Gene Silencing Inhibits Tumors in Ewing's Sarcoma Model
Scientists have used a new method of silencing genes to inhibit tumors in mice that have a form of Ewing's sarcoma, a rare and often deadly bone cancer in young adults. The findings were presented April 19th at the annual meeting of the American Association for Cancer Research (AACR) in Anaheim, California.
The project was a collaboration between the laboratories of Dr. Timothy Triche at Childrens Hospital Los Angeles and Dr. Mark Davis at the California Institute of Technology (Caltech). Dr. Triche's team has expertise on Ewing's sarcoma while Dr. Davis' team of chemical engineers has developed ways to deliver gene-silencing molecules to specific cells.
Ewing's sarcoma, which is nearly always fatal once it has spread in the body, has been traced to two chromosomes that break and exchange genetic material, activating a gene called EWS-FLI1 that is critical to the development of the tumors. Silencing the gene, however, can inhibit Ewing's tumors.
"The purpose of the study was to use the targeting of an oncogene to treat a childhood tumor," says Dr. Siwen Hu, a postdoctoral fellow at Childrens Hospital Los Angeles and the University of Southern California, who presented the research at AACR. When the treatment was given over 4 weeks, the results were "striking" and the tumor growth effectively stopped in many animals, according to Dr. Hu.
The delivery method uses a short interfering RNA (siRNA) molecule designed to silence the gene EWS-FLI1. The siRNA was packaged with sugar-containing polymers and delivered into tumor cells by a modified protein called transferrin that normally ferries iron into cells.
"We set out to design a delivery system that could work systemically in metastatic disease because you want to reach many different types of locations, wherever the tumors may be," says Dr. Davis.
The ultimate goal is to modify the method for use in people, as an intravenously administered treatment. The sugar molecules, or cyclodextrins, in the polymer that packages the siRNA have been used in pharmaceuticals and are safe for human use.
"The key issue here is that the delivery system is nontoxic and doesn't cause an immune response," explains Dr. Davis. "We've been working on this system for years."
To determine if the system was working as designed, the researchers modified two key elements - the targeting agent that carried the siRNA to tumor cells and the siRNA itself. With each modification, the system failed, suggesting to the researchers that the tumor inhibition they observed was due to the design of the siRNA and the mode of delivery.
The potential of gene silencing approaches in blocking the activity of genes involved in cancer has been suggested by many studies in cell cultures. But a major challenge has been finding ways to target tumor cells and protect the siRNA molecules from degradation so that they can have a therapeutic effect.
The new method "is a real breakthrough in systemic delivery of non-chemically modified siRNAs," comments Dr. John Rossi of the Beckman Research Institute in Duarte, Calif., who was instrumental in bringing the two laboratories together for the study.
"This is a lovely study that combined several technologies to silence a gene," said Dr. John Mendelsohn, president of the University of Texas M.D. Anderson Cancer Center, who moderated the news conference during which the study was presented. He predicted the approach would eventually be tested in human clinical trials.
Drs. Rossi and Davis recently launched a company called Calando Pharmaceuticals to develop siRNA gene-silencing therapies. Dr. Davis selected the word "calando" because in music notation it refers to a gradual decrease in volume, or silencing.
By Edward R. Winstead