Use of a novel Rpgr-null mouse line allows rapid efficacy evaluation of gene augmentation therapy

Abstract

Purpose :
Mutations in the RPGR gene account for about 70 percent of X-linked retinitis pigmentosa (XLRP) and are one of the most common causes of all RP cases. Gene augmentation therapy was shown to be capable of preserving retinal function and structure in canine or murine models of Rpgr deficiency. However, a long time period (12 to 24 months) is usually needed for preclinical efficacy study in mice with Rpgr mutations due to the slow degeneration of their retinas. In a recent report (Rao et al., Hum Mol Genet, 2016; 25: 2005), Rpgr^ko/Y mice with a heterozygous hypomorphic allele of Cep290 (Cep290^rd16/+) exhibited much earlier onset and faster progression of retinal degeneration and dysfunction than Rpgr^ko/Y mice. This mouse line (Rpgr^ko/Y;Cep290^rd16/+) was used in the current study to test if therapeutic effects can be captured in a short time period after treatment.

Methods : Each mouse, at 6 weeks of age, received subretinal injection of one micro liter (µL) AAV9-RPGR vector at a dose of 1x10⁹ vector genomes (vg) in one eye and the same volume of vehicle in the other eye. Electroretinogram (ERG) and optical coherence tomography (OCT) were performed on the treated mice when they were about 3 months (6 weeks after vector injection) and 7 months of age. At the end of the study, the mice were sacrificed for immunofluorescence analysis. Paired t test was used for statistics.

Results :
The mice exhibited significantly higher ERG amplitudes in the vector-treated eyes than the control eyes starting from 3 months of age. OCT analyses revealed longer outer segments (OS)/inner segments (IS) in the vector-treated eyes at 3 months of age, and thicker outer nuclear layers (ONL, or photoreceptor layer) at 7 months of age. In the vehicle-treated retinas, M cone opsin was not only found in the OS, its normal subcellular localization, but also in the IS and the perinuclear and synaptic regions. In contrast, M opsin was almost exclusively observed in the OS in the vector-treated retinas, similar to the wild-type retina.

Conclusions : Therapeutic effects can be observed in the Rpgr^ko/Y;Cep290^rd16/+ mice at 6 weeks after RPGR vector administration, which is much faster than previous studies using Rpgr^ko/Y mice. Use of the Rpgr^ko/Y;Cep290^rd16/+ mouse line would allow rapid efficacy evaluation of gene therapy treatment.

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