Ribozyme gene therapy for autosomal dominant retinal disease

Endothelin-2-mediated protection of mutant photoreceptors in inherited photoreceptor degeneration

Expression of the Endothelin-2 (Edn2) mRNA is greatly increased in the photoreceptors (PRs) of mouse models of inherited PR degeneration (IPD). To examine the role of Edn2 in mutant PR survival, we generated Edn2^−/− mice carrying homozygous Pde6b^rd1 alleles or the Tg(RHO P347S) transgene. In the Edn2^−/− background, PR survival increased 110% in Pde6b^rd1/rd1 mice at post-natal (PN) day 15, and 60% in Tg(RHO P347S) mice at PN40. In contrast, PR survival was not increased in retinal explants of Pde6b^rd1/rd1; Edn2^−/− mice. This finding, together with systemic abnormalities in Edn2^−/− mice, suggested that the increased survival of mutant PRs in the Edn2^−/− background resulted at least partly from the systemic EDN2 loss of function. To examine directly the role of EDN2 in mutant PRs, we used a scAAV5-Edn2 cDNA vector to restore Edn2 expression in Pde6b^rd1/rd1; Edn2^−/− PRs and observed an 18% increase in PR survival at PN14. Importantly, PR survival was also increased after injection of scAAV5-Edn2 into Pde6b^rd1/rd1 retinas, by 31% at PN15. Together, these findings suggest that increased Edn2 expression is protective to mutant PRs. To begin to elucidate Edn2-mediated mechanisms that contribute to PR survival, we used microarray analysis and identified a cohort of 20 genes with >4-fold increased expression in Tg(RHO P347S) retinas, including Fgf2. Notably, increased expression of the FGF2 protein in Tg(RHO P347S) PRs was ablated in Tg(RHO P347S); Edn2^−/− retinas. Our findings indicate that the increased expression of PR Edn2 increases PR survival, and suggest that the Edn2-dependent increase in PR expression of FGF2 may contribute to the augmented survival.

Loss of daylight vision in retinal degeneration: are oxidative stress and metabolic dysregulation to blame?

Retinitis pigmentosa is characterized by loss of night vision, followed by complete blindness. Over 40 genetic loci for retinitis pigmentosa have been identified in humans, primarily affecting photoreceptor structure and function. The availability of excellent animal models allows for a mechanistic characterization of the disease. Metabolic dysregulation and oxidative stress have been found to correlate with the loss of vision, particularly in cones, the type of photoreceptors that mediate daylight and color vision. The evidence that these problems actually cause loss of vision and potential therapeutic approaches targeting them are discussed.

Novel p.M96T variant of NRL and shRNA-based suppression and replacement of NRL mutants associated with autosomal dominant retinitis pigmentosa

Mutations in the gene encoding the transcription factor neural retina leucine zipper (NRL) are known to cause autosomal dominant (adRP) or recessive (arRP) retinitis pigmentosa (RP). In an adRP Spanish family, we detected a novel sequence variation (c.287T>C) in the NRL gene that results in the p.M96T protein change. A functional test of the ability of NRL, in conjunction with cone-rod homeobox (CRX), to transactivate a human rhodopsin (RHO) promoter was used to evaluate the pathogenic mechanisms of NRL. We found upregulation of the RHO promoter by p.M96T protein similar to that shown by other missense NRL mutations that cause adRP. Affected RP patients of the family carry the nucleotide change, although two other family members that also carry the c.287T>C variation remain asymptomatic. This result complicates the genetic counselling of the family. The pathogenic mechanisms associated with adRP NRL mutations appear to be caused by a gain of function. To suppress the negative effect of an NRL mutant, the suppression and replacement strategy seems to be the most suitable therapeutic approach capable of overcoming the mutational heterogeneity associated with NRL-linked adRP. Thus, we evaluated this methodology in the NRL gene for the first time.

