Elizabeth H. Blackburn, a pioneer in the study of telomeres—the ends of chromosomes, which play a role in aging and cancer—has always taken the unexpected path. Growing up in a family of physicians in Tasmania, Australia, she chose to enter medical research rather than medicine. Instead of studying the animals she had loved as a gift, she became fascinated with the chemical machinery of cells. At the University of Melbourne, where she lived in a women's residential college, she majored in biochemistry. For graduate school, she ventured abroad, in 1972, to the University of Cambridge. While there, Blackburn immersed herself in genetics under the mentorship of the Nobel laureate biochemist Frederick Sanger.
After three years at Cambridge, Ph.D. in hand, Blackburn was bound for a postdoctoral appointment at the University of California, San Francisco, to sequence viral DNA. But her fiancé, John W. Sedat, was headed for Yale. She switched projects and opted for Yale. Thus began a lifelong passion for telomeres.
The early-twentieth-century American geneticist Hermann J. Muller coined the term "telomere" from the Greek words telos (end) and meros (part). Muller and the American geneticist Barbara McClintock independently theorized that telomeres must serve a protective function for chromosomes, somehow keeping them separated from one another (the "naked" ends of two long, string-like chromosomes would otherwise fuse end to end). "McClintock did an amazing thing in the 1930s," Blackburn notes with admiration. No one knew about DNA at the time, but McClintock "could see and study chromosomes under the light microscope. She correctly surmised that the chromosome ends somehow stabilized the structure [of the chromosomes] during replication." Forty years after McClintock, when Blackburn decided to apply the DNA sequencing skills she had picked up at Cambridge, she was the only scientist studying telomeres. "I thought, 'Wow, I wonder what they're like?' Nobody knew. There was no hypothesis."
Blackburn's encounter with telomeres, and their associated biochemistry, began a life's work that has placed her among the world's leading cell biologists today. Telomeres have turned out to be a far more fascinating, and more important, line of biomedical research than even Blackburn originally suspected they would be. In her three decades of research and more than 120 peer-reviewed papers on the once-neglected subject, Blackburn has played a key part in major discoveries in the field of telomeres. Having joined the faculty at the University of California, San Francisco, after a fifteen-year delay in her original plans to go there, she is a mentor herself, to a number of young scientists. Together, Blackburn and her students, both former and current, have helped explain how telomeres act in protecting chromosomes from damage, in regulating cell division and cell death, and in such processes as aging and its associated diseases.
For her innovative and groundbreaking work, Blackburn has been recognized by peers, and richly honored. She is a member of the National Academy of Sciences and an elected fellow of the Royal Society of London, as well as the American Association for the Advancement of Science. She served on the President's Council on Bioethics during President George W. Bush's first administration but was dismissed in 2003 for her vocal objections to reports on aging and on stem cell research, among others. The reports, she felt, were neither balanced nor accurate reflections of the scientific fields from which they purported to draw. This April she will receive the Benjamin Franklin Medal in Life Sciences, presented annually by the venerable Franklin Institute in Philadelphia; the award has become one of the nation's most prestigious honors conferred on a scientist. Many Franklin Medal winners in science are also past or future recipients of the Nobel Prize.
For the first decade of her career, however, Blackburn toiled in relative obscurity. At her postdoctoral fellowship at Yale she joined the laboratory of cell biologist Joseph G. Gall. Gall had seen the value of working with a model organism, Tetrahymena, a pond-dwelling, single-celled ciliated protozoan. Like all eukaryotic organisms (organisms whose cells have a nucleus), including people, Tetrahymena has linear chromosomes inside the cell nucleus. What sets ciliated protozoans apart, though, is the sheer number of their chromosomes: Tetrahymena has as many as 40,000 in a single cell. (Each somatic cell of a human being carries just forty-six chromosomes.) The abundance of chromosome ends makes Tetrahymena an ideal organism for the study of telomeres, and so Blackburn set about determining their genetic sequence.
