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. 2017 Aug 2;25(8):1866-1880.
doi: 10.1016/j.ymthe.2017.05.004. Epub 2017 May 27.

Optimization of Retinal Gene Therapy for X-Linked Retinitis Pigmentosa Due to RPGR Mutations

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Free PMC article

Optimization of Retinal Gene Therapy for X-Linked Retinitis Pigmentosa Due to RPGR Mutations

William A Beltran et al. Mol Ther. .
Free PMC article

Abstract

X-linked retinitis pigmentosa (XLRP) caused by mutations in the RPGR gene is an early onset and severe cause of blindness. Successful proof-of-concept studies in a canine model have recently shown that development of a corrective gene therapy for RPGR-XLRP may now be an attainable goal. In preparation for a future clinical trial, we have here optimized the therapeutic AAV vector construct by showing that GRK1 (rather than IRBP) is a more efficient promoter for targeting gene expression to both rods and cones in non-human primates. Two transgenes were used in RPGR mutant (XLPRA2) dogs under the control of the GRK1 promoter. First was the previously developed stabilized human RPGR (hRPGRstb). Second was a new full-length stabilized and codon-optimized human RPGR (hRPGRco). Long-term (>2 years) studies with an AAV2/5 vector carrying hRPGRstb under control of the GRK1 promoter showed rescue of rods and cones from degeneration and retention of vision. Shorter term (3 months) studies demonstrated comparable preservation of photoreceptors in canine eyes treated with an AAV2/5 vector carrying either transgene under the control of the GRK1 promoter. These results provide the critical molecular components (GRK1 promoter, hRPGRco transgene) to now construct a therapeutic viral vector optimized for RPGR-XLRP patients.

Keywords: AAV; RPGR; X-linked retinitis pigmentosa; canine model; gene therapy.

