Gene-edited monkey model of Usher 1B: Assessment of gene therapy

Abstract

Purpose : Usher syndrome 1B (USH1B), characterized by congenital deafness and progressive vision loss due to MYO7A mutations, presents a compelling need for an accurate animal model. We created a monkey model of USH1B by embryonic gene editing, defined its phenotype, and tested effects of dual AAV gene therapy.

Methods : In vitro derived zygotes injected with Cas9 mRNA and sgRNAs targeting MYO7A exon 3 yielded embryos with confirmed editing of MYO7A; transfer to surrogate dams resulted in an infant with compound heterozygous mutations in MYO7A. Auditory function was evaluated by auditory brainstem responses and otoacoustic emissions. Retinal structure and function were assessed by imaging and ERG. At 8 mo, AAV8(Y733F)-smCBA-hybrid dual MYO7A (9.9e10 vg) was delivered subretinally to one eye. A WT monkey received the same treatment in both eyes, and retinas were assessed at 3 mo post-injection (PI).

Results : The infant with MYO7A mutations showed congenital deafness, vestibular dysfunction and progressive retinal degeneration. Midperipheral photoreceptor disruption was first observed at 4 mo and widespread cystoid edema by 6-8 mo. Full-field ERG photopic and scotopic a- and b-wave amplitudes were reduced by ~50% by 4 mo compared to control infants and declined progressively thereafter, while multifocal ERG showed reduced central cone function by 6 mo. In the treated eye, macular edema cleared by 2 wks PI but returned by 6 mo. ERG amplitudes showed a slowing of further loss compared to the untreated eye at 4 mo PI, but no differences by 10 mo PI. Abnormalities appeared by 3 mo PI in the treated eye and were more prominent by 6 mo, and included mottled fundus hypofluorescence and fluorescein angiographic hyperfluorescent foci around the arcades; OCT showed irregular RPE with hyperreflective clumps extending into photoreceptor layers. In the WT monkey, similar changes were seen in both eyes at 3 mo; histology and IF showed large pigmented cells extending from the RPE that were negative for RPE65, CD3, CD20, CD68, CD163, Ki67 and caspase 3, but positive for vimentin.

Conclusions : In a first gene-edited NHP model of USH1B, dual AAV8-MYO7A gene therapy resulted in transient slowing of functional loss but also pigmentary abnormalities that showed similarities to the chorioretinal atrophy seen in some human patients receiving voretigene neparvovec-rzyl. This model may help to elucidate the nature of this pathology and define mitigating factors.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.