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For the first time, biologists are trying to get the CRISPR gene-editing machinery directly into mature human sperm, rather than into fertilised embryos. The work is still at an early stage but could lead to a new way to听prevent inherited diseases.

Around half a dozen teams have used the CRISPR genome-editing method to alter the DNA of human embryos but it is still far from clear whether this approach is safe. One issue is that the embryo can start dividing before its DNA is corrected, meaning the faulty gene is fixed in some cells in the resulting embryo but not all cells 鈥� a phenomenon called mosaicism.

So Diane Choi and her colleagues at the Center for Reproductive Medicine in New York are experimenting with delivering DNA coding for the CRISPR machinery to mature sperm cells rather than to embryos. At the听European Society of Human Reproduction and Embryology conference in Barcelona today, they听will announce that they may have found a way to do this that allows the sperm to remain relatively healthy, and potentially capable of fertilising eggs.

鈥淚n theory all single-gene disorders transmittable by the male can be treated if we are able to successfully use CRISPR-Cas9 on sperm,鈥� says Choi.

Electric zap

Gene-editing could in theory be used to prevent fathers from passing on听a wide range of genetic disorders. For example,听it could enable two people who both have a disorder like cystic fibrosis or sickle cell aneamia to have a child together who doesn鈥檛 have their condition.

The work is still at an early stage though, and Choi鈥檚 team have not yet shown whether the gene-editing machinery works inside the sperm. If it does work, it may do so only after the sperm fertilises an egg, rather than in the sperm itself.

鈥淚f we could do the editing in sperm, this would be fantastic. But it鈥檚 a big 鈥榠f鈥欌�� says Tony Perry of the University of Bath in the UK, a leading expert on CRISPR who was not involved in the latest work. 鈥淚 think it鈥檚 unlikely but I would love to be proved wrong.鈥�

One way to get DNA inside cells is to zap them with electricity. The challenge with sperm, which come wrapped in a tough, fibrous cuticle, is doing this without disabling them. 鈥淎s soon as you try to modify the membrane to put stuff in, you stop motility,鈥� says team member Gianpiero Palermo.

The team say a single 1100-volt pulse of electricity, just a fiftieth of second long, does the trick, though it does reduce sperm motility by half. So the team first selected the strongest and healthiest sperm before zapping. This meant that the resulting sperm were still听good enough swimmers to be used in IVF, rather than needing to be injected into an egg.

The big question is what happens next. For gene editing to take place, the CRISPR protein and the RNA that guides it to the right spot need to be made from the DNA that has been added to the sperm. The CRISPR/RNA complex then needs to find the target gene. For their research, the team are targetting a gene called laminin 伪1 which is associated with infertility.

When the CRISPR/RNA complex finds a target gene, it then cuts it. Finally the cell鈥檚 repaire machinery needs to kick in to repair the cut in such a way that the gene sequence is changed.

Will it work?

But it鈥檚 not clear if any of these things will happen in sperm. For instance, sperm DNA is very tightly packed, so the CRISPR complex might be unable to search through it to find the target gene. So Palermo thinks the editing might not take place until after the sperm fertilises an egg. If so, there would still be a risk of mosaicism, although for treating some conditions, like Duchenne鈥檚 muscular dystrophy, this wouldn鈥檛 matter.

鈥淭here鈥檚 still a lot to understand and discover about the sperm nucleus, which makes our project more difficult,鈥� says Choi. 鈥淏ecause we are in our initial steps of our project, I can鈥檛 tell you which theory is right or wrong.鈥�

Perry says that several teams around the world have tried hard to genetically modify mature sperm using older techniques, without success. But no one else has tried with CRISPR, as far as he knows.

If sperm cannot be modified with CRISPR either, an alternative approach would be to edit the genomes of the stem cells that give rise to spermatozoa. This is not yet possible in humans but is widely expected to be done within years.

It听is already possible for many people to prevent passing most genetic disorders on to their children, without any need for gene editing. This can be done by fertilising a number of eggs using IVF, and screening for an embryo that will not go on to develop a genetic disorder. This approach involves discarding some embryos though, and some people think it is ethically preferable to fix embryos with faulty genes rather than throw them away.

However, there may be cases where both parents have genetic disorders in their DNA, meaning that the method of screening IVF embryos will not work for them.

Read more: First results of CRISPR gene editing of normal embryos released

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