Rescue of photoreceptor degeneration by curcumin in transgenic rats with P23H rhodopsin mutation

The P23H mutation in the rhodopsin gene causes rhodopsin misfolding, altered trafficking and formation of insoluble aggregates leading to photoreceptor degeneration and autosomal dominant retinitis pigmentosa (RP). There are no effective therapies to treat this condition. Compounds that enhance dissociation of protein aggregates may be of value in developing new treatments for such diseases. Anti-protein aggregating activity of curcumin has been reported earlier. In this study we present that treatment of COS-7 cells expressing mutant rhodopsin with curcumin results in dissociation of mutant protein aggregates and decreases endoplasmic reticulum stress. Furthermore we demonstrate that administration of curcumin to P23H-rhodopsin transgenic rats improves retinal morphology, physiology, gene expression and localization of rhodopsin. Our findings indicate that supplementation of curcumin improves retinal structure and function in P23H-rhodopsin transgenic rats. This data also suggest that curcumin may serve as a potential therapeutic agent in treating RP due to the P23H rhodopsin mutation and perhaps other degenerative diseases caused by protein trafficking defects.

Genetics and molecular pathology of Stargardt-like macular degeneration

Stargardt-like macular degeneration (STGD3) is an early onset, autosomal dominant macular degeneration. STGD3 is characterized by a progressive pathology, the loss of central vision, atrophy of the retinal pigment epithelium, and accumulation of lipofuscin, clinical features that are also characteristic of age-related macular degeneration. The onset of clinical symptoms in STGD3, however, is typically observed within the second or third decade of life (i.e., starting in the teenage years). The clinical profile at any given age among STGD3 patients can be variable suggesting that, although STGD3 is a single gene defect, other genetic or environmental factors may play a role in moderating the final disease phenotype. Genetic studies localized the STGD3 disease locus to a small region on the short arm of human chromosome 6, and application of a positional candidate gene approach identified protein truncating mutations in the elongation of very long chain fatty acids-4 gene (ELOVL4) in patients with this disease. The ELOVL4 gene encodes a protein homologous to the ELO group of proteins that participate in fatty acid elongation in yeast. Pathogenic mutations found in the ELOVL4 gene result in altered trafficking of the protein and behave with a dominant negative effect. Mice carrying an Elovl4 mutation developed photoreceptor degeneration and depletion of very long chain fatty acids (VLCFA). ELOVL4 protein participates in the synthesis of fatty acids with chain length longer than 26 carbons. Studies on ELOVL4 indicate that VLCFA may be necessary for normal function of the retina, and the defective protein trafficking and/or altered VLCFA elongation underlies the pathology associated with STGD3. Determining the role of VLCFA in the retina and discerning the implications of abnormal trafficking of mutant ELOVL4 and depleted VLCFA content in the pathology of STGD3 will provide valuable insight in understanding the retinal structure, function, and pathology underlying STGD3 and may lead to a better understanding of the process of macular disease in general.

Gene therapy in the Retinal Degeneration Slow model of retinitis pigmentosa

Human blinding disorders are often initiated by hereditary mutations that insult rod and/or cone photoreceptors and cause subsequent cellular death. Generally, the disease phenotype can be predicted from the specific mutation as many photoreceptor genes are specific to rods or cones; however certain genes, such as Retinal Degeneration Slow (RDS), are expressed in both cell types and cause different forms of retinal disease affecting rods, cones, or both photoreceptors. RDS is a transmembrane glycoprotein critical for photoreceptor outer segment disc morphogenesis, structural maintenance, and renewal. Studies using animal models with Rds mutations provide valuable insight into Rds gene function and regulation; and a better understanding of the physiology, pathology, and underlying degenerative mechanisms of inherited retinal disease. Furthermore, these models are an excellent tool in the process of developing therapeutic interventions for the treatment of inherited retinal degenerations. In this paper, we review these topics with particular focus on the use of rds models in gene therapy.

