“RETROVIRAL VECTORS TO DECREASE BONE MARROW TRANSPLANT COMPLICATIONS
Researchers have proven the safety of using retroviral vectors to decrease the risk of graft versus host disease (GvH), a major complication that may arise from bone marrow transplants. These findings were presented at the 8th Annual Meeting of theAmerican Society of Gene Therapy (ASGT) in St. Louis.
Major complications of bone marrow transplants can result when T-cells from the donor are infused with the bone marrow of the recipient. The T-cells play an important role in restoring the patient’s immune system to fight infections. However, these same T-cells can also attack the recipient’s body resulting in graft versus host disease.
Previous research has proven the success of using retroviral vectors carrying a “suicide gene” that eliminates T-cells in a patient’s body if they cause severe GvH, but otherwise allows the T-cells to have their beneficial effects supporting immunity to infections or relapse.
A research team led by Fulvio Mavilio and Claudio Bordignon from the University of Modena and Reggio Emilia and the San Raffaele Hospital in Milan, Italy, addressed the safety aspects of this approach in light of the complication of the development of leukemia in three of eleven patients treated for X-linked SCID that used retroviral vectors inserted into bone marrow stem cells to treat their immune deficiency.
Researchers analyzed the gene modified T-cells of patients treated with retroviral vectors and determined that the presence of integrated vectors had minimal effects on the genes being expressed in the cells. The study noted that over 40 bone marrow transplant patients have been treated with retroviral vectors, using more than 100 billon cells, without any complications seen. These findings are significantly different than those from the XSCID trial. They suggest that the risk of gene therapy differs specifically on the target cell type (T cells vs. bone marrow stem cells) or the gene being transferred (suicide gene vs. XSCID gene).
DEVELOPMENT OF LEUKEMIA IN A PRIMATE PROVIDES CLUES
A study reporting the development of leukemia in a primate five years after undergoing gene therapy allowed researchers to discover a gene that may contribute to the development of cancer. These findings were presented at the 8th Annual Meeting of the American Society of Gene Therapy (ASGT) in St. Louis.

The development of leukemia in three patients from the XSCID trial has become a major concern in the field of gene therapy. The critical question is whether the leukemia in the XSCID patients was going to be typical of all gene therapy trials, or whether it was related to that particular disease, and not be a problem elsewhere.
Ruth Seggewiss, MD, from the NHLBI, NIH, and colleagues report the first occurrence of leukemia in a primate five years after undergoing gene transfer. The animal was initially transplanted with blood stem cells containing a defective retroviral vector, which contained a “marker gene” allowing investigators to easily identify the cells carrying the virus. The virus contained also a gene that gave resistance to certain chemotherapy drugs.
Contrary to the X-SCID trial, the virus used in this animal did not contain a gene that enabled the virus carrying blood stem cells to live longer. Also, the animal was healthy when it received the gene marked blood stem cells, opposite from the children that developed the leukemia in the X-SCID trial, making them more likely to develop leukemia.
However, this particular animal showed special features before developing the leukemia. Researchers noted that in the first year after undergoing gene therapy, the animal had very high levels of “marked cells” opposite of the levels usually found in non-human primates. They discovered this high level was due to one hematopoietic stem cell clone that dominated the whole hematopoiesis in the first year after the gene therapy. Hematopoiesis is the process of producing and developing new blood cells.
Researchers found that the leukemia cells contained two different virus insertions. They discovered that one of the insertions was in the gene BCL2-A1, which could prevent cells from receiving a normal death signal. Researchers believe is it is this gene that made it more likely for the animal to develop leukemia. They think the animal acquired an additional mutation in the following years that led to the development of the disease.
Researchers believe the leukemia that developed in the primate was not caused by gene therapy, but that it may have made it more likely to occur. These findings indicate the need for further research to better evaluate the risks and benefits of using certain gene therapy procedures.
GENE THERAPY CURES SEVERE INHERITED DISEASE
Researchers have developed a successful gene therapy for curing the inherited disease Phenylketonuria, commonly known as PKU, in mice, according to findings presented at the 8th Annual Meeting of the American Society of Gene Therapy in St. Louis. Untreated children with PKU develop severe mental retardation due to a deficiency of a liver enzyme called phenylalanine hydroxylase (PAH).
Li Chen, PhD and Savio Woo, PhD, from the Mount Sinai School of Medicine, used an integrase system of bacterial origin to insert the PAH gene into specific locations in the chromosomes of liver cells in PKU mice, which is a faithful model of the human disease. The PAH gene provided the treated PKU mice with the needed enzyme in the liver and cured all symptoms of the disease.
The bacterial integrase system of gene transfer provides the unique advantage that the therapeutic genes are inserted into specific positions in the mouse and human genomes where no other genes exist and once integrated the therapeutic genes can be expressed strongly and permanently in the treated cells. Compared with other gene delivery systems where the therapeutic genes are inserted randomly in the genomes of target cells, there is much reduced risk of the therapy causing chromosomal damage and/or other side effects. Thus the bacterial integrase system can potentially be developed into an effective and safe method for gene therapy of inherited disorders in patients.
The American Society of Gene Therapy is a professional non-profit medical and scientific organization dedicated to the understanding, development and application of gene and related cell and nucleic acid therapies and the promotion of professional and public education in the field.
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