Geriatric Disorders Author: Ricki Lewis
SIDEBAR : TELOMERE BIOLOGY RESOURCES
Chromosome tips, called telomeres, have long fascinated geneticists because they protect chromosomes from degradation. And recent research has revealed another vital role for telomeres: As they shrink with each cell division, they keep a biochemical tally of the number of divisions remaining in a cell's lifetime. If this precise control goes awry, the biochemical consequences could be dire, such as uncontrolled growth or premature aging.
The role of telomeres in controlling cell division suggests applications in treating cancer and geriatric diseases. "It is a fascinating story, and we are right in the middle of its evolution," says Jerry Shay, a professor of cell biology and neurosciences at the University of Texas Southwestern Medical Center, Dallas. Shay's use of the word "middle" is telling-telomere biologists caution that applications of the new knowledge rest on learning more. "The research is in an exciting stage now, but our understanding of telomeres is not nailed down yet," comments Daniel Gottschling, an associate professor of molecular genetics and cell biology at the University of Chicago.
At the center of telomere research is telomerase, an enzyme built of RNA and protein that controls telomere length. Teams from Geron Corp. in Menlo Park, Calif., and Cold Spring Harbor Laboratory on Long Island, N.Y., recently sequenced the RNA part of human telomerase, which includes a template for the six DNA bases repeated at all human telomeres (J. Feng et al., Science, 269:1236-41, 1995). Telomerase aligns the template with an exposed DNA strand at the telomere and fills in the appropriate DNA bases. If telomerase is expressed, telomeres lengthen; if not, they shorten. When telomeres shorten to a critical length, cell division ceases.
Interest in telomere biology is soaring. "There has been a recognition that telomeres are important in human aging and disease. We've seen a large increase in the number of papers with 'telomere' or 'telomerase' in the title," says Titia de Lange, an associate professor in the laboratory of cell biology and genetics at Rockefeller University. Attendance at a session entitled "Telomeres and Telomerase" at the American Association for Cancer Research held last March in Toronto supports her claim-1,100 people showed up, compared with the few hundred at a typical session. And telomeres are making headlines in a range of publications, because interest in gerontology and cancer is universal. For example, The Economist ran articles on telomeres in the context of the genome project (336:73-4, Aug. 19, 1995) and aging (334:65-6, Jan. 7, 1995).
Telomere biology began early in the century, when cytogeneticists noted that chromosomes lacking tips fuse and vanish during cell division. In 1961, Leonard Hayflick, a staff scientist at the Wistar Institute in Philadelphia from 1957 until 1968, reported that human cells in culture divide a certain number of times, reaching a maximum "Hayflick limit" (L. Hayflick, P.S. Moorhead, Experimental Cell Research, 25:585-621, 1961). This observation would become critical in identifying telomeres' role as cell-division counters.
A decade later, James Watson, director of the Cold Spring Harbor Laboratory and codiscoverer of DNA's structure, presented a molecular view of telomeres. He described the "lagging strand problem," which predicts telomeres will shorten with each division (J.D. Watson, Nature, 239:197-200, 1972). During DNA replication, the two strands of the double helix duplicate in opposite directions. The strand proceeding from the chromosome tip inward lacks a small piece at its end. If this piece isn't replaced, Watson suggested, the chromosome would shorten with each cell division.
Alexy M. Olovnikov, a scientist at the Institute of Biochemical Physics in the Russian Academy of Sciences in Moscow, connected Hayflick and Watson's ideas (A.M. Olovnikov, Journal of Theoretical Biology , 41:181-90, 1973). He proposed that each DNA replication depletes telomere material to a point that signals the cell to stop dividing.
To probe the biochemical nature of telomeres, Joseph Gall and Elizabeth Blackburn, professors in the department of molecular biology at the University of California, Berkeley, investigated a common pond resident, Tetrahymena thermophila (E.H. Blackburn, J.G. Gall, Journal of Molecular Biology, 120:33-9, 1978). When these organisms form sex cells, their chromosomes shatter, then replicate explosively, yielding thousands of telomeres. Gall and Blackburn discovered that Tetrahymena's telomeres consist of many copies of a short DNA sequence.
Blackburn and graduate student Carol Greider then found how Tetrahymena keeps its telomeres long-an enzyme extends them. This ensures that the single-celled organism does not rapidly deplete itself into extinction (C.W. Greider, E.H. Blackburn, Cell, 43:405-10, 1985). In a 1987 paper (C.W. Greider, E.H. Blackburn, Cell, 51:887-96), Blackburn and Greider named the enzyme "telomerase."
Unlike the elongating telomeres of Tetrahymena, those in most somatic (nonsex) cells in multicellular organisms, including humans, shorten (C.B. Harley, A.B. Futcher, C.W. Greider, Nature, 345:458-60, 1990). Once staff scientist Robert Moyzis and his colleagues at Los Alamos National Laboratory had identified the six-base sequence (TTAGGG) repeated in human telomeres, researchers had a yardstick to measure telomeres (J. Meyne et al., Proceedings of the National Academy of Sciences , 86:7049-53, 1989). They could do so by using a restriction enzyme (HaeIII) that cuts proximally to the first repeat, and then determining the size of the resulting fragment. A polymerase chain reaction-based assay is also useful for measuring telomeres.