Anti-clarin-1 AAV-delivered ribozyme induced apoptosis in the mouse cochlea

Usher syndrome type 3 is caused by mutations in the USH3A gene, which encodes the protein clarin-1. Clarin-1 is a member of the tetraspanin superfamily (TM4SF) of transmembrane proteins, expressed in the organ of Corti and spiral ganglion cells of the mouse ear. We have examined whether the AAV-mediated anti-clarin ribozyme delivery causes apoptotic cell death in vivo in the organ of Corti. We used an AAV-2 vector delivered hammerhead ribozyme, AAV-CBA-Rz, which specifically recognizes and cleaves wild type mouse clarin-1 mRNA. Cochleae of CD-1 mice were injected either with 1mul of the AAV-CBA-Rz, or control AAV vectors containing the green fluorescent protein (GFP) marker gene (AAV-CBA-GFP). Additional controls were performed with saline only. At one-week and one-month post-injection, the animals were sacrificed and the cochleae were studied by histology and fluorescence imaging. Mice injected with AAV-CBA-GFP displayed GFP reporter expression of varying fluorescence intensity throughout the length of the cochlea in the outer and inner hair cells and stria vascularis, and to a lesser extent, in vestibular epithelial cells. GFP expression was not detectable in the spiral ganglion. The pro-apoptotic effect of AAV-CBA-delivered anti-clarin-1 ribozymes was evaluated by TUNEL-staining. We observed in the AAV-CBA-Rz, AAV-CBA-GFP and saline control groups apoptotic nuclei in the outer and inner hair cells and in the stria vascularis one week after the microinjection. The vestibular epithelium was also observed to contain apoptotic cells. No TUNEL-positive spiral ganglion neurons were detected. After one-month post-injection, the AAV-CBA-Rz-injected group had significantly more apoptotic outer and inner hair cells and cells of the stria vascularis than the AAV-CBA-GFP group. In this study, we demonstrate that AAV-CBA mediated clarin-1 ribozyme may induce apoptosis of the cochlear hair cells and cells of the stria vascularis. Surprisingly, we did not observe apoptosis in spiral ganglion cells, which should also be susceptible to clarin-1 mRNA cleavage. This result may be due to the injection technique, the promoter used, or tropism of the AAV serotype 2 viral vector. These results suggest the role of apoptosis in the progression of USH3A hearing loss warrants further evaluation.

Adeno-associated virus-vectored gene therapy for retinal disease

Recombinant adeno-associated viral (AAV) vectors have become powerful gene delivery tools for the treatment of retinal degeneration in a variety of animal models that mimic corresponding human diseases. AAV vectors possess a number of features that render them ideally suited for retinal gene therapy, including a lack of pathogenicity, minimal immunogenicity, and the ability to transduce postmitotic cells in a stable and efficient manner. In the sheltered environment of the retina, AAV vectors are able to maintain high levels of transgene expression in the retinal pigmented epithelium (RPE), photoreceptors, or ganglion cells for long periods of time after a single treatment. Each cell type can be specifically targeted by choosing the appropriate combination of AAV serotype, promoter, and intraocular injection site. The focus of this review is on examples of AAV-mediated gene therapy in those animal models of inherited retinal degeneration caused by mutations directly affecting the interacting unit formed by photoreceptors and the RPE. In each case discussed, expression of the therapeutic gene resulted in significant recovery of retinal structure and/or visual function. Because of the key role of the vasculature in maintaining a healthy retina, a summary of AAV gene therapy applications in animal models of retinal neovascular diseases is also included.

Applications of nucleic acid technology in the CNS

This review examines applications of nucleic acid technology in the form of catalytic nucleic acids (ribozymes and DNAzymes) and RNA interference (RNAi) in the CNS. The basic mechanism of catalytic nucleic acids and RNAi is reviewed, and potentials and problems highlighted. Recent advances in chemical modifications and delivery techniques are summarized. Applications in the CNS, including their use in primary neuronal cells, organotypic slice culture and the brain in vivo are further discussed.

P2Y(2) receptor agonist INS37217 enhances functional recovery after detachment caused by subretinal injection in normal and rds mice

PURPOSE

To evaluate the effects of INS37217 on the recovery of retinal function after experimental retinal detachment induced by subretinal injection.

