The optic nerve death in ADOA is usually caused by a fault on the OPA1 gene, which takes care of producing the OPA1 protein. Looking into the research around ADOA, we found several researchers who have actually succeeded in editing the OPA1 gene under laboratory conditions. This is very important, as correcting the protein stops the slow cell-death in ADOA and the slow visual decline.
The lead geneticist of the study, professor Michael Cheetham from the UCL Institute of Ophthalmology was kind enough to summarize the study in layman’s terms, give some pointers on what can we expect from the research, and how this work may eventually contribute to a medication.
Can you please describe the research in layman’s terms?
Inherited changes in the OPA1 gene produce a faulty protein, which leads to altered energetics and cell death in the optic nerve cells, causing ADOA. Our team corrected the changes in the gene in some patient derived stem cells using the newly developed crispr/cas9 method of gene editing. Correction of the gene fault corrected the protein, restored the energetics and reduced the cell death under lab conditions.
What’s the conclusion in one or two sentences?
The study shows that correcting the inherited changes in the gene can rescue the defects caused by the faulty protein in the optic nerve cells. We have shown this targeting the part of the OPA1 gene causing the ADOA+ mutation.
Can you please explain in a bit more detail how you edited the OPA1 gene (technical)?
Genetic faults in gene called OPA1 are the most common reason for people to have ADOA. The OPA1 gene encode a large protein called OPA1 which basically is a switch that regulates the energy activity of the optic nerve cell using a factor called GTP. The changes in OPA1 that cause ADOA are spread across the gene and affect different part of the protein and usually mean no protein is made. Sometimes spelling mistakes (which change one building block for another) in the part of the protein that binds the GTP lead to a faulty protein that is produced and can affect how the other normal copy of the protein works. These individuals can have more widespread symptoms with other tissues affected called DOA+. It was one of these changes in the OPA1 protein that we corrected in this study. This is important because this approach not only produces that second needed copy but it also removes the faulty ‘bad’ protein and stops it having a bad effect in the optic nerve cell.
While a number of mutations on the OPA1 gene cause ADOA, in this study we targeted a very specific part of the OPA1 gene to correct the OPA1 proteins. This is mainly the way the CRISPR technology we used works – it is a great breakthrough for research because it enables us to target almost anywhere in the genome using something called a guide RNA, which directs an enzyme Cas9 to cut a specific place, and a repair template which then replaces the damaged part of the gene through a process called homology directed repair (HDR). This HDR is something cells do to correct mistakes in their DNA as they grow.
But the downside of this method is that it is very specific – this is very much an approach that has to be developed and tested for each different DNA change. The good news is that we have recently been able to target other changes in OPA1 with new guides and repair 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 most of the time they are not). However, some of the inherited changes in OPA1 are quite common so for people that share the same change the same tools can be used.
Having said that people are trying different approaches where they replace large chunks of genes in a less specific way – but these are far less efficient and have not been tried in OPA1 yet.
How can these results contribute to a medication against ADOA?
Now we have a system set up to produce gene-edited optic nerve stem cells, which provide the necessary infrastructure for further work. We can use these gene edited stem cells to make optic nerve cells in the laboratory dish, and use them to understand the disease process in a human optic nerve cell. When searching for a treatment, we can also use these gene edited stem cells to compare to the original patient cells and test how well any therapies work. Basically, we are ready to begin testing therapies with biotech companies or other academic labs. We can also try and develop efficient methods to edit the changes in optic nerve cells.
Can this approach be tried in actual ADOA patients?
There are several technological challenges before we can use this therapy in live patients. Actual optic nerve cells are not stem cells, and in this study we used the fact that these cells are dividing with the HDR technology. Optic nerve cells in an ADOA patient no longer divide, and thus this approach would not work without modification. We are thinking about various approaches how to do gene repair in a live optic nerve cell, but scientists such as ourselves are still in the early stages in developing a practical therapy. What we know for sure is that if we repair the gene, it can stop the cell death in stem cells and we hope the same will happen in the optic nerve.
Does such gene editing actually improve vision, or just helps the retinal ganglion cells to survive (much) longer? So can such gene therapy reverse the visual decline, or only stabilize vision?
This approach is like many currently being developed and tested – it would be aimed primarily at helping the optic nerve cells survive longer. It is possible that as part of this the remaining optic nerve cells might function better and vision could improve not just stabilise, but it will not return or replace cells that have already been lost. So it would have limited use for someone that was very severely affected.
As can be seen, this is very exciting development, which we will follow in the coming years!
Written by Peter Makai