What is Familial Exudative Vitreoretinopathy (FEVR)? What are the mouse models for it?

Familial Exudative Vitreoretinopathy (FEVR) is a group of rare hereditary retinal disorders characterized by abnormal retinal vascular development, leading to incomplete peripheral retinal vascularization and subsequent retinal ischemia. The avascular retina in FEVR results in hypoxia and stimulates the growth of new blood vessels that extend into the vitreous, ultimately leading to late-stage complications including vitreoretinal traction, subretinal exudation and hemorrhage beneath the macula, retinal folds, tractional retinal detachment, and macular displacement.

The clinical presentation of FEVR often exhibits asymmetry and can vary markedly between affected members of the same family. Mild cases may be asymptomatic, while severe cases can present with profound vision loss. Currently, there are no specific drugs for curing FEVR, but treatment options such as laser therapy and vitrectomy surgery can help alleviate symptoms and preserve vision ^[1].

Figure 1. Retinal Fold Phenotype in Familial Exudative Vitreoretinopathy (FEVR) ^[2].

Familial Exudative Vitreoretinopathy (FEVR): Gene Mutations & Research Mouse Models

Vascular development is intricately regulated by various signaling pathways, among which the Wnt signaling pathway is one of the key regulatory systems. The Norrin/β-Catenin signaling pathway is an important branch of the Wnt signaling cascade, exerting a pivotal role in the development of blood vessels in the eyes and ears ^[3]. In this pathway, regulation primarily occurs through the formation of complexes between Norrin and its receptors (FZD4, LRP5, and TSPAN12), facilitating the promotion of retinal vascularization. Currently, mutations in at least nine genes are known to lead to Familial Exudative Vitreoretinopathy (FEVR) – including NDP, FZD4, LRP5, TSPAN12, ZNF408, KIF11, RCBTB1, CTNNB1, and JAG1 – collectively accounting for around 50% of all FEVR cases worldwide. These genetic mutations disrupt the normal regulation of the Wnt signaling pathway, resulting in abnormal retinal vascular development and symptoms like retinal ischemia ^[4].

Figure 2. Pathway of Norrin/β-Catenin Signaling Regulation in Retinal Vascular Development ^[4].

1. Norrin (NDP)

The NDP gene encodes Norrin protein, which is associated with the growth and development of retinal vasculature. The Norrin protein binds to FZD4 protein, forming a high-affinity ligand-receptor pair. Together with the auxiliary component TSPAN12, it promotes the entry of β-Catenin into the nucleus, inducing FZD4 and LRP5-dependent activation of the classical Wnt pathway. The Wnt signaling pathway plays a crucial role in the growth and development of the eyes, and defects in this pathway can impact this process and play a significant role in the pathogenesis of FEVR and Norrie disease ^[5].

Hence, mutations in the NDP gene can affect the function of Norrin protein, subsequently influencing the Wnt signaling pathway and leading to occurrence of FEVR. For mice, knockout of the Ndp gene results in delayed development of superficial retinal vasculature, inability to form deep retinal vasculature, and formation of lesions resembling microaneurysms, which reflect a typical retinal vascular defect phenotype ^[6].

Figure 3. Defects in Superficial Retinal Vasculature and Absence of Deep Capillary Network in Ndp-KO Mice ^[6].

2. Frizzled-4 (FZD4)

As a receptor of the Wnt signaling pathway, FZD4 activates this pathway by encoding  frizzled-4 protein that binds to Wnt proteins via interactions with norrin, thereby promoting normal retinal vascular development. Mutations in the FZD4 gene prevent its regular binding to Wnt proteins, leading to the inhibition of the Wnt signaling pathway. This anomaly can result in the formation of venous aneurysms and retinal ischemia symptoms. Additionally, mutations in the FZD4 gene might also influence the regulation of other signaling pathways such as the TGF-β and Notch pathways, thereby impacting retinal vascular development and maintenance. These two mechanisms together may contribute to the occurrence or exacerbation of FEVR ^[7].

Similar to the phenotype observed in Ndp knockout (KO) mice, homozygous Fzd4 KO mice (-/-) experience significant effects on vascular development in the retina and inner ear, displaying retinal stress phenotypes. The absence of Fzd4 leads to various phenotypic outcomes, including delayed migration of retinal surface endothelial cells, elimination of secondary and tertiary vascular branching in the retina, significant delay in the programmed degradation of the vitreous vascular system, progressive dilation and degradation of cochlear vessels, and progressive disruption of cerebellar vasculature. Research has further suggested that Fzd4 deficiency might also lead to non-vascular phenotypes, such as brain degeneration and auditory impairments ^[8].

Figure 4. Phenotype of Retinal Vascular Defects in homozygous Fzd4-KO Mice (-/-) ^[8].

3. Tetraspanin-12 (TSPAN12)

The transmembrane protein encoded by the TSPAN12 gene, Tetraspanin-12, is also an important member of Wnt signaling pathway. TSPAN12 interacts with FZD4 and participates in the regulation of Wnt signaling pathway, facilitating normal retinal vascular development. Mutations in TSPAN12 gene result in phenotypes similar to those caused by FZD4 mutations, both leading to venous aneurysm formation and retinal ischemia symptoms. These mutations impact retinal vascular development and maintenance through their effects on signaling pathways such as VEGF and Notch ^[9].

Similarly, Tspan12 gene knockout mice exhibit phenotypes similar to Fzd4 knockout mice, including the formation of venous aneurysms, abnormal retinal vascular branching, and vessel regression. Additionally, Tspan12 deficiency in mice leads to visual impairments, retinal ischemia, as well as developmental abnormalities in other organs and tissues, such as heart anomalies, liver abnormalities, and skeletal deformities ^[10].

