Corrective Gene Therapy for RPGR-XLRP Rescues a Canine Model at Mid-Stage Disease

Abstract

Purpose: Mutations in the RPGR gene are the most common cause of X-linked RP in man, a condition that is currently still incurable. However, we have recently shown in two canine models that AAV-mediated gene transfer of human RPGRORF15 cDNA can rescue photoreceptors when delivered prior to the onset, or at an early stage of degeneration. We now investigate whether gene therapy delivered at a more advanced stage of disease can still provide a positive outcome.

Methods: An AAV2/5 vector construct (titer: 1.51 x 10¹¹ vg/ml) carrying full-length human RPGRORF15 cDNA under the control of a hIRBP promoter was injected subretinally in three 12-wk-old XLPRA2 dogs. At that age, there is on-going cell death and the ONL thickness is reduced by ~ 40%. In addition, one XLPRA2 dog was injected shortly after the onset of disease (5.1 wks of age) as an early disease stage control. Contra-lateral eyes were either injected with BSS, or received a similar dose of viral construct intravitreally. Photoreceptor structure and function was assessed by means of non-invasive retinal imaging (cSLO/ SD-OCT) and ERG at 39 and 42 weeks of age, respectively.

Results: In vivo retinal imaging showed preserved ONL thickness in the treated retinal areas. ERG function was greater in treated than in control eyes, with more than an 8-, 1.2-, and 1.6-fold difference in the amplitudes of rod, mixed rod-cone, and cone 29 Hz flicker responses, respectively. ONL thickness was better preserved in the animal treated at 5.1 weeks than in the 3 dogs injected at 12 weeks of age, and ERG amplitudes were in average 1.6-fold higher.

Conclusions: These results expand our recently published findings by showing that a sustained and beneficial effect on photoreceptor structure and retinal function can be achieved even when delivering RPGR gene augmentation at a mid-stage of XLRP disease. This has important translational application given that most patients are likely to have relatively advanced disease at the time of treatment.

Keywords: 538 gene transfer/gene therapy • 696 retinal degenerations: hereditary • 648 photoreceptors