Nanoparticle cDNA Delivery In Animal Models of Retinal Dystrophies

 

Compacted DNA (cDNA) nanoparticles are being tested as vehicles for transferring genes into cells of the eye. In this compacted form, the DNA can enter the nuclear membrane of cells, where it must be delivered to be effective.  Nanoparticle delivery is being researched as an alternative to virus-mediated gene delivery.

 

Following injection of the nanoparticles into the eye, gene expression has been documented in photoreceptors, retinal pigment epithelial cells, retinal ganglion cells, and in other cells of the eye. Researchers have now shown that the nanoparticles can be targeted to specific locations by controlling the injection site (e.g., subretinal, intravitreal) and that gene expression levels are dose-dependent and sustained. In animal models of inherited retinal dystrophies, subretinal injections transfected most photoreceptors and produced nearly normal levels of gene expression. This produced significant improvements in photoreceptor function and structure and restoration of retinal phenotype. No toxicity was associated with the treatment which makes it promising as a safe non-viral delivery mechanism for supplying therapeutic genes to the retina and other tissues of the eye.

 

(M.I. Naash)

Light-stimulated LiGluR Drives Visual Response in Retinal Ganglion Cells in RP Mice

Along these same lines was a report by John Flannery and colleagues who used an adeno-associated virus (AAV2) to deliver the engineered light-activated glutamate receptor (LiGluR) to the retinal in several different mouse models of retinitis pigmentosa. LiGluR successfully integrated into retinal ganglion cell membranes and, when prompted by light of various wavelengths, responded by opening ion channels, which triggered action potentials. This technique is promising for retinal conditions where photoreceptors cells are damaged or degenerated but retinal ganglion cells remain intact. The researchers further demonstrated that the depolarization started by the light stimulation was propagated along the visual pathway to the visual cortex. One of the beauties of this optogenetics approach is that light is non-invasive. The technique requires no implanted electrodes or other devices. The light can be projected onto discrete or diffuse regions of the retina and at various depths to produce responses that will hopefully result in recognizable visual patterns in humans with late stage retinal degeneration.

(N. Caporale, K.D. Kolstad, I. Tochitsky, D. Dalkara, T. Huang, D. Trauner, R.H. Kramer, Y. Dan, E.Y. Isacoff, J.G. Flannery)

 

Increased Macular Thickness in CNTF Therapy Attributed to ONL

Weng Tao and her colleagues reported at ARVO 2010 their additional findings about the encapsulated cell technology (ECT) intraocular implant containing genetically modified cells that secrete ciliary neurotrophic factor (CNTF). The implant releases CNTF in a sustained fashion. The hope is that it will protect photoreceptor cells in patients with retinal degenerative disorders. CNTF is being tested in clinical trials in patients with retinitis pigmentosa and has demonstrated a positive biological effect.

 

One of the effects of CNTF is a dose-dependent, statistically significant increase in macular thickness involving the photoreceptor cell layer. In the current report, the researchers reported on their use of Fourier domain (fd)OCT to investigate the source of the thickening. (fd)OCT, compared to other OCT technologies, greatly enhances depth resolution in tissues and increases the detail scientists can see and the measurements they can make of the retina. Their results showed that the thickness is due primarily to a difference in the width of the outer nuclear layer (ONL). The ONL is composed of the cell bodies of the rods and cones. The thickening could represent increased transcriptional and metabolic activity of the cells. Whether it presages functional improvement is yet to be determined.

 

The next step for the scientists is a Phase II efficacy study in RP patients to determine whether CNTF can slow degeneration of cells and increase visual activity.

(D.G. Birch, K.G. Locke, H. Patel, W. Tao)

 

Gene Therapy Clinical Trial Aims to Identify a Subset of LHON Patients and Carriers

 

Researchers at ARVO 2010 reported on enrollment (n=46) in a gene therapy clinical trial involving Leber hereditary optic neuropathy (LHON) patients. The ultimate goal of the research is to improve visual function by replacing a mutated gene in retinal ganglion cell mitochondrial DNA (mtDNA).

 

LHON was the first neurodegenerative disease for which a mitochondrial pattern of inheritance was described. It follows a maternal inheritance pattern and is characterized by retinal ganglion cell degeneration, retinal nerve fiber degeneration, and vision loss. Three primary mutations (at nucleotide pair 3460, 11778, and 14484) are generally agreed to be the primary causes of most cases of LHON.  The scientists are studying carriers and patients with the G1178A mutation. The G1178A mutation results in an amino acid substitution in the ND4 gene product. There is currently no treatment for LHON.

 

The mitochondrial ND4 gene provides instructions for making a protein called NADH dehydrogenase 4. The protein is part of an enzyme complex involved in energy production. The goal of treatment is to rescue retinal ganglion cells using AAV-mediated intravitreal delivery of a normal ND4 gene.

 

(B.L. Lam, W.J. Feuer, V. Porciatti, F. Abukhalil, A. Morante, J.R. Guy)

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