By: Matt Varga Gene Therapy

Background There have been many major breakthroughs in the field of medicine over the last several decades. Research and the use of gene therapy has led to several of these important breakthroughs. Gene therapy uses DNA that encodes a functional therapeutic gene in order to replace a mutated gene. Recent successful treatments include: Leber’s Congenital Amaurosis (rare inherited eye disease) X-linked SCID (autoimmune deficiency) Adrenoleukodystrophy (disorder that leads to progressive brain damage) Even though many diseases have been successfully treated, many diseases such as, Parkinson’s disease (degenerative disease of the CNS) or severe Spinal Cord (SC) injuries still remain a mystery.

Current Therapeutic Approaches Parkinson's disease (PD) is the most common neurodegenerative movement disorder, affecting more than 2% of all individuals over 60 years of age. Current gene therapeutic trials expressing neurotrophic factors in the brain predominantly use the adeno-associated virus serotype 2. High expression of neurotrophic factors in neurons may impose a serious safety issues. There is currently no successful therapeutic strategies for SC injuries.

CNS Damage The failure of axons to regenerate is a major obstacle for functional recovery after central nervous system (CNS) injury. After injury in the CNS, axons proximal to the injury don’t re- grow. Distal axons degenerate and neurons die. Proximal axons retract and glial scar (fluid-filled cyst) seals the injury and prohibits axonal growth around it. axons Degenerating axons Macrophages & neutrophils Astrocytes Glial Scar oligodendrocytes (transplanted Schwann cells)

Previous Research Research by Martin Schwab found that CNS myelin from glial scars inhibits axonal growth. Due to the ligand Nogo released by myelin. Nogo can be inhibited by several molecules such as, Semaphorin 4D and Ephrin B3. This leads to increased axonal growth and some restored function.

New Reserach 1) PTEN/mTor Pathway 2) Cellular Implantation

PTEN/mTor Pathway The CNS has an inhibitory extrinsic environment (glial scars) and there is also diminished intrinsic (cellular pathways) regenerative capacity of mature CNS neurons (Park, 2008). The research has found that by modulating the PTEN/mTor pathway allowing the CNS is able to promote axon regeneration. This finding could lead to impactful clinical treatments to restore CNS function (Park, 2008). Deletion of PTEN (phosphatase and tensin homolog), a negative regulator of the mammalian target of mTOR pathway, in adult retinal ganglion cells (RGCs) promotes robust axon regeneration after nerve injury. Reactivating this pathway leads to axon regeneration.

Cellular Implantation (stem cell grafting) The use of stem cells is also another possibility to be used in therapeutic trials. Cell transplantation may be used in different non-exclusive ways to promote axonal regeneration and functional recovery after CNS damage (specifically spincal cord injuries). Can be used to enhance regenerative capacity, provide a scaffold for regeneration of severed axons, or replace lost cells. Results showed that short term lived, engineered hENP cells lead to increase in motor functions for rats with injured spinal cords.

Negative Effects The Semaphorin 4D and Ephrin B3 ligands are tumor suppression cells. An over expression of these ligands leads to an increase in tumor appearance(Nakada, Drake, Niska, Berens, 2006). For cell grafting, rats injected with hENP cells that would not engineered resulted in worsening of their condition (Perrin, 2010). This has dramatic implications. Stem cells can help restore axon growth, however, grafting of long term surviving hENPs was found to worsen the damage. Given the increasing number of clinical trials of cell grafting for SCI patients, more research needs to be performed because of the possible detrimental effect of this therapy (Perrin, 2010).

Anaylsis CNS damage causes devastating pathology with currently no effective treatment of any symptom. New research can lead to novel therapeutic techniques that can lead to effective treatments. There have been successful attempts to grow axons in a damaged CNS. However, there are negative effects with some of the techniques used now. As of now, it looks as though the mTor pathway is the best choice to be used for clinical trials. However, this method only yields about 10% axon re-growth. Results are promising but more research is needed.

References Drinkut, Anja, et al. "Efficient Gene Therapy for Parkinson's Disease using Astrocytes as Hosts for Localized Neurotrophic Factor Delivery." Molecular Therapy 20.3 (2012): Print. Nakada, M., Drake, K. L., Nakada, S., Niska, J. A., Berens, M. E. “Ephrin-B3 Ligand Promotes Glioma Invasion through Activation of Rac1.” Cancer Research 66 (2006). Print. Park, K. K., Lie, K., Hu, Y., Smith, P. D., Wang, C., Cai, B. Xu, B., Connolly, L., Kramvis, L., Sahin, M., He, Z. “Promoting Axon Regeneration in the Adult CNS by Modulation of the PTEN/mTor Pathway.” Science 322 (2008): Print. Perrin, F. E., Boniface, G., Serguera, C., Lonjon, N., Serre, A., Prieto, M., Mallet, J., Privat, A. “Grafted Human Embryonic Progenitors Expressing Neurogenin-2 Stimulate Axonal Sprouting and Improve Motor Recovery after Severe Spinal Cord Injury.” Plos One, 5.12 (2010). Print. Sheridan, Cormac. "Gene Therapy Finds its Niche." Nature biotechnology 29.2 (2011): Print.