Viral-mediated vision rescue of a novel AIPL1 cone-rod dystrophy model

Clinical and Molecular Characterization of AIPL1-Associated Leber Congenital Amaurosis/Early-Onset Severe Retinal Dystrophy

PURPOSE

This study aimed to characterize the clinical features, genetic findings and genotype-phenotype correlations of patients with Leber congenital amaurosis (LCA) or early-onset severe retinal dystrophy (EOSRD) harboring biallelic AIPL1 pathogenic variants.

DESIGN

Retrospective case-series.

METHODS

This study consecutively enrolled 51 patients from 47 families with a clinical diagnosis of LCA/EOSRD harboring disease-causing variants in the AIPL1 gene, from October 2021 to September 2023. Molecular genetic findings, medical history, and ophthalmic evaluation including visual acuity (VA), multimodal retinal imaging and electrophysiologic assessment were reviewed.

RESULTS

Of the 51 patients (32 with LCA and 19 with EOSRD), 27 (53%) were females, and age at last review ranged from 0.5-58.4 years. We identified 28 disease-causing AIPL1 variants, with 18 being novel. In patients with EOSRD, the mean (range) VA was 1.3 (0.7-2.7) logMAR and 1.3 (0.5-2.3) logMAR for right and left eyes respectively, with an average annual decline of 0.03 logMAR (R2 = 0.7547, P < 0.01). For patients with LCA, the VA ranged from light perception to counting fingers. Optical coherence tomography imaging demonstrated preservation of foveal ellipsoid zone in the 5 youngest EOSRD patients and 9 LCA children. Electroretinography showed severe cone-rod patterns in 78.6% (11/14) of patients with EOSRD, while classical extinguished pattern was documented in all patients with LCA available for the examination. The most common mutation was the nonsense variants of c.421C>T, with am allele frequency of 53.9%. All patients with EOSRD carried at least one missense mutation, of whom 13 identified with c.152A>G and 5 with c.572T>C. Twenty-six patients with LCA harbored two null AIPL1 variants, while 18 were homozygous for c.421C>T, and 6 were heterozygous for c.421C>T with another loss-of-function variant.

CONCLUSIONS

This study reveals distinct clinical features and variation spectrum between AIPL1-associated LCA and EOSRD. Patients harboring at least one non-null mutations, especially c.152A>G and c.572T>C, were significantly more likely to have a milder EOSRD phenotype than those with two null mutations. Residual foveal outer retinal structure observed in the youngest proportion of patients suggests an early window for gene augmentation therapy.

Neuroinflammation in retinitis pigmentosa: Therapies targeting the innate immune system

Retinitis pigmentosa (RP) is an important cause of irreversible blindness worldwide and lacks effective treatment strategies. Although mutations are the primary cause of RP, research over the past decades has shown that neuroinflammation is an important cause of RP progression. Due to the abnormal activation of immunity, continuous sterile inflammation results in neuron loss and structural destruction. Therapies targeting inflammation have shown their potential to attenuate photoreceptor degeneration in preclinical models. Regardless of variations in genetic background, inflammatory modulation is emerging as an important role in the treatment of RP. We summarize the evidence for the role of inflammation in RP and mention therapeutic strategies where available, focusing on the modulation of innate immune signals, including TNFα signaling, TLR signaling, NLRP3 inflammasome activation, chemokine signaling and JAK/STAT signaling. In addition, we describe epigenetic regulation, the gut microbiome and herbal agents as prospective treatment strategies for RP in recent advances.

Clinical and functional analyses of AIPL1 variants reveal mechanisms of pathogenicity linked to different forms of retinal degeneration

Disease-causing sequence variants in the highly polymorphic AIPL1 gene are associated with a broad spectrum of inherited retinal diseases ranging from severe autosomal recessive Leber congenital amaurosis to later onset retinitis pigmentosa. AIPL1 is a photoreceptor-specific co-chaperone that interacts with HSP90 to facilitate the stable assembly of retinal cGMP phosphodiesterase, PDE6. In this report, we establish unequivocal correlations between patient clinical phenotypes and in vitro functional assays of uncharacterized AIPL1 variants. We confirm that missense and nonsense variants in the FKBP-like and tetratricopeptide repeat domains of AIPL1 lead to the loss of both HSP90 interaction and PDE6 activity, confirming these variants cause LCA. In contrast, we report the association of p.G122R with milder forms of retinal degeneration, and show that while p.G122R had no effect on HSP90 binding, the modulation of PDE6 cGMP levels was impaired. The clinical history of these patients together with our functional assays suggest that the p.G122R variant is a rare hypomorphic allele with a later disease onset, amenable to therapeutic intervention. Finally, we report the primate-specific proline-rich domain to be dispensable for both HSP90 interaction and PDE6 activity. We conclude that variants investigated in this domain do not cause disease, with the exception of p.A352_P355del associated with autosomal dominant cone-rod dystrophy.