What she discovered was very curious: Telomeric DNA is made up of short, simple, repeating sequences of nucleic acids. (Much longer, more complex runs of nucleic acids make up the DNA sequences that constitute genes.) Soon she and other investigators found similar patterns of repeating DNA segments in the telomere sequences of other species though the number of repeats varied from organism to organism. For instance, Tetrahymena has strings of TTGGGG repeated about 50 times, and humans have strings of TTAGGG repeated about 2,000 times. (T, A, and G stand for the nucleic acids thymine, adenine, and guanine, respectively.)
That evidence and some other results led Blackburn to suspect that the simple sequences were performing a more complex function, and that something else in the cell was controlling the telomeres. Her colleagues remained politely interested but unimpressed, even as she was working out how the sequences were maintained over time. "After we described this work, I would go to meetings and be the last speaker, in the last session of the day" Blackburn says.
[pagebreak]From McClintock's era on, biologists had simply accepted the idea that telomeres somehow cap the end of a chromosome, just as plastic caps the ends of a shoelace and protects it from becoming unraveled. The cell is normally vigilant about detecting and repairing breaks in its chromosomes. In so doing, though, the cell could mistake an unprotected chromosome end for a break and attempt to fuse it to another chromosome end. The telomeres prevent that from happening. Blackburn was determined to find out how, but it would be many more years before she would have enough clues to sketch the process, and much about it remains unknown.
"We now think that there's something that hides the chromosome ends in plain sight," Blackburn says. "The cell sees the ends, but instead of hiding—and we still don't understand how they do this—the cell turns the recognition of the ends into a response appropriate to the telomeres. It's a very dynamic process, not like a passive shoelace end, and that was not expected at all."
Another cellular enigma was how telomeres manage to maintain their length and, hence, their functionality. By the early 1970s, biochemists realized that the normal process of DNA replication could not copy a chromosome all the way to its end. Consequently, with every chromosome replication and cell division, the telomeres should theoretically get shorter. Eventually, without anything to arrest the process, the telomeres would get so short that any further chromosome replication would cut into the genes themselves, and the cell would die. Because bacterial cell lines can live and divide for thousands of generations, chromosome shortening became a paradox known as the "end-replication problem."
There was much speculation about how the cells might solve the problem, but no empirical explanation. "In biology you can wave your hands and make up all these paper schemes" Blackburn says. "But the key thing was to show in the test tube that there really was a tangible mechanism." So in the mid-1980s Blackburn, who was running a laboratory at the University of California, Berkeley, and an especially determined graduate student named Carol W. Greider went back to Tetrahymena to figure out how cells preserve their telomeres. "We normally think of genetic material as sacred," Blackburn says. "But [Tetrahymena] chop up their somatic [non-germ line] chromosomes and add new repeat DNA [sequences] to the ends."
Blackburn had hypothesized that an undescribed enzyme within Tetrahymena cells was building new telomeric sequences. Another enzyme that assembles strands of DNA, called DNA polymerase, was already known. But DNA polymerase builds a new strand of DNA by using a single strand of the double helix that forms a chromosome as a copy template. The new strand is a complementary copy of the original. (Nucleic acids that make up DNA always pair with their complement: adenine with thymine, and cytosine with guanine.)
Unlike DNA polymerase, Blackburn and Greider's mystery enzyme, which they called telomerase, would have to build telomere sequences from scratch, with no template. To find the enzyme, Greider mixed synthetic telomeres, created in the laboratory, with extracts of Tetrahymena cells. The synthetic telomeres, Greider and Blackburn had reasoned, would be extended only if the Tetrahymena extracts contained the hypothesized telomerase enzyme. To their delight, the synthetic telomeric DNA grew longer, proving the existence of telomerase.
Their newly discovered enzyme turned out to be a remarkable molecular complex. Like most enzymes, telomerase contains protein. But the telomerase complex also includes a single molecule of RNA, a chemical cousin to DNA. "Telomerase is a collaboration between RNA and a protein" Blackburn explains. No one understands exactly how the two work together, but what is known is that the RNA codes for short segments of DNA that are added piece by piece to the ends of telomeres. Thus telomerase restores bits of telomere lost during cell division
The finding came as a surprise, Blackburn recalls, because "people had thought that only bad things, like the HIV virus, did this conversion of RNA to DNA. But here is a molecule that does this, not for evil, but for a critical function necessary for continued life." She now suspects that telomerase is an ancient molecule, a relic from a prebiotic world dominated by RNA reactions, rather than by proteins and DNA.
