AAV‐Mediated Gene Transfer Protects Against Mitochondrial Optic Neuropathy in Mice

Mutations affecting mitochondria lead to a variety of diseases that have been hard to replicate in animals, because of the difficulty in manipulating mitochondrial DNA and because of embryonic lethality. We have used Adeno‐associated virus (AAV) mediated RNA knockdown to produce two mouse models of Leber Hereditary Optic Neuropathy (LHON). Patients with LHON usually harbor mutations in subunits of Complex I of the respiratory chain and experience loss of vision beginning around age 20. In mice, blocking synthesis of subunits of this enzyme leads to the pathologic hallmarks of the disease—death of retinal ganglion cells, demyelination and loss of axons in the optic nerve. Using these mouse models, we tested two approaches for gene therapy of LHON. In the first, we delivered the gene for manganese superoxide dismutase (SOD2) to limit the accumulation of reactive oxygen species that result from Complex I defects. Delivery of SOD2 caused an 18% increase (p=0.001) in the survival of retinal ganglion cells and a proportional increase in the thickness of the optic nerve. In the second strategy, we re‐coded the mitochondrial gene for the ND4 subunit of Complex I, attached a transport sequence to its amino terminus and expressed it as a nuclear gene‐‐allotopic expression. Treatment of the optic nerve with AAV expressing this protein led to a 13% increase (p<0.01) in diameter of the optic nerve relative to eyes treated with a control virus. Thus both approaches prevented tissue injury resulting from defects to Complex I. These provide the first evidence in animals that gene therapy for mitochondrial disease may be possible.