The power to change your DNA EQUINOX GRAPHICS/SCIENCE PHOTO LIBRARY

A fourth paper describing attempts to correct defective genes in human embryos using CRISPR is about to be published. A report in MIT Tech Review says the results are the most promising so far but the lack of detail means it is not possible to judge whether this really is the case.

Several groups around the world are working on gene editing in human embryos, using the revolutionary CRISPR technique. Some want to do this to study embryonic development, with the ultimate aim of improving fertility treatments. Others are interested in its potential for correcting genetic diseases.

Is working on human embryos moral? See our expert discussion at New Scientist Live

To date, just three teams – all in China – have published results in scientific journals. The first two studies used defective embryos and got poor results. The third study, revealed by New Scientist in March, got more promising results but it was based on editing just six normal embryos.


Now a team led by Shoukhrat Mitalipov of Oregon Health and Science University is about to publish details of a bigger study based on editing “many tens” of embryos. In 2013, Mitalipov was the first to derive stem cells from human embryos created by cloning adult cells.

Mosaic problem

Mitalipov’s team has reportedly made progress in tackling the biggest safety issue in editing embryos: mosaicism. This occurs when genome editing does not take place until after the DNA of a fertilised egg has begun to divide, meaning only some cells in the resulting embryo have the desired change, and any resulting children might still develop the disease that was supposed to be prevented.

The trouble with mosaicism is that there is no sure way to detect it before implanting an embryo into the uterus. High rates of mosaicism were seen in the first three CRISPR human embryo studies.

Now Mitalipov’s team has apparently managed to reduce the rate of mosaicism by injecting the CRISPR machinery at an earlier stage, at the same time as the eggs were fertilised with sperm. Until the numbers are published, it will not be clear to what extent this reduced mosaicism.

Mosaicism is also a problem for the hundreds of teams creating gene-edited animals, so many groups are working on ways to reduce it. One sure way to prevent it would be to edit the genes of sperm or eggs rather than of embryos.

The other big issue with the idea of using genome editing to correct genetic diseases is that there is actually little need for it. The vast majority of serious genetic diseases, such as cystic fibrosis, can already be prevented by various forms of screening, such as generating several embryos by IVF and implanting only the ones that don’t carry a genetic disease.

Read more: Boom in human gene editing as 20 CRISPR trials gear up