An estimated 250,000 people in the United States have one of the 185 primary immunodeficiency diseases, according to the Immune Deficiency Foundation.
There are several forms of SCID, but the most common type is linked to the X-chromosome and affects only males. Unlike boys, who have a Y chromosome, girls have a second X chromosome that is able to function for the faulty one.
Boys with SCID-X1 have a severe defect in both the T- and B-lymphocyte systems, causing serious and sometimes life-threatening infections within the first few years of life, including pneumonia, meningitis or bloodstream infections.
In two previous gene therapy trials in Paris and London, 18 of the 20 boys had a complete repair of their immune system, according to Williams, but five developed cancer as a consequence of the virus vector used to deliver the "payload" of repaired gene.
As a result the Food and Drug Administration stopped the trial for child patients in the United States. But in the latest trial, based in Germany, Britain and the United States, scientists "fished out where the vector landed and saw that it was next to a gene that was oncogenic (cancer-causing)," said Williams.
Now, they have changed the virus vector to deliver the repaired gene more safely.
"The bottom line is the vector seems to work as well as the old vector," he said. "We have to wait longer to know whether any of the children develop leukemia before we can say for sure."
The two children who have been in the study the longest have been cancer-free for more three years, which is about the point those in the previous studies had developed the cancer.
The findings of the Dana-Farber/Boston Children's study were presented this week at the annual meeting of the American Society of Hematology in New Orleans by pediatric hematologist/oncologist Dr. Sung-Yun Pai.
She treats 6-year-old Aidan Seymour of Springfield, Mass., who was diagnosed with SCID-X1 at the age of seven months. At the time, gene therapy was not available, so he underwent a successful blood cord transplant, which can be done with an unmatched donor. In order to minimize the risk for graft versus host disease, he had chemotherapy first.
"We had hope, but there was risk," said his mother, Kirstin Seymour, 37. "It is an art, not a science."
At the time, a cord transplant seemed the best option.
"I can say as a parent, that if I had had that opportunity for gene therapy, 100 percent it would probably have been my course of action," she said.
Today, Aiden is doing well and takes no medications, but Seymour worries about whether Aidan will ever be able to have children or if he will develop a secondary cancer because of chemotherapy.
"At this point, they say he should be able to live a long, happy, healthy life like everyone else," said Seymour.
But Aidan can never get a second transplant from the blood cord donor because of the anonymous nature of the transplant, so gene therapy might provide promise for him.
"It allows the body to cure itself from its deficits, and that is pretty amazing," said Seymour.
Williams is also optimistic about the research.
"Because this is very experimental in human beings, we can't answer if these children are cured with any surety yet," he said. "But because we put the correction into the blood stem itself and those stem cells last a lifetime, it is likely a long-term correction of the child."
Gene therapy technology may also help children with other genetic diseases, most notably sickle cell anemia.
"One of the great things about this trial is that it has multiple institutions around the world working together," Williams said.