Gene therapy – which aims to replace "bad" genes with useful ones – has yet to live up to its promise largely because of problems delivering genes to the right place in the body. What’s more, the viruses many gene therapy techniques use can arouse unwanted immune reactions.
Researchers at Jefferson Medical College have developed a system that sidesteps viruses, and they hope, some of the inherent problems with their use. Scientists, led by Eric Wickstrom, PhD, professor of microbiology and immunology, and graduate research assistant Stephen H. Cleaver, both of Jefferson Medical College and Thomas Jefferson University in Philadelphia, have used DNA and bacterial proteins to deliver a gene to a specific place in a piece of human DNA.
They have used a "transposon," or a naturally mobile piece of DNA, from the E. coli bacterium as a gene delivery vehicle, inserting a gene for antibiotic resistance into a segment of DNA.
"This is the first time it [gene therapy] was done this way," says Dr. Wickstrom, who is also a member of Jefferson’s Kimmel Cancer Center. Their results appear August 22 in the journal Gene.
"The transposon inserted in exactly the right place and expressed its product, in this case, an antibiotic-resistant gene." The technique has several advantages, he says. "It avoids using viruses that cause immune reactions, it puts the gene in a precise, known location as opposed to anywhere in the cell’s DNA, and it makes only one copy."
There are significant problems with viral methods, Dr. Wickstrom says. In traditional gene therapy methods, there is a question of how many gene copies may be made in each individual and where in the cell’s DNA a copy may go. "The genes could go anywhere in the genome. This may be okay, or they could interact inappropriately with other genes, such as a tumor suppressor gene, knocking out its function and starting a cancer."
Dr. Wickstrom points out that his current tests so far have been only in bacteria to date. "We would like to avoid these problems and still have an effective method," he explains. "We’ve only done this in bacteria – not a higher organism, though this was with a human genomic sequence."
The Jefferson team was awarded a patent for the technique and a new grant for more than $660,000 from the National Institutes of Health to continue to develop their method of gene transfer in yeast and mice.
Dr. Wickstrom notes that gene therapy with and without viruses currently is being studied in clinical trials by many researchers in this country and overseas. In his recent book, "Clinical Trials of Genetic Therapy with Antisense DNA and DNA Vectors," he and his co-authors described a variety of preclinical and clinical studies in the United States and other countries.
He adds "the ability to turn off or correct individual disease-causing genes, or to replace them at will in a patient’s cells provides a powerful therapeutic intervention in genetic diseases.
"It’s clear that all disease results from incorrect gene expression, one way or another," he notes. "By turning off some genes or correcting others, we may be able to treat each disease at its point of origin." Eventually, Dr. Wickstrom hopes to use his nonviral method to provide normally functioning genes in diseases such as hemophilia, sickle cell anemia, muscular dystrophy, and phenylketonuria, and therapeutic genes to treat cancer.
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