Scientists restore eyesight in mice with glaucoma and age-related visual impairment by delivering youth-promoting genes.
Many aging studies focus on how molecules called methyl groups that get tagged onto DNA affect how genes are regulated. An increase in these tags, a process called methylation, on DNA usually indicates reduced gene activation. With time, changes in methylation patterns cause cells to read the wrong genes, leading to their malfunction and age-related diseases. Analyzing how these DNA methylation patterns change with age has allowed scientists to predict future health and lifespan in animals as well as people.
Sinclair and colleagues from Harvard Medical School published a study in Nature where they reversed methylation patterns in eye cells of aged mice and mice with glaucoma to restore vision. They were able to dial back the biological age of cells by restoring youthful DNA methylation patterns in the mouse eye cells (retinal ganglion cells).
“Our study demonstrates that it’s possible to safely reverse the age of complex tissues such as the retina and restore its youthful biological function,” said senior contributor to the study David Sinclair in a press release.
Methylation patterns change with age so that genes typically have more methyl group tags on them with passing time. This has allowed researchers to generate DNA methylation clocks to determine a person’s biological age.
To dial back the DNA methylation clock of aged or injured eye cells, the team of researchers used an innocuous virus called an adeno-associated virus (AAV) to deliver three youth-restoring genes–Oct4, Sox2, and Klf4–into the retinas of mice. These youth-promoting genes are normally switched on during the development of the embryo. They are part of a group of genes that scientists refer to as the Yamanaka factors.
When Sinclair and colleagues used AAV to deliver the three youth-restoring genes to the eye cells of both young and old mice, they saw the regeneration of damaged axons, the cellular extension that sends signals to other cells. The researchers used forceps to crush the optic nerve, which contains a bundle of axons from eye cells projecting to the brain, and then analyzed the damaged axons of mice that received the AAV delivering Oct4, Sox2, and Klf4. They saw that older and younger mice alike showed axon regeneration and increased axon structural integrity with the gene delivery.
The Yamanaka factors are known to reverse DNA methylation patterns from aging, so the scientists tested whether delivery of these genes reversed the methylation patterns linked with aging to promote axon regeneration. Four days after injury, the eye cells showed accelerated DNA methylation age, but the delivery of Oct4, Sox2, and Klf4 reversed these DNA methylation changes. Lower levels of enzymes called TET1 and TET2 that diminish DNA methylation on genes eliminated the effects of the gene delivery on axon regeneration. This indicated that restoration of youthful DNA methylation patterns via Oct4, Sox2, and Klf4 gene delivery was necessary for axon regeneration.
The researchers then delivered the three genes to mice with glaucoma, which facilitated the recovery of vision. Half of the visual acuity lost from increased eye pressure was restored when the three genes were delivered by AAV. They found similar results in restoring vision to older mice with Oct4, Sox2, and Klf4 gene delivery.
“Regaining visual function after the injury occurred has rarely been demonstrated by scientists,” said Meredith Gregory-Ksander, an assistant professor of ophthalmology and co-author on the study in a press release. “This new approach, which successfully reverses multiple causes of vision loss in mice without the need for a retinal transplant, represents a new treatment modality in regenerative medicine.”
This study showed that delivery of the Yamanaka factor genes could restore vision in a glaucoma model as well as in aged mice. The researchers found that reversing methylation patterns toward a more youthful state was necessary for vision restoration.
“What this tells us is the clock doesn’t just represent time — it is time,” said David Sinclair in the press release. “If you wind the hands of the clock back, time also goes backward.” Future research needs to reproduce these results before testing the procedure on people, but the possibility exists that such a procedure could one day lead to vision restoration and tissue repair in aged humans.