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Kaitlyn Calabro, Sanford Boye, Kevin Tyler Mccullough, Shreyasi Choudhury, Paul D Gamlin, Shannon Elizabeth Boye; Investigating the utility of mice as a model for developing clinically relevant dual AAV vectors to treat USH1B. Invest. Ophthalmol. Vis. Sci. 2017;58(8):4098.
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© ARVO (1962-2015); The Authors (2016-present)
Efforts are underway to develop a dual AAV-based approach to treat MYO7A-Usher syndrome 1B. While MYO7A is indispensable for the function/survival of human photoreceptors (PR), retinas of Myo7a null mice don’t degenerate, and exhibit only mild functional deficits, including a reported impairment in rhodopsin trafficking through the PR connecting cilium. Differences in PR structure between species may account for this discrepancy. In primates, MYO7A localizes to calyceal processes which are absent in mice. The purpose of our study was to further interrogate whether Myo7a -/- mice can provide reliable outcome measures of gene therapy.
We assessed whether the absence of MYO7A significantly impacted rhodopsin trafficking in mouse PRs by breeding newly established Myo7a knock out mice (Myo7a KO) with mice containing knock-in human rhodopsin-GFP fusion (Rho-GFP) to generate Myo7a-/- :Rho-GFP+/-. Myo7a +/-:Rho-GFP+/- littermates served as controls. Rho(P23H) transgenic:Rho-GFP+/- mice were generated for comparison. Retinal function and structure were assessed by ERG and OCT at P30, 60 and 90. RHO-GFP localization was compared in retinal cross sections with microscopy. Second, we explored the distribution of MYO7A in mouse vs. macaque. Protein from neural retina and retinal pigment epithelium (RPE) were evaluated by western blot to determine the relative distribution of MYO7A.
The absence of MYO7A had no impact on rhodopsin-GFP trafficking. Additionally, no differences were observed in retinal function/structure out to P30 relative to controls. Analysis of older mice is ongoing. MYO7A expression was essentially restricted to the neural retina in adult macaque whereas, in mouse, the majority of MY07A was in the RPE.
Localization of rhodopsin cannot serve as a metric for evaluating therapeutic efficacy in Myo7a KO mice. Our results suggest that MYO7A does not play a role in rhodopsin trafficking in mouse. In agreement with this, we see a strikingly different pattern of MYO7A expression in mouse vs. macaque. These results suggest that dual AAV-mediated gene therapies must be designed to recapitulate the endogenous pattern of MYO7A expression in primate retina. However, Myo7a-/- mouse models have utility for evaluating the relationship between efficiency of vector recombination and MYO7A expression.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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