The retina has been at the forefront of translational gene therapy. Proof-of-concept that gene therapy could restore vision in a large animal led to the initiation of the first successful clinical ...trials and, in turn, to the recent approval of the first gene therapy product for an ocular disease. As dozens of clinical trials of retinal gene therapy have begun, new challenges are identified, which include delivery of large genes, counteracting gain-of-function mutations, and safe and effective gene transfer to diseased retinas. Advancements in vector design, improvements of delivery routes, and selection of optimal timing for intervention will contribute to extend the initial success of retinal gene therapy to an increasing number of inherited blinding conditions.
The first FDA-approved gene therapy product for an inherited disease is an adeno-associated virus (AAV)-based vector for a form of blindness.
Clinical development of gene therapy for dozens of rare and common retinal diseases is ongoing, although improvements will be required to broaden the success of retinal gene therapy to a growing number of conditions.
Transduction of the retina from the vitreous and delivery of genes larger than 5kb are long-standing goals of the field, for which promising results have been recently obtained in animal models.
The recent evolution of designer nucleases, including CRISPR/Cas9, allows to edit toxic gain-of-function mutations even in terminally differentiated neurons.
Therapeutic approaches which are independent of the underlying mutations are being developed that are based on delivery of either neurotrophic factors to slow down retinal degeneration or optogenetic molecules to sensitize neurons to light.
Selenoprotein N (SEPN1) is a broadly expressed resident protein of the endoplasmic reticulum (ER) whose loss-of-function inexplicably leads to human muscle disease. We found that SEPN1 levels ...parallel those of endoplamic reticulum oxidoreductin 1 (ERO1), an ER protein thiol oxidase, and that SEPN1's redox activity defends the ER from ERO1-generated peroxides. Moreover, we have defined the redox-regulated interactome of SEPN1 and identified the ER calcium import SERCA2 pump as a redox-partner of SEPN1. SEPN1 enhances SERCA2 activity by reducing luminal cysteines that are hyperoxidized by ERO1-generated peroxides. Cells lacking SEPN1 are hypersensitive to ERO1 overexpression and conspicuously defective in ER calcium re-uptake. After being muscle-transduced with an adeno-associated virus driving ERO1α, SEPN1 knockout mice unmasks a myopathy that resembles the dense core disease due to human mutations in SEPN1, whereas the combined attenuation of ERO1α and SEPN1 enhances cell fitness. These observations reveal the involvement of SEPN1 in ER redox and calcium homeostasis and that an ERO1 inhibitor, restoring redox-dependent calcium homeostasis, may ameliorate the myopathy of SEPN1 deficiency.
Retinal gene therapy with adeno-associated viral (AAV) vectors holds promises for treating inherited and noninherited diseases of the eye. Although clinical data suggest that retinal gene therapy is ...safe and effective, delivery of large genes is hindered by the limited AAV cargo capacity. Protein trans-splicing mediated by split inteins is used by single-cell organisms to reconstitute proteins. Here, we show that delivery of multiple AAV vectors each encoding one of the fragments of target proteins flanked by short split inteins results in protein trans-splicing and full-length protein reconstitution in the retina of mice and pigs and in human retinal organoids. The reconstitution of large therapeutic proteins using this approach improved the phenotype of two mouse models of inherited retinal diseases. Our data support the use of split intein-mediated protein trans-splicing in combination with AAV subretinal delivery for gene therapy of inherited blindness due to mutations in large genes.
Retinal gene therapy with adeno‐associated viral (AAV) vectors is safe and effective in humans. However, AAV's limited cargo capacity prevents its application to therapies of inherited retinal ...diseases due to mutations of genes over 5 kb, like Stargardt's disease (STGD) and Usher syndrome type IB (USH1B). Previous methods based on ‘forced’ packaging of large genes into AAV capsids may not be easily translated to the clinic due to the generation of genomes of heterogeneous size which raise safety concerns. Taking advantage of AAV's ability to concatemerize, we generated dual AAV vectors which reconstitute a large gene by either splicing (trans‐splicing), homologous recombination (overlapping), or a combination of the two (hybrid). We found that dual trans‐splicing and hybrid vectors transduce efficiently mouse and pig photoreceptors to levels that, albeit lower than those achieved with a single AAV, resulted in significant improvement of the retinal phenotype of mouse models of STGD and USH1B. Thus, dual AAV trans‐splicing or hybrid vectors are an attractive strategy for gene therapy of retinal diseases that require delivery of large genes.
