Elizabeth Blackburn

As a teenager, Elizabeth Blackburn refused to learn how to type. Tapping on the keyboard pointed toward the stereotypical life of a young woman in a menial office job—and she wanted to do something that she considered substantial. Biology always interested Blackburn, and two books steered her toward research. Madame Curie told how Marie grew up poor in Poland, hungered in a chilly Paris garret while studying at the Sorbonne, and teased out tiny quantities of radium from tons of pitchblende. These romantic images enchanted the suburban Australian youngster. Curie’s relentless pursuit of education and science, despite discrimination and other cultural barriers, conjured up an individual who relished her work—and this impression resonated with Blackburn. Later, George Gamow’s stories about Mr. Tompkins, who travels through his own body, convinced her that studying molecules would unveil biological mechanisms at a level that nourished her curiosity.

In the mid 1970s, Blackburn joined Joseph Gall’s laboratory as a postdoc, aiming to sequence the DNA at chromosomal ends. These regions interested her because they possess special characteristics. Chromosome breaks fuse with one another but their natural ends are stable. Unknown structures—which scientists had dubbed telomeres—must seal their tips. Furthermore, a quirk in the mechanism of DNA replication should nibble one strand’s terminus every time a cell divides, yet chromosomes in most cells don’t shrink. Something must replenish them.

For her experimental material, Blackburn exploited a creature from pond scum. This organism, Tetrahymena thermophila, accumulates large numbers of linear minichromosomes—and thus, telomeres. Using radioactive DNA subunits, Blackburn began sequencing. One day, a spot on her X-ray film—visible even in the darkroom’s red light—jumped out. She knew she’d see such a signal only if the same sequence recurred many, many times. She had discovered a string of Cs—part of what would turn out to be the telomeric repeat, CCCCAA, of Tetrahymena DNA.

By showing that the Tetrahymena sequences can protect linear yeast chromosomes from degradation and that a particular yeast sequence functions similarly, she and Jack Szostak demonstrated in 1982 that distantly related organisms cap their chromosomes with reiterated DNA. These observations and others presented new puzzles. Common models could not explain how an organism could attach its characteristic telomeric repeat to DNA ends from another creature, nor how the number of telomeric sequences varied among chromosomes and cells. No known molecular machine could perform these feats. Blackburn wondered whether an undiscovered enzyme constructs telomeres on DNA ends.

Graduate student Carol Greider joined Blackburn’s lab to begin in earnest the hunt for the hypothetical protein. Using a synthetic telomere in a test tube, she sought a substance from Tetrahymena cells that could add the distinctive repeated DNA sequence. With this approach, Greider and Blackburn identified the enzyme that creates telomeric DNA in 1985. They called it “telomerase” and established that it contains an RNA plus the expected protein component. Blackburn and her team then demonstrated that the RNA serves as the template for telomeric DNA.

Blackburn triggered an explosion in the study of telomeres and telomerase. This field now touches many biomedical arenas, including aging, cancer, and stem cells.

Author: Evelyn Strauss, Ph.D.

Carol Greider:
Carol Greider learned early to ignore obstacles. Her undiagnosed dyslexia landed her in remedial spelling classes as a child, and from the age of six—when her mother died—she had to find support in nontraditional places and chart her own course. Perhaps in part because she grew to expect impediments, Greider honed her ability to maneuver around them. In Germany for a year when she was 12 years old, she earned Fs in English class diction exercises because she spelled words backwards, yet she navigated the city bus system without knowing the language.

In college, Greider explored multiple areas of biological research before finding a home for how she thinks. With a biochemical approach, she could perturb a single experimental component and assess how that change affects the system. This method satisfied her mind’s yearning for decisive answers. Weak GRE scores brought automatic rejections from most graduate schools, but Greider’s research experience and stellar grades caught the attention of the admissions committee at the University of California, Berkeley. At the interview, faculty member Elizabeth Blackburn impressed Greider with her charisma and enthusiasm about an offbeat topic that Greider knew nothing about—telomeres, the protective caps at chromosome ends. Blackburn had proposed that an undiscovered molecular machine adds the repeated DNA sequences that characterize these structures. This idea challenged conventional wisdom. Unsure whether such an enzyme even existed, Greider set about tracking it down—an especially bold move for a graduate student, who might have wanted a safe project.

