They Can See Clearly Now: Groundbreaking Study Regenerates Sight in Mice Through Epigenetic Reprogramming
Once thought to be an irreversible process, researchers may have discovered a novel way to regenerate sight. Published in Nature in December 2020, Lu et al. detailed their innovative experiments in mice that reprogrammed cells in the eye to revert to their younger selves. For the one billion people worldwide currently living with vision impairment or loss, the results from this study provide a hopeful message that repairing and restoring sight may soon be on the horizon.
How retinal cells and their projections make up the basics of sight
Although all organs and tissues can deteriorate with age, one of the first to lose its ability to regenerate and repair itself is the central nervous system (CNS). Cells in the eye called retinal ganglion cells (RGCs) are a vital component of the CNS — although they don’t reside in the brain, they are still considered neurons. RGCs are necessary for sight, as they line the inner surface of the retina to process light into visual information as it enters the eye. From there, RGCs relay this visual information to the brain via their axons, the long finger-like fibers that project from RGCs to form the optic nerve.
Damaged RGCs can only regenerate their axons in early life; this reparative ability ceases within the first few days following birth. A progressive loss of RGCs and their axons causes glaucoma, one of the leading causes of blindness in older adults. Once damaged, RGCs and their axons were thought to be unable to regenerate themselves — until now, that is.
Epigenetic noise and DNA methylation: theories of aging
Researchers have been recently popularizing the “information theory of aging” to explain why we age. This theory says our cells lose epigenetic information with age. The epigenome – the network of compounds controlling if and when genes turn on or off – can get “marked” with various chemicals. After one too many markers accumulate, the epigenome can’t direct genes to perform in the same way it used to, leading to dysfunctional cells and accelerated aging.
One of those markers is DNA methylation, which is thought to be a biological proxy of age. As the name describes, DNA methylation adds compounds called methyl groups to DNA, which can change the way that genes are activated. We all have specific DNA methylation patterns that were created during our early development; however, with the passing years, these patterns change, leading to aging and disease.
Lu and colleagues hypothesized that somewhere deep down, our cells retain a copy of their youthful epigenetic information – and uncovering this hidden pattern may reprogram cells to reverse aging. The tricky part is that they had to develop a way to reset the age-accumulated epigenetic noise without harming the cell’s overall identity and function.
Previous research has found that four transcription factors, which are proteins that turn genes on or off by binding to nearby DNA, essentially reset the clock on DNA methylation, erasing the epigenetic markers left behind. However, one of the transcription factors can cause tumor development and, in some cases, death. So, the researchers of this study took out that toxic gene and instead used just three: OCT4, SOX2, and KLF4, referring to the trio as “OSK”.
A trio of gene activators regenerates axons in mice
After ensuring OSK’s safety — inducing the gene trio in mice showed no signs of tumor growth or adverse health outcomes — the researchers studied its effects in mice with optic crush injuries. This is as bad as it sounds — they literally crushed the optic nerve, damaging axons and killing RGCs.
After treating these mice with OSK, their axons regenerated and their RGCs survived longer. Amazingly, the RGC axon fibers regenerated far enough to extend into the optic chiasm, which is where the optic nerve connects to the brain.
These results held strong for both young and old mice with optic nerve damage. In 12-month old aging mice, OSK treatment effectively doubled their RGC survival rate — similar to results seen in young mice. While axon regeneration was not as pronounced in older mice compared to their younger counterparts, a 3-week OSK treatment extension in the aging mice allowed the axons to catch up in regenerative growth.
OSK + a reduction in DNA methylation = sight restored
Next, the team looked at OSK’s effects on DNA methylation. After optic crush injuries, OSK significantly mitigated the DNA methylation pattern that the injury sped up.
They wondered if these beneficial changes to DNA methylation were necessary for RGC survival and axon regeneration. By inactivating two enzymes required to remove methylated DNA, OSK’s vision-boosting effects were severely diminished. This indicates that reducing DNA methylation is essential for OSK to work its sight-restoring magic.
OSK turns back the epigenetic clock
Lu and colleagues also looked at OSK’s effects on mice with glaucoma, as this eye condition is a leading result of RGC death. Once again, OSK effectively restored axon density, significantly recovering half of the visual acuity that was lost in these glaucoma-ridden mice.
Next, they tested mice experiencing vision loss from natural aging — no optic crush injuries, no induced glaucoma. In these naturally-aged mice, OSK treatment restored vision loss and RGC activity. Additionally, OSK reset 90% of the genes that were altered during aging, effectively restoring epigenetic patterns to youthful levels.
Translating the research, from mice to men
Last but not least, the researchers tested out OSK in human neuron cells. They saw similar results as seen in the mice: OSK was able to attenuate axon loss, increasing both the number and length of axons. In human cells, OSK also counteracted the accumulation of DNA methylation that is seen with age.
We don’t know yet whether these results will translate to living human beings. But the results from this series of experiments suggest that epigenetic reprogramming may soon be a therapeutic option for restoring sight and repairing vision loss.
In this study, Lu et al. exhibited a safe reversal of vision loss in several models — mice with optic nerve damage, glaucoma, and natural aging, as well as human neuron cells — using OSK transcription factors. Inducing these genes effectively regenerated axons, improved the survival rate of retinal ganglion cells, and reset the epigenetic programming to youthful levels.
The researchers determined that manipulating DNA methylation patterns is a necessary step to produce these beneficial, sight-restoring effects. Although the study was only done with mice and cellular models, research with humans may not be far behind. But we’ll have to wait and see.
Lu Y, Brommer B, Tian X, et al. Reprogramming to recover youthful epigenetic information and restore vision. Nature. 2020;588(7836):124-129. doi:10.1038/s41586-020-2975-4
Takahashi K, Yamanaka S. Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell. 2006;126(4):663-676. doi:10.1016/j.cell.2006.07.024