Nonhuman primate model of Usher Syndrome Type 1B: course of retinal degeneration and initial gene therapy results

Abstract

Purpose : More accurate animal models are urgently needed for retinal degenerative diseases, especially those not replicated in rodent models. Usher syndrome 1B (USH1B), characterized by congenital hearing loss and progressive vision loss due to mutations in the MYO7A gene, presents a compelling need for such a model. This work aimed to create a non-human primate (NHP) model of USH1B, characterize its phenotype, and test whether novel dual AAV vectors were therapeutic in the retina.

Methods : Rhesus oocytes were fertilized in vitro, and zygotes were injected 16 hours later with Cas9 mRNA and sgRNAs designed to target exon 3 of MYO7A. Embryos with confirmed MYO7A editing were transferred to surrogate dams. Peripheral blood mononuclear cells, skin, and cheek cells from a resulting infant were sequenced to confirm MYO7A editing. Infant auditory function was evaluated by auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE). Retinal structure and function were assessed by multimodal retinal imaging and electroretinography (ERG). At 8 months, the left eye was treated subretinally with AAV8(Y733F)-smCBA-MYO7A dual vectors.

Results : Sequencing of infant genomic DNA verified biallelic mutations of the MYO7A gene with no detectable off-target effects. Auditory testing at 1, 2, and 6 months showed no detectable ABR or DPOAE responses. Abnormal photoreceptor structure was first observed in the mid-periphery at 4 months of age. Cystoid edema appeared in the midperipheral inner nuclear layer at 6 months and in foveal Henle’s fiber layer by 8 months. Photopic, flicker and scotopic ERG responses showed an approximate 50% reduction of a- and b-wave amplitudes by 4 months compared to control infants and declined progressively through 8 months. After injection of dual AAV-MYO7A vectors, transient clearing of macular edema was seen in the treated eye. At 4 months post-treatment, the ERG showed a halting or slowing of further loss in amplitude compared to the untreated eye.

Conclusions : We have shown for the first time the ability to use gene editing to create an NHP model of an inherited retinal disease that closely mirrors the human disease phenotype. Initial results of dual AAV-MYO7A vector- based gene therapy show slowing of functional loss, providing the first evidence for retinal efficacy of dual AAV vectors in a phenotypically accurate, translational model of USH1B.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.