A new gene therapy technique has been used to successfully reverse sickle cell disease for the first time, scientists report.

While this is just one case study involving a single French teenager, the early signs are encouraging, and the therapy could eventually lead to an effective treatment for the millions of people with this crippling disease worldwide.

Sickle-cell disease occurs when one of the proteins making up a type of haemoglobin we use to carry oxygen through our body takes a slightly different form.

This small change is enough to make the red blood cells they occupy lose elasticity, deforming them into a curved 'sickle' shape and risking clumps of cells piling up as they struggle to slip through blood vessels.

But by using a virus to insert genes for the correct form of this protein into the bone marrow of a French teenager, researchers have been able to restore the elasticity to the patient's blood cells.

After 15 months of therapy, the patient is off medication, and while it's far too early to say he's been functionally cured, it's a case of 'so far so good' for this pioneering kind of treatment.

"So far the patient has no sign of the disease, no pain, no hospitalisation," one of the team, Philippe Leboulch from the University of Paris in France, told the BBC. "He no longer requires a transfusion so we are quite pleased with that."

"But of course we need to perform the same therapy in many patients to feel confident that it is robust enough to propose it as a mainstream therapy."

Sickle cell disease causes a condition called ischemia – an interruption of the flow of oxygen to parts of the body that causes pain, organ damage, and in some cases, eventually death.

The only existing long-term treatment is a bone marrow transplant, a high-risk and difficult operation that not everyone is eligible for.

But because sickle cell disease involves a mutation of just a small part of the body's genetic code – producing an abnormal beta-globin protein known as haemoglobin S – it's a prime candidate for genetic therapy treatments that could reverse the mutation.

Although there are still concerns over how modern gene therapy techniques could be abused, scientists now have the know-how to 'correct' certain genetic mutations.

In this case scientists removed bone marrow stem cells from the teen's body and added a specially made virus, designed to recode the cells to produce normal haemoglobin again. The cells were then transfused back into the patient.

Doctors are reporting that half the patient's red blood cells are now regular and healthy, and he hasn't needed any blood transfusions since three months after his first treatment.

We've come a long way – this type of treatment was first tested on mice back in 2001 – but there's still a great way to go before we can say we have an effective, long-term treatment for sickle cell disease.

A number of other gene therapy techniques are currently being tested, and not all of them are as promising as this one, but scientists are continuing to tweak their approach to find a therapy that can work for the majority of patients, while still being affordable.

"I've worked in gene therapy for a long time and we make small steps and know there's years more work," Deborah Gill, from the gene medicine research group at the University of Oxford, who wasn't involved in this study, told the BBC.

"But here you have someone who has received gene therapy and has complete clinical remission – that's a huge step forward."

The research has been published in the New England Journal of Medicine.