May 11, 1999 HOUSTON, May 5, 1999 -- A material used for years in common processes such as shampoo manufacturing, paper production and water filtration may one day bring DNA to a patient's cells by way of a safer, nonviral gene therapy method.
Poly(ethylenimine) (PEI), a polymer, has been used to successfully transfer genes into living cells. For the first time, researchers at Rice University and the University of Texas-Houston Health Science Center have tracked the path that this polymer--which acts as an artificial virus--and the DNA it carries take through the cell. They have shown that the complex ends up--intact--in the cell's nucleus, where the new DNA can be read and put to work.
Antonios (Tony) Mikos, Rice professor of bioengineering, W.T. (Terry) Godbey, Rice graduate student in biochemistry and cell biology, and Kenneth Wu, professor and director of the Division of Hematology and Vascular Biology Research Center at the UT-Houston Health Science Center, labeled PEI/DNA complexes, which are joined by their electrostatic attraction to each other, with fluorescent markers and followed the trail as they moved through the cell to the nucleus.
"This study improved our understanding of the transport of DNA into the nucleus and provided new knowledge for the design of better and safer nonviral carriers for gene delivery," Mikos said.
The findings are published in the April 27 issue of the Proceedings of the National Academy of Sciences.
"We ran into something a little bit surprising--that PEI actually delivers the DNA to the nucleus without separating from the DNA--they get in as intact structures," Godbey said. "That's important because there are other polycationic [positively charged] systems that exist and if our carrier gets into the nucleus, other polycationic carriers might also get in." The researchers caution, however, that the effect the PEI has on the cell needs to be known before this method could be used to treat disease.
They found that the PEI/DNA complexes attach to cell membranes and then join into clumps that are taken into the cell. Once inside the cell, rather than being broken down and used by the cell's machinery, the complexes move into the nucleus, where the desired gene is turned on.
The researchers labeled both the DNA and the PEI separately in different colors, to verify that the complex gets into the nucleus intact.
Exactly how the DNA-polymer complex enters cell nuclei is not yet explained, but the researchers suggest the PEI/DNA could become coated with phospholipids, which help regenerate the cell membrane. The coated complexes might fuse with the membrane, and then release the PEI/DNA into the nucleus. More research needs to be done to determine the mechanism.
Gene therapy has traditionally taken advantage of a virus's ability to inject its DNA into cells, but with the viral agents come risks, such as an immune response. Researchers have wanted to find safer, nonviral agents to gain access to cells.
Both viral methods and nonviral methods using liposomes, which can fuse with the cell membrane to gain entry, have been used in clinical trials to treat Cystic Fibrosis and several forms of cancer. "The results that we're getting with the polymer rival if not exceed the results achievable with liposomes," Godbey said.
Said Mikos, "One of the great potentials of this method that uses synthetic polymers is not only that it is much safer than viral systems but one can modify the polymer and incorporate specific ligands so it can deliver the DNA to the desired cell type for targeted gene therapy."
In addition to Cystic Fibrosis and cancer, the technique is also promising for cardiovascular applications such as balloon angioplasty. During the procedure, gene therapy could be used to deliver genes aimed at slowing down the rapid proliferation of cells which causes scarring and a re-narrowing of the artery. It could also be used in growing tissue-engineered bone for repairing bone defects by delivering growth factors that increase the proliferation of cells.
Mikos and Godbey point out that it is a versatile method. With the PEI/DNA complex, they are not limited by the gene or genes that they want to deliver. PEI/DNA can be injected into the patient intravenously; gene therapy with PEI can be done in the lab, such as in growing new bone, and then introduced into the body; and it could be introduced during surgery.
"The ultimate goal of this approach will be to apply it to a clinical situation that will treat a disease or try to repair a defect in a tissue," Mikos said. "However, before we get to that point there are some very basic questions we have to address about the fate of the polymer itself and also in trying to understand the mechanism before this system reaches the clinic," he cautioned.
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