Kohn’s Advantage
By bringing promising laboratory results directly to sick patients, Donald Kohn sees first hand when his research bears fruit.
Stem cells broke into the public consciousness in 2001, but they have been on Donald Kohn’s mind for more than a decade.
Much of the public talk has been about the potential stem cells have for curing diseases and replacing bad body parts. But Kohn and his colleagues at Childrens Hospital Los Angeles have been using stem cells to directly attack several diseases. It is a research track that he firmly believes is running in the home stretch. Among the diseases are pediatric AIDS, severe combined immune deficiency (SCID) and sickle cell anemia.
Kohn, M.D., a professor of pediatrics at the Keck School of Medicine of USC, directs the John Connell Gene Therapy Program at the Research Institute of Childrens Hospital. He has been examining the potential for utilizing gene transfer to cure diseases since the 1980s, when he was a fellow at the National Institutes of Health in Bethesda, Md.
The basic idea then, as now, was to reverse a disease process by inserting new genes into a patient as a kind of “silver bullet” that would strike at the basic disease mechanism. One idea was to insert “good genes” in hopes they would supplant the ones causing the disease. Another was to insert genes that would curtail the function or activity of a disease-causing gene. Still another was to fix a patient’s malfunctioning genes in the laboratory and then give the repaired genes back to the patient.
There are not many “Eureka!” moments in the progress of Kohn’s research, but each success holds such promise that he and his colleagues find much motivation to stay the course.
Along the trail, there are numerous challenges. A central one: while wonders can be achieved in laboratory cell cultures and animal models, when it is a human patient—and a small child to boot—there is little margin for error.
Another challenge is that while it was possible even in the mid-1980s to insert a gene into the nucleus of a cell in a test tube or Petri dish, where it could assimilate itself into the cell’s DNA, it was much more difficult to accomplish this in lab animals. It was nearly impossible with human patients.
To transfer the genes, the delivery vehicle of choice turned out to be viruses, whose propensity for infecting other cells and forcing them to replicate the viral DNA or RNA was turned to the scientists’ advantage.
Kohn and others hit on the idea of using stem cells to carry the genetic messages they wished to send. Stem cells are undifferentiated—not yet developed into liver or brain or T-cells. They show a remarkable property to become any kind of cell—which is their job in life.
Stem cells are formed in embryos, but are also found in other places: in bone marrow, in blood and in umbilical cords, where they abound.
But in the early days, using stem cells was a far-out approach.
When Kohn joined the NIH laboratory of pediatric immunologist R. Michael Blaese in 1985, he made a fortunate choice. Blaese and W. French Anderson (who is at the Keck School of Medicine of USC now) were pioneers in developing the first gene therapy treatment. Their target was SCID, a rare disease caused by a defect in the gene that makes the enzyme ADA. Without ADA, the immune system could not develop normally. Patients had to be protected from any possible infection; some were kept alive inside plastic bubbles.
The tactic of Anderson and Blaese was to insert normal ADA genes into white blood cells called T-cells that normally circulate in the blood. The idea was that the altered white cells, central to immunity, would gradually restore the patient’s immune system.
Kohn was a skeptic. He thought the repaired genes would fare better if placed in stem cells in bone marrow. T-cells were short-lived, his thinking went, while stem cells endured.
“I had stem cell envy,” Kohn recalls. “That was the really interesting stuff. T-cells were dead end cells.” He adds ironically: “Great foresight on my part, since T-cells turned out to be used for the first gene therapy.”
The Blaese-Anderson gene therapy worked, although the infusion of altered genes had to be repeated. And the results were clouded because the NIH required that the patients, girls then ages 4 and 9, had to be simultaneously treated with a drug called PEG-ADA, a form of the missing enzyme. But it was the first attempt at gene therapy and yielded information that guides researchers to this day.
When he moved to Childrens in 1987, Kohn was determined to focus on stem cells, knowing it would be harder to insert genes into them than into white blood cells. But the potential for cures was better, he believed. He received strong support from a man who was to be his close ally, USC immunologist Robertson Parkman, M.D.