METHODS

Subretinal injections of 1 micro L of fluorescent microbeads, saline, or INS37217 (1-200 micro M) were made by the transvitreal method in normal (C57BL/6) mice and in mice heterozygous for the retinal degeneration slow (rds) gene. Control, mock-injected animals underwent corneal puncture without injection. Histologic and ERG evaluations were made at 0 to 1 and 8 hours, and 1, 3, 7, 10, 14, and 60 days post injection (PI). DNA fragmentation was evaluated by terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphate-biotin nick end labeling (TUNEL).

RESULTS

A single subretinal injection of saline solution containing fluorescent beads caused a histologically evident retinal detachment and distributed the microbeads to almost all the subretinal space. Spontaneous reattachment occurred within 24 hours after injection and was accompanied by evident retinal folding that appeared largely resolved by 6 days later. Relative to controls, injection of saline resulted in approximately 40% recovery of dark-adapted a-wave amplitude at 24 hours PI and gradually improved to approximately 90% of controls at 2 months PI. Subretinal injection of saline containing INS37217 (10 micro M) significantly increased rod and cone ERG of normal and rds(+/-) mice at 1 and 10 days PI, when compared with injection of saline alone. Additionally, INS37217 reduced the number of TUNEL-positive photoreceptors and the enhanced rate of reattachment.

CONCLUSIONS

Enhancement of ERG recovery by INS37217 is likely due to reduced retinal folding and cell death associated with detachment. These results support the use of INS37217 to help restore function after therapies that involve subretinal administration of drugs in animal models of retinal diseases.

Comparison of the clinical expression of retinitis pigmentosa associated with rhodopsin mutations at codon 347 and codon 23

PURPOSE

To examine the difference in expression of retinitis pigmentosa from mutations at codon 23 and codon 347 or rhodopsin; to report a novel mutation in rhodopsin.

METHODS

Goldmann perimetry (solid angle of I4e isopter) and electroretinographic amplitudes (square root transform of a response ratio) were analyzed for 24 patients with mutations at codon 347 (15 with Pro347Ala, 2 with Pro347Gln, 6 with Pro347Leu, and 1 with a novel Pro347Cys change) and 41 patients with mutations at codon 23 (6 with Pro23Ala; 35 with Pro23His).

RESULTS

When all patients with mutations at codons 347 and 23 were compared, loss of visual fields was significantly worse in patients with codon 347 changes (P =.0003). Only rod responses of the electroretinograms were significantly different between the two groups (P =.048). Specific comparison of Pro347Ala with Pro23Ala using regression analysis demonstrated significant differences in severity between codon 23 and codon 347 patients for b-wave amplitudes of rod (P =.0069), cone (P =.039) and maximum combined response (P =.049). The solid angle of the I4e isopter was also significantly different (P =.025) between the groups after controlling for age. Modeling age by group for Pro347Ala comparison produced an R(2) of.44.

CONCLUSION

We reconfirmed that rhodopsin-related retinitis pigmentosa from mutations involving codon 347 produces a more severe phenotype than that involving codon 23. Accurate modeling of disease was shown to be possible by incorporating the effects of a patient's age and specific genotype. Therefore, both of these variables must be considered in prognostic counseling and subject recruitment for future therapeutic trials.

Gene therapy for genetic and acquired retinal diseases

We present an overview of the current status of basic science and translational research being applied to gene therapy for eye disease, focusing on diseases of the retina. We discuss the viral and nonviral methods being used to transfer genes to the retina and retinal pigment epithelium, and the advantages and disadvantages of each approach. We review the various genetic and somatic treatment strategies that are being used for genetically determined and acquired diseases of the retina, including gene replacement, gene silencing by ribozymes and antisense oligonucleotides, suicide gene therapy, antiapoptosis, and growth factor therapies. The rationales for the specific therapeutic approaches to each disease are discussed. Schematics of gene transfer methods and therapeutic approaches are presented together with a glossary of gene transfer terminology.