Targeted deletion of Crb1/Crb2 in the optic vesicle models key features of leber congenital amaurosis 8

The Crb1 and 2 (Crumbs homolog 1 & 2) genes encode large, single-pass transmembrane proteins essential for the apicobasal polarity and adhesion of epithelial cells. Crb1 mutations cause degenerative retinal diseases in humans, including Leber congenital amaurosis type 8 (LCA8) and retinitis pigmentosa type 12 (RP12). In LCA8, impaired photoreceptor development and/or survival is thought to cause blindness during early infancy, whereas, in RP12, progressive photoreceptor degeneration damages peripheral vision later in life. There are multiple animal models of RP12 pathology, but no experimental model of LCA8 recapitulates the full spectrum of its pathological features. To generate a mouse model of LCA8 and identify the functions of Crb1/2 in developing ocular tissues, we used an mRx-Cre driver to generate allelic combinations that enabled conditional gene ablation from the optic vesicle stage. In this series only Crb1/2 double knockout (dKO) mice exhibited characteristics of human LCA8 disease: locally thickened retina with spots devoid of cells, aberrant positioning of retinal cells, severely disrupted lamination, and depigmented retinal-pigmented epithelium. Retinal defects antedated E12.5, which is far earlier than the stage at which photoreceptor cells mainly differentiate. Most remarkably, Crb1/Crb2 dKO showed a severely attenuated electroretinogram at the eye opening stage. These results suggest that human LCA8 can be modeled in the mouse by simultaneously ablating Crb1/2 from the beginning of eye development. Importantly, they also indicate that LCA8 is caused by malfunction of retinal progenitor cells during early ocular development rather than by defective photoreceptor-Muller glial interaction, a mechanism proposed for RP12.

RDH12 Mutations Cause a Severe Retinal Degeneration With Relatively Spared Rod Function

Purpose

To describe the retinal phenotype of pediatric patients with mutations in the retinol dehydrogenase 12 (RDH12) gene.

Methods

Twenty-one patients from 14 families (ages 2-17 years) with RDH12-associated inherited retinal degeneration (RDH12-IRD) underwent a complete ophthalmic exam and imaging with spectral domain optical coherence tomography (SD-OCT) and near infrared and short-wavelength fundus autofluorescence. Visual field extent was measured with Goldmann kinetic perimetry, visual thresholds with dark-adapted static perimetry or with dark-adapted chromatic full-field stimulus testing (FST) and transient pupillometry.

Results

Visual acuity ranged from 20/40 to light perception. There was parafoveal depigmentation or atrophic maculopathies accompanied by midperipheral intraretinal pigment migration. SD-OCT revealed foveal thinning in all patients and detectable but thinned outer nuclear layer (ONL) at greater eccentricities from the fovea. Photoreceptor outer segment (POS) signals were only detectable in small pockets within the central retina. Measurable kinetic visual fields were limited to small (<5-10°) central islands of vision. Electroretinograms were reported as undetectable or severely reduced in amplitude. FST sensitivities to a 467 nm stimulus were rod-mediated and reduced on average by ∼2.5 log units. A thinned central ONL colocalized with severely reduced to nondetectable cone-mediated sensitivities. Pupillometry confirmed the psychophysically measured abnormalities.

Conclusions

RDH12-IRD causes an early-onset, retina-wide disease with particularly severe central retinal abnormalities associated with relatively less severe rod photoreceptor dysfunction, a pattern consistent with an early-onset cone-rod dystrophy. Severely abnormal POS but detectable ONL in the pericentral and peripapillary retina suggest these regions may become targets for gene therapy.