Figures

Figure 1
Figure 1
Recombinant AAV Vectors Used in the Study (A and B) rAAV vectors containing a human IRBP promoter (IRBP Pro) or human GRK1 promoter (GRK1 Pro) driving expression of a humanized enhanced GFP reporter gene (eGFP). (C–F) rAAV vectors containing a human GRK1 promoter or human IRBP promoter driving expression of different versions of the human RPGR cDNA. hRPGRstb and hRPGRco are two versions of human RPGRexon1-ORF15, with nucleotide sequence differences in exon ORF15 described in Figure S5. With the exception of bGH pA not being included in constructs (D) and (E), all other elements (SV40 SD/SA and poly A signals) within these constructs are identical. TR, AAV2-inverted terminal repeats; SV40 SD/SA, simian virus 40 splice donor/splice acceptor element; SV40 pA, simian virus 40 polyadenylation signal; bGH pA, bovine growth hormone polyadenylation signal.
Figure 2
Figure 2
GFP Expression in Macaque Photoreceptors 8 Weeks Post-subretinal Injection with High Titers of AAV2/5 Using IRBP or GRK1 Promoters (A) En face cSLO fundus image (displayed as equivalent right eye) showing GFP fluorescence in the central left retina of NHP #090365 treated with 100 μL of AAV2/5-IRBP-eGFP at 1 × 1012 vg/mL. Black arrow, fovea; asterisk, approximate location of immunolabeling images A4–A6. Retinal cross sections temporal to (A1), approximately within (A2), and nasal to (A3) the foveal pit, immunostained for cone arrestin (red) and counterstained with DAPI (blue) reveal a lack of rAAV2/5-IRBP-mediated GFP expression in central cones. White arrows in panels (A1) and (A3) delineate the eccentricity (∼1,000 μm) from the fovea at which IRBP-driven GFP expression limited to rods is observed. High-magnification images in this region (A4–A6) reveal the location of rod cell bodies (outlined with white line) that lack cone arrestin labeling (A4). GFP expression is restricted to these cone arrestin-negative cells (A5 and A6). (B) En face cSLO fundus image showing GFP fluorescence in the peripheral right retina of NHP #090365 treated with 100 μL of AAV2/5-IRBP-eGFP at 1 × 1012 vg/mL. Black arrow, fovea; asterisk, approximate location of immunolabeling images B1–B3. rAAV2/5-IRBP-mediated GFP expression is found in rod, but not cone photoreceptors, as evidenced by a lack of GFP expression (white arrows, red Xs in panel B2) in cone-arrestin-positive cells (yellow arrows, panels B1 and B3). En face cSLO fundus image showing GFP fluorescence in the central right (C) and peripheral left (displayed as equivalent right eye) (D) retinas of NHP #AV136 treated with 90–100 μL of AAV2/5-GRK1-eGFP at 1 × 1012 vg/mL. Black arrow, fovea; asterisk, approximate location of immunolabeling images D1–D3. Immunolabeling in retinal cross sections reveal AAV2/5-GRK1-mediated GFP expression in central cones (C1–C3), peripheral cones (white arrows in D1–D3) and rods (D1–D3). Scale bars, 10° (A–D), 100 μm (A2 and C1–C3), 33 μm (A1 and A3), and 17 μm (A4–A6, B1–B3, and D1–D3).
Figure 3
Figure 3
Dose Response Function and Long-Term Stability after Gene Therapy Intervention at Early- and Mid-stages of RPGR Disease (A–C) WT control ONL thickness topography (A) compared to injected RPGR mutant (B and C) dogs. WT control map is the mean of 12 eyes (age 16–198 weeks; mean = 56 weeks). RPGR mutant maps are representatives from a cohort of 16 eyes injected subretinally with BSS control or two titers of AAV2/5-GRK1-hRPGRstb at early- (B) (5 weeks) or mid-stage (C) (12 weeks) disease and imaged at age 102–106 weeks. Treatment boundaries are based on fundus photographs of the bleb taken at the time of the injection (dotted lines), and, if visible, demarcations apparent on infrared imaging at the time of scanning (dashed lines). All eyes shown as equivalent right eyes with optic nerve and major blood vessels (black), tapetum boundary (yellow), and fovea-like region (white ellipse) overlaid for ease of comparison. T, temporal; N, nasal retina. Z489-OD and similar labels designate the individual animal and eye. (D) Difference of ONL thickness from mean control (ONL fraction in log10 units) at individual retinal locations in uninjected and injected WT control and RPGR mutant eyes. For each grouping of injected eyes (lanes b–h), the retinal loci outside the largest treatment boundary are shown on the left of the lane, and loci inside the boundary are shown on the right. Colors differentiate exposure to vector (green, Tx) versus BSS or no exposure (red, UnTx). Uninjected WT control eyes are shown with gray. **p < 0.05, t test. Numbers of eyes contributing to each lane are shown.
Figure 4
Figure 4
