Retinitis Pigmentosa 17 (RP17) Cell Models for Research

Retinitis Pigmentosa 17 (RP17) is a rare inherited retinal dystrophy caused by mutations in the PRPF31 gene. According to the World Health Organization (WHO), retinitis pigmentosa (RP) affects approximately 1 in 4,000 people worldwide, with RP17 accounting for about 5-10% of autosomal dominant RP cases. The disease typically presents with night blindness in adolescence, followed by progressive peripheral vision loss and eventual central vision loss in later decades. There is no cure, and current treatments are limited to supportive measures. The clinical impact is significant, as RP17 leads to legal blindness in most patients by age 40-50. The National Cancer Institute (NCI) does not track RP17 as it is not a cancer, but the genetic and cellular mechanisms are of high interest for gene therapy and CRISPR-based research.

RP17 is an ideal model for studying RNA splicing defects and photoreceptor degeneration. The PRPF31 gene encodes a splicing factor, and mutations lead to haploinsufficiency, affecting pre-mRNA splicing in retinal cells. This provides a clear mechanistic link between a single gene defect and a complex degenerative phenotype. Public datasets, such as those from NCBI Gene and ClinVar, provide extensive mutation information. Open questions include the tissue-specificity of splicing defects and the potential for modifier genes. Gene-edited cell models, such as PRPF31 knockout or knock-in lines, are essential for dissecting these mechanisms and testing therapeutic strategies.

The primary pathogenic mechanism in RP17 is haploinsufficiency of PRPF31, leading to defective pre-mRNA splicing. This affects the expression of many genes, particularly those involved in photoreceptor function and survival. Key pathways include:

• • Splicing dysregulation: PRPF31 is a component of the U4/U6.U5 tri-snRNP complex, essential for spliceosome assembly. Reduced levels impair splicing efficiency, leading to aberrant transcripts and reduced protein production.
• • Photoreceptor apoptosis: Defective splicing of genes critical for photoreceptor structure (e.g., rhodopsin) and function leads to cellular stress and apoptosis.
• • Oxidative stress: Impaired splicing may affect antioxidant defense genes, increasing oxidative damage in retinal cells.
• • Mitochondrial dysfunction: Some studies suggest that PRPF31 mutations affect mitochondrial function, contributing to energy deficits in photoreceptors.

Data from ClinVar and NCBI Gene.

The splicing defect in RP17 impacts multiple signaling networks:

• • Photoreceptor-specific gene expression: Genes like RHO, PDE6B, and CNGA1 are mis-spliced, leading to impaired phototransduction.
• • Apoptotic pathways: Upregulation of pro-apoptotic factors (e.g., BAX) and downregulation of survival factors (e.g., BCL2) in response to cellular stress.
• • Unfolded protein response (UPR): Accumulation of misfolded proteins triggers ER stress and UPR activation.
• • Inflammatory signaling: Microglial activation and complement pathway dysregulation contribute to retinal degeneration.
• • Metabolic pathways: Altered glucose metabolism and mitochondrial dysfunction are observed in RP17 models.

Organoids derived from patient iPSCs are valuable for studying retinal development and degeneration. They recapitulate photoreceptor differentiation and can be used for drug screening. However, organoids are complex and less amenable to high-throughput studies compared to cell lines.

• • PDX models: Not commonly used for RP17 as it is not a cancer.
• • GEMM (Genetically Engineered Mouse Models): Prpf31 knockout mice exhibit retinal degeneration, but homozygous knockout is embryonic lethal. Heterozygous mice show late-onset degeneration, mimicking human haploinsufficiency.
• • Induced models: CRISPR-generated Prpf31 mutations in mice are used to study disease mechanisms.
• • Zebrafish models: prpf31 morpholino knockdown causes retinal defects, useful for drug screening.
• • Rat models: Similar to mouse models, but less common.

CRISPR-Cas9 technology enables the creation of isogenic cell lines with precise PRPF31 mutations. These models are essential for studying the functional consequences of specific mutations and for drug development. Examples include:

• • PRPF31 knockout cell lines: Complete loss of function, mimicking severe haploinsufficiency.
• • PRPF31 knock-in lines: Introduction of patient-specific point mutations (e.g., c.1115G>A) to study dominant-negative effects.
• • Reporter lines: PRPF31 promoter-driven fluorescent reporters to monitor gene expression.

These sequence-verified models are commercially available from various sources, accelerating research without the need for in-house gene editing. They are ideal for high-throughput screening and mechanistic studies.

Gene-edited cell lines are used to validate the function of PRPF31 and its interacting partners. For example, PRPF31 knockout lines can be used to identify genes whose splicing is dependent on PRPF31 via RNA-seq. Additionally, complementation assays with wild-type or mutant PRPF31 can determine the functional impact of specific mutations. These models also enable the study of modifier genes that influence disease severity.

Isogenic pairs (wild-type vs. PRPF31 knockout) are used in high-throughput screens to identify compounds that rescue splicing defects or prevent photoreceptor death. These screens can identify drugs that upregulate PRPF31 expression or enhance spliceosome activity. Resistance modeling is less relevant for RP17, but drug resistance to therapies like gene therapy vectors can be studied.

CRISPR-based synthetic lethality screens can identify genes that, when knocked down, are lethal in PRPF31-deficient cells but not in wild-type cells. These synthetic lethal partners may serve as therapeutic targets. Additionally, secretome analysis of PRPF31 knockout cells can identify secreted biomarkers for disease progression.