Editing the genes of the human embryo can have unintended consequences for human health and for society as a whole. Therefore, when a Chinese scientist used this method in an attempt to make children more resistant to HIV, many were quick to condemn the move as premature and irresponsible. Nature asked researchers what prevents this procedure from being considered an acceptable clinical practice.
Attempts to make inheritable changes in the human genome have been controversial. Here's what you need to do to make this technique safe and acceptable.
Six months after the wedding, Jeff Carroll and his wife decided not to have children. Carroll, a 25-year-old former U. S. Army corporal, just learned that he has a mutation that causes Huntington's chorea, a genetic disorder that damages the brain and nervous system and invariably leads to premature death. About four years ago, his mother was diagnosed with the disease, and now he has learned that he too will almost certainly get sick.
Faced with a 50% chance of passing on the same grim fate to their children, the couple decided that children were out of the question. “We just closed the topic,” says Carroll.
While still in the army, he began studying biology in the hope of better understanding his illness. He learned that there is such a procedure as preimplantation genetic diagnosis, or PGD. Carroll and his wife could practically rule out the possibility of mutation transmission through in vitro fertilization (IVF) and embryo diagnostics. They decided to try their luck, and in 2006 they had twins without Huntington's mutation.
Carroll is now a researcher at Western Washington University in Bellingham, where he is applying another technique that could help couples in his plight: CRISPR genomic editing. He has already used this powerful tool to alter the expression of the gene responsible for Huntington's disease in mouse cells. Because Huntington's chorea is caused by only one gene, and its consequences are so devastating, it is this disease that is often cited as an example of a situation in which gene editing in the human embryo - a procedure that can cause changes inherited by future generations, and therefore is controversial - can really be justified. But the prospect of using CRISPR to alter this gene in human embryos still worries Carroll. “This is a huge milestone,” he says. - I understand,that people want to pass it as soon as possible - including me. But in this matter all ambitions must be dropped. The procedure can have unforeseen consequences for human health and for the whole society. It will take decades of research before the technology is safe, he said.
Public opinion about editing genes to prevent disease is generally positive. But Carroll's reticence is shared by many scientists. When news broke last year that a Chinese biophysicist was using genome editing to try to make children more resistant to HIV, many scientists were quick to condemn the move as premature and irresponsible.
Since then, several researchers and scientific societies have called for a moratorium on editing the inherited human genome. But such a moratorium raises an important question, says embryologist Tony Perry of the University of Bath, UK. “When can it be removed?” He says. - What conditions need to be met for this?
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Nature has asked researchers and other stakeholders what is preventing genetic gene editing from being considered an acceptable clinical method. Some scientific problems can probably be overcome, but it may be necessary to change clinical trials practice and find broader consensus on the technology for a method to be certified.
Past the Target: How Many “Mistakes” Can You Make?
Genome editing is technically challenging, but what has attracted the most attention is the potential for unwanted genetic changes, says Martin Pera, a stem cell researcher at Jackson's Bar Harbor, Maine lab. Yet, he adds, this is the problem that is likely to be the easiest to solve.
The most popular gene editing method is the CRISPR-Cas9 system. The mechanism itself is borrowed from some bacteria, which use it to protect against viruses by cutting DNA with the Cas9 enzyme. A scientist can use a piece of RNA to direct Cas9 to a specific region in the genome. However, as it turns out, Cas9 and similar enzymes cut DNA in other places, especially when there are DNA sequences in the genome that are similar to the desired target. These "side" incisions can lead to health problems, such as changing a gene that inhibits tumor growth can lead to cancer.
Researchers have tried to develop alternatives to the Cas9 enzyme that may be less prone to error. They also developed versions of Cas9 that give a lower error rate.
The error rate varies depending on which region of the genome the enzyme is targeting. Many gene editing enzymes have only been studied in mice or human cells grown in culture, not in human embryos. The error rate can be different in mouse and human cells, as well as in mature and embryonic cells.
The error count does not have to be zero. A small amount of DNA changes occur naturally every time a cell divides. Some say that certain background changes may be acceptable, especially if the method is used to prevent or treat a serious illness.
Some researchers think the CRISPR error rate is already low enough, Perry says. “But - and I think this is a pretty big 'but' - we have not yet figured out the specifics of editing human eggs and embryos yet,” he said.
Target, but not so: how accurate should genomic editing be?
A bigger problem than side effects can be DNA changes that are targeted but unwanted. After Cas9 or a similar enzyme cuts the DNA, the cell is left to heal the wound. But cell repair processes are unpredictable.
One form of DNA repair, or repair, is non-homologous end-attachment, which removes some of the DNA letters at the cut - a process that can be useful if the goal of editing is to disable the expression of a mutant gene.
Another form of repair, called homologous repair, allows researchers to rewrite the DNA sequence by providing a sample that is copied at the site of the cut. It can be used to correct a condition such as cystic fibrosis, which is usually caused by a deletion (loss of a portion of a chromosome) in the CFTR gene.
Both processes are difficult to control. Deletions caused by non-homologous end joining can vary in size, forming different DNA sequences. Homologous repair allows for better control over the editing process, but it occurs much less frequently than deletions in many cell types. Studies in mice could make CRISPR genomic editing more accurate and efficient than it is now, says Andy Greenfield, a geneticist at the Harwell Institute of the UK Medical Research Council, near Oxford. Mice breed large offspring, and so researchers have many attempts to achieve successful editing and get rid of all mistakes. The same cannot be said about human embryos.
It is not yet clear how effective targeted homologous repair will be in humans, or even how exactly it will work. In 2017, one group of scientists used CRISPR-Cas9 in human embryos to correct gene variants associated with heart failure. The embryos were not implanted, but the results showed that the modified cells were used as a template for DNA repair with the mother's genome, rather than the DNA template provided by the researchers. This may prove to be a more reliable way to edit the DNA of human embryos. But since then, other researchers have reported that they have not been able to replicate these results. "We don't yet fully understand how DNA repair occurs in embryos," says Jennifer Doudna, a molecular biologist at the University of California, Berkeley.“We need to do a lot of work with other types of embryos in order to understand at least basic things.”
Researchers are developing ways to solve the problems associated with DNA repair. Two papers published in June discuss the CRISPR system, which can insert DNA into the genome without disrupting both strands, thereby bypassing dependence on DNA repair mechanisms. If the systems successfully pass further testing, they may allow researchers to better control the editing process.
Another approach is to use a technique called basic editing. The basic editors contain a disabled Cas9 along with an enzyme that can convert one letter of DNA to another. The disabled Cas9 directs the base editor to a section of the genome, where it chemically modifies DNA directly without cutting it. Research published in April showed that some of these basic editors can also make unintended changes, but work continues to improve their accuracy.
"Basic editing does not currently meet our criteria," says Matthew Porteus, a pediatric hematologist at Stanford University in California. "But you can imagine that it will get better over time."
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Heidi Ledford