Bardet-Biedl syndrome-8 (BBS8) protein is crucial for the development of outer segments in photoreceptor neurons

Bardet-Biedl syndrome (BBS) is an autosomal recessive ciliopathy characterized by developmental abnormalities and vision loss. To date, mutations in 21 genes have been linked to BBS. The products of eight of these BBS genes form a stable octameric complex termed the BBSome. Mutations in BBS8, a component of the BBSome, cause early vision loss, but the role of BBS8 in supporting vision is not known. To understand the mechanisms by which BBS8 supports rod and cone photoreceptor function, we generated animal models lacking BBS8. The loss of BBS8 protein led to concomitant decrease in the levels of BBSome subunits, BBS2 and BBS5 and increase in the levels of the BBS1 and BBS4 subunits. BBS8 ablation was associated with severe reduction of rod and cone photoreceptor function and progressive degeneration of each photoreceptor subtype. We observed disorganized and shortened photoreceptor outer segments (OS) at post-natal day 10 as the OS elaborates. Interestingly, loss of BBS8 led to changes in the distribution of photoreceptor axonemal proteins and hyper-acetylation of ciliary microtubules. In contrast to properly localized phototransduction machinery, we observed OS accumulation of syntaxin3, a protein normally found in the cytoplasm and the synaptic termini. In conclusion, our studies demonstrate the requirement for BBS8 in early development and elaboration of ciliated photoreceptor OS, explaining the need for BBS8 in normal vision. The findings from our study also imply that early targeting of both rods and cones in BBS8 patients is crucial for successful restoration of vision.

Gene and Cell Therapy for AIPL1-Associated Leber Congenital Amaurosis: Challenges and Prospects

Leber congenital amaurosis (LCA) caused by AIPL1 mutations is one of the most severe forms of inherited retinal degeneration (IRD). The rapid and extensive photoreceptor degeneration challenges the development of potential treatments. Nevertheless, preclinical studies show that both gene augmentation and photoreceptor transplantation can regenerate and restore retinal function in animal models of AIPL1-associated LCA. However, questions regarding long-term benefit and safety still remain as these therapies advance towards clinical application. Ground-breaking advances in stem cell technology and genome editing are examples of alternative therapeutic approaches and address some of the limitations associated with previous methods. The continuous development of these cutting-edge biotechnologies paves the way towards a bright future not only for AIPL1-associated LCA patients but also other forms of IRD.

ARL2BP, a protein linked to retinitis pigmentosa, is needed for normal photoreceptor cilia doublets and outer segment structure

The outer segment (OS) of photoreceptor cells is an elaboration of a primary cilium with organized stacks of membranous disks that contain the proteins needed for phototransduction and vision. Though ciliary formation and function has been well characterized, little is known about the role of cilia in the development of photoreceptor OS. Nevertheless, progress has been made by studying mutations in ciliary proteins, which often result in malformed OSs and lead to blinding diseases. To investigate how ciliary proteins contribute to OS formation, we generated a knockout (KO) mouse model for ARL2BP, a ciliary protein linked to retinitis pigmentosa. The KO mice display an early and progressive reduction in visual response. Before photoreceptor degeneration, we observed disorganization of the photoreceptor OS, with vertically aligned disks and shortened axonemes. Interestingly, ciliary doublet microtubule (MT) structure was also impaired, displaying open B-tubule doublets, paired with loss of singlet MTs. On the basis of results from this study, we conclude that ARL2BP is necessary for photoreceptor ciliary doublet formation and axoneme elongation, which is required for OS morphogenesis and vision.

PDGF-C and PDGF-D in ocular diseases

PDGFs and their receptors are critical regulators of numerous tissues and organs, including the eye. Extensive studies have shown that PDGFs and their receptors play critical roles in many ocular neovascular diseases, such as neovascular age-related macular degeneration, retinopathy of prematurity, and proliferative vitreoretinopathy. In addition, PDGFs and PDGFRs are also important players in ocular diseases involving the degeneration of retinal neuronal and vascular cells, such as glaucoma and retinitis pigmentosa. Due to their critical roles in the pathogenesis of many blinding ocular diseases, the PDGFs and PDGFRs have been considered as important target molecules for the treatment of eye diseases. PDGF-C and PDGF-D are relatively new members of the PDGF family and are potent angiogenic and survival factors. Recent studies have demonstrated their important roles in different types of eye diseases. Thus, modulating PDGF-C and PDGF-D activities may have therapeutic values for the treatment of ocular neovascular and degenerative diseases. This review mainly summarizes the recent advances on PDGF-C and PDGF-D biology in relationship to some major ocular diseases.

CLINICAL PROGRESS IN INHERITED RETINAL DEGENERATIONS: GENE THERAPY CLINICAL TRIALS AND ADVANCES IN GENETIC SEQUENCING

PURPOSE

Inherited retinal dystrophies are a significant cause of vision loss and are characterized by the loss of photoreceptors and the retinal pigment epithelium (RPE). Mutations in approximately 250 genes cause inherited retinal degenerations with a high degree of genetic heterogeneity. New techniques in next-generation sequencing are allowing the comprehensive analysis of all retinal disease genes thus changing the approach to the molecular diagnosis of inherited retinal dystrophies. This review serves to analyze clinical progress in genetic diagnostic testing and implications for retinal gene therapy.

