Genome editing – obstacles emerge?

Post by James Lowe, who leads the pig strand as part of the TRANSGENE: Medical Translation in the History of Modern Genomics project. His research into the pig genome project is funded by a European Research Council Horizon 2020 Programme Starting Grant. See the TRANSGENE website for more information on the project: www.stis.ed.ac.uk/transgene​

CRISPR-Cas9-biologist

Guide to CRISPR-Cas9, produced by ‘J LEVIN W’. Reproduced under a Creative Commons Attribution-Share Alike 4.0 International license. Available online at: commons.wikimedia.org/wiki/File:CRISPR-Cas9-biologist.jpg

In the last month, two developments have potentially stymied the seeming promise of genome editing, the set of tools by which particular stretches of DNA can be selectively cut out of the genome. One was a research finding, the other a court judgement. Both cut across the seeming advantages that genome editing has over other methods of genetic manipulation, such as transgenics, the transfer of genes from one species to another.

Unlike transgenics and other examples of genetic modification where a gene or other sections of DNA are added to the recipient genome, the editing of DNA by tools such as the CRISPR-Cas9 complex were thought to be targeted and precise. When DNA is added by genetic modification techniques such as microinjection of DNA into the nucleus of the cell or using a viral vector to transport the desired addition, it is unknown where in the genome the added DNA will be inserted, and therefore the consequences of this addition are uncertain.

Last year, a paper was published contending that genome editing by CRISPR-Cas9 induced significant ‘off-target’ changes to the genome, alterations far beyond the site to which the editing activity was supposed to be focused. That publication suffered stinging critiques and was later retracted by the journal. The researchers who wrote the paper had not adequately accounted for the existing genetic variation between individuals in determining whether off-target changes had occurred. Off-target modifications are not the primary concern of people working with CRISPR-Cas9, however, not least because the tools to improve the targeting of the system and the means to detect off-target changes appear to be rapidly improving.

The problems instead relate to unwanted ‘on-target’ changes, and interference in the processes associated with p53, a protein involved in tumour-suppression. The on-target changes, according to research conducted at the Wellcome Sanger Institute, include deletions of thousands of base pairs of DNA and rearrangements of DNA.

Two groups (one in the USA, the other mainly Scandinavian) have also demonstrated that the double-stranded breaks initiated by CRISPR-Cas9 activate the p53 pathway, which acts to arrest cell division and may cause the cell to commit suicide. This is a sensible response to signs of DNA damage, as this tends to increase the risk of the cell becoming cancerous. In a developing organism, however, this process acts as an agent of selection, reducing the relative number of cells with an active, functioning p53 pathway that helps to suppress tumour-formation, while leaving cells in which mutations have made p53 inactive or malfunctioning free to divide and increase in relative proportions.¹

The implication is that genome editing can help generate organisms with a large pool of cells that have one of the main safeguards against the development of cancer missing.
The response to this has been sanguine, with calls for continued improvement of the tools and understanding of the processes of DNA repair. The Wellcome team highlight the crucial role of establishing what baseline normal variation exists between the genomes of different individuals, so that meaningful conclusions can be drawn from the development methods to detect unwanted alterations to the target genome.

European_Court_of_Justice_(ECJ)_in_Luxembourg_with_flags_0017_(1674479483).jpg

The European Court of Justice, Luxembourg. Photograph by Cédric Puisney. Reproduced under a Creative Commons Attribution 2.0 Generic license. Available online at: https://bit.ly/2okBmfX

The court judgement has greater potential to limit the application of genome editing beyond the laboratory, at least in the European Union. Earlier this year, a formal opinion written by Michal Bobek, the Advocate General of the Court of Justice of the European Union suggested that products of genome editing would not need to be regulated in the same way as products of genetic modification. After all, genome editing was analogous to a mutation, and the products of it could potentially occur without human intervention. Genetic modification, on the other hand, involved the addition of a gene, a negligible possibility in nature.

In July, however, the court ruled that products of genome editing should indeed be deemed to be genetically modified organisms, and therefore governed under the strict provisions of the GMO Directive. While clarity on the regulatory status of genome editing may be welcome to scientists, the decision appears not to be, though there is a recognition by some advocates of genome editing such as Ottoline Leyser that the law itself is flawed, and that the judges were constrained by this. The implications of the judgement are still unclear. It may not affect laboratory research much, but in placing considerable regulatory obstacles in the way of applications of genome editing, it will likely influence whether the fruits of that research end up on our plates, at least in Europe.

Notes –

  1. On p53, see the general account of the history of research into this pathway by Sue Armstrong, ‘p53: The Gene that Cracked the Cancer Code

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