Gene Therapy Developed in Mice Shows Promise For Treating Retinal Diseases

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Mount Sinai researchers have developed a gene therapy in a mouse model that protects optic nerve cells and helps to prevent vision loss, which they hope could be used for treating retinal degenerative diseases with no current treatment.

As described in the journal Cell, the team demonstrated how using gene therapy to reactivate a key enzyme known as CaMKII and its downstream signaling in retinal ganglion cells (RGCs) provided robust protection against continued vision loss or impairment in multiple disease and injury models.

“Neuroprotective strategies to save vulnerable retinal ganglion cells are desperately needed for vision preservation,” said senior author Bo Chen, PhD, associate professor of ophthalmology and neuroscience, and director of the ocular stem cell program at the Icahn School of Medicine at Mount Sinai.

“We uncovered evidence for the first time that CaMKII is a key regulator of the survival of retinal ganglion cells in both normal and diseased retinas, and could be a desirable therapeutic target for vision preservation in conditions that damage the axons and somas of retinal ganglion cells … If we make retinal ganglion cells more resistant and tolerant to the insults that cause cell death in glaucoma, they might be able to survive longer and maintain their function.”

Chen and colleagues described their studies in a paper titled, “Preservation of vision after CaMKII-mediated protection of retinal ganglion cells,” in which they concluded, “ … our results demonstrate that CaMKII is a key regulator of RGC survival in normal and diseased retinas and could be a general therapeutic target for RGC protection and vision preservation in a spectrum of pathological conditions.”

Glaucoma is the leading cause of irreversible visual impairment worldwide. The disorder is estimated to have affected 76 million people in 2020, with numbers rising to potentially 112 million by 2040, according to figures cited by the researchers. The disease results from irreversible neurodegeneration of the optic nerve, the bundle of axons from retinal ganglion cells that transmits signals from the eye to the brain to produce vision. While available therapies can slow vision loss by lowering elevated eye pressure, some glaucoma will progress to blindness despite normal eye pressure.

The major barrier to restoring vision loss from glaucoma and other retinal diseases and injuries is that the long nerve axons—which allow retinal ganglion cells to process visual information by converting light that enters the eye into a signal transmitted to the brain—do not regenerate. Some disorders that affect RGCs affect the nerve cell soma. This is the main body of the nerve cell from which axon branches off along the optic nerve to the brain. “Some conditions injure the RGC soma, including excitotoxicity and retinal ischemia, whereas others injure the RGC axon, including optic nerve transection, compression, intracranial hypertension, and glaucoma,” the team pointed out. And this means that neuroprotective strategies to preserve both the RGC axons and somas are urgently needed. “An ideal therapeutic approach would be to identify a universal target that effectively protects RGC somas and axons from diverse insults in a broad spectrum of pathological conditions if one exists.”

The Mount Sinai researchers investigated whether CaMKII (calcium/calmodulin-dependent protein kinase II) could play such a therapeutic role. The CaMKII pathway regulates key cellular processes and functions throughout the body, including retinal ganglion cells in the eye. Yet the precise role of CaMKII in retinal ganglion cell health is not well understood. Inhibition of CaMKII activity, for example, has been shown to be either protective or detrimental to retinal ganglion cells, depending on the conditions. “The precise role of CaMKII in regulating RGC survival under physiological and pathological conditions is far from understood,” the team noted. “Importantly, the fundamental question remains whether CaMKII could be a therapeutic target to protect RGCs from diverse insults, and ultimately preserve vision.”

For their studies, using an antibody marker of CaMKII activity, Chen’s team discovered that CaMKII pathway signaling was compromised whenever retinal ganglion cells were exposed to toxins or trauma from a crush injury to the optic nerve, suggesting a correlation between CaMKII activity and retinal ganglion cell survival.

Searching for ways to intervene, they found that activating the CaMKII pathway with gene therapy proved protective to the retinal ganglion cells. The gene therapy approach was designed to introduce a more active type of CaMKII into the original retinal ganglion cells to boost their activity. The modified version of CaMKII, with a mutated amino acid, was transferred to the targeted cells using an FDA-approved adeno-associated viral vector system.

The team tested the enzyme across a wide range of injury and disease animal models, including optic nerve damage, excitotoxicity (where nerve cells are destroyed by the overactivation of glutamate receptors that result in damage to the cell structure), and two glaucoma models that mimicked the pathophysiology of human disease with both high and normal intraocular pressure.

The investigators’ studies indicated that CaMKII regulated the survival of retinal ganglion cells across many of these pathologies, and that in the small-animal excitotoxicity model, insults to the retinal ganglion cell’s somas or injury to optic nerve axons led to inactivation of CaMKII and its downstream signaling target CREB (or cAMP response element binding protein). “Intriguingly, we found that reactivation of CaMKII and CREB provided robust protection for retinal ganglion cells,” noted Chen, who is also the McGraw family vision researcher at Icahn Mount Sinai, “and that CaMKII-mediated protection slowed down the disease progression in both glaucoma models.”

Through their experiments, the scientists demonstrated that administering the gene therapy to mice just prior to the toxic insult (which initiates rapid damage to the cells), and just after optic nerve crush (which causes slower damage), led to increased CaMKII activity and robustly protected retinal ganglion cells. The results indicated that among gene therapy-treated mice, 77% of retinal ganglion cells survived 12 months after the toxic insult compared with 8% in control mice. Six months following optic nerve crush, 77% of retinal ganglion cells had survived versus 7% in controls. Increasing retinal ganglion cell survival rates translated into greater likelihood of preserved visual function, according to cell activity measured by electroretinogram, and patterns of activity in the visual cortex.

Encouragingly, three different vision-based behavioral tests confirmed that the treated animals exhibited sustained visual function. In a visual water task, the mice were trained to swim toward a submerged platform on the basis of visual stimuli on a computer monitor. Depth perception was confirmed by a visual cliff test based on the mouse’s innate tendency to step to the shallow side of a cliff. Lastly, a looming test determined that treated mice were more apt to respond defensively (by hiding, freezing, or tail rattling) when shown an overhead stimulus designed to simulate a threat, compared with untreated mice.

“Our results for the first time provide in vivo evidence that CaMKII is a key regulator of RGC survival in normal and diseased retinas and could be a promising therapeutic target for vision preservation in a variety of pathological conditions typically characterized by degeneration of RGCs,” the investigators concluded. “Our research showed that CaMKII could indeed be a valuable therapeutic target to save retinal ganglion cells and preserve vision in treating potentially blinding diseases like glaucoma,” added Chen, who is a winner of the Pew Scholars in the Biomedical Sciences award given to young investigators showing outstanding promise. “The fact that manipulation of CaMKII would involve a one-time transfer of a single-gene adds to its vast potential to treat serious retinal conditions in humans. The next step is testing this in larger animal models, which may pave the way for starting clinical trials.

Mount Sinai has filed patent applications for the technology through Mount Sinai Innovation Partners (MSIP), the commercialization arm of the health system, which is in active discussions companies, with a view to progressing the treatment to the clinic.

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