New gene editing technology – Expert Reaction

US researchers claim to have developed a new type of gene editing called ‘prime editing’ that is more precise and creates fewer off-target byproducts compared to CRISPR–Cas9.

They say their technique can target mutations in particular sections of DNA, perform precise insertions and deletions, all while avoiding double-stranded DNA breaks (a common criticism of CRISPR-Cas9).

They tested their technique over 175 times in cells, and were able to correct the primary genetic causes of sickle cell disease and Tay Sachs disease, and they say they could use it to correct around 89 per cent of genetic diseases.

The SMC gathered expert comment on the report, feel free to use these comments in your reporting.

Dr Hilary Sheppard, Senior Lecturer, School of Biological Sciences, University of Auckland, comments:

“We have the ability to edit DNA in human cells using a gene-editing tool called CRISPR/Cas9. However, whilst transformative to the field of biological research and biomedicine, this technology does have some drawbacks. A paper released in Nature this week describes the development of a new and improved gene-editing tool called ‘prime editing’ which could help to overcome some of the gene-editing issues we are presently grappling with.

“One of the difficulties with the current CRISPR/Cas9 gene-editing technology is that to work effectively a double-stranded DNA break has to be made in the cell’s DNA. This can have undesired consequences and often leads to the majority of cells containing slightly incorrect gene-edits. The ‘prime editing’ approach is a CRISPR/Cas9-based tool that has been carefully engineered to eliminate the need for creating a double-stranded DNA break. As a result, precise editing efficiency is greatly increased. In addition, other engineered improvements result in dramatically reduced off-target effects and, importantly, include the ability to make a wide range of gene-edits. In a series of elegant experiments the authors created over 175 different types of gene-edits, including clinically relevant edits to genes involved in sickle cell and Tay-Sachs disease.

“The proof-of-principle data presented in this paper suggests that, in theory, we could target the majority (89%) of known pathogenic human genetic variants. However, there is much more work to be done to more fully understand this approach before we can safely apply this in the clinic. For example, as the authors themselves indicate, off-target effects were not assessed across the genome. In addition, the bulk of the work was carried out in laboratory cell-lines, and so further research using clinically relevant cell-types is required, and these tend to be harder to edit.

“So, we may be able to ‘fix’ known human variants associated with disease, but the ability to do so in the right cell type and in a clinically relevant manner may be some time away. Nevertheless, this is an exciting development that may address some of the issues and expand the current capabilities of gene-editing.”

No conflict of interest

Professor Peter Dearden, Director, Genomics Aotearoa, University of Otago, comments:

“The paper from Anzalone et al. describes the development of a new tool for gene editing, developed to solve some of the problems with CRISPR-Cas9 gene editing that have been identified. The key problems being solved here are to increase the probability that the edit you want to make IS made, and that changes to the genome don’t happen in places you don’t want to change.

“In normal gene editing, an enzyme that cuts DNA is sent to a specific sequence in the genome. The very act of cutting a bit of DNA gives molecular geneticists the ability to change the DNA sequence around the cut site or to insert a novel bit of DNA. This technology is occasionally problematic as it is hard to control exactly what change is caused, and such changes can also be caused when the enzyme cuts the wrong site elsewhere in the genome. This is not a problem in most cases because, with modern genomics, we can screen modified organisms to find the ones that have only the desired change.

“The problem is that if, for example, these technologies are to be used to correct mutations in human genes that are linked to disease, we really want those changes to be accurate and to only be the desired changes.

“Anzalone et al. have developed a very clever way to ensure only the expected change is made, and that off-target changes are greatly reduced. They do this by modifying the CRISPR-Cas9 system such that instead of cutting the DNA at a specific site, no cuts are made, and the change to the DNA sequence is made by copying a template provided by the researchers. In this way, off-target cuts are also not made, and the change in sequence produced is exactly that required. The authors go on to demonstrate the improved precision of this technique in human cells grown in culture.

“This is a very smart piece of technology that makes gene-editing more precise. Such innovations are very likely as scientists around the world race to develop this technology. This technique, however, does not address ethical, legal and social concerns about gene editing human embryos, nor does it escape the fact that the use of this system produces genetically modified organisms under New Zealand’s regulations.”

No conflict of interest.