**15. Gene therapy**

With emerging evidence for the molecular basis in glaucoma- pathophysiology, the disease may be interrupted by targeting key sites once the genetic expression is known. Studies of micro-RNA such as miRNA-125b has led to the understanding of the key sites for targeted down-regulation of messenger RNA which is thought to add to the oxidative stress induction of inflammation and astrogliosis in alzheimer's disease [78]. Alzheimer's disease, parkinson and glaucoma are thought to have a similar neurodegenerative basis (molecular and cellular pathways for neuronal cell loss) [78]. Hence gene therapy for glaucoma and other neurode‐ generative disorders may be where medical management is headed. Target sites include uveoscleral outflow site, surgical (trabeculectomy) site, ciliary apparatus, retina and optic nerve head (neuroprotection) [4].

Gene therapy would be helpful in preventing neurodegeneration using anti apoptotic genes, bcl-2 and bcl-x [2]. Another mechanism is blocking the apoptotic pathway with deprenyl (monamine oxidase inhibitor). It is proposed that it stabilizes the mitochondrial membrane potential, preventing the release of cytochrome c which can activate capsases (Figure 1) [2, 87].

Targeting antioxidant genes is a promising strategy for future management of glaucomatous neurodegeneration. Researchers used cloned extracellular superoxide dismutase (ECSOD) or catalase (CAT), carried on recombinant adeno associated virus intravitreally in mice. The mice were euthanized and optic nerve volume, myelin fibre area, axonal cell loss and RGC loss evaluated Inital response showed a 15 fold increase in ECSOD and 3.3-fold in CAT [88]. After six months the authors reported 29% reduction in RGC loss, 36% in ON demyelination, and reduction in axonal loss by 44% all compared to control eyes, indicating that antioxidant gene therapy will prove an invaluable adjunct to current glaucoma therapy.

The use of short 21 siRNA by intracameral and intravitreal injection to silence the unwanted expression of glaucoma genes particularly in the trabecular meshwork is being studied. The effects are so far temporary, and such siRNAs need the assistance of developed nanoparticles, such as magnetic nanoparticles, to enter target cells [89]. Chemical approach include the use of *cationic lipids (liposomes*) used in vitro, and shows promise for in vivo application via

Strategies for Neuroprotection in Glaucoma http://dx.doi.org/10.5772/53776 219

The cause of glaucoma and ultimately retinal ganglion cell death is multifactorial. At present there is no cure for glaucoma and the mainstay of treatment medically and surgically is to control the IOP. However, this conventional approach of lowering IOP is merely a secondary or indirect approach to the real problem. Current studies show that glaucoma is a neurode‐ generative disease with neuroprotection and possibly neuroregeneration and neuro enhance‐ ment as the future treatment modality. Modified Koch's postulates have been applied in the experimental neuroprotective research. Ultimately the retinal ganglion cell death whether primary or secondary (bystander result) must be stopped and the neurons preserved. The clinical application of most of these experimental neuroprotective strategies still has yet to pass through randomized controlled clinical trials before they can be accepted. The future holds much promise as to possible effective neuroprotective strategies, however, much research is

intracameral route with the target tissue being trabecular meshwork [4].

**16. Conclusion**

still yet to be done.

**Author details**

**References**

Lizette Mowatt1,2\* and Maynard Mc Intosh3

3 St Joseph Hospital, Kingston, Jamaica

\*Address all correspondence to: lizettemowatt@yahoo.com

2 University Hospital of the West Indies, Mona, Jamaica

hancement. Ophthalmology. 2012 ;119:979-86.

1 Faculty of Medical Sciences, University of the West Indies, Jamaica

[1] Chang EE, Goldberg JL. Glaucoma 2.0: neuroprotection, neuroregeneration, neuroen‐

#### **15.1. Administering gene therapy**

Administration of gene therapy must ideally be safe, repeatable, have low immunogenicity, and carry low infectious and mutagenic potential, modification of Koch's postulates [4]. Because viral vectors have the ability to maintain stable DNA within the target nucleus, they are preferred over non-viral vectors.

#### *15.1.1. Viral vectors*

*Adenoviral vectors (Ad*), non-enveloped replication-deficient recombinant viruses were the first to be used in gene therapy research [4,89]. They show high level of tropism for post mitotic and highly specialized cells, and have been known to reproduce TM cells with high accuracy. They have application in Muller cell and RPE cell replication as well. Studies have been done with Adenovirus (Ad) mediated intravitreal delivery of BDNF. However, repeated injections have been found to cause severe inflammation in experimental models [2, 90]

*Adeno-associated Viruses (AAV).* This is an integrating vector known to show efficient delivery to target tissues. AAVs do not carry viral genes, therefore they have no unwanted pathoge‐ nicity, immunogenicity, nor significant inflammation upon sub retinal application [2]. In the last 4 years the use of AAV vector in the delivery of gene therapy has met some success in human trials, but the effect is limited to RGC survival. Though the vectors may target trabec‐ ular meshwork cells, they are not very active there [89].

*Herpes Simplex Virus (HSV).* This virus has shown promise in glaucoma research and therapy as it is able to transduce trabecular meshwork,ciliary body epithelial, and retinal ganglion cells. The injected derivative however has been found to carry risks of inflammation, toxicity, and limited duration of gene expression [4].

*Lentiviral vectors.* These single strand RNA viruses can incorporate trabecular meshwork and RGC DNA by reverse transcription, with both neuroprotective and IOP lowering potential. Combining several enzymes such as cyclooxygenase and prostaglandin pathway enzymes increases their IOP-lowering properties. [90]

#### *15.1.2. Non-viral vectors*

*Naked DNA Injection.* Work done with naked DNA as plasmid vectors expressing chloram‐ phenicol acetyl transferase has shown promise in the possible control of wound healing after trabeculectomy. The plasmid injected in the bleb or under collagen shield has resulted in a 30 fold increase in the activity of the enzyme. [4].

The use of short 21 siRNA by intracameral and intravitreal injection to silence the unwanted expression of glaucoma genes particularly in the trabecular meshwork is being studied. The effects are so far temporary, and such siRNAs need the assistance of developed nanoparticles, such as magnetic nanoparticles, to enter target cells [89]. Chemical approach include the use of *cationic lipids (liposomes*) used in vitro, and shows promise for in vivo application via intracameral route with the target tissue being trabecular meshwork [4].
