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What the researchers actually did to the embryos
The team led by Dieter Egli (Columbia University) worked on human embryos at the single-cell stage, just after fertilization. They targeted three specific genes. The first, PCSK9, controls the production of "bad" cholesterol. Disabling it would reduce the risk of heart attack. The other two, HBG1 and HBG2, produce a blood protein in the fetus. Mimicking them could alleviate diseases like sickle cell disease. For each gene, the researchers replaced one chemical letter: they turned an adenine (A) into a guanine (G).
To understand, picture DNA as a very long sentence written with four letters: A, T, G, C. Each letter is called a nucleotide. A single misplaced letter can cause a serious disease. Here, base editing acted like an eraser that removed an A and wrote a G instead. Across all embryos, this operation succeeded three out of four times for PCSK9, but only one out of two times for HBG1/2. Why the difference? According to the study, it all depends on the small guide RNAs, the molecules that direct the tool to the right spot.
The technical advantage is real: base editing caused neither large mutations nor chromosomal abnormalities, unlike what was observed with older CRISPR-Cas9. This is a world first on healthy embryos. Yet the team remains cautious. They noted that introducing the tool in RNA form blocked embryo development. So they had to use another method, in protein form, which is more stable. Despite these advances, the problem of mosaicism persists: some cells retain the original version of the gene. As long as this flaw exists, no doctor will offer this technique in a clinical setting.
Why modifying an embryo divides specialists so much
On one side, scientists see a medical breakthrough. Preventing a child from being born with a serious genetic disease, such as cystic fibrosis or Huntington's disease, would be a revolution. Base editing makes this hypothesis more realistic than it was five years ago. But on the other side, ethicists are sounding the alarm. For them, opening this door means accepting the risk of eugenic abuses. One day, people might modify not a disease, but eye color, height, or even intellectual traits.
Chinese researcher He Jiankui experimented with this in 2018: he implanted modified embryos using a dangerous technique, and two baby girls were born. Since then, the global scientific community has condemned this practice.
Yet in the United States, no federal law bans the genetic modification of embryos for reproductive purposes. Only a lack of public funding is holding back research. Dieter Egli says he opposes any genetic "enhancement," but he acknowledges that his work could be misused. Hank Greely, a bioethicist at Stanford, sums up the problem: with a few million dollars, a private clinic could attempt the venture.