Innovations 879

turned off. The shorten, Figure 1 Innovations until eventually the cells stop dividing. This fact, correlation number one, suggests that shortened Of mice and men telomeres might signal the cell to become senescent, a stable state in Geron Corporation which the cell remains metabolically active but no longer divides. Calvin Harley now knows that he has Senescent cells accumulate as little chance of curing in mice. organisms age, and their altered gene Whether he can do it in humans expression may contribute to various remains an open question. age-related diseases. Harley is chief scientific officer of The last thing that cancerous cells Geron Corporation (Menlo Park, want to do is to senesce, and sure California), a company formed in 1992 enough they reactivate to exploit the possible involvement of activity (by a mechanism that remains an enzyme called telomerase in unknown) and maintain the length of cancer and aging. The simplicity and their telomeres. The finding that Mouse chromosomes stained for telomeric elegance of the telomerase theory has ~85% of malignant cancer biopsies are repeat DNA. Image courtesy of Geron. landed it on the cover of both Cell and positive for telomerase, correlation Time, and in the pages of both Science number two, has encouraged Geron to The first involves mutation of an and the New Yorker. search for specific inhibitors of oncogene or tumor suppressor to But it is a theory that was initially telomerase as cancer therapeutics. allow unregulated growth. This is based more on correlation than enough to keep them going for a causation, and recent results in mice The mouse that kept on roaring while, but then the telomeres shorten have left some researchers with To test the outcome of telomerase and cells enter a state called crisis. doubts. Most are keeping an open inhibition, Maria Blasco of the The cells that get through crisis have mind, given the uncertain translation Universad Autonoma Madrid, and increased levels of telomerase. of mouse results to humans. “The Carol Greider of Johns Hopkins Mouse cells have a single period arguments at this stage are purely University made knockout mice of slowdown which may be theological,” says Elizabeth lacking the gene for telomerase RNA. equivalent to crisis. The mouse cells Blackburn of the University of Perhaps because of the long lacking telomerase were just as California, San Francisco. “There is telomeres that they are born with, the efficient as normal cells at getting no scientific evidence, only opinion.” mice live for as long as their parents through this block, so it seems that and can generate cancer-causing cells. re-expression of telomerase is not a A theory is born The knockout mice lose telomeric critical event in this mouse version of Telomerase solves a problem first DNA at the rate of ~4.8 kb per in vitro immortalization. recognized by James Watson in 1972. generation. By the sixth generation For the in vivo test, Blasco and The duplication of DNA is without telomerase, 56% of cells Greider added oncogenes to cells unidirectional, and it starts with an derived from the mice have the wrong cultured from sixth generation mice, RNA primer. Even if a primer numbers of chromosomes, 5% have then injected the cells into attaches to the very end of a undetectable amounts of telomeric immunodeficient (nude) mice. chromosome (the ), primer repeats at one or more chromosome Although these cells had divided over removal makes the new chromosome ends, and the average length of 300 times without telomerase, they a little bit shorter. Telomerase telomeric repeats has shrunk from were still capable of forming tumors. corrects this erosion by making more ~44 kb to ~14 kb. Bad things, one Greider is concerned that the copies of the protective sequence at suspects, are just around the corner. association of telomerase with some the end of the chromosome But in Blasco and Greider’s could be just correlation. (Figure 1). Telomerase has both an current description of the sixth Last year Titia DeLange of RNA component, which provides the generation, they find little evidence Rockefeller University in New York template for catalysis, and a recently of telomerase’s importance in cancer reported that telomerase levels discovered protein component with and . increased even in those cancerous reverse-transcriptase motifs. Their first test was growth in mouse cells that still had long In germ cells, telomerase actively culture. Cultured human cells must telomeres. “Just because a tumor maintains the ends of chromosomes. jump over two barriers to avoid expresses telomerase,” says Greider, But in most other cells telomerase is senescence and become immortal. “that doesn’t mean it needs it.” 880 Chemistry & Biology 1997, Vol 4 No 11

