By David N. Leff
Nude mice, it's well known, lack the immune defenses to protect themselves against non-mouse invaders. That's why countless numbers of these hairless rodents get injected under the skin with human tumor cells, in laboratory tests of anti-cancer strategies and drugs.
Of course, the researchers must make allowances for the simple fact that, although they're the most popular animal model we've got, mice are not 100 percent interchangeable with people in their cellular and molecular reactions. One little-known difference between Mus musculus and Homo sapiens is that all of a mouse's cells contain the enzyme telomerase. (This is the polymerase that creates telomeres - the protective tip ends of mammalian chromosomes.) In humans, only a few select cells come into the world equipped with this key enzyme.
"The great majority of normal human cells," observed molecular and clinical oncologist William Hahn, "don't have telomerase. The minority that do - cells like sperm or ovaries - are generally self-renewing stem-cell populations. For example, hematopoietic - blood-forming - stem cells are telomerase- positive. So are crypt cells in your epidermis." (See BioWorld Today, April 20, 1998, p. 1.)
"Everyone presumes," Hahn went on, "that these germ-line cells have to turn over many more times than the differentiated somatic cells around them. And because of that they need to have this telomerase enzyme, which allows them a longer life span."
Why mice have telomerase in all their somatic and germ-line cells is a subject of speculation, Hahn pointed out. "The way we think about it is that human cells possess many more barriers than do mouse cells to prevent them from becoming cancerous. It might be that as mice have a lot shorter lives than humans do, so the chance that cancer will develop in their lifetime is much less. That's impossible to prove," he continued, "but it's one way of thinking about why mouse cells are much easier to transform, and turn into cancer cells."
Considering that lab mice spend their short lives helping human researchers defeat cancers, Hahn's remark may seem a bit counterintuitive. He is a post-doctoral fellow in the laboratory of molecular biologist Robert Weinberg, at the Whitehead Institute for Biomedical Research in Cambridge, Mass.
Hahn explained the apparent incongruity of deliberately trying to render healthy cells malignant: "In Dr. Weinberg's laboratory over the last 15 or 20 years," he recalled, "one of the goals has been to try to understand, at a molecular level, what are the changes that take a normal cell and turn it into a cancer cell. And that search has taken the lab through oncogenes and tumor suppressors and the cell cycle, as well as a number of other tumorigenic pathways."
Telomerase - An Unusual Suspect ...
"That undertaking a few years ago," Hahn continued, "led some of the postdocs in the lab to start working on telomerase, which is expressed in the great majority of human cancers, and not expressed in most normal cells. More specifically, in 1983 the lab published a finding that pairs of oncogenes - so-called collaborating oncogenes - could take a normal rat or mouse cell and convert it into a cancer cell. But when they did the same experiment in human cells," he recounted, "it never worked. Nobody understood why it didn't work, but saw it as a paradox of cancer."
Fast forward 15 years. Today's issue of Nature, dated July 29, 1999, carries a climactic report titled: "Creation of human tumor cells with defined genetic elements." Its lead author is Hahn, and senior author, Robert Weinberg.
"We asked a simple question," Hahn told BioWorld Today: "'If we took two collaborating oncogenes and added telomerase, then put all three of these genes into a normal human cell, would we get a cell that now can form a tumor?' And that's indeed what we're reporting in Nature."
One of the co-author's two cooperative oncogenes encoded the simian virus 40's large-T antigen. The other was ras. "We know that ras is not mutated in every cancer," Hahn explained, "but that in a lot of cancers there are mutations in its pathway. Also, that the large-T oncogene of SV40 inactivates the p53 and retinoblastoma tumor suppressors, but it certainly is not involved in human cancers. So our future work," he added, "will be to try to figure out if we take genes that we know are involved in human cancers - say breast or prostate - can we recapitulate these results by using the identified genes that caused the cancers, and are seen in tumors in those tissues?"
Their third gene - like the third key on a three-key safe-deposit box - expressed the catalytic subunit of the telomerase enzyme. Then they infected two normal human cell types, skin fibroblasts and kidney epithelial cells, with this triple-threat package. In vitro, the transfected cells showed their new malignant colors by piling up on each other, rather than anchoring to the dish in a healthy monolayer.
"Then we took those cells," Hahn narrated, "and put them into mice. When we looked to see whether a tumor would form or not, we found that only when we had introduced all three genes into the cells did they make tumors. No other combination of one gene or two genes alone gave us that result."
... Now A Potential Drug Target
"This is one of the first pieces of evidence," Hahn pointed out, "that telomerase really is important in making a tumor, and not just a marker that came along for the ride, making a cancer cell line. Because of that, it validates the idea that telomerase is a target to start making drugs against."
He and his co-authors believe that this conversion has that potential for developing anticancer therapeutics, "but not immediately. What they'd like to do now," he said "is try to determine whether these oncogenic pathways required to transform a normal cell work in all cancers, or just certain ones. Then down the road," he concluded, "identify agents that affected those pathways in particular, so you would have a new arsenal of chemotherapy drugs that not only - hopefully - would be more effective, but more specific and less toxic."