Synopsis
The Authors provide proof‐of‐concept of gene therapy for two inherited retinal degeneration conditions by using dual AAV vectors. This allows the expansion of AAV cargo capacity for gene therapy of syndromes that require the transfer of large genes.
Dual AAV vectors significantly expand AAV cargo capacity in the retina.
Unlike dual AAV overlapping vectors, trans‐splicing and hybrid vectors efficiently transduce photoreceptors.
Subretinal delivery of dual AAV vectors improves the retinal phenotype of murine models of inherited blinding conditions due to mutations in large genes.
The Authors provide proof‐of‐concept of gene therapy for two inherited retinal degeneration conditions by using dual AAV vectors. This allows the expansion of AAV cargo capacity for gene therapy of syndromes that require the transfer of large genes.
The aim of this study was to show the clinical data of long-term (3-year) follow-up of 5 patients affected by Leber congenital amaurosis type 2 (LCA2) treated with a single unilateral injection of ...adeno-associated virus AAV2-hRPE65v2.
Clinical trial.
Five LCA2 patients with RPE65 gene mutations.
After informed consent and confirmation of trial eligibility criteria, the eye with worse visual function was selected for subretinal delivery of adeno-associated virus (AAV2-hRPE65v2). Subjects were evaluated before and after surgery at designated follow-up visits (1, 2, 3, 14, 30, 60, 90, 180, 270, and 365 days, 1.5 years, and 3 years) by complete ophthalmic examination. Efficacy for each subject was monitored with best-corrected visual acuity, kinetic visual field, nystagmus testing, and pupillary light reflex.
Best-corrected visual acuity, kinetic visual field, nystagmus testing, and pupillary light reflex.
The data showed a statistically significant improvement of best-corrected visual acuity between baseline and 3 years after treatment in the treated eye (P<0.001). In all patients, an enlargement of the area of visual field was observed that remained stable until 3 years after injection (average values: baseline, 1058 deg(2) vs. 3 years after treatment, 4630 deg(2)) and a reduction of the nystagmus frequency compared with baseline at the 3-year time point. Furthermore, a statistically significant difference was observed in the pupillary constriction of the treated eye (P<0.05) compared with the untreated eye in 3 patients at 1- and 3-year time points. No patients experienced serious adverse events related to the vector in the 3-year postinjection period.
The long-term follow-up data (3 years) on the 5-patient Italian cohort involved in the LCA2 gene therapy clinical trial clearly showed a stability of improvement in visual and retinal function that had been achieved a few months after treatment. Longitudinal data analysis showed that the maximum improvement was achieved within 6 months after treatment, and the visual improvement was stable up to the last observed time point.
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Inherited retinopathies (IRs) are common and untreatable blinding conditions inherited mostly as monogenic due to mutations in genes expressed in retinal photoreceptors (PRs) and in retinal pigment ...epithelium (RPE). Over the last two decades, the retina has emerged as one of the most favorable target tissues for gene therapy given its small size and its enclosed and immune-privileged environment. Different types of viral vectors have been developed, especially those based on the adeno-associated virus (AAV), which efficiently deliver therapeutic genes to PRs or RPE upon subretinal injections. Dozens of successful proofs of concept of the efficacy of gene therapy for recessive and dominant IRs have been generated in small and large models that have paved the way to the first clinical trials using AAV in patients with Leber congenital amaurosis, a severe form of childhood blindness. The results from these initial trials suggest that retinal gene therapy with AAV is safe in humans, that vision can be improved in patients that have suffered from severe impairment of visual function, in some cases for decades, and that readministration of AAV to the subretinal space is feasible, effective, and safe. However, none of the trials could match the levels of efficacy of gene therapy observed in a dog model of the disease, suggesting that there is room for improvement. In conclusion, these results bode well for further testing of AAV-mediated retinal gene therapy in patients with other monogenic and complex forms of blindness.
