Investigating tolerability of subretinally delivered dual AAV-MYO7A vectors in non-human primate (NHP)

Abstract

Purpose : Mutations in MYO7A are associated with Usher syndrome 1B (USH1B) and cause loss of retinal structure and function in patients. To date, proof of concept experiments for a dual AAV vector-based treatment of USH1B have only been performed in mouse models. Unlike patients, Myo7a mutant mice do not exhibit retinal degeneration and have only mild functional deficits. Photoreceptor toxicity has been observed following injection of dual vectors in C57BL/6J mice. This may be due to MYO7A over-expression, differences in MYO7A distribution and/or structural differences in photoreceptors between mouse and man. The purpose of our study was to test the tolerability of dual AAV5-MYO7A vectors in a clinically relevant species, Macaca fascicularis.

Methods : Two male macaques were subretinally injected in their right eyes with simple overlap dual AAV5 vectors designed to express full length human MYO7A carrying a C-terminal HA tag (MYO7A-HA). Front half vectors contained either the smCBA or GRK1 promoter and back half vector was same throughout. Vectors were delivered at total concentration of 5e12 vg/mL (2.5e12 vg/ml each of front and back vector). Left eyes received either AAV5-CBA-GFP or AAV5-GRK1-GFP at an equal concentration. Four subretinal blebs, 30-140 ul in size were created in each eye. Retinal structure and function were assessed at baseline, and at 1 and 2 months post-injection with scanning laser ophthalmoscopy (SLO) and mfERG, respectively. At 2 months post-injection, animals were euthanized, eyes were enucleated and retinas analyzed for the presence of vector mediated MYO7A transcript and protein expression. Identical dual vectors were subretinally injected in Myo7a-/- mice at the same concentration.

Results : OCT scans were unremarkable post injection except for limited cellular disorganization at some injection sites and a small area of abnormal reflectivity in the right eyes. In macaques, mfERG responses in the left eyes were normal post-injection, while the right eyes showed modest declination in mfERG responses at 1 month that essentially resolved by 2 months post-injection. Similarly, no loss of retinal structure/function was observed in treated Myo7a-/- mice treated with dual vectors. Protein and transcript analysis are ongoing.

Conclusions : Simple overlap dual AAV5-MYO7A vectors were well-tolerated in NHP and Myo7a-/- mouse retina.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.