The latest case, a boy who was treated as an 8-month-old in 2002, was diagnosed about five weeks ago -- just two weeks after the team had begun gene therapy on a new patient, Fischer said in a phone interview. He said he immediately stopped the study again and, although he is not regulated by the FDA, notified the agency.
The FDA does not generally tell the public about clinical trials it has put on hold, and officials declined to say this week what actions they had taken in light of the latest diagnosis. But U.S. researchers involved in two experiments similar to the one in France told The Washington Post that the agency had recently directed them to suspend their studies. One experiment is being led by Harry L. Malech and Jennifer Puck at the National Institutes of Health, who have treated two patients so far. The other, led by Donald B. Kohn of the University of Southern California, has enrolled four patients.
Library of Life: Interactive graphic shows the makeup of human genes. (Flash Required)
Fischer said the recently diagnosed boy in France is undergoing chemotherapy and emphasized that the other cancer survivor -- along with the other dozen or so children treated previously -- is doing well.
"They can cope with infections. That tells us efficacy is there," Fischer said. "There is a future in gene therapy."
But in light of the monkey's case, Fischer and others acknowledged it will be many years before the final verdict is in.
The monkey, whose September death has not been previously publicly disclosed, was one of 42 being watched by NIH scientists for delayed effects of gene therapy. It is the only one to have developed cancer so far, said study leader Cynthia E. Dunbar of the National Heart, Lung and Blood Institute. But with an average follow-up of about three years so far, it is too soon to know how many might show delayed effects, she said.
Unfortunately, she added, most research teams kill their animals less than a year after using them in gene therapy experiments.
"It's extremely expensive, but this shows how important it is" to study the animals longer, said Dunbar, who called the fatal cancer "very concerning."
In both the human and monkey gene therapy experiments, the mouse virus used -- known as a murine retrovirus -- is the most common means of delivering DNA. The virus's predilection to interact with genes that can cause cancer has led many scientists to look for better delivery systems.
Among the most promising, several scientists said, are genetically modified "lentiviruses" -- including the human immunodeficiency virus (HIV), which causes AIDS -- in part because they appear less likely to trigger the kind of cell replication that adds up to cancer.
To date the FDA has approved three human studies involving infusions of genetically engineered lentiviruses, according to NIH records -- all of them restricted to patients who already have AIDS.
Mark A. Kay, a professor of pediatrics and genetics at Stanford University and president-elect of the American Society of Gene Therapy, said he remains hopeful that gene therapy will prevail.
"This is a devastating side effect," he said of the cancers, speaking for himself and not for the society. "But taking a disease that is pretty much fatal . . . if you can get a 60 or 70 percent cure rate, you have to balance that out."