Lolita Petit University of Massachusetts Medical School
Hemant Khanna University of Massachusetts Medical School
Claudio Punzo Ph.D. University of Massachusetts Medical School
Diseases of the Eye That Cause Vision Loss and Blindness Are Ideal Targets for Emerging Therapeutic Approaches
Vision is considered by many to be the most important of our five senses. It is a highly complex process that requires the coordinated activity of numerous components in the eye and the brain. The initial steps are performed by the retina, which is the light-sensitive neuronal tissue situated at the back of the eye. When light reaches the retinal rod and cone photoreceptors, photons are absorbed by a photopigment, which activates a cascade that converts the light signal into an electrochemical signal. This is done in collaboration with the retinal pigment epithelium (RPE), which regenerates the visual chromophore. Electrochemical signals are then transferred through bipolar cells to ganglion cells, where they are converted into action potentials that are sent to the brain. Consistent with the crucial role of the retina in vision, the majority of diseases that lead to blindness are caused by an acquired or inherited degeneration of the retina.
As a gene therapy target, the retina is a particularly well-suited organ for therapeutic interventions. The retina is a small tissue, highly compartmentalized, immune-privileged, and easily accessible. Optical transparency of the eye enables safe evaluation of reporter gene expression and therapeutic effects by noninvasive methods, such as electroretinography (ERG), funduscopy, and optical coherence tomography (OCT). These favorable factors, along with a thorough knowledge of the molecular pathogenesis of many retinal diseases, the development and characterization of animal models that mimic human diseases, and the advances in gene delivery tools, have fueled a rapid development of multiple gene therapy strategies for several forms of retinopathies. This review is focused on emerging strategies that use gene therapy to combat vision loss, particularly for the treatment of retinal diseases caused by mutations that directly affect the photoreceptors.
Gene Replacement Therapy for LCA2: The First Success of Ocular Gene Therapy
The most successful example of ocular gene therapy was the gene replacement therapy for RPE65, Leber's congenital amaurosis 2 (LCA2), an early onset form of autosomal recessive retinal degeneration caused by mutations in theRPE65 (RPE-specific 65 kDa protein) gene. RPE65 encodes an isomerase expressed mainly in the RPE that is critical for recycling the visual chromophore involved in the visual cycle. Mutations in RPE65 result in defective visual pigment formation (both rhodopsin and cone opsin), hence severely affecting photoreceptor function and vision. Large amounts of opsin apoprotein in photoreceptors, as well as accumulation of toxic retinyl esters in the RPE, are thought to promote the progressive death of photoreceptors. Clinical phenotype analyses revealed that the degenerative component of RPE65-LCA2 starts at an early age in patients with a functional loss that is much larger than expected for the amount of cells retained. It is this phenotype that provided a very good starting point for a gene-based intervention for this disorder.
Several murine and canine models of LCA2 have shown marked functional benefits with gene therapy. In particular, results obtained in the Rpe65−/− Briard dog yielded deep excitement in the field because of its more human-like eye anatomy and immune system. The first study was carried out by subretinal delivery of recombinant adeno-associated virus (AAV) 2 vectors expressing the wild-type canine Rpe65 cDNA under the control of the ubiquitous chicken β-actin (CBA) promoter. This study revealed a dramatic improvement in photoreceptor function and vision in treated dogs. Subsequent dog studies extended the use of other AAV serotypes, including AAV1, AAV4, and AAV5, and different promoters. Improvement of vision persisted for over 11 years after a single injection of the vector. In addition, successful restoration of both cone and rod function was achieved in 20 out of 22 treated eyes at more advance stages of the disease (dogs over 2 years of age).
Based on these preclinical studies, four separate phase I–II clinical trials were initiated, which yielded promising results after subretinal administration of AAV2-hRPE65 vectors (NCT00481546, NCT00516377, NCT00643747, NCT00749957; Supplementary Table S1; Supplementary Data are available online at www.liebertpub.com/hum). Although the designs of these studies varied with respect to the use of the promoter driving the expression of RPE65, the volume of vector injected, and the surgical protocol, the data collectively demonstrated safety of AAV2 delivery to the retina. Remarkably, patients in all trials exhibited several aspects of visual improvements within a few months after treatment, though with varying degrees. These results generated excitement in the field. Subsequent treatment of the second eye of LCA2 patients previously treated with the same vector demonstrated both safety and efficacy, indicating that subretinal administration of AAV2 is feasible even in the case of preexisting immunity against the vector capsid. These promising results justified the initiation of a phase III clinical trial (NCT00999609) evaluating the treatment of both eyes in patients over 8 years of age. Thus far the trial is confirming the previous visual improvements (Spark Therapeutics Press Release 10/05/2015). AAV2-hRPE65 is expected to become the first approved gene therapy product in the United States, marking a pivotal step for the entire gene therapy field. In addition, a phase I/II clinical trial (NCT01496040) evaluating the effects of an alternative vector with increased specificity for the RPE (AAV4-RPE65-hRPE65) was recently completed.
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Human Gene Therapy, published by Mary Ann Liebert, Inc., is the premier, multidisciplinary journal covering all aspects of gene therapy. The Journal publishes in-depth coverage of DNA, RNA, and cell therapies by delivering the latest breakthroughs in research and technologies.The above article was first published in the June 2016 issue of Human Gene Therapy with the title “Advances in Gene Therapy for Diseases of the Eye”. The views expressed here are those of the authors and are not necessarily those of Human Gene Therapy, Mary Ann Liebert, Inc., publishers, or their affiliates. No endorsement of any entity or technology is implied.