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The team tried to tweak the gene responsible for ß-thalassaemia, a potentially deadly blood disorder, using a germ line editing technique known as CRISPR/Cas9.

CRISPR technology precisely changes target parts of genetic code.

Unlike other gene-silencing tools, the CRISPR system targets the genome’s source material and permanently turns off genes at the DNA level.

The DNA cut – known as a double strand break – closely mimics the kinds of mutations that occur naturally, for instance after chronic sun exposure.

But unlike UV rays that can result in genetic alterations, the CRISPR system causes a mutation at a precise location in the genome.

When cellular machinery repairs the DNA break, it removes a small snip of DNA. In this way, researchers can precisely turn off specific genes in the genome.

The team studied the ability of the CRISPR/Cas9 system to edit the gene called HBB responsible for ß-thalassaemia

Huang’s group studied the ability of the CRISPR/Cas9 system to edit the gene called HBB.

The team injected 86 embryos and then waited two days for the CRISPR technique to work.

Nature reports that of the 71 embryos that survived, 54 were genetically tested. The tests found that 28 were successfully spliced, but only a few contained the replacement genetic material.

‘If you want to do it in normal embryos, you need to be close to 100 per cent,’ Huang told Nature. ‘That’s why we stopped. We still think it’s too immature.’

Until now scientists have concentrated on using genome editing techniques on somatic cells – non-reproductive cells like those found in skin, muscle, nerves, bone and liver for example – to help them study diseases.

Modifying the DNA of human embryos so that DNA changes are preserved in the germ line – the sperm and eggs – means such alterations can be passed on to future generations.

Such work is already illegal in many countries around the world including the UK, but is permitted in the US and China.

A number of other groups around the world are thought to be working on techniques to genetically modify human embryos.

Xingxu Huang, from Shanghai Tech University in China has been seeking permission to perform the technique on discarded human embryos after having successfully using it to modify monkey embryos that developed into live animals.

Professor George Church, a geneticist at Harvard Medical School is also said to be developing techniques that could be used to modify human embryos.

Dr Luhan Yang, a scientist with Church’s group, recently outlined proposals to edit the human germline in an interview with Technology Review.

She proposed using immature egg cells from the ovaries taken from women undergoing surgery for ovarian cancer.

The immature egg would be grown and coaxed into dividing before a gene-editing technique known as CRISPR would be used to correct the genetic fault that was causing the ovarian cancer.

However, the project is not thought to be underway yet and it is not known if it has been approved.

Dr Yang, however, has been reported to have been involved in a similar project in China.

British and US scientists have already expressed concerns over the ethics of the latest experiment, with one doctor calling for the research to be banned.

Dr David King, from watchdog Human Genetics Alert, said: ‘This news emphasises the need for an immediate global ban on the creation of genetically modified designer babies.

‘It is critical that we avoid a eugenic future. If China does not want to get the reputation of being the wild west of the new eugenics it must join the many countries – including the UK -which would ban such research.’

Dr George Daley, a stem cell biologist at Harvard Medical School, said the experiment poses a ‘cautionary tale’.

He said: ‘Their study should be a stern warning to any practitioner who thinks the technology is ready for testing.’

Professor Shirley Hodgson, an expert in cancer genetics at St George’s, University of London, said: ‘This is a significant departure from currently accepted research practice. Can we be certain that the embryos were indeed non-viable?

‘The fact that there were a number of “off-target” mutations is clearly a worry.’

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