Two Genes Are Better Than One: Combination Approach Selectively Kills Pancreatic Cancer Cells

Columbia University College of Physicians & Surgeons (P&S) researchers are reporting that a gene that stops tumor growth--called mda-7--together with a snippet of DNA that turns off the K-ras cancer gene killed pancreatic cancer cells in test tube experiments. The combination approach also prevented tumor growth in animals. The findings are being reported in the Aug. 28th issue of the Proceedings of the National Academy of Sciences.

"I think the observations are very provocative and, with a little tweaking, may lead to an effective therapy for a devastating and aggressive disease," says Dr. Paul B. Fisher, professor of clinical pathology at P&S and lead investigator for the study.

An estimated 29,200 people in the United States will be diagnosed with pancreatic cancer in 2001 and about 28,900 will die of the disease, according to the American Cancer Society. About 2 percent of people with pancreatic cancer live at least one year after the cancer is found, and very few survive five years. Current surgical, chemotherapy, and radiation treatments are not very effective.

Dr. Fisher decided to test the combination approach because his prior research had shown the mda-7 protein could stop the growth of a variety of cancer cells, but not pancreatic cancer cells. "The mda-7 protein is a good candidate as a possible therapy because it does not have a deleterious effect on normal cells," Fisher says. In fact, clinical studies elsewhere are testing the safety of mda-7 in other types of cancers.

K-ras is a type of protein inside a cell involved in controlling cell division and cell signaling. Scientists believe a change in the K-ras gene, which occurs in 85 percent to 95 percent of pancreatic cancers, may lead to tumors, or the uncontrolled growth of cells. Mutated ras proteins, which includes altered K-ras, are targets in several ongoing cancer clinical trials.

In the study, Dr. Fisher and researchers in his laboratory genetically engineered an adenovirus, a common virus that causes respiratory problems, to contain the mda-7 gene. The virus delivers the mda-7 gene to pancreatic cells but cannot reproduce, thereby persisting in cells only for a few generations. Adenovirus has been used previously as a carrier in gene therapy experiments.

Using what is called antisense methodology, Dr. Fisher also worked with a small piece of DNA, 18 base pairs long, that acts to prevent the formation of the K-ras protein. Such DNA, when introduced inside a cell, blocks the initiation of K-ras gene expression and prevents the protein from being made. Scientists have also investigated giving patients pieces of antisense DNA for treatment in a variety of diseases.

In the PNAS study, the researchers found the mda-7 gene and the antisense K-ras DNA combination killed pancreatic cancer cells in test tube experiments after three days of treatment. Pancreatic cancer cells continued to grow if they were treated with either the mda-7 alone or the antisense K-ras DNA. The scientists also showed the combination prevented tumor growth in animals.

The investigators now believe they understand why mda-7 introduced on its own cannot kill pancreatic cancer cells. Although the gene is expressed and a chemical intermediary, messenger RNA, is manufactured, the actual functioning protein is not made in these cells. But when the scientists also blocked K-ras, the cells were able to make mda-7 protein.

Dr. Fisher's laboratory now is working to improve the way the mda-7 gene and K-ras antisense DNA get into pancreatic cancer cells. While the two pieces of DNA were introduced separately in the current research, getting both into one delivery system, called a bipartite virus, might be even more efficient, Dr. Fisher says.

P&S associate research scientists Dr. Zao-zhong Su, Dr. Irina V. Lebedeva and Dr. Rahul V. Gopalkrishnan participated in the research as did Dr. Cy Stein, associate professor of clinical medicine (in pharmacology).

Collaborators on this project included Dr. John C. Reed, scientific director, Burnham Institute, La Jolla, Calif., Dr. Neil I. Goldstein, vice president, DGI Biotechnologies, Edison, N.J., and Dr. Paul Dent, associate professor, Medical College of Virginia, Richmond.

The Michael and Stella Chernow endowment and the Samuel Waxman Cancer Research Foundation supported the research. Dr. Fisher also is Director of Neuro-oncology Research and the Michael and Stella Chernow Urological Cancer Research Scientist at P&S.

The 6-inch long pancreas is a gland located in the abdomen that makes pancreatic juices, including hormones and insulin that help digest food.

Pancreatic cancer has such a low survival rate because it is difficult to detect early. By the time a person has symptoms, such as jaundice or abdominal pain, the cancer may have reached a large size and have spread to other organs.

The National Cancer Institute says patients with any stage of pancreatic cancer can be considered candidates for clinical trials because of the poor outcome of existing therapies, although conventional treatment can alleviate symptoms.

The risk of getting pancreatic cancer increases with age. Smoking and diabetes are other risk factors for pancreatic cancer.