Hope for humans (and Geron) Another uncertainty involves a What was old is new again Harley cites results from yeast poorly characterized pathway named The idea that telomerase is involved (telomerase inhibition causes alternative lengthening of telomeres in cell senescence, itself only a small senescence) and cultured human (ALT). Roger Reddel of the part of the aging process, somehow cells (telomerase antisense causes Children’s Medical Research got translated into the idea that crisis) as a counter to Greider’s Institute in Sydney has described telomerase controls lifespan. “But findings. And he is not deterred by this pathway in cultured cells that that was never proposed by anyone,” the mouse results. “The initial lack detectable telomerase activity says Greider. Although Geron has generations of mice are not a good but acquire long telomeres. As in images of elderly but spry individuals model for telomerase biology in yeast, this lengthening may work by sprinkled liberally through its humans,” he says. Humans start off non-reciprocal recombination promotional materials, Geron is not with many fewer telomeric repeats between telomeric repeats, in which planning to stop people from aging. than mice, and some of the cancers DNA is copied from one telomere to They are, however, looking at that Geron hopes to treat have another. What is important for Geron how the cells with shortened particularly diminutive telomeres. is how often ALT happens in vivo in telomeres affect several age-related “I never thought that the telomerase humans. With this and other issues diseases. Geron researchers found approach was going to be a universal unresolved, David Kipling of the that telomeres shorten twice as fast in cure for all cancers,” says Greider. University of Wales at Cardiff says endothelial cells lining arteries “I think there will be some specific that “treating real people with real compared to those lining veins, cancers that will be good targets.” tumors is the only true experiment.” probably because the turbulence in One of the worst-kept secrets in arteries causes higher cell turnover. molecular biology is that mice Diagnostics Consistent with this, telomere loss beyond the sixth generation have Approximately half of Geron’s rates are highest for the sections of significant problems, but until this research is focussed on finding a arteries subject to the most work is published none of the authors telomerase inhibitor. A project with turbulence. Accumulating senescent are discussing it. These later less glamour but arguably less risk is cells may then lead to atherosclerosis. generations of mice may vindicate the investigation of telomerase Judith Campisi of the University parts of the telomere hypothesis, but activity as a marker for cancerous of California, Berkeley, has found they also raise the question of how cells. In this field correlation is that senescent cells accumulate as long a telomerase inhibitor will take enough; even if telomerase activation skin ages. She can see senescent skin to start working. Harley has always is a passive bystander in tumor cells by staining for the enzyme acknowledged that there will be a lag, formation it can be a valuable marker. β-galactosidase at neutral pH (it is a time in which the existing telomere Telomerase is associated with active in normal cells only at low pH). is gradually eaten away even as the over 80% of malignant tumors. Only Senescent skin cells produce too little cancer grows. He estimates the delay 14% of benign and premalignant collagen and too much collagenase, in human cells to be 20 population growths, and 6% of tissues adjacent and the combination may contribute doublings, equivalent in theory to a to tumors, test positive. Many of to the fragility of aging skin. million-fold increase in cell number. these ‘false positives’ may be true Geron is using the chips made by But Harley has calculated that, with positives, as the telomerase activity Synteni, Inc. (see Chemistry & Biology, the extensive cell death that is assay is especially sensitive, capable 4, 157–158) to search for more genes common in tumors, the tumor mass of detecting from one to ten that are expressed differentially in will increase by only several-fold. telomerase-positive cells. senescent cells, and hopes to modify This is a disadvantage that Geron “Telomerase is here to stay,” says that expression with small molecules. claims it can manage. “Every cancer Jerry Shay of the University of Texas These newer research directions therapy on the market today has a Southwestern. “It’s an extraordinarily may be good insurance if humans limitation,” says Harley. “We didn’t important new marker.” turn out to be as indifferent to the expect telomerase inhibitors to be Telomerase screening of easily loss of telomerase as mice. any different — we don’t expect available cells could replace invasive Meanwhile, supporters like Shay are them to be a magic bullet. But we do biopsies and tissue pathology as an ready to test the inhibitors. “There’s expect them to be more universal indicator of tumorigenicity. The an optimistic and a pessimistic way and safer than current therapies.” samples collected would include to spin any story,” he says. “But Harley plans to use telomerase urine (bladder cancer), sputum (lung, these trials will be done either way.” inhibitors to prevent metastases and and head and neck cancer), blood relapses that start from just a few enriched for epithelial cells (various William A. Wells, Biotext Ltd cells, not as treatments for large metastatic cancers) and nipple- 1095 Market Street #516, San Francisco, primary tumors. aspirated fluid (breast cancer). CA 94103-1628, USA; [email protected].