Two AAV2/5-mediated RPGR gene augmentation constructs driven by the GRK1 promoter rescue photoreceptor structure and function in a canine model of RPGR X-linked retinitis pigmentosa

Abstract

Purpose : A frameshift mutation in RPGR ORF15 causes an early onset (~ 5 wks of age) and rapidly progressive retinal degeneration in the XLPRA2 dog. Long-term rescue has been achieved by AAV2/5-mediated delivery at early, mid, and late stage disease of a stabilized version of human RPGR cDNA under the control of the IRBP promoter. A 292 nucleotide fragment of the human GRK1 promoter appears to promote transgene expression more efficiently than an IRBP promoter in cones of NHPs. Thus, we now compare the short-term efficacy of two AAV2/5 constructs carrying (under the control of the GRK1 promoter) either a codon-optimized RPGR cDNA (RPGR1) or the same stabilized RPGR cDNA (RPGR2) used in previous canine studies.

Methods : Seven XLPRA2 dogs (9 eyes) were treated at ~ 6 wks of age with a subretinal injection (70 μl; titer: 7.2 x 10¹¹ vg/ml) of either AAV2/5-GRK1-RPGR1 (5 eyes) or AAV2/5-GRK1-RPGR2 (4 eyes) and followed up for either 12 or 18 wks post-injection. Rescue of photoreceptor structure and function was assessed by clinical examination, cSLO/OCT retinal imaging, ERG, and histology/IHC at termination.

Results : Both vectors were clinically well tolerated. Rod-mediated ERG function was improved in 4 out of 5 RPGR1-treated eyes and 3 out of 3 RPGR2-treated eyes in comparison to non-injected eyes. At these ages persistent cone-mediated ERG function in untreated eyes precluded accurate assessment of rescue of cone function. Significant and similar preservation of ONL thickness was seen with both constructs in the treated areas of all 9 treated eyes by OCT imaging, and confirmed histologically. IHC revealed that RPGR transgene expression was found predominantly in rods and in rare cones in the treated area of all injected eyes but was associated with improved structure of both rods and cones, and correction of rod and cone opsin mislocalization. Rescue of bipolar cell dendritic arborization abnormalities also was found.

Conclusions : In this study rescue of both rods and cones in a canine model of XLRP was achieved following AAV2/5-GRK1-RPGR vector treatment. No significant differences in efficacy were observed when comparing a previously tested stabilized RPGR cDNA to a codon-optimized version.

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