2013 Progress Report – Executive Summary
Progress on Translational Path
Our primary TPath is in restoration of vision in animal models of the blinding disease retinitis pigmentosa (RP) in which photoreceptor cells in the retina degenerate leaving behind the other cell layers of the retina, but with no sensitivity to light. Our approach is to synthesize chemical photoswitches that can be delivered by the well-tolerated clinical approach of intravitreal injection and which endow these remaining cells with a light response of their own. We are pursuing two variant strategies:
1) The 1 component approach, in which the photoswitch enters the surviving cells, associates with the ion channels of those cells and render the channels light-sensitive so that they open and close in response to changes between light and dark in a manner that generates neural activity. Although the photoswitches likely enter all of the surviving cell types of the diseased retina, they selectively impact the firing of the output cells of the retina, the retinal ganglion cells (RGCs). The attraction of this approach is its dependence on just a small chemical compound.
2) The 2 component approach, in which the photoswitch is covalently attached to a specific amino acid that is introduced into a receptor, where the receptor is introduced into a specific retinal cell type via a viral gene delivery (we use the AAV virus for its flexibility, cell-specific targeting and because it is best established for clinical use). In this case, the identity of the light-receptive protein is defined as is the cell type in which it resides. This makes it possible to select whether to target the output RGCs or, alternatively, the upstream bipolar cells, which could provide for advantages of processing that occurs in normal vision as information flows through the retinal circuit (indeed we see one such advantage in the preservation of center-surround receptive fields). Moreover, it makes it possible to select either ion channel receptors whose activation directly generates currents, or to select G protein coupled receptors (GPCRs), which activate ion channels through a cascade that could result in signal amplification. This approach depends on both the small chemical compound and the viral introduction of the gene encoding the modified receptor (even though the receptor comes from the organism). While clinical trials using AAV-mediated gene introduction are on-going, there is an extra burden of development for such clinical approaches.
As we enter our final 2 years of NDC support with FY13, which starts on August 1, 2013, we have dropped several strong scientific projects that did not merge into the clinical path in order to accelerate the effort on vision restoration so that we could file an IND for each of the above approaches in the final year of support.
Fy13 will be spent selecting a best 1 component photoswitch and a best 2 component photoswitch – receptor – cell type combination. This will involve continued work in genetic models of RP in mouse and dog, as well as a laser- induced local photoreceptor degeneration model in macaque. It will also include two new complimentary model systems: i) A genetic photoreceptor degeneration model in rat, which provides the low cost and ease of assay of the mouse, but with a larger eye in which it is easier to deliver known quantities of the photoswitch so that proper dose- response relationships can be determined and that a therapeutic window can be assessed; and ii) Toxicity assays in normal pigs, which have large eyes with a gelid vitreous, similar to humans, and where larger numbers of animals are available in surgical labs at University of Washington. The work will also involve a much more informative brain imaging assays and behavioral assays of vision than what we did so far, so that for the first time we will be able to know the acuity of the restored vision, the speed of object motion that will still allow for clear vision.
Progress in the past year
Progress in the past year included 18 publications, including in Nature, Science Translational Medicine, Nature Cell Biology, Nature Neuroscience, Neuron (2) and PNAS (2). Two of the papers generated considerable attention in the print and broadcast media. The first of these, published in Neuron, described the first 1 component restoration of vision (work that has now been greatly surpassed by the 2nd generation 1 component photoswitch, which is described in a paper that is currently under review). The second of these, published in Science Translational Medicine, described a method for directed evolution of the AAV capsid to endow it with selective tropism to specific cell types in the retina to enable targeted gene therapy, an essential step for the 2 component approach.
Dissemination of information about our program was amplified through 86 presentations by the investigator group at various US and international forums.