A Center for Cancer Research scientist has genetically engineered cancer-prone mice that carry cells that switch on a cancer-causing gene spontaneously, generating lung and other cancers much like humans do. The technique for generating the mice may be used to model many kinds of human cancers in mice.
In an article published in the April 26 issue of Nature, Tyler Jacks, associate professor of biology and Howard Hughes Medical Institute (HHMI) investigator, and his colleagues reported that they used a "hit-and-run" approach to producing gene alterations in mice whose cells harbor an inactivated form of the K-ras cancer gene, or oncogene.
Mutations in K-ras are highly prevalent in human cancers, occurring in 90 percent of pancreatic tumors, 50 percent of colon tumors and 30 percent of non-small-cell lung cancers. Previous mouse models of these forms of cancer have been informative, said Professor Jacks, but they have not accurately recapitulated the kind of spontaneous mutations that characterize cancers involving K-ras. Some of the obstacles have been technical in nature. For example, he said, creating a mouse embryo with cells that contain mutated K-ras, which is a dominant mutation, would either be severely damaging or lethal to the embryo.
"Researchers have tried to overcome the problem of dominance by making transgenic mouse strains that only express the dominant oncogene in cells of a given tissue," said Professor Jacks. "The problem is that neither embryonic expression nor such tissue-specific transgenic mice recapitulate what one finds in normal human cancer, where individual cells acquire an oncogenic mutation, but they are otherwise surrounded by normal cells.
"Our strategy was to create a kind of genetic Trojan horse," he said. They did this by introducing latent K-ras genes into mice. The genes were inactivated because they had duplicated segments of DNA that prevented the genes from activating themselves.
"Then as the mice grew, individual cells underwent rare spontaneous, sporadic recombination events that deleted one copy of the duplicated sequence, such that the K-ras gene was activated, and this initiated tumor development," Professor Jacks said.
The technique was based on the hit-and-run gene-targeting technology developed by HHMI investigator Allan Bradley at Baylor College of Medicine. The two-part technique consists of "hitting" cells with an inserted mutated gene and then allowing the recombination event to "run," activating the inserted gene.
"We believe that this new model more accurately mimics tumor development in humans, which may lead to new insights into the genetic changes that occur in human disease," said Professor Jacks. "We can isolate tumors at various stages of progression, and thus discover genes and processes that accompany tumorigenesis."
In an additional experiment, the scientists also produced K-ras mice with a mutant form of p53, a tumor suppressor gene whose malfunction is known to spur the progression of K-ras-based human lung cancers.
Although his laboratory will focus mainly on lung cancer, their studies of the mice have already offered new insights into other K-ras-related cancers. "These mice don't develop colon tumors, although we might expect them to because K-ras is mutated in human colon cancer at high frequency," said Professor Jacks. "This tells us that the order of mutation matters in colon cancer, which is caused by multiple mutations." The idea that the order of mutation is important is an argument that has been advanced by HHMI investigator Bert Vogelstein at Johns Hopkins University, based on his studies of human colon cancers, he said.
Particularly promising, said Professor Jacks, is the immediate potential for using the new mouse model to test both chemotherapies and chemopreventives for lung cancer.
Strategies to prevent such lung cancers, which formerly required expensive and long-term clinical trials in humans, could be tested quite readily using the new mouse strain, he added.
Co-authors of the paper include former MIT graduate student Leisa Johnson, current laboratory researchers Kim Mercer and David Tuveson (also at Dana-Farber Cancer Institute) and Roderick Bronson, a pathologist from the Tufts University Schools of Medicine and Veterinary Medicine.
Support for the early stages of this project came from the MIT/Charles Reed Fund.
This is an edited version of an article that originally appeared on the web site for HHMI news.
A version of this article appeared in MIT Tech Talk on May 9, 2001.