METHODS

A literature search of PubMed and OMIM was conducted to relevant articles in inherited retinal dystrophies.

RESULTS

Next-generation genetic sequencing allows the simultaneous analysis of all the approximately 250 genes that cause inherited retinal dystrophies. Reported diagnostic rates range are high and range from 51% to 57%. These new sequencing tools are highly accurate with sensitivities of 97.9% and specificities of 100%. Retinal gene therapy clinical trials are underway for multiple genes including RPE65, ABCA4, CHM, RS1, MYO7A, CNGA3, CNGB3, ND4, and MERTK for which a molecular diagnosis may be beneficial for patients.

CONCLUSION

Comprehensive next-generation genetic sequencing of all retinal dystrophy genes is changing the paradigm for how retinal specialists perform genetic testing for inherited retinal degenerations. Not only are high diagnostic yields obtained, but mutations in genes with novel clinical phenotypes are also identified. In the era of retinal gene therapy clinical trials, identifying specific genetic defects will increasingly be of use to identify patients who may enroll in clinical studies and benefit from novel therapies.

Gene Therapy in a Large Animal Model of PDE6A-Retinitis Pigmentosa

Despite mutations in the rod phosphodiesterase 6-alpha (PDE6A) gene being well-recognized as a cause of human retinitis pigmentosa, no definitive treatments have been developed to treat this blinding disease. We performed a trial of retinal gene augmentation in the Pde6a mutant dog using Pde6a delivery by capsid-mutant adeno-associated virus serotype 8, previously shown to have a rapid onset of transgene expression in the canine retina. Subretinal injections were performed in 10 dogs at 29–44 days of age, and electroretinography and vision testing were performed to assess functional outcome. Retinal structure was assessed using color fundus photography, spectral domain optical coherence tomography, and histology. Immunohistochemistry was performed to examine transgene expression and expression of other retinal genes. Treatment resulted in improvement in dim light vision and evidence of rod function on electroretinographic examination. Photoreceptor layer thickness in the treated area was preserved compared with the contralateral control vector treated or uninjected eye. Improved rod and cone photoreceptor survival, rhodopsin localization, cyclic GMP levels and bipolar cell dendrite distribution was observed in treated areas. Some adverse effects including foci of retinal separation, foci of retinal degeneration and rosette formation were identified in both AAV-Pde6a and control vector injected regions. This is the first description of successful gene augmentation for Pde6a retinitis pigmentosa in a large animal model. Further studies will be necessary to optimize visual outcomes and minimize complications before translation to human studies.

Extended conformation of the proline-rich domain of human aryl hydrocarbon receptor-interacting protein-like 1: implications for retina disease

Mutations in the primate-specific proline-rich domain (PRD) of aryl hydrocarbon receptor-interacting protein-like 1 (AIPL1) are thought to cause Leber congenital amaurosis or dominant cone-rod dystrophy. The role of PRD and the mechanisms of PRD mutations are poorly understood. Here, we have examined properties of hAIPL1 and effects of the PRD mutations on protein structure and function. Solution structures of hAIPL1, hAIPL11-316 with PRD truncation, and the P351Δ12 and P376S mutants were examined by small angle X-ray scattering. Our analysis suggests that PRD assumes an extended conformation and does not interact with the FK506-binding and tetratricopeptide domains. The PRD truncation, but not PRD mutations, reduced the molecule's radius of gyration and maximum dimension. We demonstrate that hAIPL1 is a monomeric protein, and its secondary structure and stability are not affected by the PRD mutations. PRD itself is an extended monomeric random coil. The PRD mutations caused little or no changes in hAIPL1 binding to known partners, phosphodiesterase-6A and HSP90. We also identified the γ-subunit of phosphodiesterase-6 as a novel partner of hAIPL1 and hypothesize that this interaction is altered by P351Δ12. Our results highlight the complexity of mechanisms of PRD mutations in disease and the possibility that certain mutations are benign variants. Mutations in the proline-rich domain (PRD) of human AIPL1 cause severe retinal diseases, yet the role of PRD and the mechanisms of PRD mutations are unknown. Here, we describe a SAXS-derived solution structure of AIPL1 and functional properties of disease-linked AIPL1-PRD mutants. This structure and functional analyses provide a framework for understanding the mechanisms of PRD in disease.