Figure 5. Phenotypes of Microaneurysms and Delayed Degradation of Vitreous Vasculature in Tspan12-KO Mice ^[10].

4. Low-density lipoprotein receptor-related protein 5 (LRP5)

The LRP5 gene encodes a transmembrane protein (low-density lipoprotein receptor-related protein 5) that acts as a key component of the LRP5/LRP6/Frizzled co-receptor group that is involved in the Wnt signaling pathway. LRP5 activates the Norrin/β-Catenin signaling pathway by binding to Wnt proteins, participating in the regulation of retinal vascular formation. Loss-of-function mutations in LRP5 gene prevent its binding to Wnt proteins, leading to Wnt signaling pathway abnormalities and resulting in abnormal retinal vascular development, including venous aneurysm formation and retinal ischemia symptoms, ultimately causing FEVR. Additionally, LRP5 is involved in regulating bone density and cholesterol metabolism. Loss-of-function mutations in the LRP5 gene can also lead to osteoporosis-pseudoglioma syndrome (OPPG). Conversely, gain-of-function mutations in LRP5 can lead to abnormally increased bone density [11]. Therefore, LRP5 gene knockout (KO) mice not only exhibit ocular disease phenotypes such as retinal vascular abnormalities and poor retinal development, but also present skeletal phenotypes like skeletal deformities and low bone density, as well as cardiovascular phenotypes such as cholesterol metabolism abnormalities and hypercholesterolemia ^[12-14]. Compared to Ndp and Fzd4 gene KO mice, Lrp5-KO mice display milder retinal lesions and exhibit non-retinal vascular phenotypes such as skeletal deformities and metabolic abnormalities.

Figure 6. Defective Vitreous Vasculature Degeneration in Lrp5-KO Mice ^[12].

5. β-catenin (CTNNB1)

The CTNNB1 gene encodes the β-catenin protein, a cytoplasmic adhesion protein that, together with cadherins and α-catenin, forms adherens junction complexes. These complexes regulate cell growth and adhesion, playing a crucial role in the construction and maintenance of epithelial cell layers. It is generally known that loss of the CTNNB1 gene can lead to CTNNB1 syndrome, a severe neurodevelopmental disorder. Recent research has found that certain functional loss-of-function mutations in CTNNB1 gene can also result in disorders related to retinal defects, such as NEDSDV (Neurodevelopmental disorder with spastic diplegia and visual defects) and Familial Exudative Vitreoretinopathy (FEVR) ^[15-16].

Studies in mice have shown that systemic (homozygous) knockout (KO) of the Ctnnb1 gene leads to embryonic lethality, abnormal embryonic development, neural tube defects, liver developmental anomalies, kidney developmental anomalies, and severe cardiac developmental phenotypes, however, with limited reports on retinal phenotypes. Recent research has revealed that endothelial-specific knockout of Ctnnb1 reduces activity of Norrin/β-catenin pathway, affecting retinal vascular development. In this mouse model, retinal vascular development is inhibited, displaying a FEVR-like phenotype ^[17]. Additionally, endothelial cell-specific deletion of the Ctnna1 gene, which encodes α-catenin, also presents a FEVR-like phenotype similar to Ctnnb1-deficient mice, providing another avenue for studying this genetic disorder in a tissue-specific model ^[18].

Figure 7. Endothelial Cell-Specific Ctnnb1 Knockout Mice Show Phenotypes Similar to FEVR^[17].

6. Kinesin Family Member 11 (KIF11)

KIF11 is another important causative gene for FEVR, independent of the Norrin/β-Catenin signaling pathway. The gene encodes a motor protein (Kinesin Family Member 11) involved in various biological processes, including cell division and microtubule dynamics. KIF11 is also a crucial factor in retinal progenitor cells, regulating microtubule dynamics and participating in the growth and branching of retinal vasculature. In humans, inactivating mutations in KIF11 are associated with a multi-organ syndrome characterized by varying combinations of retinal vascular hypoplasia with or without microcephaly, lymphedema, choroidal retinal dysplasia, microcephaly, and/or intellectual disability. Studies suggest that mutations in KIF11 in some FEVR patients may impact the development and stability of retinal vasculature, although the exact mechanisms remain unknown. Therefore, Kif11 gene knockout mouse models have been used to investigate the relationship between KIF11 mutations and FEVR.

Recent research indicates that endothelial cell-specific Kif11 knockout (KO) leads to severe developmental delays in the mouse retinal vascular system and mild developmental delays in the cerebellar vascular system ^[19]. The phenotype of endothelial cell-specific Kif11-cKO (conditional KO) mice is highly similar to that of endothelial cell-specific Ctnnb1-cKO mice, further supporting the close connection between KIF11 loss and FEVR.

Figure 8. Severe Retinal Vascular Growth Delay in Mice with Early Endothelial Cell-Specific Kif11 Deletion ^[19].

Cyagen Rare Disease Research Resources - Gene-Edited Mouse Models

Genome-editing mouse models play a crucial role in the research of rare disease mechanisms and evaluation of drug efficacy. Cyagen offers thousands of independently developed genome-editing mouse strains, including various gene knockout (KO) and conditional (e.g. tissue-specific) KO models for rare diseases, such as NDP, FZD4, and LRP5. Additionally, our customized model development and CRO services can be tailored to your research needs, allowing you to expedite your research projects.

References:

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