Abstract
Purpose :
AAV-mediated RPGR gene augmentation therapy has been shown to rescue photoreceptors for more than 2 years when delivered at early (ONL thickness: ~ 100% of normal) mid (~ 60% of normal) and late (~40-50% of normal) stages of disease in the XLPRA2 dog, a canine model of XLRP caused by a frameshift mutation in RPGR-ORF15. Dogs from these previous studies were followed up (> 6.5 years post-injection) to evaluate the long-term effects of this gene therapy on retinal and visual function.
Methods :
5 XLPRA2 dogs underwent regular full-field ERG recordings and visual behavior testing that included performance in an obstacle-avoidance course under 7 different conditions of ambient illuminations (0.003 to 646 Lux) and ability to identify a low intensity (10-7 mW/cm2) blue (470 nm) flickering (3 Hz) LED light in a forced 2-choice Y maze. These dogs had previously been subretinally-injected in one eye at either early (n=1), mid (n= 2), or late (n=2) stage disease with an AAV2/5 vector (titer: 1.5 x1011 vg/mL) carrying a stabilized version of the human RPGR cDNA (hRPGRstb) under control of the human IRBP promoter. ERG and visual behavior parameters from the treated and contralateral/control eyes were compared at all time-points.
Results :
All 5 dogs showed in the treated eyes robust rod, mixed rod-cone, and cone ERG waveforms and amplitudes that were preserved throughout the 6.5 - 6.9 year post-injection period while in the untreated eyes rod-mediated responses were no longer recordable by 1 year of age, and mixed rod-cone and cone responses were lost by ~ 3 years of age. Transit time and number of collisions in the obstacle avoidance course were significantly lower for treated versus control eyes of all 5 dogs particularly under scotopic/mesopic (0.03-1 Lux) conditions, and worsened over time in the control eyes. Ability to identify in the Y maze a dim blue light that stimulates rods was lost as early as 2.8 years of age in control eyes but was preserved in 4 of 5 treated eyes for > 6.5 years. In 1 of 5 eyes (mid stage Tx) this visual function was lost by 4.2 years of age.
Conclusions :
Retinal and visual function was preserved for more than 6.5 years following RPGR gene therapy in a naturally-occurring canine model of XLRP with an AAV2/5-IRBP-hRPGRstb vector. Long-term stability of both rod and cone-mediated function supports use of this stabilized transgene in the context of a clinical trial.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.