The most risky aspect of gene editing is accidentally editing the wrong gene, causing problems that can be passed down generations.
Reproductive biologist Shoukhrat Mitalipov led a team of scientists from Oregon Health & Science University, Portland, and the Salk Institute, California, as well as collaborators in South Korea and China, to make a pretty huge breakthrough in gene editing. They targeted a dangerous mutation in a gene which makes seemingly healthy adults prone to sudden and fatal heart attacks. Using their technique, they were able to get rid of the mutation in embryos that would’ve gone on to become a baby that would’ve grown up without the disease. So, thanks to Mitalipov & co., it would now seem that we are closer to a future where disease-causing mutations in genes can be corrected to ensure healthy babies. As if the nature of his research wasn’t newsworthy enough, there was a further flutter of excitement thanks to a mysterious leak.
What was the leak?
Mitalipov’s paper was due to be published in Nature journal on August 2 (until the date and time of publishing, studies are under strict embargo). But somehow, parts of the media caught wind of it almost a week earlier (examples here and here). There is still no clarity on how this happened. The jury is also out on what the implications of such high profile leaks in scientific research are.
What is the gene that was edited?
The gene is called MYBPC3. We all have two copies of it in each of our cells. It plays a major role in maintaining the structure and function of the heart. About 1 in 500 (0.2%) of us inherits a mutation in one of our MYBPC3 genes. This makes us susceptible to a condition called hypertrophic cardiomyopathy (HCM). It manifests in young, otherwise healthy, carriers in the form of sudden heart attacks. In fact, HCM is known as the most common form of sudden death among young athletes and is even more alarmingly common among Indians. Some studies say that more than 4% of Indians carry this mutation.
A parent who is a carrier of a mutated gene has a 50% chance of passing the defect down to his or her offspring – provided the other parent’s gene does not have the mutation.
When do you edit a gene?
An adult human body has billions of cells in it, so correcting a gene defect in adults is not an option. An ideal patient for gene editing would have just one cell so that, if the mutation in that single cell is corrected, the newly healthy cell can divide to more healthy cells and eventually develop into a healthy individual. The single cell from which the ball of development starts rolling is called a zygote. It is formed from the fusion of the egg and the sperm cells. It then multiplies to form an embryo and, over the next six weeks, a developing foetus.
How did Mitalipov do it?
The most risky aspect of gene editing is accidentally editing the wrong gene, causing problems that can be passed down generations. This forces us to confront many ethical issues (see below), and this is also why gene editing studies on human embryos is either banned or tightly regulated around the world. So first, Mitalipov had to receive the adequate permissions and scientific and ethical reviews in the US before his team was allowed to work with human embryos. The scientists used sperm samples from an HCM patient. Among the two copies of the gene this individual’s cells possessed, the MYBPC3 gene was mutated in one while the other was healthy. The eggs were obtained from a healthy female donor (both copies of whose gene were healthy), and they were fertilised with the sperm cells in the lab.
A child born from such a union would have a 50% chance of inheriting the HCM mutation from the father. At this juncture, the scientists used a five-year-old gene-editing technology called CRISPR/Cas9. CRISPR/Cas9 works like a customised pair of scissors: they programmed it such that it cut the gene at the start of the mutated part (in this case a tiny missing section in the MYBPC3 gene). The cell notices this cut and proceeds to repair it using the other copy of the gene as a template. And since the other copy doesn’t have the mutation, the cell repairs the cut into a fully healthy one. Thus, the process began with the presence of a mutation and ended without it.
Previous experiments with CRISPR-Cas9 in human zygotes have not been very successful. Sometimes the embryos of edited zygotes were found to consist of a mosaic of cells, some with the mutation and some without. To avoid this, Mitalipov started one step earlier. He injected CRISPR/Cas9 into the egg along with the sperm itself, rather than after fertilisation. In this way, he was able to produce embryos whose every cell was edited. Ergo, no mosaicism.
Ever since its discovery in 2012 by Jennifer Doudna, of the University of California, Berkeley, and Emmanuelle Charpentier, from the Helmholtz Centre for Infection Research, Germany, this technique has captured the imaginations of people around the world because of the ease, precision and efficiency with which it allows scientists to edit genes. It was even a favourite to win the 2015 Chemistry Nobel Prize, though it finally didn’t.
What was the breakthrough?
All 58 of Mitalipov’s mutation-carrying embryos were cut at the right spot. Forty-two of them were successfully edited. That means the chance of healthy offspring rose from 50% without this procedure to 72.4% with. The fact that the team was able to accomplish this without mosaic embryos (with both defective and corrected cells) is highly encouraging. If their methodology – injecting the sperm and the CRISPR/Cas9 molecule simultaneously into the egg – is tested and shown to consistently prevent mosaicism, it would mean one less giant hurdle for the future of gene editing. Also significant is the fact that they did not seem to have induced any unwanted mutations in non-target areas.
What should we watch out for?
Just because they detected no off-target mutations need not mean there weren’t any. In his blog, American biologist Paul Knoepfler expressed optimism at the quality of this study but also noted: “I do not believe they can be quite so confident about ‘no off-target activity’, when as best as I can tell they did not look thoroughly in enough embryos and cells and in an unbiased manner at the whole genome to really be sure about this.” Discussions about this are sure to follow in the coming weeks.
Mitalipov leaves no speculation about his aspirations for this research. He is known to have a strong opinion, that basic research such as this must go on to become treatments available to people who need it. His other major contribution was developing mitochondrial replacement therapy – a way to prevent babies from inheriting a life-threatening disease from the mother by using DNA from a female donor. These babies would now have DNA from three adults rather than two, earning them the tag ‘three-parent babies’. Despite the controversies that accompanied this, at least two ‘three-parent’ babies have been born in Mexico and, reportedly, in Ukraine.
Much less successful attempts to edit human embryos (until now, all from China) have been followed by deep debates about the need for such technologies. The fear of them heralding an era of designer babies, where parents can pick and choose traits considered more desirable, opens up an ocean of ethical implications. Moreover, mistakes in editing can introduce more problems that may persist in future generations. Given that the easy and efficient CRISPR/Cas9 technology is in the picture and is presumably here to stay, it is good that progress is being made to make it a safer method.
In-vitro fertilisation techniques today already make genetic screening methods for embryos available so that doctors can ensure only healthy ones are transferred into the uterus. So we may not always have to resort to gene editing. Nevertheless, the future of gene editing as a therapeutic technique will depend not just on breakthroughs like these but also on the success of studies that attempt to reproduce these results – and on all stakeholders mindfully considering the implications.