Scientists in the US have repaired a disease-causing mutation in the DNA of early-stage human embryos, on 2 August, taking a step closer to engineering babies free of inherited disorders.

The team's successful use of the CRISPR ‘gene editing’ tool in viable embryos was hailed as a technical feat by outside experts, who called at the same time for deeper debate on the ethics of altering human DNA.

The lab-created, edited embryos were not allowed to develop beyond a few days, when they comprised a handful of cells.

This was an improvement on the 50 percent odds for embryos that result naturally from a couple in which one partner carries the coding error.

With further improvement, said study co-author Paula Amato of the Oregon Health & Science University, the method “can potentially be used to prevent transmission of genetic disease to future generations.”

Further research is needed to determine the technique's safety, and to boost its accuracy to as close to 100 percent as possible before it can be used to create embryos meant to develop into healthy babies.

But there is also the prospect of avoiding heritable, genetic diseases that can handicap or kill.

It is a type of disorder, like Huntington's Disease, that requires an abnormal gene from just one parent.

The researchers used sperm from a donor carrying the mutation, and eggs from healthy women. They injected the CRISPR-Cas9 editing tool into the eggs at the same time as the sperm – a major departure from what other teams have done.

In previous CRISPR studies in China, the gene-editing tool was added only after fertilisation.

Those teams had problems with ‘mosaicism’, which occurs when some cells in an embryo are corrected, and some not.

In the new study, just one embryo turned out a mosaic.

For Peter Braude, a reproductive health expert from King's College London, the study showed that ‘germline genome editing has moved from future fantasy to the world of possibility.’

The debate about using it in practice, he added, ‘needs to run to catch up.’

Currently, the only way to avoid heritable disease in assisted reproduction, is to fertilise eggs in the lab, analyse the DNA of the resulting embryos, and eliminate those containing errors.

In 2015, a UN bioethics committee called for a stop to human embryo gene editing for fears it could be used to modify the human race.

Last year, Britain granted scientists permission to edit embryo DNA in research on the causes of infertility and miscarriages.

And in February this year, a US science advisory committee said such modification should be allowed in future to eliminate disease.

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