Some envision biomedical research as a steady march, moving one resolute step after another toward medical progress. But many fundamental discoveries happen when researchers meander—and indulge that most childlike of traits, curiosity.
So it was with chromosome tips, or telomeres, whose mysterious nature had vexed biologists for decades. For solving that mystery, HHMI investigator Jack Szostak of Harvard Medical School and Massachusetts general Hospital shared the 2009 Nobel Prize in Medicine or Physiology with Elizabeth Blackburn of the University of California, San Francisco, and Carol Greider of the Johns Hopkins University School of Medicine.
As early as the 1930s, pioneering geneticists had learned that when chromosomes are severed by x-ray irradiation, the broken ends behave differently from natural chromosome tips: The broken chromosome fragments reattach to each other and insinuate themselves into other chromosomes; chromosomes capped by telomeres don't.
By the 1960s, the mystery had only intensified. By then scientists had determined the structure of DNA and its means of replication. It became clear that the enzyme that copies DNA, called DNA polymerase, works by extending a preexisting piece of DNA rather than assembling DNA solely from its molecular components. At the very tip of a chromosome there is no preexisting piece to extend, yet somehow cells replicate DNA at their chromosome tips every generation.
To shed light on telomeres, Elizabeth Blackburn first found an organism with lots of them: a pond-dwelling, single-celled protozoan called Tetrahymena that produces small, linear minichromosome by the thousands. Beginning in 1975, she isolated these minichromosomes and analyzed the nucleotide sequence at their tips. The result was unusual, to say the least: between 20 and 70 successive repeats of a specific six-nucleotide sequence. Blackburn presented her results at a conference in 1980. In the audience that day was Szostak.
Szostak had discovered that linear DNA added to yeast cells is short-lived: it is either chewed up or incorporated into the cell's chromosomes. In contrast, Blackburn had found that Tetrahymena telomeres stabilize the protozoan's DNA. Szostak was struck by the difference, and after Blackburn spoke, he approached her and proposed an experiment. What if they attached telomeres from Tetrahymena to the tips of yeast linear DNA? Would the Tetrahymena telomeres stabilize the hybrid minichromosomes in yeast? It was a long shot. Yeast diverged from Tetrahymena long ago, and the two organisms have had eons to evolve different types of telomeres. But maybe, just maybe, the transplanted Tetrahymena telomeres would stabilize the hybrid minichromosomes. If so, then telomere structures were widely conserved–and probably of fundamental biological importance.
Remarkably, the experiment worked. The hybrid minichromosomes remained intact in yeast, generation after generation. Szostak knew that they could now be used to clone yeast telomeres. From the hybrid minichromosome, Szostak now excised one of the two Tetrahymena telomeres, and joined it to random pieces of yeast DNA. Only yeast DNA with the yeast telomere attached would be propagated in growing yeast. The strategy worked. Szostak had cloned the yeast telomere.
By comparing the DNA sequence of the yeast and Tetrahymena telomeres, the two researchers discovered that while both had characteristic six-nucleotide repeats, their respective sequences differed slightly. What's more, yeast continually adds DNA to the tips of Tetrahymena telomeres, and that DNA is characteristic of yeast telomeres. That suggested that an enzyme existed that added telomere DNA to chromosome tips.
Carol Greider, then a graduate student in Blackburn's laboratory, began working to isolate that enzyme biochemically. In December 1984, she added a 24-nucleotide fragment of telomere DNA to an extract of Tetrahymena cells. The extract added six-nucleotide snippets to that fragment, one snippet at a time, proving that an enzyme is responsible. Greider and Blackburn dubbed the responsible enzyme “telomerase.” They purified it and showed that it contained both protein and RNA. Later they learned that RNA serves as a template to help construct the telomere.
As Greider pursued telomerase biochemically, Szostak and a graduate student in his lab, Vicki Lundblad, were curious what role it played in cells. By screening for yeast that were defective in telomere maintenance, Lundblad identified a mutant in which telomeres grew shorter each generation. The mutant yeast strain, called EST1 for “ever shorter telomeres,” began to lose chromosomes and die after about 50 generations—unlike normal yeast, which reproduce indefinitely into healthy daughter cells. The results suggested that maintaining long enough telomeres keeps cells youthful, and that gradually losing them leads to premature aging. Later, scientists showed that telomeres shorten as cultured human cells age and die after dozens of generations, and that adding telomerase back to these cells keeps them young.
These results in turn suggested that activating telomerase might keep cancer cells perpetually youthful, and that blocking telomerase might help slow the growth of cancers. In recent years, an experimental telomerase-fighting drug and an experimental telomerase vaccine have been developed to help treat cancer.
Today telomere research has bloomed into a major research enterprise, with implications for understanding aging, stem cells, and cancer. Francis Collins, director of the National Institutes of Health, said that the discoveries by Greider, Blackburn and Szostak “offer a classic example of how basic science research driven by investigators' curiosity can illuminate our understanding of health and disease.” Szostak agrees that curiosity-driven research is critical: “It's only by tackling the fundamental questions that you can make large advances.”
Photo: Mark Wilson