Safe gene-editing for babies still blocked by mosaicism

mosaicism blocks – A US team reports improved CRISPR base editing in healthy embryos with far fewer unwanted mutations. The work revives hopes for safer germline gene editing—but a single, stubborn barrier remains: mosaicism, where not every cell carries the intended edit, leavi

In the wake of a 2018 revelation from a rogue researcher in China—CRISPR used to create three gene-edited children—the scientific community didn’t just recoil. It focused on the central fear: the CRISPR method was not safe, with a very high risk of harmful mutations.

So when new results from the US arrive, they land differently. A team in the US used an improved form of CRISPR. known as base editing. to edit healthy embryos and reported that it can be done without introducing unwanted mutations. The question quickly becomes the one regulators and bioethicists keep circling: are we finally close to safety enough to consider gene-editing babies?.

The answer, for now, is no—because one major obstacle still isn’t solved: base editing doesn’t work in every cell. And in an embryo, that is the difference between a fix and a gamble.

To understand why, you have to start with how CRISPR works. The first CRISPR version uses a protein called Cas9. Cas9 latches onto guide RNA that helps it find a specific spot in the genome. Once it arrives, Cas9 cuts through both strands of DNA. The cell’s repair process often makes mistakes, introducing small mutations that can disable genes.

Even when CRISPR-Cas9 works as intended, it remains destructive. DNA cut ends can be reattached in the wrong places, creating large mutations and chromosomal abnormalities. Over the years, many improved approaches have emerged. Base editors. for example. are designed to change a single DNA letter to another and cut only a single strand of DNA—aiming for precision with much less chance of something going seriously wrong.

Base editing is no theoretical promise. The technique has already saved lives, and a number of trials are under way, including studies testing it as a treatment for conditions that result in very high cholesterol.

But embryos are not adults. For people treated with gene editing, imperfections can sometimes be tolerated because only a fraction of cells need to be successfully edited. A common example is the liver: often only a fifth of cells need to be corrected to treat a disease.

A human embryo is different. In an embryo, the edits have to work perfectly because that embryo will give rise to every cell in the body. In other words, success in the wrong cells isn’t just incomplete—it can be misleading.

The promise of base editing in embryos was first suggested in 2017 by a team in China. In a small study, the researchers used human embryos discarded during IVF because of abnormalities. They reported that base editing made the desired change in almost every embryo with very few unintended changes.

Now Dieter Egli at Columbia University in New York and his colleagues have taken the next step with a larger study. They worked with healthy two-cell embryos donated by parents, reporting broadly similar results. The team tried making two changes. One was successfully made in three-quarters of cells, with no unwanted changes. The other change worked only in around half of the cells, and it often caused unwanted changes.

The researchers point to an explanation that sounds technical but is crucial to safety: guide RNAs used to direct the editing. They believe that better design and testing of guide RNAs should help avoid off-target effects.

Even if guide RNA design improves, the bigger problem sits somewhere else. Base editing didn’t work in every cell in each embryo—a problem known as mosaicism. If a mosaic embryo develops into a child. only some of the cells in their body will carry the intended change. That means the edited child could still develop the disease that the gene editing was meant to prevent.

The implications are stark. The three gene-edited children growing up in China may all be mosaics—an unsettling detail that underscores why safety is not just about avoiding unintended mutations at the genome level. It’s about making sure the intended change lands everywhere it needs to.

And there is currently no way to be sure a gene-edited embryo isn’t a mosaic. When there is a risk of children inheriting a serious disease. clinicians can remove a single cell from IVF embryos for genetic testing. That approach could be used for gene-edited embryos too—but if the embryo is mosaic. testing a single cell isn’t enough to reveal what’s happening across the developing body.

So even these latest results, promising as they are, are not likely to convince regulators that germline gene editing is safe. Germline gene editing is the kind that would affect a person’s reproductive cells and can be passed on.

For now, mosaicism has to be solved first.

One proposed path would be to edit gene-carrying cells before fertilisation—using gene-edited sperm or eggs. If the editing is done before an egg is fertilised and begins to divide, mosaicism should not occur. This hasn’t been done in humans yet.

But a recent claim from a start-up has raised the possibility of getting there through lab-generated cells. The start-up said it can generate sperm in the lab from sperm stem cells. If that claim holds up, it should be possible to gene-edit those sperm stem cells before fertilisation.

That sort of approach might bring gene editing for children into the realm of safer feasibility. Whether society should allow it at all is a separate argument—one that will likely intensify as the science edges closer to what regulators demand.

CRISPR base editing gene editing babies germline gene editing mosaicism Dieter Egli Columbia University IVF embryo genetic testing high cholesterol trials