Quantifying transduction efficiencies of unmodified and tyrosine capsid mutant AAV vectors in vitro using two ocular cell lines

PURPOSE

With the increasing number of retinal gene-based therapies and therapeutic constructs, in vitro bioassays characterizing vector transduction efficiency and quality are becoming increasingly important. Currently, in vitro assays quantifying vector transduction efficiency are performed predominantly for non-ocular tissues. A human retinal pigment epithelial cell line (ARPE19) and a mouse cone photoreceptor cell line, 661W, have been well characterized and are used for many retinal metabolism and biologic pathway studies. The purpose of this study is to quantify transduction efficiencies of a variety of self-complementary (sc) adeno-associated virus (AAV) vectors in these biologically relevant ocular cell lines using high-throughput fluorescence-activated cell sorting (FACS) analysis.

METHODS

ARPE19 and 661W cells were infected with sc-smCBA-mCherry packaged in unmodified AAV capsids or capsids containing single/multiple tyrosine-phenylalanine (Y-F) mutations at multiplicity of infections (MOIs) ranging from 100 to 10,000. Three days post infection fluorescent images verified mCherry expression. Following microscopy, FACS analysis was performed to quantify the number of positive cells and the mean intensity of mCherry fluorescence, the product of which is reported as transduction efficiency for each vector. The scAAV vectors containing cone-specific (sc-mCARpro-green fluorescent protein [GFP]), rod-specific (sc-MOPS500-eGFP), retinal pigment epithelium (RPE)-specific (sc-VMD2-GFP), or ubiquitous (sc-smCBA-GFP) promoters were used to infect both cell lines at an MOI of 10,000. Three days post infection, cells were immunostained with an antibody raised against GFP and imaged. Finally, based on our in vitro results, we tested a prediction of transduction efficiency in vivo.

RESULTS

Expression from unmodified scAAV1, scAAV2, scAAV5, and scAAV8 vectors was detectable by FACS in both ARPE19 and 661W cells, with scAAV1 and scAAV2 being the most efficient in both cell lines. scAAV5 showed moderate efficiency in both ARPE19 and 661W cells. scAAV8 was moderately efficient in 661W cells and was by comparison less so in ARPE19 cells; however, transduction was still apparent. scAAV9 performed poorly in both cell types. With some exceptions, the Y-F capsid mutations generally increased the efficiency of scAAV vector transduction, with the increasing number of mutated residues improving efficiency. Results for single scAAV1 and scAAV8 capsid mutants were mixed. In some cases, efficiency improved; in others, it was unchanged or marginally reduced. Retinal-specific promoters were also active in both cell lines, with the 661W cells showing a pattern consistent with the in vivo activity of the respective promoters tested. The prediction based on in vitro data that AAV2 sextuple Y-F mutants would show higher transduction efficiency in RPE relative to AAV2 triple Y-F capsid mutants was validated by evaluating the transduction characteristics of the two mutant vectors in mouse retina.

CONCLUSIONS

Our results suggest that this rapid and quantifiable cell-based assay using two biologically relevant ocular cell lines will prove useful in screening and optimizing AAV vectors for application in retina-targeted gene therapies.

Whirlin replacement restores the formation of the USH2 protein complex in whirlin knockout photoreceptors

PURPOSE

Whirlin is the causative gene for Usher syndrome type IID (USH2D), a condition manifested as both retinitis pigmentosa and congenital deafness. Mutations in this gene cause disruption of the USH2 protein complex composed of USH2A and VLGR1 at the periciliary membrane complex (PMC) in photoreceptors. In this study, the adeno-associated virus (AAV)-mediated whirlin replacement was evaluated as a treatment option.

METHODS

Murine whirlin cDNA driven by the human rhodopsin kinase promoter (hRK) was packaged as an AAV2/5 vector and delivered into the whirlin knockout retina through subretinal injection. The efficiency, efficacy, and safety of this treatment were examined using immunofluorescent staining, confocal imaging, immunoelectron microscopy, Western blot analysis, histologic analysis, and electroretinogram.

RESULTS

The AAV-mediated whirlin expression started at two weeks, reached its maximum level at 10 weeks, and lasted up to six months post injection. The transgenic whirlin product had a molecular size and an expression level comparable to the wild-type. It was distributed at the PMC in both rod and cone photoreceptors from the central to peripheral retina. Importantly, the transgenic whirlin restored the cellular localization and expression level of both USH2A and VLGR1 and did not cause defects in the retinal histology and function in the whirlin knockout mouse.

CONCLUSIONS

Whirlin transgene recruits USH2A and VLGR1 to the PMC and is sufficient for the formation of the USH2 protein complex in photoreceptors. The combined hRK and AAV gene delivery system could be an effective gene therapy approach to treat retinal degeneration in USH2D patients.