For the next six years, Kohn developed the techniques he would need to implant altered stem cells into a patient. In 1993, he had the opportunity to test his ideas on three babies born with ADA deficiency. Kohn and his team used viruses into which they had placed working ADA genes to infect stem cells taken from each of the infants’ umbilical cords. Only a few years before, scientists had discovered that cord blood was rich in stem cells. And Blaese had demonstrated that corrected cells would have an advantage over defective cells. Also, because the stem cells were the infants’ own, there would be no problems with tissue rejection.
But there was that troubling business of the need to give the infants PEG-ADA. This time, Kohn gradually lowered the PEG-ADA dose while carefully monitoring the ongoing health of the babies. This enabled the Childrens Hospital team to calculate with some accuracy the effect of the altered stem cells on replenishing the immune system’s T-cell counts.
The results so far are encouraging; the children continue to be monitored closely, and their health is good. But the number of cells that contain the gene is low and the children continue to receive PEG-ADA. A second trial using improved gene transfer and expression techniques developed in Kohn's lab was initiated Sept 1, 2001.
Kohn and his collaborators at Childrens also are using gene therapy for treating pediatric AIDS. While annual transmissions of HIV from mothers to their newborns has decreased dramatically in recent years, from an estimated 2,000 babies infected nationwide in 1994 to 200-300 per year currently—the outlook is not good. Infection rates in adults have begun to increase and current treatment regimens have limitations, especially in the immature immune systems of children.
Kohn and his colleague Joseph A. Church, M.D., began their first clinical trial of gene therapy to treat HIV infections in children in 1997. Buoyed by the success of that trial, which sought to determine whether the approach would be safe, they have launched another nationwide clinical trial with children.
Since the infectious agent in AIDS is a virus—human immunodeficiency virus—Kohn and the other scientists at the Research Institute have created a gene they hope will block replication of HIV. The gene is being inserted into bone marrow stem cells of children infected with HIV, using a disabled mouse virus as a transporting “vector.” Because stem cells in children proliferate rapidly, the hope is that the stem cells with their HIV-blocking cargo will overwhelm the infection that depresses the children’s immune system.
The first trial proved safe, but not effective. So Kohn, Church and their co-investigators at the University of Michigan, Drs. Steven King and Keith Bishop, have developed a more powerful gene and are currently treating a dozen children in the Childrens AIDS Center at CHLA.
The children will be closely followed for two years to determine whether the anti-HIV gene is multiplying and playing its role in controlling the disease.
Because children’s bone marrow seems more receptive to gene transfers, and because the thymus gland acts as a regulator for T-cell traffic, the Childrens researchers believe very young children have a better chance of responding to this treatment regimen. The thymus shuts down after puberty.
“If this gene transfer procedure doesn’t work in children who have a healthy thymus, and a more active bone marrow, it probably won’t work in adults,” says Kohn. “So we learn more by working with children.”
Kohn is lucky. Research, in the final analysis, is a learning cycle—search for an answer, find it, raise new questions, then search again. Kohn’s bench-to-bedside process enables him to take promising laboratory results directly to sick patients. When he sees the effect in the patient, he returns to his laboratory to make adjustments, test new hypotheses, try new things. Then, back to the bedside.
It is a process called translational medicine. At each step, Kohn and his colleagues acquire new knowledge and gradually advance their stem cell therapies, working closely with their patients.
It might well be called Kohn’s Advantage. Working closely with patients gives him both motivation and unique insight as he pursues exquisitely difficult solutions to the challenges that diseases present. Many scientists who work far removed from a clinical setting lack the opportunity to see at first hand whether their research bears fruit when tried in patients.
And they never get to see the hope that flashes in the eyes of patients with hopeless diseases—hope mirrored in the eyes of physician-scientists like Donald Kohn.
Earlier stories by Candace Pearson and Kate Vozoff were used in this report.
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