Photo by Todd McNaught.
Clinical Research Division scientists have developed a new way to transfer genes into adult stem cells, a process that could lead to improved gene therapy for treating human diseases, including cancer.
By manufacturing the gene-therapy vector - a weakened virus used as a delivery vehicle for therapeutic genes - in human cells instead of mouse cells, researchers enhanced the transfer of genes into stem cells about threefold.
The strategy will be used in clinical trials to treat Fanconi anemia, a rare genetic disorder that results in destruction of the bone marrow.
Dr. Hans-Peter Kiem, whose laboratory led the research, said a major obstacle to success in gene therapy has been the inability to transfer efficiently genetic material into blood-forming stem cells, which are then transplanted into a patient to correct a genetic defect.
Two- to three-fold increase
"Progress in gene therapy has been made in a step-wise fashion," he said. "So even a two- to threefold increase in gene transfer to stem cells is an important advance."
Conducted by postdoctoral fellow Dr. Peter Horn and research technician Julie Morris in Kiem's laboratory in collaboration with Drs. Max Topp and Stanley Riddell, the study appears in the Dec. 1 edition of Blood.
Gene therapy tries to introduce healthy versions of genes into cells of tissues that lack them. The strategy requires a gene-delivery system, or vector, to transfer DNA into cells. Viruses are attractive vectors because they can be engineered to infect specific cell types and modified to reduce or eliminate their pathogenic properties. Particularly useful are retroviruses, which insert their genetic material into the chromosomes of host cells and become a permanent fixture of the cell's DNA.
Stable persistence of the genes in tissue requires transfer into long-lived adult stem cells that can self-renew, producing specialized cell types that make up a tissue. Blood-forming, or hematopoietic, stem cells are the most accessible adult stem cells, as they can be isolated from circulating blood.
Researchers typically grow large quantities of gene-therapy viral vectors in mouse "packaging" cells, which release mature viral particles that can be collected to infect stem cells. But factors released by the mouse cells can interfere with the ability of the viruses to successfully enter stem cells.
To overcome this problem, Horn and colleagues used human packaging cells to produce viral vectors. The researchers found that packaging the vector in human cells resulted in a two- to threefold increase in gene-transfer efficiency to stem cells compared to using virus prepared in mouse-packaging cells. Packing in human cells resulted in stable gene-transfer levels up to 25 percent, suggesting that such cells do not produce factors that inhibit viral transfer.
Based on these and previous results from his lab, Kiem's group can efficiently transfer potentially therapeutic genes to blood stem cells to treat patients with genetic diseases, such as children with Fanconi anemia, a severe anemia that also increases cancer susceptibility.
In addition, Kiem's lab, Dr. George Georges in the Clinical Research Division and colleagues at Cincinnati Children's Hospital reported at last month's American Society for Hematology (ASH) annual meeting that their improved gene-transfer strategy also may allow for better gene-therapy strategies for the treatment of cancer.
In their study, the researchers introduced a drug-resistance gene into stem cells to protect blood cells from the damaging effect of chemotherapy. This approach could make the administration of chemotherapy to solid-tumor patients more tolerable.
Furthermore, they demonstrated that drug-resistance genes also can be introduced into donor stem cells for stem-cell transplantation, a therapy for leukemia and other blood disorders. This approach may be of particular use for a modified stem-cell transplant procedure known as a non-myeloablative or "mini"-transplant, a procedure that employs minimal doses of radiation and immunosuppression followed by an infusion of donor stem cells that attack the cancer. Use of drug-resistant stem cells could allow for less toxic treatment of persistent or recurrent disease following the transplant procedure.