As the creator of “CRISPR babies” approaches his release from prison, where does embryo editing stand? | Science
Biophysicist He Jiankui, who served a 3-year sentence for creating the world’s first genetically modified babies, could be released from a Chinese prison this week, Science has learned. He largely secretly uses the CRISPR genome editor to edit the DNA of human embryos and implant them in two women, which has led to three births, sparking ethical outrage and fears for the health of the babies (including little is known). This did not, however, end basic research into human embryo editing.
The response to He’s announcement in November 2018 has been “stern and dynamic,” says Fyodor Urnov, who studies CRISPR-based genome editing at the University of California, Berkeley. For now, Urnov sees no circumstances that would justify efforts to genetically modify babies. But he strongly supports using CRISPR to correct disease-causing mutations after birth, without causing heritable changes to a human genome, and laments that “we’ve poured a pot of tar on gene editing.” And Urnov and others believe that, used responsibly and safely, embryo editing could eventually prove a powerful tool against disease in rare circumstances. In laboratory studies, they continue to explore the possible avenues and the many obstacles.
Work continued with little notice. “The pandemic has pushed this topic out of people’s mainstream attention,” says Alta Charo, a bioethicist emeritus and attorney at the University of Wisconsin, Madison, who notes that surveillance measures intended to stop rogue experiments like He are stalled project, including a global registry of preclinical research on inherited genome editing.
This type of registry might have noted a study reported last week in which a research team working with surplus human embryos from in vitro fertilization (IVF) clinics showed how CRISPR could rid a newly fertilized egg of an extra copy. of a chromosome – a problem that can lead to Down syndrome and other medical conditions. Other groups are studying how to introduce inherited genetic changes via human sperm or eggs. There are “a lot of people pushing the envelope” in this regard, says Robin Lovell-Badge, a developmental geneticist at the Francis Crick Institute, although few, if any, think the work is ready for the clinic. “We’re still waiting for better tools,” says developmental biologist Shoukhrat Mitalipov of Oregon Health & Science University.
Initial concerns about designer babies centered on CRISPR’s lack of rigor. The DNA-cutting enzyme, which is one of its two components, occasionally cuts at unintended points, and even if the cut is targeted, the cell’s gene-repair equipment can scramble adjacent DNA by inserting or removing bases, potentially creating new damage. Indeed, a study of CRISPR-edited human embryos found that 16% had these “unintended editing outcomes” at the targeted DNA level, a group led by Crick’s Kathy Niakan reported last year in the Proceedings of the National Academy of Sciences.
Genetic screening of modified IVF embryos may not detect these errors. Although CRISPR is introduced just after fertilization, at the single-cell stage, its action is not necessarily immediate. “The change can occur at the two- or four-cell stage, so not all cells are the same,” Lovell-Badge explains, a phenomenon called mosaicism. Improperly modified and unmodified cells can easily go unnoticed, as an embryo is examined by taking a sample of its cells at the 5-day stage, when it contains around 100 cells. “If you have mosaicism, then you don’t know what you have in the rest of the embryo,” says Lovell-Badge.
Columbia University stem cell researcher Dietrich Egli hopes to find a way to start and stop CRISPR at the single-cell stage of the embryo, thereby preventing mosaicism. Meanwhile, his group has found a specific type of CRISPR modification for an embryo that significantly reduces the risk of unintended DNA changes.
One of the most common abnormalities seen when IVF clinics examine embryos, especially those made with eggs from older people, is the presence of one or three copies of certain chromosomes rather than the normal two. In a preprint published on bioRxiv on March 10, Egli’s group demonstrated a strategy for trisomy, a wandering third chromosome. The scientists showed they could target an extra copy of the paternal or maternal chromosome with a CRISPR cut at or near its centromere, the DNA-protein structure that holds the different arms of a chromosome together. The extra chromosome then falls apart during cell division. Unintended on- or off-target changes would theoretically not matter because CRISPR would, in effect, destroy the entire DNA sequence.
Mosaicism could still be a problem if CRISPR doesn’t correct trisomy in all cells of an early embryo, but Egli notes that when those embryos have a mix of cells with normal and abnormal chromosomes, a natural “rescue mechanism” generally seems to eliminate abnormal cells. “There are still many obstacles,” he said. “We could have given it a different title, ‘Correction of Trisomy 16 in the Human Embryo’, and we might have created more buzz and news articles, but we didn’t think it was was appropriate because it indicates that you are going to do this clinically tomorrow, which is absolutely not the case.
Researchers studying CRISPR in human embryos face obstacles beyond science. In the United States, Congress prohibits government funding of human embryo research, forcing Egli, Mitalipov and others to rely on foundations, academic institutions or corporations. The legislation also prevents the US Food and Drug Administration from evaluating even therapies that modify human embryos.
Editing the DNA of egg cells or sperm precursor cells can avoid some of these obstacles. It also circumvents what Kyle Orwig, a reproductive biologist at the University of Pittsburgh, calls “a numbers problem.” Even under the best of circumstances, IVF clinics could only create, modify, and test a small number of embryos for any given couple, giving them little chance of getting it right.
Altering the cells that give rise to sperm could improve the odds. Researchers have already taken these spermatogonial stem cells from mice and grown millions of them. This allows for rigorous quality control of CRISPR edits: scientists can track down stem cells that have the right edit, without unintended DNA changes, and clone them en masse, again checking for errors. Then they can transplant those cells into the testicles where they should produce mature sperm, Orwig says. Indeed, rodents with edited sperm stem cells have been used to create offspring with a desired DNA editing.
Turning this basic research into a way to help potential parents won’t be easy. “The barrier is that we don’t yet know how to maintain human spermatogonial stem cells in culture,” says Orwig. His team is exploring a different route to creating modified sperm stem cells: “reprogramming” adult human cells into a stem cell state and trying to get them partly on the pathway that creates sperm. Other groups hope that reprogrammed adult cells could one day produce human eggs, which could then be modified in large numbers.
Unfortunately, in mice, spermatogonial stem cells only survive when placed in newborn animals, which is not a realistic option for humans. As a first step in determining whether the program could work in humans, Orwig’s team is currently recruiting men who have become infertile due to cancer treatment and whose testicular tissue or cells were frozen before chemotherapy. or radiotherapy. The team plans to isolate the spermatogonial stem cells from the thawed tissue and then inject them, unmodified, into the owner’s testicle to see if this produces viable sperm.
The 3 years since he went to prison have seen glimmers of progress in hereditary editing of the human genome, but many scientists say the heightened awareness of CRISPR’s shortcomings has underscored the recklessness of embryo transplantation modified with the technology available today. One exception is Russian geneticist Denis Rebrikov, one of the few scientists after the He scandal to openly advocate implanting modified embryos into people. “We’ve done a lot of validation experiments, and now we’re confident that we can move into real clinical use,” says Rebrikov.
Lovell-Badge speaks for most researchers when he says such confidence is unwarranted. Stick to lab work on embryo editing for now, he advises. “People should do as much preclinical research as possible, and let’s find out if it’s doable.”