New gene therapy approaches may be able to treat forms of vision loss that currently have no treatment options, and improve the treatment of others.
Researchers at Case Western Reserve University and the University of California at Irvine have demonstrated proof of principle that such correction is possible in Leber congenital amaurosis (LCA), a disease where a point mutation in the RPE65 gene, which codes for the protein retinoid isomerase, leads to retinal degeneration and blindness.
The authors published their findings in the October 19, 2020, online issue of Nature Biomedical Engineering.
Gene therapy for LCA was first tested more than a decade ago. It was initially successful in improving vision, and follow-up studies indicated that improvements in light sensitivity could be followed by improvements in perception for relatively long periods of time, due to slow rewiring processes in higher brain areas.
However, the earliest attempts at gene therapy did not lead to permanent improvements in vision. Instead, visual gains typically appear to fade after a few years.
There are several possible explanations for the temporary nature of the gains, including that the vector-based gene therapy that is currently approved to treat LCA does not lead to permanent expression of the corrected gene.
Loss of AAV-based gene expression is a more general concern for gene therapies, prompting a search for more durable alternatives. One possibility is so-called base editing, where one DNA base is directly converted to another.
Base editing has the potential to be a relatively simple method for correcting a sizable fraction of genetic disorders. And because it corrects an individuals' own DNA rather than delivering extraneous DNA, the resulting changes would be expected to be permanent.
In the work now published in Nature Biomedical Engineering, the team showed that an adenine base editor was able to correct the disease-causing mutation with a frequency of roughly 30% in adult mice, leading to "restored RPE65 expression and retinoid isomerase activity, and near-normal levels of retinal and visual functions." Homology-directed repair has also been used to genetically correct LCA-causing RPE65 mutations, but corrected a much lower proportion of cells.
Another paper published this week expanded the use of gene therapy in a different direction.
Researchers from Nanoscope Technologies reported in the October 22, 2020, online issue of Gene Therapy that they had improved vision in mice with retinal degeneration by delivering, via an AAV2 vector, an engineered version of opsin that was more highly sensitive to light than its normal counterparts.
Opsins are proteins that implement visual signal transduction, essentially turning photons into neuronal activity that allows the brain to perceive light. They are expressed in the bipolar cells that link rod and cone photoreceptors to the retinal ganglion cells that make up the optic nerve.
There are a number of retinal diseases, including age-related macular degeneration (AMD) and retinitis pigmentosa, whose root cause is damage to photoreceptors. Although lack of input from sensory receptors will ultimately cause reorganization of the CNS as higher brain regions are reassigned to new functions, those long-term consequences could, in theory, be prevented by developing alternative input mechanisms.
Retinal implants that stimulate retinal ganglion cells directly, as well as optogenetic activation of bipolar cells, are being developed to provide such alternate inputs. However, retinal implants are limited in the amount of resolution they can provide. Optogenetic approaches in turn work only in intense light conditions, and "long-term active stimulation with high intensity light poses the risk of retinal damage and adds complexity to the treatment," the authors wrote.
The Nanoscope team addressed these issues by first developing a highly sensitive opsin that could sense ambient light, and delivered that opsin, which they called MCO-1, directly into bipolar cells.
Gene therapy led to protein expression for at least 6 months, and the treatment enabled otherwise blind mice to sense light, and to use it to orient themselves in behavioral tests, demonstrating that their vision was good enough to confer practical benefits.
In a prepared statement, PaekGyu Lee of the National Eye Institute, which funded the approach, said that "if this optogenetic approach using cells spared in degenerated retina can prove to be effective in vision restoration in humans, beyond light perception, it could offer a valuable alternative to the retinal prosthesis approach for people with late-stage retinitis pigmentosa." Nanoscope plans to initiate a clinical trial of the approach by the end of 2020 (Suh, S. et al. Nat Biomed Eng 2020, Advanced publication; Batabyal, S. et al. Gene Ther 2020 Adv. Publication).