Efficient mutagenesis of the rhodopsin gene in rod photoreceptor neurons in mice

Dominant mutations in the rhodopsin gene, which is expressed in rod photoreceptor cells, are a major cause of the hereditary-blinding disease, autosomal dominant retinitis pigmentosa. Therapeutic strategies designed to edit such mutations will likely depend on the introduction of double-strand breaks and their subsequent repair by homologous recombination or non-homologous end joining. At present, the break repair capabilities of mature neurons, in general, and rod cells, in particular, are undefined. To detect break repair, we generated mice that carry a modified human rhodopsin-GFP fusion gene at the normal mouse rhodopsin locus. The rhodopsin-GFP gene carries tandem copies of exon 2, with an ISceI recognition site situated between them. An ISceI-induced break can be repaired either by non-homologous end joining or by recombination between the duplicated segments, generating a functional rhodopsin-GFP gene. We introduced breaks using recombinant adeno-associated virus to transduce the gene encoding ISceI nuclease. We found that virtually 100% of transduced rod cells were mutated at the ISceI site, with ∼85% of the genomes altered by end joining and ∼15% by the single-strand annealing pathway of homologous recombination. These studies establish that the genomes of terminally differentiated rod cells can be efficiently edited in living organisms.

DICER1 deficit induces Alu RNA toxicity in age-related macular degeneration

Geographic atrophy (GA), an untreatable advanced form of age-related macular degeneration, results from retinal pigmented epithelium (RPE) cell degeneration. Here we show that the microRNA (miRNA)-processing enzyme DICER1 is reduced in the RPE of humans with GA, and that conditional ablation of Dicer1, but not seven other miRNA-processing enzymes, induces RPE degeneration in mice. DICER1 knockdown induces accumulation of Alu RNA in human RPE cells and Alu-like B1 and B2 RNAs in mouse RPE. Alu RNA is increased in the RPE of humans with GA, and this pathogenic RNA induces human RPE cytotoxicity and RPE degeneration in mice. Antisense oligonucleotides targeting Alu/B1/B2 RNAs prevent DICER1 depletion-induced RPE degeneration despite global miRNA downregulation. DICER1 degrades Alu RNA, and this digested Alu RNA cannot induce RPE degeneration in mice. These findings reveal a miRNA-independent cell survival function for DICER1 involving retrotransposon transcript degradation, show that Alu RNA can directly cause human pathology, and identify new targets for a major cause of blindness.

AAV delivery of wild-type rhodopsin preserves retinal function in a mouse model of autosomal dominant retinitis pigmentosa

Autosomal dominant retinitis pigmentosa (ADRP) is frequently caused by mutations in RHO, the gene for rod photoreceptor opsin. Earlier, a study on mice carrying mutated rhodopsin transgenes on either RHO + / +  or RHO + /- backgrounds suggested that the amount of wild-type rhodopsin affected survival of photoreceptors. Therefore, we treated P23H RHO transgenic mice with adeno-associated virus serotype 5 (AAV5) expressing a cDNA clone of the rhodopsin gene (RHO301) that expressed normal opsin from the mouse opsin promoter. Analysis of the electroretinogram (ERG) demonstrated that increased expression of RHO301 slowed the rate of retinal degeneration in P23H mice: at 6 months, a-wave amplitudes were increased by 100% and b-wave amplitudes by 79%. In contrast, nontransgenic mice injected with AAV5 RHO301 demonstrated a decrease in the ERG, confirming the damaging effect of rhodopsin overproduction in normal photoreceptors. In P23H mice, the increase in the ERG amplitudes was correlated with improvement of retinal structure: the thickness of the outer nuclear layer in RHO301-treated eyes was increased by 80% compared with control eyes. These findings suggest that the wild-type RHO gene can be delivered to rescue retinal degeneration in mice carrying a RHO mutation and that increased production of normal rhodopsin can suppress the effect of the mutated protein. These findings make it possible to treat ADRP caused by different mutations of RHO with the expression of wild-type RHO.

XIAP therapy increases survival of transplanted rod precursors in a degenerating host retina

PURPOSE

To assess the survival of rod precursor cells transplanted into the Rd9 mouse, a model of X-linked retinal degeneration, and the effect of antiapoptotic therapy with X-linked inhibitor of apoptosis (XIAP) on preventing cell loss.

METHODS

Dissociated retinal cells from P4 Nrlp-GFP mice were transplanted into the subretinal space of 2-, 5-, and 8-month-old Rd9 mice. Histology, immunohistochemistry, and quantification of integrated cells were performed every month for up to 3 months after transplantation. XIAP delivery to donor cells was accomplished by transfection with adenoassociated virus (AAV-XIAP). Intraretinal activation of immune modulators was assessed using a quantitative real-time polymerase chain reaction-based immune response array.

