Genetics and Genomics
Gene Therapy: On the road again
by Michelle F. Pflumm, PhD
David Vetter got his very own spacesuit in 1977, and for 30 minutes, he could finally explore the outside world. But then, unable to fight off the simplest infections, he had to return to his germ-free bubble. He died from complications of a bone marrow transplant at the age of 12.
Twenty-five years later, children like David with severe combined immunodeficiency (SCID) still face an uncertain future. New gene-therapy-based strategies provide great promise, but fatal complications have stalled their progress toward the clinic. Now, a team of experts hopes to change that with a new fleet of gene delivery vehicles.
SCID: a fatal disease
The most common form of "bubble boy" syndrome, SCID-X1, strikes boys shortly after birth; without intervention, most die from bacterial or viral infections before their first birthday.
"These children have no mechanism to fight off even a simple cold virus," explains David Williams, MD, director of Translational Medicine at Children's Hospital Boston and chief of the Division of Hematology/Oncology. "Simple infections become deadly infections."
The only curative treatment is bone marrow transplantation, but most children, like David, never find a perfectly matched marrow donor. Parents often step up to the plate, but provide only a partial match, so their children typically must remain on immunoglobulin transfusions for life.
A promising alternative
Gene therapy emerged as another option in the 1990s. It is similar to a bone marrow transplant, but uses the childâ€™s own cells instead of cells from a donor. The cells are removed, the blood stem cells are isolated and their genetic defect is corrected. The corrected cells are reintroduced into the patient, and go on to create the specialized cells needed for a functional immune system, ultimately curing the disease.
The critical component of gene therapy is the "vector," a debilitated virus that serves as the gene delivery vehicle. This virus infects the cell, carrying the therapeutic gene, and, depending on the virus used, inserts that gene into the chromosomes.
A healthy copy of the affected gene is introduced into the patient's stem cells by means of a gene delivery vehicle called a vector, a genetically altered virus that can penetrate cells but does not cause ongoing infection. The process begins by isolating blood stem cells from the patient's bone marrow. The therapeutic gene, inserted into the vector's genome, is then delivered into the patient's stem cells. The patient's stem cells, now corrected for the defect, are infused back into the patient in the clinic.
Two teams in England and France pioneered the gene therapy strategy for SCID-X1, and by 2000, 18 out of 20 treated children had been cured of the disease. But the team had little time to celebrate. Five children developed leukemia as a complication of gene therapy, causing one of them to die. The Food and Drug Administration placed a hold on U.S. trials in SCID-X1, deciding that this therapeutic strategy was not worth the risk of a second potentially fatal disease.
United we stand
Williams, like his collaborators around the world, refused to be deterred. He knew that gene therapy could sometimes be curative, while bone marrow transplantation could lead to potentially fatal graft-versus-host disease, in which children are actually attacked by the newly transplanted immune system.
"The advantage of gene therapy is that you are using the patient's own cells and repairing them," explains Williams. "The risk of having graft-versus-host disease is minimal, probably zero."
With collaborators Adrian Thrasher, MD, of Great Ormond Street Hospital, London, who led the original trials in England, and Chris Baum, MD, of Hannover Medical School in Germany, an expert on gene delivery vectors, Williams founded the Transatlantic Gene Therapy Consortium (TAGTC). Its mission: bring gene therapy back into the clinic--safer and cancer-free.
"The commitment of researchers and investigators all around the world was simply amazing," Williams noted. "As the TAGTC, we could now share resources and expertise in an effort to move forward as fast as possible."
Back to the lab
Joining forces, the French and British teams that led the original trials took a closer look at the treated patients who contracted leukemia, and zeroed in on the culprit. A small stretch of DNA in the gene delivery vehicle had switched on a cancer-causing gene when it was inserted into the patient's genome.
The team had a choice to make. They could switch to one of the newer vectors being developed, but not yet tested in humans. Instead, they chose to work with the same retroviral gene delivery vehicle that had cured the children of SCID-X1.
"We have a lot of experience with these vector systems which we developed," explains Williams. "What we are trying to do is improve upon a vector that's already been used successfully, just make it more safe."
To guard against side effects, the researchers removed the cancer-triggering elements and powered down the expression of the SCID-X1 gene. Subsequent laboratory testing of the redesigned vehicle suggested a significantly reduced risk of activating cancer-causing genes. Furthermore, in mouse models, the animals were cured of SCID and remained cancer-free.
A New Hope
With this safer vector, the FDA gave the team the green light. An international trial, now enrolling patients, will soon treat a total of 20 children with SCID-X1 at Children's Hospital Boston, Mattel Children's Hospital at UCLA, Cincinnati Children's Hospital, Great Ormond in London and the Hopital Necker-Enfants Malades in Paris.