In Duchenne muscular dystrophy (DMD) the absence of dystrophin at the sarcolemma delocalizes and downregulates nitric oxide synthase (nNOS); this alters S-nitrosylation of HDAC2 and its chromatin ...association. We show that the differential HDAC2 nitrosylation state in Duchenne versus wild-type conditions deregulates the expression of a specific subset of microRNA genes. Several circuitries controlled by the identified microRNAs, such as the one linking miR-1 to the G6PD enzyme and the redox state of cell, or miR-29 to extracellular proteins and the fibrotic process, explain some of the DMD pathogenetic traits. We also show that, at variance with other myomiRs, miR-206 escapes from the dystrophin-nNOS control being produced in activated satellite cells before dystrophin expression; in these cells, it contributes to muscle regeneration through repression of the satellite specific factor, Pax7. We conclude that the pathway activated by dystrophin/nNOS controls several important circuitries increasing the robustness of the muscle differentiation program.
► Dystrophin controls gene expression by nNOS relocalization and HDAC2 nitrosylation ► Epigenetic control of miRNA expression is involved in DMD pathogenesis ► miR-1 controls G6PD expression and the redox state of the cell ► miR-206 contributes to satellite cell differentiation by repressing Pax7
The expansion of the CRISPR-Cas toolbox is highly needed to accelerate the development of therapies for genetic diseases. Here, through the interrogation of a massively expanded repository of ...metagenome-assembled genomes, mostly from human microbiomes, we uncover a large variety (n = 17,173) of type II CRISPR-Cas loci. Among these we identify CoCas9, a strongly active and high-fidelity nuclease with reduced molecular size (1004 amino acids) isolated from an uncultivated Collinsella species. CoCas9 is efficiently co-delivered with its sgRNA through adeno associated viral (AAV) vectors, obtaining efficient in vivo editing in the mouse retina. With this study we uncover a collection of previously uncharacterized Cas9 nucleases, including CoCas9, which enriches the genome editing toolbox.
Liver gene therapy with adeno‐associated viral (AAV) vectors is under clinical investigation for haemophilia A (HemA), the most common inherited X‐linked bleeding disorder. Major limitations are the ...large size of the F8 transgene, which makes packaging in a single AAV vector a challenge, as well as the development of circulating anti‐F8 antibodies which neutralise F8 activity. Taking advantage of split‐intein‐mediated protein trans‐splicing, we divided the coding sequence of the large and highly secreted F8‐N6 variant in two separate AAV‐intein vectors whose co‐administration to HemA mice results in the expression of therapeutic levels of F8 over time. This occurred without eliciting circulating anti‐F8 antibodies unlike animals treated with the single oversized AAV‐F8 vector under clinical development. Therefore, liver gene therapy with AAV‐F8‐N6 intein should be considered as a potential therapeutic strategy for HemA.
Synopsis
Liver directed AAV‐intein mediated protein trans‐splicing results in stable therapeutic levels of F8 in mice thus representing a novel therapeutic strategy for haemophilia A.
Split‐intein mediated protein trans‐splicing (PTS) allows reconstitution of large proteins via adeno‐associated viral (AAV) vectors in mouse liver.
The highly secreted and active F8‐N6 (N6) variant has been adapted to fit into AAV intein vectors.
A single systemic injection of AAV‐N6 intein targets liver of hemophilic mice resulting in stable therapeutic levels of F8 without eliciting anti‐F8 antibodies at the vector doses used.
Liver directed AAV‐intein mediated protein trans‐splicing results in stable therapeutic levels of F8 in mice thus representing a novel therapeutic strategy for haemophilia A.
Because of its favorable anatomical and immunological characteristics, the eye has been at the forefront of translational gene therapy. Dozens of promising proofs of concept have been obtained in ...animal models of inherited retinal degenerations (IRDs), and some of them have been relayed to the clinic. The results from the first clinical trials for a congenital form of blindness have generated great interest and have demonstrated the safety and efficacy of intraocular administrations of viral vectors in humans. However, this progress has also generated new questions and posed challenges that need to be addressed to further expand the applicability of gene therapy in the eye, including safe delivery of viral vectors to the outer retina, treatment of dominant IRDs as well as of IRDs caused by mutations in large genes, and, finally, selection of the appropriate IRDs and patients to maximize the efficacy of gene transfer. This review summarizes the strategies that are currently being exploited to overcome these challenges and drive the clinical development of retinal gene therapy.
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