A new treatment is able to effectively cure people with the rare clotting disorder haemophilia B, according to doctors, in a major breakthrough.
The therapy - described by one patient as “transformational” - corrects a rare genetic mutation that causes severe bleeding that can last for long periods, either spontaneously or as a result of an injury.
The rare disorder is caused by a change on the factor IX gene on the X-chromosome.
Normally, when a person cuts themselves, clotting factors mix with blood cells called platelets to make the bleeding stop.
But people with haemophilia lack clotting factors and are at risk of heavy bleeding.
The level of the IX gene in the blood of haemophiliacs is too low complete the clotting process. As a result, bleeding can also occur inside the body, from joints and from the skin.
But a medical team from Royal Free Hospital and University College London now say the majority of adults living with haemophilia B could be cured within three years.
During the trial to test the therapy, developed with biotechnology company Freeline Therapeutics, participants were given an engineered virus which delivered the instructions for manufacturing the missing factor IX gene to the liver - which in turn triggers the production of the clotting protein.
One participant, Elliott Mason, of London, told BBC News he now feels “completely normal” as a result of a single infusion that took just one hour to drip into his system.
“I’ve not had any treatment since I had my therapy. It’s all a miracle really. Well it’s science, but it feels quite miraculous to me,” he added.
A common preventative treatment for the disorder is to replace the missing or reduce factor IX with a lab-made substitute delivered via regular injections, usually given weekly at home.
The jabs can cost between £150,000 and £200,000 per patient per year and patients can suffer debilitating joint damage.
But, according to the findings of the trial, nine out of 10 patients offered the new treatment no longer need their clotting injections.
Leader author Pratima Chowdary, a consultant haematologist at the Royal Free Hospital and professor at University College London, said: “Removing the need for haemophilia patients to regularly inject themselves with the missing protein is an important step in improving their quality of life.
“The long-term follow up study will monitor the patients for durability of expression and surveillance for late effects.”
She told the BBC that there was a “transformational impact” about a year after the therapy, when recipients realised they “don’t need to worry about [their] haemophilia at all”.
Patients on the trial had to take immune-suppressing drugs over several weeks to several months, to prevent their immune systems from rejecting the therapy.
While the treatment was generally well-tolerated, all patients experienced some form of side-effects, with an abnormal blood clot in one who received the highest FLT180a dose and had the highest levels of the protein.
Freeline co-founder Professor Amit Nathwani, from UCL, who co-authored the study, said: "Gene therapy is still a young field that pushes the boundaries of science for people with severe genetic diseases."
He said the new trial adds to "the growing body of evidence that gene therapy has the potential to free patients from the challenges of having to adhere to lifelong therapy or could provide treatment where none exists today."
The trial, the findings of which were published in the New England Journal of Medicine, is the latest in a string of advances in treating both haemophilia A, which is caused by a mutation in a different gene, and B.
Since the liver continues to develop before the age of 12, doctors hope the therapy can be offered from that point onwards.
It is not yet known what the cost of the new gene therapy will be or how long it remains effective for, though current estimates suggest at least a decade.
Around 85 per cent of haemophiliacs have haemophilia A, which is caused by a lack of the blood clotting factor VIII.
