600. Human Gene Therapy for a Synaptic Disease: X-Linked Retinoschisis (XLRS)

Abstract

We have initiated a clinical trial for an ocular condition that has prominent synaptic dysfunction caused by mutations in an extracellular matrix protein, RS1 (retinoschisin). This report describes our findings on dissection of the molecular pathology at the synapse and reversal of the structural and functional defects when RS1 is introduced into mature Rs1-KO mouse retina using an AAV8 vector. We have started a human trial (NCT02317887) using the same vector, which will probe the synaptic plasticity in XLRS patients. RS1 mutations result in XLRS, a rare but recognized ocular condition that causes splitting of the neural retinal layers and results in intraretinal cysts. Medical attention generally was focused on the structural abnormalities of XLRS. However, visual function testing shows a selective depression of the electroretinogram (ERG) b-wave despite a normal a-wave, indicating intact photoreceptor responses but deficient post-synaptic signaling.The AAV8-RS1 vector for the murine studies and the clinical trial contains a self-complementary vector genome with the human RS1 promoter and the RS1 cDNA. For synaptic structural studies in adult Rs1-KO retina, 2.5e9 viral vector genomes (vg)/eye were administered by intravitreal injection at postnatal day 30. Retinal morphology, function and pathology were evaluated 2 months post injection, with contralateral untreated eyes as controls.The photoreceptor-depolarizing bipolar cell (DBC) synapse initially develops normally in Rs1 -KO mice, but the mGluR6-TRPM1 signaling cascade elements in the post-synaptic structure are progressively delocalized to the dendritic shaft and soma of the DBC. Concomitantly, the DBC resting membrane potential is altered. This molecular pattern of post-synaptic disruption is different from several other murine night-blindness models which share a “negative-going” ERG response that characterizes murine and human XLRS. Following AAV8-RS1 gene transfer to the XLRS mouse eye, TRPM1 and the signaling molecules return to their proper location at the dendritic tips, and the proper DBC resting membrane potential is restored, along with the ERG b-wave.The clinical trial utilizing the same vector is a prospective, dose-escalation, single-center study conducted at the National Eye Institute. One eye of each participant receives the RS1 gene vector given by intravitreal injection. While the primary outcome is safety, the dose range is scaled to probe possible efficacy.Invoking synaptic plasticity for therapeutic repair would be a desirable treatment strategy for a number of neurological conditions. These findings in Rs1 -KO mice provide insight into the molecular pathology of XLRS disease and demonstrate remarkable plasticity of a critical synapse in the visual system. Reversal of synaptic pathology by an AAV8-RS1 gene construct demonstrates novel possible therapeutic avenues for this and other diseases involving synaptic pathology.