The Guardian view on adjusting DNA: a new world
A hope that embryos could be purged of a genetic disease has been fulfilled in part. However, we are some way off reimplanting modified embryos into their mothers – for all the right reasons
By Editorial Board
Aug 3 2017
The news that a team of scientists from America and Korea have corrected a genetic defect in single-cell embryos is tremendous. In the short term it affirms the revolutionary principles of the gene editing technique known as Crispr – or, formally, Crispr-Cas9 . In the medium term it holds out the prospect – if the law changes – of eliminating some single-gene defects from entire families, since embryos treated in this way will no longer transmit the defective form of the gene. In the long term, the prospects for widespread genetic manipulation of humans are chilling as well as exhilarating.
Crispr makes it possible to manipulate a genome in the way that we can alter the words in a word processor. Very small identifying stretches of DNA around a defective gene can be targeted and then deliberately broken. The hope is that when the cell repairs the break, it does so without the original error. When the DNA is copied at the next cell division, only a functioning version of the gene is reproduced. When this is done early enough, at the embryo stage, the change does not last only for the patient’s lifetime, but is transmitted to all their descendants, too.
Changing words or genes successfully requires in both cases that we grasp their meanings, and here lies one of the dangers that have made most scientists extremely cautious about applying the technique to humans. The process by which genes are used to build the bodies around them is extremely complex and we understand what happens when it goes wrong much better than when it goes right. Crispr, although it appears to be very precisely targeted, can make errors analogous to a badly framed search-and-replace request, changing gene sequences that are far from the intended targets.
Previous attempts to extend this technique to human embryos, in China, are reported to have run into problems with mosaicism, where some, but not all, the copies of the relevant gene are changed, so that the embryo develops with different regions expressing different variants of the same gene. The effects can be appealing when applied to cats. It is not desirable in other contexts. That is one of the technical problems that the latest experiments have overcome. Another was that the right part of the genome was edited. In the latest paper researchers looked at an inheritable heart disease caused by an error in a single gene which affects about 1 in 500 adults. Even when it is not fatal, it causes great misery, since a diagnosis means that you could die at any moment, and may transmit this condition to your children.