RESULTS

GFP-positive rod precursors were able to integrate into the outer nuclear layer (ONL) of the Rd9 retina. Transplanted cells underwent morphologic differentiation with the formation of inner and outer segments and synaptic projections to bipolar cells. Integration of donor cells into the ONL increased as a function of host age at the time of transplantation. The number of integrated cells was maximal at 1 month after transplantation and then decreased with time. Survival of integrated cells was significantly increased when donor cells were pretreated with AAV-XIAP. We did not detect any donor cell-specific activation of inflammation within the host retina.

CONCLUSIONS

Survival of integrated cells decreases with time after transplantation but can be significantly increased with XIAP antiapoptotic therapy. Preventing programmed cell death through XIAP therapy may be an important component of future therapeutic retinal cell transplantation strategies.

Ab-externo AAV-mediated gene delivery to the suprachoroidal space using a 250 micron flexible microcatheter

Background

The current method of delivering gene replacement to the posterior segment of the eye involves a three-port pars plana vitrectomy followed by injection of the agent through a 37-gauge cannula, which is potentially wrought with retinal complications. In this paper we investigate the safety and efficacy of delivering adeno-associated viral (AAV) vector to the suprachoroidal space using an ab externo approach that utilizes an illuminated microcatheter.

Methods

6 New Zealand White rabbits and 2 Dutch Belted rabbits were used to evaluate the ab externo delivery method. sc-AAV5-smCBA-hGFP vector was delivered into the suprachoroidal space using an illuminated iTrackTM 250A microcatheter. Six weeks after surgery, the rabbits were sacrificed and their eyes evaluated for AAV transfection using immunofluorescent antibody staining of GFP.

Results

Immunostaining of sectioned and whole-mounted eyes demonstrated robust transfection in all treated eyes, with no fluorescence in untreated control eyes. Transfection occurred diffusely and involved both the choroid and the retina. No apparent adverse effects caused by either the viral vector or the procedure itself could be seen either clinically or histologically.

Conclusions

The ab externo method of delivery using a microcatheter was successful in safely and effectively delivering a gene therapy agent to the suprachoroidal space. This method presents a less invasive alternative to the current method of virally vectored gene delivery.

Gene therapy rescues cone structure and function in the 3-month-old rd12 mouse: a model for midcourse RPE65 leber congenital amaurosis

PURPOSE

RPE65 function is necessary in the retinal pigment epithelium (RPE) to generate chromophore for all opsins. Its absence results in vision loss and rapid cone degeneration. Recent Leber congenital amaurosis type 2 (LCA with RPE65 mutations) phase I clinical trials demonstrated restoration of vision on RPE65 gene transfer into RPE cells overlying cones. In the rd12 mouse, a naturally occurring model of RPE65-LCA early cone degeneration was observed; however, some peripheral M-cones remained. A prior study showed that AAV-mediated RPE65 expression can prevent early cone degeneration. The present study was conducted to test whether the remaining cones in older rd12 mice can be rescued.

METHODS

Subretinal treatment with the scAAV5-smCBA-hRPE65 vector was initiated at postnatal day (P)14 and P90. After 2 months, electroretinograms were recorded, and cone morphology was analyzed by using cone-specific peanut agglutinin and cone opsin-specific antibodies.

RESULTS

Cone degeneration started centrally and spread ventrally, with cells losing cone-opsin staining before that for the PNA-lectin-positive cone sheath. Gene therapy starting at P14 resulted in almost wild-type M- and S-cone function and morphology. Delaying gene-replacement rescued the remaining M-cones, and most important, more M-cone opsin-positive cells were identified than were present at the onset of gene therapy, suggesting that opsin expression could be reinitiated in cells with cone sheaths.

CONCLUSIONS

The results support and extend those of the previous study that gene therapy can stop early cone degeneration, and, more important, they provide proof that delayed treatment can restore the function and morphology of the remaining cones. These results have important implications for the ongoing LCA2 clinical trials.

Molecular anthropology meets genetic medicine to treat blindness in the North African Jewish population: human gene therapy initiated in Israel

The history of the North African Jewish community is ancient and complicated with a number of immigration waves and persecutions dramatically affecting its population size. A decade-long process in Israel of clinical-molecular screening of North African Jews with incurable autosomal recessive blindness led to the identification of a homozygous splicing mutation (c.95-2A > T; IVS2-2A > T) in RPE65, the gene encoding the isomerase that catalyzes a key step in the retinoid-visual cycle, in patients from 10 unrelated families. A total of 33 patients (four now deceased) had the severe childhood blindness known as Leber congenital amaurosis (LCA), making it the most common cause of retinal degeneration in this population. Haplotype analysis in seven of the patients revealed a shared homozygous region, indicating a population-specific founder mutation. The age of the RPE65 founder mutation was estimated to have emerged 100-230 (mean, 153) generations ago, suggesting it originated before the establishment of the Jewish community in North Africa. Individuals with this RPE65 mutation were characterized with retinal studies to determine if they were candidates for gene replacement, the recent and only therapy to date for this otherwise incurable blindness. The step from molecular anthropological studies to application of genetic medicine was then taken, and a representative of this patient subgroup was treated with subretinal rAAV2-RPE65 gene therapy. An increase in vision was present in the treated area as early as 15 days after the intervention. This process of genetically analyzing affected isolated populations as a screen for gene-based therapy suggests a new paradigm for disease diagnosis and treatment.

