In vivo gene therapy in young and adult RPE65-/- dogs produces long-term visual improvement

Defects in the RPE65 gene, which is selectively expressed in the retinal pigment epithelium (RPE), result in blindness and gradual photoreceptor cell degeneration. Experiments were conducted to assess the efficacy of gene replacement therapy in restoring retinal function in RPE65-/- dogs. Long-term effects of RPE65 gene therapy were assessed using visual behavioral testing and electroretinographic (ERG) recordings at 10-12 weeks and 6-9 months after surgery in five affected dogs. Subretinal injections of similar dosages of two constructs were performed in affected dogs at the ages of 4-30 months: rAAV.RPE65 into one eye and, in four of five dogs, rAAV.GFP contralaterally. Before surgery all RPE65-/- dogs were behaviorally blind with either no or very low-amplitude ERG responses to light stimuli. Marked improvements in visual behavior and ERG responses were observed as early as 4 weeks after surgery in affected animals. Except for light-adapted 50 Hz ERG flicker responses, all ERG parameters tested increased significantly in the eyes treated with the rAAV.RPE65 construct at the early follow-up. Gradual progressive improvements in ERG responses were observed in the RPE65-treated eyes over time. An unexpected finding was that on long-term follow-up, marked improvement of photopic ERG responses were also observed in the contralateral control eye in both young and older dogs. These results are promising for future clinical trials of human patients with retinal degenerative diseases, such as Leber congenital amaurosis, that result from RPE65 gene defects.

Matrix metalloproteinase (MMP) expression in experimental choroidal neovascularization

PURPOSE

Matrix metalloproteinases (MMP) are a family of proteolytic enzymes that degrade basement membrane and extracellular matrix proteins. To gain information on the possible role of MMPs in choroidal neovascularization (CNV), we have analyzed the mRNA expression of MMP-2 and MMP-9, two forms of MMPs implicated in ocular neovascularization, in a rat model.

METHODS

Choroidal neovascularization was induced in pigmented rats by krypton laser photocoagulation of the fundus whereafter eyes were enucleated at 1, 3, 5, 7, 10 and 60 days. Antisense and sense riboprobes were generated using DNA complementary to MMP-2 and MMP-9, and mRNA expression was analyzed using in situ hybridization.

RESULTS

In the untreated eyes MMP-2 mRNA expression was weakly detected in cells within the choroid. In laser-treated eyes MMP-2 mRNA expression was markedly increased and mainly localized to macrophage-like and retinal pigment epithelial (RPE)-like cells invading the choroid, subretinal space and inner retina. This increase in MMP-2 mRNA expression peaked at day 10 whereafter a decline was detected. MMP-9 mRNA expression was low in untreated eyes and did not increase following laser treatment.

CONCLUSION

The results show that MMP-2 mRNA expression is increased in experimental CNV, and support of a role for MMP-2 in the development of CNV in age-related macular degeneration.

Senescence of the retinal pigment epithelium

Senescence of human cells has largely been studied as an in vitro phenomenon resulting from replicative exhaustion. The literature contains many studies of retinal pigment epithelium (RPE) cells which document replicative senescence. Several studies by Burke and others illustrate the relationship between donor age and replicative lifespan, the relationship between geographical location of RPE in the posterior pole and replicative lifespan, and the phenomena of altered cellular morphology and decreased culture saturation density for senescent RPE cells. Other studies have focused on the alterations of the expression of specific genes or the alteration of enzymatic activities during the senescence of RPE cells in vitro. Recently, a technique utilizing a histochemical staining procedure for beta galactosidase has been developed which identifies senescent cells. Normal beta galactosidase histochemistry which identifies the lysosomal form of the enzyme is performed at pH 4.0, while senescence-associated beta galactosidase activity is observed at pH 6.0 and is observed in the cytoplasm. We have studied the replicative senescence of human RPE cells in vitro using this procedure and have also measured the length of chromosomal telomeres to identify the aging of cultures in vitro. Our results show that RPE cultures accumulate beta galactosidase positive cells as a function of the number of population doublings and that these data correlate with the shortening of chromosomal telomeres to a functional limit observed for many human cell types at senescence. We have also recently extended this work to the development of a senescence-associated beta galactosidase procedure for observing senescent RPE cells in vivo. Basically, the same histochemical procedure is used with a post-staining bleaching step to clearly visualize staining in the RPE. Our first studies were performed on globes from Rhesus monkeys at a variety of ages from 1 year to 29 years of age. The results show the accumulation of beta galactosidase positive cells in the older monkey eyes. We have also examined several human eyes in an attempt to observe whether any relationship exists between beta galactosidase staining and age, pathology (diabetes, basal laminar deposits), and geographical location (macula vrs. periphery). These studies represent a first effort to determine if senescent RPE are present in vivo. It will be important to extend these studies so that these data might be expressed on a quantitative bases.