[pagebreak]With their molecule in hand, Blackburn and others could start to tease out how telomerase works in the cell. From work in the 1960s it was known that human cells grown outside the body, unlike the ceils of single-celled organisms, have a limited life span. After some twenty to fifty divisions (a number thought to be highly dependent on cell type), human cells stop dividing and enter a static phase known as senescence. Could telomeres be functioning as a clock that tells cells when they have reached the end of their line?
Greider left Blackburn's laboratory in Berkeley in 1988, for a postdoctoral fellowship at Cold Spring Harbor Laboratory, in Long Island, New York. There she discovered that the telomeres in laboratory-grown human skin cells get shorter with every cell division. The idea took hold that shortened telomeres could be a signal to the cell that its genetic material is getting old and is at risk of losing its integrity--in short, the shortened telomeres become the canaries in the coal mine that tell a cell it is dangerous to continue dividing.
Greider's finding led to speculation that the telomerase gene is turned off in normal cells; that telomerase remains active only in other actively dividing cells, such as immune cells and germ cells. "We know now that there's a smidgen of telomerase in just about all cells, and that it is protecting telomeres," says Blackburn. "But there's not enough telomerase to keep up with the shortening. With time, she adds, "the telomeres will gradually run down."
Intriguingly, human telomeres vary in length from individual to individual. Telomeres in centenarians, for instance, are longer than one would expect. Could longer telomeres be protecting longlived people? After all, centenarians live longer in part because they don't die from the diseases that kill most of their age cohorts. Perhaps robust telomeres and extra telomerase are helping protect them against heart disease and other diseases.
An important link between telomerase, disease, and aging was identified in 2001, with the discovery of a genetic mutation responsible for a rare disease called dyskeratosis congenita. People with the condition are born with only one functioning gene for telomerase, and as a result, their telomeres shorten rapidly. They show some signs of premature aging, such as gray hair in their teenage years, but the most dire effect is that they usually die in early adulthood or middle age from bone marrow failure and a resulting inability to fight infections. "It's a striking reminder that we need a lot of self-renewal and telomerase in immune cells," says Blackburn. Immune cells have to multiply rapidly when they meet an antigen. Without sufficient telomerase, those cells cannot survive enough cell divisions to overcome the invader.
Once it became clear that telomere shortening might have a role in cell aging and, conversely, that long telomeres might somehow contribute to human longevity, Blackburn's colleagues began to take notice. The once-quiet field exploded, and the cumulative citations for "telomerase" in medical and biological journals skyrocketed. As others began working on telomeres and telomerase, new insights into disease and aging have come to light. With them has come the potential for developing new treatments against some of humanity's most intractable killers.
One recent discovery is that shortened telomeres do not necessarily spell imminent cell death, or even loss of vitality; the more important factor is whether enough telomerase is available in the cell nucleus to rescue and protect the remaining telomere ends. Remarkably, available telomerase turns out to be at least one key to the ability of cancer cells to circumvent the genetic safeguards of normal cell senescence.
In a malignant tumor, cancer cells divide and multiply indefinitely, becoming immortal, runaway tissue that consumes all the resources that would otherwise go to healthy tissue. In the early 1990s Greider and others found that the telomerase concentration in cancer cells is 100 times higher than it is in normal cells. The elevated telomerase occurs both in cancer-cell lines grown in the laboratory and in ovarian tumors growing in the body.
[pagebreak]Somehow, then, on the road to becoming malignant, cancer cells switch on the telomerase gene before the telomeres become too short for cell division. Surprisingly, the telomeres in cancer cells are often much shorter than the telomeres in the cells of surrounding tissue—evidence that the cancer cells had already begun to replicate (and their telomeres had begun to shorten) at breakneck speed, before the telomerase came back on the scene to perform its vital function.