Novel properties of tyrosine-mutant AAV2 vectors in the mouse retina

Vectors based on adeno-associated virus serotype 2 (AAV2) have been used extensively in many gene-delivery applications, including several successful clinical trials for one type of Leber congenital amaurosis in the retina. Many studies have focused on improving AAV2 transduction efficiency and cellular specificity by genetically engineering its capsid. We have previously shown that vectors-containing single-point mutations of capsid surface tyrosines in serotypes AAV2, AAV8, and AAV9 displayed significantly increased transduction efficiency in the retina compared with their wild-type counterparts. In the present study, we evaluated the transduction characteristics of AAV2 vectors containing combinations of multiple tyrosine to phenylalanine mutations in seven highly conserved surface-exposed capsid tyrosine residues following subretinal or intravitreal delivery in adult mice. The multiply mutated vectors exhibited different in vivo transduction properties, with some having a unique ability of transgene expression in all retinal layers. Such novel vectors may be useful in developing valuable new therapeutic strategies for the treatment of many genetic diseases.

Leber hereditary optic neuropathy gene therapy clinical trial recruitment: year 1

OBJECTIVE

To describe the patient profiles of the Leber hereditary optic neuropathy (LHON) Gene Therapy Clinical Trial, year 1. This study aims to identify and characterize affected patients and carriers with the G11778A mutation in mitochondrial DNA for planned gene therapy that will use "allotopic expression" by delivering a normal nuclear-encoded ND4 gene into the nuclei of retinal ganglion cells via an adeno-associated virus vector injected into the vitreous.

METHODS

Patients with LHON with visual loss as well as asymptomatic maternally related family members were molecularly screened for ND1, ND4, and ND6 mutations in mitochondrial DNA commonly associated with LHON. All patients and maternal relatives also underwent complete neuro-ophthalmic examination, automated visual field testing, pattern electroretinogram (PERG), and OCT3.

RESULTS

Twenty-five subjects with LHON and 21 carriers positive for the G11778A mitochondrial DNA mutation were recruited. Three additional mutations in the ND4 gene, G11719A, G11947A, or G11914A, were detected. Mean retinal nerve fiber layer (RNFL) thickness was 78.3 μm up to 32 months after visual loss. It was 63.5 μm for all affected patients and 100.7 μm for carriers (P < .01). Mean PERG amplitude was lower in affected patients (40% of normal) than in carriers (94% of normal) (P < .01). Four carriers with PERG amplitudes less than 75% of normal had Early Treatment Diabetic Retinopathy Study acuity more than 20/25, mean defect more than -2 dB, and average RNFL thickness more than 80 μm.

CONCLUSIONS

Potential candidates for future gene therapy may include affected patients, as late as 32 months after loss of vision, with mildly reduced RNFL thickness or carriers with low PERG amplitudes and normal RNFL thickness, if the PERG amplitude is a predictor of conversion to LHON in these carriers.

Induction of rapid and highly efficient expression of the human ND4 complex I subunit in the mouse visual system by self-complementary adeno-associated virus

OBJECTIVE

To demonstrate the high efficiency and rapidity of allotopic expression of a normal human ND4 subunit of complex I in the vertebrate retina using a self-complementary adeno-associated virus (scAAV) vector for ocular gene delivery to treat acute visual loss in Leber hereditary optic neuropathy (LHON).

METHODS

The nuclear-encoded human ND4 subunit fused to the P1 isoform of subunit C of adenosine triphosphate synthase (ATPc) mitochondrial targeting sequence and FLAG epitope was packaged in scAAV2 capsids or single-stranded (ss) AAV2 capsids. These constructs were injected into the vitreous cavities of mice. The contralateral eyes were injected with scAAV-green fluorescent protein (GFP). One week later, pattern electroretinograms and gene expression of the human ND4 subunit and GFP were evaluated. Quantitative analysis of ND4FLAG-injected eyes was assessed relative to Thy1.2-labeled retinal ganglion cells (RGCs).

RESULTS

Pattern electroretinogram amplitudes remained normal in eyes inoculated with scAAV-ND4FLAG, ssAAV-ND4FLAG, and GFP. Confocal microscopy revealed the typical perinuclear mitochondrial expression of scAAV-ND4FLAG in almost the entire retinal flat mount. In contrast, scAAV-GFP expression was cytoplasmic and nuclear. Relative to Thy1.2-positive RGCs, quantification of scAAV-ND4FLAG-positive RGCs was 91% and that of ssAAV-ND4FLAG-positive RGCs was 51%.

