The Guardian  Ian Sample Science Editor 9-28-17

Instead of cutting right through the double strand of the DNA helix like Crispr, the new technique changes single letters, or bases, of the G, T, A and C that make up the genetic code. Photograph: Alamy Stock Photo

Scientists have used the technique, also known as ‘base editing’, for the first time in human embryos to change a single letter in a faulty gene.

Researchers in China have used a procedure described as “chemical surgery” to mend harmful mutations in human embryos for the first time.

The scientists found that it was possible to repair a faulty gene that gives rise to a serious blood disorder called beta thalassemia which can be caused by one misspelling in the DNA code.

None of the embryos treated in the experiments were used to produce babies, and doing so would be illegal in the UK and many other countries. But the work proves that the method, known in genetics as “base editing”, could be an effective way to prevent inherited diseases. Base editing was dubbed  “chemical surgery” by its inventor, David Liu at Harvard University.

In the past decade, scientists have developed a range of powerful tools to edit the genetic code, the most popular of which is called Crispr-Cas9. The method uses enzymes to make precision cuts in faulty genes which the body can then repair with the correct DNA.


What is Crispr?

In 2015, scientists led by Junjiu Huang at Sun Yat-sen University in Guangzhou, China, tried to use Crispr to correct abnormal beta thalassemia genes in human embryos  without much success. In the latest breakthrough, the same team turned to base editing instead.

To perform the experiments, Huang first created a batch of cloned embryos. His team took skin cells from patients with beta thalassemia, removed their DNA-containing nuclei, and dropped them into donor eggs that had their own nuclei removed. The eggs then developed into early stage embryos that carried the beta thalassemia mutation.

Base editing uses similar enzymes to Crispr, but instead of cutting right through the double strand of the DNA helix, the enzymes instead change single letters, or bases, of the G, T, A and C that make up the genetic code. For many patients with beta thalassemia, the disease is caused by a mutation that switches an A for a G.

Writing in the journal  Protein and Cell, the researchers show that the new procedure worked to some extent. Humans carry two copies, or alleles, of every gene and in many cases both versions have to be “healthy” to avoid disease. In the study, base editing sometimes repaired only one faulty gene rather than both, creating so-called mosaic embryos that had both normal and mutant cells. “This looks very promising, but all the embryos from which they were able to get information from more than one cell were clearly mosaic, with some cells still carrying both mutant alleles,” said Robin Lovell-Badge at the Francis Crick Institute in London.