organoids derived from induced pluripotent stem cells (iPSCs) recapitulate retinal development and allow for modeling of RP in a 3D context. They can be generated from patients with specific RP mutations and used for drug screening and gene editing studies.

Animal models for RP include:

• • Genetically engineered mouse models (GEMMs): Knock-in mice carrying RHO mutations (e.g., P23H), RPGR knockout mice, and PDE6B mutant mice (rd1) are widely used.
• • Chemically induced models: Administration of N-methyl-N-nitrosourea (MNU) induces photoreceptor degeneration.
• • Rat models: Royal College of Surgeons (RCS) rats have a mutation in MERTK, leading to RPE dysfunction.

These models are essential for preclinical testing of gene therapies and pharmacological agents.

CRISPR-Cas9 gene editing enables the creation of isogenic cell lines with specific RP-associated mutations. For example:

• • RHO knockout cell lines: Generated by introducing frameshift mutations in RHO to model loss-of-function.
• • RHO P23H knock-in cell lines: Introduce the common dominant mutation to study protein misfolding.
• • RPGR knockout cell lines: Model X-linked RP and ciliary dysfunction.

These gene-edited cell models are commercially available and sequence-verified, providing reproducible tools for drug discovery and functional genomics. They allow for high-throughput screening and mechanistic studies without the variability of primary cells.

Gene-edited cell lines are used to validate the function of RP-associated genes. For example, knocking out RPGR in RPE1 cells leads to defects in ciliogenesis, confirming its role in ciliary function. Similarly, introducing RHO mutations into ARPE-19 cells allows for the study of protein trafficking and ER stress. These models enable the identification of downstream pathways and potential therapeutic targets.

Isogenic pairs (wild-type vs. mutant) are used for drug screening to identify compounds that rescue the mutant phenotype. For instance, screening for small molecules that reduce ER stress in RHO P23H knock-in cells can identify potential therapies. Additionally, gene-edited cells can be used to study resistance mechanisms to existing treatments, such as resistance to gene therapy vectors.

CRISPR-based synthetic lethality screens in RP cell models can identify genes that, when silenced, selectively kill mutant cells. This approach can reveal novel drug targets and biomarkers for patient stratification. For example, a screen in RPGR knockout cells might identify genes essential for survival in the absence of RPGR, which could be targeted therapeutically.