CONCLUSION

Treatment of acute visual loss due to LHON may be possible with a normal human ND4 subunit gene of complex I, mutated in most cases of LHON, when delivered by an scAAV vector. Clinical Relevance Unlike most retinal degenerations that result in slowly progressive loss of vision over many years, LHON due to mutated mitochondrial DNA results in apoplectic, bilateral severe and usually irreversible visual loss. For rescue of acute visual loss in LHON, a highly efficient and rapid gene expression system is required.

Functional and behavioral restoration of vision by gene therapy in the guanylate cyclase-1 (GC1) knockout mouse

Background

Recessive mutations in guanylate cyclase-1 (Gucy2d) are associated with severe, early onset Leber congenital amaurosis-1(LCA1). Gucy2d encodes guanylate cyclase (GC1) is expressed in photoreceptor outer segment membranes and produces cGMP in these cells. LCA1 patients present in infancy with severely impaired vision and extinguished electroretinogram (ERG) but retain some photoreceptors in both their macular and peripheral retina for years. Like LCA1 patients, loss of cone function in the GC1 knockout (GC1KO) mouse precedes cone degeneration. The purpose of this study was to test whether delivery of functional GC1 to cone cells of the postnatal GC1KO mouse could restore function to these cells.

Methodology/Principal Findings

Serotype 5 AAV vectors containing either a photoreceptor-specific, rhodopsin kinase (hGRK1) or ubiquitous (smCBA) promoter driving expression of wild type murine GC1 were subretinally delivered to one eye of P14 GC1KO mice. Visual function (ERG) was analyzed in treated and untreated eyes until 3 months post injection. AAV-treated, isogenic wild type and uninjected control mice were evaluated for restoration of visual behavior using optomotor testing. At 3 months post injection, all animals were sacrificed, and their treated and untreated retinas assayed for expression of GC1 and localization of cone arrestin. Cone-mediated function was restored to treated eyes of GC1KO mice (ERG amplitudes were ∼45% of normal). Treatment effect was stable for at least 3 months. Robust improvements in cone-mediated visual behavior were also observed, with responses of treated mice being similar or identical to that of wild type mice. AAV-vectored GC1 expression was found in photoreceptors and cone cells were preserved in treated retinas.

Conclusions/Significance

This is the first demonstration of gene-based restoration of both visual function/vision-elicited behavior and cone preservation in a mammalian model of GC1 deficiency. Importantly, results were obtained using a well characterized, clinically relevant AAV vector. These results lay the ground work for the development of an AAV-based gene therapy vector for the treatment of LCA1.

Retinal disease in Rpe65-deficient mice: comparison to human leber congenital amaurosis due to RPE65 mutations

PURPOSE

To quantify the retinal disease in Rpe65-deficient mice across a wide age span and compare the results to those in humans with Leber congenital amaurosis (LCA) caused by RPE65 mutations.

METHODS

Full-field electroretinograms (ERGs) were recorded from wild-type (C57BL/6; Rpe65(+/+)) and Rpe65(-/-) mice at ages ranging from ∼1 month to 2 years. A physiologically based model of rod phototransduction activation was used to determine photoreceptor (P3) cell components of ERG photoresponses. A bipolar (P2) cell component was also derived. Photoreceptor and inner retinal thickness measurements were made by using optical coherence tomography in human RPE65-LCA.

RESULTS

Age-related declines in ERG photoreceptor and bipolar amplitudes were present in the Rpe65(-/-) mouse. The loss of photoresponse amplitude with age in the mutant mice paralleled reported losses of photoreceptor nuclear layer thickness over the same age range. Unexpectedly, the early activation phase of photoresponses in Rpe65(-/-) mice accelerated with age as amplitude decreased; this was not a feature of Rpe65(+/+) mice. Inner retinal dysfunction increased with age in the mutant mice. Human RPE65-LCA patients had retinal degeneration and loss of photoreceptors in the first decade of life. Unlike the mouse model, there were no examples of a normal photoreceptor complement. Abnormal thickening of the inner retina occurred with increasing loss of photoreceptors.

CONCLUSIONS

The differences in time course of murine and human RPE65-deficiency diseases suggests that preclinical efficacy testing of therapeutic modalities would be most informative when the murine disease becomes comparable to early human disease, toward the end of the first year of life in Rpe65(-/-) mice.

Diabetic eNOS-knockout mice develop accelerated retinopathy

PURPOSE

Dysfunction of endothelial nitric oxide synthase (eNOS) has been implicated in the pathogenesis of diabetic vascular complications. This study was undertaken to determine the role of eNOS in the development of diabetic retinopathy (DR), by investigating the functional consequences of its deficiency in the diabetic state.