Long-Term Preservation of Retinal Function after Gene Therapy with AAV2/5-GRK1-hRPGRstb in Dogs Treated at Mid-stage Disease (A) Representative ERG traces of rod (−1.74 log cd.s.m−2), mixed rod-cone (1.01 log cd.s.m−2) recorded dark adapted, and cone (1.01 log cd.s.m−2) responses to single stimuli or 29-Hz cone flicker (0.76 log cd.s.m−2) recorded light adapted at 103 weeks of age in an RPGR mutant dog treated at 12 weeks of age with 150 μL of a low viral titer (1.51 × 1011 vg/mL). (B) Mean (± SD) of all rod and cone ERG results recorded at 103 weeks of age from three RPGR mutant dogs treated at mid-stage disease with 150 μL of a low viral titer (1.51 × 1011 vg/mL). (C) Representative ERG traces in an RPGR mutant dog treated at 12 weeks of age with 150 μL of a high viral titer (1.51 × 1012 vg/mL). (D) Mean (± SD) of all rod and cone ERG results recorded at 103 weeks of age from three RPGR mutant dogs treated at mid-stage disease with 150 μL of a high viral titer (1.51 × 1012 vg/mL). Tx, treated; Ctrl, contralateral BSS injected; p ≤ 0.07; *p < 0.05; **p < 0.001 from paired t test between treated and contralateral eyes.
Figure 5
Figure 5
Long-Term Durability of Visual Behavior Rescue after Gene Therapy with AAV2/5-GRK1-hRPGRstb in RPGR Mutant Dogs Treated at Mid-stage Disease (A) Visually guided behavior in a forced two-choice Y maze of RPGR mutant dogs treated at 12 weeks of age and tested during six sessions between 116 and 127 weeks of age. The performance (mean ± SD) of treated versus contralateral eyes (20 trials per eye per session) are shown from dogs (n = 3 eyes per treatment group) injected with 150 μL of two different titers of viral vector (left and right panels). (B) Visually guided navigational skills in an obstacle-avoidance course under eight different ambient illuminations at 120–123 weeks of age. Top panels show the transit time (mean + SD) and bottom panels show the number of collisions (mean + SD) from the treated versus contralateral eyes of the same dogs shown in (A). Tx, treated; Ctrl, contralateral BSS injected; *p < 0.05; **p < 0.001 from comparisons between treated and contralateral eyes using the paired t test.
Figure 6
Figure 6
Comparative Efficacy of Two RPGR Gene Constructs at Preserving ONL Thickness (A and B) ONL thickness topography in individual eyes injected at 5 to 6 weeks of age with 70 μL of a AAV2/5-GRK1-viral vector (titer: 7.2 × 1011 vg/mL) carrying either hRPGRstb (A) or hRPGRco (B) gene constructs compared with uninjected control eyes. Treatment boundaries are based on fundus photographs of the bleb taken at the time of the injection (dotted lines), and, if visible, demarcations apparent on infrared imaging at the time of scanning (dashed lines). All eyes shown with optic nerve and major blood vessels (black), tapetum boundary (yellow), and fovea-like region (white ellipse) overlaid for ease of comparison. N, nasal retina. Z522-OD and similar labels designate the individual animal and eye. (C) Schematic describing the two methods of analysis performed to compare the efficacy of the two gene constructs. Analysis 1 uses an intra-retinal control and compares the paired loci across the treatment boundary in each eye. Analysis 2 uses the contralateral control eye and compares the loci within the treatment boundary (dashed outline) with loci at corresponding locations in uninjected contralateral eye. UnTx (red squares), untreated; Tx (green squares), treated with vector. (D) Treatment effect (difference in ONL thickness between treated and untreated retina in log10 units), quantified at ∼12 weeks after treatment using intra retinal (left) and contralateral (right) control to evaluate efficacy of hRPGRstb and hRPGRco gene constructs. Symbol with error bars represents mean (± SD) treatment effect for an individual eye in each group of RPGR mutant dogs. N.S., not significant, t test.
Figure 7
Figure 7
Histological and Immunohistochemical Comparison of Two RPGR Gene Constructs (A) Pseudocolor maps of ONL thickness at ∼17 weeks of age of the eyes of an RPGR mutant dog injected at 5.7 weeks of age with 70 μL of AAV vectors (titer: 7.2 × 1011 vg/mL) carrying either hRPGRco or hRPGRstb gene constructs. Dotted lines correspond to the border of the bleb based on fundus photographs taken at the time of the injection, and dashed lines correspond to demarcations apparent on infrared imaging at the time of imaging. Eyes are shown as equivalent right eyes with optic nerve and major blood vessels (black), tapetum boundary (yellow), and fovea-like region (white ellipse) overlaid for ease of comparison. N, nasal retina; T, temporal retina. (B–F) Retinal morphology and immunohistochemistry at 23 weeks of age of retinas shown in (A). (B1) H&E-stained section across the treatment boundary shown as a red bar in (A). (B2) H&E stain, higher magnifications view within the untreated (UnTx) and treated (Tx) areas. (C) IHC labeling of the two human RPGR transgene products. (D) Cone arrestin (hCA, red) and rod opsin (Rho, green) double IHC. (E) M/L-opsin IHC labeling. (F) S-opsin IHC labeling. Hoechst 33342 nuclear stain (blue) was used in (C–F). Asterisks, artifactual disruption during sectioning; white arrows point to opsin mislocalization in cones.

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