In a recent study sponsored by the National Institutes of Health, a team of scientists developed a new mouse model for Leber hereditary optic neuropathy (LHON), a vision disorder that results from genetic mutations that damage the mitochondria, organelles responsible for producing the body’s energy. The research team determined that gene therapy can be used to improve vision in mice.
In England, LHON affects about 1 in every 30,000 people. It is an inherited form of vision loss that causes blurry vision, with eyesight tending to worsen over time with severe loss of sharpness (visual acuity) and color vision. The disease is caused by a loss of retinal ganglion cells, and the most common genetic mutation is on the mitochondrial gene ND4.
Mitochondria have their own DNA (mtDNA), and DNA mutations in this organelle lead not only to LHON but also other conditions. “This study marks an important contribution to research on LHON, and in efforts toward an effective therapy. But the implications are even broader, because the approaches that the investigators used could aid therapy development for a vast array of other mitochondrial diseases,” said Maryann Redford, D.D.S, M.P.H., a program director in Collaborative Clinical Research at NIH’s National Eye Institute.
Until now, “there was no efficient way to get DNA into mitochondria,” said John Guy, M.D., professor of ophthalmology and director of the ocular gene therapy laboratory at the Bascom Palmer Eye Institute, University of Miami Miller School of Medicine. His lab was one of the first ones to create a method that targets the mtDNA in mice and humans. The new study findings have been recently published in the journal Proceedings of the National Academy of Sciences, in an article entitled “Consequences of zygote injection and germline transfer of mutant human mitochondrial DNA in mice“.
Fifteen years ago, Dr. Guy started searching for a gene therapy to substitute a copy of the ND4 gene in mitochondria. However, the viruses used in gene therapy were able to invade the body’s cells and penetrate the nucleus, but were poor at targeting the mitochondria.
To address this issue, the team used the fact that mitochondria imports cellular proteins that they cannot make themselves. By connecting a portion of these proteins to the outer shell of the virus, researchers were able to introduce the virus into the mitochondria. This altered virus was crucial for the development of a LHON mouse model and to test gene therapy for this disease.
To develop the LHON mouse model, the team loaded the virus with a defective ND4 copy carrying the mutation that is present in approximately 70% of the LHON cases. Researchers have also included DNA coding for a red fluorescent protein, so they could observe the virus. Subsequently, the virus was injected into fertilized mouse egg cells to create the mouse model. The animals expressed the virally encoded ND4 mutation in their eyes, and through electroretinogram, researchers observed that the mice had atrophy (shrinkage) of the optic nerve, loss of retinal ganglion cells, and visual response decline.
To create the LHON gene therapy, the team inserted the natural human ND4 gene into the same virus. This mixture was injected into the animal’s eye and remarkably led to an improvement in visual function. When the mixture was given to normal mice, researchers observed that the virus carrying ND4 did not cause any adverse effects on vision.
“Now we’ve shown that we can improve visual function after it’s been lost,” concluded Dr. Guy.
This research is helping an ongoing clinical trial study (NCT02161380), led by Dr. Guy, which is assessing the safety of a similar gene therapy strategy in patients with LHON .