Children's Hospital Boston serves as the lead investigative site for the U.S. portion of this trial and has received a grant of approximately $5 million from the National Institute of Allergy and Infectious Diseases of the NIH to carry out the trial. The patients' stem cells will be genetically corrected at the Dana-Farber Cancer Institute.
The children will be tested at six months to see if they are indeed cured of SCID-X1, and will be followed for 15 years to be sure that they don't develop leukemia.
"There is a strong emphasis on safety," says Luigi Notarangelo, MD, director of the Research and Molecular Diagnosis Program on Primary Immunodeficiencies at Children's. "We will monitor these patients very, very carefully."
One size does not fit all
With a potentially life-saving cure for this rare immunodeficiency disease, researchers had hoped these same retroviral delivery vehicles could be used to treat more common genetic blood diseases such as sickle cell anemia and the thalassemias. But so far, they have not succeeded, unable to get sufficient expression of the healthy gene to cure the disease.
The problem is that genes that are introduced often need to be switched on at just the right time, in the right place and in the right cells. The complex regulatory regions of these genes that control their expression are often extremely large, and often are unstable and switch off over time.
Lentiviruses have recently emerged as an alternative gene delivery vehicle, appearing to power more therapeutic genes stably and long-term. And better still, these viral vectors appear to hop into the genome outside of gene regulatory regions, and therefore may be less likely to switch on cancer-causing genes.
"Lentivirus vectors appear to choose different parts of the genome into which to insert the gene payload and therefore may prove to be safer," explains Williams. "The target cells also appear to require less manipulation in the laboratory for successful gene transfer. This characteristic of the vector may be important for some specific human diseases."
Scientists at Brigham and Women's Hospital, led by Philippe Leboulch, MD, are using lentivirus-based systems to develop a treatment strategy for beta-thalassemia and sickle cell disease. The team has successfully treated mice and is currently testing the therapy in a small group of patients with these diseases in France.
At Children's Hospital Boston, Sung-Yun Pai MD, of the Division of Hematology/Oncology, will lead a trial testing out a similar lentiviral strategy to treat Wiskott-Aldrich syndrome, a rare genetic disorder affecting boys that causes both platelet deficiency and immunodeficiency.
"Like SCID, Wiskott-Aldrich syndrome is typically treated with bone marrow transplantation, and is plagued with the same problems of donor availability and graft-versus-host disease," says Pai. "Gene therapy promises to obviate these problems. We are particularly excited to offer this therapy to older boys without a suitable donor because they are at higher risk of graft-versus-host disease."
When stem cells are in short supply
But gene therapy is not always an option. In patients with Fanconi anemia, the very disease results in blood stem cells that are fragile, tricky to handle and simply too few and far between. Scientists simply cannot easily generate enough corrected cells to treat the disease.
Teaming up with George Daley, MD, PhD, director of the Stem Cell Transplantation Program at Children's, and Alan D'Andrea, chief of Radiation and Cancer Biology at Dana-Farber, Williams hopes to change that by creating more blood stem cells for patients with Fanconi anemia in the laboratory.
Their plan, funded by a new $4 million translational research grant from the NIH, is to obtain skin cells from the patients and correct them by inserting the critical gene. Then, these cells would be genetically reprogrammed and used to make new blood cells to treat the patients' disease.
"Reprogramming technology may revolutionize medicine in the future by allowing the generation of needed cells for regenerative medicine in many diseases," says Williams. "Already, we are beginning to utilize this technology to gain a new understanding of the pathophysiology of human diseases, including Fanconi anemia."
Still the one
Gene therapy is finally beginning to deliver its promise. More and more children suffering from immunodeficiencies will soon be able to live their lives, free of the fear of infections or graft-versus-host disease. With further research, Williams hopes these lifesaving strategies can reach thousands more children--with other blood diseases and diseases beyond the blood system.
"It is clear that the many years of research investment and development are now paying off," says Williams. "We believe that continued investment in basic and translational research using this technology will lead to additional advances, bringing new therapies to children worldwide."
SCID-X gene therapy trial launched
The Wall Street Journal features the gene therapy trial recently launched at Children's Hospital Boston for "bubble boy" syndrome.
Fanconi anemia initiative
David Williams, MD, teams up with George Daley, MD, PhD, director of the Stem Cell Transplantation Program and Fanconi anemia expert Alan D'Andrea, MD, of the Dana-Farber Cancer Institute to develop a strategy to more effectively treat children with Fanconi anemia.
"Boy in the Bubble"
Wired's slideshow of the story of SCID patient David Vetter, who because of his illness had to live inside a plastic bubble for twelve years.