If telomerase could somehow be inactivated, malignancies would presumably stop before they could spread to other parts of the body, establish new malignancies, and do their extensive damage. (Blackburn suspects, nonetheless, that cancer cells may be able to subvert telomere shrinkage in other ways as well.) Hence, blocking the production of telomerase has become an attractive target for cancer therapies, particularly if they can home in on specific tissue and avoid cells, such as immune cells, that depend on telomerase to keep the body healthy. For the investigators in Blackburn's lab, as well as for geneticists at other universities and within the biotech industry, telomerase blockers have become an important, emerging line of research.
Cancer is by no means the only cell-damager associated with telomere length. In 2004 Blackburn joined forces with Elissa S. Epel, a psychiatrist and clinical colleague at the University of California, San Francisco, to test the role of psychological stress in aging at the cellular level. "We started with the observation [of Epel's] that people look really old and drawn when they have chronic worries and stress in their lives," Blackburn explained. "But we had no hypothesis about whether we'd see an effect on telomeres in the cell. Nobody knew, so I said we should just look."
Blackburn, Epel, and Richard M. Cawthon, a geneticist at the University of Utah in Salt Lake City, conducted a study of thirty-nine women, ages twenty to fifty, who had been caring for a child suffering from a serious chronic illness, such as autism or cerebral palsy. Those women, presumably highly stressed, were compared to a control group of nineteen mothers of healthy children. Stress was quantified in part by the number of years each woman in the test group had been caring for an ill child. That number was combined with other objective measures of stress, including so-called oxidative stress (damage to DNA caused by "free radicals"), one of the major risk factors for cardiovascular disease.
The investigators discovered a clear correlation between the number of years a woman had been caring for her sick child and shortening of telomeres. The stressed women also had lower levels of telomerase in their white blood cells and higher levels of oxidative stress. Moreover, the investigators found that the perceived stress in their test group, as measured by a subjective battery often questions called Cohen's Perceived Stress Scale, was also correlated with shorter telomeres and lower telomerase levels in the blood cells. The finding held whether the mother had an ill child or not. "We didn't expect to see such a clear relationship right across the full range," Blackburn says. "Elissa crafted a beautiful study where she had a well-controlled group of individuals, and the relationship between stress and telomere length really held." In other words, a woman's perception of the level of her own stress is correlated with her body's cellular response. As far as Blackburn and Epel can determine, this result was the first time a mind-body link that reaches into the cell was established.
"Of course, now we want to Understand exactly how stress is affecting the cell," says Blackburn. "Stress is changing hormones in your blood and bathing the cells in something that's different. So that's what we're trying to do in the lab: figure out what things influence telomerase."
Blackburn credits much of her success to supportive research environments and the resulting opportunity to pursue curiosity-driven science. "Thank goodness I don't work in industry," she says. "You can do really good research in industry, but you have to stay on some kind of goal-directed line. [At universities] you're still goal directed, but you can be more creative."
She remains keenly aware of the importance of scientific mentors—in her case, Sanger at Cambridge and Gall at Yale. "Sanger was supportive in a quiet way," she says. "He loved being in the lab and liked talking about science. It was important to feel that I could always converse with him." Gall was equally supportive. "Joe Gall was famous for having a good proportion of women postdocs who had done well," Blackburn says. "He would announce to the lab when one of his former students or postdocs got tenure. Joe realized it was important to be sending a positive message."
In her turn, Blackburn is quite serious about her own role as a mentor, particularly for women in science. "Carol Greider said the fact that I had a child was encouraging to her," Blackburn recalls. "It's important to show that you don't have to give your entire life over to science, that you can be successful by being smart and efficient and not always working long hours and weekends."
When Blackburn thinks about the future direction of her lab's work on telomerase, she has a two-pronged approach. "I would like to go deep into the chromosome and really understand what is happening structurally and functionally around the telomeres. This is a dynamic, robust system, like a buzzing bazaar with all sorts of molecules coming and going. I would love to understand the dynamics."
But it's also important to her that such deep knowledge be" applied to healing the body. What can knowledge add to the understanding of how things can go wrong? How can it help treat cancer, chronic stress, and heart disease? "We want to exploit knowledge of telomerase and telomeres to develop therapies at a cellular level," she says. Ambitious goals, to be sure. But considering how far Elizabeth Blackburn has already pushed the study of telomeres, such goals could well be within her grasp.