The Retina as a Candidate Tissue for Gene Therapy

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Ophthalmic involvement in the genetic basis of retinal disease has increasingly advanced due to the recent explosion in biotechnology. Ocular pathologies have been recognized for centuries, but many are now being reclassified as their genetic etiologies undergo intense investigation at the molecular level. This increases the potential for developing new DNA-based treatments for inherited and acquired eye diseases.

The etiologies of blinding diseases, such as AMD, are often obscured by secondary complications and therefore diagnosed at a late stage when adjacent ocular tissues are also affected. The impact of gene transfer intervention may be significant in the management of such diseases as we continue to understand early promoting mechanisms underlying their progression. Prerequisites for effective DNA-based intervention in the eye include:

1. Knowledge of the etiology of the disease

2. Characterization of the gene(s) associated with the disease

3. Effective delivery of the therapeutic gene

4. Low toxicity and immunological tolerance of the gene transfer vector

5. Targeting transgenes to the specific affected area in the eye

6. Expression of the delivered gene at a therapeutic level

7. Long-term gene expression for reversing or correcting the disease

Diseases of the retina are good targets for gene transfer approaches. Visual function hinges on two critical layers of the retina: the outer nulcear layer, which contains the photoreceptor cells, and the retinal pigment epithelium, which lies adjacent to the photoreceptors. The relationships between the RPE and photoreceptor cells is critical not only to normal function but also to the pathology of several blinding hereditary diseases, including retinitis pigmentosa, Leber's congenital amaurosis, and macular degeneration.

Delivery of a therapeutic product to the RPE and photoreceptor cells is likely to be most successful if the delivery system is designed to target those specific cells. Because of the many compartments and structural complexity of the eye, it is easy for innoculated material to get trapped and diffused in any of the eye spaces and cell layers, reducing the availability of the transgene to target sites. In addition, the expression of certain genes is controlled through spatial and temporal regulation. Administration of high vector titers is, therefore, necessary to achieve reasonable levels of gene expression at the target site for attenuation of the disease. Attempts are currently underway to minimize widespread effects of an exogenous gene by using tissue- or cell-specific promoters to achieve regionally restricted gene transfer.

Some of the most widely used vectors in ocular gene delivery include the retrovirus, the adenovirus, and adeno-associated viruses. The choice of viral vectors, the design of efficient delivery systems, and the development of well-characterized animal models of retinal diseases are key elements for gene therapy to be successful in ocular pathologies.

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