Each year, millions of people around the world are affected by diseases caused by mutations that occur in the very early stages of development.

But many of those diseases could soon cease to exist, thanks to a gene editing technique that uses the controversial CRISPR-Cas9 system.

In a world first, scientists have used the technique to correct a mutation for a heart condition in embryos, so that the defect would not be passed on to future generations.

The findings could pave the way for improved IVF outcomes, as well as eventual cures for some of the thousands of diseases caused by mutations in single genes.

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The work is a collaboration between the Salk Institute, Oregon Health and Science University (OHSU) and Koreas Institute for Basic Science.

Professor Juan Carlos Izpisua Belmonte, an author of the study, said: Thanks to advances in stem cell technologies and gene editing, we are finally starting to address disease-causing mutations that impact potentially millions of people.

Gene editing is still in its infancy so even though this preliminary effort was found to be safe and effective, it is crucial that we continue to proceed with the utmost caution, paying the highest attention to ethical considerations.

In the study, the researchers were able to correct a mutation that causes an inherited heart disease, called hypertrophic cardiomyopathy (HCM).

HCM is an inherited disease of your heart muscle, where the muscle wall of your heart becomes thickened.

A: An incredibly powerful gene-editing tool that is transforming the way DNA is manipulated and modified. First demonstrated in 2013, it is based on a system bacteria use to defend themselves against invading viruses.

A: In its most basic form, the gene editing tool kit consists of a small piece of RNA a genetic molecule closely related to DNA and an enzyme protein called Cas9.

The RNA component is programmed to latch onto a specific DNA sequence. Then Cas9 slices through the strands of DNA, like a pair of molecular scissors.

A: By cutting away precisely targeted elements of DNA, active genes can be switched off. Defective parts of a gene can also be removed, allowing the fault to be repaired.

A: Here, nature comes into play. Once a piece of DNA has been snipped out in a cell, natural repair systems kick in to try to repair the damage.

More advanced gene editing systems include additional template DNA the cell can use to mend the break, making it possible to re-write the genetic code.

This was what the scientists conducting the new research planned to do. In the event, the embryos went their own way.

Instead of adopting the researchers template, their cells exploited the fact that only one copy of the gene carried by sperm was defective.

They based their repairs on the other, functioning, copy of the gene inherited from the women who donated their eggs for the research.

A: A lot more research has to be done before the technique is shown to be safe and effective enough to be used in the clinic.

Also, altering nuclear DNA in a developing embryo is currently illegal.

A change in the law would be needed before such treatments can be considered, and this would involve addressing some profound ethical questions.

If in future gene editing of embryos is given the green light, it could potentially prevent thousands of diseases being passed onto future generations.

It is caused by a mutation in the MYBPC3 gene, and those affected have a 50 per cent chance of passing the disease on to their own children.

Using a skin biopsy from a man with HCM, the researchers generated stem cells to use in their study.

The researchers used a technique based on CRISPR-Cas9 a genetic tool that can cut and paste small sections of DNA, deleting or repairing flawed genes to correct the HCM mutation in the cells.

CRISPR-Cas9 works as a pair of genetic scissors designed to cut the DNA near the position of the mutation.

The cut is then spontaneously repaired by the cell with different mechanisms: one repairs the DNA without leaving any trace, while the other introduces some unwanted insertions or deletions of a few base pairs near the cutting site.

While previous studies have injected CRISPR-Cas9 after IVF, they faced problems due to mosaicism in which embryos have some repaired cells, and others that carry the mutation.

To overcome this issue, the researchers injected the CRISPR-Cas9 and the sperm into the egg at the same time.

Using this technique, they found that the mosaicism did not occur.

During testing, CRISPR-Cas9 cut the DNA at the correct position in all tested embryos.

Forty-two out of the 58 embryos tested did not carry the HCM mutation.

In other words, this technique increased the probability of inheriting the healthy gene from 50 per cent to 72.4 per cent.

The highly controversial technique is still at an early experimental stage.

There is no question of any attempt being made to create babies with the genetic modification, which would be illegal both in the US and the UK.

But a leading member of the team has hinted that first steps towards bringing the treatment to patients could take place in the UK under the direction of the fertility regulator the Human Fertilisation and Embryology Authority (HFEA).

Dr Shoukhrat Mitalipov, from Oregon Health and Science University (OHSU) in Portland, said in a telephone briefing with journalists: Maybe .. (the) HFEA might take a lead on this, but Im quite sure before these clinical trials can go on they have to go through, I believe, Parliament to change a law.

So there is still a long road ahead, particularly if you want to do it in a regulatory way.

US regulatory barriers to such research are so high they could be insurmountable.

