Gene therapy frees 'bubble babies'

Two infants, aged 8 and 11 months, received new genetic material to combat a life-threatening disease called Severe Combined Immunodeficiency (Scid).

Usually, patients with Scid are forced to live in tightly-controlled, sterile "bubbles" to avoid threats to their non-existent immune systems.

A bone marrow transplant is the conventional treatment for such patients. But the two treated children have shown "striking" improvements and now live normally after their novel therapy.

Gene therapy trials in the US have attracted much criticism over fears that patients were being harmed by the experimental treatments.

Viral infection

Scid is caused when a defective copy of a gene on the X chromosome is inherited.

The corrupted gene means crucial components of the immune system do not develop and Scid patients are left fatally vulnerable to even slight infections such as a cold sore or common childhood diseases like chicken pox.

Scientists at the Hospital Necker in Paris, France, began the therapy by taking bone marrow from the patients and sorting out a set of blood stem cells from the marrow.

The cells were infected with a retrovirus carrying the correct, replacement gene. After three days of repeated infection, the scientists transplanted the cells back into the patients.

Rapid improvement

As early as 15 days later, the team detected new cells bearing the correct version of the gene, along with rising numbers of fully functional and diverse immune cells.

Currently, the two patients have immune cell counts similar to healthy children of their age. The scientists also tested the patients' newly-developed immune systems with tetanus, diphtheria, and polio vaccinations, and found that the infants produced the correct antibodies for each.

The infants' return to a normal immune system has lasted over 11 months without side effects, says study co-author Alain Fischer. They are no longer in protective isolation and both live at home without any treatment.

A third patient is experiencing similar progress four months after the gene transfer.

Ideally, Dr Fischer says, the children will be monitored for the rest of their lives, both to ensure their continued health and to monitor the long-term success of the therapy.

Secret of success

Dr Fischer believes that the key to the therapy's success lies "not in the technique, but in the disease itself."

In the Scid cases, cells with the normal gene seem to enjoy a significant selective advantage, multiplying rapidly until their numbers overwhelm their mutated cousins.

"This means that even a poorly efficient gene therapy technique - one that only introduces a few cells with the right gene - may work as a treatment," said Dr Fischer.