Greider sought a substance from Tetrahymena thermophila—a single-celled creature found in fresh water—that could add the telomeric sequences to an artificial telomere. After nine months, she came into the lab on Christmas Day 1984 and developed an X-ray film that identified the radioactively labeled DNA molecules in her experimental reaction. Emerging from the darkroom, she saw a ladder pattern that suggested the presence of the substance she sought: Each rung was exactly six DNA-subunits larger than the preceding one. Her heart raced because the result looked so promising, but she didn’t want to be fooled by hope. Over the next six months, she conducted dozens of experiments to rule out dull explanations for the data. Finally her results convinced her that she had unearthed an enzyme that repeatedly adds a particular sequence to the
ends of chromosomal DNA.

Greider and Blackburn discovered that the enzyme is composed not just of protein; it also contains an RNA component. In her own lab, Greider isolated the gene that encodes the RNA module and showed that it is essential for telomerase activity.

Greider and colleagues subsequently established that telomeres shorten progressively in cultured human cells, a process that triggers cell suicide or a state in which the cell neither divides nor dies. This observation led to the idea that telomere attrition contributes to age-related diseases—and that cancer cells owe their immortality in part to reactivation of telomerase. Greider now continues probing connections between telomeres, cancer, stem cells, and age-related human diseases.

Author: Evelyn Strauss, Ph.D.

Vicki Lundblad:
In junior high school, Vicki Lundblad threw herself into science fair projects. Once she tested whether skin substances repel mosquitoes—and mistakenly released more than 100 insects into her house. Despite her initial enthusiasm for experimental work, she backed off from the pursuit in high school and immersed herself in music. She played the cello for hours each day, relishing the demands and creative opportunities. Lundblad holds a similar attitude toward research.

After college, where she vacillated between mathematics and biology, she decided to enter graduate school in biology. There, she heard a talk by Jack Szostak about his studies with Elizabeth Blackburn on telomeres, the caps that protect chromosome tips. Szostak and Blackburn had proposed the existence of an enzyme that adds DNA sequences to chromosome ends, thus enabling their maintenance, given that the cell-division process whittles down these termini. A year later, in 1983, Lundblad joined Szostak’s lab as a postdoc and strategized how to find this hypothetical molecular machine. She reasoned that telomeres of yeast with a faulty version of the enzyme, now called telomerase, would gradually shorten over many generations. Eventually this erosion would eat into sequences that signal DNA health; without that indicator of well-being, the cells would stop duplicating. By identifying yeast with those properties, she would unearth genes for constituents of the enzyme or its assistants.

In 1989, Lundblad found one such gene, which she named EST1 (for ever shorter telomeres), and we now know that the Est1 protein is a telomerase subunit that regulates its activity. Subsequent work extended Lundblad’s idea about the connection between short telomeres and cell-division capacity to mammals. Like yeast, many human cells fizzle in culture dishes when their telomeres have shrunk too far. This phenomenon underlies the body’s inability to rejuvenate particular tissues after injury or as we age.

Lundblad had noticed that a small proportion of yeast with flawed EST1 escape its lethal consequences. She and Blackburn discovered that these cells—despite inadequate Est1—rebuilt withering chromosome ends. They thus unveiled a telomere-replenishing system that did not rely on telomerase and predicted that similar schemes exist in mammalian cells. This idea proved correct. Some human cancers employ the telomerase-independent mechanism to refurbish telomeres, thus fostering unbridled proliferation.

Fueled by her success at identifying EST1, Lundblad sought additional genes involved in the chromosome-end reconstructing process when she established her own lab. She designed an approach that would expose not only participants in the bare-bones test-tube reaction, but also elements that govern telomerase’s behavior in living cells. This ambitious and painstaking project—as part of it, her team individually transferred 35,000 yeast colonies from petri dishes into culture broth—uncovered three additional genes involved in the telomerase pathway.

In parallel with Tom Cech, Lundblad found that the product of one of these genes resembles enzymes that copy RNA to make DNA—a hallmark of telomerase. The scientists had pinpointed the enzyme’s core.

As predicted, Est proteins also control telomerase’s conduct. For instance, Est1 recruits the enzyme to chromosome extremities. Lundblad is uncloaking additional essential roles the Est proteins play as she discerns how cells revitalize the crucial structures at their chromosome ends.

Author: Evelyn Strauss, Ph.D.