New Addition to Ophthalmic Models: Autosomal Recessive Retinitis Pigmentosa (arRP) - Rho KO Mice

Retinitis Pigmentosa (RP) is a group of inherited retinal degenerative diseases that affect over 1.5 million people worldwide. The primary pathological feature is the progressive apoptosis of photoreceptor cells, leading to night blindness, narrowing of the visual field, and ultimately potential blindness. ^[1] Among the numerous pathogenic genes, mutations in the RHO gene are one of the most common causes, especially playing a key role in autosomal dominant RP (adRP), which is the focus of most RHO-related research, mouse model development, and therapeutic exploration. In rare cases, homozygous functional loss mutations in the RHO gene can also cause autosomal recessive RP (arRP). However, research on the mechanisms and model development for this type of arRP has been relatively insufficient. ^[2] To address this gap, Cyagen has developed and launched the Rho KO mouse model. This model simulates the loss of RHO function, providing a powerful tool for studying the pathogenesis of arRP and developing intervention strategies.

Figure 1. Types of RHO Gene Mutations Associated with Retinitis Pigmentosa (RP). ^[2]

The RHO Gene and Retinitis Pigmentosa (RP)

The global prevalence of Retinitis Pigmentosa (RP) is approximately 1 in 4000, and its pathological feature is the gradual apoptosis of rod and cone photoreceptor cells. ^[3] Patients typically present with night blindness in the early stages, followed by progressive constriction of the visual field (tunnel vision), and in severe cases, can lead to complete blindness. The RHO gene is one of the most common pathogenic genes for RP. It encodes rhodopsin, which is located in the rod cells, and combines with 11-cis-retinal to form a photopigment. Under light conditions, it initiates a visual signal transduction cascade. As a G-protein coupled receptor, rhodopsin converts light signals into neural signals after absorbing photons. This process is not only fundamental to scotopic vision but also crucial for maintaining the survival of rod cells and the structural stability of the retina. ^[4]

Figure 2. The Function of Rhodopsin Protein and the Impact of Different Mutation Types on Its Function. ^[2]

Mutations in the RHO gene are one of the major causes of genetic diversity in Retinitis Pigmentosa (RP). Over 150 types of RHO mutations associated with retinal diseases have been reported. These mutations primarily lead to autosomal dominant RP (adRP), accounting for approximately 25%-30% of adRP cases. In rare cases, they can also cause autosomal recessive RP (arRP) or congenital stationary night blindness (CSNB). ^[5] The pathogenic mechanisms primarily include:

• Gain-of-function (GOF) mutations: These mutations cause misfolding or functional abnormalities of the rhodopsin protein, which is toxic to rod cells, triggering apoptosis and ultimately leading to adRP. For example, the P23H mutation results in protein misfolding and retention in the endoplasmic reticulum (ER), triggering ER stress and subsequently leading to cell death. ^[6]
  
  
• Loss-of-function (LOF) mutations: The loss of function in both alleles leads to an inability to produce functional rhodopsin, which causes degeneration of the outer segment of rod cells and ultimately progresses to arRP. ^{[2, 5-6]}
  
  

Figure 3. RHO Gene Mutations Associated with Autosomal Dominant RP (adRP). ^[2]

Cyagen Rho KO Mouse Model

Previous studies on RHO gene mutations in mice have largely focused on adRP, particularly the P23H mutation models, which are primarily used to simulate the protein toxicity mechanisms in dominant inheritance patterns. ^{[2, 7-8]} However, to better understand arRP caused by the complete loss of RHO function and evaluate therapies aimed at restoring or replacing rhodopsin function, loss-of-function animal models are essential. Cyagen's newly launched Rho KO mouse model (Product No. C001700) completely knocks out the endogenous Rho gene, blocking rhodopsin expression, and precisely simulates the pathological state caused by RHO null mutations. This homozygous knockout model faithfully reproduces the rhodopsin deficiency phenotype found in human arRP, making it a critical tool for studying this type of recessive genetic retinal disease. Key phenotypic features include:

• Retinal Outer Nuclear Layer (ONL) Thinning
  At 8 weeks of age, the Rho KO mice exhibit a significant reduction in the thickness of the retinal outer nuclear layer (ONL), indicating severe loss of rod cells and reflecting notable retinal degeneration.

Figure 4. Fundus Morphology, Optical Coherence Tomography (OCT), and Fluorescein Angiography (FFA) Results of 8-Week-Old Homozygous Rho KO Mice and Wild-Type Mice (WT).

• Abnormal Electroretinography (ERG)
  Electroretinography (ERG) testing shows that the amplitudes of the scotopic a-wave, b-wave and photopic a-wave of the B6-RHO-KO mice were significantly reduced, indicating impaired photoreceptor function and reduced electrophysiological activity.

Figure 5. Electroretinography (ERG) Results of 8-Week-Old Wild-Type Mice (WT) and Homozygous Rho KO Mice.

Conclusion

In summary, Cyagen's Rho KO mouse model (Product No. C001700) successfully simulates the severe retinal degenerative changes caused by the complete loss of RHO gene function. This model exhibits significant photoreceptor cell loss (ONL thinning) and severe impairment of retinal electrophysiological function (reduced ERG amplitudes). The model not only provides a powerful tool for studying the pathogenesis of recessive inherited retinal diseases such as arRP but also establishes an important preclinical research platform for evaluating novel intervention strategies like gene therapy and cell therapy. Additionally, Cyagen offers a variety of ophthalmic disease research models, including wild-type (WT) and adRP mouse models with the classic P23H mutation in the humanized RHO gene, to meet the research needs for different types of retinal diseases.

Cyagen RHO Gene-Related Ophthalmic Disease Research Models

References
[1]Hartong DT, Berson EL, Dryja TP. Retinitis pigmentosa. Lancet. 2006 Nov 18;368(9549):1795-809.
[2]Athanasiou D, Aguila M, Bellingham J, Li W, McCulley C, Reeves PJ, Cheetham ME. The molecular and cellular basis of rhodopsin retinitis pigmentosa reveals potential strategies for therapy. Prog Retin Eye Res. 2018 Jan;62:1-23.
[3]Hofmann KP, Lamb TD. Rhodopsin, light-sensor of vision. Prog Retin Eye Res. 2023 Mar;93:101116.
[4]Meng D, Ragi SD, Tsang SH. Therapy in Rhodopsin-Mediated Autosomal Dominant Retinitis Pigmentosa. Mol Ther. 2020 Oct 7;28(10):2139-2149.
[5]Hofmann L, Palczewski K. The G protein-coupled receptor rhodopsin: a historical perspective. Methods Mol Biol. 2015;1271:3-18.
[6]Vingolo EM, Mascolo S, Miccichè F, Manco G. Retinitis Pigmentosa: From Pathomolecular Mechanisms to Therapeutic Strategies. Medicina (Kaunas). 2024 Jan 22;60(1):189.
[7]Barwick SR, Smith SB. Comparison of Mouse Models of Autosomal Dominant Retinitis Pigmentosa Due to the P23H Mutation of Rhodopsin. Adv Exp Med Biol. 2023;1415:341-345.
[8]Vasudevan S, Senapati S, Pendergast M, Park PS. Aggregation of rhodopsin mutants in mouse models of autosomal dominant retinitis pigmentosa. Nat Commun. 2024 Feb 16;15(1):1451.