In the US, taxpayer funds cannot pay for research that destroys human embryos.

And Congress has banned the US Food and Drug Administration (FDA) from even considering the possibility of human clinical trials involving embryos with edited inherited genes.

More liberal Britain has already blazed a trail by becoming the first country officially to sanction mitochondrial replacement therapy (MRT), seen by some as opening the door to designer babies.

The researchers also found that human embryos have an alternative DNA repair system, where the Cas9-induced cuts in the DNA coming from the sperm are repaired using the healthy eggs DNA as a template.

This explained why the remaining 27.6 per cent embryos still had the HCM mutations.

Additionally, the researchers found that there were no off-target changes made during the testing.

Some people are voicing their opposition to the gene-editing technology.

Dr David King, director of Human Genetics Alert, said: If irresponsible scientists are not stopped, the world may soon be presented with a fait accompli of the first GM baby.

We call on governments and international organisations to wake up and pass an immediate global ban on creating cloned or GM babies, before it is too late.

There is absolutely no medical need to use this technology to avoid the birth of children with genetic diseases, since genetic selection techniques can prevent their birth, where that is appropriate.

So scientists racing to develop this technology must be driven by something else: irresponsible technological enthusiasm, the desire for fame, or the financial gain of being the first to market designer babies.

Dr Jun Wu, one of the papers first authors, said: Our technology successfully repairs the disease-causing gene mutation by taking advantage of a DNA repair response unique to early embryos.

During testing, none of the embryos were allowed to develop beyond five days after conception.

But had they produced offspring, those with the repair would no longer be at risk of developing HCM, or passing the defective gene onto their own children.

Dr Shoukhrat Mitalipov, who also worked on the study, said: Every generation on would carry this repair because weve removed the disease-causing gene variant from that familys lineage.

By using this technique, its possible to reduce the burden of this heritable disease on the family and eventually the human population.

While the results are extremely promising, the researchers warn that they are very preliminary, and that further studies will be needed to make sure there are no unwanted side effects.

Professor Belmonte said: Our results demonstrate the great potential of embryonic gene editing, but we must continue to realistically assess the risks as well as the benefits.

Dr Daniel Dorsa, senior vice president for research at OHSU added: The ethical considerations of moving this technology to clinical trials are complex and deserve significant public engagement before we can answer the broader question of whether its in humanitys interest to alter human genes for future generations.

But not everyone is happy about the study, and claim that it is the first step in the development of designer babies.

Dr David King, director of Human Genetics Alert, said: If irresponsible scientists are not stopped, the world may soon be presented with a fait accompli of the first GM baby.

We call on governments and international organisations to wake up and pass an immediate global ban on creating cloned or GM babies, before it is too late.

There is absolutely no medical need to use this technology to avoid the birth of children with genetic diseases, since genetic selection techniques can prevent their birth, where that is appropriate.

So scientists racing to develop this technology must be driven by something else: irresponsible technological enthusiasm, the desire for fame, or the financial gain of being the first to market designer babies.

James Clapper, US director of national intelligence was right to call the creation of GM babies a weapon of mass social destruction.

Charlie Gard would not have been saved by gene editing his embryo in the way described by Dr Shoukhrat Mitalipov and his fellow scientists.

The technique worked for the heart failure condition hypertrophic cadiomyopathy because the disorder is due to a fault in a single gene inherited from one parent.

Charlies illness, infantile onset encephalomyopathy mitochondrial DNA depletion syndrome (MDDS) is an autosomal recessive disorder, which only manifests itself if the gene fault is inherited from both parents.

The disease leads to a loss of mitochondrial DNA, housed in cellular power plants that supply energy to vital organs.

Because of the gene defect Charlie was unable to transfer energy to his muscles, kidneys and brain.

Although it affects mitochondrial DNA, the rare condition is triggered by a fault in the cell nucleus passed down by both a childs mother and father.

The American researchers admitted that fixing such a recessive genetic error caused by two mutated copies of a gene would be far more challenging.

This is because the repair they carried out depended on having one good copy of the gene.

The scientists used a molecular scissors technique called Crispr-Cas9 to snip away precisely targeted elements of defective DNA carried by fertilising sperm.

Once the dysfunctional DNA was removed, Mother Nature took over as the embryos own repair systems fixed the damage using the good gene copy inherited from the egg donor mothers as a template.

Without the mothers functioning genes, it is unlikely the fix would have succeeded.

Although the scientists introduced their own healthy gene template, at the end of the day this played no part in the repair.

Charlie died on July 28, aged 11 months, after being at the centre of a painful legal battle between his parents and Londons Great Ormond Street Hospital.

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