Gene augmentation therapy in a large animal model of CNGB1 retinitis pigmentosa

Abstract

Purpose : Currently there is no cure for retinitis pigmentosa (RP). Gene augmentation therapy is in clinical trials for a number of retinal dystrophies but not for RP due to mutations in rod photoreceptor genes. CNGB1 mutations are reported to account for ~4% of autosomal recessive RP. The purpose of this study was to assess the potential of gene augmentation therapy for CNGB1-RP using a large animal model.

Methods : Five young Cngb1-/- dogs (8 eyes) received gene augmentation therapy using a rAAV AAV5-hGRK1-cCngb1 vector via a subretinal injection. A titer of 1x10¹² vg/mL was used in one eye and 5x10¹² vg/mL in 7 eyes. Dark- and light-adapted electroretinograms (ERGs) were recorded prior to injection and monthly post-injection. Vision was assessed using a four exit-choice vision-testing device from dim (rod only vision) lighting levels up to normal room lighting. One dog was euthanized at each of 3, 6 and 9 months post-injection and the eyes processed for immunohistochemistry (IHC).

Results : Pretreatment Cngb1-/- dogs had an elevated dark-adapted ERG response threshold and decreased amplitudes. The results of vision testing indicated that they could not see at the lowest light levels and choice of exit tunnels was random. At one month post treatment all the eyes that received the higher titer vector had a dramatic improvement in rod mediated ERG responses. They had a ~2 log unit lowering of dark-adapted ERG response threshold and markedly increased a- and b-wave amplitudes. The eye injected with rAAV at 1x10¹² vg/mL had slight improvement in ERG responses. All dogs showed improvement in visual testing performance, being able to choose the correct exit at the lowest lighting level. ERG rescue was maintained at least until 9 months post treatment.
IHC in the examined animals showed appropriate localization of Cngb1 (rod outer segments) in the treated retinal regions. Untreated regions were not labeled by the anti-Cngb1 antibody. Cnga1 expression was normal in the treated retinal regions, in untreated regions there was a lack of Cnga1 immunolabeling.

Conclusions : rAAV gene augmentation therapy rescues rod function in Cngb1-/- dogs. Treated dogs will be monitored to see if treatment preserves structure as well as restoring function. Gene augmentation therapy shows promise for the treatment of CNGB1-RP.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.