Assessment of AAV vector tropisms for mouse and human pluripotent stem cell-derived RPE and photoreceptor cells

Adeno-associated viral vectors are showing great promise as gene therapy vectors for a wide range of retinal disorders. To date, evaluation of therapeutic approaches has depended almost exclusively on the use of animal models. With recent advances in human stem cell technology, stem-cell derived retina now offers the possibility to assess efficacy in human organoids in vitro. Here we test 6 AAV serotypes (AAV2/2, AAV2/9, AAV2/8, AAV2/8T(Y733F), AAV2/5 and ShH10) to determine their efficiency in transducing mouse and human pluripotent stem cell (PSC)-derived RPE and photoreceptor cells in vitro. All the serotypes tested were capable of transducing RPE and photoreceptor cells in vitro. AAV ShH10 and AAV2/5 are the most efficient vectors at transducing both mouse and human RPE, while AAV2/8 and ShH10 achieved similarly robust transduction of human ESC-derived cone photoreceptors. Furthermore, we show that hESC-derived photoreceptors can be used to establish promoter specificity in human cells in vitro. The results of this study will aid capsid selection and vector design for pre-clinical evaluation of gene therapy approaches, such as gene editing, that require the use of human cells and tissues

Long-Term Effect of Gene Therapy on Leber's Congenital Amaurosis

BACKGROUND Mutations in RPE65 cause Leber’s congenital amaurosis, a progressive retinal degenerative disease that severely impairs sight in children. Gene therapy can result in modest improvements in night vision, but knowledge of its efficacy in humans is limited. METHODS We performed a phase 1–2 open-label trial involving 12 participants to evaluate the safety and efficacy of gene therapy with a recombinant adeno-associated virus 2/2 (rAAV2/2) vector carrying the RPE65 complementary DNA, and measured visual function over the course of 3 years. Four participants were administered a lower dose of the vector, and 8 were administered a higher dose. In a parallel study in dogs, we investigated the relationship among vector dose, visual function, and electroretinography (ERG) findings. RESULTS Improvements in retinal sensitivity were evident, to varying extents, in six participants for up to 3 years, peaking at 6 to 12 months after treatment and then declining. No associated improvement in retinal function was detected by means of ERG. Three participants had intraocular inflammation, and two had clinically significant deterioration of visual acuity. The reduction in central retinal thickness varied among participants. In dogs, RPE65 gene therapy with the same vector at lower doses improved vision-guided behavior, but only higher doses resulted in improvements in retinal function that were detectable with the use of ERG. CONCLUSIONS Gene therapy with rAAV2/2 RPE65 vector improved retinal sensitivity, albeit modestly and temporarily. Comparison with the results obtained in the dog model indicates that there is a species difference in the amount of RPE65 required to drive the visual cycle and that the demand for RPE65 in affected persons was not met to the extent required for a durable, robust effect. (Funded by the National Institute for Health Research and others; ClinicalTrials.gov number, NCT00643747.

Investigation of stem cell-derived retinal pigmented epithelium transplantation

Retinal pigment epithelium (RPE) cells perform a variety of roles that are principally directed at maintaining retinal function and homeostasis and their loss causes conditions such as age-related macular degeneration (AMD) and Stargardt disease (STGD). Regeneration of the loss or dystrophic RPE cells with stem cell-derived RPE may provide a potential therapeutic option.   STGD is the commonest form of juvenile-onset, inherited macular degeneration. In order to determine the safety and efficacy of embryonic stem (ES) cell-derived RPE transplantation for the treatment of STGD, twelve subjects with STGD received a suspension of hES-derived RPE cells in escalating dose cohorts. Following the intervention, areas of sub retinal pigmentation were noted in all participants, suggestive of engraftment and survival. Transplanted hES-derived RPE cells were often observed overlying regions of atrophic Bruch's membrane (BrM). There was no evidence of tumorigenicity, immune adverse events or other serious safety concerns related to the transplanted cells. Furthermore, there was no significant change in visual function in the study eye of any participant.   In order to improve the efficacy of ES-derived RPE transplantation, I developed an improved rodent model of retinal degeneration that features focal regions of atrophic BrM. I used a diode laser to selectively ablate RPE and observed focal regions of RPE atrophy with corresponding changes in choroidal vasculature that resemble those observed in retinal degenerative disease. Moreover, transplantation of human ES- and human induced pluripotent stem cell (iPS)-derived RPE resulted in re-population and restoration of RPE morphology post ablation. Although transplanted ES-derived RPE survived well on healthy BrM, attachment and survival of RPE is compromised on BrM that exhibits AMD or senescent changes. A potential strategy to promote survival and engraftment where there is damaged BrM is to deliver ES-derived RPE on a carrier substrate. I used a bespoke electrospun scaffold consisting of poly(e-caprolactone) and seeded this with either hESC or hiPSC-derived RPE. Transplantation of ES-derived RPE resulted in a functional monolayer of RPE with correct orientation and polarity on a biodegradable, porous and biocompatible membrane. These studies support further work in large animal models using ES-derived RPE scaffolds to restore the RPE in the presence of a compromised Br