In-vivo gene therapy successfully restored the immune system in basset pups with X-linked severe combined immunodeficiency, or XSCID, a life-threatening genetic disorder that effectively disables the immune system. Researchers from the University of Pennsylvania School of Veterinary Medicine and the National Institute of Allergy and Infectious Disease injected a retrovirus containing a corrective version of the gene responsible for XSCID, an important proof of principle for the technique of "in-vivo" gene therapy. Their findings are presented in the April 15 issue of the journal Blood.
In humans, XSCID affects one in 100,000 boys, resulting from the inheritance of a faulty gene on one of the mother's X-chromosomes. It often proves fatal before the child's first birthday. The disease first came to public attention in the late 1970s with the "Bubble Boy," David Vetter, who lived his entire life in a sterilized environment in order to protect him from outside germs. Vetter died in 1984.
The only treatment for XSCID is through a bone-marrow transplant from a normal donor designed to replenish the hematopoietic stem cells that are capable of constantly renewing new functional immune cells, or, more recently, gene therapy that works by replacing the defective gene with a normal gene in the patients own cells. Gene therapy has been put to curative use against this disease in humans resulting in successful immune reconstitution in 10 of 11 boys in a 1999 French study. This clinical trial consisted of the standard ex-vivo approach to gene therapy in which bone-marrow cells were taken out of the body of the affected boy, cultured in vitro for five days with exposure to a retroviral vector containing the normal gene and transplanted back into the patient. Three of the boys unfortunately developed a T-cell leukemia attributed to the gene therapy resulting in the cessation of this study.
"Although ex-vivo gene therapy has been shown to be capable of restoring normal immune function in XSCID boys, there are several potential problems with this approach," said Peter J. Felsburg, professor of immunology at Penn's School of Veterinary Medicine. "The number of gene-corrected bone- marrow stem cells that can be transplanted back into the patient is limited to correcting the potentially low number of bone-marrow stem cells harvested from the patient. In addition, the manipulation and culturing of the cells outside the body may alter their ability to provide for long-term generation of new immune cells."
The Penn researchers and their NIAID colleagues, led by Drs. Suk See Ting-De Ravin and Harry L. Malech took a different approach by directly injecting the retrovirus vector containing the corrective gene into the bloodstream of XSCID dogs with the hope of correcting the defective hematopoietic stem cells within the patient. The therapy completely restored immune function in three of the four dogs the researchers treated. The fourth dog received the lowest dose of the retrovirus vector the virus that had been engineered to pass on the gene leading Felsburg and his colleagues to believe that there is a lower limit to the dose before the treatment becomes effective.
Since the boys involved in the French gene-therapy study did not develop leukemia until more than three years after the treatment began, the researchers have been particularly interested in knowing the long-term consequences of the trial. At 16 and 18 months following treatment, the two dogs involved in the long-term study have maintained their immune systems and remain, effectively, cured of XSCID, with no adverse side effects.
"The results of this study show that this in-vivo approach to gene therapy may be a viable alternative for not only gene therapy of XSCID but perhaps other hematologic and immunologic diseases, thereby eliminating any potential detrimental effects of the ex-vivo manipulation and culture of cells that is required by current clinical gene therapy protocols," Felsburg said. "In addition, this approach would make it easier to perform gene therapy outside of specialized medical centers."
Felsburg's research was funded by grants from the National Institute of Allergy and Infectious Disease of the National Institutes of Health.
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