The road to treatment 7
In ADOA, the death of the optic nerve is usually caused by an error in the OPA1 gene. The OPA1 gene ensures the production of the OPA1 protein, which is important for properly functioning optic nerve cells. There are several researchers who have managed to edit the OPA1 gene under laboratory conditions. This is very important, because the repaired protein stops both the slow cell death of the optic nerve and the slow decline of vision.
The lead geneticist of a recent study, Professor Michael Cheetham of the University College London Institute of Ophthalmology, was willing to summarize the study in layman's terms, explain what we can expect from the research and how this work could ultimately contribute to a cure for ADOA.
Can you describe the research in layman's terms?
Hereditary changes in the OPA1 gene produce a defective protein, leading to lower energy levels in optic nerve cells and ultimately cell death. This is the cause of ADOA. Our team corrected the changes in the gene by deriving stem cells from patients, using the newly developed CRISPR/CAS9 gene editing method. Correction of the gene mutation restored the protein, increased the energy level of optic nerve cells and reduced cell death under laboratory conditions.
What is the conclusion of the study in one or two sentences?
The study shows that correcting the hereditary changes in the gene can prevent the defects in the optic nerve cells caused by the defective protein. We have corrected the OPA1 gene at the location that causes the ADOA+ mutation and showed that the optic nerve cells do not indeed die.
Can you explain in a bit more detail how you edited the OPA1 gene (technically)?
ADOA is usually caused by genetic errors in the OPA1 gene. The OPA1 gene codes for the production of a large protein called OPA1. The OPA1 protein is essentially a switch that controls the energy activity of the optic nerve cell using a factor called GTP. The changes in OPA1 that cause ADOA are spread throughout the gene, affect different parts of the protein and usually mean that no protein is made. Sometimes spelling errors (which change one type of building block into another) in the part of the protein that binds to GTP lead to production of a defective protein. The defective protein can then affect the functioning of the other normal copy of the protein. In this case, more diverse symptoms may develop with other affected tissues, this is the case in people with ADOA+. We also corrected this deviation in the OPA1 protein in this study. This is important because this approach not only produces that second copy needed, but also removes the defective 'bad' protein and prevents it from having a bad effect on the optic nerve cell.
Several mutations on the OPA1 gene can cause ADOA. However, in this study we focused on a very specific part of the OPA1 gene to correct the OPA1 proteins. This is essentially how the CRISPR technology we used works – it's a great breakthrough for research because it allows us researchers to make changes almost anywhere in the genome. Using something called an RNA guide, which directs an enzyme Cas9 to cut at a specific site, and a repair template that then replaces the damaged part of the gene through a process called homologous directed repair (HDR). This HDR is something cells do to correct errors in their DNA as they grow.
But the disadvantage of this method is that it is very specific – this is an approach that must be developed and tested for each different DNA change. The good news is that we've recently been able to target other changes in OPA1 with new guides and fix templates, but the ability to target multiple or different targets with the same guide would mean that they would have to be very close together (and usually they aren't Like this). However, some of the inherited changes in OPA1 are quite common, so for people who have the same abnormality, the same tools can be used.
That said, there are attempts to replace large chunks of genes in a less specific way – but these are much less efficient and have not yet been tried in OPA1.
How can these results contribute to a drug against ADOA?
Now we have established a system to produce optic nerve stem cells with edited genes. These provide the necessary infrastructure for further research. We can use these gene-edited stem cells to make optic nerve cells in laboratory dishes and use them to understand the disease process in a human optic nerve cell. In the search for a treatment, we can also use these gene-edited stem cells to make comparisons with the original patient cells and to test how well possible therapies work. In short, we are ready to start testing therapies at biotech companies or other academic laboratories. We can also try to develop efficient methods to manipulate the changes in optic neurons.
Can this approach be tried in real ADOA patients?
There are several technological challenges before we can use this therapy in living patients. Real optic nerve cells are not stem cells, and in this study, HDR technology requires the cells to divide. Optic nerve cells in an ADOA patient no longer divide, so this approach would not work without modifications. We are considering different approaches to restore genes in a living optic nerve cell, but scientists like us are still in the early stages of developing a practical therapy.
What we know for sure is that if we repair the gene, the repaired gene can stop cell death in stem cells and we hope the same will happen in the optic nerve.
Does such gene editing improve vision, or does it just help the retinal ganglion cells survive (much) longer? So can such a gene therapy reverse visual decline, or just stabilize vision?
This approach is similar to many others currently being developed and tested – it would primarily aim to help optic nerve cells survive longer. It is possible that as part of this the remaining optic nerve cells will function better in addition to stabilizing vision. However, this approach cannot replace cells that have already been lost. So it would be of limited use to someone very badly affected by ADOA.
Apparently this is a very exciting development, which we will follow in the coming years!
By Peter Makai
