Gene augmentation therapy restores vision in a CaBP4-mutant canine model of cone-rod synaptic disorder

Abstract

Purpose : Inherited retinal degenerations (IRDs) are a leading cause of blindness. While a variety of treatments are being developed to address specific IRDs, we currently have a poor understanding of the degenerate retina’s ability to reshape, and restore visual function after treatment. Calcium binding protein-4 (CaBP4) is necessary for the normal development of photoreceptor synaptic morphology and visual function. We identified a mutation in CaBP4 in a pedigree of dogs with a progressive IRD. We used the CaBP4-mutant canine model to examine functional and structural recovery following gene therapy.

Methods : Exome sequencing was performed. Visual function and retinal structure of treated and untreated mutant-dogs were assessed in vivo with ERG, visual behavior and SD-OCT. Gene therapy (AAV8(733)-GRK1-CaBP4) was delivered subretinally to 4 eyes (n=3 dogs) at different stages of the disease. Enucleated globes from affected (n=3) and unaffected animals (n=4) were evaluated for structural changes in treated and untreated eyes with histology, IHC and TEM. Antibodies included CaBP4, CtBP2, GPR179, S and M/L opsin, SNAP-25, hCAR, calbindin, RetP1 and PNA. Western blot for CaBP4 was performed on retinal samples.

Results : A missense mutation in CaBP4 was identified. Affected animals exhibited reduced and mislocalized CaBP4 (IHC, Western blot). Photoreceptor synaptic organization was severely disrupted with a thin outer plexiform layer, and synaptic ribbons appearing immature in affected animals. Impaired photoreceptor synaptic transmission in affected animals was shown by a lack of scotopic and photopic ERG b-wave. Gene therapy restored synaptic transmission as shown by the presence of relatively normal ERG b-waves. SD-OCT and histology supported retinal layer preservation. Rescue was maintained for over two years in one dog (both eyes treated). Two dogs with established photoreceptor layer loss also showed vision and ERG rescue. Visual performance assessed with a 4-choice task was improved in scotopic conditions, showing a 4-fold reduction in time to complete the task, and 50% increase in success for treated eyes (n=3 dogs).

Conclusions : CaBP4-mutant dogs have profound photoreceptor synaptic dysfunction and develop a progressive IRD. Gene augmentation therapy is successful in restoring visual function in treated dogs showing photoreceptor synaptic plasticity.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.