Image copyright Science Photo Library Image caption Muscle strength in the legs of mice was improved when researchers used the gene editing therapy

Researchers in the US have used gene editing to treat mice with Duchenne muscular dystrophy.

A team at Duke University used a system known as CRISPR-Cas9 to delete DNA that was preventing cells from producing a protein essential for muscle function.

And a virus was used to deliver DNA alterations into the cells of mice.

Writing in the journal Science, the team say when they injected the therapy direct into the legs of adult mice, it resulted in improved muscle strength.

When they injected it into the bloodstream - tests showed improvements in muscles responsible for heart and lung function.

Chris Nelson, who led the research, said: "A major hurdle for gene editing is delivery.

"We know what genes need to be fixed for certain diseases, but getting the gene editing tools where they need to go is a huge challenge.

"The best way we have to do it right now is to take advantage of viruses, because they have spent billions of years evolving to figure out how to get their own viral genes into cells."

Snipping through DNA

CRISPR-Cas9 is a simple, fast and inexpensive gene editing system discovered three years ago, which is now being used by laboratories all over the world.

It combines sections of synthetic DNA - CRISPRs - with a protein known as Cas9.

Rather like a sat nav, Crispr scans the genome looking for the right location and then uses the Cas9 protein as molecular scissors to snip through the DNA.

DMD is a genetic disease that gradually causes weakness and loss of muscle function.

Humans with the condition lack normal dystrophin, a protein found in muscles, which helps to protect muscles from injury.

In patients with the disease, the muscles become damaged and eventually stop working.

The inherited disease usually affects only boys - about a 100 babies a year in the UK are affected. Few of those affected live beyond their early 30s.

'Exciting'

Although this research was in mice, it has raised hopes of a treatment for humans at some point in the future.

Prof Peter Braude, emeritus professor of obstetrics and gynaecology from King's College London, said: "This is an exciting paper demonstrating in laboratory mice the feasibility of a non-controversial use of the new CRISPR-Cas9 gene editing technology to treat a serious and slowly lethal genetic disease."

Prof Adrian Thrasher, from the Institute of Child Health and Great Ormond Street Hospital, described it as an "important study demonstrating proof of principle of gene editing in vivo for neuromuscular disease, but still some way to go before translatable to human subjects."

Different approach

In a second study, also in Science, teams at Harvard University used CRISPR-Cas9 gene editing in egg and sperm cells of mice carrying DMD.

Eighty per cent of baby mice born following germline modification showed successful DNA editing.

Germline alterations - changes to the DNA that are passed down the generations - is a more controversial use of gene editing.

It raises ethical issues, as the technology could in theory be used to produce "designer" humans, with embryos being tweaked to enhance their physical or mental abilities.

Others argue it could be a vital tool for curing serious genetic conditions like DMD, Huntington's Disease or Cystic Fibrosis.

The team at Harvard also used gene editing to treat DMD-affected mice a few days after birth via injections into the abdomen, muscles or the back of the eye, with each method improving muscle function.

A third team at University of Texas Southwestern Medical Center, published in Science, also used gene editing to treat mice with a defective dystrophin gene.