METHODS

Diabetes was induced in eNOS-knockout (eNOS(-/-)) and C57B/6 mice by streptozotocin (STZ) injection. Retinal vasculature was evaluated by albumin extravasation, to quantitatively measure vascular permeability, and by trypsin-digested retinal vascular preparations, to quantify acellular capillaries. Gliosis was evaluated by immunofluorescent techniques. Retinal capillary basement membrane thickness was assessed by transmission electron microscopy. Total retinal nitric oxide level was assessed by measuring nitrate/nitrite using a fluorometric-based assay, iNOS expression was examined by real-time PCR.

RESULTS

Diabetic eNOS(-/-) mice exhibit more severe retinal vascular permeability than age-matched diabetic C57BL/6 mice, detectable as early as 3 weeks after diabetes induction. Diabetic eNOS(-/-) mice also show earlier onset and an increased number of acellular capillaries, sustained gliosis, and increased capillary basement membrane thickness. Total nitric oxide (NO) level was also increased, concomitant with elevated iNOS expression in diabetic eNOS(-/-) retina.

CONCLUSIONS

Diabetic eNOS(-/-) mice exhibit A significantly wider range of advanced retinal vascular complications than the age-matched diabetic C57BL/6 mice, supporting the notion that eNOS-derived NO plays an essential role in retinal vascular function. This mouse model also faithfully replicates many of the hallmarks of vascular changes associated with human retinopathy, thus providing a unique model to aid in understanding the pathologic mechanisms of and to develop effective therapeutic strategies for diabetic retinopathy.

rAAV2/5 gene-targeting to rods:dose-dependent efficiency and complications associated with different promoters

A prerequisite for using corrective gene therapy to treat humans with inherited retinal degenerative diseases that affect primarily rods is to develop viral vectors that target specifically this population of photoreceptors. The delivery of a viral vector with photoreceptor tropism coupled with a rod-specific promoter is likely to be the safest and most efficient approach to target expression of the therapeutic gene to rods. Three promoters that included a fragment of the proximal mouse opsin promoter (mOP), the human G-protein coupled receptor protein kinase 1 promoter (hGRK1), or the cytomegalovirus immediate early enhancer combined with the chicken beta actin proximal promoter CBA) were evaluated for their specificity and robustness in driving GFP reporter gene expression in rods, when packaged in a recombinant adeno-associated viral vector of serotype 2/5 (AAV2/5), and delivered via subretinal injection to the normal canine retina. Photoreceptor specific promoters (mOP, hGRK1) targeted robust GFP expression to rods, while the ubiquitously expressed CBA promoter led to transgene expression in the retinal pigment epithelium, rods, cones and rare Müller, horizontal and ganglion cells. Late onset inflammation was frequently observed both clinically and histologically with all three constructs when the highest viral titers were injected. Cone loss in the injected regions of the retinas that received the highest titers occurred with both the hGRK1 and CBA promoters. Efficient and specific rod transduction, together with preservation of retinal structure was achieved with both mOP and hGRK1 promoters when viral titers in the order of 10¹¹ vg/ml were used.

Gene therapy rescues cone function in congenital achromatopsia

The successful restoration of visual function with recombinant adeno-associated virus (rAAV)-mediated gene replacement therapy in animals and humans with an inherited disease of the retinal pigment epithelium has ushered in a new era of retinal therapeutics. For many retinal disorders, however, targeting of therapeutic vectors to mutant rods and/or cones will be required. In this study, the primary cone photoreceptor disorder achromatopsia served as the ideal translational model to develop gene therapy directed to cone photoreceptors. We demonstrate that rAAV-mediated gene replacement therapy with different forms of the human red cone opsin promoter led to the restoration of cone function and day vision in two canine models of CNGB3 achromatopsia, a neuronal channelopathy that is the most common form of achromatopsia in man. The robustness and stability of the observed treatment effect was mutation independent, but promoter and age dependent. Subretinal administration of rAAV5-hCNGB3 with a long version of the red cone opsin promoter in younger animals led to a stable therapeutic effect for at least 33 months. Our results hold promise for future clinical trials of cone-directed gene therapy in achromatopsia and other cone-specific disorders.

Self-complementary AAV-mediated gene therapy restores cone function and prevents cone degeneration in two models of Rpe65 deficiency

To test whether fast-acting, self-complimentary (sc), adeno-associated virus-mediated RPE65 expression prevents cone degeneration and/or restores cone function, we studied two mouse lines: the Rpe65-deficient rd12 mouse and the Rpe65-deficient, rhodopsin null ('that is, cone function-only') Rpe65(-/-)::Rho(-/-) mouse. scAAV5 expressing RPE65 was injected subretinally into one eye of rd12 and Rpe65(-/-)::Rho(-/-) mice at postnatal day 14 (P14). Contralateral rd12 eyes were injected later, at P35. Rd12 behavioral testing revealed that rod vision loss was prevented with either P14 or P35 treatment, whereas cone vision was only detected after P14 treatment. Consistent with this observation, P35 treatment only restored rod electroretinogram (ERG) signals, a result likely due to reduced cone densities at this time point. For Rpe65(-/-)::Rho(-/-) mice in which there is no confounding rod contribution to the ERG signal, cone cells and cone-mediated ERGs were also maintained with treatment at P14. This work establishes that a self-complimentary AAV5 vector can restore substantial visual function in two genetically distinct models of Rpe65 deficiency within 4 days of treatment. In addition, this therapy prevents cone degeneration but only if administered before extensive cone degeneration, thus supporting continuation of current Leber's congenital amaurosis-2 clinical trials with an added emphasis on cone subtype analysis and early intervention.

