About a decade ago, David Williams, MD, set out to solve a problem. The chief of Dana-Farber/Children’s Hospital Cancer Center’s Hematology/Oncology division wanted to treat Fanconi anemia (FA)—a rare, inherited bone marrow failure disease—using gene therapy. In the process, he’d be able to replace patients’ faulty bone marrow cells with ones corrected for the genetic defect that causes FA.
There was one big problem though. “The main bar to attempting gene therapy in FA is that you need to be able to collect a certain number of blood stem cells from a patient in order to be able to give enough corrected cells back,” he says. “In our early clinical trials, we were unable to provide enough corrected stem cells to reverse the bone marrow failure we see in these patients.”
One way around the supply issue would be to create the necessary blood stem cells from FA patients’ own cells by first reprogramming skin cells into what are called induced pluripotent stem (iPS) cells. Using one of several methods, scientist can reboot mature skin cells into an immature, stem cell-like state—essentially turning the cells’ biological clocks back to a time when they could grow into anything the body might need.But then Williams came up against another problem: Cells from FA patients turned out to be very difficult to reprogram into iPS cells.
To see if he could make his gene-therapy plan work, he and a member of his laboratory, Lars Mueller, MD, joined forces with iPS cell expert George Daley, MD, PhD, director of the hospital’s Stem Cell Transplantation Program, and FA specialist Alan D’Andrea, MD, at Dana-Farber/Children’s Hospital Cancer Center, to root out the cause of the FA cells’ stubbornness. From the beginning, the group suspected that the problem likely lay in the very defect that causes FA in the first place: a genetic mutation that shuts off some of the molecular machinery cells use to repair damage to their own DNA.
It turned out that they were right: As they reported last month in Blood, the group found that cells from FA patients and from mouse models of the disease cannot fix the genetic damage that occurs during the reprogramming process, damage that normal cells can fix on their own. “For reprogramming to work, cells need a working DNA repair pathway,” says Williams, “but that’s the pathway that’s defective in FA.”
As a result, in FA cells the reprogramming process stalls and the cells enter a state of senescence—they become “old” and stop growing or dividing.
But there was good news in the study, too. The researchers also found two potential ways around the problem. First, as suggested by others, they tried correcting the FA defect in the skin cells before attempting reprogramming (rather than the other way around), combining the Williams’ lab’s expertise in gene transfer and that of the Daley lab in cell reprogramming methods. Second, during the reprogramming process they kept the cells in air that had much less oxygen than the air we typically breathe, making use of the D’Andrea laboratory’s discovery that cells with the FA defect are easily damaged by oxygen and oxygen byproducts.
This second strategy allowed them to reprogram FA cells without correcting the FA defect first, albeit at low efficiency—something researchers at other institutions had suggested would be impossible. “The ability to create iPS cells that retain the FA defect is critical if we are to develop iPS-based human FA disease models in the laboratory,” Williams explains.
All told, the results mean that the barriers to reprogramming cells from FA patients into iPS cells are not as insurmountable as had been thought. So what does this knowledge mean for Williams’ gene therapy plans?
“The technology for creating iPS cells isn’t yet ready for clinical application, though it remains a promising way to potentially grow large numbers of cells for gene therapy for FA,” he says. “But the technology is evolving, and this study is a great example of how really innovative basic research into the nature of the reprogramming process, involving laboratories with very different expertise, could one day benefit patients directly.”