DNA minivectors provide new gene therapy tool

HOUSTON — (October 21, 2010) —
Tiny wound-up circular bits of DNA that carry a short “hairpin” of RNA could prove a solution to the quest for a non-toxic, long-lived gene therapy vector designed to silence cancer genes in tumor cells, said researchers from Baylor College of Medicine and The Methodist Hospital Research Institute in a report that appears in the current issue of the journal Gene Therapy.

At a minimum, this Minivector™ DNA will prove a valuable laboratory tool, said Dr. Lynn Zechiedrich, associate professor of molecular virology and microbiology at BCM and one of the report’s senior authors. “In the best case scenario, it could prove a valuable therapeutic tool that can work on cells that are difficult to manipulate. This may help clinicians treat some diseases.”

Minivector DNA avoids the toxicity of viruses, lasts longer than plasmids (large DNA circles containing bacterial sequences toxic to human cells) and survives and continues gene delivery longer than small interfering RNAs (siRNA), currently used to silence the genes in the laboratory research setting. Plasmids are as much as 10 times larger than Minivectors, and siRNAs, while small, are too unstable in blood to translate to patient treatment.

“Plasmids are too large and toxic,” said Zechiedrich. “Smaller is better. Also, deleting toxic bacterial sequences from the tiny circles prevents bad side effects.”

Lymphoma is the fifth most common cancer in the United States, and a type called anaplastic large cell lymphoma is the most common T-cell lymphoma in children and the second most common aggressive systemic T-cell lymphoma in adults. Doctors treat it with a specific regimen of a cocktail of many anti-cancer drugs.

“Because this regimen can cause side effects, researchers are seeking a specific therapeutic approach” said Dr. Youli Zu, associate professor in the Methodist Hospital Research Institute and a senior author of the report. However, while he has been able to suppress the growth of lymphoma with siRNA in animals, he has found siRNA to be unstable in blood, extremely limiting its value as a treatment.

Zechiedrich and Zu found that the Minivectors are stable in human blood for at least 48 hours, compared to only half an hour for siRNA and two hours for plasmids.

In addition, Minivectors are highly effective in taking the DNA into lymphoma cells, specifically shutting down cellular cancer genes that resulted in death of the lymphoma cells.

“Clinically, Minivectors should advance gene-targeting therapy specific for lymphoma, as well as other types of tumors, with little or no side effects on normal tissue in cancer patients,” said Zu.

“Using these Minivectors will be trickier in diseases of organs that are not as easily accessed as blood, but we are working on that now,” said Zechiedrich.

Minivectors are measured in nanometers and consist of no more than a few hundred DNA base pairs. They are highly effective in taking the DNA into cells and also very efficient in silencing the targeted genes. In this case, they target cancer genes.

The postdoctoral fellows who carried out this research are Dr. Nianxi Zhao of The Methodist Hospital Research Institute and Dr. Jonathan M. Fogg of BCM.

Funding for this work came from the National Institutes of Health, The Methodist Hospital Research Institute, and the Burroughs Wellcome Fund.

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