Washington: For the first time scientists have been able to "edit" the genetic code, a breakthrough which they say could pave the way for new treatments for some hereditary diseases.

Using this innovative genome editing technique, which hones in on the precise location of mutated DNA, scientists at the Children's Hospital of Philadelphia in the US have treated the blood clotting disorder hemophilia in mice.

This is the first time that genome editing, which precisely targets and repairs a genetic defect, has been done in a living animal and achieved clinically meaningful results, the researchers said.

In this new study, published online in journal Nature, the researchers used two versions of a genetically engineered virus (Adeno-Associated virus, or AAV) -- one carrying enzymes that cut DNA in an exact spot and the other carrying a replacement gene to be copied into the DNA sequence. All of this occurred in the liver cells of living mice.

"Our research raises the possibility that genome editing can correct a genetic defect at a clinically meaningful level after in vivo delivery of the zinc finger nucleases," said study leader, Katherine High, a gene therapy expert at the Children's Hospital.

The researchers managed to persuade cells in mice to repair a faulty gene but, instead of recreating the flawed piece, the cells generated a healthy one.

The faulty gene was the one responsible for hemophilia, meaning that the process cured the mouse of the hereditary condition, which can be life-threatening in humans.

The improvements persisted over the eight months of the study, and showed no toxic effects on growth, weight gain or liver function, suggesting that the treatment was well-tolerated. The scientists hope the discovery could help develop better treatments for conditions affecting the immune system, bone marrow and liver. Up to a third of genetic diseases are caused by a single faulty gene.

"We established a proof of concept that we can perform genome editing in vivo, to produce stable and clinically meaningful results," said High.

"We need to perform further studies to translate this finding into safe, effective treatments for hemophilia and other single-gene diseases in humans, but this is a promising strategy for gene therapy."

However, she said it may take around two decades or so before the process could be used in human trials.

"The clinical translation of genetic therapies from mouse models to humans has been a lengthy process, nearly two decades, but we are now seeing positive results in a range of
diseases from inherited retinal disorders to hemophilia," High said.

"In vivo genome editing will require time to mature as a therapeutic, but it represents the next goal in the development of genetic therapies," she added.