Retinal ganglion cell gene therapy and visual system repair

Recent clinical trials have shown that the use of replication deficient viral vectors to genetically modify cells in the retina can be of therapeutic benefit in the treatment of certain inherited degenerative conditions that compromise photoreceptor, and hence visual, function. This review is focussed primarily on the use of recombinant adeno-associated viral (rAAV) vectors to target neurons in inner retina, specifically retinal ganglion cells (RGCs). Genetic modification of RGCs may be of value in various ophthalmic conditions in which there is documented loss of RGCs or damage to their centrally projecting axons. Such conditions include glaucoma, optic neuritis, vascular disruption or trauma, and neurological degenerative conditions such as Alzheimer's disease. Furthermore, because the retina and optic nerve (ON) form part of the CNS, the visual system is a useful experimental model in which to study the molecular and cellular mechanisms that underlie degenerative as well as regenerative responses of adult CNS neurons after injury. Gene therapy studies from a number of laboratories are first reviewed, involving not only rAAV-based treatments but also application of lentiviral and adenoviral vectors. Recent work from our own laboratory is then summarized, in which intravitreal injection of rAAV2 serotype vectors is used to introduce growth promoting genes into injured RGCs. rAAV encoding a secretable form of ciliary neurotrophic factor (CNTF) has proved to be particularly effective in promoting RGC survival and axon regeneration after optic nerve crush or after transection followed by a peripheral nerve autograft. In the latter situation we have found that RGCs and their regenerated axons are maintained for at least 15 months after the initial injury. We have also combined rAAV gene therapy with pharmacotherapy to determine if cAMP elevation and additional intravitreal injections of growth factors can act synergistically with vector-based delivery of growth-promoting genes.