Infants with SCID are essentially born without a functioning immune system. Without treatment, they rarely survive past their first birthday and can be killed by infections as innocuous as the common cold. Such children were once kept isolated in sterile environments, giving rise to the term "bubble boy". Their unusual predicament has caught national attention and has been featured in movies and TV shows. The survival rate in recent years has soared with the advent of widespread screening tests and refinement of life-saving procedures such as bone marrow transplants. But such treatments have come with complications, often leaving children dependent on regular infusions of immunoglobulin or causing their new immune systems to attack their own bodies. Dr Ewelina Mamcarz, one of the study leaders on the breakthrough gene therapy treatment, at St Jude Children's Research Hospital in Memphis. Credit:AP The new gene therapy - developed by St Jude Children's Research Hospital and UCSF Benioff Children's Hospital in San Francisco and published in The New England Journal of Medicine - corrects the genetic defect in the DNA of babies soon after they're born, prompting their bodies to generate missing parts of their immune system. In interviews, the researchers said they first extracted bloodstem cells from the infants' bone marrow. They used a modified virus as a vehicle to deliver the correct copy of a defective gene into those patients' stem cells. Those corrected cells were then reinfused back into the patient, where they proliferated and created healthy immune cells.

To avoid accidentally turning on cancer-causing genes as previous gene therapies did, the researchers built "insulators" into the virus so neighbouring genes wouldn't be affected when the virus made its insertion into the DNA. In another innovation, the researchers gave their infant patients a tiny amount of chemotherapy to clear existing cells from the bone marrow before they reinfused the treated cells, giving the corrected cells a better chance to take hold. The announcement was bittersweet for many at St Jude Children's Research Hospital. Their team leader and senior author on the paper, Brian Sorrentino, spent the final months of his life racing against a fatal cancer to finish his work on the experimental treatment. Loading Sorrentino was diagnosed at a young age with Hodgkin's lymphoma, said his widow, Suzanne Sorrentino. Because better treatments did not yet exist, his doctors used radiation that weakened his heart and produced the lung cancer that killed him last November. "The loss of hair, the pain and agony of the treatment, it's what made him determined to find new treatments for other diseases in kids," his widow said.

Since the 1990s, Sorrentino had been working alongside other immunologists on gene therapy for SCID, but his work took on new urgency in the final 18 months of his life after his cancer diagnosis, Suzanne said. When he could no longer get to the hospital, colleagues at St Jude started arranging conference calls and setting up shop at Sorrentino's home dining table. By then, it was apparent that the experimental procedure was succeeding. Several of their infant patients had grown into toddlers with normal immune systems. "He told me he thought he was saved as a kid from Hodgkin's because there was something he thought he should be doing with his life," said Suzanne, her voice cracking as she recalled one of the last conversations with her husband. "With the trial working, and the children doing so well, he felt it was a sign that he achieved what he was supposed to. That it was time for him to go." In recent years, as screening newborns for SCID has become mandatory across the country, experts have found the disease to be more common than previously thought - occurring in as many as 1 in 50,000 infants. David Vetter, seen here in 1982, was born with immune deficiency and lived in a protective enclosure. He died in 1984 through complications from an experimental bone marrow transplant aimed at ending his isolation. Credit:AP There are several forms of SCID. Gene therapy for a variant called ADA-SCIDS is available in Europe. But a cure for the most common form - X-linked SCID, which is only found in boys - had remained elusive.

The disease drew national attention in the 1970s with the case of David Vetter. News outlets chronicled his life as he grew up cocooned in plastic. His story spawned the 1976 John Travolta movie The Boy in the Plastic Bubble. Even after Vetter's death, something about the disease's vulnerable isolating effects kept it in the national consciousness, making a famous cameo in a Seinfeld episode and becoming the premise of a panned 2001 Jake Gyllenhaal movie, Bubble Boy. Until now, the most effective therapy for SCID had been transplanting blood-forming stem cells from a tissue-matched sibling donor, but most patients lack such donors. Half-matched and unrelated donors have also proved lifesaving but often resulted in complications. Gael Jesus Pino Alva appears to have been cured of an "incurable" condition. Credit:AP The first successful gene therapy trial, in 2000 by French doctors, gave many researchers and patients hope that they could fix the molecular underpinnings of the disease instead of simply treating its symptoms. But when leukemia was found in some of those patients, it temporarily halted gene therapy trials in the US and much of the world and sent dispirited researchers searching for answers.

"The leukemia stopped the field for a long time and rightfully so, because people had to figure out what went wrong," said Jordan Orange, a pediatric immunologist at Columbia University, who was not involved in the St Jude study but helped develop a gene therapy for another immunodeficiency disease called Wiskott-Aldrich. "But that led us to the point now where we're finally beginning to see actual cures to incurable conditions." The Washington Post