With Respect to the Elders: Calvin Harley's Geron Corp. applies principles of cell senescence to research treatments for aging disorders. Telomere shortening explained the Hayflick limit. It also suggested that cancer cells, which ignore the limit, might have long telomeres and active telomerase. In support of this hypothesis, telomerase activity was detected in HeLa cells (G.B. Morin, Cell, 59:521-7, 1989), which are a commonly used immortal cell line, and in ovarian cancer cells (C. Counter et al., PNAS, 91:2900-4, 1994).
Following the classic genetic approach of disrupting a gene to reveal its protein's function, researchers predicted that disabling telomerase in cells that normally express it would shorten chromosomes. Gottschling screened for pre-existing telomerase mutants in yeast, and found that these cells indeed have shrinking telomeres (M. Singer, D. Gottschling, Science, 266:404-9, 1994). With her colleagues, Greider-who in 1988 moved to Cold Spring Harbor Laboratory and is now a senior staff scientist there-used an antisense RNA that silences human telomerase's RNA template, which caused HeLa cells to die after 23 to 26 divisions (J. Feng et al., Science, 269:1236-41, 1995). "These results indicate that human telomerase is a critical enzyme for the growth and proliferation of immortal tumor cells," says Calvin Harley, a coauthor of the paper and vice president of research at Geron.
Geron Corp. was founded in 1992 to apply principles of cell senescence to treat disorders of aging (see story on page 1). One project tracks unusually short telomeres as a marker of accelerated aging in cells forming blood vessel inner linings (E. Chang, C.B. Harley, PNAS, 92:11190-4, 1995). Such cellular aging raises the risk of heart attack and stroke. The reported work was done in cultured cells, but the researchers plan to develop a test to predict atherosclerosis risk. "This is the first evidence utilizing the techniques of telomere biology to suggest that the aging of vascular cells contributes to atherosclerosis and, possibly, other cardiovascular diseases," notes Harley. "It also suggests that telomere length might be useful in predicting the onset of atherosclerosis."
Cancer Studies: Rockefeller's Titia de Lange investigates telomerase activity in peripheral blood cells. Halting telomere extension in cancer cells might open a new treatment approach. The first step toward applying telomere biology to cancer treatment is to establish correlations between telomere length (and too-active telomerase) and the cancerous state. So far, the data strikingly support the presence of telomerase activity in cancer cells, but not in healthy cells. This is largely the work of Shay's laboratory in Dallas, in collaboration with researchers at Hiroshima University School of Medicine and Geron.
"We've looked at more than 1,000 human primary cancers, and 90 percent of them have telomerase activity," Shay reports (N.W. Kim et al., Science, 266:2011-5, 1995). A telomerase assay may also have prognostic value (E. Hiyama et al., Nature Medicine, 1:249-55, 1995). "There is a good correlation with the stage of disease. The more advanced the cancer, the higher the telomerase activity," Shay adds. He and his collaborators have found this to be true for several types of cancer.
Particularly exciting is a correlation between increased telomerase activity and severity of illness seen in breast cancer (E. Hiyama, Journal of the National Cancer Institute, 88:116-22, 1996). Shay suggests that surgeons might be able to use a telomerase test to choose an appropriate treatment. A tumor with low telomerase activity might be effectively treated with a lumpectomy, whereas a tumor with high telomerase activity would require a mastectomy and chemotherapy.
The telomerase-cancer connection is not absolute, prompting investigators to urge caution in developing a telomerase-inhibiting drug. Unspecialized "stem" cells, such as those in the skin and small intestine, show weak telomerase activity. Titia de Lange and her group were surprised to detect low telomerase activity in peripheral blood cells, which are mature, specialized cells. The team had been assaying telomerase activity in leukemias and checked white blood cells from healthy blood or bone-marrow donors as controls (D. Broccoli, J.W. Young, T. de Lange, PNAS, 92:9082-6, 1995). "We hadn't expected telomerase activity in those cells-T and B cells-because others had shown that telomeres shorten in those cells," de Lange comments.
Telomerase level in blood cells isn't sufficient to lengthen the telomeres, Shay says, but even weak activity in healthy cells means that blocking telomerase may cause side effects. "We will watch in particular for potential side effects on hematopoietic and peripheral blood cells" in developing telomerase inhibitors as drugs, Harley remarks.
A second reason researchers cite for caution in inhibiting telomerase is that cells may have evolved ways to circumvent disabled telomerase. For example, some human cancer cell lines have long telomeres but no telomerase activity. These cells must keep their telomeres another way, researchers surmise. "In yeast, it is clear that there are [other] pathways for telomere maintenance," explains Virginia Zakian, a professor of molecular biology at Princeton University. "If you knock down telomerase, you see suppressor cells that can survive without it."
If cells have backup ways to lengthen telomeres, blocking telomerase to treat cancer could backfire by "naturally selecting" cancer cells that can survive without telomerase. "In a tumor, you'd be selecting something with a growth advantage. So you might have to hit two things, target more than one pathway," Zakian adds. Although researchers at Geron are already searching for telomerase inhibitors, telomere biologists say it's too soon to think about clinical trials. "It is a nice story because there are nice correlations. But we're not yet sure of the biology, and we can't manipulate it yet. So I'm a healthy skeptic," declares Gottschling. Still, the convergence of several lines of basic research, combined with convincing correlations between increased telomerase level and cancer severity, is making telomere biology a deservedly hot topic. Says Shay: "The optimism is justified for now."
SIDEBAR : TELOMERE BIOLOGY RESOURCES
Ricki Lewis, a freelance science writer based in Scotia, N.Y., and is the author of several biology textbooks. She is online at firstname.lastname@example.org.