Reversal of blindness in animal models of leber congenital amaurosis using optimized AAV2-mediated gene transfer

Abstract

We evaluated the safety and efficacy of an optimized adeno-associated virus (AAV; AAV2.RPE65) in animal models of the RPE65 form of Leber congenital amaurosis (LCA). Protein expression was optimized by addition of a modified Kozak sequence at the translational start site of hRPE65. Modifications in AAV production and delivery included use of a long stuffer sequence to prevent reverse packaging from the AAV inverted-terminal repeats, and co-injection with a surfactant. The latter allows consistent and predictable delivery of a given dose of vector. We observed improved electroretinograms (ERGs) and visual acuity in Rpe65 mutant mice. This has not been reported previously using AAV2 vectors. Subretinal delivery of 8.25 x 10(10) vector genomes in affected dogs was well tolerated both locally and systemically, and treated animals showed improved visual behavior and pupillary responses, and reduced nystagmus within 2 weeks of injection. ERG responses confirmed the reversal of visual deficit. Immunohistochemistry confirmed transduction of retinal pigment epithelium cells and there was minimal toxicity to the retina as judged by histopathologic analysis. The data demonstrate that AAV2.RPE65 delivers the RPE65 transgene efficiently and quickly to the appropriate target cells in vivo in animal models. This vector holds great promise for treatment of LCA due to RPE65 mutations.

Figures

Figure 1. Recovery of adeno-associated virus 2 (AAV2) vector following dilution and passage through administration devices

(a) Stock AAV2. RPE65 vector diluted to a target working concentration of 1 × 1011 vector genome (vg)/ml in phosphate-buffered saline (PBS) supplement (+PF68) or not (−PF68) with Pluronic F68 (PF68) (0.001%) was drawn into 1-mL syringes, vector was passed through device A, B or device C, and the concentration of recovered vector was measured by quantitative- polymerase chain reaction (Q-PCR). (b) AAV2.RPE65 vector diluted to a target working concentration of 1 × 1011 vg/ml in PBS supplemented with PF68 (0.001%) was drawn into 1-ml syringes, incubated for 2 or 20 hours, then passed through device A or device B, and the concentration of recovered vector measured by Q-PCR. (c) AAV2.LacZ vector subjected to the same procedure described for AAV2.RPE65 in b was tested for transduction titer as described in Materials and Methods. Quantitative post-device recovery of vector titer and functional activity provide assurance of consistent vector dosing under conditions of immediate or delayed use that may occur in a surgical suite.

Figure 2. Improvement in visual acuity in mice correlates with immunohistochemical localization of RPE65 in Rpe cells after sub-retinal injection with AAV2-RPE65

(a) Post-treatment optokinetic (Optomotry) responses of treated and control eyes of four rd12 (Rpe65 null) mice and of a control (phenotypically normal) C57Bl/6 mouse. There is significant improvement in visual acuity in eyes of all rd12 mice injected subretinally with AAV2-RPE65 compared with the control eye (left eye of animal 1). *P ≤ 0.005; **P ≤ 0.02. Testing was performed 1 month post-injection. The experimenter was unaware of the age, and treatment paradigm of the tested animals and the direction of rotation of the sinusoidal pattern presented to the animal. Mice were tested under photopic conditions during their daytime light cycle and four separate trials were performed per animal. RPE65 protein is detectable by immunohistochemistry in (b) the rpe (only) of the right, subretinally injected eye, but not in (c) the contralateral, uninjected (control) eye. These are the eyes from animal #1, shown in a. Upper panels in b and c show RPE65-specific fluorescence (arrows); lower panels show 4,6-diamidino-2-phenylindole (DAPI) fluorescence. Immunohistochemistry was performed 1 month post-injection. GCL, ganglion cell layer; INL, inner nuclear layer; ONL, outer nuclear layer; RPE, retinal pigment epithelium.

Figure 3. Nystagmus wave forms of both eyes of dog BR334 (a) before and (b) 4 weeks after subretinal injection in the left eye

a Nystagmus is present in both eyes with comparable characteristics, consisting of a large amplitude (~4° peak-to-peak, p-p), lower-frequency (~3 Hz) predominantly jerk waveform combined with a smaller amplitude (~1–2° p-p), high-frequency (~10 Hz) pendular oscillation. b One month post-treatment, the jerk component has been eliminated, leaving only the underlying pendular oscillation. At times, the amplitude of the nystagmus in the left (subretinally injected) eye is approximately half of that in the right eye. This study presents additional evidence of the strong yoking between the movements of the two eyes, with smooth pursuit having dominance over nystagmus.

Figure 4. Electroretinogram responses in a phenotypically normal dog; an untreated RPE65 mutant dog (BR336) and at the 5-week and 3-month timepoints after bilateral subretinal injections in an affected dog (BR332)

Waveforms developing after exposure to a light intensity series ranging from 0.000577 to 10.26 cd.s/m2 (from the bottom to the top of the series) are shown. Note the waveforms present in the third to lowest light intensity of both eyes in BR332 at the 5-week timepoint as compared with the control animal M580 where a waveform developed at the second to lowest light intensity. Arrows, waveform developing at lowest light intensity. Arrowheads, the first appearance of a photoreceptor “a-wave” in the light intensity series.

Figure 5. Timecourse and amplitudes of electroretinograms (ERGs) in treated and control dogs

(a) Same timing of ERG responses in eyes of a phenotypically normal dog (M580) and an affected dog (BR334 left) injected subretinally with AAV2.RPE65. The waveform for BR334 is shown at the 5-week timepoint; (b) Comparison of ERGs at 3 months after injection between Lancelot (BR33), and BR332 and BR334. Note that the amplitudes, waveforms, and timecourse of the waveforms are similar between Lancelot and the subretinally injected eyes of BR332 and BR334. Photoreceptor response is more sensitive in BR332 OU and BR334 left as judged by the presence of a waves (arrowheads) after a stimulus of lower light intensity than resulted in “a waves” in Lancelot. The “a waves” in BR332 also have larger amplitude than those in Lancelot (after they were elicited at identical light intensities).

Figure 6. Minimal histopathological changes are identified after sub-retinal injection of AAV2-RPE65

(a) In the majority of the treated retinas, cell layers [ganglion cell layer (gcl) inner nuclear layer (inl), and outer nuclear layer (onl)] are of normal thickness and retinal pigment epithelium (rpe) appears normal. Focal abnormalities in these injected eyes include: (b) healed retinotomy sites (one per eye) which are characterized by a transretinal defect (apparent at the left of the lower panel) and, occasionally, cystic structures (stars) in the adjacent neural retina. Arrows from the low magnification view in the lower panel indicate the region shown at higher magnification in the upper panel; (c) focal detachment with subretinal microglia (arrows); (d) focal defects in reattachment of the neural retina with resultant outpocketing of this tissue; (e) lesions characteristic of RPE65 deficiency, including multifocal rpe atrophy (thin arrow), rpe hypertrophy (arrowheads), and atrophy of the overlying photoreceptor layer (resulting in a thinned onl). Lesions such as those identified histologically in c and d correlated with (f, g) retinal folds visible in retinal eyecups prior to sectioning. Arrows in f, g indicate “ripples” where the neural retina had not reapposed completely to the underlying rpe.