Selective tropism of the recombinant adeno-associated virus 9 serotype for rat cardiac tissue

BACKGROUND

Cardiac gene transfer may serve as a novel therapeutic approach for heart disease. Numerous serotypes of recombinant adeno-associated virus (rAAV) have been identified with variable tropisms to cardiac tissue.

METHODS

Both in vitro and in vivo experiments were undertaken to compare cardiac tropisms of rAAV-2, 5, 7, 8 and 9. For the in vitro studies, 10(7) vector genome (vg) of rAAV-2, 5, 7, 8 or 9 were used to transduce both rat neonatal cardiac myocytes (RNCM) and fibroblasts (RNCF). For the in vivo studies, 4 x 10(10) vg of rAAV-2, 5, 7, 8 or 9, and 4 x 10(11) vg of rAAV8 or 9 were administered in 5-day-old rats via a relatively non-invasive intracardiac injection. One and two months post-administration, green fluorescent protein (GFP) expression in tissues was visualized and GFP mRNA was quantified by the real-time polymerase chain reaction.

RESULTS

At 3 days post-viral transduction, rAAV9 and rAAV2 produced the highest transducing efficiency in RNCM. Only rAAV2 elicited any transduction in the RNCF. The results obtained in vivo indicated that the order for transduction efficiency in the heart was: rAAV9 > rAAV8 > rAAV7 > rAAV2 = rAAV5. The transduction efficiency order in the liver was: rAAV2 > rAAV5 > rAAV7 > rAAV8 > rAAV9. Injection of a higher dose (4 x 10(11) vg) of rAAV9 provided more widespread and highly cardiac-selective GFP expression in the heart than rAAV8. Zero to minimal expression of GFP was found in the lung and kidney for both doses of all rAAV serotypes utilized.

CONCLUSIONS

Collectively, the results obtained in the present study suggest that rAAV9 provides the most selective and stable transduction efficiency in cardiac tissue, and this expression was primarily exhibited in cardiac myocytes.

Self-complementary AAV5 vector facilitates quicker transgene expression in photoreceptor and retinal pigment epithelial cells of normal mouse

To clarify whether transduction efficiency and cell type specificity of self-complementary (sc) AAV5 vectors are similar to those of standard, single-stranded AAV5 vectors in normal retina, one micro liter of scAAV5-smCBA-GFP vector (1 x 10(12) genome-containing particles/ml) and AAV5-smCBA-GFP vector (1 x 10(12) genome-containing particles/ml) were subretinally or intravitreally (in both cases through the cornea) injected into the right and left eyes of adult C57BL/6J mice, respectively. On post-injection day (PID) 1, 2, 5, 7, 10, 14, 21, 28 and 35, eyes were enucleated; retinal pigment epithelium (RPE) wholemounts, neuroretinal wholemounts and eyecup sections were prepared to evaluate green fluorescent protein (GFP) expression by fluorescent microscopy. GFP expression following trans-cornea subretinal injection of scAAV5-smCBA-GFP vector was first detected in RPE wholemounts around PID 1 and in neuroretinal wholemounts between PID 2 and 5; GFP expression peaked and stabilized between PID 10-14 in RPE wholemounts and between P14 and P21 in neuroretinal wholemounts with strong, homogeneous green fluorescence covering the entire wholemounts. The frozen sections supported the following findings from the wholemounts: GFP expression appeared first in RPE around PID 1-2 and soon spread to photoreceptors (PR) cells; by PID 7, moderate GFP expression was found mainly in PR and RPE layers; between PID 14 and 21, strong and homogenous GFP expression was observed in RPE and PR cells. GFP expression following subretinal injection of AAV5-smCBA-GFP was first detected in RPE wholemounts around PID 5-7 and in neuroretinal wholemounts around PID 7-10; ssAAV5-mediated GFP expression peaked at PID 21 in RPE wholemounts and around PID 28 in neuroretinal wholemounts; sections from AAV5 treated eyes also supported findings obtained from wholemounts: GFP expression was first detected in RPE and then spread to the PR cells. Peak GFP expression in RPE mediated by scAAV5 was similar to that mediated by AAV5. However, peak GFP expression mediated by scAAV5 in PR cells was stronger than that mediated by AAV5. No GFP fluorescence was detected in any retinal cells (RPE wholemounts, neuroretinal wholemounts and retinal sections) after trans-cornea intravitreal delivery of either scAAV5-GFP or AAV5-GFP. Neither scAAV5 nor AAV5 can transduce retinal cells following trans-cornea intravitreal injection. The scAAV5 vector used in this study directs an earlier onset of transgene expression than the matched AAV5 vector, and has stronger transgene expression in PR cells following subretinal injection. Our data confirm the previous reports that scAAV vectors have an earlier onset than the standard, single strand AAV vectors (Natkunarajah et al., 2008; Yokoi et al., 2007). scAAV5 vectors may be more useful than standard, single-stranded AAV vector when addressing certain RPE and/or PR cell-related models of retinal dystrophy, particularly for mouse models of human retinitis pigmentosa that require rapid and robust transgene expression to prevent early degeneration in PR cells.

