Gene Delivery Method Yields Surprising Finding

A substance found in common consumer products like shampoo and paper may also have future applications in gene therapy, according to findings by researchers at Rice University and The University of Texas-Houston Health Science Center.

Gene therapy is a means of introducing genetic material into a cell that would not normally express that gene, or a way of altering the normal behavior of a cell. However, according to WT (Terry) Godbey, Rice Ph.D. candidate in biochemistry and cell biology, most cell types will not accept DNA on their own. As a result, a carrier, or transfection agent, is needed to deliver the DNA into the cell. Viral agents can inject DNA into cells, but with the viruses comes a risk of an immune response.

That's where the material called poly(ethylenimine) or PEI enters the equation. PEI is a nonviral polymer that carries many positive charges. Its charges are an important characteristic because when PEI interacts with DNA's negative charge, the two bond.

Godbey, along with Antonios (Tony) Mikos, Ph.D., Rice professor of bioengineering, and Kenneth Wu, M.D., Ph.D., Roy and Phyllis Huffington Chair, professor and director of the Division of Hematology and Vascular Biology Research Center at UT-Houston Health Science Center, tracked the PEI/DNA structure's path into the cell and made a surprising finding.

"No one had tracked PEI delivery into cells until this point and the big surprise was that the PEI got into the nucleus along with the DNA," says Godbey. "And that sparks a whole lot of additional questions that need to be addressed." The findings were published in the April 27 issue of the Proceedings of the National Academy of Sciences.

The researchers tracked the path of the PEI/DNA complex into the nucleus, labeling both materials in a different color. 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. (See illustrations)

The finding that the PEI also made its way into the nucleus, remaining attached to the DNA, raises many issues that researchers say will need to be addressed before this polymer could ever be considered in human clinical application. Chief among the considerations are: What happens to the polymer once it is inside the nucleus? Does the polymer have an effect on the genes that are already there? And does PEI's presence in the nucleus have any effect on the host gene expression?

"We need to know how PEI works, why it works, and any potential hazards that may be associated with it and possibly design a new carrier based on the information we learn from PEI," says Godbey. "The fact that we can do some of these things in vitro like we are doing is a big step. We need to take several more steps before we can use this in a clinical setting, but every step is one closer."

"PEI acts as a model carrier to help us understand the physical characteristics of the carrier for gene delivery," says Dr. Mikos. "Eventually, we may want to design a new synthetic polymer that preferably would be degradable."

According to Dr. Mikos, the advantage of using a nonviral carrier to transport the DNA into the cell is that, not only will it be safer, but researchers can tailor the properties of the polymer as needed for a specific application.

This research could have applications for gene therapy in cardiovascular disease, cancer and hereditary diseases.

"We need to have a good vector and PEI could be one of those. The first question is safety," says Dr. Wu, who is studying its application for the treatment of cardiovascular disease. "We're looking to see if, when the DNA is introduced to endothelial cells, would it also cause the activation of some other gene product or interfere with the function of the endothelial cells."

Gene therapy using a nonviral material could also be used to correct bone defects that result from tumors or trauma by delivering or producing locally the molecules that are needed to initiate bone regeneration.

Whatever the application, the research will require the expertise of several fields - bioengineering, biochemistry, cell biology and medicine - to produce a polymer that is safe and effective in a clinical setting. "This is the beauty of working with colleagues at the Texas Medical Center and Rice. This could not have been accomplished solely at one institution because it requires the integration of engineering, life sciences and medical sciences," says Dr. Mikos.

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©2006 Texas Medical Center

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