Achromatopsia as a potential candidate for gene therapy

Achromatopsia is an autosomal recessive retinal disease involving loss of cone function that afflicts approximately 1 in 30,000 individuals. Patients with achromatopsia usually have visual acuities lower than 20/200 because of the central vision loss, photophobia, complete color blindness and reduced cone-mediated electroretinographic (ERG) amplitudes. Mutations in three genes have been found to be the primary causes of achromatopsia, including CNGB3 (beta subunit of the cone cyclic nucleotide-gated cation channel), CNGA3 (alpha subunit of the cone cyclic nucleotide-gated cation channel), and GNAT2 (cone specific alpha subunit of transducin). Naturally occurring mouse models with mutations in Cnga3 (cpfl5 mice) and Gnat2 (cpfl3 mice) were discovered at The Jackson Laboratory. A natural occurring canine model with CNGB3 mutations has also been found. These animal models have many of the central phenotypic features of the corresponding human diseases. Using adeno-associated virus (AAV)-mediated gene therapy, we and others show that cone function can be restored in all three models. These data suggest that human achromatopsia may be a good candidate for corrective gene therapy.

Efficient expression of self-complementary AAV in ganglion cells of the ex vivo primate retina

PURPOSE

To evaluate the efficiency of self-complementary adeno-associated virus (scAAV)-mediated gene expression of green fluorescent protein (GFP) or the allotopic human ND4 subunit of complex I in ganglion cells of the primate retina.

METHODS

ScAAV2 containing the cDNA encoding the humanized GFP or allotopic ND4 subunit of complex I under the control of the cytomegalovirus (CMV) immediate early gene enhancer and short chicken beta-actin promoter-exon1-intron1 (CBA) was injected into the vitreous cavity of five primate eyes after enucleation. Following incubation in standard Dulbecco's Modified Eagle Medium (DMEM) culture media overnight at 37 degrees C with 5% CO(2), retinal flat mounts were probed with monoclonal GFP or FLAG antibodies overnight followed by counterstaining with anti-mouse IgG conjugated to cy2. For identification of retinal ganglion cells (RGCs), the retinal whole mounts were also stained with a Brn3a or Thy1.2 (protein expressed in RGCs. domain) antibody, then counterstained with cy3 or cy2. Immunofluorescence and colocalization were assessed using confocal microscopy. Quantitative analysis of GFP, ND4FLAG, Brn3a, or Thy1.2 expressing cells was performed using Image J software.

RESULTS

While the endogenous fluorescence of GFP was seen in a few retinal cells, GFP and ND4FLAG immunofluorescence was plentiful. The immunosignals were restricted to the inner retina and colocalized to slightly more than half of all cells expressing Brn3a or Thy1.2, suggesting efficient expression in RGCs.

CONCLUSIONS

Our findings suggest that the hybrid CMV enhancer-CBetaA promoter can play an efficient role in targeting primate RGCs following intravitreal gene delivery using the scAAV2 vector. Donated ex vivo primate eyes may serve as a model system for testing RGC expression before in vivo intravitreal injections of this and perhaps other AAV serotypes.

Human RPE65 gene therapy for Leber congenital amaurosis: persistence of early visual improvements and safety at 1 year

Human gene therapy with rAAV2-vector was performed for the RPE65 form of childhood blindness called Leber congenital amaurosis. In three contemporaneous studies by independent groups, the procedure was deemed safe and there was evidence of visual gain in the short term. At 12 months after treatment, our young adult subjects remained healthy and without vector-related serious adverse events. Results of immunological assays to identify reaction to AAV serotype 2 capsid were unchanged from baseline measurements. Results of clinical eye examinations of study and control eyes, including visual acuities and central retinal structure by in vivo microscopy, were not different from those at the 3-month time point. The remarkable improvements in visual sensitivity we reported by 3 months were unchanged at 12 months. The retinal extent and magnitude of rod and cone components of the visual sensitivity between 3 and 12 months were also the same. The safety and efficacy of human retinal gene transfer with rAAV2-RPE65 vector extends to at least 1 year posttreatment.